Dissertations / Theses on the topic 'Composite laminate design'

Genetic algorithms are well known for being expensive optimization tools, especially if the cost for the analysis of each individual design is high. In the past few years, significant effort has been put forth in addressing the high computational cost GAs. The research conducted in the first part of this thesis continues this effort by implementing new multiple elitist and variable elitist selection schemes for the creation of successive populations in the genetic search process. The new selection schemes allow the GA to take advantage of a greater amount of important genetic information that may be contained in the parent designs, information that is not utilized when using a traditional elitist method selection scheme. By varying the amount of information that may be passed to successive generations from the parent population, the explorative and exploitative characteristics of the GA can be adjusted throughout the genetic search also. The new schemes provided slight reductions in the computational cost of the GA and produced many designs with good fitness' in the final population, while maintaining a high level of reliability. Genetic algorithms can be easily adapted to many different optimization problems also. This capability is demonstrated by modifying the basic GA, which utilizes a single chromosome string, to include a second string so that composite laminates comprised of multiple materials can be studied with greater efficiently. By using two strings, only minor adjustments to the basic GA were required. The modified GA was used to simultaneously minimize the cost and weight of a simply supported composite plate under different combinations of axial loading. Two materials were used, with one significantly stronger, but more expensive than the other. The optimization formulation was implemented by using convex combinations of cost and weight objective functions into a single value for laminate fitness, and thus required no additional modifications to the GA. To obtain a Pareto-optimal set of designs, the influence of cost and weight on the overall fitness of a laminate configuration was adjusted from one extreme to the other by adjusting the scale factors accordingly. The modified GA provided a simple yet reliable means of designing high performance composite laminates at costs lower than laminates comprised of one material.
Master of Science

Advanced composite materials have seen major market growth in recent years due to their high strength and low weight properties. These materials are often made using a process that creates a composite laminate by stacking several composite layers together. However, the design, analysis and optimization of laminate composite materials are often a labor intensive process when done manually. This thesis discusses CAD independent algorithms that are integrated into commercial CAD tools to streamline these processes. Methods have been developed to automatically create 3D ply geometry for a laminate composite lay-up, streamline the creation of a laminate composite finite element model and optimize the composite lay-up for a multi-layered laminate composite part. Integrating a CAD independent geometry kernel into the NX laminate composite design automation application significantly improves the run time of that application. In addition, the automated composite finite element tool creates laminate composite finite element models that are more detailed than those made with zone based methods. This tool will save engineers, who are making laminate composite finite element models manually, dozens of hours of work per model. The automated composite finite element tool can also be integrated into an optimization framework, used in conjunction with a method to automatically apply boundary conditions, to create an effective optimization of a laminate composite part.

This thesis describes the development of a general Fortran 90 framework for the solution of composite laminate design problems using a genetic algorithm (GA). The initial Fortran 90 module and package of operators result in a standard genetic algorithm (sGA). The sGA is extended to operate on a parallel processor, and a migration algorithm is introduced. These extensions result in the distributed genetic algorithm with migration (dGA). The performance of the dGA in terms of cost and reliability is studied and compared to an sGA baseline, using two types of composite laminate design problems. The nondeterminism of GAs and the migration and dynamic load balancing algorithm used in this work result in a changed (diminished) workload, so conventional measures of parallelizability are not meaningful. Thus, a set of experiments is devised to characterize the run time performance of the dGA. The migration algorithm is found to diminish the normalized cost and improve the reliability of a GA optimization run. An effective linear speedup for constant work is achieved, and the dynamic load balancing algorithm with distributed control and token ring termination detection yield improved run time performance.
Master of Science

To meet the need of lightweight chassis in the near future, a technological step of introducing anisotropic materials like Carbon Fibre Reinforced Plastics (CFRP) in structural parts of cars is a possible way ahead. Though there are commercially available tools to find suitability of Fibre Reinforced Plastics (FRPs) and their orientations, they depend on numerical optimization and complexity increases with the size of the model. Nevertheless, the user has a very limited control of intermediate steps. To understand the type of material system that can be used in different regions for a lightweight chassis, especially during the initial concept phase, a more simplified, yet reliable tool is desirable.The thesis aims to provide a framework for determining fibre orientations according to the most-ideal loading path to achieve maximum advantage from FRP-materials. This has been achieved by developing algorithms to find best-fit material orientations analytically, which uses principal stresses and their orientations in a finite element originating from multiple load cases. This thesis takes inspiration from the Durst criteria (2008) which upon implementation provides information on how individual elements must be modelled in a component subjected to multiple load cases. This analysis pre-evaluates the potential of FRP-suitable parts. Few modifications have been made to the existing formulations by the authors which have been explained in relevant sections.The study has been extended to develop additional MATLAB subroutines which finds the type of laminate design (uni-directional, bi-axial or quasi-isotropic) that is suitable for individual elements.Several test cases have been run to check the validity of the developed algorithm. Finally, the algorithm has been implemented on a Body-In-White subjected to two load cases. The thesis gives an idea of how to divide the structure into sub-components along with the local fibre directions based on the fibre orientations and an appropriate laminate design based on classical laminate theory.

Optimization is a decision making process to solve problems in a number of fields including engineering mechanics. Bio-inspired optimization algorithms, including genetic algorithm (GA), have been studied for many years. There is a large literature on applying the GA to mechanics problems. However, disadvantages of the GA include the high computational cost and the inability to get the global optimal solution that can be found by using a honeybee-inspired optimization algorithm, called the New Nest-Site Selection (NeSS). We use the NeSS to find optimal parameters for three mechanics problems by following the three processes: screening, identifying relationships, and optimization. The screening process identifies significant parameters from a set of input parameters of interest. Then, relationships between the significant input parameters and responses are established. Finally, the optimization process searches for an optimal solution to achieve objectives of a problem. For the first two problems, we use the NeSS algorithm in conjunction with a third order shear and normal deformable plate theory (TSNDT), the finite element method (FEM), a one-step stress recovery scheme (SRS) and the Tsai-Wu failure criterion to find the stacking sequence of composite laminates and the topology and materials for doubly curved sandwich shells to maximize the first failure load. It is followed by the progressive failure analysis to determine the ultimate failure load. For the sandwich shell, we use the maximum transverse shear stress criterion for delineating failure of the core, and also study simultaneously maximizing the first failure load and minimizing the mass subject to certain constraints. For composite laminates, it is found that the first failure load for an optimally designed stacking sequence exceeds that for the typical [0°/90°]₅ laminate by about 36%. Moreover, the design for the optimal first failure load need not have the maximum ultimate load. For clamped laminates and sandwich shells, the ultimate load is about 50% higher than the first failure load. However, for simply supported edges the ultimate load is generally only about 10% higher than the first failure load. For the atmospheric spray process, we employ the NeSS algorithm to find optimal values of four process input parameters, namely the argon flow rate, the hydrogen flow rate, the powder feed rate and the current, that result in the desired mean particle temperature and the mean particle velocity when they reach the substrate. These optimal values give the desired mean particle temperature and the mean particle velocity within 5% of their target values.
Ph. D.

