Present Understanding of Fatigue in Bulk NanoMaterials

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Outline . Fatigue life approachRole of strengthening mechanisms in fatigueFatigue mechanismsCyclic softeningShear bandingmicrocrackingFatigue crack growth resistance in ultra-fine grain and nano-materials Modeling of Fatigue . . Fatigue Life. - Coffin-Manson law. . . . . . . . . - Basquin law.

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Present Understanding of Fatigue in Bulk NanoMaterials

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2. Outline Fatigue life approach Role of strengthening mechanisms in fatigue Fatigue mechanisms Cyclic softening Shear banding microcracking Fatigue crack growth resistance in ultra-fine grain and nano-materials Modeling of Fatigue

3. Fatigue Life

4. Fatigue Life Diagram At small strains the elastic component determines the fatigue life STRENGTH At large strains the plastic component is dominant DUCTILITY

5. Enhanced Ductility in Bulk Nano-Structured Metals

6. How to Enhance the High Cycle Fatigue Performance?

7. Mechanisms of Strengthening Grain size reduction Lattice dislocation accumulation (strain hardening) Solid-solution strengthening Precipitation strengthening

10. Equal Channel Angular Pressing 1975, V.M.Segal, Minsk, USSR

11. Microstructure

13. Fatigue Limit Remarkable improvement of fatigue limit can be achieved after grain refinement by SPD Crack initiation

16. Comparison of Fatigue Performance of Nano-crystalline and Ultra-Fine Grain Materials No significant improvement of fatigue properties in the Nano-structured state

17. Precipitation hardening Solid solution and precipitation hardening is very effective to improve the fatigue strength

18. Low Cycle Fatigue LCF properties of UFG or Nano-materials are lower than those of their conventional coarse grain counterparts (under constant plastic strain amplitude) Limited ductility

19. Crack Growth Rate

20. Deformation mechanisms Primary role of dislocation activity

21. Hysteresis Loop Modeling U.Essmann and H.Mughrabi, (1979), H.Mughrabi (1988) One-parametric model for dislocation kinetics

22. Conclusions Extreme grain refinement down to the sub-micron and nano-scopic scale improves the fatigue performance substantially. Limited ductility of nano-materials determines the lower than expected low-cycle fatigue properties and crack growth resistance. The full potential of achieving favourable combinations of grain refinement and other strengthening factors for further enhancement of fatigue performance has certainly not been explored and exploited in nano-materials. A better understanding of the specific mechanisms underlying the response of these materials to cyclic loading may lead to microstructures optimised with respect to the fatigue performance and overall mechanical behaviour. Control over impurity content and a judicious choice of a chemical composition and thermal treatment has the highest potential for the utmost high-cycle fatigue improvement

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