Connect, no.79, September 1996
For over 30 years there have been tests to help establish fracture toughness of parent materials, weld metals and heat affected zones. Although testing procedures for parent materials have been standardised, this has not been the case for weld material. Researchers at TWI have helped develop procedures for weld fracture toughness testing which will be implemented as a British Standard.
By carrying out extensive investigations, an understanding of the testing procedure variables which affect fracture toughness measurement has been developed. Special procedures must be used for testing welds because of variations such as inhomogeneous nature of welds, effect of non-uniform shape of specimen, crack position and presence of residual stresses. Previous lack of a testing standard has meant that different results are possible from virtually identical welds, due to differences in inter-laboratory procedures.
This situation is about to change with the issue of a Draft British Standard specifically for weld fracture toughness testing - Method for determination of KIc, critical CTOD and critical J values of welds in metallic materials, Part 2 of BS 7448. This will be fully compatible with the existing Part 1, which is concerned with parent material testing.
TWI has played a major role in formulating this draft standard. Considerations covered include crack location in the weld metal or heat affected zone, requirements for pre-test or post-test metallography, and how to cope with 'pop-in'. The procedures drafted ensure that welding residual stresses do not affect the test result, and because test specimens are often taken from the final product they have relaxed the tolerances on dimensions.
For further information contact us or the BSI (document 96/706339).