Nonlinear effects of micro-cracks on long-wavelength symmetric Lamb waves

• Published in: Ultrasonics Vol. 90; pp. 98 - 108
• Main Authors: Rjelka, Marek, Köhler, Bernd, Mayer, Andreas
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 01.11.2018
• Subjects: Effective material properties; FEM; Harmonic generation; Hertzian contact; Lamb waves; Micro-cracks; Non-destructive evaluation; Nonlinear ultrasound; Penny-shaped cracks; Perturbation theory; Perturbation theory; Lamb waves; Hertzian contact; Non-destructive evaluation; Penny-shaped cracks; Micro-cracks; Nonlinear ultrasound; Harmonic generation; Effective material properties; FEM
• ISSN: 0041-624X; 1874-9968
• DOI: 10.1016/j.ultras.2018.06.001

•Flat micro-cracks and micro-cracks with interior structure simulated.•Simple material models for cracked solids with effective parameters from simulations.•Non-analytic behavior of stress-strain relations depending on micro-crack type.•Non-analyticity yields unusual nonlinear properties of S0 Lamb waves.•Quantitative predictions and simple expressions for growth rates of harmonics. For an elastic medium containing a homogeneous distribution of micro-cracks, an effective one-dimensional stress-strain relation has been determined with finite element simulations. In addition to flat micro-cracks, voids were considered that contain a Hertzian contact, which represents an example for micro-cracks with internal structure. The orientation of both types of micro-cracks was fully aligned or, for flat micro-cracks, totally random. For micro-cracks with Hertzian contacts, the case of random orientation was treated in an approximate way. The two types of defects were found to give rise to different degrees of non-analytic behavior of the effective stress-strain relation, which governs the nonlinear propagation of symmetric (S0) Lamb waves in the long-wavelength limit. The presence of flat micro-cracks causes even harmonics to grow linearly with propagation distance with amplitudes proportional to the amplitude of the fundamental wave, and gives rise to a static strain. The presence of the second type of defects leads to a linear growth of all harmonics with amplitudes proportional to the power 3/2 of the fundamental amplitude, and to a strain-dependent velocity shift. Simple expressions are given for the growth rates of higher harmonics of S0 Lamb waves in terms of the parameters occurring in the effective stress-strain relation. They have partly been determined quantitatively with the help of the FEM results for different micro-crack concentrations.