TWI Industrial Member Report Summary 831/2005
By H Pisarski
When limited fracture toughness data are available, the Master Curve approach is a powerful engineering method providing a quantitative description of the lower part of the ductile to brittle transition curve of ferritic steels. In homogenous material, the Master Curve approach characterises fracture toughness when failure is by cleavage and occurs under small-scale yielding conditions. By conducting a relatively small number of tests (as few as six), an indexing temperature can be defined. Using this temperature, the lower part of the transition curve is described for any specimen thickness with a specified confidence. The approach has been developed and used by the nuclear industry for pressure vessel steels. Because of the potential power of the Master Curve approach, its application to non-nuclear structural steels is being explored.
This report outlines the basic assumptions of the weakest link fracture model that provides the fundamental basis to the Master Curve approach to fracture. The equations used to characterise fracture in homogeneous steels are described and examples are provided illustrating its application to non-nuclear structural steels. This is followed by a description of developments of the Master Curve approach to the analysis of welds. Examples are given of applying these methods to the characterisation of fracture toughness of weld metals and heat affected zones (including materials in the as-welded condition) using available data. This review is intended to identify strengths and limitations of the approach to welds so that its range of application can be defined better. In addition, it is intended to provide a precursor to an experimental programme to help target future research.
The transition curve described by the Master Curve is claimed to have a unique shape, independent of steel type. The position of this curve along the temperature axis is located by the indexing temperature, To, which is derived from the scale parameter of the Weibull distribution of fracture toughness. To is considered to be a material property. It is this aspect that is examined in this report with particular reference to parent material, welds and heat affected zones in non-nuclear structural steels.
The objective of the project is to provide guidelines on the application and use of the Master Curve approach to fracture toughness evaluation of welds in ferritic steels. This report is concerned with the first phase in which the objective is a critical review of published information on the background to Master Curve approach and its developments for analysing data from inhomogeneous steel materials such as welds.