VARIABLE-AMPLITUDE FATIGUE OF A DUAL-PHASE SHEET STEEL SUBJECTED TO PRESTRAIN

The present work concerns variable-amplitude strain-controlled fatigue of a dual-phase sheet steel (R(M) = 444 MPa and R(P0.2) = 266 MPa). The material was tested in the as-received condition as well as after ageing and prestraining by 10% uniaxially in the direction of fatigue loading. Four variable-amplitude strain sequences were considered with different types of overstrain cycles interspersed each tenth cycle. The experimental investigation indicated a sequence effect and a weak positive influence on the fatigue life of the prestrain operation. During constant-amplitude testing at fatigue lives higher than about 10(5) cycles the prestrained material exhibited a mean stress and the as-received material did not. However, under variable-amplitude testing these mean stresses were found to vanish owing to the overstrain cycles. Several different life-prediction methods based on linear damage accumulation were applied to the variable-amplitude sequences used. The methods took due account of the experimentally determined mean stresses and turned out to predict the fatigue lives properly.