Abstract: Based on the research of Hahn-Tsai and Jones -Nelson nonlinear theories, an engineering method for calculation of the nonlinear stress-strain response of fiber-reinforced unidirectional laminas and laminates, under multiaxial loading is established, On the use of Jones-Nelson equation, the author presents a new approach to determine constants of the equation, which is more reasonable and effective than Jones-Nelsons, Moreover, stress superposition theorem was successfully introduced to the equation extensions which ensure continuity and being applicable for whole stress-space to Jones-Nelson equation. The main flow charts and partial results of computer programs were also given in this paper, and it has practical significance for engineers to apply the nonlinear constitutive equations in structure design.

Key words: Composite materials, Nonlinear behavior, Stress-Strain relationship