# Fatigue Damage Evaluation of Aviation Aluminum Alloy Based on Strain Monitoring
## 3 Experimental Results and Analysis  
### Experiment Program  
The aerospace aluminum alloy 7075 was used as the test material in this study. It underwent a solution treatment and artificial aging (T6) heat treatment process. The heat treatment process began with subjecting the alloy to a solid solution treatment at a temperature range of 465-477 \({}^{\circ}\)C for a duration of 1-2 h. Afterward, the alloy was cooled and subjected to an artificial aging treatment at a temperature range of 120-180 \({}^{\circ}\)C for a duration of 6-24 h. The fabrication process involved hot rough-rolled quenched plates, and the detailed chemical composition can be found in Table 2. The average grain size of the material was measured to be approximately 88 \(\upmu\)m using SEM electron microscopy (Thermo Fisher Scientific Inc., Waltham, MA, USA), following the standard metallographic procedure [54]. Subsequently, EBSD experiments were conducted to investigate the crystal orientation of the material. A step size of 4 \(\upmu\)m was used in these experiments. Based on the research by [PERSON] and [PERSON] et al. [55], the geometrically necessary dislocation density (GND) of each pixel point during the EBSD scanning process can be calculated using the strain gradient theory:  
\[\rho^{GND}=\frac{2\theta_{i}}{ub} \tag{19}\]  
where \(\theta_{i}\) denotes the orientation difference, \(b\) denotes the Burger vector, and \(u\) is the unit length of the pixel point.  
To determine the precise mechanical properties of the material, a tensile test was performed on a standard specimen measuring 205 mm in length and 25 mm in width, as shown in Figure 3a. The resulting tensile stress-strain curves were subsequently compared with the simulation results presented in Figure 2b. The strong correlation between the experimental and simulated tensile curves validates the accuracy of the material parameter.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \(E\) (GPa) & \(\
u\) & \(h\) & \(c\) (MPa) & \(b_{0}\) & \(b_{sat}\) & \(\gamma_{0}\) & \(g^{0}\) (MPa) & \(n\) \\ \hline
71.5 & 0.33 & 40 & 203 & 16 & 0.0 & 0.02 & 203 & 20 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Simulation parameters of 7075-T6 aerospace aluminum alloy.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Element** & **Si** & **Fe** & **Cu** & **Mn** & **Mg** & **Cr** & **Zn** & **Ti** & **Al** \\ \hline Atomic Content (\%) & 0.16 & 0.37 & 1.7 & 0.19 & 2.8 & 0.22 & 5.9 & 0.03 & Bal. \\ \hline \hline \end{tabular}
\end{table}
Table 2: 7075-T6 aircraft aluminum alloy main composition.  
In comparison to the tensile test, the fatigue test involves a grooved specimen. The parameters of the specimen are shown in Figure 3b, where the thickness of the test block is 5 mm. The fatigue test is conducted using an Instron 8801 fatigue tester and the control mode is strain control. The specific fatigue parameters can be found in Table 3. In order to observe changes in surface organization during fatigue, the specimen's surface is sanded and polished using sandpaper with different mesh sizes (800, 1000, 1200, 1500, 2000, 2500, and 3000). Strain gauges are attached to the notch to monitor strain changes before the initiation of fatigue microcracks. The overall test program is illustrated in Figure 4. For dynamic strain gauge measurements, this paper utilizes the DH3823 8-channel strain collector with a collection frequency of 1 kHz. The experimental setup is shown in Figure 4b.  
Figure 4: Schematic diagram of the strain monitoring fatigue system: (**a**) fatigue testing machine; (**b**) strain acquisition system.  
Figure 3: Experimental specimen design: (**a**) tensile specimen design scheme; (**b**) strain fatigue test design scheme.