Experimental methodology for the measurement of plasticity on metals at high strain-rates

Abstract

An experimental methodology has been developed for the tensile characterisation of ductile isotropic metals at high strain-rate. This study includes the region beyond plastic instability or necking, which is rarely analysed for conventional applications. The research explores an imaging technique used to track the geometry of the specimen during tensile tests and calculate true local values of stress and strain by applying Bridgman theory [1]. To improve the quality of the images taken at high strain-rate an in-situ high speed shadowgraph technique has been developed, and to obtain better results from the images a sub-pixel accuracy edge detection algorithm has been implemented. The technique has been applied to an austenitic stainless steel. Its tensile behaviour has been assessed by testing round samples at strain-rates ranging from quasi-static to ~103 s-1. The results obtained with the proposed methodology have been validated by comparison with more conventional techniques such as video-extensometer and digital image correlation in the pre-necking region and good performance even at the highest strain-rate tested has been proved.

Optical method to study post-necking material behaviour

“…Nevertheless, its scope is very limited without the use of an additional camera: using DIC, the deformation is determined on the surface of mainly flat specimens, and the obtained data are used to solve the inverse problem by the finite element method (FEM). The most accurate methods for measuring the neck of specimens were obtained using the DSLR and high-speed cameras in works of Sancho et al [6,7] and Mirone et al [8,9]. However, Sancho does not reveals the method to calculate the radius of neck curvature.…”

“…In order to optimize the sharpness of images authors applied additional lightning as well as painted the specimens white. Sancho et al improved the test procedure in [7]. They used only one light placed behind the specimen and recorded the shadow produced by the specimen.…”

“…In this work, we investigated the possibility of using the shadowgraph technique as in [7], but we faced inextricable difficulty. The problem is that the use of backlight significantly illuminates the boundaries of specimen.…”

Application of the Method for Optical Measuring the Neck Profile to Determine the Hardening Curve of Aluminum from the Results of the Tensile Test

“…In [9,13], the material hardening curves were obtained using the Bridgman model, and in [10], using all three models mentioned above. Despite the results of the theoretical evaluation of the range of applicability of the Bridgman, Davidenkov-Spiridonova, and Ostsemin models presented in [6], the question of which model is more preferable to use requires experimental verification.…”