WEB Evaluating the defect tolerance of nano-structured, Cu alloyed steelsThursday (28.05.2020) 09:50 - 10:10 Room 2
For a reliable design of safety relevant components, a sound knowledge of the material’s mechanical and especially cyclic properties is indispensable. Numerous investigations have demonstrated that microstructural defects, i.e. pores or nonmetallic inclusions, have a huge impact on fatigue lifetime [Mura02, Bamb16]. Therefore, the defect type and size, as well as the material’s defect tolerance have to be considered. For the evaluation of defect tolerance, Murakami’s √area approach [Mura02] is commonly used. However, previous investigations [Bamb16] have shown that instrumented cyclic indentation tests (CITs), enable a more efficient analysis of the material’s defect tolerance.
Moreover, the results in [Bamb16] demonstrate that the cyclic hardening potential of steels can be increased by Cu precipitates, resulting in a higher ability of the material to counteract stress intensities at microstructural notches. This improves the defect tolerance and hence, the fatigue lifetime.
To get a deeper understanding of the effect of Cu precipitates on defect tolerance, different heat treatment states of Cu alloyed steels were investigated by means of CITs, enabling the identification of material conditions with sufficiently different defect tolerance. For all conditions, specimens with artificial defects were characterized in fatigue tests to explicitly evaluate the defect tolerance in dependency of Cu precipitation state. These artificial defects had significantly different sizes, enabling the determination of the Kitagawa-Takahashi diagram [Zerb16] by considering the fatigue strength of defect free specimens.
Besides of utilizing Murakami’s √area concept [Mura02], different aspects are analyzed in the present work to evaluate the materials’ defect tolerance: i) reduction of fatigue lifetime at a given stress amplitude and defect size, ii) reduction of fatigue strength for a defined defect size and iii) critical defect size as well as iv) threshold stress intensity factor ΔKth for fatigue crack propagation, both determined by using the Kitagawa-Takahashi diagram. By using these different approaches for evaluating the defect tolerance, a significant influence of Cu precipitates on defect tolerance is found. Moreover, a higher cyclic hardening potential, determined in CITs, correlates excellently with an increased defect tolerance according to Murakami’s approach.
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