Investigation of the 2D assumption in the image‐based inertial impact test

Abstract

The image‐based inertial impact (IBII) test has shown promise for measuring properties of composites at strain rates where existing test methods become unreliable due to inertial effects (> 102 s−1). Typically, the IBII tests are performed with a single camera, and therefore, to use surface measurements for material property identification, it is necessary to assume that the test is two‐dimensional. In this work, synchronised ultra‐high‐speed cameras are used to quantify the relevance of this assumption when nonuniform, through‐the‐thickness loading is applied to interlaminar samples. Initial experiments revealed that an angular misalignment of approximately 1° between the impact faces of the waveguide and projectile created a bending wave that propagated along the sample behind the axial pulse. Even under these conditions, consistent measurements of stiffness were made by assuming a linear distribution of the behaviour through‐the‐thickness. When the misalignment was reduced to 0.2°, the effects on single‐sided measurements were significantly reduced. The two alignment cases were compared to show that three‐dimensional loading had a small effect on stiffness identification (approximately 5% bias) relative to failure stress (approximately 30% bias). This study highlights the importance of impact alignment for reliable characterisation of the interlaminar failure stress and was used to establish guidelines for diagnosing loading issues from single‐sided measurements.

High strain rate elasto‐plasticity identification using the image‐based inertial impact (IBII) test part 2: Experimental validation

“…The experimental data presented in this study were obtained before the work in Van Blitterswyk et al [34] Therefore, it is possible that the presence of 3D loading explains some of the differences observed between test samples as well as the increase in scatter between the quasi‐static and dynamic test data. Analysis of the exact errors induced by 3D loading for elasto‐plasticity would require a similar systematic study with back‐to‐back imaging as presented in Van Blitterswyk et al [34] …”

“…A key assumption that is used to analyse IBII test data using the VFM is that the kinematic fields must be uniform through the thickness of the sample such that the volume integrals collapse to 2D. The validity of this assumption was analysed experimentally in Van Blitterswyk et al [34] for the case of elastic modulus identification and tensile failure stress identification for a quasi‐brittle material. In this study, it was found that a bending wave created by projectile misalignment leads to small bias on the elastic modulus but a significant bias on the tensile failure stress.…”