Residual Strain Analysis via a Non-Bonded Interface Technique in Comparison to a Finite Element Model
Abstract
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.