Full-field strain measurements at high rate on notched composites tested with a tensile Hopkinson bar

Abstract

is an open access repository that collects the work of Arts et Métiers ParisTech researchers and makes it freely available over the web where possible. SUMMARYThe present paper deals with full-field strain measurement on open-hole tensile specimens submitted to high strain rate through a Hopkinson bar device. After having briefly explained the methodology (using an Ultra-High speed camera), the strain results obtained on a first test are shown leading to a discussion on the non-uniform state of loading induced by the Hopkinson bar system. Then, a first attempt to identify stiffnesses on this dynamic case from only the strain and acceleration maps using the Virtual Fields Method is introduced. The results are extremely promising considering the limited quality of the measurements.

Generalized Stress–Strain Curves for IBII Tests on Isotropic and Orthotropic Materials

“…At this stage, the reason for this remains unknown and will have to be investigated by undertaking more tests in the future. As also expected, no significant response could be obtained for Q 11 and Q 12 as this 45 • configuration does not generate enough fibre strains to be able to measure them with sufficient accuracy. Tests on shallower angle specimens will be conducted in the future, even though the fibre stiffness is known to exhibit little strain rate dependence and is therefore a less critical parameter to study [29].…”

“…Equations O1 and O2 are symmetrical with respect to 11 and 22 , as expected. In practice, for highly anisotropic carbon-epoxy composites, the strain in the fibre direction, 11 , is likely to be very small. In this case, the term Q 12 11 can be neglected with respect to Q 22 22 in the second line of Eq.…”

“…The first publications to exploit this new paradigm appeared in the late 2000s [10,11] and it has flourished in the later part of the 2010s [12][13][14][15][16][17]. A modern implementation of this idea uses purely inertial test configurations, inspired by the spalling tests in [13].…”

Ultra High Speed DIC and Virtual Fields Method Analysis of a Three Point Bending Impact Test on an Aluminium Bar

“…In this case, the second term in equation (4) containing T can be eliminated by a relevant choice of virtual field so that no force measurement is required. The same idea has recently been applied to high strain rate testing on composites where the elastic stiffness components of a quasi-isotropic glass-epoxy laminate have been identified using only deformation data from an ultra high speed camera [25]. In the present application, the Virtual Fields Method will be used for the first time to reconstruct the impact force directly from the specimen's response at high strain rates with inertia effects.…”

“…It is important to note that this paper does not provide a packaged solution to such a difficult problem but is aimed at proposing a different way of looking at dynamic testing when fullfield measurements are available. The same philosophy has also been recently applied to an open-hole tensile test on fibre composites [25]. The final objective is to identify an elasto-plastic constitutive law as in [24], but at very high strain rates using ultra high speed imaging.…”