Circumferential Notched Tensile Testing for Correlation of the Stress Intensity Factor ( K I ) and Stress Corrosion Crac
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aterials can suffer sudden and unexpected cracking, under the combined action of stress and corrosion, i.e., a phenomenon known as stress corrosion cracking (SCC). Therefore, the determination of the growth rate of stress corrosion cracks of the engineering materials is very important for design engineers. Fracture toughness (KIC) is the measure of a given material’s resistance to unstable crack growth (in inert environment). Accurate determination of the stress intensity factor (KI) and its threshold value for crack propagation are major materials selection criteria for any mechanical engineering design. The crack growth rate is usually determined by measuring the length of the longest crack and then dividing this length by the time of the test. This approach is rather simplistic because it assumes that the cracks initiate at the start of the test, including the longest crack, which may not necessarily always be the case. More importantly, with progressing
R. RIHAN, Former Student, and R.N. IBRAHIM, Associate Professor, Department of Mechanical Engineering, and R.K. SINGH RAMAN, Associate Professor, Departments of Mechanical Engineering and Chemical Engineering, are with Monash University, Melbourne, Australia 3800. Contact e-mail: [email protected] Manuscript submitted May 15, 2007. Article published online April 29, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
crack, the KI is likely to increase as the load bearing area decreases (under self-loading condition), necessitating a need for a reliable correlation between KI and crack velocity. For an alloy-environment system conducive to SCC, the determination of KI in plane strain fracture toughness tests requires a specimen of considerably large dimensions compared to the plastically deformed zone at the crack tip. The threshold stress intensity for crack propagation under the influence of environment at the crack tip (i.e., KISCC) is a critical parameter for design and life assessment of components exposed to corrosive environment. The KISCC and correlation between KI and crack velocity have traditionally been determined using compact tension (CT) and double cantilever beam (DCB) specimen geometries, both of which require bulky specimens.[1] Specimens for traditional test methods (viz. CT and DCB) are expensive to manufacture, requiring expensive testing infrastructure, particularly if the tests are to be carried out under high-temperature and pressure conditions. These limitations of traditional testing also put a restriction on the affordability of the number of test points, whereas generation of reliable KISCC would necessitate a considerably large number of tests. This article presents the use of circumferential notched tensile (CNT) testing as a rapid, accurate, and cost-effective testing for correlation between KI and crack velocity and for the determination of KISCC (which are immensely important to design engineers). Caustic SCC is a common problem in the alumina processing and pulp-and-paper processing industries. Mild steel, the most common co
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