The main challenge in the design of vibration energy harvesters is the optimization of energy outcome relative to the applied excitation to reach a higher efficiency in spite of the weakness of ambient energy sources. One promising principle of vibration converters is magnetoelectricity due to the outstanding properties of magnetostrictive and piezoelectric laminate composites, which provide interesting possibilities to harvest energy from low amplitude and low frequency vibration with relatively high energy outcome. For these devices, ensuring high deformations in the magnetostricive layers, improvement of the magnto-mechanical and the electro-mechanical couplings are highly required for the optimization of the energy outcome. This thesis primarily aims to develop a model based harvester design for magnetoelectric (ME) converters. Based on a comprehensive understanding of the complex energy flow in magnetoelectric transducers, several design parameters are investigated. For instance, magnetostriction in a Terfenol-D plate is investigated by means of atomic force microscopy under similar conditions as within magnetoelectric transducers. A novel measurement approach was successfully developed to detect the evolution of magnetic domains and measure deformations in a Terfenol-D plate in response to externally non-uniform applied magnetic fields. Furthermore, a finite element model is developed to predict the induced voltage in the ME transducer as a response to the magnet’s displacement, corrected based on atomic force microscopy measurements, and used for the design of the harvester. The presented three- dimensional model takes into consideration the nonlinear behaviour of the magnetostrictive and piezoelectric materials. Additionally, three novel converters having different magnetic circuits are designed and analysed analytically based on Lindstedt-Poincaré method. The effects of the structure parameters, such as the nonlinear magnetic forces, the magnetic field distribution and the resonance frequency are discussed, and the electric output performances of the three designed converters are evaluated. In order to improve both mechanical and electrical coupling between the piezoelectric and the magnetostrictive layers, a bonding technique at room temperature is proposed which uses conductive polymer nanocomposites. Two magnetoelectric transducers are fabricated based on this technique having 1 wt.% and 2 wt.% concentration of multiwalled carbon nanotubes in epoxy resin. Another magnetoelectric transducer is fabricated by a classical technique for comparison purposes. In order to validate the design, a series of demonstrators are designed and fabricated according to the simulation and optimization results. The proposed design is composed by a cantilever beam, a magnetic circuit with several magnet arrangements and a magnetoelectric transducer, which is formed by a piezoelectric PMNT plate bonded to two magnetostrictive Terfenol-D layers. In this design, external vibrations are converted to magnetic field changes acting on the magnetostrictive layers leading to deformations, which are transmitted directly to the piezoelectric layer. The converters are tested under harmonic excitations and real vibration profiles reproduced by an artificial vibration source. Different parameters were investigated experimentally including the magnetic forces between the transducer and the magnetic circuit and the used bonding technique. Tuning the resonance frequency of the ME converter is also addressed using a simple screw/nut system, which allows to control the relative position and therefore the magnetic forces between the magnetic circuit and the transducer. The magnetoelectric transducer bonded with 2 wt.% concentration of multiwalled carbon nanotubes shows better output performances than the two other ME transducers under similar excitations. A maximum power output of 2.42 mW is reached under 1 mm applied vibration at 40 Hz. This performance presents an improvement of minimum 20 % of the reached energy outcome by other magnetoelectric vibration converters using single ME transducer at comparable applied excitations.
Die größte Herausforderung bei der Konstruktion von Vibrations-Energiewandlern ist die Optimierung der gewonnenen Energie im Verhältnis zur angewandten Anregung, um trotz schwacher Umgebungsenergiequellen einen hohen Wirkungsgrad zu erreichen. Ein vielversprechendes Prinzip von Vibrationswandlern ist die Magnetoelektrizität aufgrund der hervorragenden Eigenschaften von magnetostriktiven und piezoelektrischen Verbundwerkstoffen, die interessante Möglichkeiten bieten, Energie aus niederfrequenten Schwingungen mit kleinen Amplituden zu gewinnen. Bei diesen Wandlern ist die Sicherstellung hoher Verformungen in den magnetostriktiven Schichten, die Verbesserung der magnetisch-mechanischen und der elektromechanischen Kopplungen für die Optimierung des Energieertrages sehr wichtig. Diese Arbeit zielt in erster Linie auf die Entwicklung eines modellbasierten Entwurfs für magnetoelektrische (ME) Wandler ab. Basierend auf einem umfassenden Verständnis des komplexen Energieflusses in magnetoelektrischen Wandlern werden mehrere Entwurfsparameter untersucht. So wird beispielsweise die Magnetostriktion in einer Terfenol-D-Platte mittels Rasterkraftmikroskopie unter ähnlichen Bedingungen untersucht wie in magnetoelektrischen Wandlern. Dabei wurde eine neuartige Messmethode erfolgreich entwickelt, um die Entwicklung von magnetischen Domänen zu erfassen und die Deformation in einer Terfenol-D-Platte als Reaktion auf extern ungleichmäßig angelegte Magnetfelder zu messen. Darüber hinaus wird ein Finite-Elemente-Modell entwickelt, um die induzierte Spannung im ME-Wandler als Reaktion auf die Verschiebung des Magneten vorherzusagen, welches auf der Grundlage von Atomkraftmikroskopie Messungen korrigiert und für den Entwurf des Energiewandlers verwendet wird. Das vorgestellte dreidimensionale Modell berücksichtigt das nichtlineare Verhalten der magnetostriktiven und piezoelektrischen Materialien. Zusätzlich werden drei neuartige Wandler mit unterschiedlichen Magnetkreisen nach dem Lindstedt-Poincaré Verfahren konzipiert und analytisch analysiert. Die Auswirkungen der Strukturparameter, wie die nichtlinearen Magnetkräfte, die Magnetfeldverteilung und die Resonanzfrequenz, werden diskutiert und die elektrischen Ausgangsleistungen der drei ausgelegten Wandler ausgewertet. Um die mechanische und elektrische Kopplung zwischen der piezoelektrischen und der magnetostriktiven Schicht zu verbessern, wird eine bei Raumtemperatur prozessierbare Verbindungstechnik vorgeschlagen, bei der leitfähige Nanokomposite verwendet werden. Zwei magnetoelektrische Wandler werden basierend auf dieser Technik mit einer Konzentration von 1 wt.% und 2 wt.% an mehrwandigen Kohlenstoff-Nanoröhren in Epoxidharz hergestellt. Ein weiterer magnetoelektrischer Wandler wurde zu Vergleichszwecken mit einer klassischen Technik hergestellt. Für die Validierung des Entwurfes wird eine Reihe von Demonstratoren entsprechend den Simulations- und Optimierungsergebnissen konstruiert und gefertigt. Der vorgeschlagene Entwurf besteht aus einem Trägerbalken, einem Magnetkreis mit mehreren Magnetanordnungen und einem magnetoelektrischen Wandler, der aus einer piezoelektrischen PMNT-Platte besteht, die mit zwei magnetostriktiven Terfenol-D-Schichten verbunden ist. Bei dieser Konstruktion werden externe Schwingungen in Magnetfeldänderungen umgewandelt, die auf die magnetostriktiven Schichten wirken und zu Verformungen führen, die direkt auf die piezoelektrische Schicht übertragen werden. Die Wandler werden unter harmonischen Anregungen und mit realen Schwingungsprofilen getestet, die von einer künstlichen Schwingungsquelle reproduziert werden. Verschiedene Parameter wurden experimentell untersucht, darunter die magnetischen Kräfte zwischen dem Wandler und dem Magnetkreis sowie die verwendete Verbindungstechnik. Die Abstimmung der Resonanzfrequenz des ME-Wandlers erfolgt ebenfalls über ein einfaches Schrauben-Mutter-System, das es ermöglicht, die relative Position und damit die magnetischen Kräfte zwischen Magnetkreis und Wandler zu steuern. Der magnetoelektrische Wandler, der mit einer Konzentration von 2 wt.% mehrwandiger Kohlenstoff-Nanoröhrchen verbunden ist, zeigt bessere Ausgangsleistungen als die beiden anderen ME-Wandler bei ähnlichen Anregungen. Eine maximale Ausgangsleistung von 2,42 mW wird bei 1 mm angelegter Vibration bei 40 Hz erreicht. Diese Leistung stellt eine Verbesserung von mindestens 20 % im Vergleich zu anderen magnetoelektrischen Schwingungsumrichtern dar, welche mit einem einzigen ME-Wandler bei vergleichbaren Anregungen getestet werden.

The manufacture, laminate design, and modeling of a part with complex geometry are explored. The ultimate goal of the research is to produce a model that accurately predicts part stiffness. This is validated with experimental results of composite parts, which refine material properties for use in a final prototype part model. The secondary goal of this project is to explore manufacturing methods for improved manufacturability of the complex part. The manufacturing portion of the thesis and feedback into material model has incorporated a senior project team to perform research on manufacturing and create composite part to be used for experimental testing. The senior project was designed, led, and managed by the author with support from the committee chair. Finite element modeling was refined using data from coupon 3-point bend testing to improve estimates on material properties. These properties were fed into a prototype part model which predicted deflection of composite parts with different layups and materials. The results of the model were compared to experimental results from prototype part testing and 3rd party analysis. The results showed that an accurate mid-plane shell element model could be used to accurately predict deflection for 2 of 3 experimental parts. There are recommendations in the thesis to further validate the models and experimental testing.

