Short-life fatigue under combined stresses

Abstract

Most of the short-life fatigue tests have been carried out in push-pull or in reversed bending. Many machine parts and structural elements, however, are subjected to combined rather than uniaxial fatigue loads. This work is intended to demonstrate a possible approach to solving the problem of short-life fatigue failures due to a multiaxial stress-strain system. The paper includes a review of the available literature relevant to the problem. The more important experimental findings are summarized and presented in the form of diagrams. Various approaches toward the solution of the problems are discussed, and a new approach is proposed. It is demonstrated in this paper that the use of an ‘equivalent-strain parameter’ calculated from the Mises law is not suitable in the description of the problem. It was found that when the strain Δ∊ in the direction of the maximum stress is adopted as a strain parameter and a stress-dependent parameter L is introduced, all of the results can be described by the following equation:         Δ∊ Na = C where n = f( np, L) and C = f( Cp, L),     Δ∊ = cyclic strain range,      N = number of cycles to failure,      np = slope of the log ∊-log N curve which represents a push-pull fatigue test,      L = stress parameter,      Cp = constant in the log ∊-log N curve which represents a push-pull fatigue test. These functions have been determined approximately. It is recognized that, because of lack of experimental data, the present work can indicate only a possible and somewhat speculative approach and that there is a great need for further experimental work.