Crack Arrestment of Laminated Composites

In this paper the potential of the buffer strip concept for providing fracture control in graphite/epoxy laminated composite panels subjected to uniaxial tension static and fatigue loads is evaluated. The specific buffer strip design consists of replacing the 0-deg and 90-deg plies in a π/4 laminate with ±45-deg plies along strips parallel to the uniaxial load direction. A number of specimens were fabricated and tested. The specimens were 8 by 20-in. panels of 8-ply (0, ±45, 90)_s Thornel 300/SP286 graphite/epoxy laminates. Two 1-in. wide buffer strips were symmetrically placed 2 in. apart (3 in. center-to-center) and parallel to the longer sides of the panels. Initial flaws consisting of slits and circular holes of various sizes and orientations were machined at the midsection between the two buffer strips. Initial fracture and ultimate failure strengths of the panels were determined. In addition, several panels were fatigue cycled prior to static failure to assess the influence of fatigue loads on post-fatigue performance.

The static tests of the panels not previously fatigue tested showed that crack arrestment occurred when the initial flaw size exceeded 0.50 in. That is, the crack arrest capability was found to depend on the initial flaw size. For panels with 0.75 and 1.00-in. flaws, an initial fast crack was arrested in the buffer strips and the panels subsequently withstood approximately 20 percent more load. The crack initiation load for the panels which were previously fatigue cycled increased by approximately 10 percent as compared to the crack initiation load for an identical panel not previously fatigued. The improvement is attributed to development of zones of crazing and delamination in the crack tip region during fatigue cycling which reduce the local stiffness and the stress concentration. Fractographic examination of the failure surfaces confirmed the existence of this zone.