Existing test and analytical methods (theoretical and numerical) are normally restricted to thin laminate components, which cannot accurately represent the 3D stress state behaviour of the so called Ultra Thick Laminates (UTL) structures. Thus, it is necessary to expand the scope of application of the current numerical methods to accurately predict the out-of-plane delamination failure associated with these types of structures (mainly due to the transverse shear stresses and interlaminar stresses). The overall objective of this work is to address the following research objectives: • To assess the functionality, advantages and limitations of different solid element formulations, including layered solid elements that are available in commercial Finite Element codes, applied to the mechanical response prediction of UTL composite components (thicknesses up to 30 mm are considered). • To perform a design exploration and optimisation of constant thickness UTL composite component in terms of the orientation of a varying and repeatable stacking sequence of an eight ply Non-Crimped Fabric, in order to assess the design implications on performance. In order to achieve the above stated objectives a standard, flexible and expandable FE based design exploration methodology (at a ply level) for UTL composite components is proposed, which considers a commercial FE tool (ANSYS), and a data management system and optimisation tool (ISIGHT), through the use of layered solid elements (SOLID186 and SOLID191, 20-node layered solid elements). Application of manufacturing design rules (for reducing the number of feasible stacking sequences to be evaluated) is also considered, in order to reduce the computational cost of such a study, as well as to present a practical solution from the manufacturing point of view. Initially, in-plane and out-of-plane capabilities of various layered element formulations and modelling strategies where evaluated for thin and thick laminate applications against known analytic solutions (CLT, etc), in order to understand the key parameters and the accuracy limitations of each formulation. This led to practical recommendations for pre and post processing of thick laminate FE models, such as for the number of layered solid elements required as a function of the thickness of the UTL component to effectively predict the magnitude and variation in transverse shear stress across the thickness. The application of this research was demonstrated on the design exploration and performance optimisation of a UTL composite specimen (with constant thickness) under a 3-point bending test (linear static analysis), for which experimental results were available. The individual ply orientations are the design variables considered, and the performance was assessed through the vertical displacement of the component and the maximum transverse shear stress value. This exploration of the design space did identify other possible configurations that may have a better performance than the baseline (Biax), considering only the maximum transverse shear stress values as directly responsible for the delamination failure. However, these improved designs may present a higher number of plies failed or a higher failure index (Tsai-Wu failure criteria). Further experimental studies are required to further explore the design space, but this work represents the starting point and possible approaches for development of robustness and weight optimisation of UTL composites are proposed.

Thesis (M.S. in Mechanical Engineering)--Naval Postgraduate School, December 1990.
Thesis Advisor(s): Shin, Phillip Y. "December 1990." Description based on title screen as viewed on March 28, 2010. DTIC Descriptor(s): Thickness, optimization, layers, laminates, finite element analysis, height, stiffening, orientation(direction), blades, solutions(general), buckling, equations, fibers, loads(forces) DTIC Identifier(s): Design optimization, laminates, blade stiffened plate, buckling Author(s) subject terms: Design optimization, blade stiffened plate, buckling Includes bibliographical references (p. 81-84). Also available in print.

Thesis (M.S.)--West Virginia University, 2003.
Title from document title page. Document formatted into pages; contains xiv, 180 p. : ill. Includes abstract. Includes bibliographical references (p. 115-118).

A new specimen design for determining the compression strength of thick unidirectional laminate composites has been developed using finite element simulations and validated by experimental testing. The computational models included parts of the testing fixture. The materials used for experiments were carbon fibre/epoxy T300/914 from Hexcel Composites and IM7/8552. An understanding has been developed to explain why, using the standard, parallel sided design, for testing specimens in compression and using the ICSTM fixture, specimens using a laminate thicker than 2 mm do not fail in an acceptable way. Initially, simulation and experimental parametric studies were carried out to investigate the effects of loading and design conditions on the fixture and specimen in order to change the stress distribution in the 2 mm thick, parallel sided, 10 mm x 10 mm gauge section specimen. In addition, in order to optimise the specimen itself, different adhesives for bonding end tabs to the laminate were investigated, as were the end tab design and material used in their manufacture. Subsequent simulations showed that the use of an extended and waisted gauge length of either circular or s-shaped profile both caused thick laminate specimens to fail close to the centre of the gauge length. The predicted strength being similar to that measured for a 2 mm thick, parallel sided specimen using the optimised design. Experimental compression strength data from thick laminate specimens with the circular and s-shaped profiles machined into the gauge section validated the finite element results; the strengths achieved being almost identical to those for the 2mm thick laminates. Results from the analysis of the standard design and some preliminary work on the waisted design were presented at a conference [52]. Results for further work on the waisted design and experimental details have been reported in [51] and [75].

A general purpose constraint handling technique for genetic algorithms (GA) is developed by borrowing principles from multi-objective optimisation. This is in view of the many issues still facing constraint handling in GA, particularly in the number of control parameters that overwhelms the user, as well as other GA parameters, which are currently lacking in heuristics to guide successful implementations. Constraints may be handled as individual objectives of decreasing priorities or by a weighted-sum measurement of normalised violation, as would be done in multi-objective scenarios, with full consideration of the main cost function. Rather than the unnecessary specialisation seen in many new heuristics proposed for GA, the simplicity, generality and flexibility of the technique is maintained, where several options such as partial or full constraint evaluation, tangible or Pareto-ranked fitness, and implicit dominance evaluation are presented. By reducing the number of constraint evaluations, these options increase the probability of discovering optimal regions, and hence increase GA efficiency. Studies in applications to a constrained numerical problem, and to the design of realistic composite laminate plates and structures, serve to demonstrate the ease of implementation and general reliability in heavily constrained problems. The difference in the dynamics of partial or full violation knowledge showed that while the former reduced the overall number of constraint evaluations performed, the latter compromises for the expense of full constraint evaluations in the reduced number of GA generations, whether in terms of discovering feasible regions or optimal solutions. The benefit of partial or full constraint evaluations is subjective, as it ultimately depends on the trade-off in the computational cost of constraint evaluations and GA search.

Acknowledging the goal of reduced aircraft weight, there is a need to improve on conservative design techniques used in industry. Minimisation of laminate in-plane elastic energy is used as an appropriate in-plane performance marker to assess the weight saving potential of new design techniques. MATLAB optimisations using a genetic algorithm were used to find the optimal laminate variables for minimum in-plane elastic energy and/or damage tolerance for all possible loadings. The use of non-standard angles was able to offer equivalent, if not better in-plane performance than standard angles, and are shown to be useful to improve the ease of manufacture. Any standard angle laminate stiffness was shown to be able to be matched by a range of two non-standard angle ply designs. This non-uniqueness of designs was explored. Balancing of plus and minus plies about the principal loading axes instead of themanufacturing axes was shown to offer considerable potential for weight saving as the stiffness is better aligned to the load. Designing directly for an uncertain design load showed little benefit over the 10% ply percentage rule in maintaining in-plane performance. This showed the current rule may do a sufficient job to allow robustness in laminate performance. This technique is seen useful for non-standard angle design that lacks an equivalent 10% rule. Current use of conservative damage tolerance strain limits for design has revealed the need for more accurate prediction of damage propagation. Damage tolerance modelling was carried out using fracture mechanics for a multi-axial loading considering the full 2D strain energy and improving on current uni-axial models. The non-conservativeness of the model was evidenced to be from assumptions of zero post-buckled stiffness. Preliminary work on conservative multi-axial damage tolerance design, independent of thickness, is yet to be confirmed by experiments.

