A new test method has been developed to apply high through-thickness compressive stresses simultaneously with in-plane tension using a simple straight sided specimen in a biaxial testing machine. Tensile tests carried out on cross-plied IM7/8552 carbon/epoxy specimens showed a strong interaction between the stress components, with an approximately linear reduction of in-plane tensile strength with through-thickness compressive load. Finite element analysis indicated that extremely high through-thickness compressive stresses over 1400 MPa were present, well in excess of the unidirectional transverse strength of the material of 305 MPa. A failure criterion based on the difference between the in-plane tensile stress and the through-thickness compressive stress gave conservative results.
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