Cleavage Fracture of Steel: Improved Consideration of Carbide Failure and Stress Variation

Abstract

The use of Local Approach (LA) to cleavage fracture models offers a mechanistic alternative to global approaches for predicting fracture behaviour. LA models calculate a probability of cleavage initiation at individual points throughout the fracture process zone (FPZ) ahead of a crack-tip. This calculation incorporates the behaviour of cleavage initiators, typically carbides and the sharp micro-cracks they nucleate, the population and size distribution of these initiators, and the mechanical fields local to these defects. This work investigates the weaknesses of current LA methods, which limit their ability to be used predictively to assess the effects of temperature, material degradation and defect geometry on the structural integrity of Reactor Pressure Vessels. A comprehensive review of the current understanding of cleavage fracture mechanisms and LA methods concluded that current LA methods make significant, and sometimes conflicting, assumptions about cleavage initiators. These include assumptions about the size and density of micro-cracks and how damage at the length scale micro-cracks could influence their behaviour. A new approach is presented herein, which operates at a lower length scale than current LA models and explicitly simulates individual particles and micro-cracks in the FPZ. This allows a closer modelling of the underlying mechanics of cleavage fracture and the removal of some of the simplifying assumptions noted above. The new approach incorporates: • An improved model of the deformation of elastic, brittle carbides in a ductile matrix, giving an improved calculation of the stresses developed within carbides and their rupture. • A calculation of micro-crack size incorporating the depth of extension into the matrix at nucleation, based upon the available strain energy for the creation of new surface. • How the populations of micro-cracks and voids evolve during loading, incorporating nucleation, void growth, and the coalescence of both types of defects. • The redistribution of load as the microstructure changes and how this influences subsequent failures (thus considering ductile damage at a local level, which could form the basis of a coupled ductile-cleavage model in future work). The results presented strongly suggest that the micro-crack population, in terms of its size distribution and density, and the stresses applied to micro-cracks, vary greatly with plasticity. As 1) these factors collectively control cleavage fracture, and 2) how plasticity develops around a crack-tip is strongly influenced by load, temperature, irradiation and crack-tip geometry, an improved accounting of these factors is considered necessary to allow LA models to be used predictively.

Date of Award	31 Dec 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Andrey Jivkov (Supervisor) & Lee Margetts (Supervisor)