Correlation Between Material Certificate Data and Damage Mechanics Parameters in the Upper-Shelf Regime for S690Q Steels

Abstract

Calibration of ductile damage models typically involves significant experimental and reverse engineering effort, due to their stress-state dependent nature. Having access to the calibrated damage parameters for a range of materials could enable finite-element analyses of the fracture performance of the different materials in a structural detail and aid in material selection based on specific criteria such as the ductility or the toughness of the detail. By performing a parametric study and regression analysis using a previously validated rate-and temperature-dependent damage-plasticity model, considering steels with a yield strength between 730 MPa and 850 MPa, this paper presents correlations between easily available material certificate properties and calibrated damage model parameters, developed for the purpose of modelling the standardised single-edge notched bending (SENB) crack-tip opening displacement (CTOD) fracture toughness test. First, the correlations provide a tool for quickly estimating the calibrated parameters for the type of material considered in the study, so that damage mechanics modelling can be used in subsequent parametric investigations and design considerations concerning SENB fracture toughness testing. Second, the correlations give insight into the trends and relative importance of the plasticity and damage parameters in relation to the variations in engineering properties from the widely used tensile test and Charpy test, which are the fracture elongation A and Charpy energy C_v, respectively. By relating the properties obtained from these tests, which involve known stress states at key locations in the test specimens, to the strains in stress-state-dependent damage initiation locus, physical insight is obtained regarding the role of the normalised damage parameters, which are the damage initiation strains. Such information is useful both for understanding how plasticity parameters are related to fracture toughness and for the future calibration of similar materials. Finally, it is found that parametrically varying the yield strength σ_y and the yield-to-tensile strength ratio σ_y/σ_u simultaneously according to observed empirical trends has a small effect on the correlations between notch toughness and damage initiation strains when compared to the significantly larger effect of A. Since the damage initiation strains are directly correlated to the fracture behaviour, the σ_y /σ_u ratio’s small influence suggests that it might not be a good choice as a lo cal ductility indicator for a material in terms of ductile fracture, although it is so used in practice, a point which remains to be ascertained through further study of fracture toughness testing and simulation in light of the present findings. By the same token, the existing correlations between C_v and fracture toughness, which have not taken the effect of variations in A and σ_y/σ_u into account, should be reconsidered in light of these effects.