Effect of a hard artificial asperity on the crack closure behavior in an annealed SAE 1015 steel M. Okayasu a , Z. Wang a, * , D.L. Chen b a Department of Materials Science and Engineering, University of Toronto, 184 College Street, Toronto, Ont., Canada M5S 3E4 b Department of Mechanical and Industrial Engineering, Ryerson University, 350 Victoria Street, Toronto, Ont., Canada M5B 2K3 Received 3 March 2004; received in revised form 16 November 2004; accepted 16 December 2004 Available online 24 May 2005 Abstract The load–crack opening displacement (COD) curves and deformation characteristics in the vicinity of a hard arti- ficial asperity in an annealed SAE 1015 steel were studied. The artificial asperity was found to have a significant effect on the trend of the load–COD curves. The lower portion of the load–COD curves in the unloading phase exhibited a con- vex shape without the asperity, but a concave shape with the asperity. The concave shape, signifying the acceleration in the COD decrease, was further verified by varying the size of the asperity, conducting special compression tests and elastic–plastic load–COD tests. The plastic deformation in the vicinity of both asperity and crack tip was studied via microhardness tests, etching techniques, and finite element analysis. Based on the experimental observations, a modified crack closure process model was proposed, where three stages of the unloading curve was defined: (i) the asperity does not contact the upper crack face, (ii) a process where both the asperity and the specimen material deform elastically, and the elastic-wedge model is applicable, and (iii) the plastic deformation of the specimen material adjacent to the asperity occurs, thus resulting in the concavely shaped load–COD curves. An equation was proposed to estimate the COD val- ues, in which the plastic deformation both at the crack tip and at the asperity was considered. The residual COD cal- culated from the proposed equation was found to be consistent with the experimental results. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: SAE 1015 steel; Crack closure; Plastic deformation; Crack opening displacement (COD); Artificial asperity 1. Introduction Since the crack closure behavior was first observed by Christensen in 1963 [1] and defined by Elber in 1970 [2], fatigue crack growth tests have been performed to identify the effect of crack closure on the 0013-7944/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfracmech.2004.12.006 * Corresponding author. Fax: +1 416 978 4155. E-mail address: zhirui.wang@utoronto.ca (Z. Wang). Engineering Fracture Mechanics 72 (2005) 2106–2127 www.elsevier.com/locate/engfracmech