FATIGUE PERFORMANCE OF NITINOL TUBING'WITH AEOF 25°C
`Tracy Lashley Lopes, Xiao-Yan Gong, and Christine 'Irépanier
`Nitinol Devices &. Components, 47533 Westinghouse Drive, Fremont, CA 94539
`
`ABSTRACT
`The purpose of this study is to assess the fatigue behavior of Nitinol tubing aged at different tem—
`peratures 850-500" C) and times to achieve a 25° C i2” C Af. Fatigue properties were determined
`in tension at a mean strain of 1.5% and alternating strain magnitudes of 0.2%, 0.3%, and 0.4%.
`Although there are insufficient data to draw statistical conclusions from this preliminary investiga-
`tion, it is important to point out that there did not appear to be any major differences in the samples *
`tested despite the differences in microstructure and therefore in plateau strength. Still, among the
`heat-treatment temperatures evaluated, 500°C produces the smallest amount or” R—phase with the
`most fatigue-stable microstructure, and therefore is the recommended choice for good reproducible
`device performance.
`
`KEYWORDS
`
`Tension, Fatigue, Mechanical Properties, Af, Aging
`
`INTRODUCTION
`Like many metallic biomaterials, Nitinol’s mechanical properties are determined by the thermome—
`chanical processing of the alloy. In addition, Nitinol’s mechanical behavior at body temperature is
`strongly dependent upon its transformation temperatures [1,2]. Several implant devices, such as
`self—expanding Nitinol stents, rely on the superelastic properties of the material, which are affected
`by the Af transformation temperature of the alloy [3]. As a result, optimization of the mechanical
`properties of Nitinol for medical device applications is achieved by using various aging treatments
`to target a specific Af transformation temperature. Aging treatments in the 350 to 500° C range are
`routinely used to adjust the transformation temperatures of Nitinol medical devices [1]. Small
`changes in time and temperature have shown to have little effect on the performance of the device.
`
`311
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`Edwards Exhibit 1021, p. 1
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`312
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`Although Nitinol’s fatigue properties have been previously investigated [4], few studies have been
`conducted on the effect of aging'on fatigue properties. Therefore, the purpose of this study is to
`assess the fatigue behavior of Nitinol tubing aged at different temperatures and times to achieve a
`25° C i2“ C Af.
`
`EXPERIMENTAL METHODS
`
`-
`
`-
`-
`
`-
`
`-
`
`As—received Ni50_gTi49'2 Nitinol tubing of 0.072 in. Outer Diameter (OD) by 0.051 in. Inner
`Diameter (ID) was measured using the bendfifree recovery method [5] to determine the initial
`Af temperature prior to any heat treatments.
`6-in. pieces of the tubing were heat treated to the times and temperatures listed in Table 1.
`One sample of each group was then measured using the bend-free recovery method to
`determine the final Af temperature (see Table 1). Another sample from each group was further
`characterized by Differential Scanning Calorimetry (DSC) [6].
`To avoid sample breaking near or inside the grip during the fatigue test, the outside diameter of
`the tubes was electropolished such that the gauge section had a slightly smaller outside
`diameter of 0.071 inches [7].
`The mechanical testing was performed on a MTS 858 Mini Bionix test system. A 2000N
`pneumatic grip system with MTS serrated grip surfaces of dimension 38mm by 58mm were
`used in the test. The test consists of a loading and unloading sequence from 0 to 6% strain and
`then a fatigue test at 1.5% mean strain with cut-Off cycle counts of one million, i.e., if the
`samples do not break at one million cycle counts, the test is stopped. The 1.5% mean strain is
`consistent with the in viva mean strain that SMART® stents experience [8]. Alternating strain
`magnitude for fatigue tests Were chosen to be 0.2%, 0.3% and 0.4%. All mechanical testing
`was performed at 37° C using circulated warm air.
`
`Table 1 Temperature, Time and Average Final Afof the Samples Used in the Study
`
`
`
` Heat Treat Temperature C” C) Film? (min) Final Af(° C)
`
`350
`35
`23
`
`
`
`
`400
`
`8
`
`l
`
`27
`
`450
`5
`26
`
`
`25
`90
`500
`
`
`We evaluated the following properties: (1) Af temperature after each heat treatment using the bend-
`free recovery method, (2) transformation temperatures using DSC testing, and (3) fatigue life and
`loading/unloading plateaus after aging.
`
`‘
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`
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`i
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`Edwards Exhibit 1021, p. 2
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`

