Although a lot of interface crack problems were previously treated, few solutions are available under arbitrary material combinations. This paper deals with periodic interface cracks in a bonded infinite plate and a central interface in a bonded infinite plate. Then, the effects of material combination and relative crack length on the stress intensity factors are discussed. A useful method to calculate the stress intensity factor of interface crack is presented with focusing on the stress at the crack tip calculated by the finite element method. For periodic interface cracks, it is found that the stress intensity factors are controlled by the bimaterial parameter ε alone and increase with increasing ε and relative crack length. For a central interface crack, the relationship between the dimensionless stress intensity factors FI, FII and relative crack length a/W are obtained under arbitrary material combinations. It is found that FI has the maximum value when α = 0.2, β = 0.3 and minimum value when α = 1.0, β = 0 independent of a/W. On the other hand, FII has the maximum value when α = 0.1, β = 0 and minimum value when α = 0.2, β = 0.3.
The asymptotic solution of an interfacial corner between dissimilar crystals is expressed by the theory of anisotropic linear elasticity. However, it is not elucidated if the stress field obtained by the molecular analysis corresponds with the anisotropic linear elastic solution. We analyzed the stress fields around interfacial corners between dissimilar crystals using the molecular statics, and compared them with the asymptotic solutions. The stress intensity factor (SIF) is one of the basic fracture mechanics parameters, but there are few applications of the SIF to the fracture criteria of interfacial corners. We proposed a new definition of the SIFs of an interfacial corner between dissimilar anisotropic materials in our previous paper. In this study, the mixed mode fracture criteria of interfacial corners between dissimilar crystals modeled by the molecular statics were evaluated using the proposed SIFs.
The failure analysis of a high pressure hydrogen transducer which was actually used in a test equipment of 70MPa hydrogen gas exposure vessel was carried out. A crack existed at the root of flange of diaphragm in the transducer and a small amount of hydrogen leaked through the crack. EDSD analysis and Vickers hardness measurement confirmed that a diaphragm material was a precipitation hardened stainless steel SUS630. SEM observation showed that intergranular fracture surfaces with the size of about 20μm were formed at the root of flange and that dimple, quasi-cleavage and intergranular fracture were sequentially observed away from the flange root. The stress analysis and hydrogen invasion analysis of diaphragm were conducted. Using the plate of SUS630, the hydrogen diffusion coefficient and solubility, and the threshold stress intensity factor of hydrogen assisted cracking under static loading were measured. The diaphragm was fixed to the pressure transducer by bolt end. The elastic principal stress at the flange root by bolt end was about three times higher than the tensile strength of 1358MPa of the diaphragm. It was presumed that the 20-μm-long intergranular crack initiated at the flange root by the high principal stress and propagated by hydrogen embrittlement.
Influence of hydrogen on tensile and Charpy impact properties of quench-tempered low-alloy steels, SCM435s for four types of storage cylinders in 35MPa hydrogen stations, was investigated. After tensile and Charpy impact specimens were exposed to hydrogen gas at 100MPa and 85°C, tensile tests were conducted at room temperature in air under a cross-head speed of 1mm/min, and Charpy impact tests were conducted at temperatures from -100 and 100°C in air. Residual hydrogen contents of these specimens were also measured by TDA (Thermal Desorption Analysis). The tensile strength of hydrogen-exposed specimens, whose residual hydrogen contents ranged from 0.29 to 0.44 mass ppm, were nearly equal to that of unexposed specimens, while the reduction of area of hydrogen-exposed specimens were about 10% lower than that of unexposed specimens. The Charpy impact properties of hydrogen-exposed specimens were consistent with those of unexposed specimens. Microstructures of these materials were observed by EBSD (Electron Back Scatter Diffraction). EBSD analysis clarified that the fracture appearance transition temperature became lower, when the block grain size of martensite became smaller. The upper-shelf absorbed energy increased with decreasing the block grain size. Fracture thoughness values of the materials were estimated from the upper-shelf absorbed energy ; then, evaluation of LBB (Leak-before-break) of 35MPa hydrogen storage cylinders was conducted.
The axial fatigue tests with stress rations of -1.0 and 0.1 were conducted in order to investigate the effects of the laser peening (LP) treatment on the fatigue strength and the fatigue crack behaviors in the rolled aluminum alloy A7050 for aircraft structures. The LP treatment was effective for fatigue strength improvement in the fatigue lives before 2∼3 × 106cycles, but the treatment reduced the strength after the cycles at the both stress ratio conditions. Fatigue cracks initiated at the surface on the higher stress amplitude levels, but the cracks initiated in the internal positions on the lower stress levels. From the observation results of fatigue crack behaviors, it was clear that the LP treatment could control the crack initiation and the propagation behaviors. The fatigue strength behaviors by the LP treatment were evaluated by the stress intensity factor range including the residual stress induced by the LP treatment.
