Atsushi Kanda

Lamb wave generation using nanosecond laser ablation to detect damage

This paper proposes a non-contact damage detection method based on Lamb waves generated by laser ablation (LA). Previously, Lamb waves generated by contact-type sensors such as acoustic emission or piezoelectric zirconate titanate devices have been studied to detect damage. Lamb wave generation systems with embedded contact-type excitation devices to objective structures to be inspected may quickly realize large-area damage detection on a huge object such as an aircraft. However, replacing contact-type devices with non-contact devices in Lamb wave generation systems, the systems will have sufficient potential to excite under the specific conditions such as submerged target structures in liquid and high-temperature substances. The LA-generated Lamb waves that have amplitudes several hundred times larger than those generated by conventional laser-thermoelastically generated Lamb waves are of advantage from the viewpoint of the signal-to-noise ratio in the measurements. When the laser fluence reaches 1012–1014 W/m2, which is greater than that for laser-thermoelastic regime, a LA regime is induced. The amplitudes of the LA-generated Lamb waves might be higher than those of the laser-thermoelastically generated Lamb waves; this is within the scope of the assumption. Since the LA process entails a number of nonlinear processes such as melting, vaporization, and sublimation, it is important to confirm that LA could generate a Lamb wave and its mode. In this paper, Lamb waves that contain broadband frequency elements of more than several hundred kHz are generated by non-contact impulse excitation using LA, which is common in vibration tests in the high-frequency range, laser peening, propulsion of micro-aircraft, bolt loosening diagnosis, etc. The present method is evaluated by comparing the measured and calculated propagation phase and group velocities of the Lamb waves. Furthermore, the feasibility of our approach is demonstrated by non-contact damage detection against an aluminum alloy 2024 plate with a crack.

Wavelet analysis of unsteady shock-wave motion on two-dimensional airfoil with vortex generators

We visualized the shock wave-boundary layer interaction on a two-dimensional transonic airfoil with and without vortex generators (VGs) by using a fast-framing focusing schlieren imaging. Image processing extracted a time-space trajectory of the shock wave motion from a time-series of the schlieren images. The shock trajectories were analyzed by using the Morlet wavelet. The shock motions in the cases without VGs were quite periodic. The Fourier analysis well extracted the characteristics of the shock motion such as frequency etc. However, the shock motions in the cases with VGs were not periodic. Therefore, the Fourier analysis is not applicable for this case. The wavelet analysis with the statistical significance test gave quantitative measure of change in the shock oscillation around the buffet frequency due to installation of VGs, such as amplitude and intermittency. The wavelet spectrograms revealed that the installation of VGs did not prevent from generating the buffet but just reduced the amplitude of the buffet oscillation.

Analysis of the Effect of a Wrinkle on the Elastic Wave Propagation in Membrane Structures

As a new technique for the detection of the wrinkle without cameras, a method utilizing the propagation of elastic waves is suggested. A kind of elastic waves is highly affected by the existence of curvature and has a different propagation status between plane and wrinkled areas. The relationship between the frequency of elastic waves, the curvature of wrinkles and the transmittance of the waves is discussed. It is derived that the waves have high transmittance if the frequency is higher than a value defined by phase velocity and the curvature. The vibration in the membrane was measured by the Multi-axis Vibration

Detection of wrinkles in membrane structures by elastic wave propagation

Wrinkles are one of the critical factors that degrade the performance of space membrane structures. This paper proposes the method to detect wrinkles by observing propagation of the elastic waves at the boundary between the wrinkled area and the plane area. A wrinkle is modeled as a set of parts of an annulus, and the effects of properties of the membrane on the reflectance of elastic waves are investigated. Finally, the proposed identification method is experimentally demonstrated.