Fatigue life of structural steel components significantly depends on local material properties, such as microstructure and mechanical characteristics. In order to enhance durability and facilitate light-weight design, the application of advanced high-strength materials is essential. Fracture mechanical methods, considering complex local stress states and load-spectra during operation, are commonly applied to assess fatigue life and define inspection intervals.
This paper focuses on the effect of various load sequences on the fatigue crack growth. Therefore, uniaxial loaded flat specimens with V-shaped notches and four-point rotary bending loaded round specimens with semi-elliptical notches made of mild and high-strength steel material are investigated. The test procedure incorporates simplified block load-spectra, overloads, and underloads at different stress ratios and load levels. A specific optical measurement system and evaluation routine is set-up to detect the surface crack length during testing. In addition, supplementary crack length measurement techniques, such as crack gauges and microscopical analysis, are applied for validation. Special attention is given on retardation and acceleration effects, and their interaction with the applied load scenario. Fracture surface analysis provides information about the crack front geometry and the behavior during crack propagation. The aim of the experimental work is to determine material depended crack growth parameters focusing on both constant and variable amplitude loading.
Based on the experimentally evaluated fracture mechanical characteristics, numerical and analytical linear-elastic fracture mechanical assessments are performed. Herein, common retardation models, such as presented by Willenborg, are applied and the results are compared to the tested material behavior. Finally, the potential of advanced high-strength compared to mild steels for complexly loaded structural components, and recommendations for fracture mechanical analyses incorporating load sequence effects is presented.
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|Präsentation||Crack growth measurement by optical observation and crack gauge||The figure describes the implemented crack length measurement methods and a representative a/N-curve.||363 KB||Download|