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1. Introduction 7-series aluminum alloys include Al-Zn-Mg series and Al-Zn-Mg-Cu series alloys; these alloys possess a range of advantages such as low density, high specific strength, good workability, and excellent weldability. With the continuous improvement of heat treatment processes and microalloying techniques applied to aluminum alloys, their mechanical properties have been significantly enhanced, and their overall performance has also seen a substantial improvement. It has been widely used in aerospace, construction, vehicles, bridges, engineering equipment, and large pressure vessels. The rapid development of modern industry has placed higher demands on the strength, toughness, and stress-corrosion resistance of 7-series aluminum alloys. However, there is another phenomenon: in various industries, aluminum alloy equipment occasionally experiences subtle fractures, and it is difficult or even impossible to detect any plastic deformation before the fracture occurs. This type of fracture causes irreparable damage to human life and property safety. Numerous experiments have shown that these fractures are caused by material fatigue; under prolonged exposure to alternating loads, microcracks occur on the surface or, more notably, inside the material. This paper mainly studies cracks induced by fatigue in aluminum alloys and the analysis of fatigue fracture surfaces. Such research is of great significance for future production safety. Images 2. The development history of Series 7 aluminum alloys. In the 1920s, German scientists developed an Al-Zn-Mg alloy; however, due to its poor resistance to stress corrosion, it was not utilized in industrial applications. From the early 1930s until the end of World War II, various ** studies have shown that the Cu element can improve the resistance of aluminum alloys to stress corrosion. Here, a large number of Al-Zn-Mg series alloys have been developed, so research on these alloys has been neglected. Countries such as Germany, the United States, the Soviet Union, and France have successfully developed alloys such as 7075, B93, and D683 based on Al-Zn-Mg-Cu series alloys. It is currently widely used in the aerospace industry; however, an optimal combination of strength, toughness, and stress corrosion resistance has not yet been achieved. In the 1950s, German scientists introduced the alloys AlZnMg1 and AlZnMg2, which possessed excellent welding properties, drawing attention to Al-Zn-Mg series alloys. During this period, American researchers added elements such as Zr, Mn, and Cr to the AlZnMg1 alloy, resulting in the development of 7004 and 7005 alloys. These alloys possess excellent weldability and resistance to stress corrosion, and they are widely used in the welding industry. The only drawback is that its process performance is poor. Japanese scientists attempted to reduce the Mg content in the alloys and increase the Zn/Mg ratio, resulting in the development of ZK60 and ZK61 alloys, which improved the weldability and processability of these alloys but significantly reduced their strength. During the same period, the former Soviet Union also developed 1915 and 1933 alloys, which also had relatively low strength. To overcome the problem of low strength, the 7020 alloy was developed in the 1970s, featuring high strength and good weldability. Later, attention was focused on Al-Zn-Mg series aluminum alloys. In the early 1980s, American scientists, based on the 7075 alloy, adjusted the contents of certain alloying elements in order to address the issue of relatively high susceptibility to stress corrosion in actual production and to meet certain special requirements; as a result, many new types of alloys were developed. In contrast, research on 7-series aluminum alloys in China started relatively late; in the 1980s, Al-Zn-Mg series aluminum alloys were developed by research institutes in Northeast China and Beijing. Currently, the main alloy products include 7050, 7075, 7175, etc. In the mid-1990s, the Beijing Institute of Aeronautical Materials developed the 7A55 ultra-high-strength aluminum alloy using conventional semi-continuous casting methods; in recent years, a still stronger 7A60 alloy has been created. In the development of Al2Zn2Mg series aluminum alloys, domestic efforts are mostly limited to imitation, with very little independent development. Image 3. Classification of aluminum alloy fatigue. 3.1 Definition of fatigue: Fatigue fracture is a delayed fracture caused by alternating loads and stresses. The stress level at which such fracture occurs is usually lower than the material’s tensile strength σb; sometimes it is even lower than the yield strength σs. Under normal circumstances, fatigue failure does not involve significant plastic deformation; its deformation is mainly characterized by brittle fracture. It is a sudden and extremely dangerous mode of failure that occurs without any warning, and is difficult to detect or prevent. Fatigue of aluminum alloys can be classified into three categories based on the causes of fatigue failure: thermal fatigue, corrosion fatigue, and mechanical fatigue. 3.2 Thermal fatigue Thermal fatigue of aluminum alloys is a type of fatigue failure that occurs under the combined action of alternating stresses and thermal stresses. External constraints and internal constraints are two necessary conditions for the occurrence of thermal fatigue. External constraints prevent the material from expanding freely, while internal constraints create temperature gradients that cause the material to expand; however, these constraints give rise to thermal stresses and strains. After a certain number of cycles, this leads to the initiation and propagation of cracks. Zhang Wenxiao et al. studied the homogeneous and heterogeneous thermal fatigue properties of LD8 aluminum alloy, and investigated the thermal fatigue life under different conditions using elasto-plastic fracture mechanics methods. 