The growing number of applications of composite materials in aerospace and naval structures along with advancements in manufacturing technologies demand continuous innovations in the design of composite structures. In the traditional design of composite laminates, fiber orientation angles are constant for each layer and are usually limited to 0, 90, and ±45 degrees. To fully benefit from the directional properties of composite laminates, such limitations have to be removed. The concept of variable-stiffness laminates allows the stiffness properties to vary spatially over the laminate. Through tailoring of fiber orientations and laminate thickness spatially in an optimal fashion, mechanical properties of a part can be improved. In this thesis, the optimal design of variable-stiffness fiber-reinforced composite laminates is studied using an emerging numerical engineering optimization scheme based on the cellular automata paradigm. A cellular automaton (CA) based design scheme uses local update rules for both field variables (displacements) and design variables (lay-up configuration and laminate density measure) in an iterative fashion to convergence to an optimal design. In the present work, the displacements are updated based on the principle of local equilibrium and the design variables are updated according to the optimality criteria for minimum compliance design. A closed form displacement update rule for constant thickness isotropic continua is derived, while for the general anisotropic continua with variable thickness a numeric update rule is used. Combined lay-up and topology design of variable-stiffness flat laminates is performed under the action of in-plane loads and bending loads. An optimality criteria based formulation is used to obtain local design rules for minimum compliance design subject to a volume constraint. It is shown that the design rule splits into a two step application. In the first step an optimal lay-up configuration is computed and in the second step the density measure is obtained. The spatial lay-up design problem is formulated using both fiber angles and lamination parameters as design variables. A weighted average formulation is used to handle multiple load case designs. Numerical studies investigate the performance of the proposed design methodology. The optimal lay-up configuration is independent of the lattice density with more details emerging as the density is increased. Moreover, combined topology and lay-up designs are free of checkerboard patterns. The lay-up design problem is also solved using lamination parameters instead of the fiber orientation angles. The use of lamination parameters has two key features: first, the convexity of the minimization problem guarantees a global minimum; second, for both in-plane and bending problems it limits the number of design variables to four regardless of the actual number of layers, thereby simplifying the optimization task. Moreover, it improves the convergence rate of the iterative design scheme as compared to using fiber angles as design variables. Design parametrization using lamination parameters provides a theoretically better design, however, manufacturability of the designs is not certain. The cases of general, balanced symmetric, and balanced symmetric with equal thickness layers are studied separately. The feasible domain for laminates with equal thickness layers is presented for an increasing number of layers. A restricted problem is proposed that maintains the convexity of the design space for laminates with equal thickness layers. A recursive formulation for computing fiber angles for this case is also presented. On the computational side of the effort, a parallel version of the present CA formulation is implemented on message passing multiprocessor clusters. A standard parallel implementation does not converge for an increased number of processors. Detailed analysis revealed that the convergence problem is due to a Jacobi type iteration scheme, and a pure Gauss-Seidel type iteration through a pipeline implementation completely resolved the convergence problem. Timing results giving the speedup for the pipeline implementation were obtained for up to 260 processors. This work was supported by Grant NAG-1-01105 from NASA Langley Research Center. Special thanks to our project monitor Dr. Damodar R. Ambur for his technical guidance.
Ph. D.

This Ph.D. thesis represents a summative report detailing research processes and outcomes from investigating the ultimate and serviceability limit state short- and long-term behaviour and design of timber-concrete composite floors. The project enables the realization of a semi-prefabricated LVL-concrete composite floor system of up to 15 m long using 3 types of connection. Design span tables which satisfy the ultimate and serviceability limit state short- and long-term verifications for this system form the novel contribution of this thesis. In quantifying the behaviour of timber-concrete composite floors, 5 different experimental phases have been carried. 9 major achievements in meeting 9 sub-objectives have been concluded: 1) Three best types of connection system for timber-concrete composite floors have been identified; 2) The characteristic strength and secant slip moduli for these connections have been determined; 3) The short-term behaviour of the selected connections defined by their pre- and post-peak responses under collapse load has been established; 4) An analytical model for the strength evaluation of the selected connections based on the different possible modes of failure has been derived; 5) Easy and fast erected semi-prefabricated timber-concrete composite floor has been proposed; 6) The short-term ultimate and serviceability limit state behaviour of timber-concrete composite floor beams under collapse load has been investigated; 7) The long-term behaviour of chosen connections defined by their creep coefficient has been determined; 8) The long-term behaviour of timber-concrete composite floor beams under sustained load at serviceability limit state condition has been investigated; and 9) Design example and span tables for semi-prefabricated timber-concrete composite floors that satisfy both the ultimate and serviceability limit state in the short- and long-term using the gamma-method have been developed.

La thèse propose une formulation générale de la conception optimale des composites stratifiés, exprimée comme un problème d’optimisation avec contraintes en fonction de tous les paramètres constitutifs du stratifié et en utilisant la méthode polaire. Dans l’approche présentée il n’est pas nécessaire d’introduire d'hypothèses simplificatrices, que ce soit sur les critères de conception ou sur l’espace de recherche. D’autre part, une partie importante de ce travail concerne la production d’une nouvelle version de l’algorithme génétique BIANCA: il s’agit d’un algorithme génétique multi population, capable de résoudre des problèmes difficiles d’optimisation avec contraintes et multi objectif, dont les problèmes formulés pour la conception optimale des composites stratifiés. Dans la dernière partie, l’algorithme BIANCA est testé pour la résolution des problèmes formulés selon l’approche basée sur la méthode polaire; de nombreux cas d'intérêt sont traités en conception de composites stratifiés. Appliquée à une série étendue de cas d'exemple, la méthode polaire-génétique montre son efficacité et sa robustesse
In this thesis, we introduce a new global approach to the optimal design of laminated composites. This method uses the polar representation of plane tensors and the design of laminates is formulated as a constrained optimization problem without any pre-defined simplifying hypothesis. The design variables are all the constitutive parameters of the laminate. The first part of the thesis concerns the creation of a suitable genetic algorithm, BIANCA, able to handle any constrained/unconstrained and multi-objective optimization problems, and which is rich in its architecture and information representation. The genetic algorithm BIANCA is successfully applied to the resolution of optimal design problems for composite laminates, as they are formulated by our polar approach. Several practical cases are treated, and through an extended series of examples, we show the effectiveness and robustness of the polar-genetic method for the optimal design of composite laminates

The effect of structural optimization on control of stiffened laminated composite structures is considered. The structural optimization considered here, is the maximization of structural frequencies of the structure subject to maximum weight and frequency separation constraints and an upper bound on weight. The number of plies with a given orientation and the stiffener areas form the two sets of design variables. As the number of plies is restricted to integer values, the optimization problem considered belongs to the class of nonlinear mixed integer problems (NMIP). Several efficiency measures are proposed to reduce the computational cost for solution of the optimization problem. Savings in computer time due to each of the measures is discussed. The control problem is solved using the independent modal space control technique. This technique greatly simplifies the evaluation of the sensitivity of the performance index with respect to the individual frequencies. The effect of different optimization schemes on the control performance is considered. To reduce the probability, that conclusions drawn from numerical results, are purely coincidental, a large number of cases has been studied. It has been concluded that sufficient improvement in control performance can be achieved through structural optimization.
Ph. D.

A numerical simulation model has been developed which describes the manufacture of composite laminates from fiber-reinforced polyimide (PMR-15) matrix prepreg materials. The simulation model is developed in two parts. The first part is the volatile formation model which simulates the production of volatiles and their transport through the composite microstructure during the imidization reaction. The volatile formation model can be used to predict the vapor pressure profile and volatile mass flux. The second part of the simulation model, the consolidation model, can be used to determine the degree of crosslinking, resin melt viscosity, temperature, and the resin pressure inside the composite during the consolidation process. Also, the model is used to predict the total resin flow, thickness change, and total process time. The simulation model was solved by a finite element analysis. Experiments were performed to obtain data for verification of the model. Composite laminates were fabricated from ICI Fiberite HMF2474/66C carbon fabric, PMR-15 prep reg and cured with different cure cycles. The results predicted by the model correlate well with experimental data for the weight loss, thickness, and fiber volume fraction changes of the composite. An optimum processing cycle for the fabrication of PMR-15 polyimide composites was developed by combining the model generated optimal imidization and consolidation cure cycles. The optimal cure cycle was used to manufacture PMR-15 composite laminates and the mechanical and physical properties of the laminates were measured. Results showed that fabrication of PMR-15 composite laminates with the optimal cure cycle results in improved mechanical properties and a significantly reduced the processing time compared with the manufacturer's suggested cure cycle.
Ph. D.

