Constraint Effects in Fracture

Assuring structural integrity using weld flaw assessment procedures based on fracture mechanics is now well established and accepted in safety critical components such as nuclear power plants, pipelines and offshore structures.

BS 7910:2013, R6 and API 579-1/ASME FFS-1 provide guidelines for carrying out these assessments and require the fracture toughness of the material at the crack tip to be determined. This can be derived from J-integral, CTOD or KIc tests, and there has been much discussion on how to make the small-scale laboratory test specimens representative of the structure ultimately being analysed.

For the case of ferritic steels, elastic-plastic fracture toughness has long been recognised as dependent on specimen thickness, and it is normal practice to test specimens of the same thickness as that of the structure to be analysed. Several standards cover testing of these so-called ‘high-constraint’ specimens, in which the crack tip is subjected to in-plane bending, restricting plastic flow. The use of such tests in conjunction with standard flaw assessment procedures has a good record in preventing failure of safety-critical plants and structures.

An additional factor to consider is that of in-plane constraint; hydrostatic crack-tip stresses in a tension-loaded structure are typically lower than those in a specimen under bending, giving the structure a higher effective toughness than that determined by conventional small-scale tests. This is routinely exploited in, for example, the offshore pipeline industry, where testing of tension-loaded (SENT) specimens (to model tensile loading of girth welds) is commonplace.

Standardised methods are now available for both low-constraint testing and the constraint-based assessment of structures using R6 and BS 7910.

This seminar will examine the roots of constraint-based assessment, providing case studies, validation and an overview of current research.