Detection of disbonds in adhesively bonded aluminum plates using laser-generated shear acoustic waves

Abstract

Adhesively bonded metals are increasingly used in many industries. Inspecting these parts remains challenging for modern non-destructive testing techniques. Laser ultrasound (LU) has shown great potential in high-resolution imaging of carbon-reinforced composites. For metals, excitation of longitudinal waves is inefficient without surface ablation. However, shear waves can be efficiently generated in the thermo-elastic regime and used to image defects in metallic structures. Here we present a compact LU system consisting of a high repetition rate diode-pumped laser to excite shear waves and noncontact detection with a highly sensitive fiber optic Sagnac interferometer to inspect adhesively bonded aluminum plates. Multiphysics finite difference simulations are performed to optimize the measurement configuration. Damage detection is performed for a structure consisting of three aluminum plates bonded with an epoxy film. Defects are simulated by a thin Teflon film. It is shown that the proposed technique can efficiently localize defects in both adhesion layers.

Non-contact detection of ultrasound with light – Review of recent progress

“…Visualization of wrinkles and local ply angle orientation [69] : (h) - photograph of wrinkled sample, (i) – LU image of the sample structure overlapped with the locally computed ply angle. Evaluation of disbonds in adhesively bounded aluminum plates using laser-generated shear acoustic waves [70] : (j) – diagram of the sample, (k) – measurement diagram, (l) – LU C-scan at the depth of plate bounding, indicating a Teflon inclusion in the structure. …”

Multi-mode laser-ultrasound imaging using Time-domain Synthetic Aperture Focusing Technique (T-SAFT)

“…They can be calculated as follows, and satisfy the excitation conditions for a line source under the thermoelastic mechanism [28] : where and are the normalised amplitudes of the longitudinal and shear waves, respectively; is the included angle between the ultrasound and the surface direction; and and are the slowness defined by the reciprocal of the wave speeds and , respectively. It was previously demonstrated that in a 3-D situation, the ultrasound directivity generated by a line source in the 2-D plane perpendicular to the length of the laser line source is similar to that of the point source [17] , [29] , [30] . To study the influence of the directivity of different wave modes, the calculated half-magnitude directivity patterns of longitudinal and shear waves for aluminum are shown in Fig.…”

“…Second, the LUT provides not only possibilities for both wide-band and narrow-band generation, but also for multi-wave-mode generation. In addition, different modes of waves can carry physical information with special directivity patterns [14] , [15] , [16] , relating to the location of surface cracks, thickness of the specimen, and internal defects [17] , [18] , [19] . The LUT also has a high spatial resolution [20] , which is important for detecting and imaging tiny defects (i.e.…”

Non-destructive laser-ultrasonic Synthetic Aperture Focusing Technique (SAFT) for 3D visualization of defects

“…As shown in Fig.1 (a), in previous LU-SAFT studies, the generation (represented by “G” and the red arrows) and the detection (represented by “D” and the green arrows) are coincident (scenario ①). In practical LU tests, to avoid the thermal expansion’s influence caused by generation laser irradiation on the detection point, the generation and detection laser spots are often separate [6] , [22] as shown by scenario ② in Fig.1 (a). In both scenarios, the actual ultrasound propagation paths are indicated by solid lines with arrows, while the equivalent ultrasound propagation paths in the exploding source approximation of the F-SAFT algorithm are indicated by dashed lines with arrows.…”