Studies of texture in nuclear graphites using laser ultrasonic line source measurements

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Laser ultrasonic line sources have been used to study the ultrasonic properties of nuclear graphites. These materials exhibit varying degrees of porosity and texture that relate directly to the conditions imposed during material processing-extruded materials display significant texture while the anisotropy of molded materials is significantly lower. Both the texture (related to grain orientation) and porosity impact the long term performance of graphite under service conditions and methods are needed to assess the microstructural states of these materials during service. Laser ultrasonic measurements can be used to assess aspects of material microstructure by measuring longitudinal and shear wavespeeds as a function of propagation direction and polarization. While porosity-related effects are independent of propagation direction for materials with spherical pores, material texture (related to preferred grain orientation) produces anisotropic wave propagation effects. In particular, propagation perpendicular to extrusion directions can produce shear wave birefringence effects that can be used to assess texture. Ultrasonic measurements in this work were made using laser ultrasonic methods that yield waveforms that can be interpreted using elastodynamic models for wave propagation in anisotropic materials. In particular, models for laser ultrasonic line sources in transversely isotropic materials have been used to simulate laser sources in nuclear graphites. The effects of optical penetration (related to material porosity) have been incorporated to produce synthetic waveforms that can be used to extract modulus information from experimental measurements. Current results hold open the opportunity for porosity and texture assessment using limited sets of ultrasonic measurements.