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`FATLGUE PERFORMANCE OF NITINOL TUBING WITH AF 0F 25°C 313
`
`RESULTS AND DISCUSSIONS
`
`Mechanical and Thermal Properties
`
`
`600
`
`500
`
`1.5% Strain
`
`(MPa)
`
`Stress
`
`0
`
`J__
`
`I
`
`J—
`
`0%
`
`1%
`
`2%
`
`3%
`
`4%
`Strain
`
`5%
`
`6%
`
`7%
`
`Figure 1 Stress-strain graph osz'tinol tubing heat treated at 450° C tested at 37° C.
`
`The typical stress«strain responses of the tubing after the 450° C heat treatment process at 37° C is
`presented in Figure l. The 1.5% mean strain used in the fatigue experiments is indicated on the fig—
`ure. With this lower heat—treatment temperature, the tangential stiffness decreases at lower strain,
`which indicates the presence of a significant fraction of the R—phase. This is further confirmed with
`the DSC tests performed on the same heat-treated sample (Figure 2). There is a known variation
`between the bend-free recovery method and the DSC method when determining the Af temperature
`due to the influence of strain. It is important to note that the DSC results were used as reference
`only and that all Af temperatures were set using the bend~free recovery method.
`
`Samples heat treated at 500° C (Figure 3), have a noticeable increase in the tangential stiffness over
`those heat treated at 450° C. This indicates that the R—phase is less of a factor at this temperature as
`indicated in Figure 4. The 1.5% mean strain for this heat treatment corresponds to the approximate
`onset of the plateau stress.
`
`Figures 5 and 6 show the stress—strain responses of samples heat treated at 350° C and 400° C before
`and after the 106 cycle fatigue tests. As can be seen in these figures, it appears that the displacement
`cycling modified the tangential stiffness by eliminating the R—phase inflection. This indicates that
`the R—phase interacted with the fatigue-induced dislocations thereby stabilizing the structure. Addi—
`tional mechanical and DSC testing is required to verify these observations.
`
`The plateau stress as a function of the hea~ treat temperature is plotted in Figure 7. Clearly there is
`a nonlinear relationship between plateau stress and aging temperature used to obtain an Afof 25° C.
`It is interesting to note that these thermal treatments led to a plateau stress range from 300 MP3 to
`over 500 MPa.
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`Edwards Exhibit 1021, p. 3
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`314
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`10
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`a.
`,2#1
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`-57
`
`m
`
`-10
`
`
`
`
`
`-15 ,
`450
`
`.
`-100
`
`.
`t
`O
`-50
`Temperature (”C)
`
`.
`50
`
`100
`
`Figure 2 DSC results ofNitinol tubing heat treated at 450° C.
`
`
`
`400
`
`350 ,
`
`300 ~
`
` 1.5% Strain I
`
`
`
`
`
`M01O
`
`
`
`Stress(MPa) MOO
`
`_x UTCD
`
`100 ,
`
`50
`
`O
`
`0%
`
`l__l_
`1%
`
`|
`2%
`
`l
`3%
`
`—L
`4%
`Strain
`
`_I_
`5%
`
`6%
`
`7%
`
`Figure 3 Stress—strain graph ofNitinol tubing heat treated 511500” C tested at 37° C.
`
`Fatigue Properties
`
`Fatigue testing 011 thin—walled tubes is nontrivial as pointed out by Tabanli at al. [9]. Sample fail-
`ures inside or ve1y close to the grips are still possible even with the proper gauge section reduction.
`Grip failures are normally attributed to excessive load from the glip; therefore these fatigue data
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`Edwards Exhibit 1021, p. 4
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`

