Large Strain Four-Point Bending of Thin Unidirectional Composites

A large deformation four-point bending fixture was used to test thin unidirectional composite laminas primarily used in strain energy deployable space structures. The tests allowed investigation of the large strain elastic constitutive behavior of two aerospace-grade intermediate modulus carbon fibers, a high modulus carbon fiber, and a structural glass fiber. Thermoset plastic coupons reinforced with these fibers and ranging in thickness from 0.1 to 0.5mm were tested. A nonlinear empirical constitutive model was used to represent fiber axial tensile and compressive behavior and through a structural model, predict coupon flexural response and estimate constitutive model parameters. Intermediate modulus carbon fibers were found to be significantly nonlinear with modulus linearly increasing with strain. At failure, the fiber tangent modulus in tension was 2 to 3.1 times higher than in compression. The modulus of glass fibers was essentially constant. High modulus carbon fibers exhibited a more complex response with flexural stiffness initially slightly increasing followed by a moderate reduction in stiffness and finally, a sharp reduction in stiffness and failure.