# Fatigue Damage Evaluation of Aviation Aluminum Alloy Based on Strain Monitoring
## 3 Experimental Results and Analysis
### Analysis of Simulation and Test Results  
This paper aims to investigate the correlation between fatigue damage and microstrain by varying the fatigue times to achieve different fatigue damage states. To characterize the surface fatigue damage state, the electron backscattering (EBSD) technique [56] is employed. EBSD tests are primarily conducted above the notch, which represents the stress concentration point. EBSD maps corresponding to different fatigue durations are generated, and the geometrical mean dislocation densities are calculated using the formula in Equation (19). Figure 5 displays the GND maps of specimens that did not undergo fatigue failure. Figure 6 presents the recorded strain changes from resistance strain gauges during fatigue testing of the specimen. The strain data is shown with an envelope above and below it, and the initial value is zeroed as shown in Figure 6b. The results display the strain changes on two surfaces of the specimen, namely surface A and B. At this stage, the cracks have extended to approximately 925 \(\upmu\)m. Due to the presence of coarse grains, the main mechanism of crack extension is perforation fracture. Figure 7a illustrates the metallographic image of crack expansion at the notch on the B-side, which corresponds to a strain growth of 100 \(\upmu\)s. As the cycles progress to around 25,000 cycles, the crack length continues to increase. In order to establish a clearer relationship between fatigue damage and strain, a new batch of test materials was produced to obtain varying fatigue cycles and different strain changes in the fatigue damage test materials. EBSD tests were conducted on the surface to analyze the micro-mech  
Figure 5: EBSD above the specimen notch when not fatigued: (**a**) GND; (**b**) histogram of GND. \begin{table}
\begin{tabular}{c c} \hline \hline \multicolumn{1}{c}{**Fatigue Parameters**} \\ \hline Loading Frequency/Hz & 5 \\ Strain Amplitude & 0.15\% \\ Strain Ratio & 0.1 \\ Load Waveform & Simple Harmonic Vibration \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Fatigue test parameters. Following the fatigue test program outlined in Section 3.1, this paper fabricated aluminum alloy specimens and subjected them to different numbers of fatigue cycles: 3000, 5000, 7000, 10,000, and 11,500. The fatigue parameters used in the experiments are provided in Table 3. The strain changes on the A and B surfaces were monitored under various fatigue conditions, as shown in Figure 8.  
Figure 6: Map of real-time strain monitoring data: (**a**) plot of strain real-time acquisition data; (**b**) plot of data after taking envelope homogenization. Figure 7: Crack extension metallograph at the notch: (**a**) 25000 cycles; (**b**) 11500 cycles. Figure 8: Fatigue cycles versus strain change: (**a**) 3000 cycles; (**b**) 5000 cycles; (**c**) 7000 cycles; (**d**) 10,000 cycles; (**e**) 11,500 cycles. Observing Figure 8e, it is evident that the strain growth is 26 \(\mu\epsilon\) when the fatigue duration reaches approximately 11,500 cycles. Metallographic observation on the B side of the notch at this point reveals a crack length of 46\(\mu\)m, as shown in Figure 7b. Meanwhile, metallographic experiments were conducted on the A side, where no cracks were produced, and the strain growth was only 17 \(\mu\epsilon\). Figure 8d also shows that the strain change rule is not consistent on both sides. However, in Figures 6b and 8a-c, the strain monitoring data on both sides are essentially similar. This discrepancy can be attributed to variations in the surface treatment of specimen A and B sides, the processing of the notch, and the different states of the notches on the two surfaces. These factors introduce variations and offset issues in the strain data. Nevertheless, this disparity also highlights the feasibility of monitoring the fatigue state through strain. To investigate additional surface damage-strain relationships, an EBSD test was conducted above the notch of the specimen prior to crack initiation. [PERSON] et al.