A Novel Image-Based Inertial Impact Test (IBII) for the Transverse Properties of Composites at High Strain Rates

Abstract

Current methods for testing the high strain rate properties of composites require multiple assumptions that limit achievable strain rates. Therefore, this study presents a new method for testing the transverse properties of composites at high strain rates using ultra-high speed imaging. The image-based inertial impact test developed here uses the reflection of a compressive stress wave to generate tensile stress in the specimen. Throughout the test, full-field displacement measurements are taken. The acceleration and strain fields are then derived from the displacement fields. The acceleration is then used to calculate the average stress in the specimen. This paper describes the optimisation of the experimental configuration using simulations and the experimental validation of the technique. The elastic modulus and tensile strength were identified at strain rates of ∼ 2000 s −1 . The results showed an increase of 8% in elastic modulus and an increase of 57% in strength compared to quasi-static values.

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

“…The advantage of this representation is that one can immediately see if the behaviour is linear, and witness possible damage or fracture, as in [13] or [25]. It was shown later [19] that two other rigid-body virtual fields could be used: u ⋆ x = 0 and u ⋆ y = 1 , and u ⋆ x = y and u ⋆ y = −x . They represent virtual rigid body movements in vertical translation and in-plane rotation.…”

“…The principle was first published in 2014 [9] on an elastically isotropic material. It was then refined and extended to several classes of materials under the name 'Image-Based Inertial Impact' (IBII) test [18][19][20][21]. This test consists of an edge-on impact of a thin rectangular test specimen with all other edges free from any constraints.…”

“…This sends a compression wave into the specimen which reflects off the free edge as a tensile wave. The pulse length can be designed either to load and unload the specimen in compression [9,18] or to develop enough tension after wave reflection so that tensile fracture occurs [19][20][21].…”

“…In the case of isotropic materials, this requires the a priori knowledge of Poisson's ratio or the assumption of uniaxial stress [9]. Poisson's ratio can be obtained using the full VFM analysis [19], but it would be convenient to enable both stiffness components to be identified with a stressgauge approach, which is the object of the present paper. For orthotropic materials tested 'on-axis' (i.e.…”

“…with orthotropy axes aligned with the specimen axes), this is also possible. However, when a unidirectional composite is tested in the 90 • orientation, Poisson's effect can be neglected because of the high stiffness at 0 • [19,21]. The advantage of this second approach is that it leads naturally to spatially resolved properties, and provides an easy way to plot 2D curves from which the departure from possible linearity / elasticity is easily appraised.…”