A unique fracture in fatigue of age-hardened Al alloys and its macro and microscopic aspects

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  • 時効硬化Al合金における特異な疲労破壊とその巨視的および微視的様相


<p>There are many studies reported that a fatigue crack propagated in a tensile mode macroscopically and the fracture surface was mainly occupied by striations in many metals. Whereas, a unique fracture in which a crack propagated in a macroscopic shear direction accompanied by ductile facets was observed in some aluminum alloys under specific conditions. In the present study, fatigue tests of age-hardened Al alloys of extruded 2017-T4 and 7075-T6 were conducted in relative humidity environments of 25% and 85% under rotating bending and ultrasonic loading conditions to clarify the crystallographic feature of a shear mode crack and propose its growth mechanism. Many facets showing a feature of shear mode crack propagation with an equivalent size to the grain size were observed at the fracture surface under both conditions of rotating bending in high humidity and ultrasonic loading irrespective of humidity. In addition, it was confirmed that the angle between the loading axis and the growth direction of the shear mode crack composed a constant value, ~35°, relating to the marked texture in the propagation process of the macroscopic shear mode crack. However, a crack growth rate was lower in the ultrasonic loading than in the rotating bending in high humidity. Based on their differences in occurrence conditions of the shear mode crack, two mechanisms for this unique propagation were proposed as follows; that is, one was a shear mode crack occurred by the promotion of the slip deformation to one direction due to hydrogen generated by reaction of Al alloy with water vapor in high humidity, and the other was a crack by the suppression of the deformation to one direction due to re-welding of crack faces under ultrasonic loading.</p>


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