5.1 Sonic velocity measurements are useful for comparing materials with similar elastic properties, dimensions, and microstructure.

5.2 Eq 1 provides an accurate value of Young's modulus only for isotropic, non-attenuative, and non-dispersive materials of infinite dimensions. For non-isotropic graphite, Eq 1 can be modified to take into account the Poisson's ratios in all directions. As graphite is a strongly attenuative material, the value of Young's modulus obtained with Eq 1 will be dependent on specimen length. If the specimen lateral dimensions are not large compared to the wavelength of the propagated pulse, then the value of Young's modulus obtained with Eq 1 will be dependent on the specimen lateral dimensions. The accuracy of the Young's modulus calculated from Eq 1 will also depend upon the uncertainty in Poisson's ratio and its impact on the evaluation of the Poisson's factor in Eq 2. However, a value for Young's modulus can be obtained for many applications, which is often in good agreement with the value obtained by other more accurate methods, such as in Test Method C747. The technical issues and typical values of corresponding uncertainties are discussed in detail in STP 1578.5

5.3 If the grain size of the carbon or graphite is greater than or about equal to the wavelength of the sonic pulse, the method may not be providing a value of Young's modulus representative of the bulk material. Therefore, it would be recommended to test a lower frequency (longer wavelength) to demonstrate that the range of obtained velocity values are within an acceptable level of accuracy. Significant signal attenuation should be expected when the grain size of the material is greater than or about equal to the wavelength of the transmitted sonic pulse or the material is more porous than would be expected for an as-manufactured graphite.

Note 1: Due to frequency dependent attenuation in graphite, the wavelength of the sonic pulse through the test specimen is not necessarily the same as the wavelength of the transmitting transducer.

5.4 If the sample is only a few grains thick, the acceptability of the method's application should be demonstrated by initially performing measurements on a series of tests covering a range of sample lengths between the proposed test length and a test length incorporating sufficient grains to adequately represent the bulk material.