6. NDE for Advanced Materials
Failure mechanisms in fiber composites are a function of many parameters such as constituent properties, lamination geometry, state of stress, etc. They can be viewed at the microscopic level of fiber-matrix interaction, at the single lamina level and at the macroscopic laminate level. The basic failure mechanisms at the microscopic level include tensile, compressive or shear fracture of the matrix, bond failure of the fiber-matrix interface and tensile or compressive (buckling) failure of the fibers. In angle-ply laminates the first failure in the form of cracks parallel to the fibers occurs when the strain limit of the weakest ply (usually at 90-deq. to the load) is exceeded. This first ply failure is usually not catastrophic. The subsequent behavior of the laminate depends on its notch sensitivity and interlaminar shear strength. Failure patterns are highly influenced by free edges where a three-dimensional state of stress exists with high localized interlaminar shear and normal stresses. A variety of failure modes, not always predictable by theory, have been observed around notches in boron/epoxy, glass/epoxy and graphite/epoxy composites using experimental strain analysis techniques. In uniaxially loaded quasi-isotropic laminates with holes, regions of high strain concentration with nonlinear response develop at characteristic locations around 22.5-deg. off the horizontal axis. Failure starts at these off-axis points in the form of local delamination and fiber breakage and propagates across the width of the plate. The effects of lamination geometry, ply stacking sequence, biaxial stress and hole diameter on failure are discussed. In the case of cracks, failure at the top of the crack usually takes the form of a damage zone consisting of ply subcracking along fiber directions, local delamination and fiber breakage. Final failure occurs when this damage zone reaches some critical size. One noteworthy result was that the strength of quasi-isotropic graphite/epoxy laminates is independent of notch geometry and that there is a critical notch size b!low which the material becomes notch insensitive.
Daniel, I. M., "Failure Mechanisms in Fiber-Reinforced Composites" (1978). Proceedings of the ARPA/AFML Review of Progress in Quantitative NDE, September 1976–June 1977. 34.