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Sensing & Measurement

Laser vibrometer measures surface acoustic waves for nondestructive testing

A noninvasive ultrasonic optical technique provides a wealth of data that aids in locating and characterizing damage to structures.
29 November 2006, SPIE Newsroom. DOI: 10.1117/2.1200611.0377

In assessing the safety of a structure, a nondestructive testing (NDT) technique should be able to detect the position of any defects and calculate their size. Most NDT techniques are thus necessarily complex, require qualified staff, and are unsuited to automation.1 In this article, we describe an experimental NDT methodology for crack detection using surface acoustic waves (SAWs) and optical laser Doppler vibrometer measurements.2 We evaluate whether ultrasonic laser measurements appropriately detect damage and imperfections in structural components.

SAWs are elastic waves that are confined to the surface of a material. They travel along, penetrating about one wavelength into the surface (where the depth of penetration is defined by the depth at which 95% of the wave energy is transmitted).3,4 We chose an ultrasonic optical technique instead of other currently-available methods (e.g., ultrasonic scanning, magnetic particle inspection, x-ray imaging, and imaging techniques in general5) for two reasons. First, using the laser as a receiving ultrasound transducer enables remote access to the structure being tested. Second, laser scanning a region of interest makes it possible to obtain many measurements regarding the incident, transmitted, and reflected waves. These measurements, in turn, elucidate the behavior of the SAWs and of the crack itself.

In the presence of surface discontinuities such as surface roughness and cracks, part of an incident wave is reflected. The rest is transmitted. Hence, gauging SAWs in the vicinity of a discontinuity allows one to infer information about the state of the surface. The material we tested was the slat track of an Airbus A320. This part of the plane is the movable support structure connecting the wing with the leading-edge slats (see Figure 1). A hydraulic test rig was used to induce in the structure the kind of fatigue damage that aircraft components are prone to develop over their lifetime.

Figure 1. Shown is an Airbus A320 and the slat track used for testing.

During the tests, a SAW transducer, connected to a pulse generator, was positioned on a wedge, ∼1cm from the crack. The laser scanned the surface of the slat track starting from the transducer position. The output signal was measured using a Hewlett-Packard digital oscilloscope with a 200MHz sampling frequency and 8bit resolution: see Figure 2(a) and (b).

Figure 2. (a) A transducer generates surface acoustic waves (SAWs) in the slat track. The laser detects waves in the material. (b) The scheme illustrates the experimental setup.

Measurements were taken over a region of 2cm (1cm before the advancing crack and 1cm after it) for the first test and 6mm (3mm before and 3mm after) for the second test. A total of 100 scan points were sampled for each test: see Figure 3(a) and (b). We acquired 2000 time samples at each scan point. These figures clearly show the incident, reflected, and transmitted ultrasonic waves and the position of the crack. It is also interesting to observe the interference phenomena, owing to overlap of the incident and reflected waves, at the crack front.

Figure 3. (a) Measurements taken over a region of 2cm show SAWs partially reflected from a crack. (b) A closer look taken over a region of 6mm provides more detail.

The crack was well detected by SAWs. In many cases, however, detection alone is not sufficient to ensure complete monitoring of the material being tested. Indeed, estimating crack depth using laser Doppler vibrometer measurements of SAWs is an important complement to NDT techniques. To understand what happens in materials with cracks of varying depths, we repeated the tests using steel beams—which have structural properties similar to those of the slat track—with slots of known depth (i.e., 0.2, 0.4, 0.6, and 0.8mm).

The spectrum of the transmitted wave clearly showed that the crack provokes a low-pass filter effect and that the cutoff frequency is a function of its depth (see Figure 4). But for crack depths greater then 0.4mm, the wave transmitted by the incident signal is very weak. Better results might be obtained by studying the reflected SAW and observing the corresponding high-pass filter effect as a function of crack depth.

Figure 4. The crack acts as a low-pass filter. The cutoff frequency is a function of its depth.

In summary, our technique gave good results in terms of crack detection. In every case, the location of the crack was very clear. Moreover, laser scanning makes it possible to gather a large number of measurements. Applying all this information to estimating crack depth is a principal future goal of our work.

Roberto Longo
Departement of Mechanical Engineering, Vrije Universiteit Brussel
Brussels, Belgium

Roberto Longo received his master's degree in electronic engineering at the Universitá Politecnica delle Marche, Ancona, Italy, in 2005. He is now working toward his PhD in the Department of Mechanical Engineering at Vrije Universiteit Brussel. His major fields of interest are ultrasound and laser techniques for nondestructive testing, identification and modeling techniques, and biomedical damage detection and analysis.

Steve Vanlanduit, Patrick Guillaume  
Vrije Uiversisiteit Brussel
Brussels, Belgium

1. Federal Aviation Administration,
Aircraft inspection, repair, and alterations,
Aviation Supplies and Academics, 2001.
4. I. A. Viktorov,
Rayleigh and Lamb Waves: Physical Theory and Applications,
Plenum Press, New York, 1967.
5. L. Cartz,
Nondestructive testing,
ASM International, 1995.