Fatigue Fracture Process of Extruded and Heat-Treated Al-Zn-Mg Alloys under Uniaxial Cyclic Loading with Positive Stress Ratio

Abstract

Fatigue fracture process of extruded and heat-treated Al-Zn-Mg alloys, i.e., 7075- and 7150-alloys, under uniaxial cyclic loading with positive stress ratio, i.e., R = 0.25, has been investigated. Several tens’ micrometers wide preexisting “pits” on surfaces have been confirmed as the preferential fatigue crack initiation sites of the original 7000 series alloys. And, after secondary surface treatment, i.e., surface layer removal, metallic compounds or scratches induced during specimen machining have become possible fatigue crack initiation sites. Cycles to failure of the surface-layer-removed specimens were much longer than the specimens before the surface removal. We have classified the entire fatigue process of the original extruded and heat-treated 7000 series alloys as follows. Crack Initiation Stage: Cleavage crack starts from the interface between several tens’ micrometers wide preexisting “pit” and matrix. Striation Incubation Stage: Cleavage crack without remarkable striation propagates into the specimen. Striation Stage: Striations propagate and create the several millimeters wide semicircle “Fatigue Crack Zone”. Final Fracture Stage: Ductile dimple fracture stage with the final one cycle follows the “Fatigue Crack Zone” and leads the specimen to failure. We have quantitatively estimated the period of each stage and revealed that the lengthiest stage is the combined stage of the crack initiation and the striation incubation. When the cycle to failure reaches six digits number, the period of the combined stage becomes around 90% of the cycle to failure.

Fig. 4 High-speed camera visions of the fatigue fracture sequence of the dialogue-ballooned specimen of Fig. 3. LD means the loading direction. Each frame shows crack appearances at (a) 0, (b) 40,005, (c) 58,095, (d) 65,003, (e) 69,696 and (f) 69,697 loading cycles, respectively. (online color)