# Structural Health Monitoring of Walking Dragline Excavator Using Acoustic Emission
## 2 Materials and Methods
### AE Source Characterization  
Mechanical tests were carried out on specimens made of the same steel as the dragline back legs to determine the AE parameters of a fatigue crack. Specimens were produced from 16G low-alloyed steel (marked in accordance with Russian state standard) sheets using laser cutting. The chemical composition of the studied steels is presented in Table 1.  
The shape and overall dimensions of the specimens (Figure 3a) were selected on the basis of Russian state standard GOST 25.506-85. Thus, a series of rectangular specimens with a thickness of 5 mm and with an edge notch were prepared. The width is equal to \(b=50\) mm, and the length \(l=350\) mm was chosen due to the condition of \(l\geq 2b\) and taking into account the distance between the grips. The notch width \(e\) also should meet the requirement (\(e\leq 0.06b\)) and was selected as \(e=4\) mm, the opening angle \(\Theta\) corresponded to (\(30^{\circ}\leq\Theta\leq 60^{\circ}\)) and was \(\Theta=45^{\circ}\), and the notch depth \(h=10\) mm. A fatigue crack was grown on each specimen by cyclic tensile loading using the pulsation cycle (cycle asymmetry coefficient \(R_{c}=0\)) with a frequency of 5 Hz and a maximum cycle load of \(\sigma_{max}\approx 0.6\cdot\sigma_{y}\), where \(\sigma_{y}\) is a yield stress. The number of loading cycles was chosen so that the fatigue crack developing under cyclic loading in the lateral notch region would reach a certain length. Nominal crack lengths ranged from 3 to 15 mm. Thus, the total length of the notch and crack was approximately 15-28 mm depending on the specimen. After the crack growth, the specimens were tested under static tension using a loading scheme adopted during industrial AE testing (Figure 3b) [18]. A series of experiments was done for 10 specimens with identical parameters. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multicolumn{1}{c}{} & \multicolumn{6}{c}{**The Content of Chemical Elements (\% wt.)**} \\ \hline
**C** & **Mn** & **Si** & **Cr** & **Ni** & **Al** & **Mo** & **S** & **P** \\ \hline
0.14–0.18 & 1.0–1.2 & 0.15–0.3 & \(\leq\)0.2 & \(\leq\)0.3 & 0.02–0.05 & \(\leq\)0.8 & \(\leq\)0.02 & \(\leq\)0.025 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The chemical composition of the 16G low-alloyed steel. Figure 3: The design of the test specimen: (**a**) scheme of the specimen; (**b**) diagram of the loading cycle. The A-Line 32D AE system (LLC \"Interunis-IT\") was chosen during the experiment. It was equipped with four measuring channels including the PAEF-014 preamplifier and GT200 resonant sensor (LLC \"GlobalTest\", resonant frequency 180 kHz). The intrinsic noise of the equipment, preamplifier, and AE sensor was 26 dB. The discrimination threshold was selected as 45 dB (6 dB higher than the noise level of the testing machine). Figure 4 presents the dependence of the cumulative AE hits depending on the current stress and time. Cumulative AE hits have a power-law dependence of the \(\sigma/\sigma_{y}\) relation which is typical for the crack presence. The number of AE hits registered during the loading of the specimens varied in the range from 118 to 240 with an average value about of \(N_{\Sigma}\) = 185. The number of AE hits depends on the loading stress. For more objective evaluation of the material emissivity the Palmer-Heald model was implemented. According to the Palmer-Heald model,  
\[N_{\Sigma}\ =\ D\cdot a\cdot\left(sec\left(\frac{\pi}{2}\frac{\sigma}{\sigma_{y}} \right)\ -\ 1\right) \tag{1}\]  
where \(\sigma\) is the actual stress; \(\sigma_{y}\)--yield stress; \(a\) is half the length of the crack; \(D\)--dimensional coefficient of the model (1/mm), depending on the characteristics of the material, temperature, and type of stress-strain state. It is presented in [19] that the multiplicative coefficient of the Palmer-Heald model \(D\) can be considered as parameters that may be applied to describe the material AE emissivity independent of loading conditions, crack length, and stress intensity factor.