EFFECTS OF A SINGLE OVERLOAD EVENT ON THE FATIGUE CRACK GROWTH RATE OF A LOW ALLOYED ROTOR STEEL

摘要

The aim of this work is to study the effect of the overload on the fatigue crack growth rate properties of a low alloyed steel used for rotor disk. On one hand, experimental fatigue tests during which a single overload event is applied are performed on CT specimens. Different loading conditions are imposed in order to study the effects of these parameters on the retardation of the fatigue crack due to the overload. On the other hand, two dimensions elastic plastic Finite Element calculations of crack propagation using nodes release method were used to estimate the effects of a single overload event on the fatigue crack growth rate. Different loading conditions, as for the experimental tests, are used in order to study numerically the effects of these parameters on the retardation of the fatigue crack due to the overload. The experimental and numerical results show the decrease of the crack growth rate due to the overload. This decrease depends on different parameters as overload ratio, stress ratio used for the constant amplitude cyclic loading and AK at which the overload is applied. From experimental test results, it can be observed that the decrease is as significant as the overload ratio is high, and as the AK at which overload is applied and stress ratio are low. Numerical results show similarities with experimental results, for instance the decrease of the fatigue crack growth is linked to the increase of the overload ratio or to the decrease to the AK at which overload is applied. Differences are also observed i.e. the increase of the stress ratio seems to increase the effect of the overload in the numerical calculations in contrary of the experimental results. By comparing to the numerical results, the quality of the results obtained from simplified models has been assessed in regard of the overload effect. A modified Kim and al. model seems to be representative of the different effects of the overload on the fatigue crack growth rate. The future work to be done consists to improve the comparison between experimental and numerical studies.