# Identification of Deformation Stage and Crack Initiation in TC11 Alloys Using Acoustic Emission
## 4 Analysis of Experimental Results  
### AE Signals during Deformation and Damage of TC11 Titanium Alloy  
Figure 3 shows the stress-strain curve and the amplitude change of the AE signal of the TC11 titanium alloy during material deformation. Generally, metal deformation could be divided into four stages, namely the elastic stage, yield stage, strengthening stage, and necking stage [36]. It should be mentioned that a large number of nonferrous metals, including TC11 titanium alloys, do not have an obvious yield point [37]. The stress-strain curve of the TC11 specimen during the elastic deformation is not strictly linear, indicating that the plastic deformation occurs at this stage. Thus, the elastic deformation and yield stage were classified as the first stage. In this work, the deformation of the TC11 specimens was composed of four stages, including the elastic-yield stage, strengthening stage, necking stage, and fracture stage. In the elastic-yield stage (\(oa\) segment), a large number of AE signals were produced at this stage, mainly due to the large amount of dislocation movement and grain boundary slip inside the material. This also means that the plastic deformation of the material occurred in the elastic stage. The results are similar to those reported by the authors of [38], in which the loading applied to the low-carbon structural steel apparently affected the acoustic activity. An increased acoustic activity was found when achieving the critical density of the fatigue microracks. Over the strengthening stage (\(ab\) segment), plastic deformation intensified with the increase of the load. The material began to harden, resulting in a great reduction of the free dislocation movement, thus decreasing the number of corresponding AE signals. At the necking stage (\(bc\) segment), the specimen underwent a great plastic deformation, in which the dislocation freedom became extremely small, and the dislocation congestion was serious. So, the quantity of the AE signal at this stage was also small, which can be attributed to the formation of a microcrack in the TC11, which decreased the acoustic activity [39]. At the fracture stage (\(cd\) segment), the sharp drop in the load at point c (1280s) and the sharp increase in the amplitude of the AE signal predicted the occurrence of cracks. At the same time, a visible macro crack can be observed at point I (1287s) by the micro camera. Obvious crack growth can \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sensor** & **Resonant Frequency/kHz** & **Frequency Range/kHz** & **Threshold/dB** \\ \hline S1/S3 & 150 & 50–400 & 48 \\ S2/S4 & 650 & 100–1000 & 38 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Details of Acoustic Emission (AE) sensors. Figure 2: Experimental test system and sensor layout. also be observed at point II, as shown in Figure 4. At this stage, the specimen began to initiate micro cracks at weak stress points and continued to expand until the final fracture happened. Although this section lasts for a short time, a large amount of AE signals are still generated [40]. The changes of the AE signals after the loading reflect the evolution of the TC11 damage in the fatigue process. These changes are related to primary dislocation-hardening at the early stage of fatigue and propagation of micro cracks at the later stage of fatigue [41]. Figure 5 shows the typical time- and frequency-domain AE signals obtained form the TC11 titanium alloy at different stages of deformation. From the signal spectrum, it can be seen that during the first three stages of the testing, the signal amplitude and frequency components over 50-200 kHz remained mostly unchanged. These signals mainly corresponded to the friction between the test piece and the fixture and the \(H_{\text{NAE}}\) by the hydraulic system, as described in Section 2. In comparison, the signals over 200-500 kHz changed significantly in the four stages of material deforming, which corresponded to the \(H_{\text{IAE}}\).