The CTOA as a parameter of resistance to crack extension in pipes under internal pressure

Abstract

Different methods of CTOA measurements are described: Optical microscopy coupled with digital image correlation, analytical analysis of experimental load-displacement curves or simulation by Finite Element method. Using a Modified Compact Tension (MCT) specimen at room temperature, tests are performed to measure the value of the CTOA of API 5L X65 pipe steel. The influence of thickness on CTOA has been studied and explained through a “triaxial stress constraint”. Crack extension is modelled by the finite element method using the CTOA criterion coupled with the node release technique. Crack velocity, arrest pressure and crack extension at arrest have been determined. Values of the CTOA are not intrinsic to materials. Like other measures of fracture toughness, it is sensitive to geometry and loading mode. This sensitivity can be described by a constraint parameter. For the thickness effect, the constraint parameter Tz is very appropriate.