A B S T R A C T The demonstration of leak before brake (LBB) based on fracture mechanics requires information on the initial size of a defect, initiation of crack growth from the inherent defect and subsequent crack growth rates. In the present paper the prediction methodologies have been tested for three different full scale pipes geometry experimentally tested data. The prediction accuracy of two SIF solutions available in the literature has also been judged. The effect of fatigue crack closure and corrections needed in the numerical prediction methodology using FEM have also been included. The results showed that the FEM could fairly predict the fatigue crack initiation and crack growth life of full-scale piping components having a constant depth crack profile.Keywords crack growth; cracked pipes; fatigue loading; finite element analysis; stress intensity factor.
I N T R O D U C T I O NFailure of piping components under normal operating conditions well below the allowable stress given by codes can often be due to flaws. Crack-like defects could develop at the manufacturing/installation stage or it could develop as a result of continued operation, with cyclic loading and material deterioration being the two major causes for defect initiation and propagation. 1 This necessitates detailed flaw evaluation of different piping components such as straight pipes, elbow and branch tees. In the primary heat transporting system of power plants, piping components are subjected to high stresses due to temperature fluctuations, vibration, etc. Based on the results of investigations of failures conducted by Nishida and his co-operating researchers, failures are generally caused directly or indirectly by fatigue. 2 Such failures show that conventional stress analysis is not sufficient to guarantee component integrity under operational condition. Design criteria for the prevention of fatigue failure have evolved from the concept of almost infinite life to the presentday fail-safe design philosophy. Therefore, fail-safe design criteria such as leak-before-break (LBB) based on fracture mechanics concepts are being adopted. An important consideration in this methodology is the selection of materials with high fracture toughness. This design criterion requires the demonstration of integrity of the piping Correspondence: S. Saxena. system by showing that unstable crack growth will not occur before a crack penetrates the wall thickness, nor will it occur for a through-wall leakage size flaw. The demonstration of LBB design criteria requires information on the size of initial defects, initiation of crack from the inherent defect and subsequent propagation rate of the initiated crack till the crack penetrates the wall thickness under fatigue loading. Thereafter, the crack will grow in the circumferential direction under cyclic loading till the acceptable leakage size flaw yields the detectable leakage rate. 3 Therefore, various tests are required for the determination of fatigue crack initiation, fatigue crack growth and fracture resi...