Stress imaging by guided wave tomography based on analytical acoustoelastic model.

A nondestructive method ( M) for stress characterization in plate-like structures is proposed. In this method, the acoustoelastic effects (AEEs) on Lamb and shear horizontal guided waves are used to reconstruct a nonuniform multiaxial stress field. The development of M starts by deriving an analytical acoustoelastic model (An-AEM) to predict AEEs induced by a triaxial stress tensor as a function of the stress components, its orientation, the wave propagation direction, and three acoustoelastic coefficients (AECs). The AECs are independent of stress but specific to each mode. The An-AEM allows one to retrieve the three components of the stress tensor and its orientation from AEEs, assuming the stress to be uniform in the plane of the plate and through its thickness. To deal with stress that is nonuniform in the plane, the An-AEM is combined with time-of-flight straight ray tomography to enable stress field reconstruction. Numerical simulation is used to illustrate how such reconstruction can be performed. It is shown that in some cases, stress components can be reconstructed with arbitrary accuracy, and in other cases, the tensorial nature of stress renders the accuracy of its reconstruction dependent on spatial variations of the stress orientation.

Effects of multiaxial pre-stress on Lamb and shear horizontal guided waves.

A theoretical model is derived to extend existing work on the theory of acoustoelasticity in isotropic materials subjected to uniaxial or hydrostatic loadings, up to the case of arbitrary triaxial loading. The model is applied to study guided wave propagation in a plate. The semi-analytical finite element method is adapted to deal with the present theory. Effects of triaxial loading on velocities of Lamb and shear horizontal (SH) modes are studied. They are non-linearly dependent on stress, and this nonlinearity is both frequency-dependent and anisotropic. Velocity changes induced by the effect of stress on the plate thickness are shown to be non-negligible. When a stress is applied, both Lamb and SH modes lose their simple polarization characteristics when they propagate in directions different from the principal directions of stress. The assumption that effects induced by a multiaxial stress equal the sum of effects induced by each of its components independently is tested. Its validity is shown to depend on frequency and propagation direction. Finally, the model is validated by comparing its predictions to theoretical and experimental results of the literature. Its predictions agree very well with measurements and are significantly more accurate than those of existing theories.

Mechanisms of elastic wave generation by EMAT in ferromagnetic media.

The present paper deals with the problem of elastic wave generation mechanisms (WGMs) by an electromagnetic-acoustic transducer (EMAT) in ferromagnetic materials. The paper seeks to prove that taking into account all the WGMs must be a general rule to quantitatively predict the elastic waves generated by an EMAT in such materials. Existing models of the various physical phenomena involved, namely magnetic and magnetostrictive, electromagnetic, and ultrasonic, are combined to solve the multiphysics wave generation problem. The resulting model shows that WGMs (i.e., electromagnetic force, magnetostrictive strain, and magnetic traction) strongly depend on material properties and EMAT design and excitation. To illustrate this, four magnetic materials (nickel, AISI410, Z20C13, and low carbon steel) with similar elastic but contrasting electromagnetic properties are studied. A given EMAT of fixed excitation and geometry yields WGMs with highly different amplitudes in these materials, with a WGM dominant in one material being negligible in another. Experimental results make it possible to validate the accuracy of certain predictions of the model developed. In summary, the present work shows that considering all WGMs is the general rule when working with ferromagnetic materials. Furthermore, it offers a generic model that can be integrated into various numerical tools to help optimize EMAT design and give reliable data interpretation.