Effect of periodic underloads on fatigue crack growth in three steels used in hydraulic turbine runners

The aim of the present work is to study the interaction between two loading cycles in hydraulic turbine runners, i.e. baseline cycles which are the result of power output variations and periodic underloads which correspond to runner start/stop sequences. In order to make better evaluations of fatigue lives, there is a need to determine the real crack growth as compared to those predicted by linear damage summation. This comparison is made for three wrought steels, AISI 415, ASTM A516, and AISI 304L. Fatigue tests were run under both constant amplitude loading and periodic underloads at two constant values of maximum stress intensity factor, K-max. The crack growth under periodic underloads was faster than that predicted by linear damage summation for the A516 and 304L steels, while almost no acceleration was found for the 415 steel. The acceleration factors reached the highest values under low K-max and high load ratio of baseline cycles. High tensile residual stresses and strain hardening at the crack tip caused by underloads contribute to crack growth acceleration during the subsequent baseline cycles. (C) 2015 Elsevier Ltd. All rights reserved.