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New Approach to Determine Fracture Toughness from the Small Punch Test

The so-called "Local approach" is introduced to determine ductile fracture from the small punch test by means of a micro-mechanical model introduced by Gurson and modified by Needleman and Tvergaard.. By using the observed load-deflection curve from the small punch test, reverse finite element analysis is performed to determine the material plastic property (?-? curve) and the damage parameters. With material plastic property and the damage parameters known, a compact tensile (CT) specimen can be modeled by a finite element analysis. The J-integral value can be derived from the calculated load-load line displacement curve, and the amount of crack extension can be estimatde by the length of the damaged elements, in which the void fraction reaches the void volume fraction at final fracture. Thus a J-R curve can be created and JIC can be determined according to ASTM E 1820-08. Experimental results for both the small punch tests and the standard fracture tests. have been analysed and verifications carried out by using standard uni-axial tensile tests and fracture CT tests. A good agreement is found between the test results and the analytic prediction.