Transient Analysis of Fatigue Crack Propagation under a Step Increase in Loading,

摘要

In a previous report,steady state crack propagation at a constant stress intensity factor range was analyzed on the basis of a cumulative fatigue damage model. In this model,the points ahead of a crack tip were assumed to constitute a series of uniaxial fatigue specimen and fatigue crack propagation was viewed as the successive fatigue failure of these specimens. In this report an extension of the previous analysis to a situation involving an increase in the stress intensity factor range from a lower level to a higher level is carried out. The analysis indicates that there is a transient zone through which the crack has to grow before attaining the full steady state crack propagation rate corresponding to the higher load level. Numerical results have been computed for three steels of comparable ductility but widely varying yield strength The results show that both the crack length increment and the number of cycles involved in the transient period decrease with increasing yield strength. Also,for any given steel,the crack length increment during the transient increases with the load ratio but the number of cycles is independent of the load ratio. (Author)