Determination of rigidities of fiber-reinforced plastic laminates using holographic interferometry

Abstract

Introduction F the conventional fiber-reinforced plastic (FRP) laminates, the bending and torsional rigidities can be calculated either according to the rule of mixture using the elastic moduli of constitutive fiber and matrix and the volume fraction, or according to the classic lamination theory (CLT) using the elastic moduli of the prepreg. However, some of the advanced composite laminates, such as carbon-carbon composites, are manufactured through a repeated process of carbonization and graphitization at high temperature, which is likely to result in a change of material properties as well as the volume fraction of the fiber and matrix. Thus, the calculation based on the virgin-material properties fails. In such a situation, the bending and the torsional rigidities must be directly evaluated from the experiment. Laser holographic interferometry is useful for visualizing and recording the out-of-plane deflection of the whole plate. In application to a dynamic case, Aprahamian and Evensen identified the mode shapes of a cantilever beam, comparing them with Timoshenko beam theory. In static cases, Yamaguchi and Saito, and Jones and Bijl applied laser holographic interferometry to Cornu's method for the measurement of Poisson's ratio of isotropic plates. One of the authors developed Cornu's method to determine the ratio of the bending rigidities of orthotropic laminates. Marchant and Snell' proposed an experimental method for determining the flexural stiffness of aluminum and carbon composite plates by using holographic interferometry. Although the validity of their method has been confirmed for specially orthotropic laminates, three rhombic test specimens must be cut from a hexagonal laminate, and the curvatures must be carefully examined. In this study, we propose yet another measurement technique for the rigidities of the rectangular orthotropic laminates. Here, by the bending rigidities we mean Z>n, D12, and £>22, and by the torsional rigidity, we mean Z)66 in the following constitutive relation: