# Fatigue Damage Evaluation of Aviation Aluminum Alloy Based on Strain Monitoring
## 3 Experimental Results and Analysis
### Analysis of Simulation and Test Results
(2017); [PERSON] (2018) analyzed the dislocation characteristics and deformation-induced martensitic transformation of 304 and 316 stainless steels under asymmetric cyclic loading. The study revealed that work hardening plays a crucial role in influencing the fatigue behavior of stable austenitic stainless steel under such loading conditions. Moreover, using X-ray diffraction, they quantitatively assessed the dislocation densities in austenitic and ferritic stainless steels during cyclic loading and found a linear variation with low circumferential fatigue life. In ferrite, helical dislocations decreased with decreasing life, while in austenite, they remained relatively unchanged. In summary, the arrangement of dislocations during cyclic loading is dependent on the applied load, and dislocations in both materials generally exhibit an increasing and then stabilizing relationship with fatigue cycles. Subsequently, an analysis of the distribution of geometrically necessary dislocation density was performed. Following cyclic loading, the specimen showed an overall trend of dislocation density initially increasing and then decreasing, as shown in Figure 9. A significant concentration of geometrically necessary dislocations was observed at grain boundaries. Figure 9b displays the GND map of the fatigue specimen above the notch at 3000 cycles. EBSD analysis of the fatigue specimen reveals that the early stage of fatigue in hard aluminum alloy corresponds to the hardening stage. At this point, the specimen exhibits a work-hardening condition. However, the strain monitoring remains nearly constant. This can be attributed to the current measurement method not having reached a higher level of accuracy. Additionally, the strain is less pronounced during the work-hardening stage, as shown in Figure 8a,b. During this phase, the number of slipped grains increases, leading to an increase in dislocation density. As a result, the fatigue process enters the softening stage, characterized by a decrease in dislocation density and the formation of a strong slip band. [PERSON] et al. (2018) also observed that the work-hardening and work-softening states change at different fatigue cycles, affecting the plastic deformability. Within this robust slip band, microcracks start to emerge, indicating the beginning of the cyclic saturation state. At this point, the monitored strain gradually begins to increase. In low circumferential fatigue, the strain gradually increases due to cyclic loading controlled by machining softening during strain loading, while machining hardening hinders plastic deformation. The appearance of cracks intensifies the change in strain, making it more pronounced. Utilizing fatigue test data, this study predicts the fatigue crack emergence life under various fatigue loading conditions through the crystal plasticity model. The crystal plasticity model, detailed in Section 2.1, is employed to calculate and analyze the plastic deformation of the polycrystalline model. Both macroscopic and fine scales of strains are meticulously examined, and the resulting strain variations in the simulation results are analyzed. Cyclic loading of the crystal plasticity model proposed above is conducted, and Figure 2c illustrates the cyclic stress-strain curves of the 10 th cycle at a strain amplitude of 0.15%, compared with the experimental results. It is evident that the model parameters obtained in Table 1 align with the experimental results. Figure 10 presents a comparison of the clouds of the two FIPs discussed in Section 2.2. These FIP clouds represent specimens after 40 fatigue cycles at a 0.3% strain amplitude and 0.1 stress ratio. It is evident that there is a significant accumulation of cumulative plastic slip and strain energy dissipation closer to the notch. This accumulation indicates a high likelihood of crack generation due to stress concentration in this region. The FIP critical value near the grain boundary is the primary criterion for assessing fatigue crack initiation life.