## 1 Introduction
This paper focuses on the surface crack detection of the solid cylinder. Identification methods for the surface crack location and surface crack depth of a solid aluminum cylinder will be discussed in detail, using a numerical method. Generally, the types of ultrasonic waves used to detect the surface crack are Lamb waves [29; 30], Rayleigh waves [31], circumferential surface waves [26], and circumferential guided waves [32; 33]. In our work, we use circumferential surface waves to detect the surface crack of a solid cylinder. Traditional techniques for the identification of the surface crack rely on monitoring the reflection (pulse-echo) or changes in the amplitude of transmission (pitch-catch) of the incident wave caused by the presence of a surface crack [34; 35]. However, the changes in the amplitude of reflection and transmission are often too weak to be detected for small defects relative to the wavelength of the generated Rayleigh wave. The scanning laser line source (SLLS) technique was proposed as an alternative inspection method to overcome these size limitations [36]. The SLLS technique is based on monitoring the changes of the amplitude or frequency of laser ultrasonic as a laser source scans over a crack. When the laser scanning is close to a surface-breaking defect in a metal sample, the peak-to-peak amplitude of the ultrasonic Rayleigh wave increases significantly compared to the Rayleigh wave that is generated in the far field [34; 37; 38]. Results of previous experiments and numerical simulations showed that this signal enhancement effectively identified surface cracks on plates [34; 37]. In fact, surface-breaking defects often have a complicated geometry other than the \"normal slot\" used in many calibration tests [39]. The signal enhancement factor could be used to identify the position of branched defects and provide an idea of the defect geometry. For the angled surface defect, near field enhancements in the amplitude and frequency of surface waves could potentially be used to identify different angled cracks in real samples. When compared to the SLLS method, the scanning of the detection point may be a better method for locating angled surface defects [40]. Although many scholars have adopted the signal enhancement of the SLLS method to identify the surface crack of plates, very few studies have focused on the detection of the surface crack on the solid cylinder, using signal enhancement. In comparison to the surface waves propagating on plates, surface waves propagating on the solid cylinder demonstrate characteristics of phase shift and are more susceptible to interference from other types of waves. This makes the surface crack detection of a solid cylinder based on the signal enhancement method more difficult to implement. Therefore, this paper explores the signal enhancement method for laser scanning detection of the surface crack in a solid cylinder. The optimization approach of detection position in SLLS is proposed for the location of the surface crack on the solid aluminum cylinder and the SLLS peak of signal enhancement within a limited scope is adopted to identify the surface crack depth of a solid aluminum cylinder. In light of the complexity of the generation of laser ultrasonics and the interactive process of the acoustic wave with the surface crack, a numerical method is suitable for dealing with these complicated processes due to its flexibility in modeling complicated geometry and the feasibility of obtaining full field numerical solutions [41; 42]. Furthermore, numerical modeling of laser ultrasonics can allow direct studies on the interplay of the various factors, leading to the improvement of experimental design [42; 43]. In this paper, an explicit finite element model of laser ultrasonics is developed for the acquisition of laser ultrasonic field data in a cylinder and a fully coupled thermal-stress method is considered here. Following that, we explore the interaction of laser ultrasonics with the surface crack of the solid cylinder in detail.