Effect of prestrain on stretch-zone formation during ductile fracture of Cu-strengthened high-strength low-alloy steels
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I. INTRODUCTION
THE fracture behavior of ductile materials is usually characterized by elastic-plastic fracture mechanics parameters such as the J-integral. Unlike in the linear-elastic fracture-toughness testing, where crack initiation is easily detectable due to its brittle nature, a crack in a ductile material will first blunt, extend virtually, and then grow continuously in a stable manner. The various stages of crack extension in a ductile material are shown in Figure 1. Determination of crack-initiation toughness in elasticplastic materials requires experience, because the procedures for evaluating a singular unequivocal ductile fracturetoughness value are not straightforward. Virtual crack extension, prior to stable tearing, in ductile materials leaves a signature of the crack-initiation phenomena called the stretch zone. This is a featureless region between the end of the fatigue precrack front and the onset of ductile tearing. The formation, shape, and size of the stretch zone are characteristics of the material. In highly ductile materials, the stretch zone would normally have two components: a horizontal component called the stretchzone width (SZW) and a vertical relief of the crack front called the stretch-zone depth (SZD), as depicted in Figure 2. Both the SZW and SZD are closely related to the ductile fracture toughness of a material, and many researchers have measured characteristic stretch-zone parameters on the fracture surface of specimens and attempted to use them as an aid in determining the critical ductile fracture toughness.[1–10] The measured SZW or SZD values can be used as convenient markers for identifying the crack initiation point on the experimentally generated crack-growth-resistance (R) curve. There is no unanimity among researchers in their choice S. SIVAPRASAD, S. TARAFDER, V.R. RANGANATH, and S.K. DAS, Scientists, are with the National Metallurgical Laboratory, Jamshedpur 831007, India. K.K. RAY, Professor, is with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur 721302, India. Contact e-mail: [email protected] or shiva@nmlindia. org Manuscript submitted May 11, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
of employing the SZW or SZD for ductile fracture-toughness estimation. Some researchers have used the SZW,[4–8] while others argue that SZD measurements are more appropriate than SZW[9,10,11] for obtaining ductile fracture toughness. While the SZW can be measured directly from the fracture surface of a specimen under a scanning electron microscope (SEM), SZD measurements are difficult to obtain, mainly due to the problems associated with identifying the various stages of the fracture process while observing the specimen end-on. A methodology for SZD measurements is, therefore, necessary. Apart from accurately determining the ductile fracture toughness, sometimes it is necessary to examine the nature of the variation of the ductile fracture toughness of a material when factors affecting the toughness, such as the yield s
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