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Laser ultrasonic assessment of the effects of oxidation and microcracking on the elastic moduli of nuclear graphites

Structure-property relationships in bulk materials can be important in establishing sensing methods to assess the state of material microstructure. This is particularly true for nuclear graphites since these materials can display anisotropy and can become oxidized under service conditions. Both anisotropy and oxidation-induced porosity can influence structural integrity and this must be confirmed during the lifetime of the material. In this work, laser ultrasonic methods were used to measure the elastic response of various graphite grades with a focus on NBG-18, NBG-25 and IG-110. Results of laser ultrasonic measurements in oxidized graphites show that the moduli decrease with increasing porosity. This variation can be interpreted successfully using effective medium models if the characteristics of graphite porosity are taken into account. Results of laser ultrasonic shear wave birefringence measurements can be used to assess elastic anisotropy. This anisotropy can be expressed in terms of the orientation distribution coefficients W400 and W200. Interpretation of results requires the use of appropriate property-averaging schemes.