# Fatigue Damage Evaluation of Aviation Aluminum Alloy Based on Strain Monitoring
## 1 Introduction
[PERSON] [21] investigated the influence of grain orientation and hardened particles on the fatigue performance of friction stir welded (FSW) joints in aluminum alloys. In these studies [18; 19; 20; 23; 24; 25], the prediction of metal fatigue life primarily involved exploring new fatigue indicating factors (FIPS). FIPS is a fatigue life prediction method based on crystal plasticity finite elements, which attributes the plastic deformation of crystalline materials to the slip of dislocations on the slip surface. Unlike macroscopic finite element analysis, the FIP analysis method closely reflects the actual situation and can reveal the fatigue failure mechanism from a microscopic perspective. However, there is limited research on the online monitoring of fatigue damage, with most studies being theoretical. In the field of acoustic NDT, [PERSON] [27] monitored fatigue crack extension by diffraction of elastic waves at the crack tip and compared it with the conventional DC potential drop method. [PERSON] [28] analyzed the acoustic emission signals during tension and fatigue of 7075-T6 aluminum alloy with various coating combinations to assess the damage and its mechanisms during fatigue testing. [PERSON] [29] introduced a novel patch antenna sensor that monitors crack extension and opening by observing the resonant frequency shift of the antenna, enhancing the sensitivity of crack growth monitoring with sub-millimeter resolution. [PERSON] [30] developed a model for damage detection, localization, and quantification based on a sensing chip. Strain data from the peaks in the slope are used for crack identification, and localization algorithms are developed based on different orders of magnitude of strain. [PERSON] [31] introduced a large-area strain sensing technique using a soft elastomer capacitor (SEC) to monitor plastic deformation and large crack openings in low circumferential fatigue cracks. [PERSON] [32] utilized strain gauges to create an automated Structural Health Monitoring System (SHMS) for monitoring specific segments of a contemporary military aircraft's structure. In a study evaluating fatigue damage in relation to strain, [PERSON] et al. [33] characterized the fatigue damage evolution of CFPR using DIC and infrared thermography, among other methods. The experimental results indicated that fatigue damage parameters characterized by local transverse strains could predict changes in fatigue life for unnotched samples, with more accurate predictions for tensile and compressive fatigue. In a study conducted by [PERSON] [34], the crack extension process of aluminum alloy materials was investigated. The study established a relationship between local strain and crack length, and characterized fatigue crack extension based on local strain. Additionally, an inverse data algorithm was used for pattern recognition of crack length, and the particle filtering algorithm was employed for fatigue life prediction. The crack-monitoring algorithm proposed in this study is capable of detecting crack extensions at the millimeter level and exhibits a robust crack extension index. Previous research on fatigue damage has primarily focused on macroscopic cracks, with fewer studies on online monitoring of early fatigue damage. Therefore, this paper aims to investigate the relationship between fatigue damage and strain in the early stage of metal. This study utilizes crystal plastic finite element (CPFEM) to simulate the low-cycle fatigue behavior of 7075-T6 aluminum alloy. The predicted fatigue life is analyzed using two different fatigue indicator factors (FIP), and the surface strain changes of the specimen are compared with the results from monitored fatigue experiments. Furthermore, fatigue surface damage is analyzed using EBSD. The objective of this study is to establish a strain-based model for early monitoring of metal fatigue damage, in order to provide early warning. The accuracy of the model is experimentally verified.