Laser ultrasonics could be an attractive solution for the nondestructive testing of structures in harsh environments Longitudinal waves generated in the ablation regime are especially well suited to internal defect detection because they provide a higher signal-to-noise ratio in comparison to ultrasonic waves generated under a thermo-elastic regime, while their propagation direction normal to the surface enables the simplified interpretation processing of received signals when the defect echoes are analyzed The internal defect detection using laser-generated longitudinal waves in the ablation regime was investigated numerically and experimentally, and a numerical model to simulate the generation and propagation of ultrasonic waves in the ablation regime was developed This model was based on the simulation of ultrasonic generation and propagation caused by the net reaction force that was directly converted from the laser intensity absorbed onto the surface This model was also extended as a model for internal defect detection A steel specimen containing artificial internal defects was fabricated and inspected by using the through-transmission (T-scan) and pulse-echo (B-scan) modes Clear amplitude reduction was observed in the transmitted waves at the defect positions in T-scan images, while B-scan images clearly showed the defect echoes arriving at different times depending on the depth location of internal defects These results demonstrate that longitudinal waves excited in the ablation regime can be effectively used for internal defect detection

Internal defect detection using laser-generated longitudinal waves in ablation regime

In this article, the interaction of a laser-generated Rayleigh wave with a subsurface crack has been simulated in detail. The finite element method (FEM) was used to model a subsurface hollow with a rectangular shape, which was used to represent the subsurface crack. By studying the contributions of all of the reflected and mode-converted signals from the subsurface crack, the arrival time of those signals and the mechanism underlying their occurrence could be fully explained. The first negative peak of the reflected waves originated from the top corner of the subsurface crack. The arrival time of this peak first decreased and then increased as the depth of the subsurface crack increased, which was due to the mode conversion when the diffracted bulk mode interacted with the free surface. However, when the depth of the crack remained the same and the height of the subsurface crack increased, the arrival time of the first negative peak of the reflected waves remained the same but the arrival time of the following reflected peaks maintained a linear relationship with the height of the crack. Furthermore, the sources and mechanism of the reflected waves were studied. It was found that the interval time of the peaks was dependent on the height of the crack, and a method for measuring the height of subsurface cracks has been proposed in this paper. The results of the simulation enable the quantitative measurement of the size of subsurface cracks and the height measuring method has the potential to measure the height of subsurface defects and structures.

Numerical simulation of the interaction of laser-generated Rayleigh waves with subsurface cracks

Many acousto-optic paratellurite-based devices use a slow shear bulk wave for interaction with light beams. Strong bulk absorption of this wave leads to a significant inhomogeneous crystal heating, a drift of material constants, and to the consequent change in light diffraction patterns. Current paper is focused on a study of thermal regimes in acousto-optic devices. A method of laser ultrasonic time-of-flight thermography of solid-state devices is proposed. The method is used to experimentally evaluate temperature maps of an acousto-optic dispersive delay line with different acoustic pump frequencies. The comparison of obtained results with thermal imaging is carried out, showing good agreement within measurement error.

Laser-ultrasonic temperature mapping of an acousto-optic dispersive delay line

Laser-induced surface acoustic waves for measurement of elastic constants based on mode conversion

In this study, the widths of subsurface lateral rectangular defects were successfully gauged using laser-generated Rayleigh waves. Finite element analysis was used to simulate the interaction between Rayleigh waves and subsurface defects. The results of the simulation show that diffracted bulk modes resulting from the interaction between Rayleigh waves and subsurface defects may interact with the free surface. Modes are converted into Rayleigh waves that propagate along the surface. The arrival time of reflected mode-converted Rayleigh waves remains constant, even when the height of subsurface defects increases. However, as the depth increases, the arrival time of reflected mode-converted Rayleigh waves changes due to mode conversion caused by diffracted bulk modes interacting with the free surface. The influence of mode conversion was quantitatively analyzed and taken into account in the width measurement method for subsurface lateral defects. Experimental results are consistent with simulation results and show that Rayleigh waves induced by a pulsed line laser are sensitive to subsurface defects. Finally, the proposed method may be potentially useful for width measurements of other types of subsurface structures.

Laser-generated Rayleigh wave for width gauging of subsurface lateral rectangular defects

Ceramic matrix composites (CMCs) are poised to revolutionize jet engine technology by enabling operation temperatures well beyond those possible with current superalloys, while reducing active cooling requirements and engine weight. Manufacturing of parts formed by silicon-carbide (SiC) fibers in a SiC matrix is now well advanced, with the first non-structural static components entering service in 2017 with the CFM Leap® engine that uses SiC/SiC turbine shrouds. In order to expand the scope of application of CMCs to rotating parts, such as turbine blades, much work is being conducted to understand and characterize the modes of failure of these materials at temperatures beyond ∼ 1100 ° C . In this context, the ability of nondestructively monitoring the formation and progression of damage in CMCs specimens during high-temperature mechanical testing is critical. However, the elevated temperature precludes the possibility of using sensors placed in direct contact with the specimen and therefore severely restricts the range of available NDE techniques. This paper provides the first experimental assessment of the feasibility of noncontact laser ultrasonic inspection of SiC/SiC flat coupons. An Nd: Yag laser is used to excite ultrasonic waves on one side of the specimen while a Michelson interferometer detects the signals emerging on the other side at the epicenter position. The lasers are mounted on synchronized linear stages to form C-scans as in conventional immersion ultrasonics while ablation damage to the surface of the specimen is prevented by operating the lasers at low power density. Despite the complex microstructure of the SiC/SiC material it is found that the measured waveforms are remarkably similar to those observed when conducting the same tests in aluminum specimens. Moreover, it is shown that it is possible to image interlaminar defects caused by impacts, and monitor crack opening under tensile load. Finally, very good signal stability is observed when temperature is increased from 25 to 1250 °C which confirms the feasibility of laser monitoring at high temperature and is consistent with the good thermal stability of ceramic materials.

J. Friedl

Papers

Noncontact Laser Ultrasonic Inspection of Ceramic Matrix Composites (CMCs)