On some aspects of material behavior relating microstructure and ultrasonic higher harmonic generation

This article investigates some important aspects of material behavior responsible for acoustic nonlinearity. Even though the discussion is based on a specific constitutive model used for studying higher harmonic generation, the conclusions drawn are valid in a general context. Three aspects of material behavior, namely tension-compression asymmetry, shear-normal coupling and deformation induced anisotropy are presented. The role of each in the generation of higher harmonics along with the plausible microstructural features that contribute to such behavior is discussed. First and foremost, tension-compression asymmetry is identified to cause second (even) harmonic generation in materials. Then shear-normal coupling is identified to cause generation of secondary waves of different polarity than the primary waves. In addition, deformation induced anisotropy due to the presence of residual stress/strain and its contribution to acoustic nonlinearity is qualitatively discussed. Meso-scale modeling aspects to accurately predict the effect of microstructure on higher harmonic generation are emphasized throughout.