New unloading compliance correlation for throughwall circumferentially cracked pipe to measure crack growth during fracture tests

Abstract

Crack growth is generally measured during fracture experiment of specimen or component. The unloading compliance technique is commonly used for this purpose because of its simplicity. It infers the crack length from unloading compliance of cracked component. The pre‐requisite of this technique is the availability of an equation that correlates crack length with unloading compliance. While such correlations are available for compact tension and three‐point bend specimens, it is not available for big components such as pipe or pipe bend. Development of such a correlation for throughwall circumferentially cracked (TCC) straight pipe under bending, therefore, forms the objective of the present study. However, the challenge to develop such correlation for TCC pipe is that the equation should contain not only crack length as a function but also the current deformation or load level as a parameter. This is attributed to the fact that the circular cross section of the pipe ovalizes during deformation leading to change of bending stiffness of the cracked body. New compliance correlations have been proposed for TCC pipe under bending load considering these complexities. Elastic‐perfectly plastic material behaviour has been assumed to characterize the material stress–strain response. However, it has been shown that error due to this approximation with respect to the actual stress–strain behaviour is negligible if one chooses flow stress equal to average of yield and ultimate strength. The proposed correlations are expressed in terms of normalized parameters to make them independent of specific values of geometric dimensions such as radius, thickness and span length of four‐point bending loading system. Effectiveness of this normalization has also been verified by carrying out a sensitivity study.

An η‐compliance method to estimate the J‐Δa curve for pipes with a circumferential surface crack

“…Huang et al 20 have developed constraint-corrected J-Δa curve from standard SE(B) specimens using nine different constraint parameters for pipeline steels based on 3D finite element (FE) analyses. Chattopadhyay et al 38 have proposed new compliance relations for through-wall, circumferentially cracked pipes under bending for the crack growth measurement. Pluvinage et al 21 have presented a comprehensive review on the constraint and stress gradient approaches for fracture toughness, based on four different constraint parameter, L , β, Q and T. Recent studies [22][23][24][25][26][27] advocate the use of single-edge-notched tension, SE(T), specimens to measure the J-Δa curve for surface-cracked pipes due to the closer constraint condition between an SE(T) specimen and a surfacecracked pipe than that between a deeply notched SE(B) or C(T) specimen and a surface-cracked pipe.…”

“…Kim et al 37 have studied the ductile fracture behaviour of steel pipes with two circumferential surface cracks under pure bending, through the numerically implemented stress-modified finite strain model and the element-size dependent critical damage concept. Chattopadhyay et al 38 have proposed new compliance relations for through-wall, circumferentially cracked pipes under bending for the crack growth measurement. Most of the experimental works previously focus on through-wall cracked pipes.…”