ASTM Student Grant Paper
Contact Information
Janet Gbur, PhD Student
Case Western Reserve University
White Building, Room 334
10900 Euclid Avenue
Cleveland, Ohio 44106
JLG120@case.edu
330-398-0838
Title
Janet Gbur
John J. Lewandowski, PhD Advisor
Case Western Reserve University
White Building, Room 520/522
10900 Euclid Avenue
Cleveland, Ohio 44106
JJL3@case.edu
216-368-4234
Mechanical characterization of 316 LVM wires: A comparative study of flex bending fatigue and rotating bending
fatigue and its utility in fatigue testing for biomedical applications
Abstract
Mechanical characterization of 316 LVM stainless steel was investigated across a range of fatigue life cycles in
rotating-bending fatigue and flex bending fatigue. A comparison of the material behavior between the techniques was
performed, holding the environment, test temperature, and wire diameter constant while varying the applied strain.
Fractography was performed and qualitative assessments were made regarding the fracture surface and mode of failure. As
expected, the rotating bending tests produced lower fatigue lives than samples tested in flex bending due to the different
amount of material sampled during the test.
Introduction
The increased demand for miniaturized implantable devices poses new questions in the design and deployment of
implants in addition to the need of addressing specific processing, characterization, and ultimately, reliability testing of the
device material(s). Therefore, it is important to develop test methods that can examine the dynamic loading conditions
imposed on the device during deployment as well as ensure that functionality is maintained during long term cyclic fatigue.
The materials chosen for these biomedical applications must be able to support all of these critical parameters and in order
to best evaluate and optimize a material for an application, an extensive knowledge of how the material responds to the
applied stressors under loading and failure modes become critical. In cardiovascular applications for example, intravascular
stents must be able to provide support to the vessel and must exceed an FDA requirement of 400 million cycles (Yan, Yang
and Qi, 2006). Materials such as 316 LVM stainless steel provide a wide range of properties that can meet the
requirements of these developing implantable devices. 316 LVM is an austenitic stainless steel used in the manufacture of
surgical implants. It undergoes an electric-arc melt and vacuum arc re-melt in order to refine the homogeneity and purity of
the material and is compliant to ASTM F138. The material exhibits good corrosion resistance in the annealed condition and
good ductility in the cold worked condition (Fort Wayne Metals 2013).
Testing to represent the exact anatomical and physiological conditions these devices experience is difficult. What
can be done as a step toward a first approximation to fatigue life in these materials and respective environments is to adapt
existing strain-controlled dynamic tests to represent the fundamental cyclic bending fatigue incurred by the material.
Background
Flex bending fatigue provides a useful means of establishing fatigue life under strain-controlled conditions. ASTM
E796-94 (Reapproved 2000) is intended for use with metallic foils but has been adapted to studies for wire and cables used
in biomedical applications (Benini 2010, Lavvafi 2013, Lewandowski et al 2008, Lewandowski, Varadarajan, Smith 2008).
The testing apparatus has the capability to evaluate metallic foils of known geometry and apply either a fully reversed
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Janet Gbur
bending test condition, R = -1 or a zero-to-tension bending test condition, R = 0 depending on whether the sample is placed
between the test mandrels or above or below the mandrels. In the case of R = -1, the wire is bent around a set of rollers and
through a pair of mandrels that move vertically, applying constant strain. The wire is secured in a carrier which is attached
to a nominal dead weight in order ensure the wire maintains constant contact with the mandrel surfaces during the test.
The maximum stress occurs at the maximum stroke of the mandrels thereby applying an alternating tension and
compression (Engelmaier 1982). Those elements in the material in contact with the mandrel surfaces experience the
greatest surface strains. A schematic of the test and image of the test apparatus with a loaded sample is shown in Figure 1.
A)
B)
Figure 1. Flex bending test set up. A) Schematic of mandrel and roller arrangement and dead weight (ASTM E796-94). B)
Image of test machine noting break detectors, mandrels, and dead weight.
