The Modified Quadric Surfaces Criterion for Composite Materials

A modification of the generalized quadric surfaces criterion for composite materials is presented in this paper. The unique features of the generalized quadric surfaces criterion are maintained. Thus, it is shown that the entire closed failure envelope of the modified quadric surfaces criterion is composed of piecewise quadric surfaces. And no biaxial tests are required for the determination of the coefficients about the stress interaction terms (i.e., σ _i σ _j ). Consider the fact that for isotropic and orthotropic materials loaded in principal material directions, there is no coupling between the in-plane and shear responses. To further simplify the use of failure criterion in engineering design, it is reasonable to assume the coefficients of in-plane and shear coupling terms to be zero in the modified quadric surfaces failure criterion. The validity of the modified quadric surfaces criterion is examined with experimental results from off-axis uniaxial strength tests of unidirectional fiber composites, and angle-ply composite laminates. Comparisons were made with other well-known anisotropic failure theories and the generalized quadric surfaces criterion as well.