Finite element analysis of a shear punch test
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THE shear punch test (SPT) is a testing technique in which a small amount of material can be used. This is an important advantage for evaluating the mechanical properties of a material when its availability is limited. The data obtained from an SPT can be used to predict the tensile yield and ultimate strengths.[1–10] A linear correlation between tensile and SPT shear strengths has been reported by several researchers.[4,9–13] The SPT has been used for testing nuclear irradiated materials,[4,5] biomaterials,[6,7] and composites.[8] The test is similar to a blanking process in which a solid flat cylindrical punch shears a disc specimen through a die at constant speed. The thicknesses of the discs used are usually in the range of a few hundred microns and the diameters are in the range of 5 to 10 mm. During testing, the load P on the punch is measured as a function of the punch displacement d. The average shear stress t is calculated using the relation[1] t5
P 2prt
[1]
where t 5 the specimen thickness and r 5 the average of the punch and die radii. The t vs d curve obtained during the SPT shearing operation has many similarities to tensile test curves (e.g., elastic loading to the yield point, plastic deformation, and an ultimate stress at failure). There is no standard procedure for determining the yield point in an SPT. Three procedures have been proposed by different investigators: (1) initial deviation from linearity along the elastic loading line,[1,2,3,5,6,7,9] which can be viewed as "0 pct strain offset, (2) 1 pct offset in the shear strain g, which was defined using the pure shear approximation[14] g5
d c
[2]
where d 5 the punch displacement and the clearance c 5 the difference between the punch and die radii, and (3) 1 pct R.K. GUDURU, Ph.D. Student, and R.O. SCATTERGOOD, C.C. KOCH, and K.L. MURTY, Professors, are with the Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695-7907. Contact e-mail: [email protected] A.V. NAGASEKHAR, Ph.D. Student, is with the School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798. Manuscript submitted September 8, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS
offset in the normalized displacement d/t. The normalized displacement procedure was proposed by us[10] to eliminate an effect of specimen thickness on the SPT curves; t vs d/t curves were found to be independent of the specimen thickness. The thickness effect can influence SPT yield point measurements when procedures (1) and (2) are used. To predict the tensile yield stress values from SPT data, the empirical correlations should be established between SPT and tensile test results for standardized testing conditions (e.g., loading rates, temperature, punch-die configurations). We reported the following yield stress correlation for tests carried out using a range of different metals and alloys,[10] s ¼ 1:77t
[3]
where t 5 the SPT yield stress for 1 pct normalized displacement offset and s 5 the tensile yield stress for 0.2 pct strai
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