Development of the calculation for cyclic strain has been done in literature and was used for the analysis of this
work (Costello 1997). The strain in the wire can be approximated by the bending of a wire over a mandrel where the radius
of curvature, ρ, is related to the bending moment, M, and E is the modulus of elasticity as shown in Equation 1.
𝑀=
𝐸𝐼𝑤𝑖𝑟𝑒
𝜌
(1)
Moment of inertia of the wire, Iwire can be calculated from the diameter of the wire, d, according to Equation 2.
𝐼𝑤𝑖𝑟𝑒 =
𝜋𝑑4
64
(2)
The normal stress due to bending, σ, is given by Equation 3, where the maximum stress occurs at y = d/2.
𝜎=
𝑀𝑦
𝐸𝑦
=
𝐼𝑤𝑖𝑟𝑒
𝜌
(3)
Therefore the strain range for R = -1 (fully reversed cycling) is given by Equation 4 noting that the radius of curvature of the
mandrel is much larger than the wire diameter.
∆𝜀 =
2𝜎 𝑑
=
𝐸
𝜌
(4)
Rotating bending fatigue also provides a method of establishing fatigue life under strain-controlled conditions on a
known wire geometry and applying fully reversed bending test conditions, R = -1. While an ASTM test standard is not
currently available for this particular test, procedures did follow the current ASTM work item and recommendations from
the machine manufacturer. The load case is modeled after the pure bending of a linear-elastic beam wherein the wire is
held in loop geometry with the ends held in parallel and the loop maintained in a single plane. One end of the wire is fixed
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Janet Gbur
in a driven chuck while the other end is either free to rotate in a bushing or secured in a second driven chuck to provide the
respective wire rotation. The maximum stress occurs at the apex of the loop, alternates in tension and compression, and
elements at the extreme edges of the wire experience the greatest surface strains (Wagner and Eggeler, 2006). A schematic
of the test and image of the test apparatus with a loaded sample is shown in Figure 2.
A)
B)
Figure 2. Rotating bending test set up. A) Schematic of loop showing wire length (L), loop radius (R), loop height (h),
distance between collets and/or collect and bushing (C), and load (P) (Positool). B) Image of test machine noting dual driven
collets, wire guides to maintain plane of wire, and the radius of curvature.
The test method, provided by the manufacturer (Positool, Valley Technologies) provides a series of calculations to
determine the appropriate wire length and center distance between the collets in order to obtain a particular applied
strain. Based on the loop geometry, the center distance between the collets can be calculated by Equation 5 where E is the
elastic modulus, d is the wire diameter and σ is the test stress.
𝐶 = 1.198 ∗
𝐸𝑑
𝜎
(5)
The minimum bending radius, ρmin, test length, L, and the total and length of the wire, TL, can be calculated from C, noted in
Equation 6.
𝜌𝑚𝑖𝑛 = 0.417𝐶
𝐿 = 2.19𝐶
𝑇𝐿 = 𝑐ℎ𝑢𝑐𝑘 𝑑𝑒𝑝𝑡ℎ + 𝑏𝑢𝑠ℎ𝑖𝑛𝑔 𝑑𝑒𝑝𝑡ℎ + 𝐿
(6)
Finally, the strain in bending, εa, can be determined from Equation 7.
𝜀𝑎 =
𝑑/2
∗ 100%
𝜌𝑚𝑖𝑛
(7)
Research Statement
The proposed hypothesis of this study was that the type of heat treatment on 316 LVM wire affects the
mechanical properties, notably the fatigue life. Secondly, the choice of type of fatigue test produces differences in the
amount of mater subjected to high alternating stresses, potentially leading to different fatigue life results. To test the
hypothesis, uniaxial tension tests and fatigue tests in flex bending and rotating bending were performed along with optical
analyses on fractured hard and annealed 316 LVM wires. Experimental results were correlated with those from the
literature where similar tests were conducted. Such results could be used to aid evaluating the most appropriate wire
treatments and test conditions for a given application and provide further understanding of 316 LVM fatigue behavior.
