Analysis of Nonlinear Stress-Strain Behavior of Fiber-Reinf orced Composite Materials

Fiber-reinforced composite materials generally exhibit nonlinear stress-strain behavior in at least one of the principal material directions. For example, boron/epoxy and graphite/epoxy have highly nonlinear shear behavior. Moreover, boron/aluminum has nonlinear behavior transverse to the fibers as well as a shear nonlinearity, and carbon/carbon has nonlinearities in all principal material directions. The Jones-Nelson strain energy based nonlinear mechanical property model is extended to treat all nonlinearities of fiber-reinfor ced composites. The basic model will converge only up to a specific strain energy value. That limitation is eased by using new extrapolations of the stress-strain curve and mechanical property - energy curve for strain energies above available stress-strain data. These extrapolations are necessary because the strain energies of biaxial loading exceed the strain energies of uniaxial loading under which the properties are defined and because the maximum strain energies under uniaxial loading are different in the various principal directions due to orthotropy. Strains predicted with the new material model correlate well with strains measured by Cole and Pipes in uniaxial off-axis loading of boron/epoxy and graphite/epoxy. The predicted strains are also close to strains predicted by Hahn and Tsai, who use a material model with a single (shear) nonlinearity. Our model can be used for the several nonlinearities of boron/aluminu m and carbon/carbon; therefore, it is more widely applicable than the Hahn and Tsai model.