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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.

C/C composites with different porosities, produced by chemical vapor infiltration have been mechanically tested under quasi-static loading in bending modes using uniform and notched specimens. The acoustic emission (AE) method was used to monitor the damage accumulation profile during loading up to fracture, supported by optical and scanning electron microscope characterization. Three stages in the damage buildup up to fracture were observed: Stage I, with no AE activity, Stage II, gradual growth in AE counts up to an abrupt jump and Stage III, sharp increases in AE counts. Moreover, the similarity in the profile between the cumulative AE counts vs. strain data and the predicted crack density vs. strain by the micro mechanical model suggested for interlaminar cracking, indicates the importance of AE in monitoring the damage evolution in composites in terms of AE counts. Fast Fourier transform analysis of the AE waves revealed three characteristic frequencies in Stage III, which is a sign of three main micro-mechanisms of failure which control the failure progress: fiber fracture, debonding and matrix cracking seem to be the active mechanisms.

Fatigue transient effects have been investigated in AISI 304 metastable austenitic stainless steel. The present experimental study is centered on a combined mechanical/environmental interactive problem including single overload and simultaneous hydrogen charging at 296K. Generally, fracture mechanics methods were applied and analyzed indicating the phenomenological resemblance of the delayed retardation in the environmental affected material as compared to unhydrogenated specimens. Finally, a proposed formulation is discussed in which the mechanical/ environmental interactive effects are expressed by modified values of the nominal overload intensification or level factors.