316 LVM stainless steel wires were obtained from Fort Wayne Metals. Tests were conducted under straincontrolled conditions in both rotating bending fatigue and in flex bending fatigue. In the former, the loop diameter was
varied in size to obtain a high, mid-range, and low applied strain that would permit a testing span from low cycle fatigue
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Janet Gbur
(LCF) conditions to high cycle fatigue (HCF) conditions. The same variance was applied to samples tested in flex bending
fatigue through the change in mandrel diameters.
Aim 1: Tensile strength evaluation. Tensile strength of hard and annealed wires determined following ASTM E8.
Aim 2: Fatigue life evaluation. Wires for flex bending fatigue tested following ASTM. Wires for rotating bending
fatigue will follow the testing format provided by the proposed ASTM E08 work item for rotating bending fatigue.
Increases in strain will be controlled by the mandrel or loop diameter size and begin in the LCF regime and
continue through run-out in the HCF.
Aim 3: Failure analysis. Scanning electron microscopy (Hitachi S-4500 field emission gun SEM) will be utilized to
provide additional information on the surface failure as a consequence of the applied strain and time within the
fatigue cycle. Surface flaws, crack initiation and propagation, regions of fatigue, and fractography features will also
be identified.
Anticipated Results
Differences are expected between the flex bending fatigue results and rotating bending fatigue results. In the case
of flex bending, a smaller portion of the wire surface is sampled. Only the top and bottom surfaces which alternate in
tension and compression are engaged. The rotating bending fatigue samples a larger surface volume of material as the wire
undergoes alternating tension and compression with rotation producing the maximum stresses at the outer edges of the
wire. There is also an expectation based on preliminary work that the change in mandrel or loop diameter will correlate
with a change in the fatigue life of the wire. Data would be compared between test methods, material treatment (hard
versus annealed), and the effect of mandrel size on the fatigue life of the wire.
Due to the high stresses concentrated on the surface elements in rotating bending fatigue, failure is expected to
occur from the surface and progress radially inward. Ductile behavior is expected and as such elongated dimples may be
found in the fatigue region in a radial pattern while more isometric dimples will be located in the fast fracture region.
Methods
The 316 LVM stainless steel wires evaluated in this study possessed a 0.254 mm diameter and were provided in the
hard and annealed conditions by Fort Wayne Metals, Fort Wayne, IN. A summary of the alloy chemistry as provided by the
company includes (given in weight percent): 17.57% Cr, 14.68% Ni, 2.79% Mo, 1.84% Mn, 0.37% Si, 0.03% Cu, 0.03% N,
0.023% C, 0.014% P, 0.001% S, balance Fe (Fort Wayne Metals).
Tensile tests and fatigue tests (rotating bending and flex bending) were performed in the same fixed environment
(air) and test temperature (25°C).
Tension testing and subsequent analyses followed ASTM E8 in addition to work performed by Benini (2010) and
Lavvafi (2013) as the wire diameter was less than 4 mm. Testing was conducted on an Instru-Met uniaxial testing machine
(Instru-Met Corporation, Union, NJ) equipped with a 100kN load cell at a gage length of 30 mm and a displacement rate of
0.5 mm/min. Load and displacement values were collected via MTS Testworks 4 data acquisition software at 5 Hz. The
sample diameter was 0.254 mm. A variety of grips (serrated wedge, roller, rubber-lined, and aluminum-capped) were
attempted in order to minimize wire slippage and any possibility of induced stress risers on the fine diameter material,
Figures 3E-F. Slipping did occur in the roller grips, Figure 3A, and rubber-lined grips, Figure 3B and induced stress risers were
apparent with the serrated wedge grips, Figures 3C-D, even if the wire was protected inside a paper/tape sheath within the
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grips. The best result for securing the wire resulted from an aluminum cap placed over serrated block grips as show in
Figures 3 E-F.
