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The ASTM Standard C769-09 covers a procedure for measuring the sonic velocity in manufactured carbon and graphite which can then be used to obtain Young’s modulus. There are four main assumptions in this standard that require further investigation (i) homogeneity and isotropy of the tested specimens, (ii) transducer frequency that is sufficiently high to provide the required level of timing accuracy yet not affected by frequency dependent attenuation in graphite, (iii) specimen lateral dimensions much larger than the wavelength of the transmitted pulse so that dispersion and hence, distortion of the propagated pulse is minimised and, (iv). specimen lateral dimensions much larger than the wavelength of the transmitted pulse so that Young’s modulus can be calculated from the sonic velocity. This paper presents the experimental and theoretical work undertaken to provide the technical basis or compensate for the above assumptions. Specifically, the experimental work investigated sample size (length and diameter), coupling, equipment, and signal analysis effects on measured velocity; the theoretical work investigated the relation between sonic velocity and Young’s modulus at intermediate sample diameters. The experimental results on as-manufactured gilsocarbon graphite samples show that the effect of the lateral dimension is within the stated accuracy and reproducibility limit of the technique. Regarding the sample length, the experimental results provide conflicting evidence and this suggests that the parameters of the experimental setup, e.g., transducer type and frequency and type of excitation pulse could affect the results. This finding is also confirmed by the theoretical investigation, which shows that, in certain cases, the distortion of the propagated pulse is severe enough that the apparent wave speed depends considerably on sample length and on the area of the sample surface in contact with the transducer.
elastic moduli, ultrasonic measurements, graphite, small sample size
National Nuclear Laboratory, Reactor Operations Support, Central Laboratory, Sellafield,