In quest of methods for measuring 3D mechanical properties of composites

Accurate and efficient full three-dimensional characterization of mechanical properties of composite materials, including stress-strain curves and strength characteristics, is essential for understanding complex deformation and failure mechanisms of composites and optimizing material qualification efforts. This work summarizes the ongoing effort by the authors in pursuit of methods for measuring multiple mechanical properties of composite materials from a single experiment; and presents a new method enabling simultaneous assessment of the shear stress-strain curves in all three principal material planes. The method uses a small rectangular plate torsion specimen. The method is relying on digital image correlation to capture deformation including the out-of-plane strain components; and on finite element-based stress calculation. Nonlinear interlaminar shear stress-strain constitutive properties, generated in the 1-3 principal material plane, are compared with existing data for a practical carbon/epoxy tape material system. In addition, a stress-strain curve characterizing the nonlinear interlaminar shear properties in the 2-3 principal material plane at all strain levels including large shear strain, is captured in this work for the first time.