Notch Effects in Fracture and Fatigue

Conclusion

Notch effects present some similar approaches in fatigue and fracture and are characterised in fracture by the critical gross stress being less than resulting only in a reduction of the cross section and in fatigue by the fact that the Wöhler curve obtained with a notched specimen being below that obtained with a smooth specimen.

In any case, the value of maximum stress or maximum stress range at the notch tip is able to describe this effect; experimental results show that the effective critical stress or effective stress range has a lower value than the maximum stress or stress range. To model this phenomenon two major approaches are possible: the hot spot approach, the volumetric approach.

The hot spot approach considers that the stress or the stress range value, for some particular point on the stress distribution, is equal to the effective stress or stress range.

The volumetric approach assumes that fatigue or fracture needs a physical process volume. Inside this volume at any point stress plays a role which depends upon its distance to the notch tip and the stress gradient. The hot spot approach is historically more traditional but cannot explain loading mode and scale effects in fatigue and fracture.

The effective stress or stress range being less than the maximum stress or stress range cannot be explained by plastic or damage relaxation but that a part of the stress distribution is acting on the process with an average value less than the maximum one. For the particular case of a notch as a crack the volumetric approach seems more appropriate owing to the assumption of a stress singularity in which the maximum stress or stress range is infinite. There is a continuous approach between notch fracture mechanics and classical fracture mechanics.