Fatigue crack propagation in ARALL^® LAMINATES: Measurement of the effect of crack-tip shielding from crack bridging

Fatigue crack propagation behavior has been examined in a 2024-T3 aluminum alloy/aramid-fiber epoxy 32 laminated composite, ARALL^®-2 LAMINATE, with the objective of quantitatively evaluating the primary mechanisms of crack-tip shielding. Based on metallographic and crack-path sectioning and in situ compliance measurements, it is confirmed that the vastly superior (longitudinal) fatigue crack growth resistance of the laminate is primarily associated with extensive crack bridging from unbroken aramid fibers in the wake of the crack, with a smaller contribution from crack closure due to the wedging of fracture-surface asperities. The bridging phenomenon, which results in a local (near-tip) reduction in the maximum stress intensity (K_max) in the cycle, is shown to rely on controlled delamination, created by weak interfacial bonding between fibers and the epoxy matrix, which acts to limit fiber breakage. By progressively removing material from the crack wake, the length of the "bridging zone" behind the crack tip is found to be as large as 3-5 mm. Using a novel compliance-measurement scheme to evaluate the local reduction in K_max from bridging and the local increase in K_min from closure, an effective stress-intensity range (ΔK_eff), experienced at the crack tip, is estimated and shown to provide excellent agreement in normalizing seemingly non-unique crack propagation data presented in the literature in terms of the nominal (applied) stress intensity (ΔK).