WEB A fracture mechanics-based approach to fatigue of additively manufactured maraging steel specimensTuesday (26.05.2020) 10:30 - 10:50 Room 2
Recently, the authors have investigated the fatigue strength of additively manufactured (AMed) maraging steel specimens, where the effects of building orientation and ageing heat treatment were analysed. The main outcome of this preliminary study highlighted the importance of adopting a local damage parameter due to the presence of defects within AMed materials. Since the defects can be considered as a short cracks-like, it is deemed to be more appropriate the use of Linear Elastic Fracture Mechanic (LEFM) approaches than the classical nominal approaches for the characterization of the fatigue behavior of AMed materials. Starting from this assumption, in the present contribution, new axial fatigue tests were carried out on AMed maraging steel specimens produced by two different AM systems (EOS Gmbh and SISMA SpA) as well as axial fatigue tests were carried out on wrought maraging steel specimens both in annealed and in aged condition. Stress Intensity Factor (SIF) based on the Murakami’s parameter √area was selected as LEFM parameter in which its evaluation was done after failure by analysing the √area of the killer defects observed on the fracture surfaces of all the AMed specimens. Furthermore, the SIF have been also corrected by the El-Haddad-Smith-Topper parameter a0 to take into account the influence of defects on the fatigue strength of the here considered material. Due to the lack of expensive experimental data for determine the relevant material length parameter a0, a novel rapid method to approximately evaluate a0 has been proposed. In particular, it consists in matching El-Haddad-Smith-Topper model with Murakami’s expression of the threshold range of mechanically short cracks. The advantage of the adopted engineering approach is that only Vickers hardness of the material is necessary. Stress intensity factor-based design curves were obtained for all the AMed test series, showing a reduction of the scatter index from 8% to 58% with respect to the nominal stress amplitude curves (an example is reported in the figure). At the end, the stress intensity factor-based design curve was also adopted to estimate the fatigue strength of sharp V-shaped notches characterized by a reduced notch opening angle.
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