In-situ crack propagation measurement of high-strength steels including overload effects

In general, the requirements of endurable light-weight structures are challenging tasks for engineers. High-strength steels provide a major potential by replacing commonly applied construction mild steels. However, a higher notch sensitivity and increased crack propagation rates may decrease the benefit of high-strength steels and their practicability of application. Therefore, reliable assessment methods are demanded to assess fatigue life and crack propagation of welded as well as un-welded structures. For this purpose, an optical measurement system consisting of an industrial camera, telecentric lenses, and special LED components is set-up to analyze crack initiation and propagation of high-strength steels compared to common construction mild steels. Furthermore, an image processing program is developed to investigate the crack length during testing automatically. Indirect potential drop measurement method with crack gauge and optical-light microscopical investigations are utilized to calibrate the elaborated image measurement system. Constant amplitude tests including overloads are performed for high-strength and common construction mild steel specimens to specify the material's service strength. Based on the researched data, material parameters for crack propagation analysis are evaluated for these steel grades.