Three composites materials, Glare 2 fiber metal laminates, graphite/epoxy (Gr/Ep) and carbon/carbon (C/C) have been tested mechanically under quasi-static loading in uniaxial and bending modes using uniform and notched specimens. Acoustic emission (AE) technique was utilized in tracking the damage accumulation profile during loading up to fracture in terms of AE counts rate and cumulative. In addition wavelet transforms was used to process AE signals in order to obtain both frequencies and time information on the main failure mechanism and the sequential events during fracture process. This was supported by light and electron microscopies characterizations. The mechanical and acoustical responses were examined with respect to orientation and temperature effects for the Glare 2, exposure temperature effect for the Gr/Ep and porosity degree effect for the C/C. Manifestly, the AE results demonstrate different damage build-up profiles and point to a transition in failure micro-mechanisms with respect to the influence of each parameter (temperature, orientation, and density) on the specific composite tested. For the Gr/Ep, the wavelet transforms indicate the sequence of events in the fracture process, from fiber breaks followed by debonding and ending with matrix cracking. In some cases, it has been observed that the damage accumulation profile in terms of AE resembled the dependency of the crack density versus the strain predicted by a micro-mechanical damage model.
IntroductionCarbon/carbon (C/C) composites recognized also as carbon fiber reinforced carbon are characterized by their higher specific strength and stiffness of up to 3000°C in non-oxidizing atmospheres, as compared to other ceramic matrix composites. In addition, the low coefficient of thermal expansion, high thermal conductivity and high thermal shock resistance, have made this material an excellent candidate for aerospace and defence applications for more than three decades. The most popular applications that this unique composite is being used for are: brake discs, rocket nozzles, leading edges of re-entry vehicles, furnace heating, and thermal management components in space vehicles [1].The severe environmental conditions (mechanical and thermal) which critical engineering structures such as those made of C/C composites have to face, require special attention from manufacturing processes to improve their service in performance [2] aspects. As well-known, defects and flaws may be initiated and may grow as a result of the above conditions and this can upset the mechanical balance of fiber, matrix and interface that gives composites their high performance profile. For the first, the determination of the optimal parametric process, quality control using destructive and non-destructive techniques and mutual properties becomes important. For the second, damage degree development, degradation in performance residual strength, etc., have to be assessed from the structural integrity and life prediction points of view. EVA-STAR (Elektronisches Volltextarchiv -Scientific Articles Repository)
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