Irene Arias

Thermoelastic generation of ultrasound by line-focused laser irradiation

A two-dimensional theoretical model for the field generated in the thermoelastic regime by line-focused laser illumination of a homogeneous, isotropic, linearly elastic half-space is presented. The model accounts for the effects of thermal diffusion and optical penetration, as well as the finite width and duration of the laser source. The model is obtained by solving the thermoelastic problem in plane strain, rather than by integrating available solutions for the point-source, leading to a lower computational effort. The well-known dipole model follows from appropriate limits. However, it is shown that, by simple elasticity arguments, the strength of the dipole can be related a-priori to the heat input and certain material properties. The strength is found to be smaller than that of the dipoles equivalent to a buried source due to the effect of the free surface. This fact has been overlooked by some previous researchers. Excellent quantitative agreement with experimental observations provides validation for the model. Some representative results are presented to illustrate the generated field and provide insight into the relevance of the different mechanisms taken into account in the model.

A model for the ultrasonic detection of surface-breaking cracks by the scanning laser-source technique

A model for the Scanning Laser Source (SLS) technique is presented. The SLS is a novel laser based inspection method for the ultrasonic detection of small surface-breaking cracks. The generated ultrasonic signal is monitored as a line-focused laser is scanned over the defect. Characteristic changes in the amplitude and the frequency content are observed. The modelling approach is based on the decomposition of the field generated by the laser in a cracked two-dimensional half-space, by virtue of linear superposition, into the incident and the scattered fields. The incident field is that generated by laser illumination of a defect-free half-space. A thermoelastic model has been used which takes account of the effect of thermal diffusion, as well as the finite width and duration of the laser source. The scattered field incorporates the interactions of the incident field with the surface-breaking crack. It has been analyzed numerically by a direct frequency domain boundary element method. A comparison with an experiment for a large defect shows that the model captures the observed phenomena.

A cohesive model of fatigue of ferroelectric materials under electro-mechanical cyclic loading

A cohesive fatigue-crack nucleation and growth model for ferroelectric materials under electro-mechanical loading is presented. The central feature of the model is a hysteretic cohesive law which couples the mechanical and electrical fields. This law can be used in conjunction with general constitutive relations of bulk behavior, possibly including domain switching, in order to predict fatigue crack growth under arbitrary loading conditions. Another appealing feature of the model is its ability to predict fatigue-crack nucleation. Despite the scarcity and uncertainty of the experimental data, comparisons with PZT fatigue-life data are encouraging.

A Theoretical Model for the Ultrasonic Detection of Surface‐Breaking Cracks with the Scanning Laser Source Technique

In work reported at last year’s QNDE meeting, a first step towards the development of a model for the Scanning Laser Source (SLS) technique was presented, which provides an analytical formulation for the transient response of an isotropic, homogeneous, linearly elastic half‐space submitted to a pulsed laser line source operating in the thermoelastic regime. The formulation takes into account optical penetration into the material and thermal diffusion from the source, and is therefore a suitable representation not only for the far field, but also for the field near the laser source, where these effects become significant. In the present paper, we report the progress made in the numerical analysis by the Boundary Element Method of the interactions of the previously obtained laser generated field with surface‐breaking cracks. Some preliminary simulations of SLS experimental observations are presented.

Near field analysis of laser-generated ultrasound: The effects of thermal diffusion and optical penetration

A first step towards the development of a model for the Scanning Laser Source technique is presented, which entails an analytical formulation for the transient response of an isotropic, homogeneous, linearly elastic half-space submitted to a pulsed laser line source operating in the thermoelastic regime. The formulation takes into account optical penetration into the material and thermal diffusion from the source, and is therefore a suitable representation for the field near the source, where these effects become significant.