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A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 2 1 4 9 metals [13,14] . These fatigue cracks on the compressive surface in ductile engineeringmaterials will arrest as a result of crack closure effects from crack progression outside the residual tensile zone and into the compressive stress field [13,14] . The extent to which these residual tensile stress fatigue cracks grow is directly correlated to the magnitude of the far-field compressive load; i.e., higher compressive loads will result in longer crack arrest distances [12,13] . Figures 4-6 are a collection of BSE micrographs from a fractured Nitinol wire apex fatigue specimen in which crack initiation, growth, and arrest occurred on a bend intrados (inner curve) that was intentionally damaged by compres- sive bending. The sample was first crimped to 12% (tensile) strain on the extrados (outer curve) with a corresponding −15% (compressive) strain on the inner curve. The sample was then allowed to expand and was cycled at a strain am- plitude of 1.2% and a mean strain of 1.5% (outer curve) to 10 million cycles. All loading strains mentioned for this sam- ple were all calculated from FEA. This wire apex sample sur- vived fatigue testing and was subsequently inspected for cracks on the intrados. Arrested fatigue cracks are visible in Fig. 4, which is an SEM micrograph of the wire apex inner bend surface. The sample was then manually fractured to expose the surface of the arrested fatigue crack. An overview of the fracture surface is shown in Fig. 5. An arrested thumb- nail crack, measuring approximately 44 μm in radial depth, was present at the apex intrados. Further inspection of the arrested crack in Fig. 5 reveals characteristic signs of Nitinol fatigue fracture at this location including “feathering” lines that radiate back to a near-surface nonmetallic inclusion at the origin. Unique to this fracture surface, the arrested fatigue crack transitions into the overload area at a sharply defined ridge. Lastly, a shear lip with microvoid coalescence is observed at the area of final fracture in Fig. 6. ~SMST Note: This article is an excerpt from “Fractography of Ni- tinol,” Fractography, Volume 12, ASM Handbook, to be published in 2022. Visit the ASM Digital Library for more in- formation, dl.asminternational.org . For more information: Louis G. Malito, senior engineer, Ex- ponent Inc., 149 Commonwealth Dr., Menlo Park, CA 94025, 650.688.7018, lmalito@exponent.com , exponent.com. References 1. S.W. Robertson, et al., Evolution of Crack-tip Transforma- tion Zones in Superelastic Nitinol Subjected to in situ Fatigue: A Fracture Mechanics and Synchrotron X-ray Microdiffraction Analysis, Acta Mater., 55, p 6198–6207, 2007. 2. P. Adler, et al., Effects of Tube Processing on the Fatigue Life of Nitinol, Shape Mem. Superelasticity, 4, p 197–217, 2018. 3. H. Cao, et al., The Influence of Mean Strain on the High-cy- cle Fatigue of Nitinol with Application to Medical Devices, J. Mech. Phys. Solids, 104057, 2020. 4. M. Launey, et al., Influence of Microstructural Purity on the Bending Fatigue Behavior of VAR-melted Superelastic Nitinol, J. Mech. Behav. Biomed. Mater., 34, p 181–186, 2014. 5. A.R. Pelton, S.M. Russell, and J. DiCello, The Physical Metallurgy of Nitinol for Medical Application, JOM, 2003, doi. org/10.1007/s11837-003-0243-3. 6. ASTM F2063, Standard Specification for Wrought Nickel- TitaniumShape Memory Alloys for Medical Devices and Surgi- cal Implants, Annu. B. ASTMStand., 1–6, 2012, doi.org/10.1520/ F2063. 7. C.D. Beachem, Microscopic Fatigue Fracture Surface Fea- tures in 2024-T 3 Aluminum and the Influence of Crack Pro- pogation Angle upon their Formation, ASM Trans. Quart., 60, 1967. 8. L. Engel and H. Klingele, An Atlas of Metal Damage: Surface Examination by Scanning Electron Microscope. Prentice Hall, 1981. 9. T.W. Duerig, Some Unsolved Aspects of Nitinol , Mater. Sci. Eng., A 438–440, p 69–74, 2006, doi.org/10.1016/j.msea. 2006.05.072. 10. K. Gall and H. Sehitoglu,, Role of Texture in Tension- compression Asymmetry in Polycrystalline NiTi, Int. J. Plast., 15, p 69–92, 1999, doi.org/10.1016/S0749-6419(98)00060-6. 11. B. James, et al., Compressive Damage-Induced Crack- ing in Nitinol, in: SMST-2004: Proceedings of the International Conference on Shape Memory and Superelastic Technologies, p 117, 2006. 12. B. James, S. Murray, and S. Saint, Fracture Characteriza- tion in Nitinol, in: SMST-2003 Proceedings of the International Conference on Shape Memory and Superelastic Technologies, p 321–324, 2004. 13. C.N. Reid, K. Williams, and R. Hermann, Fatigue in Com- pression, Fatigue Fract. Eng. Mater. Struct., 1, p 267–270, 1979. 14. S. Suresh, Crack Initiation in Cyclic Compression and its Applications, Eng. Fract. Mech., 21, p 453–463, 1985. 1 3 FEATURE