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`.
`100 -
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`FATIGUE PERFORMANCE OF NITINOL TUBING WITH AF 0F 25°C 315
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`
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`HeatFlow(mW)
`
`—150
`
`-100
`
`—50
`
`0
`
`50
`
`100
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`Figure 4 DSC results Nitinol tubing heat treated at 500° C.
`
`Temperature (°C)
`
`600
`
`500
`
`J:C} O
`
`Original
`
`After Fatigue Test
`
`/
`
`
`
`Stress(MPa) co0CD
`
`[‘0O C)
`
`0
`
`I
`
`I
`
`0%
`
`1%
`
`2%
`
`3%
`
`4%
`
`5%
`
`6%
`
`Strain
`
`7%
`‘
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`Figure 5 Stress/strain graph ofNitz‘nol tubing heat treated at 350° C before and afterfatigue test.
`
`were not included in the analysis. Figure 8 summarizes the fatigue—tests results for samples that
`either broke at the test section or had run out at 106 cycle counts. There was no tubing failure with
`0.2% half—alternating strain for any of the aging temperatures. At 350°C and 400° C, there were
`two samples with run out at 0.4% and 0.2% half-alternating strain magnitudes, yet there was a
`
`‘
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`i
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`E
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`
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`1'
`
`1
`
`1
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`Edwards Exhibit 1021, p. 5
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`316
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`
`
`600
`
`500 ,
`
`
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`
`
`After Fatigue Test
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`
`
`Original
`\
`
`
`
`Stress(MPa)
`
`AC)C
`
`0:OO
`
`MO0
`
`£00
`
`0
`
`/
`
`
`
`
`
`0%
`
`1%
`
`2%
`
`3%
`
`4%
`Strain
`
`5%
`
`6%
`
`7%
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`Figure 6 Shem/strain graph ofNitinol tubing heat treated at 400° C before and afierfatigue test.
`
`600
`
`
`,.
`
`i
`
`01DO
`
`4:.CD0
`
`300 -
`
`
`
`
`
`
`
`
`
`LoadingPlateauStress(MP3)
`
`200
`
`
`'
`'
`'
`'
`'
`
`300
`
`350
`
`400
`
`450
`
`500
`
`550
`
`Aging Temperature ('C)
`
`Figure 7 Plateau Stress as a. function ofaging temperature for a constant Af = 25° C.
`
`break at 0.3% half-alternating strain. A repetitive test for an additional sample aged at 400° C also
`had run out at 0.2% half—alternating strain. At 450° C, two samples broke at 0.4% at the test section,
`but no break was observed at 0.2% and 0.3% half—alternating strains. At 500° C, there was a sample
`
`Edwards Exhibit 1021, p. 6
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`

`

`I 500‘0 run out
`
`
`
`
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`AlternatingStrainAmplitude
`
`0.35 -
`
`0.30 -
`
`0.25
`
`0.20 '
`
`0.15
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`FATIGUE PERFORMANCE OF NlTINOL TUBING WITH AFOF 25°C 317
`
`
`
`0.45
`
`0.40
`
`O 350'C run out
`A 400'6 run out
`
`I450‘C run out
`
`
`
`104
`
`’IU6
`105
`Cycies to Failure
`
`107
`
`Figure 8 Fatigue results of all tested tube samples.
`
`with run out at 0.2% half—alternating strain; the remaining four samples broke at 0.3% and 0.4%
`half alternating strains. A summary of the total tested samples is listed in Table 2. Typical fatigue
`failures of the tested tubes are shown in Figure 9.
`
`It is very important to point out that 1.5% mean strain was chosen also to coincide with an onset of
`stress—induced martensite for 500" C heat-treatment conditions {in bending). However, at the lower aging
`conditions, 1.5% is still nominally on the linear—elastic portion of the stress—strain curve even though the
`Afwas set to 25° C for all samples. Surprisingly, these initial results indicate that there are no significant
`differences in fatigue properties even though there are differences in material microstmctures.
`
`Figures 10a and b show typical SEM photos of a broken section. These images show typical fatigue
`fracture surfaces. It is worth mentioning that the fracture surfaces are nearly identical for all the
`eight broken samples. All samples indicated that fatigue fractures initiate from the ID of the tubing.
`This implies that the electropolishing treatment on the OD of the tubing sample reduces the surface
`flaws and defects on the tubing 0D and therefore increased the fatigue resistance on the OD sur-
`face. Conversely, since the ID surface was not electropolished, it is possible that ID surface defects
`may have played a role in decreasing the fatigue life of the specimens and may have contributed to
`some of the scatter in the data.
`
`CONCLUSIONS
`
`The scatter in the fatigue data makes it difficult to compare the fatigue characteristics of the tubing
`at different heat—treatment temperatures. Yet the data indicate that 0.2% seems to be the one million
`cycle fatigue endurance limit for the tubing aged at any temperature between 350°C and 500° C,
`consistent with previously reported tension~tension fatigue results for Nitinol microtubing [9].
`Although there are insufficient data to draw statistical conclusions from this preliminary investiga-
`
`Edwards Exhibit 1021, p. 7
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`

