Evaluation of crack growth during fracture and fatigue-crack-growth test using unloading compliance technique is well established for standard homogeneous fracture specimens . Three Point Bend specimens are widely used to measure fracture toughness and fatigue-crack-growth rates in metallic materials. A compliance functions is given in ASTM E1820-09 [5] to find out the in-situ crack length based on crack mouth opening displacement (CMOD). One of the inpute required in compliance function is the young's modulus of elasticity of the specimens material. To quantify the fracture toughness of weld joints, specimens are machined from weldments. Such specimens comprises of both weld and base materials. In general, base and weld materials have different young's modulus of elasticity (E). ASTM (E-1820) method doesn't account for the influence of mismatch in "E" values of base and weld material on the compliance function.
In this work, an effective modulus of elasticity is proposed for TPB specimens having weld center crack to account for the influence of mismatch in "E" values of base and weld material and geometry of weld on the measurement of crack length using ASTM compliance function. Numerical studies were carried out on TPB specimens. Compliances were calculated by linear elastic 3D Finite Element analysis . Wide range of relative crack length (a/w) varying from 0.3 to 0.7 and practical range of weld width were considered. Crack lengths calculated based on proposed effective modulus of elasticity were compared with the results obtained from ASTM (E-1820) technique (applicable to homogeneous specimens). Our studies revealed that, in comparison to ASTM technique , the proposed scheme for the TPB specimens having weld centre crack allows a very accurate evaluation of actual crack size.