This thesis reports on a design study, conducted for an advanced composite helmet shell for helicopter pilots. The helmet shell is expected to provide a level of protection against low velocity impacts with its weight criteria. Therefore, ergonomy, light weight, and the ability to withstand low velocity impact became the main issues for this study. For this purpose, an experimental program has been developed including low velocity impact tests on specimens. The drop height, drop weight, specimen stacking sequences and size were constant parameters. Test specimens were produced using the plate size of 220x220 mm having different thicknesses. Specimen materials were aramid, carbon, and a hybrid form of these two. Thus, the parameters of the study were specimen thickness and the material types. The impact tests are carried out on a specially designed test rig. The design decisions are made in accordance with the results of the experiments. In compliance with the lightweight and manufacturing criteria, the hybrid specimen was selected helmet shell. For the purpose of ergonomy a geometric design was also conducted from headfrom sizes of Turkish Army by using 3D design software. After specifying the composite material, manufactured helmet shell was tested in another test rig according to the ANSI Z90.1.1992. For the requirement of the acceleration level 300g, the helmet shell design was found to be successful at seven different and critical impact points.

Despite the weight savings, composite materials are vulnerable to impact loads mainly due to the alarming reduction in the compression after impact strength (CAI). This thesis exploits the potential of laminate designs with novel stacking sequences with the aim to improve the CAI strength of composite thick and thin laminates. Using experimental and numerical studies, we propose unsymmetrical stacking sequence designs and hybrid laminates (mixing different ply grades in the laminate) in the different modules of the thesis. The proposed laminates improves the CAI strength over the baseline by a maximum of 40% which demonstrates the potential of laminate design as an efficient and economic methods towards improving the impact damage tolerance of composites
Los materiales compuestos son vulnerables a las cargas de impacto principalmente a causa de la reducción de la resistencia a la compresión después de impacto (CAI, de las siglas en inglés Compression After Impact). Esta tesis investiga el potencial de los diseños de laminados con nuevas secuencias de apilamiento con el objetivo de mejorar la resistencia CAI. Mediante ensayos experimentales y simulaciones numéricas, proponemos nuevos diseños con secuencias de apilamiento asimétricas y laminados híbridos. Estos últimos mezclan diferentes grosores de capa en el laminado. Los laminados propuestos mejoran la resistencia CAI sobre la línea base en un máximo del 40%, lo que demuestra el potencial del diseño de los laminados como método eficiente y económico para mejorar la tolerancia al daño por impacto de los materiales compuestos

Le sujet de cette étude est le développement de séquences d’empilement pour la conception d'éprouvettes multidirectionnelles en matériaux composite à matrice polymère destinées à des essais de délaminage.Ces séquences permettent d’obtenir des éprouvettes multidirectionnelles qui, dans le cadre de la théorie classique des stratifiés, montrent un comportement thermoélastique qui reproduit celui des composites unidirectionnels : elles sont complètement libres de tout type de couplage élastique et ils ne développent pas des déformations résiduelles dues au cycle de cuisson.Par ailleurs, ces empilements permettent de tester n’importe quelle interface de délaminage.La conception de ces empilements est basée sur les solutions Quasi-Triviales (QT).Un algorithme pour la création d’une base de données de ces solutions a été conçu et implémenté.Grace à cela, un nombre plus grand de solutions QT et des solutions QT avec un nombre plus grand de plis que dans des études précédentes ont été trouvées.Ensuite, des critères analytiques permettant d’obtenir de nouvelles solutions à partir de la superposition des solutions connues ont été établis.Ces critères permettent d’obtenir des séquences QT avec n’importe quel nombre de couches.En combinant les solutions QT, les principes usuels de conception des stratifiés et les critères de superposition, les séquences d’empilement Fully-Uncoupled Multi-Directional (FUMD) ont été obtenues.Pour évaluer les propriétés des éprouvettes de délaminage obtenues avec ces séquences, un modèle Éléments Finis d’une éprouvette Double Cantilever Beam (DCB) a été développé pour comparer le comportement d’une séquence FUMD à celui d’autres séquences proposées dans la littérature pour les essais de délaminage.En utilisant la méthode de refermeture (Virtual Crack Closure) dans ses formulations originale et révisée, les distributions du taux de restitution d’énergie (Energy Release Rate) et ses partitions modales ont été évaluées.La séquence FUMD démontre les meilleurs résultats.Finalement, une campagne expérimentale de caractérisation du délaminage en mode I a été menée.Des éprouvettes DCB avec des séquences FUMD ont été conçues et fabriquées.Des plis tissés déséquilibrés ont été utilisés pour réduire la probabilité de changement de plan de délaminage.Les rotations des bras des éprouvettes et la forme du front du délaminage ont été étudiés pour évaluer la capacité des éprouvettes à garantir un comportement mécanique optimal lors du test de délaminage en mode I.Les résultats obtenus avec les éprouvettes FUMD sont proches de ceux obtenus avec les éprouvettes unidirectionnelles.En ce qui concerne l'énergie de rupture, les éprouvettes ayant la même interface de délaminage ont montré des valeurs très proches, même si leur rigidité était sensiblement différente.Par ailleurs, la caractérisation d'interfaces différentes a abouti à des énergies de rupture différentes, résultants des modes de ruptures non identiques.Ce travail ne représente qu'une étude préliminaire et des recherches complémentaires sont nécessaires.Néanmoins les éprouvettes de délaminage FUMD ont démontré un bon potentiel, et sont intéressantes pour les essais de délaminage
The object of this study is the development of a novel class of stacking sequencesfor the design of multidirectional polymer matrix laminated composite specimens for interlaminar fracture toughness (or delamination) tests. These sequences allow to obtain multidirectional specimens that, in the framework of Classic Laminated Plate Theory, have a thermo-elastic behaviour that closely matches that of unidirectional specimens: they are completely free from elastic couplings and they do not develop laminate-level thermally-induced deformations due to the curing process. Furthermore, they allow to test delamination interfaces between plies of any desired orientation. Because of their properties, they were labelled Fully-Uncoupled Multi-Directional (FUMD).In order to design these layups, Quasi-Trivial (QT) solutions were exploited.Firstly, an algorithm for the creation of a database of such solutions was conceived and implemented.Thanks to it, more and longer QT solutions were found than in previous studies.Then, analytical rules were established allowing to obtain new QT solutions from the superposition of known ones.These criteria allow to obtain QT sequences of any desired length, thus overcoming existing computational limitations arising when searching for QT solutions using the algorithm.Combining QT solutions with a few basic laminate design principles and the superposition criteria, FUMD stacking sequences are designed.In order to assess the properties of delamination specimens obtained with FUMD layups, a Finite Element model of a Double Cantilever Beam specimen was developed and used to compare the behaviour of a FUMD layup with that of other sequences proposed in relevant literature on the topic of delamination in multidirectional laminates.By means of the standard and of a revised Virtual Crack Closure Technique formulations, Energy Release Rate distributions and modal partitions of the specimens were evaluated.It emerged that the FUMD layup resulted in an optimal behaviour of the specimen.Eventually, a mode I interlaminar fracture toughness experimental campaign was performed.FUMD Double Cantilever Beam specimens were fabricated, along with with unidirectional ones.A UD-fabric material was used to reduce the likelihood of delamination migration.Rotations of the specimens arms and the shape of the delamination fronts were studied in order to assess the capability of the specimens to yield the correct mechanical behaviour for mode I delamination testing.For both aspects, FUMD specimens yielded results similar to those obtained with unidirectional specimens.With respect to interlaminar fracture toughness, specimens with identical delamination interface yielded similar values, even if their global stiffness was different.On the other hand, different interfaces led to different interlaminar fracture toughness, related to different fracture behaviours.While this work represents a preliminary study and further research is clearly required, FUMD delamination specimens have shown a good potential, and they may stand out as a viable solution for interlaminar fracture toughness tests.Possibly, they could be considered for an extension of the scopes of existing standard test methods to multidirectional laminates and interfaces

Carbon fiber reinforced composites are utilized in many design applications where high strength, low weight, and/or high stiffness are required. While composite materials can provide high strength and stiffness-to-weight ratios, they are also more complicated to analyze due to their inhomogeneous nature. One important failure mode of composite structures is delamination. This failure mode is common when composite laminates are subject to impact loading. Various finite element methods for analyzing delamination exist. In this research, a modeling strategy based on contact tiebreak definitions in LS-DYNA®was used. A finite element model of a low-velocity impact event was created to predict delamination in a composite laminate. The resulting delamination relative size and shape was found to partially agree with analytical and experimental results for similar impact events, while the force-time plot agreed well with experimental results. A small difference in contact time in the simulation compared to experimental testing is likely due to the omission of composite failure modes other than delamination. Experimental impact testing and subsequent vibrothermography analysis showed delamination damage in locations shown in previous research. This confirmed the validity of vibrothermography as a nondestructive evaluation technique for analyzing post-impact delamination.

