A theory of warm prestressing: experimental validation and the implications for elastic plastic failure criteria

A theoretical explanation of warm prestressing (WPS) is proposed based on the J-integral. This is defined so that only elastic strains and distortions are included in the integrand compared with the definition of J common in finite element computation where total strains and distortions are used. The physical meaning of J as used here is that it represents the force on singularities (e.g. dislocations) enclosed by the contour of integration. Therefore, it is path dependent if the contour cuts through the plastic zone. After WPS there are regions of residual plastic strain, where dislocations are immobile, due to the change in yield stress resulting from the lowering in temp. of the cracked structure. A new plastic zone, where dislocations are mobile, forms on reloading at low temp. It is postulated that failure occurs when the force on these mobile dislocations attains a critical value, Jlc. Hence the contour used to evaluate J should be chosen to include only this plastic region. The concepts and theory are experimentally validated by successfully analyzing fracture data from center-cracked Mn steel panels (Fe -- 1.04C -- 1.18Mn -- 0.20Si and Fe -- 0.94C -- 1.16Mn -- 2.27Si), which were subjected to a range of WPS loads before being fractured at liquid nitrogen temp. Similar successes were also obtained on analyzing other published experimental WPS data. It is shown that the finite element definition of J cannot explain these results.32 refs.--AA