A B S T R A C TThe shear mode crack growth mechanism in 1050 aluminium was investigated using precracked specimens. A small blind hole was drilled in the centre section of the specimens in order to predetermine the crack initiation position, and a push-pull fatigue test was used to make a pre-crack. Crack propagation tests were carried out using both push-pull and cyclic torsion with a static axial load. With push-pull testing, the main crack grew by a mixed mode. It is thus apparent that shear deformation affects the fatigue crack growth in pure aluminium. In tests using cyclic torsion, the fatigue crack grew by a shear mode. The micro-cracks initiated perpendicular and parallel to the main crack's growth direction during the cyclic torsion tests. However, the growth direction of the main crack was not changed by the coalescence of the main crack and the micro-cracks. Shear mode crack growth tends to occur in aluminium. The crack growth behaviour is related to a material's slip systems. The number of slip planes in aluminium is smaller than that of steel and the friction stress during edge dislocation motion of aluminium is lower than many other materials. Correlation between the crack propagation rate and the stress intensity factor range was almost the same in both push-pull and cyclic torsion with tension in this study.In many structural materials, fatigue cracks propagate in mode I rather than in a shear mode. In order to investigate shear mode crack propagation of metals, some special techniques had been employed by others. For example, a double cantilever system and a chevron type notched specimen were used by Murakami et al., 1 and an apparatus, which makes it possible to apply static tension and cyclic shear stress simultaneously was developed by Otsuka et al. 2 They investigated fatigue crack propagation in steel and aluminium alloys. The friction between crack surfaces is an important factor in evaluating fatigue crack propagation in the shear mode. [3][4][5] To investigate the friction effects between crack surfaces on the crack propagation law, Yu et al. 3 evaluated the fatigue crack propagation rate of a circumferential cracked specimen under cyclic torsion when the friction between crack surfaces was varied. After changing the Correspondence: C. Makabe.

In the present study, a detection method of an overload application during stress cycles under constant amplitude was investigated. Also, the effect of the tensile overload was shown at three stress ratios: R = 0, -1, and -1.5, to understand the effects of R on crack propagation after an overload. At the baseline of R = 0, after the overload, retardation in the crack propagation was observed, and the crack growth rate decreased. However, in the case of R = -1.5, the fatigue crack growth rate actually accelerated after the tensile overload. The detection of that crack propagation behavior was attempted through the information of the strain waveform h; h = εy + 1.2λx, where εx and εy are the local strains at the specimen axis, and λ is the strain range ratio Δεy/Δεx. The waveform shape of h was changed after the overloading. Also, the application of the overload could be detected by the variation of the strain range ratio λ. Especially, the present method is useful for cases of the crack propagation stage under negative R conditions.

An electrical resistance welding method was applied under atmospheric conditions by using one of metal powder medium or media mixture which was sandwiched in the space between the two solid metal bars of specimen (i.e., solid specimen material), and was compressed longitudinally by oil pressure servo control electrodes (upper and bottom) and simultaneously current was conducted to generate Joule thermal heat. In the joining experiments, a solid aluminum specimen material was used as a basis material, and was joined to another solid aluminum specimen material or one of four other solid specimen materials with different melting points by using resistance-welding apparatus. Some fundamental data on the mechanical properties of the joint were obtained by material testing. In the experiments, the specimen used as solid specimen materials in this study were pure aluminum, copper, stainless steel, carbon steel and titanium bars of solid specimen, and the powder media were aluminum, nickel and silicon powder. Proper mixed ratios of total amount of the powder media were determined for reliable joining, and material testing was prepared for mechanical properties. The obtained data were examined with the intent of optimizing the method using metal powder media between a pair of specimen materials and were compared with that of the solid specimen material, in terms of tensile strength, Vickers hardness, bending U-shape flexure stiffness. On the tensile strength and Vickers hardness, they were found to be reliable, but on bending U-shape flexure stiffness, they were not definite enough.

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The formation mechanism of the factory-roof pattern in a circumferential notched specimen was examined. To achieve observation of the material's cracks internally, plastic material, that was acrylic, was used for the test. Cyclic torsional tests were performed with and without application of static tension. A factory-roof pattern was formed in the fracture surface when cyclic torsion tests were conducted with static tension. Formation of the factory-roof pattern was found to be dependent on testing conditions. When such a pattern was formed, many small cracks were initiated by shear mode, followed by their coalescence. The new cracks' initiation at the front of the shear mode cracks could be one of the reasons for the coalescence. After the coalescence and branching of cracks, the cracks grew under the control of the maximum principal stress criterion. This behavior was examined by direct observation of the cracks. The final fracture pattern showed the initiation of many shear-mode cracks and their succeeding coalescence. From those observations, we considered the formation of a factory-roof pattern model.