TWI Industrial Member Report Summary 108/1980
By J D Harrison
The crack tip opening displacement (CTOD) approach to material selection and weld defect significance assessment has been widely used in Britain and elsewhere for several years. Application of the approach has drawn attention to a number of problems which are discussed in this report. They concern the testing technique itself, the application of the test to weldments and the use of test results in structural assessments. The general problems in testing concern the physical definition of CTOD and its calculation from a given test record, the effects of constraint, the significance of ductile tearing initiation and short arrested brittle instabilities (pop-in), and scatter, particularly in the transition regime. In testing weldments there are difficulties with fatigue cracking and crack tip location and the question of specimen design and constraint also impinges on this aspect. The point is emphasised that the welding procedure used for tests must represent the worst probable procedure for the structure under consideration. Application can be through a specific analysis but will more frequently make use of the design curve. Problems here relate to the definition of strain to be used in the design curve, the allowance that should be made for residual stress and the significance of stress gradients. It is emphasised that many of these problems relate not only to CTOD testing and assessment but to any other method applied in the elastic plastic regime. The method is under continuous development, but, in spite of the difficulties, it has been applied with considerable success to many industrial problems. Having a basis in fracture mechanics means that the method can be applied to assess the significance of weld defects. This is not possible with more traditional methods based on the Charpy V notch impact test.