TWI Industrial Member Report Summary 665/1999
A D Karstensen, M R Goldthorpe and C S Wiesner
Fracture toughness predictions using the 'local approach' to cleavage fracture are compared with experimental results and the effect of Weibull parameter variance on the predictions is investigated.
Micromechanical failure models combine the understanding of fracture behaviour on a micro-scale and the use of modern finite element techniques which allows refined stress analyses of complex structures containing flaws. Cleavage fracture is essentially a weakest link process that involves several independent variables associated with applied conditions and the microstructure of the steel. The Beremin model (also referred to as the 'local approach' to cleavage fracture) gives the probability of failure by cleavage via a two-parameter statistical Weibull distribution. The 'local approach' to cleavage fracture therefore has an advantage over conventional fracture toughness assessments because of its ability to describe the inherent scatter in cleavage fracture toughness test results. Another advantage of the 'local approach' is the potential to predict failure for a wide range of test conditions including the effect of constraint, changes in temperature, etc.
Despite the apparent advantages of the Beremin model, there are few published applications of its use in structural assessments. This is partly due to its recent development, but also because there are still some uncertainties concerning the application of the model.
This report presents work on some of the parameters which influence the predictions of cleavage fracture and supplements an investigation into the temperature and geometry dependence of the parameters of the Beremin model described in a recent TWI report by one of the authors.
The aim of this work is to explore the transferability of Weibull parameters between different temperatures, and also between different cracked geometries. The following more specific objectives shall contribute to this aim:
- To determine the effect of changes in the Weibull parameters on cleavage fracture toughness predictions.
- To determine the ability of the Beremin model to predict the fracture toughness of different geometries and/or temperatures using Weibull parameters determined from experiments at one temperature and one geometry.