A laser–EMAT system for ultrasonic weld inspection
Introduction
Electromagnetic acoustic transducers (EMATs) can generate and detect ultrasound on materials that are electrical conductors or are magnetic in nature [1], [2], [3], [4], [5]. However, in some of these materials EMATs do not have sufficient sensitivity to detect very low amplitude diffracted waves, making them an impractical proposition for non-destructive testing. One technique that can overcome this lack of sensitivity is to use a hybrid system, where a high energy pulsed laser is used to generate relatively large amplitude ultrasonic waves in a sample by illuminating the sample surface [6], [7], [8], [9], [10] with the laser beam. In the experiments described here the laser beam was weakly focused, giving rise to a strong impulsive ultrasonic source normal to the surface. Used in this way, the laser generates many different ultrasonic wave modes simultaneously [11] (e.g. shear, longitudinal and surface wave modes), and the EMAT can be designed such that it is sensitive to all of these modes simultaneously, or is particularly sensitive to one displacement direction associated with an ultrasonic wave. The resulting laser generated ultrasonic waves can be detected at the surface of the sample using an EMAT as shown in Fig. 1. The wavefront that the laser source generates in this geometry is highly divergent, which means that there are many possible routes (ultrasonic paths) that the ultrasound can take from the generation point to a point displaced from the generation point. Waves that have undergone a large number of internal reflections are more strongly attenuated due to the longer pathlength and losses at the reflections, but may still be much larger in amplitude than a scattered wave from a defect.
The major disadvantage with ‘hybrid’ laser–EMAT systems is that the ultrasonic waveform detected by the EMAT can be extremely complicated, as in general the EMAT is sensitive to different wavemodes and all the multiple reflections from each mode. As a result the signal from a defect could be ‘masked’ by other large amplitude ultrasonic arrivals. There can be many different defect types within welded components, such as lack of side wall fusion, porosity, cracking and slag inclusions, and it would not be expected that all defect types could be inspected using a single test procedure [12]. Although the laser–EMAT system described here was intended for weld defect detection, it may have many applications within general non-destructive testing. The basic technique may be used with other types of detection transducer.
Section snippets
Experimental details
A well-adhered oxide scale can greatly enhance the performance of an EMAT [13], [14], but in this paper experiments were performed with the EMAT operating on the more unfavourable bare steel surface. Some of the plates tested had also been exposed to moisture and had a surface layer of hydrous oxide (rust), which did not appreciably degrade the EMAT's detection sensitivity.
The experimental arrangement placed the laser and EMAT on opposite sides and equidistant from the weld, and they were
Results
In general, the ultrasonic scatterer or defect may be identified in one of two ways:
(i) an extra feature in a ‘normal’ A-scan corresponding to the detection of a scattered wave;
(ii) a disrupted or missing feature in the A-scan when compared with a ‘normal’ A-scan.
The ultrasonic arrivals that should be detected by the EMAT can be predicted and in some cases it is possible to obtain quantitative data from the scan. For example, it may be possible to attribute a defect to a surface crack if only
Discussion
The laser–EMAT system described here is capable of rapidly detecting a range of defect types in welded steel plates with no surface preparation other than removing oxide scale in the region along the weld (which is necessary for any ultrasonic inspection). It has been shown that a time-of-flight diffraction measurement can be made to locate defects and that other features in the data can reveal the presence of a defect within the weld. EMATs are inherently less sensitive than piezoelectric
Acknowledgments
The authors would like to thank MitsuiBabcock Technology Centre for the loan of sample II, and SLV Munchen for samples I and III. This work has been funded by a Brite-Euram Framework IV Project (Rapinspect, Project No. BE95/1389).
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