B)
D)
F)
E)
d
Figure 3. Various tensile grips. A) Roller grips, B) Rubber-lined grips, C-D) Serrated wedge grips, E-F) Aluminum-capped grips.
A)
C)
Due to the size of the wire diameter, an extensometer could not be used to determine elongation. Engineering
stress-strain curves were prepared and analyzed utilizing a 0.2% offset for the yield stress and the ultimate tensile stress
was calculated by the peak load attained. The engineering stress was calculated using Equation 8 where P is the load and Ai
is the initial cross-sectional area.
𝑆=
𝑃
𝐴𝑖
(8)
Flex bending fatigue tests were performed on a Universal Fatigue Ductility Flex Tester 3FDF (Jovil/Universal
Manufacturing Company, Danbury, CT) following ASTM E796. The constant-amplitude, strain-controlled test for fully
reversed bending, was executed by interchanging the steel mandrels with varied diameters providing flexural strains that
would cause fracture in low cycle fatigue and high cycle fatigue. The mandrels ranged from 1.27 mm diameter to 31.7 mm
diameter. Refer to Figure 1 for test set up. The total sample length was 90 mm of which 30 mm on each end was secured to
the carrier and to clamps which provided continuity. Once the specimen breaks, continuity is broken and the test ceases.
The test rate was 1 Hz to minimize heating effects and for physiological relevance in biomedical applications (i.e. blood
flow). The dead weight applying tension to the sample was 28 grams. Samples were run to failure or stopped at 106 cycles
and considered a run-out.
Rotating bending tests were performed on either Model 401 or 100 Wire Fatigue Tester (Positool Technology,
Valley Instruments, Brunswick, OH) according to manufacturer recommendations and the current ASTM work item. The
strain-controlled test for R = -1 was executed by adjusting the collet distance in order to vary the radius of curvature of the
loop. Calculations for the wire length and collet distance were determined according to Equations 5 and 6. This particular
rotating bending apparatus is an accelerated test method which runs at 60 Hz and like the flex tester, maintains continuity
through the wire connection and the test ceases when failure occurs, tripping the break detectors. Samples were run to
failure or stopped at 108 cycles and considered a run-out.
Scanning electron microscope (SEM) images were obtained by mounting a 6 mm sample from the fractured end of
the tested wire and fixing it to an aluminum, double 90°, Hitachi mount with conductive carbon tape. All images were
prepared at 5kV on a Hitachi S-4500 field emission gun SEM. Images obtained from the scanning electron microscopy were
imported into ImageJ software in order to determine final wire diameters. Reduction of area, RA, of the wire was
determined by Equation 9 with the note that most circular cross-sections will not remain constant following testing due to
anisotropy, therefore the cross-sectional area was determined considering the major, d1, and minor, d2, diameters of the
final wire.
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𝑅𝐴 = 100 ∗
𝐴𝑖 − 𝐴𝑓
𝐴𝑖
𝑤ℎ𝑒𝑟𝑒 𝐴𝑓 =
𝜋𝑑1 𝑑2
4
(9)
Results
As mentioned previously, due to the fine diameter of the wire, engineering strains could not be determined with
an extensometer and instead were calculated from the crosshead displacement. Engineering stress was calculated
according to Equation 8. The ultimate tensile strength (UTS) was measured from the maximum load attained during the
test, Table 1. Both the annealed and hard conditions exhibited a linear regime through the yield of the material. Strain
hardening was evident in both specimens with the annealed condition exhibiting the most strain hardening. As evidenced
by the UTS/yield stress ratio, there is approximately a 136% difference between the 0.2% offset yield strength between the
annealed and hard conditions and a 80.5% difference between UTS of the annealed and hard material.
