Fiber Orientation Effects on High Strain Rate Properties of Graphite/Epoxy Composites

Excellent mechanical properties such as high-strength, low weight, fatigue life, and impact resistance, as well as advanced manufacturing methods and flexibility of the stacking sequence make polymer matrix composites attractive candidates for use in several performance-oriented structures. However, relatively little is known of their response to impact loading, which usually occurs at strain rates higher than those used to measure the quasi-static mechanical properties of materials. In this study, the split-Hopkinson bar is used extensively to study the effect of fiber orientation on the compressive dynamic properties of a unidirectional IM7/8551-7 graphite/epoxy composite. The specimens were approximately 3/8. cubes, and were prepared using a water-cooled diamond grit blade. The off-axis angles were 0 (longitudinal direction), 15, 30, 45, 60, 75, and 90 (transverse direction) degrees. Quasi-static tests were also performed for comparison purposes. The high strain rates vary from 250-1100/s. The tests show that changing the fiber orientation changes the values of the ultimate strength and strain of the IM7/8551-7 graphite/epoxy composite. The results show that the ultimate strength in general decreases as the off-axis (test) angle is increased. However, the results show that the decrease is not uniform as in the case of the quasi-static tests. The strain increases for initial increase in the off-axis angle, but then the results don’t show any consistent trend for additional angle increments. A semi-empirical equation is obtained for this material system and can be modified to account for other polymer matrix composites.