`

`
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`318
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`Table 2 Fatigue Testing Summary
`
`Heat Treat Temperature (° C)
`
`Alternating Sttain (%)
`
`Cycles to Failure
`
`
`
`
`
`
`
`
`Run out
`
`
`Run out——
`
`—_
`—_
`——
`
`
`9.7 x 105
`
`
`
`
`
`
`
`
`
`
`
`
`
`0.4
`
`0.2
`
`6.8 x 104
`
`4.3 x 104
`
`4.6 x 104
`
`9.9 x 105
`
`tion, it is noteworthy that there did not appear to be any major differences in the samples tested
`despite the differences in material’s microstructure and therefore in plateau strength. As such, it
`may be speculated that the magnitude of the alternating load is less important than the alternating
`strain as reported by Gong et al [10]. Furthermore, it is important to note that all the fatigue frac—
`tures occurred on the ID of the tube samples, which was not surface treated. This may have played
`a role in the data scatter that was observed. Due to the time limitation and the complexity of the
`nature of fatigue, it is too early to draw any further conclusions from the results presented in this
`paper. Additional tests are in progress and the results will be analyzed accordingly to address the
`fatigue tolerance of the tubing aged through different heat treatments.
`
`It is also interesting to point out the effects of cyclic loading on the mechanical properties as
`shown in Figures 5 and 6. This can lead to different responses of a device during its service life,
`which is clearly an important design consideration for implantable devices. Therefore among the
`
`Edwards Exhibit 1021, p. 8
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`

`

`
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`_ _.—— 7
`.QSD IDIJIIm once iE‘ 4s: 35]
`
`..
`1‘
`
`_
`._._‘ '7
`.9 sebum Euflfl’ihl
`
`7
`4-:- e-EI
`
`Figure 10 (a) SEM image offiucture surface ofthe 350° C sample and (17) SEM image offracture
`swface of the 350° C sample
`
`heat treatment temperatures evaluated, 500° C produces the smallest amount of R—phase with the
`most fatigue-stable microstructure and is the recommended choice for a good reproducible
`device performance.
`
`ACKNOWLEDGMENTS.
`
`The authors would like to thank Dung Pham, Mike Connally and Rose Marie Ramirez of Nitinol
`Devices and Components for their assistance in sample preparation and mechanical testing. Special
`thanks also to Dr. Alan R. Pelton for inspirational discussions and providing the scientific material
`background knowledge.
`
`REFERENCES
`
`1. AR. Pelton, J. DiCello, and S. Miyazaki, MITAT 9, no. 2 (2000), p. 107.
`
`2. T.W. Duerig and R. Zadno, in Engineering Aspects ofShape Memory Alloys, eds. T.W. Duerig
`et at. (1990), p. 369.
`
`3. T.W. Duerig, D.E. Tolomeo, and M. Wholey, in MITAT 9, no. 3—4 (2000), p. 235.
`
`
`i
`
`!
`
`I
`
`
`
`Edwards Exhibit 1021, p. 9
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`

`

`320
`
`9024.9?"
`
`10.
`
`S. Miyazaki, in Engineering Aspects ofShape Memory Alloys, eds. T.W. Duerig er al. (1990),
`p. 394.
`
`F 2082-01, Annual Books ofASTM Standards.
`
`F 2004-00, Annual Books ofASTM Standards.
`
`E8M—Olel, Annual Books ofASTM Standards.
`
`X.»Y. Gong and AR. Pelton, in SMST-2003: Proceedings of the International Conference on
`Shape Memory and Snperelastic Technologies, eds. AR. Pelton and T. Duerig (Pacific Grove,
`Ca]if.: International Organization on SMST).
`
`R.M. Tabanli, N.K. Simha, and ET. Berg, Mater. Sci. and Eng. A 273—275 (1999), p. 644.
`
`X.-Y. Gong et al., in SMST—2003: Proceedings ofrhe International Conference on Shape
`Memory and Snperelostic Technologies (Pacific Grove, Ca1if.: International Organization on
`SMST), in press.
`
`
`
`
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`
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`Edwards Exhibit 1021, p. 10
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