1998 TWI/HSE Structural Integrity Seminar – Micromechanical failure models for fracture predictions

Abstract

This study focuses on the Weibull stress approach to assess the effects of constraint loss on cleavage fracture toughness (Jc). The investigation addresses the significance of the Weibull modulus m (which bears direct connection with parameter calibration schemes) on the correlation of macroscopic fracture toughness for varying crack configurations. In particular, we question the ability of current calibration procedures to provide reliable values for the Weibull parameters (m, σu) and present arguments that a non-uniqueness arises in the calibrated values, i.e., many pairs of (m, σu) provide equally good correlation of critical Weibull stress values with the distribution of measured fracture toughness values. Our analyses also show a strong sensitivity of corrected Jc-values (LSY→SSY constraint correction) on parameter m for several specimen geometries and different material flow properties; such results provide compelling support to introduce an alternative, improved calibration procedure based on a toughness scaling model. Results of a parameter study suggest typical values of m for a wide range of material flow properties and toughness values. An example illustrates application of the toughness scaling model to construct Weibull stress based, constraint corrections for measured Jc-values to provide the Weibull modulus, m, for a structural C–Mn steel (BS 4360 Gr 50D).