No cenário da produção de edificações sustentáveis, a madeira laminada colada (MLC) ocupa lugar de destaque, sobretudo pela possibilidade de emprego de madeiras provenientes de florestas plantadas. Com o propósito de amenizar os problemas de durabilidade, quando exposta às intempéries, uma solução pressupõe a associação das vigas de MLC com um tabuleiro de concreto armado, sendo as partes ligadas por meio de conexões flexíveis. Essa técnica tem sido aplicada com sucesso, especialmente por conta do expressivo acréscimo de rigidez proporcionado pela composição. No entanto, em situações de elevados carregamentos ou de grandes vãos, a aplicação de reforços com fibras sintéticas, na face tracionada das vigas de MLC, aprimora ainda mais essa técnica, refletindo-se em significativos acréscimos nas forças de ruptura. Neste trabalho avaliou-se, de forma experimental e numérica, o comportamento estrutural de vigas mistas de MLC-concreto reforçadas com fibras de vidro. Numa primeira etapa foram estudados os elementos de ligação, optando-se pelos ganchos de aço com diâmetro de 8 mm pelo seu excepcional desempenho. Em seguida foram confeccionadas as vigas mistas, com e sem reforços com fibras de vidro, registrando-se acréscimo médio de 37% no módulo de ruptura (MOR) das vigas mistas em relação às vigas de MLC, ambas reforçadas com fibras. O emprego do reforço com fibras sintéticas se justifica pela diminuição na dispersão dos resultados. Por fim, um algoritmo foi proposto para o dimensionamento das vigas mistas de MLC-concreto reforçadas com fibras de vidro, o qual, associado às avaliações numéricas e experimentais, permite ampliar os horizontes de aplicação das estruturas de madeira.
Production of sustainable constructions forms a scenario where glulam beams occupy a prominence place, because of the possibility of utilization of wood that comes from planted forests. With the intention of diminution in the durability problems, when exposed to the weather effects, a solution presupposes the association of glulam beams with a reinforced concrete slab, in which the components are linked by means of flexible connections. This technique has been applied with results, especially due to the expressive increment in stiffness provided by the composition. However, in situations where high loads or great spans are found, the application of synthetic fibers reinforcements in the tension side of glulam beams improve this technique, being reflected in significant increments in the rupture forces. In this study it was evaluated, in experimental and numerical way, the structural behavior of glulam-concrete composite beams reinforced with glass fiber reinforced polymer (GFRP). In a first stage the connection elements were studied, being opted for steel hooks with 8 mm in diameter because of their exceptional behavior. Soon after, the composite beams were made with and without GFRP reinforcements and their tests showed average increment of 37% in modulus of rupture (MOR), when the composite beams were compared to glulam beams, both reinforced with GFRP. The decrease in the variability of results justifies the use of synthetic fibers reinforcements. Finally, an algorithm was proposed for the design of glulam-concrete composite beams reinforced with GFRP. So, when associated with the experimental and numerical evaluations that were carried out, this method allows enlarging the horizons of timber structures applications.

abstract: Laminated composite materials are used in aerospace, civil and mechanical structural systems due to their superior material properties compared to the constituent materials as well as in comparison to traditional materials such as metals. Laminate structures are composed of multiple orthotropic material layers bonded together to form a single performing part. As such, the layup design of the material largely influences the structural performance. Optimization techniques such as the Genetic Algorithm (GA), Differential Evolution (DE), the Method of Feasible Directions (MFD), and others can be used to determine the optimal laminate composite material layup. In this thesis, sizing, shape and topology design optimization of laminated composites is carried out. Sizing optimization, such as the layer thickness, topology optimization, such as the layer orientation and material and the number of layers present, and shape optimization of the overall composite part contribute to the design optimization process of laminates. An optimization host program written in C++ has been developed to implement the optimization methodology of both population based and numerical gradient based methods. The performance of the composite structural system is evaluated through explicit finite element analysis of shell elements carried out using LS-DYNA. Results from numerical examples demonstrate that optimization design processes can significantly improve composite part performance through implementation of optimum material layup and part shape.
Dissertation/Thesis
Masters Thesis Civil and Environmental Engineering 2014

Natural fiber-reinforced composites are now finding extensive uses in various fields from household articles to automobiles. These composites can score high compared to common synthetic fiber-based composites, notably glass fiber-reinforced composites, in areas such as occupational safety and health, and impact on environment. The current research work is motivated by the need for exploring jute fibers as replacement for glass fibers for various engineering design applications including more demanding impact protection applications as in automotive body structures. In the current work, detailed mechanical characterization of jute-polyester (JP) composite laminates till failure has been carried out for tensile, compressive and flexural loads by varying volume fraction of jute fibers. The effect of fiber volume fraction on mechanical properties is shown. Because of the potency of closed thin-walled components as structural energy-absorbers, a comprehensive experimental study has been performed, for the first time, comparing the behaviors of various geometric sections of JP and glass-polyester (GP) composite tubes under axial quasi-static and low velocity impact loading. Additionally, for jute-reinforced plastic panels to be feasible solutions for applications such as automotive interior trim panels, laminates made of such materials should have adequate perforation resistance. Thus, a detailed comparative study has been carried out for assessing the performance of JP laminates vis-a-vis GP plates under low velocity impact perforation conditions. As high-end product design is heavily driven by CAE (Computer-Aided Engineering), the current research work has also focused on the challenging task of developing reliable modeling procedures for explicit finite element analysis using LS-DYNA for predicting load-displacement responses and failures of JP composites under quasi-static and impact loading conditions. In order to extend the applications of JP composites to structurally demanding applications, hybrid laminates made of jute-steel composites and jute with nanoclay-reinforced polyester have been investigated and the considerable enhancement of mechanical properties due to hybridization is shown. Furthermore, a comprehensive study has been conducted on the behavior of JP laminates for varying degrees of moisture content until saturation, and the efficacy of hybrid laminates in this context has been shown.

