###### Abstract  
At present, most of the research on low velocity impact of composite laminates focuses on load location and damage assessment. To provide further early warnings about structural impact damage, impact energy can be monitored and identified. For high strength composite laminates with variable thickness, in order to further accurately evaluate the impact energy, it is necessary to adopt more suitable dynamic load signal analysis and impact energy identification methods. Therefore, a new low velocity impact monitoring and identification method for composite plates with variable thickness is proposed. All impact sample signals collected by optical fiber sensor network are decomposed by whitening Empirical Mode Decomposition (EMD); the energy feature set is established according to the impact energy eigenvalue of sample signal; according to the first order component of signal decomposition, the thickness coefficient is determined and the energy feature set is modified to evaluate the actual impact energy. Meanwhile, combined with optical fiber sensing and signal processing technology, an impact energy monitoring system has been established, and the low velocity impact monitoring and identification experiments of composite laminates with variable thickness were carried out. The proposed energy identification method successfully identified 1-3 impact energy with an average error of 4.82%, and the average error of large thickness area with low sensitivity was significantly reduced from 13.25% to 5.67%. The results show that the thickness coefficient correction method based on whitening EMD can evaluate the low velocity impact energy more accurately, and the thickness coefficient correction step significantly improves the recognition performance. Structural monitoring; low velocity impact; impact energy recognition; variable thickness composite laminates; optic-fiber Bragg grating 20  
depending on the stiffness of the target, material properties and the mass and stiffness of the impact object [2]. [PERSON] et al. proposed that the types of impact can be classified according to the damage caused by impact. The main characteristics of high velocity impact are fiber fracture, while the main characteristics of low velocity impact are matrix cracking and delamination [3]. Aviation low velocity impact mainly refers to the impact of 5 J and below, mainly refers to the impact of tools falling or small particles splashing in the process of aircraft manufacturing and service. Composite laminates may revert to their original shapes after being subjected to an impact. Due to their brittleness, the composite laminate may suffer substantial internal damage without any visible damage, especially internal cracks or delamination caused by external low velocity impact. These forms of damage, most of which are hidden in the composite structure, can result in significant degradation of the mechanical properties of the composite structure [4]. Further development of these forms of damage can seriously endanger the operation of the structure and can lead to catastrophic failure of the entire structure. Therefore, the impact damage of the composite laminate structure must be monitored as early as possible. The damage mechanism of composite materials is more complicated than that of traditional isotropic materials. In addition, it is very difficult to detect the Barely Visible Impact Damage (BVID) to the composites [5]. At present, it is through regular inspection and maintenance to ensure the safety and efficiency of the structure. However, BVID may be generated before regular maintenance due to unforeseen events; in addition, impact damage caused by external impact or internal object dropouts may be overlooked during the inspection. These events can cause structural problems that can lead to serious accidents. Therefore, health monitoring of composite structures is required to detect and locate all low velocity impacts that could result in BVID [6].