Quantitative Estimation of Crack on or Near Surface Using Laser-Ultrasonic Surface Wave: Numerical Simulation

Abstract

Cracks are the most common defects in various kinds of materials. Laser ultrasonic technology has an important application in the field of nondestructive inspection of solid surface or near-surface cracks. Based on the thermoelastic coupling theory and finite element method, a numerical method to quantitative estimation of crack in the near surface area with laser-excited Rayleigh wave is established in this study. The observation position for receiving signal is set between the laser excited point and the crack. The interaction between laser-excited Rayleigh wave and surface crack is analyzed, and the time difference between two reflected wave signals is used to quantitatively estimate the surface or near-surface crack. The arrival time and amplitude of reflected Rayleigh (RR) wave are extracted to compare the effect of crack depth. The results show that the arrival time of RR wave is approximately linear with the top depth of crack, but independent with the bottom depth of crack. The amplitude of RR wave increases first and then decreases with the increase of the top depth of crack, and gradually increases with the increase of the bottom depth of crack, which provides a possible basis for practical applications and quantitative estimation of laser-excited Rayleigh wave to detect surface or near-surface cracks.

Appendix: Application to Prevention of Concrete Exfoliation

Our theory can be applied to prevent the exfoliation of the concrete structures. When a concrete structure becomes old, a drop of an exfoliated piece of concrete can happen and it may cause a serious accident. In fact, it did in Japan a few years ago. In order to prevent such accidents, we have to a priori know where the concrete exfoliation likely happens. The main (and almost all) cause of concrete exfoliation is the existence of cavities in the concrete structure close to its surface. Not so long ago, it was very usual to apply hammering test by human to detect such cavities at huge labor cost. In order to reduce such labor cost, West Nippon Expressway Shikoku Company Limited, a Japanese expressway maintenance company, developed an excellent NDI device called J-SYSTEM (https://www.w-e-shikoku.co.jp/wp-content/uploads/2021/07/4ecdce3ad2493561923042109a9fe2ed.pdf). J-SYSTEM consists of a thermal camera and a computer. The computer analyzes the thermal pictures to detect where critical cavities close to the surface locate, in whose algorithm, deep learning by artificial intelligence is applied. By NDI with J-SYSTEM, all critical cavities near the surface are completely detected and its running cost is much cheaper than the hammering test. Therefore, we can conclude that NDI with J-SYSTEM is very superior. Let us shortly review the outline of this NDI. The concrete structure gets warm in daytime by sunshine and its surface radiates heat in the evening when the temperature becomes low, which makes differences in the surface thermal pictures if there are cavities near the surface, which is photographed and analyzed by J-SYSTEM. By this character, NDI by J-SYSTEM is passive and it is possible for only limited period of time in a day and the length of possible period of time depends on the season and the weather. J-SYSTEM includes a subsystem (with a specimen) to judge when its NDI is possible, which is another superior point of J-SYSTEM. We, however, cannot inspect all concrete structures by this device. NDI by J-SYSTEM is impossible in the area where the sunshine is not sufficient in daytime. For its complement, some active NDI technique is necessary. We claim that our theory can be applied for this purpose. In this inspection, we do not have to detect the depth of the cavity (the parameter L). What we try to do is to give a rough sketch of the cavities within the depth (the parameter D) less than 1 cm. In this detection, we need not precisely detect the location of the cavity. Its rough sketch is sufficient. Since, in the practical prevention of concrete exfoliation, when the critical cavities near the surface are found, they get rid of the surface part near the cavity by hammering before it exfoliates, where the rough sketch of the cavity is sufficient. For our theory to be applied to prevent the exfoliation of the concrete structures, it is favorable to give a rough sketch of the cavities with D less than 1 cm near the whole one surface of the structure with as less laser ultrasonic waves as possible, for which our theory should be modified and generalized. It is our next problem from the viewpoint of practical applications.