Добірка наукової літератури з теми "Transonic cracks"

Fibre bundle-based reflective optical sensors are good candidates for parameter monitorisation in aero engines. Tip clearance is one of those parameters of great concern that is necessary to monitor. Within this optical technology, the evolution experienced by a custom-designed optical sensor is presented from its first configuration up to the fifth one. The performance of the last configuration is compared with those of other two optical sensors that are also based on a fibre bundle design. The comparison has been carried out in an experimental program in a transonic wind tunnel for aero engines. The proven high resolution and sensitivity of the last configuration of the optical sensor opens up the possibility to detect blade defects, cracks, etc. that could otherwise be hard to track.

Frictional motion between contacting bodies is governed by propagating rupture fronts that are essentially earthquakes. These fronts break the contacts composing the interface separating the bodies to enable their relative motion. The most general type of frictional motion takes place when the two bodies are not identical. Within these so-called bimaterial interfaces, the onset of frictional motion is often mediated by highly localized rupture fronts, called slip pulses. Here, we show how this unique rupture mode develops, evolves, and changes the character of the interface’s behavior. Bimaterial slip pulses initiate as “subshear” cracks (slower than shear waves) that transition to developed slip pulses where normal stresses almost vanish at their leading edge. The observed slip pulses propagate solely within a narrow range of “transonic” velocities, bounded between the shear wave velocity of the softer material and a limiting velocity. We derive analytic solutions for both subshear cracks and the leading edge of slip pulses. These solutions both provide an excellent description of our experimental measurements and quantitatively explain slip pulses’ limiting velocities. We furthermore find that frictional coupling between local normal stress variations and frictional resistance actually promotes the interface separation that is critical for slip-pulse localization. These results provide a full picture of slip-pulse formation and structure that is important for our fundamental understanding of both earthquake motion and the most general types of frictional processes.

Among the different available optical technologies, fibre bundle-based reflective optical sensors represent an interesting alternative for parameter monitorization in aero engines. Tip clearance is one of the parameters of great concern for engine designers and engineers. In the framework of this optical technology, three fibre-based reflective optical sensors have been compared. Two of them are custom designed and based on the same geometrical fibre arrangement, whereas the third one is commercially available and relies on a different geometrical arrangement of the fibres. Their performance has been compared in clearance measurements carried out during an experimental program followed at a transonic wind tunnel for aero turbines. The custom-designed solution that operates in the most sensitive part of its response curve proved to be by far the most reliable tool for clearance measurements. Its high resolution opens up the possibility to detect small blade features such as cracks, reflectivity changes, etc. that otherwise could not be tracked. These results show that the detection of unexpected features on blade tips may have an important effect on how the clearance is calculated, ultimately giving rise to corrective actions.

A crack driven by shear forces translating on its surfaces grows in an isotropic compressible neo-Hookean material that is initially in uniform compression. The material replicates a linear isotropic solid at small deformations, and preserves as a limit case for all deformations the incompressibility that occurs in the linear case when Poisson’s ratio becomes 1/2. A plane-strain steady state is assumed such that the crack and surface forces move at the same constant speed, whether subsonic, transonic, or supersonic. An exact analysis is performed based on superposition of infinitesimal deformations upon large, both for frictionless crack surface slip, and slip resisted by friction. The pre-stress induces anisotropy and increases the Rayleigh, rotational and dilatational wave speeds from their classical values. A positive finite fracture energy release rate arises for crack speeds below the Rayleight value and at two transonic speeds. In contrast, the transonic range in a purely linear analysis exhibits only one speed. It is found that friction enhances fracture energy release rate, and that compressive pre-stress enhances the rates for small crack speeds, but decreases it for speeds near the Rayleigh value.

In transonic interfacial crack propagating fracture problem, the generation-phase simulations were done using the moving finite element method based on Delaunay automatic mesh generation. And the contact function based on the penalty method was newly developed to consider the crack face contact near the propagating interfacial crack tip. It was succeeded to visualize in 3-dimensions the Mach shock wave emanated from the propagating crack tip. And it was tried for the transonically propagating crack problem that solving the energy flows through the contact zone or along the Mach shock wave line emitted from the crack tip. The energy flow patterns into the crack tip were also visualized. Furthermore, from the values of the separated dynamic J integrals, it was found that the dynamic J integral is non-zero even for transonic fracture region and the most of the energy release rate is provided from the more compliant material epoxy.

The near-tip behavior of the thermal field around a moving crack is investigated analytically in this work. The thermal field is characterized by a thermal Mach number defined as the ratio between the linear speed v of the moving crack and the heat propagation speed C in the solid. Mathematically, M= v/C. In the subsonic range with M < 1, a detailed comparison with the thermal diffusion model is made. In the transonic and the supersonic ranges with M ≥ 1, thermal shock waves, which separate the heat affected zone from the thermally undisturbed zone, are shown to exist in the physical domain. A swinging phenomenon for the temperature variation as a function of the thermal Mach number is found in transition from the subsonic to the supersonic ranges. Also, the r dependency of the near-tip temperature is found to be the same as that of the near-tip heat flux vector. It transits from r1/2, r, to r2 as the thermal Mach number transits from the subsonic, transonic, to the supersonic ranges.

