PSI - Issue 42

Jochen Tenkamp et al. / Procedia Structural Integrity 42 (2022) 328–335 Jochen Tenkamp / Structural Integrity Procedia 00 (2019) 000 – 000

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3.3. Fatigue damage tolerance The fatigue damage tolerance from LCF to HCF regime can be estimated by Shiozawa curves. Fig. 5a shows a stress intensity factor (SIF) based Shiozawa diagram (linear-elastic fracture mechanics). Hereby, SC-10 and SC-7 have a comparable fatigue damage tolerance (FDT) and an increased FDT to AM-10. The improvement of SC alloys is in the same range of the difference in relative S-N diagram in Fig. 4b. As the FDT correlates with the cyclic YS of the materials ( p ′ 0.2 ), the effect of process- and microstructure-induced CSS behavior was further evaluated by elastic-plastic fracture mechanics using J integral based SIF (short: J-SIF) as crack propagation driving force in the modified Shiozawa diagram in Fig. 5b. By considering the CSS behavior, the differences in Shiozawa curves between AM-10, SC-10 and SC-7 could be significantly reduced. Although, there is still a slight gap between AM and SC materials, a unified and reliable FDT assessment would be possible.

Fig. 5. (a) SIF based Shiozawa and (b) J-SIF based Shiozawa diagram of sand cast (SC) and additively manufactured (AM) alloys.

Moreover, a uniform FDT limit of around 0.90 MPa  m could be identified for all Al-Si alloys. The FDT curves and limit could be further used to generate synthetic S-N curves or Haigh curves for variations of initial defects and CSS properties. Therefore, modified Shiozawa diagrams can be used to quantify and isolate the effects of defects, CSS properties as well as stress ratio or crack opening effects. This enables a fundamental understanding of dominating fatigue mechanisms and simplifies the fatigue design of Al-Si alloys considering the material-specific defects and plasticity characteristics. 4. Conclusion and outlook Fracture mechanics have been successfully evaluated for additively manufactured (AM) and cast Al-Si alloys. The  area concept of Murakami including the light metal extension of Noguchi allowed a sufficient estimation of fatigue limit or fatigue strength at 1E7 cycles for sand cast alloys AlSi10Mg and AlSi7Mg, while the fatigue limit was overestimated for AM alloy AlSi10Mg. The assessment of fatigue damage tolerance (FDT) by Shiozawa diagram based on linear-elastic fracture mechanics allowed a defect-corrected assessment of the fatigue performance of each Al-Si microstructure. Sand cast alloys showed a comparable FDT, which was significantly increased compared to AM alloy. By modification of Shiozawa concept using effective J integral, the plasticity properties or cyclic stress-strain behavior could be identified as a dominating influence on the FDT. The higher the cyclic yield strength, the higher the FDT of Al-Si alloys. The slight difference in J integral based Shiozawa curves need to be further understood by characterization of the microstructure-related crack opening behavior as well as crack propagation mechanisms. Herewith, a unified damage tolerant design concept for Al-Si alloy can be further improved and transferred to industry applications.