Fatigue crack growth behavior of patched and un-patched structures

摘要

Aluminum alloy is an important material used for aircraft structures especially for fuselage. During service, an aircraft is subjected to severe structural fatigue loads (constant, variable or random) which can cause damage of structure. The adhesively bonded composite repair is an efficient and cost-effective method to extend the fatigue life of cracked components in advanced aerospace structures. In this study, we investigated the fatigue crack growth (FCG) behavior of aluminum cracked plate with semi circular notch without repair. In this part, effects of stress ratio, notch geometry are studied. In second part FCG behavior of cracked aluminum plate repaired with bonded composite patch are analysed. Adhesively bonded composite patch repair technique has been successfully applied to military aircraft repair and expanded its application to commercial aircraft industry recently. We investigated the fatigue crack growth behavior of thin plate repaired with bonded composite patch using the stress intensity factor range (ΔK) and fatigue crack growth rate (da/dN). The stress intensity factor of patched crack was determined from empirical result by comparing the crack growth behavior of specimens with and without repair. Fatigue crack growth of 7050 T74 aluminum alloy is investigated by using NASGRO model. For un-repaired structures, obtained results illustrate a general increase in da/dN with R for a given AK. An important effect of stress ratio, R, has been observed clearly for this material at high DK. Fatigue life and FCGRs were considerably affected by notch geometry. In repaired structures with adhesively bonded composite (Graphite/Epoxy) using single sided repair, fatigue life was increased comparatively to the unrepaired structures. FCGRs are also affected, a decreasing was shown and crack growth was retarded, this is due to reduction of stress intensity factor and the normal stress at the crack tip. Effects of some parameters are investigated on fatigue behavior of repaired structures namely orientation, number and thickness of ply in bonded composite patch.