In recent years, use of composites is increasing in most fields of engineering such as aerospace, automotive, civil construction, marine, prosthetics, etc., because of its light weight, very high specific strength and stiffness, corrosion resistance, high thermal resistance etc. It can be seen that the specific strength of fibers are many orders more compared to metals. Thus, laminated fiber reinforced plastics have emerged to be attractive materials for many engineering applications. Though the uses of composites are enormous, there is always an element of fuzziness in the design of composites. Composite structures are required to be designed to resist high stresses. For this, one requires a reliable failure criterion. The anisotropic behaviour of composites makes it very difficult to formulate failure criteria and experimentally verify it, which require one to perform necessary bi-axial tests and plot the failure envelopes. Failure criteria are usually based on certain assumption, which are some times questionable. This is because, the failure process in composites is quite complex. The failure in a composite is normally based on initiating failure mechanisms such as fiber breaks, fiber compressive failure, matrix cracks, matrix crushing, delamination, disbonds or a combination of these. The initiating failure mechanism is the one, which is/are responsible for initiating failure in a laminated composites. Initiating failure mechanisms are generally dependant on the type of loading, geometry, material properties, condition of manufacture, boundary conditions, weather conditions etc. Since, composite materials exhibit directional properties, their applications and failure conditions should be properly examined and in addition to this, robust computational tools have to be exploited for the design of structural components for efficient utilisation of these materials. Design of structural components requires reliable failure criteria for the safe design of the components. Several failure criteria are available for the design of composite laminates. None of the available anisotropic strength criteria represents observed results sufficiently accurate to be employed confidently by itself in design. Most of the failure criteria are validated based on the available uniaxial test data, whereas, in practical situations, laminates are subjected to at least biaxial states of stresses. Since, the generation of biaxial test data are very difficult and time consuming to obtain, it is indeed a necessity to develop computational tools for modelling the biaxial behavior of the composite laminates. Understanding of the initiating failure mechanisms and the development of reliable failure criteria is an essential prerequisite for effective utilization of composite materials. Most of the failure criteria, considers the uniaxial test data with constant shear stress to develop failure envelopes, but in reality, structures are subjected to biaxial normal stresses as well as shear stresses. Hence, one can develop different failure envelopes depending upon the percentage of the shear stress content. As mentioned earlier, safe design of the composite structural components require reliable failure criterion. Currently two broad approaches, namely, (1) Damage Tolerance Based Design and (2)Failure Criteria Based Design are in use for the design of laminated structures in aerospace industry. Both approaches have some limitations. The damage tolerance based design suffers from a lack of proper definition of damage and the inability of analytical tools to handle realistic damage. The failure criteria based design, although relatively, more attractive in view of the simplicity, it forces the designer to use unverified design points in stress space, resulting in unpredictable failure conditions. Generally, failure envelopes are constructed using 4 or 5 experimental constants. In this type of approach, small experimental errors in these constants lead to large shift in the failure boundaries raising doubts about the reliability of the boundary in some segments. Further, they contain segments which have no experimental support and so can lead to either conservative or nonconservative designs. Conservative design leads to extra weight, a situation not acceptable in aerospace industry. Whereas, a nonconservative design, is obviously prohibitive, as it implies failure. Hence, both the damage tolerance based design and failure criteria based design have limitations. A new method, which combines the advantages of both the approaches is desirable. This issue is also thoroughly debated in many international conference on composites. Several pioneers in the composite industry indicated the need for further research work in the development of reliable failure criteria. Hence, this is motivated to carry out research work for the development of new failure criterion for the design of composite structures. Several experts meetings held world wide towards the assessment of existing failure theories and computer codes for the design of composite structures. One such meeting is the experts meeting held at United Kingdom in 1991.This meeting held at St. Albans(UK) on ’Failure of Polymeric Composites and Structures: Mechanisms and Criteria for the Prediction of Performance’, in 1991 by UK Science & Engineering Council and UK Institute of Mechanical Engineers. After thorough deliberations it was concluded that 1. There is no universal definition of failure of composites. 2. There is little or lack of faith in the failure criteria that are in current use and 3. There is a need to carry out World Wide Failure Exercise(WWFE) Based on the experts suggestions, Hinton and Soden initiated the WWFE in consultation with Prof.Bryan Harris (Editor, Journal of Composite Science and Tech-nology)to have a program to get comparative assessment of existing failure criteria and codes with following aims 1. Establish the current level of maturity of theories for predicting the failure response of fiber reinforced plastic(FRP)laminates. 2. Closing the knowledge gap between theoreticians and design practitioners in this field. 3. Stimulating the composites’ community into providing design engineers with more robust and accurate failure prediction methods, and the confidence to use them. The organisers invited pioneers in the composite industry for the program of WWFE. Among the pioneer in the composite industry Professor Hashin declined to participate in the program and had written a letter to the organisers saying that, My only work in this subject relates to failure criteria of unidirectional fiber composites, not to laminates. I do not believe that even the most complete information about failure of single plies is sufficient to predict the failure of a laminate, consisting of such plies. A laminate is a structure which undergoes a complex damage process (mostly of cracking) until it finally fails. The analysis of such a process is a prerequisite for failure analysis. ”While significant advances have been made in this direction we have not yet arrived at the practical goal of failure prediction”. Another important conference held in France in 1999, Composites for the next Millennium (Proceedingof Symposium in honor of S.W.Tsaion his 70th Birth Day Torus, France, July 2-3, 1999, pp.19.) also concludedon similar line to the meeting held at UK in 1991. Paul A Lagace and S. Mark Spearing, have pointed out that, by referring to the article on ’Predicting Failure in Composite Laminates: the background to the exercise’, by M.J.Hinton & P.D.Soden, Composites Science and Technology, Vol.58, No.7(1998), pp.1005. ”After Over thirty years of work ’The’ composite failure criterion is still an elusive entity”. Numerous researchers have produced dozens of approaches. Hundreds of papers, manuscripts and reports were written and presentations made to address the latest thoughts, add data to accumulated knowledge bases and continue the scholarly debate. Thus, the out come of these experts meeting, is that, there is a need to develop new failure theories and due to complexities associated with experimentation, especially getting bi-axial data, computational methods are the only viable alternative. Currently, biaxial data on composites is very limited as the biaxial testing of laminates is very difficult and standardization of biaxial data is yet to be done. All these experts comments and suggestions motivated us to carry out research work towards the development of new failure criterion called ’Failure Mechanism Based Failure Criterion’ based on initiating failure mechanisms. The objectives of the thesis are 1. Identification of the failure mechanism based failure criteria for the specific initiating failure mechanism and to assign the specific failure criteria for specific initiating failure mechanism, 2. Use of the ’failure mechanism based design’ method for composite pressurant tanks and to evaluate it, by comparing it with some of the standard ’failure criteria’ based designs from the point of view of overall weight of the pressurant tank, 3. Development of new failure criterion called ’Failure Mechanism Based Failure Criterion’ without shear stress content and the corresponding failure envelope, 4. Development of different failure envelopes with the effect of shear stress depending upon the percentage of shear stress content and 5. Design of composite laminates with the Failure Mechanism Based Failure Criterion using optimization techniques such as Genetic Algorithms(GA) and Vector Evaluated Particle Swarm Optimization(VEPSO) and the comparison of design with other failure criteria such as Tsai-Wu and Maximum Stress failure criteria. The following paragraphs describe about the achievement of these objectives. In chapter 2, a rectangular panel subjected to boundary displacements is used as an example to illustrate the concept of failure mechanism based design. Composite Laminates are generally designed using a failure criteria, based on a set of standard experimental strength values. Failure of composite laminates involves different failure mechanisms depending upon the stress state and so different failure mechanisms become dominant at different points on the failure envelope. Use of a single failure criteria, as is normally done in designing laminates, is unlikely to be satisfactory for all combination of stresses. As an alternate use of a simple failure criteria to identify the dominant failure mechanism and the design of the laminate using appropriate failure mechanism based criteria is suggested in this thesis. A complete 3-D stress analysis has been carried out using a general purpose NISA Finite Element Software. Comparison of results using standard failure criteria such as Maximum Stress, Maximum Strain, Tsai-Wu, Yamada-Sun, Maximum Fiber Strain, Grumman, O’brien, & Lagace, indicate substantial differences in predicting the first ply failure. Results for Failure Load Factors, based on the failure mechanism based approach are included. Identification of the failure mechanism at highly stressed regions and the design of the component, to withstand an artificial defect, representative this failure mechanism, provides a realistic approach to achieve necessary strength without adding unnecessary weight to the structure. It is indicated that the failure mechanism based design approach offers a reliable way of assessing critically stressed regions to eliminate the uncertainties associated with the failure criteria. In chapter 3, the failure mechanism based design approach has been applied to a composite pressurant tanks of upper stages of launch vehicles and propulsion systems of space crafts. The problem is studied using the failure mechanism based design approach, by introducing an artificial matrix crack representative of the initiating failure mechanism in the highly stressed regions and the strain energy release rate (SERR) are calculated. The total SERR value is obtained as 3330.23 J/m2, which is very high compared to the Gc(135 J/m2) value, which means the crack will grow further. The failure load fraction at which the crack has a tendency to grow is estimated to be 0.04054.Results indicates that there are significant differences in the failure load fraction for different failure criteria.Comparison with Failure Mechanism Based Criterion (FMBC) clearly indicates matrix cracks occur at loads much below the design load yet fibers are able to carrythe design load. In chapter 4, a Failure Mechanism Based Failure Criterion(FMBFC)has been proposed for the development of failure envelope for unidirectional composite plies. A representative volume element of the laminate under local loading is micromechanically modelled to predict the experimentally determined strengths and this model is then used to predict points on the failure envelope in the neighborhood of the experimental points. The NISA finite element software has been used to determine the stresses in the representative volume element. From these micro-stresses, the strength of the lamina is predicted. A correction factor is used to match the prediction of the present model with the experimentally determined strength so that, the model can be expected to provide accurate prediction of the strength in the neighborhood of the experimental points. A procedure for the construction of the failure envelope in the stress space has been outlined and the results are compared with the some of the standard and widely used failure criteria in the composite industry. Comparison of results with the Tsai-Wu failure criterion shows that there are significant differences, particularly in the third quadrant, when the ply is under bi-axial compressive loading. Comparison with maximum stress criterion indicates better correlation. The present failure mechanism based failure criterion approach opens a new possibility of constructing reliable failure envelopes for bi-axial loading applications, using the standard uniaxialtest data. In chapter 5, the new failure criterion for laminated composites developed based on initiating failure mechanism as mentioned in chapter 4 for without shear stress condition is extended to obtain the failure envelopes with the shear stress condition. The approach is based on Micromechanical analysis of composites, wherein a representative volume consists of a fiber surrounded by matrix in appropriate volume fraction and modeled using 3-D finite elements to predict the strengths.In this chapter, different failure envelopes are developed by varying shear stress say from 0% of shear strength to 100% of shear strength in steps of 25% of shear strength. Results obtained from this approach are compared with Tsai-Wu and Maximum stress failure criteria. The results show that the predicted strengths match more closely with maximum stress criterion. Hence, it can be concluded that influence of shear stress on the failure of the lamina is of little consequence as far as the prediction of strengths in laminates. In chapter 6, the failure mechanism based failure criterion, developed by the authors is used for the design optimization of the laminates and the percentage savings in total weight of the laminate is presented. The design optimization of composite laminates are performed using Genetic Algorithms. The genetic algorithm is one of the robust tools available for the optimum design of composite laminates. The genetic algorithms employ techniques originated from biology and dependon the application of Darwin’s principle of survival of the fittest. When a population of biological creatures is permitted to evolve over generations, individual characteristics that are beneficial for survival have a tendency to be passed on to future generations, since individuals carrying them get more chances to breed. In biological populations, these characteristics are stored in chromosomal strings. The mechanics of natural genetics is derived from operations that result in arranged yet randomized exchange of genetic information between the chromosomal strings of the reproducing parents and consists of reproduction, cross over, mutation, and inversion of the chromosomal strings. Here, optimization of the weight of the composite laminates for given loading and material properties is considered. The genetic algorithms have the capability of selecting choice of orientation, thickness of single ply, number of plies and stacking sequence of the layers. In this chapter, minimum weight design of composite laminates is presented using the Failure Mechanism Based(FMB), Maximum Stress and Tsai-Wu failure criteria. The objective is to demonstrate the effectiveness of the newly proposed FMB Failure Criterion(FMBFC) in composite design. The FMBFC considers different failure mechanisms such as fiber breaks, matrix cracks, fiber compressive failure, and matrix crushing which are relevant for different loadin gconditions. FMB and Maximum Stress failure criteria predicts byupto 43 percent savings in weight of the laminates compared to Tsai-Wu failure criterion in some quadrants of the failure envelope. The Tsai-Wu failure criterion over predicts the weight of the laminate by up to 86 percent in the third quadrant of the failure envelope compared to FMB and Maximum Stress failure criteria, when the laminate is subjected to biaxial compressive loading. It is found that the FMB and Maximum Stress failure criteria give comparable weight estimates. The FMBFC can be considered for use in the strength design of composite structures. In chapter 7, Particle swarm optimization is used for design optimization of composite laminates. Particle swarm optimization(PSO)is a novel meta-heuristic inspired by the flocking behaviour of birds. The application of PSO to composite design optimization problems has not yet been extensively explored. Composite laminate optimization typically consists in determining the number of layers, stacking sequence and thickness of ply that gives the desired properties. This chapter details the use of Vector Evaluated Particle Swarm Optimization(VEPSO) algorithm, a multi-objective variant of PSO for composite laminate design optimization. VEPSO is a modern coevolutionary algorithm which employs multiple swarms to handle the multiple objectives and the information migration between these swarms ensures that a global optimum solution is reached. The current problem has been formulated as a classical multi-objective optimization problem, with objectives of minimizing weight of the component for a required strength and minimizing the totalcost incurred, such that the component does not fail. In this chapter, an optimum configuration for a multi-layered unidirectional carbon/epoxy laminate is determined using VEPSO. The results are presented for various loading configurations of the composite structures. The VEPSO predicts the same minimum weight optimization and percentage savings in weight of the laminate when compared to GA for all loading conditions.There is small difference in results predicted by VEPSO and GA for some loading and stacking sequence configurations, which is mainly due to random selection of swarm particles and generation of populations re-spectively.The difference can be prevented by running the same programme repeatedly. The Thesis is concluded by highlighting the future scope of several potential applications based on the developments reported in the thesis.

