Abstract
Biaxial stress tests of carbon fiber reinforced plastic (CFRP) laminates were performed to investigate a failure criterion under biaxial loading. Specimens of unidirectional CFRP prepreg sheets were subjected to a tensile load in the longitudinal fiber direction and a compressive load in the transverse fiber direction. A dedicated jig was used to perform biaxial stress tests with a commonly-used single-axis testing machine. Measurements were obtained by controlling the displacement ratio, the ratio between compressive and tensile displacements. The maximum tensile and compressive stresses were then calculated using a constitutive equation. Longitudinal tensile strength markedly dropped with increasing compressive stress in the transverse direction. The failure criterion of the biaxial stress tests was expressed as an ellipse, of which the major and minor axes were the longitudinal tensile strength and transverse compressive strength, respectively. Moreover, scanning electron microscopy (SEM) observations suggest that fiber/matrix interfacial debonding occurred due to compressive stress, and such debonding could decrease the tensile strength in the longitudinal fiber direction.
