Large Strain Behavior of Thin Unidirectional Composite Flexures

Abstract

The large strain flexure of 0.135 mm, 0.259 mm and 0.383 mm thick unidirectional carbon fiber reinforced epoxy plates was investigated. Coupons were folded in a U-shape and compressed between parallel platens while platen separation and load were measured up to failure. The test was modeling numerically using a geometrically nonlinear elastica solution method in combination with a two parameter nonlinear elastic material constitutive model based on graphite crystallite reorientation. Using a curve fitting method, the test data was used to estimate model parameters. Test results show a nonlinear material response resulting in bending stiffness softening as curvature increases. Bending failure strains were larger than expected and increased as coupon thickness decreased, ranging from 2.15% to 2.75%. Also unexpected, the failure occurred in a tensile mode on the outside coupon surface, not in a compressive mode. The results indicate thin flexures can be used at strains and curvatures larger than previously thought.