Stress intensity factors for anisotropic layered composites. II : Isolated double periodic cracks

The formulation developed in Part I for collinear cracks in alternate layers of an anisotropic laminate is extended to a system where each of the cracked layer contains a periodic array of parallel cracks. These cracks are also collinear such that they are periodic in two mutually perpendicular directions. Finite Fourier transform is applied at discrete points for one of the space variables reducing the problem to a singular integral equation. The unknown is expressible in terms of the crack opening displacement as in part I. Displayed graphically are the normalized stress intensity factor, effective stiffness of the laminate, and the interlayer stresses. The local stress intensity factor for the double array crack system is always less than that for a single isolated crack depending on the periodicity ratio. The interlayer stresses directly ahead of the crack are elevated, the intensity of which increases with decreasing distance between the crack tip ad interface. Increase in the thickness of the adjoining layer tends to decrease the interlayer stresses nearest to the crack tip, a result that is to be expected. Data are cited for boron-epoxy and graphite-epoxy composites.