Nonlinearity in the propagation of acoustic waves: Simulation and experimental validation in a creep damaged material

Abstract

A 3-Dimensional mathematical model has been developed to understand the nonlinear characteristics of an acoustic wave propagating in a creep-damaged medium. Nonlinearities were assumed to be originated from the material due to creep damage in terms of change in precipitate size, nucleation, and growth of micro voids. For carrying out mathematical simulation in this research, a nonlinear material model has been used and the dynamic value of second order elastic constants as well as density measured for the specimen after each creep interruption were incorporated into it. For studying acoustic wave propagation inside the model of Inconel 600 alloy, a common excitation and acquiring point (pulse-echo mode) has been selected via the acoustoelastic effect. The signal received has been used for calculating the acoustic nonlinearity parameter (β) which is the indicative parameter for expressing progression of damage in the material. This parameter β can be calculated as the ratio of the amplitude of the transmitted signal to the square of the amplitude of the second harmonic and is dependent on the elastic constants of the material. The simulation results obtained through the model were validated experimentally. A drastic increase in β was observed at the transition region from secondary to tertiary stage of creep in the studied material, which is due to the increase in the volume fraction of precipitates as well as micro void initiation and are the dominating cause of material failure. So, this technique can be used for assessment of localized deformation in any material much prior to failure.