Residual Strain Analysis via a Non-Bonded Interface Technique in Comparison to a Finite Element Model
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The determination of residual strains in materials after an applied load is an important quantity for the understanding of the deformation behaviour of materials. There exist many quantifying techniques to measure residual strains however these techniques have limitations when micro scale measurements are of interest. In this study a technique is developed capable of quantifying localized deformation at the microstructural level by utilizing a micro-array of pre-defined circles on the internal non-bonded interface of a split sample. By performing an indentation test on the circle array, the circles will deform along with the material. By measuring the major and minor axes of the plastically deformed circles, the residual principal total strains are determined. The results from this non-bonded interface technique (NBIT) are then compared to results from a validated non-linear finite element (FE) model. Experimental homogeneous and split samples made of AISI 4340 steel were used. FE analysis was used to examine the effect of the internal non-bonded interface which showed that the split interface caused less than a 10% difference between the split and whole samples when measuring principal major and minor strains. The residual principal major and minor strain were experimentally examined for 588.6N, 981.0N, and 1471.5N indentation forces and compared to the FE model. The results of which ranged from a percent difference of 25.99% for the principal minor strain from the 1471.4N indentation, to 69.94% for the principal minor strain from the 981.0N indentation. The large difference between the experimental and FE model was explained by the inability of the FE model to simulate the local nonhomogeneous nature of a multiphase material, as well as the measurement errors caused by human involvement. From all of this analysis it was determined that NBIT can be utilized as a reliable internal residual strain analysis technique which has the capabilities of experimentally resolving residual principle micro strains with the main limitations being the circle measurement accuracy.