Effect of specimen size, compressibility and inertia on the response of rigid polymer foams subjected to high velocity direct impact loading

Abstract

The influences of specimen length-to-diameter ratio, material compressibility, and inertia on direct impact response of high density closed-cell polymeric foam are investigated. High speed photography and stereovision digital image correlation are conducted to measure the full-field deformation response of the material subjected to direct impact. Inertia stress developed in the specimen is calculated from the acceleration distribution obtained from fullfield measurements. Total axial stress magnitude along the axis of the specimen is then reconstructed from inertia and boundary-measured stresses. It is clearly shown that there is an appreciable degree of spatial variability in strains, strain rates and stresses developed in the impacted foam specimens, whereas the degree of such axial variability is more significant at higher length-to-diameter ratios. The study is further extended to take advantage of such spatial variability to identify the rate sensitivity of the examined material over a wide range of strain rates from 1000 s-1 to 5000 s-1. The approach proposed here is shown to facilitate the identification of viscoplastic constitutive response of low impedance materials using a minimum number of tests.

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

“…on strain measurements [24]. Equation 2 was used in [9] to plot stress-strain curves for a quasi-isotropic carbon-epoxy composite, assuming either uni-axiality or a value for Poisson's ratio, see Fig.…”

A Novel Image-based Ultrasonic Test to Map Material Mechanical Properties at High Strain-rates

“…The very first example dealt with concrete spalling tests [26], and was then extended to composites [27] and metals [3,4,19]. Since then, the idea has spread and several groups worldwide are starting to use the technique [13,14,16,17,21,40,41].…”

The Image-Based Inertial Release (IBIR) Test: A New High Strain Rate Test for Stiffness Strain-Rate Sensitivity Identification

“…This would need to be investigated further using a finite element model and image deformation simulations to determine if the relevant material parameters could be identified with sufficient accuracy. The idea of identifying a rate sensitive material model using the heterogeneous strain rate fields from an inertial test has already been suggested in [12,43] and a first attempt at identifying the rate sensitive parameter of a Johnson-Cook material model was made in [44,45] for metallic materials. However, applying this type of analysis to the rate dependent stiffness of polymeric materials is an open and exciting research question.…”