Excitation-dependent nonlinear behavior of distributed microcracks

Abstract

It is well known that microcracks generate strong higher harmonics in propagating monochromatic waves. There is a large amount of literature on modeling this phenomenon, but most of these existing papers only describe one specific mechanism. For example, Zhao et al. [1] assumes that the crack faces are either open under tension or closed under compression, and in the latter case they may slide against each other. On the other hand, the Nazarov and Sutin [2] model assumes microcracks as an elastic contact of two rough surfaces, which are never completely separated by an external load. All these mechanisms depend on the level of excitation. In this research, a micromechanical model for the acoustic nonlinearity generation of microcracks is developed by combining the bilinear stiffness model and the rough surface contact model to describe the excitation-dependent nonlinear behavior of distributed microcracks. It is shown that the first and second harmonic amplitudes have the relationship: A2~A1n, with n dependent on the amplitude of excitation, and 2 ≥ n ≥ 1 for non-adhesive crack surfaces. Nanostructured ferritic alloys (NFAs) [3] are considered as an example. These materials exhibit outstanding high-temperature properties, irradiation tolerance and thermal stability, making them a leading candidate for advanced nuclear fission and fusion applications. One characteristic property of mechanically processed NFAs is their layer-like structure, with a large number of microcracks aligned in a specific direction. Nonlinear ultrasound measurements (acoustic nonlinearity, β) with longitudinal waves are used to characterize this material. The results show that these measurement techniques are sensitive to the orientation of the cracks. The model developed in this research is then used to interpret these experimental measurements and used to characterize the microcracks in a NFA specimen.