耐応力腐食割れ性に優れた高力Ａｌ‐Ｚｎ‐Ｍｇ‐Ｃｕ‐Ｌａ系押出合金の疲労き裂伝ぱ特性

Abstract

Fatigue crack growth tests on an extruded Al-8wt.% Zn-2.5wt.% Mg-1.2wt.% Cu-4wt.% La alloy were carried out using CT specimens (TL orientation). Crack closure was examined by the unloading elastic compliance technique. The main results are as follows;(1) The effect of stress ratio was observed on the crack growth rate, da/dN, vs. stress intensity factor range, ΔK, diagram. A higher stress ratio gives a faster crack growth rate at same ΔK.(2) Crack closure was observed under the stress ratio=0.1, and its degree was increasing with decreasing stress intensity factor range. On the other hand, it was not at the stress ratio=0.5.(3) The effect of stress ratio on the second region of da/dN vs. ΔK diagram disappeared when the effective stress intesity factor range, ΔKeff, was used instead ofΔK.(4) The second region of da/dN vs. ΔKeff diagram for the extruded Al-8wt.% Zn-2.5wt.% Mg-1.2wt.% Cu-4wt.% La alloy was in agreement with that for the commercial 7075T6 alloy. So, the fatigue crack growth resistance of the extruded Al-8wt.% Zn-2.5wt.% Mg-1.2wt.% Cu-4wt.% La alloy is nearly equal to that of the commercial 7075T6 alloy.