High strain rate tensile properties of a woven glass fabric composite

Abstract

A high rate Instron machine was used to study the dynamic tensile properties of a DOW DERAKANE 510A vinyl ester resin reinforced with 24 oz. woven roving glass fabric (510A/WR) composite. The composite was fabricated using two different methods: i) contact molding; and ii) the Seemann Composites Resin Infusion Molding Process (SCRIMP). The composites were tested in both the fill and warp directions at strain rates varying from 0.3 5/s. Overall, 36 tests were performed, generating ultimate stress, ultimate strain, and tensile modulus data. The results indicate that there is no significant difference in mechanical properties of either material over the 0.3 5/s interval, suggesting that the materials are strain rate insensitive. However, it is observed that at all three strain rates tested, the average ultimate stress and strain exceeded the static material properties for each material in both the fill and warp directions. For both materials, an increase in average ultimate stress with, increasing strain rate trend is identified. The tensile modulus remained unchanged with increasing strain rate 1 Graduate Research Assistant, Department of Mechanical Engineering, University of Delaware. 2 H. Fletcher Brown Professor of Mechanical & Aerospace Engineering, University of Delaware; Fellow, AIAA; Fellow, ASME; Author to whom correspondence should be addressed. Copyright © 1997 by the American Institute of Aeronautics and Astonautics, Inc. All rights reserved. for both materials in each direction. In terms of strength and stiffness, the SCRIMP material outperformed the contact molded material in both directions at all three strain rates tested. INTRODUCTION Woven fabric composites offer more balanced properties in the fabric plane than unidirectional laminae due to their bi-directional reinforcementfl]. This type of reinforcement aids in enhancing impact resistance. That, coupled with low fabrication costs and ease of handling, have made fabrics attractive for many structural applications. Although they are candidate materials for many applications where high strain rate loading is probable, little is known of their response to shock loading. Because mechanical properties vary significantly with strain rate, the use of static properties in the analysis and design of structures which undergo dynamic loadings can lead to a very conservative overweight design, or on the other hand can lead to designs which fail prematurely and unexpectedly. The use of dynamic material properties will ensure the design of composite structures which are weight efficient and structurally sound when they are subjected to dynamic loads. At present, a program is underway at the University of Delaware under the sponsorship of the Office of Naval Research (Dr. Y.D.S Rajapaske) that involves three tasks, namely: i) high strain rate testing in compression and tension, using the Split Hopkinson Pressure Bar (SHPB) facility, and the high rate Instron testing machine, and correlation and analysis of the dynamic material properties obtained; ii) examination of specimens tested using optical and electron microscopy to characterize the deformation and fracture processes; and iii) evaluation of the suitability of current models to describe the deformation and failure of these materials at high strain rates and their modifications or, whenever necessary, the development of new models. The present study, concentrates on task 1. Specifically, the high rate Instron testing machine is used to obtain tensile data. The high rate Instron machine is capable of achieving strain rates on the interval of 0.1 to 5/s. This intermediate range will fill the gap that has existed between typical static tests (0.001/s) and the work being performed on the SHPB, which yields accurate results in the strain rate 1362 American Institute of Aeronautics and Astronautics Copyright© 1998, American Institute of Aeronautics and Astronautics, Inc.