3.3 Corrosion fatigue: Metal materials that are used for extended periods in the chemical industry or in seawater are exposed to corrosive environments. In addition, they are subject to alternating loads. Compared with metal materials in normal environments, the combined effect of a corrosive environment and alternating loads significantly reduces the fatigue resistance of these materials, leading to the failure of the components and ultimately to their breakage. Gong Yuhui et al. studied the effects of different corrosion environments on the fatigue performance and crack growth rate of 7475-T7351 aluminum alloy, and found that corrosion environments significantly accelerated crack growth, whereas different corrosion environments and temperatures had little effect on the material’s low-cycle fatigue performance. Wang Cheng et al. added sodium silicate at different concentrations to aluminum alloys and found that it could inhibit pitting corrosion of the alloys, reduce crack initiation sites, and enhance the alloys’ resistance to pitting corrosion and corrosion fatigue life in sodium chloride solution. However, it had no effect on inhibiting the propagation of corrosion fatigue cracks in aluminum alloys. 3.4 Mechanical Fatigue Mechanical components experience mechanical fatigue under the action of external stresses or strains. After prolonged operation, cracks or fractures can occur even if the stress applied is below the material’s yield point. At lower levels of cyclic stress, elastic strain plays a dominant role; in such cases, the fatigue life is longer, and this phenomenon is known as high-cycle fatigue, or stress fatigue ； At higher cyclic stress levels, plastic strain plays a dominant role; in this case, the fatigue life is short. This phenomenon is known as low-cycle fatigue, or plastic fatigue. Li Rui et al. conducted high-low cycle combined fatigue tests on 2024-T3 aluminum alloy perforated plates. The study found that as the number of high-low cycle repetitions increased, the combined fatigue life decreased significantly. They also established a relationship between the number of high-low cycle repetitions and the stress amplitude ratio, and the high-low cycle combined fatigue life. However, this relationship only takes into account the effect of the number of load cycles on fatigue, without comprehensively considering other factors that influence fatigue life. Figure 4. Fatigue failure process and mechanism. The onset of the fatigue process in metallic equipment, that is, the initiation of fatigue cracks, is referred to as the fatigue origin. The fatigue source is the core of the permanent damage to the material’s microstructure. Once cracks begin to initiate, they gradually grow and merge with other cracks, eventually forming macroscopically visible cracks known as main cracks; at this point, the crack initiation stage comes to an end. Subsequently, it enters the crack propagation stage. Initially, the crack propagates stably; once it reaches a critical size, under further cyclic stress and strain, the metal material can no longer withstand the loads. As a result, the crack suddenly becomes unstable, leading to instantaneous failure and fracture of the material. In short, the fatigue failure process consists of three stages: crack initiation, crack propagation, and unstable fracture. Each stage is as follows: Crack initiation: Due to stress concentration, fatigue cracks initially originate in the weakest regions of the material’s microstructure or in areas where the stress is relatively high. In the early stage of crack initiation, its length is less than 0.05 mm to 0.1 mm; such a crack is referred to as a fatigue crack nucleus. As fatigue progresses, microcracks gradually develop into macrocracks that are visible to the naked eye. The main sites for the initiation of fatigue cracks in aluminum alloy materials are slip bands, grain boundaries, and phase interfaces. Crack propagation: After the initiation of fatigue cracks, the crack propagation stage begins. This stage is further divided into two parts. First, the crack expands inward along the primary slip system in a pure shear manner, at an extremely low rate; the extent of this expansion is on the order of a few grain lengths. Secondly, due to the obstruction at the grain boundaries, the direction of propagation gradually becomes perpendicular to the direction of the principal stress, that is, the tensile stress, resulting in the formation of fatigue striations or fatigue fringes; each fringe corresponds to the result of one cycle. In the first stage, the crack propagation speed is slow and the length is small, so the morphological characteristics of this stage are not distinct. In the second stage of transgranular propagation, the propagation rate increases as the number of cycles increases, and the extent of propagation is also quite significant; fatigue fringes can be observed under an electron microscope in most materials, and in some cases they can even be seen with the naked eye. The fatigue striations of different materials vary, and their morphologies are diverse. There are groove-like patterns that are slightly curved and parallel to the direction of crack propagation, as well as fracture surfaces that are relatively smooth with Beadley or beach patterns present. Sometimes, radial patterns centered on the source area can be observed. Fatigue striations are the most characteristic feature of fatigue fractures. Under normal circumstances, the fatigue crack propagation zone occupies a large area of the entire fracture surface. The fatigue crack growth stage is the main component of a material’s overall fatigue life. The two stages of crack propagation in different aluminum alloy materials also have varying lifespans; in specimens with a smooth surface, the time taken for propagation during the first stage accounts for the majority of the total fatigue life ； In notched specimens, the first stage is almost negligible, and the propagation in the second stage constitutes the entire life of fatigue crack growth. Crack instability: When a fatigue crack grows to a certain length—the