Towards high velocity deformation characterisation of metals and composites using Digital Image Correlation

Abstract

Characterisation of materials subject to high velocity deformation is necessary as many materials behave differently under such conditions. It is particularly important for accurate numerical simulation of high strain rate events. High velocity servo-hydraulic test machines have enabled material testing in the strain rate regime from 1-500 ε/s. The range is much lower than that experienced under ballistic, shock or impact loads, nevertheless it is a useful starting point for the application of optical techniques. The present study examines the possibility of using high speed cameras to capture images and then extracting deformation data using Digital Image Correlation (DIC) from tensile testing in the intermediate strain rate regime available with the test machines. Three different materials, aluminium alloy 1050, S235 steel and glass fibre reinforced plastic (GFRP) were tested at different nominal strain rates ranging from quasi static to 200 ε/s. In all cases DIC was able to analyse data collected up to fracture and in some cases post fracture. The use of highspeed DIC made it possible to capture phenomena such as multiple necking in the aluminium specimens and post compression failure in GFRP specimens.

Wide‐angle X‐ray scattering study of the lamellar/fibrillar transition for a semi‐crystalline polymer deformed in tension in relation with the evolution of volume strain

“…The lamellar structure that still exists strongly limits the cavitation phenomenon. Only nanovoids embedded between lamellae can develop in the polymer, and these nanovoids were detected using small‐angle X‐ray scattering measurements …”

“…We used pixels facets and an pixels stepsize (distance between two adjacent facets). The strain reference length was thus pixels and one pixel represented approximately on the specimen.…”

“…This makes it possible to accurately determine the thresholds separating the different regimes observable on the obtained curves. The volume strain measurements were obtained using 3D digital image correlation (DIC) a technique previously proved to give very accurate volume strain measurements …”

“…The volume strain measurements were obtained using 3D digital image correlation (DIC) a technique previously proved to give very accurate volume strain measurements. 22…”

Kinematic study of necking in a semi-crystalline polymer through 3D Digital Image Correlation

“…An 8 pixel facet step was considered for two adjacent facets (overlapping area of 22 pixels in longitudinal direction). The strain reference length is then 16 pixels (Eriksen et al, 2010). Taking into account the pixel size and the strain reference length, it is easy to show that it corresponds to a displacement signal std less than 0.15 lm, which is slightly smaller than 0.01 px.…”