Assessment of Fatigue Damage in Heterogeneous Materials by Application of a Novel Compliance Technique

It is well known that the reduction of the load-bearing cross section caused by fatigue damage gives rise to a characteristic extrinsic enhancement of the elastic compliance of the material. It is less well known that, in particular at higher strains, the elastic compliance effects stemming from fatigue damage are modified by the superposition of the intrinsic non-linear elastic compliance of the material. The latter can be assessed quantitatively and separated from the overall compliance effect, thus enabling the study of the extrinsic damage-induced compliance effects. The novel technique yields information on crack propagation, crack opening and crack closure and has the advantage of being applicable to plain unnotched specimens. It is particularly suited for the study of fatigued heterogeneous materials in which early fatigue damage occurs in the form of many statistically distributed microcracks some of which then coalesce into a larger main crack. Combined with a parallel replica study of surface microcracks and their coalescence, a deeper insight into the cracking process can be achieved. In the following, the potential of this new approach will be demonstrated by application to specific experimental situations. It will be shown that under favourable conditions the novel approach is a useful non-destructive tool to study the crack propagation process in fatigued plain unnotched specimens in its early and later stages.