A multilinear parametric uniaxial stress strain approach has been used to obtain the closed form nonlinear load deflection response based on strain compatibility in bending for epoxy resin materials. A constitutive model for tension and compression of resin polymeric material is proposed. The model consists of a bilinear ascending curve followed by strain softening and constant plastic flow in tension and compression. The model is described by two main parameters; modulus of elasticity in tension and strain at the proportional elastic limit point in tension, in addition to twelve non-dimensional tension and compression parameters. Moment curvature equations, deformation localization, and static equilibrium conditions were used to simulate the flexural load deflection response of a beam under three-point bending and four-point bending conditions. Results show a considerable underestimation in flexural load carrying capacity if the uniaxial tension and compression stress strain curves are directly used.