MODELING MICROSTRUCTURALLY SENSITIVE FATIGUE SHORT CRACK-GROWTH

Microstructurally sensitive fatigue short crack growth can occur in ma ny engineering components devoid of large defects. Continuum mechanics principles, including linear elastic fracture mechanics, used in dama ge tolerance design and life prediction methods are not applicable in these situations and therefore new concepts need to be developed to ch aracterize this type of growth. A microstructurally sensitive model of fatigue crack growth is presented in which the effect of microstructu re is dominant in the early stage of growth but plays a negligible rol e after the crack has gone through the transition from structure-sensi tive to structure-insensitive growth. The effect of both microstructur e and structure sensitive variables on the transition from short crack s to continuum mechanics and the conditions for crack instability lead ing to final failure are examined. The microstructural variables incor porated in the equations that describe the model are those controlling the extent and intensity of crack tip plasticity such as grain size, precipitation and dispersion hardening, strain hardening and misorient ation between grains. It is expected that the concepts developed withi n the model will form the basis for the design of new crack-resistant materials.