The effect of test machine compliance on the measured shear punch yield stress as predicted using finite element analysis

In previous research involving the use of shear punch test, it was understood that some compliance was present in the test frame and fixturing for a shear punch test, but it was assumed that this compliance would only affect the slope of the loading curve and not the perceived yield point on a shear punch test trace. The present work explores this assumption and its ramifications by simulating the shear punch test with finite element analysis (FEA). The simulations suggest that punch tip displacement is much less than previously assumed, and that for the test frames and fixtures which have been used, crosshead displacement is over an order of magnitude greater than punch tip displacement. This difference in displacements appears to be due to test machine and punch compliance, and a simple elasticity calculation of the compliance of the punch, the test machine, and a specimen gives a result which is in agreement with the FEA simulations. The effect of using punch tip displacement on the observed effective shear yield stress was evaluated using FEA-simulated shear punch tests on several different metals. Yield was measured at several different offset shear strains with a 1.0% offset shear yield strength measurement providing the best correlation with 0.2% offset uniaxial yield strength. When using the 1.0% offset shear yield values, the previously observed material-to-material variability in the tensile-shear correlation all but disappeared. Based on this work, it appears that the material-to-material variations in prior correlations between uniaxial yield strength and shear yield strength are largely due to an interplay between the large test system compliance and material-to-material differences in the work hardening behavior.