# Assessment of Fatigue Life and Failure Criteria in Ultrasonic Testing Through Thermal Analyses
## 3 Materials and Methods
### Methods  
To conduct the tests, a black coating spray paint was applied to the specimens to increase their emissivity. This step was essential for improving the accuracy of temperature measurements. Figure 3a shows the specimen geometry, while Figure 3b displays the specimen after applying the black coating.  
The experiments were conducted using a VHCF testing machine, model Shimadzu USF-2000A (Kyoto, Japan). Temperature monitoring was performed with an infrared (IR) camera, model A655 SC (FLIR, Wilsonville, OR, USA), focusing on three points along the reduced section of the specimen. Additionally, a straight-line measurement was included to track temperature variations across the specimen. This camera features a temperature measurement range of \(-40\)\({}^{\circ}\)C to \(+150\)\({}^{\circ}\)C and an accuracy of \(\pm 2\)\({}^{\circ}\)C or \(\pm 2\)% of the reading (Table 4). It provides an average sample over several cycles, which is enough to observe temperature trends.  
Ultrasonic fatigue tests were supported by a cooling system and primarily conducted with intermittent loading to enhance temperature control. The high testing frequency, which enables rapid testing, can lead to self-heating of the specimen, potentially accelerating the  
\begin{table}
\begin{tabular}{c c} \hline Distance of camera to specimen & 0.5 m \\ \hline Room temperature & 20 \({}^{\circ}\)C \\ \hline Emissivity & 0.93 \\ \hline Frames per second & 100 \\ \hline Temperature range settings & \(-40\) to 150 \({}^{\circ}\)C; 100 to 650 \({}^{\circ}\)C; 300 to 2000 \({}^{\circ}\)C \\ \hline \end{tabular}
\end{table}
Table 4: Parameters of the thermography experiments [6].  
Figure 3: (**a**) Specimen design of 34 CrNiMo6 steel (dimensions in mm); (**b**) specimen with black coating.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Steel** & \(\mathbf{\sigma_{u}}\) **(MPa)** & \(\mathbf{\sigma_{y}}\) **(MPa)** & \(\mathbf{E}\) **(GPa)** \\ \hline
34 CrNiMo6 & 900 & 760 & 207 \\ \hline \end{tabular}
\end{table}
Table 3: Mechanical properties.  
damage process. The cooling system is a type of air compressor that has a tank capacity of around 50 L, a pressure that can go up to 10 bar (145 psi), and an airflow of around 250 to 350 L per minute (L/min).  
Figure 4a illustrates the ultrasonic fatigue testing setup along with the IR camera, while the corresponding thermogram is shown in Figure 4b. The parameters used for the thermography experiments are summarized in Table 4.  
Specimens were tested under fully reversed loading conditions (R = \(-1\)) at various stress levels using a pulse-pause technique. Stress levels were chosen as a percentage of the material's ultimate strength (\(\sigma_{\mathrm{u}}\)), ranging from 30% to 66%, with varying pulse and pause durations.  
Additionally, tests were conducted both with and without cooling using a pulse-pause of 110-500 ms, targeting a total of \(10^{6}\) cycles. For these specific tests, a lower effective frequency was used to minimize wear on the testing machine and specimens. These experiments aimed to evaluate the influence of compressed air cooling on temperature evolution during the VHCF tests.  
Figure 4: (**a**) VHCF machine coupled with IR camera; (**b**) thermogram.  
Table 5 outlines the experimental conditions, including pulse-pause settings, corresponding effective frequencies (f\({}_{\text{eff}}\)), and stress amplitudes (\(\sigma_{\text{a}}\)). Tests with pulse-pause settings of 300-200 ms, 200-200 ms, 200-300 ms, and 300-500 ms were conducted to approximately \(10^{7}\) cycles.  
The S-N curve was performed using pulse-pause times with 300-500 ms that provided experiment results without temperature effect. The tests were conducted at room temperature (RT), R = \(-1\), and with air cooling. Table 6 shows the experimental conditions used to obtain the S-N curve.