Due to high stress concentrations, welded joints represent the most common locations of fatigue crack initiation in steel structures that are prone to fatigue. Welding affects material properties by the process of heating, cooling, and combining basic and additional material. Since welding is the primary process of joining elements in steel structures, it is obvious that fatigue assessment during the design and maintenance process becomes inevitable. There are many fatigue assessment methods of welded joints, but their precision remains questionable. This paper represents a review of the most common fatigue assessment methods used for welded steel joints. As a result of this review, areas that require additional research are highlighted.

The complex nature of the fatigue strength of welded steel details, which is influenced by a multitude of factors, can be improved by extending the crack initiation phase of the total fatigue life. This enhancement can be achieved through the use of the High-Frequency Mechanical Impact (HFMI) method. This study discusses the fabrication and laboratory cyclic testing of as-welded and HFMI-treated transversal and longitudinal attachments made from S690QL steel. Assessment of a two-stage model (TSM) for the evaluation of their fatigue life, previously developed by the authors, is also performed. The TSM's estimated fatigue lives of the details under consideration align well with the fatigue test results. Moreover, both the TSM model results and the corresponding fatigue test results demonstrate that the HFMI method significantly prolongs their fatigue life. The fatigue resistance of HFMI-treated details compared to as-welded details increased by 1.67 and 1.91 times for longitudinal and transversal attachments, respectively.

Welded steel details are critical components from the aspect of fatigue. Additional fatigue resistance can be achieved by the High-Frequency Mechanical Impact (HFMI) treatment. This treatment increases the crack initiation period by improving the weld geometry, introducing compressive residual stresses, and increasing the weld toe’s hardness. The study presented in this paper is based on the development and calibration of an Initiation–Propagation-based Two-Stage Model (TSM), which is, by the combination of different methods, suitable to separately consider crack initiation and crack propagation. It is shown that a TSM is able to predict the fatigue life of as-welded and HFMI-treated welded steel details, which is proven by comparing the calculated results with the results of tests on similar details given in the literature. A parametric study of the TSM is conducted for different steel grades in order to investigate the influence of steel strength and HFMI parameters on fatigue lives of a welded steel detail with longitudinal attachment.

Industrial photogrammetry is a reliable method to achieve submillimeter accuracy when mapping 2D or 3D objects. In the field of fatigue testing of steel welded details, it can be used to find a new method of crack initiation assessment. Fatigue testing is an important method for determining and predicting the durability of structural details in service. The research presented in this paper is based on a computer vision algorithm developed using the open-source code OpenCV library and the Oriented FAST and Rotated BRIEF (ORB) method to provide a solution for the assessment of crack initiation. Within this research, a method for determining the crack initiation period using polynomial functions of a certain degree is developed. The developed algorithm fully automatically determines the test specimen displacement for all imagery and assesses the crack initialization period by polynomial interpolation with a percentage threshold. The algorithm shows us the best results based on a 26th-degree polynomial with a deviation from the critical value of 5%. The validation of the algorithm was carried out using completely independently recorded data from the hydraulic press used for fatigue tests. The results of all test specimens show that the percentage accuracy of determination crack initiation period is between −0.04% for test specimens S355-TA-AW-02 and S355-TA-HFMI-03 and −0.82% for test specimen S355-TA-HFMI-03, with the mean of all results being 0.39%.