# Fatigue Damage Evaluation of Aviation Aluminum Alloy Based on Strain Monitoring
## 1 Introduction  
Extensive research has been conducted on fatigue in the past century, driven by its significant threat in terms of structural damage. Statistical evidence suggests that fatigue damage is responsible for at least half of all mechanical failures [1; 2]. Aluminum alloy, known for its strength, high specific stiffness, and ease of formability, is extensively used in critical structures of aviation and aerospace products. Aluminum alloys used in the aerospace industry are primarily classified into two categories: 2 XXX and 7 XXX. The 7 XX aluminum alloys are particularly important in engineering and are applied in fighter aircraft, launch vehicles, long-range missiles, and defense industry operations that involve demanding, complex, and severe working conditions [3]. These alloys are chosen for their high strength, toughness, good stress corrosion resistance, hot forming properties, and welding capabilities. They are extensively utilized in various aerospace components, including the upper wing, engine, tailplane front and rear beams, web, rib webs, and other parts [4; 5]. Consequently, investigating fatigue damage in aluminum alloy holds paramount significance. Early studies of fatigue failure focused on developing laboratory testing techniques and quantifying the number of failures under different fatigue loading conditions to understand fatigue life. However, these studies paid little attention to the microscopic origins of fatigue failure. In 1903, [PERSON] and [PERSON] [6] conducted a comprehensive investigation into fatigue mechanisms in metals, revealing that fatigue damage occurs due to the accumulation of minuscule irreversible plastic cyclic microstrains on metal surfaces. Building upon this, [PERSON] and [PERSON] [7; 8] proposed that the fatigue life is determined by the plastic strain amplitude. Currently, cyclic microplasticity mechanisms, based on dislocation slip, are considered the primary cause of fatigue failure. Another dislocation mechanism involving extrusion and intrusion was suggested by [PERSON] [9]. [PERSON] [10] suggests that fatigue damage usually arises from the irreversibility of cyclicslip on surfaces, leading to the formation of microcracks at localized stress concentrations, which can then propagate into the bulk. Throughout the fatigue phase, the majority of the overall fatigue life is comprised of early fatigue damage. This includes initial cyclic deformation and damage, as well as crack nucleation and microcrack propagation, which involves the growth and interconnection of microcracks. As a result, research primarily focuses on early fatigue. Currently, the investigation of early fatigue damage is categorized into models based on image-only theory, models based on crystal plasticity theory, and models based on molecular dynamics theory [11; 12; 13; 14; 15]. However, macroscopic image-only ontological models, which assume continuity and homogeneity, typically overlook the impact of microstructural factors when studying the mechanical behavior of materials. These models have certain limitations when it comes to mechanical issues related to the fine structure of materials, such as damage evolution, crack nucleation, and small crack extension. In contrast to intrinsic models at macroscopic and fine scales, microscale-based modeling involves molecular dynamics and discrete dislocation dynamics exhibits features at both temporal and spatial scales that cannot be compared to conventional experiments [14; 15; 16]. Realizing the correlation between fine-scale deformation mechanisms and macroscopic mechanical behavior is crucial in the field of crystal plasticity mechanics, particularly in the study of fatigue in polycrystalline alloys [17; 18; 19; 20; 21; 22; 23; 24; 25; 26]. For instance, [PERSON] [17] incorporated computed local plastic strain into the quasi-measurement of crack initiation to predict the fatigue life of 7075 aluminum alloy.