ICF13B

13th International Conference on Fracture June 16–21, 2013, Beijing, China Determination of fatigue crack propagation limit curves for high strength steels, and their applicability for structural elements having crack like defects János Lukács1,* 1 Department of Mechanical Engineering, University of Miskolc, H-3535 Miskolc-Egyetemváros, Hungary * Corresponding author: janos.lukacs@uni-miskolc.hu Abstract There are different documents and standards containing fatigue crack propagation limit or design curves and rules for the prediction of crack growth. The research work aimed to characterise the fatigue crack propagation resistance of different steels using limit curves, based on statistical analysis of test results and fatigue crack propagation law, and to determine fatigue crack propagation limit curves for different structural steels and wide-spreading high strength steels, and their welded joints, under mode I and mixed mode I + II loading conditions. Experiments were performed on different (high strength) steels and partially their welded joints. The specimens were cut parallel and perpendicular to the characteristic directions of the materials; therefore the specimens represent the different possible locations of cracks in the base materials and welded joints. Fatigue crack growth experiments were performed by ΔK-decreasing and constant load amplitude methods. The evaluation process consists of six steps and by means of the evaluated and selected values a simplified method can be proposed for determination of fatigue crack propagation limit curves. The determined limit curves represent a compromise of rational risk and acceptable safety. Keywords fatigue crack growth, limit curve, high strength steel, engineering critical assessment 1. Introduction Reliability of a structural element having crack or crack-like defect under cyclic loading conditions is determined by the geometrical features of the structural element and the flaws, the loading conditions, as well as the material resistance to fatigue crack propagation. There are different documents [1-3], standards and recommendations [4-6] containing fatigue crack propagation limit or design curves and rules for the prediction of crack growth [6, 7]. The background of the fatigue crack propagation limit curves and the calculations consist of two basic parts: statistical analysis of numerous experiments (fatigue crack propagation tests) and fatigue crack propagation law, frequently the Paris-Erdogan law [8], C K , dN da n = Δ (1) where da/dN is the fatigue crack growth rate, ΔK is the stress intensity factor range, furthermore C and n are material constants. The research work aimed − to characterise the fatigue crack propagation resistance of different steels using limit curves [9], [10], based on statistical analysis of test results and the Paris-Erdogan law; − determination of limit curves for different structural steels and wide-spreading high strength steels [11], and their welded joints, under mode I (tension) and mixed mode I + II (tension and shear) loading conditions. 2. Examinations 2.1. Materials and welding characteristics The most important characteristics of the investigated structural steels and high strength steels, and used welding technologies are summarized in Table 1. -1-

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