Abstract
In the authors' series study on ultrasonic nondestructive evaluation method, velocity changes of transverse waves propagating in a polycrystalline aggregate under simple and pure shear states showed the same change tendency as the development of plastic deformation. However, although no velocity changes of longitudinal wave under simple shear state were concluded, the velocity changes under pure shear state were clearly observed for both experimental and numerical results, regardless of the same stress and strain state of the two cases. Therefore, in this paper the effects of microstructural behavior on transverse and longitudinal wave velocity changes under simple and pure shear states are studied via finite element polycrystal model (FEPM). From the fact that the texture developments under simple and pure shear states are roughly the same and similar to the transverse wave velocity changes under both shear states, it is concluded that transverse wave velocity depends upon texture development mainly. On the other hand, the multiplicative amount of cross slip under pure shear state is larger than that under simple shear state and this means that the point defects induced by cross slip are much easier to introduce under pure shear state than under simple shear state, therefore we conclude that the longitudinal wave velocity depends heavily upon point defects induced by cross slip. Furthermore, longitudinal wave propagation properties under pure shear state were studied at different specimen surface morphologies. Concerned with the longitudinal wave velocity changes, the experimental results were apparently categorized into two types: (i) simple decreasing change tendency at polished surface state, and (ii) chaotic change tendencies at unpolished surface state. Therefore, from the viewpoint of longitudinal wave velocity changes showing a sensitive response to the point defects, the effects of surface roughness and crystal orientation on the amount of cross slip under pure shear state are analyzed via FEPM. The results indicate that the crystal orientation causes much more influence on the amount of cross slip than what the surface roughness does. This means that crystal orientation at the surface due to machining is responsible for the phenomenon of variety of longitudinal wave velocity change tendencies at unpolished surface state. However, the distribution of crystal orientation at the surface tends to be uniformed and its peculiarity due to machining tends to be vanished when the specimen is polished, this results in the simple decreasing change tendency of longitudinal wave velocity.