Cette thèse s’intéresse à la propagation d'une fissure dynamique à travers une membrane en élastomère (polyuréthane). Cette problématique a été étudiée expérimentalement au cours d’une étude précédente. Dans cette étude, sous certaines conditions de chargement, la vitesse de propagation des fissures dépasse la vitesse des ondes de cisaillement. De telles fissures sont appelées fissures transsoniques. Deux hypothèses principales ont été avancées dans littérature pour expliquer l'observation des fissures transsoniques. L'une d'elles repose sur la rigidification hyperélastique du matériau au voisinage de la pointe de fissure, tandis que l'autre repose sur le raidissement viscoélastique. Cette étude examine ces deux hypothèses et détermine que le raidissement viscoélastique est l'ingrédient nécessaire (et suffisant). La viscoélasticité linéaire finie a été utilisée en premier lieu. Dans un second temps, un modèle cohésif dépendant de la vitesse a été utilisé pour prédire la vitesse de propagation de la fissure. La vitesse de fissure s'est avérée indépendante de la hauteur de l'éprouvette au-delà d'un certain seuil. Un modèle viscoélastique non linéaire a également été mis en œuvre en supposant des conditions de contraintes planes. En utilisant cela, l'énergie dissipée dans le matériau en raison des effets viscoélastiques et l'énergie consommée par les processus de rupture ont été calculées explicitement. Les résultats montrent que la majorité de l'énergie de déformation est consommée sous forme de dissipation viscoélastique dans le matériau. L’énergie restante est consommée par les processus de rupture
This study aims to investigate the propagation of a dynamic crack through an elastomer membrane. The crack propagation in polyurethane elastomers was studied experimentally in an earlier study. Under certain loading conditions, crack speeds in that study were found to exceed the shear wave speed. Such cracks are called transonic cracks. Two main hypotheses were put forward in literature to explain the observation of Transonic cracks. One of them relies on the hyperelastic stiffening of the material in the vicinity of the tip, while the other relies on the viscoelastic stiffening. This study examines these two hypotheses and determines that viscoelastic stiffening is the necessary (and sufficient) ingredient. Finite Linear viscoelasticity has been used in the first instance. Once this has been established, a rate-dependent cohesive model has been used to predict the crack propagation speed. The crack speed has been found to be independent of the specimen height starting from a certain threshold. A nonlinear viscoelastic model has also been implemented assuming plane stress conditions to prevail. Using this, the energy dissipated in the bulk because of viscoelastic effects and the energy consumed by the fracture processes has been explicitly computed. The majority of the strain energy has been observed to be consumed as the viscoelastic dissipation in the bulk material. The rest is taken up by the fracture processes

碩士
國立臺灣科技大學
營建工程技術學系
82
The purpous of this work is to analyze two-dimensional plane strain problems of crack propagation with transonic speed. The method of self-similar potentials(ssp) will be used to solve the displacement and stress fields, and the method of Gauss quadrature will be applied to obtain the numerical results. In order to obtain the dynamic stress intensity factor,we will evaluate the asymptotic solution of stress and displacement fields in the vicinity of crack-tip. Finally,the energy release rate of this problem of crack propagation will be estimated by means of the J-integral. For crack propagation with transonic speed the stress and velocity fields have singular value of .epsilon.**(-0.5-H(1/s)),where .epsilon. is the distance from a point concerned to the crack-tip and the range of H(1/s) is from 0 to 0.5 .In this work we obtain negtive dynamic stress intensity factor and zero energy release rate.Thus,it is impossible for mode-I cracks to propagate with transonic speed.

A unique methodology and test rig was designed to evaluate the degradation of damaged Nozzle Guide Vanes in a transonic annular cascade in the short duration facility at the Royal Military College. A custom test section was designed which featured a novel rotating instrumentation suite. This permitted 360° multi-span traverse measurements downstream of unmodified turbine NGV rings from a Rolls-Royce/Allison A-250 turbo-shaft engine. Downstream total pressure was measured at four span-wise locations on both an undamaged reference and a damaged test article. Three performance metrics were developed in an effort to determine characteristic signatures for common operational damage such as trailing edge bends or cracked trailing edges. The highest average losses were observed in the root area, while the lowest occurred closer to the NGV tips. The results from this study indicated that multiple span-wise traverses were required to detect localized trailing edge damage. Recommendations have been made for future tests, for test rigs and for ideas to develop performance metrics.