Academic literature on the topic 'J-R křivky'

The thesis deals with the determination of fracture toughness using compact tension (CT) test specimens at elevated and high temperatures. The experimental material steel P91 designated for application at temperatures 550650°C was used. The fracture toughness in the ductile fracture region of the steel was characterized by the R curve, which characterises the resistance against crack propagation depending on the crack length. The effect of temperature on the R curve at range 23600°C was evaluated. Next the specimen size effect using three sizes of CT specimens at 23°C and the effect of loading rate (2, 0,2 a 0,02 mm/min) at 600°C was examined. The results showed that the temperature has distinct effect on the R-curve, which yields minimal values at 400°C. Only the smallest test specimen size with thickness 6.25mm showed the specimen size effect giving about 10% lower values of toughness comparing to larger specimens. The effect of loading rate was clearly distinguishable. The values of toughness varied about 20% of the toughness value comparing individual loading rate.

This thesis is focused on assessment of fracture behaviour of heterogeneous welded joint. The goal is to determine fracture behaviour at the interface of ferritic base metal and austenitic weld metal at temperature of 255°C. This kind of weld is widely used at energy industry. Fracture toughness was evaluated using CT specimens with size 0,5T in order to determine the initiation values of stable crack propagation. Fracture behaviour of the weld is correlated with results of metallography and fractography analysis. Both light and scanning electron microscopy is used for fractography analysis. An assessment of fracture behaviour of the interface of the weld joint is first of all connected with a need of suitable location of cycled crack to the interface of metals. In the case of well-prepared crack the lowest values of J-integral are observed. In other cases of slight crack deviation from the notch plane fracture values increase. Due to a large scatter of mechanical properties of heterogeneous weld joints an increased number of test specimens is needed to obtain relevant and conservative fracture toughness values. Correlation of fracture toughness with the results of fractography analysis leads to clarification of the crack trajectory and to description of fracture mechanism.

This thesis deals with a high temperature testing of fracture toughness and studies the size effect on measured values using miniature size CT specimen. Two types of specimen geometry were manufactured from P91 steel, the standard size and the quarter size specimen. J-R curves were obtained in the temperature range from 23°C to 600°C. No specimen size effect was observed at room temperature tests. The realized experiments together with fractography analysis demonstrated the drop of toughness at 400°C caused by the effect of dynamic strain aging.

The standards of fracture toughness determination prescribe size requirements for size of test specimens. In cases of limited amount of test material miniature test specimens offer one from the possibilities of fracture toughness evaluation. Because of small loaded volumes in these specimens at the crack tip the loss of constraint occur affecting measured values of fracture toughness. In such cases the size requirements for valid fracture toughness characteristics determination are not fulfilled. These specimens can be even on limits of load range of test devices and handle manipulation by their small dimensions. The important task related to these specimens is, apart from methodology of their preparation and measurement of deformations, the interpretation of measured values of fracture toughness and their possible correction to standard test specimens. Moreover, in the upper shelf region of fracture toughness quantification and interpretation of size effects is still not resolved sufficiently. This thesis is by its aims experimentally computational study focused on evaluation of size effect on fracture toughness in the upper shelf region. The size effect was quantified by testing of miniature and large specimens’ sizes in order to determine J R curves. Two geometries of miniature test specimens, there point bend specimen and CT specimen, were used. The experimental materials were advanced steels developed for applications in nuclear and power industry, Eurofer97 steel and ODS steel MA956. Finite elements analyses of realized tests together with application of micromechanical model of ductile fracture were carried out in order to evaluate stress strain fields at the crack tip in tested specimens from Eurofer97 steel. By comparison of experimental results and numerical simulations of J R curves the mutual dependencies between geometry of specimens and element sizes at the crack tip were derived. On the basis of acquired relationships, the methodology of J R curve prediction for standard specimen size from limited amount of test material was proposed. Main contribution of thesis is description of effect of material’s fracture toughness level on resistance against ductile crack propagation in miniature specimens. For material where significant crack growth occurs after exceeding the limit values of J integral (Eurofer97), the loss of constraint is considerable and highly decreases resistance against tearing. Miniature specimens then show significantly lower J R curves in comparison with standard size specimens. This effect is the opposite to the behaviour of miniature specimens in transition region. In case of material with low toughness, in which significant crack growth occurs in the region of J integral validity (ODS MA956), the effect of constraint loss is small without large impact on resistance against tearing. In such case miniature specimens demonstrate comparable J R curves as specimens of larger sizes. Next important contribution is proposed methodology for prediction of J R curve from small amount of test material using micromechanical modeling.

This master´s thesis deals with the evaluation of fracture behavior of ODS steel MA956 at high temperature range. This behavior was tested by using miniaturized CT specimens, on which were performed experiments to measure of ductile crack growth resistance curves (J-R curves). The value of the fracture toughness was determined from these J-R curves. Fracture properties were consequently evaluated by using fractographic analysis of the fracture surfaces. Structural properties of material was identified by hardness measurement and analyzed by metallographic methods. Results of the measurements show drop of the fracture toughness with respect to the increasing temperature.