Thickness-dependent lower bounds of fracture toughness of ferritic steels in the ductile-to-brittle transition regime

摘要

The inherent scatter of fracture toughness of ferritic steels in the brittle-to-ductile transition regime require statistical methods to be applied for testing and evaluation.However,for engineering purposes lower bounds of KIc such as the ASME-reference curve are often preferred since they allow deterministic worst-case predictions to be made.So the question is how to derive lower bounds of a quantity that is governed by weakest-link-statistics.Actually,neither the MC-approach nor the empirical ASME-reference curve deliver well-founded lower bounds for components of relatively small thicknesses.A theoretical model is suggested to fill this gap.The key element of the approach is the hypothesis that the weakest-link-effect is saturated at a certain thickness.The corresponding upper limit of size-dependence turned out to be close to the minimum thickness required for plane-strain conditions at the crack-front.The derived mathematical relations enables KIc to be calculated from KJc as measured on a smaller specimen.In reverse,from a lower-bound KIc as provided by the ASME-code a thickness-dependent lower bound of KJc can be obtained.The proposed model is shown to yield predictions that are consistent with experimental data as well as with the ASME-lower bound.