Titanium alloy materials are commonly used in turbine blades of aircraft engines operating in harsh environments. During their operation, they are typically affected by high temperature, high pressure, and metal fatigue, which can easily cause the formation of numerous subtle cracks. If these cracks are not detected and maintained in a timely manner in the early stages, they may continue to propagate and cause catastrophic failure to the entire titanium alloy structure, thus resulting in irreversible losses. Therefore, cracks that may occur in titanium alloy materials during their use must be detected and located. Laser ultrasonic detection is the process of creating an ultrasonic field on the surface of a material through laser irradiation, which can simultaneously excite multiple modes of ultrasonic waves. However, in engineering applications, the extraction and quantitative characterization of internal crack features remain challenging. Therefore, the effects of internal cracks on the localization, quantitative characterization, and image reconstruction of titanium alloy are investigated.

This paper focuses on the fatigue cracks of titanium alloy blades in aviation compressors. A three-dimensional (3D) solid thermal coupled ultrasonic model is established based on the thermoelastic effect using the COMSOL Multiphysics software. The laser-line-source scanning method is used to analyze the interaction mechanism between ultrasonic waves and internal and surface cracks in titanium alloy materials. Based on the amplitude distribution of reflection-mode waves in the B-scan image, different reflection echo characteristic curves are selected in the MATLAB composite overlay image algorithm to reconstruct and image the internal and surface cracks of titanium alloy. Consequently, 3D image reconstruction and quantitative characterization of crack positioning are achieved.

The imaging results of internal cracks are primarily the top scattering images of the cracks. The reconstructed center positions of the internal cracks are at 2.47, 1.48, and 1.13 mm, with three-axis relative errors of less than 2%. The crack size calculated in the y-direction is approximately 1.04 mm, with an error of approximately 4%. The imaging results of surface cracks are primarily the reflection maps of crack boundaries. The reconstructed center positions of the surface cracks are at 2.96, 1.53, and 3.81 mm, and the three-axis relative errors are all less than 3%. The crack size calculated in the y-direction is approximately 0.95 mm, with an error of approximately 5% (Fig. 8).

Based on the experimental results, the crack and excitation point based on the arrival time of the initial and reflected waves are calculated. The calculation results are consistent with the theoretical values set for prefabricated cracks. The laser ultrasonic detection error does not exceed 5% within the effective distance (Table 3).The calculated crack depth is similar to the theoretical value, thus verifying the accuracy of the reflection-wave delay peak time in characterizing the crack depth. However, at a crack depth of 0.5 mm, owing to the small crack depth, the relative deviation is 14%. For smaller crack depths, further improvement in characterization accuracy is necessitated (Table 4). The amplitudes of reflected waves with different crack widths in the experiment are extracted and used to verify the accuracy of the relationship equation. As the crack width increases, the error gradually decreases, thus indicating that the signal intensity of the reflected wave increases. The crack width is characterized by establishing a relationship equation between the amplitude of the reflected wave and the crack width. The relative error of the crack depth obtained does not exceed 3% (Table 5).

A 3D thermal structural coupled laser ultrasonic internal crack model of titanium alloy is established using COMSOL. The propagation characteristics of ultrasonic waves and the generation process of reflected and mode-conversion waves during the interaction between ultrasonic waves and cracks are analyzed. By extracting the arrival-time characteristics of reflected waves using the laser-line-source scanning method, time-domain mode reflected wave signals at different detection positions are extracted based on a synthetic aperture imaging algorithm. The localization characterization of internal and surface cracks in the 3D model is performed through layered calculation and recombination of the 3D model. The relative-error range of the reconstructed crack positions on the x- and y-axes is within 3%. The characterization error of crack size does not exceed 4%. Laser ultrasonic experiments are conducted to investigate the location, depth, and width of surface cracks. The multivariate variational mode decomposition is used to perform modal decomposition and noise reduction on the ultrasonic signals obtained by a laser ultrasonic system. The time-domain characteristics of reflected surface waves are used to characterize the crack location and depth. When the crack depth is 1 mm, the relative error is 6.9%. A decrease in the crack depth increases the error. The mapping relationship between the amplitude of reflected surface waves and crack width is analyzed. An exponential-function prediction model is constructed and the crack width is characterized, with a characterization error within 3%.