Knee Point and Post-Failure Behavior of Some Laminated Composites Subjected to Uniaxial Tension

To investigate the phenomenon of initial failure, the post-failure behavior and the behavior under unloading and reloading, uniaxial tension tests were conducted on three kinds of laminated carbon fiber reinforced plastic coupon specimens, that is, ±30-deg, 90-deg laminates, ±45-deg, 90-deg laminates and 0-deg, 90-deg laminates. The test results are compared with the authors' theory. In the experimental stress-strain diagram, the notable incremental strain at a constant stress level was observed at the knee point, that is, the point of initial failure. This indicates that the assumption of the brittle matrix of the composite is adequate for the specimens tested, and the assumption of the ductile matrix is invalid. An inverse method is presented which makes it possible to estimate the failure stresses of constituent layers and also the effective curing temperature of an individual laminate from the experimental stress-strain diagram. The experiment showed that the unloading path is in agreement with one of two possible theoretical predictions. The reloading path coincided with the unloading path.