TWI Industrial Member Report Summary 277/1985
By S J Garwood
Although crack tip opening displacement (CTOD) analysis procedures are well established for the prediction of tolerable crack sizes in ferritic steels, no specific procedures exist which incorporate a resistance (R) curve to facilitate estimation of the ductile instability conditions for a fully ductile material. This report discusses a modification to the CTOD procedure, based on the strip yield model, to enable the prediction of maximum nominal stress and the extent of stable ductile crack growth using a CTOD R-curve as input. A modified strip yield model is presented. The modified procedure is used to predict the behaviour of:
i. 500mm wide plates in 50, 90 and 110mm thick A533B steel plate
ii. 900mm wide plates in 13mm thick API 5L X56 steel parent pipe material
iii. 900mm diameter 13mm thick steel pipeline burst tests
iv. 500mm wide plates in 50mm thick 316 stainless steel plate and welds
Predictions are then compared to physical data obtained from the tests. The results indicate that the modified strip yield model, incorporating a plastic collapse estimation, can be used, with a representative resistance curve for the material, to give a good assessment of the failure conditions (i.e. fracture-controlled, or collapse) and the maximum nominal stress. However, the accuracy of the predictions is very dependent on the plastic collapse estimation. Predictions of the test results are also made using maximum load toughness values, measured in CTOD test pieces complying with BS 5762, with either the CTOD design curve or the modified strip yield model approach. Predictions give safe estimates of nominal stress. It is concluded that maximum load values provide the most practical general approach when detailed resistance curve information is not available. A generalised procedure, based on maximum load toughness and the collapse-modiied strip yield model is outlined. Finally, the potential use of the modified CTOD model for critical predictions of as-welded components, and for brittle applications, is discussed.