# Crack Initiation and Propagation Fatigue Life of Ultra High-Strength Steel Butt Joints
## 5 Summary and Conclusions  
This study deals with the determination of the crack initiation fatigue life and the subsequent macroscopic stable crack propagation life during fatigue tests of ultra high-strength steel S1100 butt joints exhibiting noticeable local undercuts. The underlying experiments show a drop of about 15% in fatigue strength if undercuts are present whereas the inverse slope of about k = 4 remains almost unchanged. Furthermore, a large scattering of the fatigue strength with the size and shape of the undercut is observed. However, the decrease is not as pronounced as one would expect based on the evaluated stress concentration factors using finite element analysis. Here, previous work in [47; 51] revealed a \(K_{t}\) of about 1.9-2.5 for regions without undercut compared to values for \(K_{t}\) from 3.5-5.4 for local undercuts. One main part of this work is the introduction of a simple and cost-effective procedure to determine the crack size during specimen fatigue testing. Therefore, a DSLR-camera is utilized to take pictures of the weld surface in specified intervals. In order to ensure comparable situations for each photo, the fatigue test is interrupted and the upper force is applied to the specimen. An implemented digital image correlation procedure compares each picture with the initial state and calculates the consequential distortion field. Excessive distortions are then identified as crack and monitored until specimen burst rupture. The procedure subsequently provides a load cycle versus surface crack length for each crack within the defined area. The related crack depth is additionally calculated using equations from the literature enabling the determination of a crack initiation load cycle number at a specified crack depth. [PERSON]'s stress averaging method is utilized to calculate the effective notch stress range for each specimen. Therefore, the results of FE-analyses for each specimen based on the measured weld geometry are taken. Hereby, values of the microstructural support length of \(\rho^{*}\) = 0.1 mm for technical crack initiation and \(\rho^{*}\) = 0.13 mm for burst fracture are obtained for minimum scattering regression analysis. These results show good accordance with the published results for similar steel grades. The second part deals with the application of the presented crack detection procedure on the fatigue tests. It reveals a comparably high portion of crack initiation phase of more than 50% fatigue life even though only specimens with undercuts are evaluated. Basically, the cycles to crack initiation tend to decrease with the increasing effective notch stress range, but quite high scattering is observed. Finally, a linear elastic crack propagation calculation is performed. The base material featured parameters for the Paris law by SENT-testing. As expected, these show a significant deviation from the suggestions according to IIW for welded joints. However, the evaluated base material S1100 parameters match published results for similar steel types and grades. The crack propagation assessment is started either at the number of cycles to crack initiation, determined with the presented optical detection procedure, or the assessment is carried out from the start of the fatigue test. Furthermore, three different sets of material parameters are used for the case study. The main conclusion of this investigation is that the crack initiation phase is not neglectable. Otherwise, the assessment tends to deliver very conservative results, especially for welds in the high-cycle fatigue domain above one million load cycles. In detail, the material parameters obtained by the IIW recommendation consistently leads to  
\begin{table}
\begin{tabular}{l c c c c c c c c c} \hline \hline  & \begin{tabular}{c} **Start Crack** \\ **Length** \\ **\(\mathbf{a_{init}}\) (mm)** \\ \end{tabular}