Issue 7

S.K. Kudari et alii, Frattura ed Integrità Strutturale, 7 (2009) 57-64; DOI: 10.3221/IGF-ESIS.07.04 57 Experimental investigation on possible dependence of plastic zone size on specimen geometry S. K. Kudari School of Mechanical Engineering, Howard College Campus, University of KwaZulu-Natal, Durban- 4041, South Africa. B. Maiti Department of Mechanical Engineering, Indian Institute of Technology, Kharagpur-721 302, India. K. K. Ray Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur-721 302, India. A BSTRACT . In this investigation the extent of the plastic zone size ahead of a crack-tip in single edge notched tension (SENT), compact tension (CT) specimens has been examined experimentally by micro-hardness technique and by elastic-plastic finite element analyses at different applied load levels. The magnitudes of the plastic zone size (PZS), r p ahead of crack-tip in the investigated specimens have been compared using normalized J-integral (J/a σ y , where, a-crack length and σ y -yield stress of the material). The results show the dependence of PZS on specimen geometry due to varied in-plane crack-tip constraint. The results also demonstrate that the existing analytical models do not explain the experimental results of PZS satisfactorily. K EYWORDS . Crack-tip plasticity; elastic-plastic material, specimen geometry, micro-hardness technique I NTRODUCTION he studies on crack-tip plastic zones are of fundamental importance in describing the process of failure from a macroscopic viewpoint and in formulating various fracture criteria. The standard ASTM procedure ASTM E1820- 99a [1] for determining fracture toughness criteria of materials require knowledge of the extent of plastic zone occurring at the crack-tips. Wang [2] has suggested that plastic deformation may be the main mechanical driving force for crack propagation. Park et al. [3] have considered that size of crack-tip plastic zone is a potential EPFM parameter connecting direct physical meaning to describe crack propagation. In EPFM, it is also known that the load intensity in ductile fracture measured as J-integral alone cannot describe the stress state accurately. This discrepancy is commonly referred as constraint issues in fracture. Yuan and Brocks [4] have considered that constraint literally is a structural obstacle against plastic deformation, which is induced mainly by geometrical and physical boundary conditions. With detailed finite element analyses they have also argued that constraint effects in a fracture specimen depend on the plastic zone size (PZS). Recently, Kudari et al. [5] h ave theoretically studied the effect of specimen geometry on plastic zone size using the J-integral to explore the constraint effects. The authors have suggested that the results can be used to obtain a specimen size requirement of J C test specimen independent of specimen geometry. The theoretical analyses of Kudari et al. [5] related to constraint effects and plastic zones have been carried out on different types of specimen configurations. These results demand substantiation with some experimental measurements of PZS. A number of investigations are available on the experimental estimation of plastic zones in metallic materials as cited in the article of Uguz and Martin [6]. But, the available results from these investigations cannot be used in a simplified manner to support the theoretical work on PZS presented by Kudari et al. [5]. The details of deformation behaviour together with determination of PZS are required to compare estimates made from theoretical analysis and experimental work. In this study an effort is made to generate experimental and theoretical estimates of PZS pertaining to the same T