The Combined Effects of Hydrostatic Pressure and Strain-Rate on the Compressive Properties of a Laminated,Multi-Directional,Graphite/Epoxy Thick-Composite

Abstract

Experimental studies of the combined effects of hydrostatic pressure and strain-rate on the compressive properties of a laminated, multi-directional graphite fiber/epoxy matrix thick-composite have been made. It has been determined that the compressive stress-strain behavior of the thick-composite is dependent significantly on hydrostatic pressure and strain-rate. The hydrostatic pressure (P) applied was 1 bar, 1 kbar, 2 kbar, and 3 kbar and the average strain-rates (ε) used for the studies were 6.51 x 10⁻²%/s (quasi-static or low strain-rate) and 1.27%/s (high strain-rate). The compressive properties—namely the Young's modulus (E), the yield strength (σγ) when one occurred, the ultimate strength (σU) or the fracture strength (σf), and the fracture strain (εf)—were determined as a function of P and ε. The E increased bilinearly with P at the low and the high is with the discontinuity point located at ~2 kbar. It is particularly important to note that there was a coupling effect of P and ε on E of the thick-composite. The coupling effect was that the value of E under simultaneous application of P and the high ε was greater than the sum of the values of E under separate application of P and the high ε. The multi-directional thick-composite behaved like a brittle material, exhibiting only linear elastic stress-strain curves, under 1 bar and the low strain-rate. It, however, underwent 2% off-set yielding when P ≥ 2 kbar under the low and the high εS. The σF remained virtually unchanged at each pressure level, regardless of strain rates used but the σF decreased with ε.