EFFECT OF CORROSIVE ENVIRONMENT ON THE FATIGUE-CRACK INITIATION AND PROPAGATION BEHAVIOR OF AL 5454-H32

Fatigue behavior of aluminum alloy 5454-H32 was studied under laborato ry air and 3 % NaCl solution environment using smooth cylindrical and notched plate specimens. Presence of 3 % NaCl environment during fatig ue loading drastically reduced alloy fatigue life. The deleterious eff ect was pronounced in both types of specimens in the long-life regions , where the fatigue lives were lowered by as much as a factor of 10. H owever, the sharply notched specimens showed only a modest reduction i n fatigue life in corrosive environment. The severe influence of the c orrosive environment in the long-life (low-stress) regime cannot be ex plained merely by the early initiation of the fatigue crack from surfa ce pits; the environmental contribution in the early crack growth regi me must also be considered an important factor. Fracture surface studi es revealed extensive pitting and some secondary cracking in the crack initiation region. It was shown that lowered fatigue life in Al 5454- H32 occurs by early initiation of fatigue cracks from surface pits. In addition, a corrosion pitting and secondary cracking process may be o perative in the small crack growth region. This could have enhanced th e early crack growth rate and thus contributed to the lower fatigue li fe in the long-cycle region.