碩士
國立臺灣科技大學
機械工程系
92
The objective of this thesis is to study the optimization of laminated composite plate. The finite element method is used to evaluate the fundamental natural frequency and the frequency response function of laminated plate. Using the anisotropic property of composite materials, one can modulate the ply angles of laminated fibers searching for the optimal orientations by genetic algorithms to promote the fundamental natural frequency, and therefore the flexural rigidity of laminated plate. In addition, the frequency response function corresponding to the working frequency is also minimized to avoid resonance happened. This thesis presents two search ways of genetic algorithms, the single-objective and the multi-objective functions, respectively. Beside, the design constraint implemented to prevent the natural frequencies from closing to the working frequency is described to increase the structure robustness. The example results show that the proposed optimization technique can search for the optimal ply angles, to increase the fundamental natural frequency and decrease the frequency response function to avoid resonance happened. The analysis results also show that in stead of the inner ply, to modulate the ply angles of the outer ply is more effective to increase the structural stiffness of laminated plate.

碩士
國立臺灣科技大學
機械工程系
96
The main purpose of this thesis is to study the optimal design of composite laminated plates. The fiber orientations and the layer thicknesses are taken to be design variables, and the weighted sum of deflection and weight is the object function. The design constraint implemented is based upon the Tsai-Hill failure criterion. The genetic algorithms and the finite element method are used to deal with the optimal design of composite laminated plates. Considering the convenience in manufacturing, the fiber orientations and the layer thicknesses are selected from a set of discrete values. In general, the strength of materials and the loads exist some uncertainties in nature. Therefore, they are considered as random variables in this study. According the optimal design is performed by using the reliability constraint equations. The several examples are illustrated to investigate the effects of the randomness of variables on the optimal results. The results show that the average weighted sum is increased when the variation of random variables or the required reliability are increased.

碩士
國立成功大學
航空太空工程學系碩博士班
95
A new algorithm has been developed to deal with the bending-stretching coupling problems of layered anisotropic plates in this thesis. By employing Stroh-like formalism for anisotropic elasticity, the fundamental solutions of displacements and tractions for the bending-stretching coupling analysis of composite laminates have been obtained. Moreover, according to the reciprocal theorem of Betti and Raleigh, the boundary integral equations for the bending-stretching coupling analysis have also been derived. Thus, the boundary element formulation for the bending-stretching coupling analysis of composite laminates is established. The aim of this thesis is focused not only on the treatment of the domain integrals and corner force, but also the position of boundary source points. Numerical results are demonstrated to show the accuracy and efficiency by comparing with either the exact solutions or with results from alternative numerical techniques. The present BEM algorithm is found to be efficient and accurate for the bending-stretching coupling analysis.

The engineering of laminated composite structures is a complex task for design engineers and manufacturers, requiring significant management of manufacturing process and materials information. Ontologies are becoming increasingly commonplace for semantically representing knowledge in a formal manner that facilitates sharing of rich information between people and applications. Moreover, ontologies can support first-order logic and reasoning by rule engines that enhance automation. To support the engineering of laminated composite structures, this work developed a novel Semantic LAminated Composites Knowledge management System (SLACKS) that is based on a suite of ontologies for laminated composites materials and design for manufacturing (DFM) and their integration into a previously developed engineering design framework. By leveraging information from CAD/FEA tools and materials data from online public databases, SLACKS uniquely enables software tools and people to interoperate, to improve communication and automate reasoning during the design process. With SLACKS, this research shows the power of integrating relevant domains of the product lifecycle, such as design, analysis, manufacturing and materials selection through the engineering case study of a wind turbine blade. The integration reveals a usable product lifecycle knowledge tool that can facilitate efficient knowledge creation, retrieval and reuse, from design inception to manufacturing of the product.