On the dependence of the Weibull exponent on geometry and loading conditions and its implications on the fracture toughness probability curve using alocal approach criterion

In a previous paper the authors assessed the probability of failure of a th ree point bend specimen, SE(B), using a local approach criterion. In that p aper the Weibull exponent, m, was derived from tests performed on round not ch bars in traction, RNB(T), following the procedure suggested by Mudry. In the present study, it is addressed the issue of the dependence of the Weib ull exponent m on geometry and loading conditions. It is shown that the amp litude and shape of the notch tip stress field and, in particular, the tria xiality characterising the stress state determines the value of the exponen t m. Tests performed on RNB(T) specimens of carbon steel 22NiMoCr37, type A 508 Cl 3, at temperatures ranging from -18 degrees C to -196 degrees C act ually indicate that m varies from similar to 6 to 40, depending on the notc h depth and root radius while for specimens carrying sharp cracks its value drops down to similar to 4. This last result seems to be consistent with t he Wallin hypothesis of a theoretical value equal to 4 for fracture mechani cs specimens with high constraint, such as C(T) or SE(B), with positive val ues of the Q-stress or T-stress and triaxiality factor, TF, approaching 2.5 . Temperature, in as long as it does not modify the stress state from plane strain to plane stress and the TF, has no effect on the value of m which i s independent of the material as well.