Mechanical and sound investigations of advanced carbon materials

Graphite (with and without different tungsten layers) and CFC have been investigated in bending and tensile experiments accompanied by sound emission recording. Attempts have been made to excite surface acoustic waves (SAWs) with the aim to characterize the different materials as well as to test new diagnostic possibilities. Graphite is an elastic and brittle material whereas CFC shows clearly quasi-plastic behaviour. Sound emission from evolving failures is easily detectable and in the case of CFC originates from the collective rupture of all fibres in a bundle. Young's modulus differs remarkably when it's deduced either from the stress-strain curves or from the sound velocity. Fatigue experiments with CFC reveal a relaxation process in an early period characterized by numerous sound events, preferably produced by friction between the fibre bundles and the matrix, followed by a very calm period,finally leading to a rupture accompanied by massive sound emission again. The capability of sound measurements to diagnose such events is clearly demonstrated. On graphite samples covered by a tungsten layer purposely excited Rayleigh-waves can be used to extract information about the surface. In a first attempt the dispersion relations of the SAWs observed on samples with W-layers of different thickness have been measured and compared with model calculations assuming the known thicknesses. The promising results reveal that the determination of thickness, Young's modulus, and density of a layer from experimental dispersion relations should be possible.