The non-linear nature of the superelastic phase transformation in NiTi means that conventional fatigue life theory is difficult to apply. The volume fraction of martensite/parent phase and its role in the fatigue mechanism is not understood. This paper considers the fatigue life of superelastic NiTi alloys when tested at 2Hz in Ringers solution. In agreement with other researchers [1] [2] it was found that low alternating strains but high mean strain values resulted in the longest fatigue lives. To support the mechanical testing data, optical and electron microscopy was used to investigate the mechanism of fatigue and the role of the NiTi microstructure

Moffat, A.J.

Morgan, N.B.

Painter, J.

English

Southampton (e-Prints Soton)

MEAN STRAIN EFFECTS AND MICROSTRUCTURAL OBSERVATIONSDURING IN-VITRO FATIGUE TESTING OF NITIN.B. Morgan(1), J Painter(2), A Moffat(2)(1) UTI Star Guide, 5000 Independence Street, Arvada, Colorado 80002, USA(2) Cranfield University, Department of Materials and Medical Sciences, Shrivenham, Swindon, WiltshireSN6 8LA, UKABSTRACTThe non-linear nature of the superelastic phase transformation in NiTi means that conventional fatigue lifetheory is difficult to apply. The volume fraction of martensite/parent phase and its role in the fatiguemechanism is not understood.  This paper considers the fatigue life of superelastic NiTi alloys when testedat 2Hz in Ringers solution. In agreement with other researchers [1] [2] it was found that low alternatingstrains but high mean strain values resulted in the longest fatigue lives. To support the mechanical testingdata, optical and electron microscopy was used to investigate the mechanism of fatigue and the role of theNiTi microstructure.KEYWORDSNiTi, Fatigue mechanisms, Martensite, Fracture surfacesINTRODUCTIONFatigue life remains one the most discussed yet least understood aspects of NiTi alloys. The FDArequirement of a fatigue life exceeding 400 million cycles for intravascular stents means that a betterunderstanding of the factors affecting fatigue life and the mechanism of crack initiation and growth isessential [3]. Although cyclic deformation by bending is by far the most likely fatigue mode of in-vivo NiTistents, fundamental mechanical properties are often best obtained from tensile studies. In an attempt tobetter understand the fundamentals of NiTi fatigue effects this paper we study uniaxial tensile fatigue dataand consider how imposed mean strains and testing temperatures affect the fatigue life. In addition, opticalmicrography and fractography will be employed to study the crack initiation and growth mechanism.EXPERIMENTALFatigue testing was carried out with specimens immersed in a body fluid simulant known as Ringerssolution that was maintained at a 38ºC using a digital water heater. All testing was performed under straincontrol using an extensometer on wires of Ti-55.9wt%Ni, straight annealed. The Af temperature of the alloyas measured by DSC was 12ºC. The wire surface was in the as-pickled condition. Fatigue data was collected at a frequency of 2Hz for three different means strains: 2%, 4%, 6% and sevendifferent alternating strain amplitudes: 0.5%, 0.75%, 1.0%, 1.5%, 2.0%, 2.5% and 3.0%. In addition to the38ºC tests the experiment was repeated at a temperature of 20ºC for a mean strain of 6% and alternatingstrain amplitudes of: 0.5%, 0.75%, 1.0%, 1.5%, 2.0%, 2.5% and 3.0%.RESULTSAlthough the data is sparse and only preliminary Figure 1 is consistent with the following observations: Athigh strain amplitudes, the mean strain has little effect upon fatigue life. However, at low strain amplitudes(1.0% and less) increased mean strain is consistent with a significant increase of fatigue life. Figure 2 plots the number of cycles to failure for 6% mean strain specimens cycled at two differenttemperatures, 38ºC and 20ºC; the data is consistent with the following observations: At high strainamplitudes the mean strain has little effect upon fatigue life. However, at 1.0% alternating strain thesamples cycled at 20ºC yield the greatest fatigue life whilst at 0.5% alternating strain the samples cycled at38ºC result in the highest fatigue life. It should be noted that these results are preliminary and that a morestatistically significant series of tests is required to reach firm conclusions.00.511.522.533.51.0E+03 1.0E+04 1.0E+05 1.0E+06 1.0E+07Cycles to Failure (N)Alternating Strain (%)2% Mean Strain4% Mean Strain6% Mean StrainFigure 1: Cycles to failure of NiTi samples cycled in tension at 38ºC. All data points are fractures, i.e.none of the samples exceeded the cycles to failure shown above00.511.522.533.51.0E+03 1.0E+04 1.0E+05 1.0E+06 1.0E+07Cycles to Failure (N)Alternating