# Crack Initiation and Propagation Fatigue Life of Ultra High-Strength Steel Butt Joints
## 3 Optical Detection of Crack Initiation and Propagation
### Experimental Setup  
The setup for this method is very simple and does not require any changes on the specimen itself (see Figure 5). A common digital SLR camera is placed around 20-30 cm off the specimen's surface. The exact distance is figured out empirically for each specimen type in order to fit the region of interest to the camera's captured area and its focus settings. In order to trigger the camera during the fatigue test, the cameras trigger button is connected to the control computer for the hydraulic test rig. The applied camera is a Canon (Ota, Tokyo, Japan) EOS 650D featuring a comparably high resolution of 18 megapixels. The lens selected for the pictures is a Canon EF-S 60 mm f/2.8 Macro USM macro lens enabling a detailed view of the region of interest without the risk of collision with the clamping devices. A lighting system consisting of an LED lamp is installed to ensure lighting independent of the time of day.  
The image acquisition is performed at the specimens' upper load \(F_{u}\) causing possible cracks to open. Before running the test, a reference image is taken at maximum loading as a basis of comparison, which is required by the subsequent post-testing digital image correlation procedure. At first, the possibility of synchronized image acquisition during fatigue testing was checked. However, as the image has to be taken exactly at the maximum load, a very short shutter time is required in order to ensure a clear picture with adverse impact on the image quality. Hence, the run cycle of the fatigue test stops at maximum load for the image acquisition for a few seconds (see Figure 6). Due to the short retention time and standard conditions around the specimen, holding the specimen at maximum load does not affect the crack initiation and propagation behaviour.  
Figure 4: Stable crack propagation curve of the investigated S1100 base material at R = 0.1.  
The image acquisition starts with a reference image before test start and continues after a certain number of load cycles \(N_{initial}\). This initial cycle number is based on previous tests and strongly depends on the load level of the respective test. The a priori fatigue testing reference image and the image taken at the initial cycle number show no distinct difference, implying early-stage crack propagation. After the initial image acquisition, the fatigue test is continued block by block. The number of load cycles between two images \(N_{interval}\) increases with the load cycle number according to Table 2. Both measures reduce the number of images to be taken without a significant decrease of the procedure's accuracy. A final image of the broken specimen at \(N_{f}\) provides insight on the failure-tripping crack in case of several initiated cracks.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Current Load Cycle** & **Number \(N\) (-)** & **Interval between Image Acquisition \(N_{interval}\) (-)** \\ \hline
0\(\leq\) & \(N\) & \textless{}100,000 & 2000 \\
100,000\(\leq\) & \(N\) & \textless{}400,000 & 5000 \\
400,000\(\leq\) & \(N\) & \textless{}800,000 & 10,000 \\
800,000\(\leq\) & \(N\) & \textless{}1500,000 & 25,000 \\
1500,000 \(\leq\) & \(N\) & \textless{}50,000 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Definition of intervals for image acquisition.  
Figure 5: Setup for image acquisition: (**a**) Schematic layout; (**b**) setup at test rig.  
Figure 6: Schematic block testing programme for image acquisition.