A single-sided, air-coupled ultrasonic NDT system based on the generation and reception of the A0 Lamb mode is used for detecting defects in plates. Transmitting and receiving transducers, being oriented at the appropriate coincidence angle for the generation and detection of the mode, are scanned along a line from one side to the other over the surface of the sample, passing the area with the defect. This contact-less NDT system is modelled in three dimensions with a Finite Element -based method. The air-coupled transmitter is modelled by the normal pressure that it locally applies on the surface of the plate, and the air-coupled receiver by integrating normal displacements over a finite area of appropriate position on the surface of the plate. In this way, beam spreading of both incident and scattered fields is considered. Numerical predictions have successfully been compared with experimental data for a through-thickness hole in an Aluminium plate and also for an impact damage in a composite sample.

3D finite element simulations of an air-coupled ultrasonic NDT system

This paper presents an experimental study of damage detection and quantification in riveted lap joints. Embedded lead zirconate titanate piezoelectric (PZT) ceramic wafer-type sensors are employed to perform in situ non-destructive evaluation (NDE) during fatigue cyclical loading. PZT wafers are used to monitor the wave reflection from the boundaries of the fatigue crack at the edge of bolt joints. The group velocity of the guided wave is calculated to select a proper time window in which the received signal contains the damage information. It is found that the fatigue crack lengths are correlated with three main features of the signal, i.e., correlation coefficient, amplitude change, and phase change. It was also observed that a single feature cannot be used to quantify the damage among different specimens since a considerable variability was observed in the response from different specimens. A multi-feature integration method based on a second-order multivariate regression analysis is proposed for the prediction of fatigue crack lengths using sensor measurements. The model parameters are obtained using training datasets from five specimens. The effectiveness of the proposed methodology is demonstrated using several lap joint specimens from different manufactures and under different loading conditions. (Some figures may appear in colour only in the online journal)

https://iopscience.iop.org/article/10.1088/0964-1726/22/10/105007/pdf

A multi-feature integration method for fatigue crack detection and crack length estimation in riveted lap joints using Lamb waves

Modelling of attenuation of Lamb waves using Rayleigh damping: Numerical and experimental studies

Modelling the scattering of guided waves by defects in three dimensions (3D) can be challenging. The most popular way to achieve this is the finite element (FE) method, at the cost of high computational load, which generally leads to a compromise between the accuracy of the results and the computational time, even when the geometry of the scatterer is simple. In this paper, we describe a procedure aimed at calculating the scattering matrix of an irregular defect in the most efficient way. The use of a frequency domain hybrid model which combines the flexibility of FE modelling and the efficiency of an integral representation of the acoustic fields allows very accurate results to be obtained with low computational load. The modelling procedure that we propose includes optimization of the size of the absorbing region and that of the mesh elements, minimization of number of incident directions, and the study of a spatial filter to smooth the geometry of the defect prior to meshing. Finally, the scattering matrix of a representative example of an irregular corrosion patch is calculated using the optimized procedure. Energy balance criteria are implemented to check the accuracy of the results.

R. Barauskas

Papers

3D analysis of interaction of Lamb waves with defects in loaded steel plates.

Evaluation of diffraction errors in precise pulse-echo measurements of ultrasound velocity in chambers with waveguide.