Notch sensitivity and size effects described by a short crack propagation model

摘要

In a series of articles begun in 1988, a model has been developed which describes the behaviour of a microstructural short crack in terms of successive blockings of dislocations at microstructural barriers, such as grain boundaries. The techniques evolved from this modelling are useful for studying a number of issues of great practical importance in fatigue design from the engineering point of view. In particular, the effect of notches on the fatigue limit. Classical explanations for notch effects (e.g. Peterson's hypothesis that the fatigue strength is determined by the stress at a certain depth below the surface, or Neuber's elementary structural unit averaging distance) are re-examined here in unambiguous and precise terms in relation to the behaviour of a short crack interacting with the microstructure under the influence of the non-uniform stress field associated with notches. An expression is presented for the notch fatigue factor, giving predictions in accordance with the experimental evidence regarding the influence of microstructural size, specimen size and notch severity. Additionally, the effect of the random distribution of grain size is also examined and a characteristic length representative of the microstructural size based on macroscopic fatigue properties is proposed.