In this study, an ultrasonic torsional fatigue testing machine was developed in order to investigate the very high cycle fatigue properties of carburized steels. The testing machine can apply more than 800MPa of shear stress amplitude at 18.3kHz. The very high cycle fatigue diagram and crack propagation behavior of carburized SCM420H were obtained by using the testing machine and compared with results of non-carburized SCM420H. Fatigue crack initiation and growth was observed in very high cycle regime for both carburized and non-carburized SCM420H. It was found that carburizing of SCM420H was effective to enhance the fatigue life in very high cycle regime. In addition, crack deflection from mode II to mode I under torsional fatigue was discussed based on the fracture mechanics.
A hierarchical model of a Monte Carlo simulation for the process of stress corrosion cracking (SCC) has been proposed to improve computational efficiency. A hierarchical simulation was developed based on the results of real size simulation which treats grain-sized micro cracks for initiation. In the real size simulation, initiation times, sites and length were assigned by random numbers based on an exponential distribution, a uniform distribution and a normal distribution, respectively, and stochastic data of initiation times, sites, lengths and aspect ratios for over 1mm cracks were obtained. In the hierarchical simulation which treats over 1mm cracks for initiation, initiation times, sites, lengths and aspect ratios were assigned by random numbers based on each distribution obtained by the real size simulation. The hierarchical simulation was carried out based on a real size simulation for a sensitized stainless steel type 304 under high-temperature and high-purity water environment. Simulation results for hierarchical and real size simulation have a good agreement with each other and computing time of analysis for large cracks can be significantly reduced by hierarchical simulation.
In recent years, the breakdowns of piping have occurred by the stress corrosion cracking in the nuclear power generation plant. To avoid serious accident, some plants did not work during several years. The stress corrosion cracking is caused by interaction of the stress and erosive environment. This study is done about the stress corrosion cracking in the combination of the SCM435 steel and the boric acid solution. This combination has the feature that corrosion happens comparatively easily. The moving finite element analyses are operated according to acquired data by the experiment. The stress intensity factor is compared with an analytical solution, and the validity of this simulation is verified. Moreover, the application of the simulation including holes is discussed based on numerical result.
When a cylindrical specimen (φ29.0mm × 12.5mm) made from sealing rubbers was exposed in high-pressure hydrogen gas, it was clarified that cracks initiated in the specimen after decompression in previous studies. However, it was not clear whether crack damage obtained from the cylindrical specimen was the same as that obtained from an O-ring specimen used for practical sealing or not, because the shapes and dimensions of these specimens were different. From this viewpoint, seven cylindrical specimens (φ13.0mm × 2.0, 4.0, 6.0, 8.0, 10.0, 12.0 mm, and φ29.0mm × 12.5mm) made from an unfilled peroxide-crosslinked EPDM composite were exposed to high-pressure hydrogen gas at a maximum pressure of 10MPa ; then, the influence of the shape of specimen on crack initiation and growth behavior was investigated. Micrometer-sized defects, facets, and notch-like regions were observed at fracture origins of internal cracks by SEM observation. It was inferred that micrometer-sized bubbles were formed at these sites by the coalescence of submicrometer-sized bubbles, and internal cracks initiated due to stress concentration of the micrometer-sized bubbles. Measurement of AE (acoustic emission) was also conducted to confirm the existence of the sub-micrometer-sized bubbles. The crack damage became more serious with an increase in the thickness of specimen as well as an increase in hydrogen pressure. Although the critical hydrogen pressures at crack initiation of the specimens with thicknesses from 2.0 to 6.0mm decreased with an increase in the thickness of specimen, those of the specimens with thicknesses from 6.0 to 12.0mm were the same. The influence of the shape of specimen on the crack damage and the critical hydrogen pressure was successfully explained in terms of retained hydrogen content and fracture mechanics.
Carbon-fiber reinforced epoxy was decomposed using subcritical water and supercritical methanol to reclaim carbon fibers. The tensile strength of the reclaimed carbon fibers was measured. Then SEM observation, XPS, and Raman spectral analysis were conducted to elucidate the change of tensile strength caused by decomposition. The tensile strength decreased by 6% in the case of decomposition with supercritical methanol, and by 12~17% with subcritical water. The surfaces of reclaimed carbon fibers were resin-free. Decomposition did not affect the fiber surface and fracture surface morphology. Subsequent XPS analysis revealed that functional groups of the carbon fiber surface had been removed. Raman spectral analysis showed decreased graphitization of the carbon fiber surface. These results imply that the fracture toughness of the carbon fiber surface decreased because of breakage of carbon-carbon bonds in the carbon fibers as a result of decomposition.
Tensile and fatigue failure behavior of C/C composite with fine woven fiber-cloth laminates was investigated in several configurations of specimens. A 3.2mm thick plate, which has the quality of machine-ability, was used for testing material. During the machining process of specimens, care was taken that the fiber directions of 0°/90° and -45°/45° orientation were set against the loading direction. Tensile and fatigue tests were performed under load control techniques. Notches were made on some specimens, and their fracture behavior was observed. Some different notch shapes were used to investigate the effect of fiber orientation on the fracture behavior of the material. The result showed that the critical fracture stresses of the specimen were affected by fiber orientation and notch shape. In addition, the shear stress conditions affected the fracture behavior.