# An Omnidirectional Near-Field Comprehensive Damage Detection Method for Composite Structures
## 1 Introduction  
Compared with traditional metal materials, composite materials have higher specific strength, specific stiffness, and strong designability, which is why they are widely used in the aerospace industry. However, the process of forming composite material-based components is extremely complex and there are many factors affecting the performance, namely, small differences in process parameters that can cause many defects. The composite structure serving process is vulnerable to many factors that may cause internal damage, such as hail impact, birds, and lightning effects. Sometimes damage cannot be detected in the early stages of production or in a timely manner and this can cause damage accumulation, which can further result in a significant decrease in structural strength and stability, and the safety of the aircraft structure could be seriously affected and its service life also be significantly shortened [1; 2; 3]. Therefore, it is of great significance to monitor structural health and identify damage in an aviation composite structure in a timely manner, and this requires the application of a structural health monitoring method. Ultrasonic guided waves have the advantage of long propagation distance, energy concentration, and convenient excitation/collection in plate and shell structures. With the extensive application of composite structures in the aerospace industry, higher attention has been paid to the application of ultrasonic guided wave-based health monitoring technology in the wing skin, connection structure, fuel tank, and similar structures. The ultrasonic phased-array damage imaging method is an activestructural health monitoring method based on a Lamb wave. By controlling the phase delay of a damage scattering signal received by a piezoelectric sensor array, the method can realize the directional scanning of a structure. Moreover, a damage scattering signal can be enhanced so that the signal-to-noise ratio of the damage signal can be improved, which is applicable to the identification of small damages in a slab structure, such as cracks and delamination. [PERSON] et al. first introduced the phased-array into piezoelectric elements to monitor plate structure [4]. [PERSON] [5], [PERSON] [6], [PERSON] [7], and [PERSON] [8] studied the basic principles of Lamb waves and ultrasonic phased-array technology and verified by experiment the practical application of this technology in aviation structural health monitoring and damage detection. A one-dimensional linear ultrasonic phased-array transducer can only monitor well the structural area between \(0^{\circ}\) and \(180^{\circ}\), while damage that is closer to \(0^{\circ}\) or \(180^{\circ}\) will be difficult to identify due to the existence of a blind spot monitoring angle. The mentioned limitation of linear arrays can be overcome by using two-dimensional (2D) arrays. [PERSON] and [PERSON] et al. proposed a two-dimensional phased-array, which can solve a larger error problem when damage is close to the array, and the omnidirectional scanning of a slab structure can be realized [9]. [PERSON] et al. proposed an improved form of a mi-zi array and combined multiple linear phased-array imaging results to realize crack monitoring of aluminum plates [6]. [PERSON] and [PERSON] studied the phased-array algorithm based on a spiral array and realized crack monitoring in aluminum plates and composites [10; 11]. [PERSON] and [PERSON] studied structure health monitoring based on a two-dimensional linear phased-array ultrasound and realized an effective identification of multiple damages in an aluminum structure used in aviation [12]. [PERSON] et al. applied phase array technology in Air Traffic Control aerospace structures to develop a set of active and passive compatible structural health monitoring systems, which were then used to determine damages in large carrier rocket composite structures [13].