technical briefs A Cumulative Damage Rule Based on Successive Reduction in Fatigue Limit ' l { ** )

Proposed Cumulative Damage Model The fatigue damage accumulation at stresses below the fatigue limit of the virgin material is accounted for in the proposed damage model as long as the stress level is greater than the "instantaneous" or "reduced" fatigue limit of the material. In the proposed model, the following premises are suitably modified and extended for multilevel fatigue cycling. (i) That all constant damage lines do converge to the knee point of the S-log N curve, as proposed by Subramanyan [l]. 2 (ii) That there is a reduction in the fatigue limit of a material due to overstressing, as proposed by Brown and Work When a specimen is subjected to a stressing at level Si for ni cycles, there exists a remaining life Nn = (A r i -n{), at the stress level Si and Nn = (Ni -n 2 i), at a stress level Si, where TJ 2 I is the equivalent number of cycles required to cause the same damage at a stress level S 2 due to stressing at stress level Si for ni cycles. The value of N21 is obtained These remaining lives when replotted on S-log JV diagram would yield a "remaining life line." The material having been stressed for a cycle ratio Ci = (wi/ JVi), will also have a reduced fatigue limit. This reduced value of fatigue limit S e2 , consequent to prestressing for a cycle ratio C\ at Si level, is then [2] where (3 is a constant, based on the yield strength of the material, and Sfi is the fatigue limit of the virgin material. If the specimen is stressed further at a second level, yet another remaining S-log N curve may be obtained using equations (1) and (2). Thus, the problem of prediction of life at the last stress level in a multilevel loading spectrum is now split up into a number of "two-level" sequences of loading each one having its ow