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As a means of determining a stress intensity factor solution, the compliance properties of an ARALL-2 laminated-sheet composite were investigated. Fatigue crack growth rate (FCGR) tests were conducted on middle crack tension (MT) specimens fabricated from a layup consisting of three sheets of 2024-T3 aluminum bonded together with unidirectional aramid fibers embedded in epoxy. Excellent fatigue crack growth properties are obtained by the presence of unbroken aramid fibers in the wake of the crack tip. These unbroken fibers act as a bridging mechanism to inhibit further crack growth. To quantify the effect of maximum fatigue load on compliance, a series of FCGR tests were performed. An effective crack length was defined to be the length of a through-the-thickness crack with the same compliance level as a fiber-bridged fatigue crack. A specimen slotted with a jeweler's saw was used to simulate a through-the-thickness crack. As maximum fatigue load increased, the difference between surface measured fatigue crack and sawcut crack length increased. Effective crack lengths were determined to be at least 10 mm shorter than surface measured crack lengths for a 76-mm-wide specimen. The bridging zone was estimated to be at least 5 mm. Compliance and stress intensity factor as functions of effective crack length were determined.
composite materials, fracture, fatigue (materials), aramid aluminum laminates, compliance measurement, effective crack length, fatigue crack growth rate testing, fiber bridging, linear elastic fracture mechanics, stress intensity factor
Aerospace engineer, George C. Marshall Space Flight Center, Huntsville, AL
Professor, Tennessee Technological University, Cookeville, TN