Table 1. Tension test results summary.
Treatment
Anneal
Hard
0.2% Offset
Yield
(MPa)
230
1370
0.2% Offset
Yield
(MPa)
300
1400
0.2% Offset
Yield Average
(MPa)
26535
138515
UTS
(MPa)
658
1489
UTS
(MPa)
624
1520
UTS
Average
(MPa)
64117
150516
Flex bending tests and rotating bending tests were performed in fully reversed cyclic fatigue at a rate of 1 Hz and
60 Hz, respectively, under strain control through the variation of mandrel sizes ranging from 1.27 mm to 31.7 mm and loop
diameters from 12.7 mm to 116 mm. Mandrel and loop diameter-life curves are illustrated in Figures 4A-B. The cyclic
strain-life curve for flex bending is provided in Figure 5A and the strain amplitude-life curve is shown in Figure 5B. In
general, an increase in the mandrel diameter results in an increase in life since this produces a decrease in cyclic strain that
also leads to an increase in life. Wires tested under flex bending with the current conditions all produced failure and no runouts were obtained. In the rotating bending tests, two strain levels exhibited run-outs in the hard wires, εa = 0.2998% and εa
= 0.2398%. Cyclic strain in flex bending, Δε, was calculated according to Equation 4 and the strain in bending for rotating
bending fatigue was calculated using Equation 7. These equations are related by the Ramberg-Osgood relationship shown in
Equation 10.
𝜀𝑎 =
∆𝜀
2
(10)
A)
B)
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Figure 4. 316 LVM mandrel and loop diameter-life curves. A) Flex bending fatigue, B) Rotating bending fatigue.
A)
B)
Figure 5. 316 LVM life curves. A) Flex bending fatigue, cyclic strain-life, B) Rotating bending fatigue, strain amplitude-life.
Fractography of the tensile specimens is shown in Figures 6A-B. The annealed specimen exhibits a greater
reduction in area compared to the hard specimen. The average reduction in area for the annealed samples was 84.7% 
1.6% and the average for the hard samples was 40.9%  1.0% Evidence of ductile fracture via microvoid coalescence is
shown in both the annealed and hard conditions.
A)
B)
Figure 6. 316 LVM tension fractography. A) Annealed B) Hard
Fatigue fractography is shown in Figures 7A-D and Figures 8A-D. The direction of crack propagation is illustrated by
the arrows in the flex bending samples in the annealed condition, Figures 7A-B. Note that the cracks were generated at the
sample surfaces and then propagated towards the midline of each sample prior to final rapid fracture (shown in the region
between the arrows). The hard wires, Figures 7C-D, exhibit a more tortuous macroscopic appearance along with a greater
change in the final cross-section geometry.
A)
B)
C)
D)
Figure 7. Flex bending fractography. A) Annealed, Nf = 4,955 cycles B) Annealed, Nf = 5,379 cycles C) Hard, Nf = 1,077 cycles
D) Hard, Nf = 2,213 cycles.
Figures 8A-D illustrate the fracture characteristics from the rotating bending tests. In the case of the annealed
wires, Figures 8A-B, some directional dimples can be observed in the direction of rotation leading to final rapid fracture
near the center of the wire. Increased cycle life in the case of Figure 8B illustrates a failure similar to those observed in flex
bending. The hard wires, shown in Figures 8C-D, again exhibit a greater change in fracture cross-section. The final rapid
fracture is illustrated by the direction of the arrows provided in these figures.
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ASTM Student Grant Paper
A)
Janet Gbur
B)
C)
D)
Figure 8. Rotating bending fractography. A) Annealed, Nf = 22 cycles B) Annealed, Nf = 14,843 cycles C) Hard, Nf = 753 cycles
D) Hard, Nf = 184,200 cycles.
