Methods for Determining the Elastic and Viscoelastic Response of Composite Materials

Two test methods for determining the longitudinal or in-plane shear stress-strain response of a unidirectional composite material are presented. These two methods are the uniaxial tension test on a ±45 deg laminate and the rail shear test on a 0/90 deg laminate. Although these tests are conducted on laminates, the laminates are special cases of geometric construction. It is shown that unidirectional shear properties can be found from the laminate test data. The stress-strain response for both methods agrees very well for glass-epoxy and graphite-epoxy composites. The nonisothermal viscoelastic shear response of a unidrectional composite is determined from tension creep tests on a ±45 deg laminate at temperatures of 81°, 125°, and 150° F. A master shear creep compliance curve is constructed by using the principle of time-temperature superposition. The other unidirectional creep compliances necessary for characterization of an orthotropic material under a state of plane stress are calculated from equations of micromechanics. Agreement is good for a comparison of the shear creep compliance calculated from micromechanics and the ±45 deg laminate. It was found that the behavior of the tensile relaxation modulus of the epoxy resin used in a typical graphite-epoxy composite is similar to the behavior of the epoxy resin used in a typical glass-epoxy. The creep behavior of any laminate can be predicted from laminated plate theory once the unidirectional material is characterized. Viscoelastic interconversion formulas are derived which relate the creep test, constant loading rate test, and dynamic test when the creep compliance can be approximated by a power law in time and linear viscoelasticity theory is valid.