critical length—the material surface can no longer withstand external loads. Upon the next loading, unstable propagation occurs, leading to rapid fracture. This stage represents the final phase of the component’s lifespan. The brief process during which instability progresses to fracture contributes negligibly to the component’s overall service life. The fracture zone that results from the rapid expansion of cracks at this stage is known as the instantaneous fracture zone; the appearance of this fracture zone varies significantly depending on the properties of the material. Image 5: Factors Affecting Fatigue Life 5.1 Internal Material Factors The fatigue properties are related to the alloy composition, which determines the alloy structure as well as the degree of strengthening ； At the same time, the microstructure of the alloy, as well as the defects present during the metallurgical process, have a significant impact on the fatigue resistance of the alloy. Crack initiation can be caused by inclusions, grain size, grain segregation, and grain boundary porosity. Zhang Tao et al. studied the fatigue properties of Al-Si cast aluminum alloys and found that the pores inevitable in the casting process, as well as the size and morphology of Si particles, all have a significant impact on the initiation of fatigue cracks in cast aluminum alloy materials ； Through his studies on the anisotropy of the fatigue properties of aluminum-lithium alloys, Zhai found that the strength is low in the rolling direction, and the fatigue properties are also the worst there; fatigue cracks tend to initiate along this direction. In contrast, the strength is higher in the thickness direction, where crack initiation is rare, and thus the fatigue properties are the best ； Ageing treatment is an effective way to improve the properties of aluminum alloys; since it alters the microstructural structure of the alloy, it also has a significant impact on its fatigue characteristics ； Sharma et al. conducted fatigue tests on AA 2219 aluminum alloy subjected to different aging treatments. The results showed that the alloy treated by natural aging and under-aging exhibited better fatigue performance, with few signs of fatigue crack initiation ； For the alloys subjected to peak aging and over-aging treatments, the crack initiation and crack propagation rates are relatively high, resulting in poor fatigue performance. 5.2 Component Status The fatigue properties of alloys are also affected by surface roughness, material size, and geometry. Surface irregularities. It is related to the uniformity of wall thickness. Suraratchai et al. studied the factors affecting the fatigue life of aluminum alloys. They conducted a finite element analysis on the surface roughness of the alloys, and the results showed that stress concentration caused by the unevenness of the material surface is the root cause of reduced fatigue life ； Xiao Ji studied the fatigue properties of 7475 aluminum alloy sheets. The specimens on the T-L plane, which performed best in the fatigue tests, were subjected to shot peening. The results showed that shot peening did not necessarily improve the fatigue strength of the specimens. During this process, while residual compressive stresses were introduced, the surface smoothness of the specimens was also compromised. Residual compressive stress will enhance the fatigue strength of the specimen, whereas excessive roughness will make the specimen surface prone to becoming a crack source. 5.3 Working conditions The magnitude of the load, the loading method, and the loading frequency are decisive factors for the fatigue life of alloy materials. Liu Gang et al. studied the fatigue properties and fatigue crack growth rate of 2E12 aluminum alloy under different stress levels. The results showed that the presence of notches reduced the fatigue strength; however, as the stress ratio increased, the fatigue strength was significantly improved ； Jian Haigen and others compared the microstructure of fatigue fracture surfaces of aluminum alloys under different stresses using metallography and scanning electron microscopy, and found that the distance between the site of fatigue crack initiation and the material surface decreased as the applied stress increased. The higher the applied stress, the smaller the area of the fatigue source region; meanwhile, the spacing between the fatigue streaks in the crack propagation zone increased. Furthermore, as the stress increased, the area of the crack propagation zone on the fracture surface decreased, while the area of the instantaneous fracture zone increased. At the same time, the material lifespan is also affected by factors such as the operating environment, including temperature and surrounding media. Through their study of the fatigue crack growth behavior of AA 2024 aluminum alloy, Gasqueres et al. found that at normal room temperature, once fatigue crack growth enters the second stage, lowering the ambient temperature to 223 K causes the crack growth to follow the pattern of the first stage again; moreover, at this point, crack growth is influenced by both temperature and air pressure. There are many factors that influence the fatigue properties of aluminum alloys. Studying the fatigue life of these materials by considering only one or a few factors is not accurate. Establishing appropriate scientific models that take all factors into account in order to accurately predict the fatigue life of the materials is an area that requires further in-depth research. It has advantages such as high efficiency, low cost, and simple processing, and is suitable for various particles and matrices; overall, this method is quite competitive. Image 6: Methods for estimating fatigue life. Since material fatigue often results in unpredictable and undetectable plastic fractures, the resulting losses are immeasurable. Therefore, estimating the fatigue life of materials has always been an important research issue. For hundreds of years, scientists from various countries have been exploring and researching.