Two modified compressive strength criteria for unidirectional composites

The compressive strength of unidirectional composites is strongly influenced by the elastic and strength properties of the fiber and matrix phases, as well as by the local geometrical properties, such as fiber volume fraction, misalignment and waviness. In this present investigation, two microbuckling criteria are examined against a large volume of measured data of unidirectional composites taken from the literature. The first criterion is based on the compressive strength formulation based on the buckling of Timoshenko' s beam. It contains a single parameter that can be determined according to the best fit to various types of polymeric matrix composites. The second criterion is based on buckling-wave propagation analogy based on the solution of an eigenvalue problem. Both criteria provide closed-form expressions for the compressive strength of unidirectional composites. We propose modifications of the two criteria by a fitting approach, for a wide range of fiber volume fractions, applied to four types of unidirectional composite systems. Furthermore, a normalized form of the two models is presented after calibration in order to compare their prediction against experimental data for each one of the material systems. The new modified criteria are shown to match a various types of unidirectional composite systems, and can be employed as practical failure criteria in the engineering design of laminated structures.