Innovative methodology for measuring residual stress in engine blocks

Abstract

Residual stress within metal components represents, in the majority of the cases, a source of a large amount of scrap and losses for the metalworking industry. Although there are methods of measuring residual stresses, the complexity of geometric shapes in several products makes it a difficult task. Current research work developed a new methodology for measuring residual stresses in areas of complex geometry, as is the case of engine blocks in the automotive industry. The strain gauge measurement technique (SMT) is used here to validate the proposed methodology since, for many years, the SMT has gained high confidence in the automotive industry for the case of measurements of residual stress in metallic products. The new methodology involves: firstly, a pair of indentation marks performed at the section of study and secondly, a cutting process which is carried out to relieve stress in the section. Replica samples were taken from both samples in every experiment to measure the distance between indentation marks. Difference between both measurements (the sample before and after the stress relief) gives us a measure of the change in deformation which is related with the residual stress by Hooke’s law (σ = F/A = Ee = E[∆e/(e0)]), where e0 is the initial spacing and Δe is the change in the spacing between indentation marks. The implementation of the replica sample allows the transfer of the distance between indentation marks by means of portable samples, which can be carried to the laboratory where an optical microscope is used together with an image analyzer to obtain accurate measurements in laboratories with a resolution of ± 2 μm. Residual stress values by the new technique were then calculated to compare with those values obtained by the strain gauge technique. Findings confirm the effectiveness of the current methodology for measuring residual stresses in complex areas and suggested a great opportunity in which residual stress measurements have not been able to obtain using any other technique.