Strain (%)Nf 38degCNf 20degCFigure 2: Cycles to failure of NiTi samples cycled in tension at a mean strain of 6.0%. All data points arefractures, i.e. none of the samples exceeded the cycles to failure shown aboveWhen discussing the mechanisms behind these fatigue effects it is worth considering the stress/strainrelationship of the mean strains and alternating strains. Figure 3 shows an idealized diagram of how thestress/strain hysteresis profiles of the samples cycled at an alternating strain of 1.5% and the samples cycledat 0.5% mean strain fit into a full superelastic transformation cycle.Figure 3: An idealized diagram of how the stress/strain hysteresis profiles of the samples cycled at analternating strain of 1.5% and the samples cycled at 0.5% mean strain fit into a full superelastictransformation cycle.At higher alternating strains the fatigue cycles rely upon considerable volumes of material transformingbetween the martensite and parent phase and will necessarily create dislocations within the structure whichmay ultimately lead to shorter fatigue lives. In addition to dislocations, the relatively high alternating strains may create defects due to discontinuitiesand inclusions within the microstructure. Figure 4 is an example of the type of fracture surfaces observed athigh alternating strains. Figure 4a is typical of a conventional fatigue failure involving high nominal stressand low to moderate stress concentration. However, in this case the wires were actually experiencing highnominal strains rather than high nominal stresses. The fatigue crack has initiated at a TiC inclusion at thesurface of the wire (Figure 4b) and the area of final fast fracture (ductile) is comparatively large. 0 1 2 3 4 5 6 7 8% StrainStress0 1 2 3 4 5 6 7 8% StrainStress0 1 2 3 4 5 6 7 8% StrainStress    (a) (b)Figure 4: An example of the fracture surfaces observed at high alternating strains. The wire diameter is0.65mm and the inclusion size approximately2µm.At low alternating strains the volume of transforming material is much less and the mismatch between thestrain in the NiTi and inclusions is much lower. Figure 5 is typical of the fracture surface observed at lowalternating strains. In comparison to Figure 4 the area of crack growth is much larger and the area of ductilefast fracture much smaller. This is typical of fatigue failure where the nominal stress is low but the stressconcentration is high. However, in this case the wires were actually experiencing low nominal strains ratherthan stresses. Unlike those samples cycled at high alternating strains the point of initiation at low alternatingstrains was diffuse and could not be traced back to inclusions.  The shear lips present on the fractured wireimply a crack front that is partially in tensile mode and partially in shear mode. Figure 5: An example of the fracture surfaces observed at low alternating strains.Figure 6 is a longitudinal view of a wire cross-section that was cycled at low alternating strain but did notbreak. The open crack tip is typical of the early stages of fatigue crack growth resulting from a highlydislocated structure.(a) (b)Figure 6: (a) Longitudinal view of a wire cross-section that was cycled at low alternating strain but did notbreak. The open crack tip is typical of the early stages of fatigue crack growth resulting from a highlydislocated structure. (b) The three stages of fatigue crack initiation, growth and failure.A cross-sectioned view of a fracture surface from the low alternating strain failures shows the classiccharacteristics of fatigue crack propagation. Stage I represents initial crack growth resulting from a highlydislocated structure. Stage II is the crack propagation approximately normal to the tensile axis whichextends over a much greater distance than Stage 1. Stage II corresponds to the crack growth area in Figure 5and is characterized by fatigue striations on the fracture surface.  Stage III is final fast fracture. Stage IIIcorresponds to the area of final fracture in Figure 5 and is characterized by micro-void coalescence leadingto ductile overload.An interesting feature of the low alternating strain fracture surfaces is microcracking. Figure 7a is typical ofthis cracking. Many of these cracks appear to be associated with a void resulting in a ‘comet’ typeappearance, Figure 7b. In addition, the fracture surface morphology at the crack tips is consistent withmartensite. Specifically, the morphology of the martensite is that sometimes described as a ‘herring-bone’type. Herringbone martensite has been shown to consist of four correspondence variants.      (a) (b)Figure 7: (a) Microcracking in the fracture surface of a NiTi sample that failed in tension fatigue. (b)Stabilzed martensite at microcrack tips.It was noted that the frequency of microcracking and occurrence of martensite at the crack tips actuallyincreased with mean strain. In other words the higher the mean strain, the greater the amount of stabilizedmartensite in the fracture surface. Figure 8 is a surface response plot constructed from the same preliminary data shown in Figure 1. It isplotted for alternating strain versus mean strain. For alternating strains above 1.5% mean strain appears tohave little effect. The fracture surfaces indicate that failure is controlled by surface inclusions and highnominal alternating strains.Below 1.5% alternating strain, the level of mean strain has a much greater effect. The increase in fatigue lifewith mean strain is consistent with lower nominal alternating strains and ‘classic’ crack initiation andgrowth. The mechanism behind the apparent increase of fatigue life (at low alternating strains) with mean strainlevel appears to be associated with microcracking and stress induced martensite. The samples tested athigher mean strains result in larger amounts of stress induced martensite. In addition, the microcracks in thesamples cycled at higher mean strains appear to be blunted by localized martensite. Figure 8: Surface response plot for alternating strain Vs. mean strain.  It is known that certain materials exhibit increased fatigue life due to phase changes ahead of the crack tip.The phase change absorbs energy and effectively toughens the material. Figure 9 describes a possiblemechanism for the longer fatigue lives and crack blunting. The advancing crack tip increases the stressintensity to a level where martensite is induced ahead of the crack tip and stabilized by residual stresses andplastic deformation. The results confirm both the importance of transformation strain and phase state withinthe material during fatigue loading.  11.522.53 24601000020000300004000050000600007000080000Cycles to Failure (N)Alternating Strain (%)Mean Strain (%)MartensiteAdvancingCrack TipMartensiteAdvancingCrack TipFigure 9: The stabilization of martensite ahead of the crack tipThe effect of testing temperature on the fatigue life of the alloys is counterintuitive. As the testingtemperature is increased so the stress required for transformation increases. With this in mind it isreasonable to assume that the fatigue life would decrease with testing temperature. Figure 2 shows that athigh alternating strains this is possibly true. However, below 1.0% alternating strain the specimens cycled atthe higher temperature display the longer fatigue lives. It should be noted that this data is preliminary and assuch no firm statistical conclusions should be drawn, however the results are consistent with such a theory.Consistent with the increased fatigue life due to higher mean strains, it is possible that the highertransformation stress at the higher temperature results in larger amounts of stabilized martensite and/or anincreased rate of stabilization. Figure 10 supports this theory. Martensite variants can be seen on theunetched surface of the wire after fatigue testing. The variants appear to be of the same type, i.e. they are allstabilized in the same orientation. Figure 10: Stabilized martensite variants on the surface of NiTi wire after fatigue testingCONCLUSIONS The results are consistent with high mean strains and low alternating strains resulting in the highestfatigue lives High alternating strain failures tend to be associated with inclusions within the alloy Crack initiation points for low alternating strain failures tend to be more diffuse and are not usuallyassociated with inclusions Higher mean strains resulted in higher amounts of martensite in the fracture surface Extended fatigue lives appear to correspond to increasing amounts of stabilized martensite variants Microcracking within the fracture surface appeared to be associated with stabilized martensiteimplying a possible blunting/energy absorption mechanism prolonged fatigue lives REFERENCES1. W.J. Harrison and Z.C. Lin in SMST-2000: Proceedings of the Third International Conference onShape Memory and Superelastic Technologies (S.M. Russell and A. Pelton, eds.), Pacific Grove,California, USA, 2000, p. 391.2. T. Duerig, A. Pelton and D Stöckel, Mat. Sci. and Eng. A, Vols. 273-275, 1999, p. 149.3. A.R. Pelton, J. DiCello and S Miyazaki in SMST-2000: Proceedings of the Third InternationalConference on Shape Memory and Superelastic Technologies (S.M. Russell and A. Pelton, eds.),Pacific Grove, California, USA, 2000, p. 361

Mean strain effects and microstructural observation during in-vitro fatigue testing of NiTi

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