STRESS INTENSITY FACTORS FOR SEMIELLIPTIC INTERNAL SURFACE CRACKS IN AUTOFRETTAGED THICK-WALLED CYLINDERS

Stress intensity factors for internal semi-elliptical surface cracks i n autofrettaged cylinders with and without internal pressures applied are presented. The three-dimensional finite element based displacement method with the crack tip square-root singularity of stresses and str ains simulated is used to evaluate the stress intensity factors along the crack front. Both allowing and disallowing the overlapping of crac k faces are considered in this investigation, the latter being simulat ed by considering crack surface contact through a kind of interface el ement introduced into the cracked area. The residual stress distributi on assumed to act on the crack face is obtained according to Tresca's yield criterion with the material assumed to be elastic-perfectly plas tic. Three different overstrains of autofrettage are chosen. The resul ts show that the stress intensity factor is generally underestimated i f the crack contact that has actually occurred is ignored, which may l ead to a danger in the assessment of either fracture strength or fatig ue life. Implications of the stress intensity factor results are also briefly discussed, particularly for the prediction of fatigue lives, a nd it is shown that the full autofrettage treatment might be the most beneficial for increasing the fatigue life of cracks initiated from th e inner core.