1 Copyright © 2014 by CSME Proceedings of The Canadian Society for Mechanical Engineering International Congress 2014 CSME International Congress 2014 June 1-4, 2014, Toronto, Ontario, Canada Effect of Crack Front Curvature on CMOD Compliance and Crack Length Evaluation for Single-edge Bend Specimens Zijian Yan Department of Civil and Environmental Engineering The University of Western Ontario London, Ontario, Canada Wenxing Zhou Department of Civil and Environmental Engineering The University of Western Ontario London, Ontario, Canada e-mail: wzhou@eng.uwo.ca Abstract— This paper presents three-dimensional (3D) finite element analyses of single-edge bend (SE(B)) specimens to investigate the impact of the crack front curvature on the crack mouth opening displacement (CMOD) compliance and the crack length predicted from the CMOD compliance. Specimens with the average crack length aave/W of 0.3, 0.5 and 0.7 and thickness-to-width ratio B/W of 1, 0.5 and 0.25 are analyzed. The curved crack front is assumed to be bowed symmetric and characterized by a power-law expression. The impact of the elastic modulus used in the equation to predict the crack length from the CMOD compliance is also investigated. The results indicate that the crack front curvature has a negligible impact on the CMOD compliance and the accuracy of the crack length evaluated from the CMOD compliance. Keywords- curved crack front; fracture toughness test; SE(B) specimen; CMOD compliance; ASTM E1820-11 I. INTRODUCTION Fracture toughness resistant curve of ductile material, such as the J-integral or crack tip opening displacement (CTOD) resistance curve, is an important input of the structural integrity analysis and usually measured on small-scale specimens, e.g. three-point single edge bend (SE(B)) specimens. The unloading compliance method proposed by Clarke et al. [1] is widely used in fracture toughness test standards, e.g. ASTM E1820-11 [2], to develop the toughness resistance curve from one single specimen. The crack length in the toughness resistance curve is typically predicted from the measured crack mouth opening displacement (CMOD) compliance of the specimen. As specified in ASTM E1820-11 [2], all machine notched specimens need to be fatigue pre-cracked to simulate natural cracks before the resistance curve testing. The fatigue pre- cracking often introduces curved as opposed to straight crack fronts, as illustrated in Fig. 1. The shape of the curved initial crack front is largely affected by specimen dimensions, notch machining conditions, fatigue pre-cracking conditions and residual stress distributions [3]. Furthermore, the crack growth during the test is in general non-uniform across the crack front. The crack generally grows faster at the mid-plane as a result of the high local stress triaxiality, and grows slower near the free surfaces due to the near plane stress conditions [3]. Steenkamp [4] investigated the influence of crack front curvature on the specimen compliance using two-dimensional (2D) plane strain finite element analyses for SE(B) specimens with the same average crack length but different degree of crack front curvature. He concluded that for the same average crack length by increasing the crack front curvature, the specimen compliance would decrease and for the same degrees of crack front curvature, the effect of curvature on compliance became more pronounced with increasing crack length. However, the actual state of stress in the remaining ligament of a three- dimensional (3D) specimen is not plane strain [5], and how the crack front curvature impacts the evaluated crack length from CMOD compliance has not been investigated in previous studies. ASTM E1820-11 specifies the allowable deviation of a curved crack front from a straight front based on the so-called nine-point measurement method. It requires that none of the nine physical measurements of the initial (final) crack size differ by more than 0.05B from the average initial (final) crack length aave obtained from the nine measurements, where B is the thickness of the specimen. Test specimens that do not meet this criterion are deemed unacceptable and therefore rejected. In this regard, the other objective of the present study was to examine the necessity of this crack front straightness criterion in ASTM E1820-11 when evaluating the crack length. In this study, a systematic 3D finite element analyses of plane-sided SE(B) specimens with a wide range of thickness-to- width ratios, average crack lengths and crack front curvatures was carried out. The CMOD compliance value for the specimen with a straight crack front was compared with the value obtained from a specimen with a curved crack front but having the same average crack length. For a given specimen with either a straight or curved crack front, the crack length predicted from the CMOD compliance was compared with its actual average crack length. The impact of the elastic modulus used in the equation relating CMOD compliance to the crack length was also investigated. It is observed that the crack front curvature has a negligible impact on the evaluated crack length and the errors in crack length