微粒子ピーニング処理を施したA7075アルミニウム合金の疲労特性

Fatigue strength and crack propagation behavior of fine particle peened A7075 aluminum alloy was investigated. Residual stress due to fine particle peening treatment was about -350 MPa at the surface and gradually declined inside and disappears at about 80 μm from the surface. The fatigue strength at 107 cycles of the fine particle peened specimen was 1.3 times larger than that of the unpeened specimen. The crack propagation behavior was also investigated under three kinds of stress ratios. The displacement distribution around the crack tip was measured in detail by the digital image correlation method to determine the effective stress intensity factor range and crack tip opening stress. Fine particle peeing contributes to suppressing the crack opening especially when the crack length is small. It was also clarified that the retardation effect of crack propagation by the fine particle peening treatment is remarkable when the stress ratio is small. If the crack starts to open, the crack opening ratio is almost the same as the unpeened condition and this means that the effective stress intensity factor range can be calculated from the crack opening displacement. When the crack propagation rates are arranged by the effective stress intensity factor range, all data collapse into a single narrow scatter band. Therefore, the effective stress intensity factor range dominates the surface crack propagation rate, even when the material is subjected to fine particle peening treatment.