Four Point Bending of Thin Unidirectional Composite Laminas

A large deformation four point bending fixture was used to test thin unidirectional composite plates under large flexural strains. The tests allowed investigation of the large strain elastic constitutive behavior of intermediate modulus (IM7 and IM10) and high modulus (M55J) carbon fibers and structural fiberglass (S2). Thermoset plastic coupons reinforced with these fibers and ranging in thicknesses from 0.1 mm to 0.5 mm were tested. A nonlinear empirical constitutive model was used to represent fiber axial tensile and compressive behavior and through a structural model, predict coupon nonlinear flexural response and estimate constitutive model parameters. Intermediate modulus carbon fibers were found to be significantly nonlinear while glass fibers where essentially linear. High modulus carbon fibers exhibited a more complex flexural response with stiffness initially slightly increasing followed by a moderate reduction in stiffness and finally, a sharp reduction in stiffness and failure.