Discussion
The observed ductility in tension tests of 316 LVM stainless steel are as expected and compare favorably to data
generated by Lavvafi (2013). That study showed an average 86% reduction in area for 316 LVM annealed wires, only a 1.5%
difference from this current work. Additionally, average values for the UTS and 0.2% offset yield strength are reasonably
close to with those found by Lavvafi as shown in Table 2.
Table 2. Tension data comparison to Lavaffi (2013).
Treatment
Anneal
Hard
0.2% Offset
Yield Average
(MPa)
Lavvafi
346
1200
0.2% Offset
Yield Average
(MPa)
26535
138515
Percent
Difference
0.2% Offset Yield
(%)
26.5
14.3
UTS
Average
(MPa)
Lavvafi
681
1415
UTS
Average
(MPa)
64116.8
150515.8
Percent
Difference
UTS
(%)
6.1
6.2
In general, whether considering flex bending fatigue or rotating bending fatigue, an increase in the mandrel size or
increase in the radius of curvature of the loop, respectively, will lead to an increase in fatigue life, as shown in prior works
(Benini 2010, Lavvafi 2013, Lewandowski et al 2008, Lewandowski, Varadarajan, Smith 2008). Data shown in Figures 4A-B
illustrate the same phenomenon. For the same cyclic strain, under flex bending, the annealed wires show improved
performance compared to the hard wires in the mid to low cycle fatigue (LCF) regime. However, it appears that as the cyclic
strain decreases, the hard wires may provide a longer life in the low strain, high cycle fatigue regime (HCF). The data was
compared to work by Lavvafi, Figure 9, and falls within the framework of that study. Significant scatter does exist in both
data sets (flex bending and rotating bending), especially in the high cycle fatigue (HCF). The sources of this variability are
under investigation.
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Figure 9. Strain-life data for 316 LVM annealed and hard wires obtained via flex bending data in comparison to Lavvafi
(2013).
A comparison of the flex bending and rotating bending tests show the superior performance of the hard wires
tested in flex bending fatigue for the same applied strain (via mandrel/loop diameter = 12.7 mm, 25.5 mm, 31.7 mm),
Figure 10. As mentioned previously, the flex bending system relies on the vertical movement of a set of mandrels that the
wire alternates between during each cycle. Only the surfaces that come in contact with the mandrel experience the
maximum alternating bending behavior. In contrast, the rotating bending test adds the component of rotation, thereby
capturing the full circumference of the sample, and this larger surface area/volume of material undergoes the fully reversed
bending behavior (Lewandowski et al 2008). Both tests, by basic mechanics, sample the surface of the wire where the
maximum stresses are located due to the nature of the test geometry. It is for this reason that surface finish and surface
defects may be particularly detrimental to the fatigue life of the wires tested in this manner.
Figure 10. Comparison of 316 LVM annealed and hard flex bending data to Lavvafi (2013).
Conclusions
The determination of the UTS and reduction in area of the annealed and hard wires matched what would be
expected qualitatively and fell within the rage of previously reported values for similar tests on 316 LVM stainless steel.
Variations between the flex bending fatigue results and rotating bending fatigue results also agreed with what
would be expected based on fundamental mechanics and followed trends noted in literature. The accelerated testing
capable in rotating bending allows for engineers to quickly ascertain the effects of wire quality on fatigue life. Furthermore,
since the sample volume that is tested is larger than that observed in the flex bending fatigue, the rotating bending may
provide a worse-case-scenario for evaluation of fatigue performance for certain applications.
References
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Reserve University. May 2010.
Engelmaier W. A method for the determinations of ductility for thin metallic materials. Formability of Metallic
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Fort Wayne Metals. 316 LVM product information. http://www.fwmetals.com/316lvm-stainless-steel.php. Accessed
March 2013.
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ASTM Student Grant Paper
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Lavvafi H. Effects of laser machining on structure and fatigue of 316 LVM biomedical wires. Doctoral Dissertation, Case
Western Reserve University. May 2013.
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