ASTM E2948-14
(Test Method)Standard Test Method for Conducting Rotating Bending Fatigue Tests of Solid Round Fine Wire
Standard Test Method for Conducting Rotating Bending Fatigue Tests of Solid Round Fine Wire
SIGNIFICANCE AND USE
5.1 A method for obtaining fatigue strain (stress) at a specific life is of interest to the wire manufacturer, designer and consumer. The method is useful in production control, material acceptance and determination of the fatigue strain (stress) of the wire at a specific fatigue life, that is, fatigue strength. Rotating bending fatigue testing of small diameter solid round wire is possible by looping a specimen of predetermined length through an arc of 90° to 180°. The bending strain (stress) is determined from the geometry of the loop thusly formed. The methodology is capable of high frequency testing provided the temperature of the test article is constant and there is no adiabatic heating of the wire. A constant temperature can be maintained by immersing the specimen in a constant temperature fluid bath or test media. This makes it practical to quickly test a sufficient number of specimens to provide a statistical frequency distribution or survival probability distribution of fatigue life at a given strain (stress). Fatigue life information is useful to ascertain wire in-service durability and to assess, for example, the effects of melt practice and cold work processing.
SCOPE
1.1 This test method is intended as a procedure for the performance of rotating bending fatigue tests of solid round fine wire to obtain the fatigue strength of metallic materials at a specified life in the fatigue regime where the strains (stresses) are predominately and nominally linear elastic. This test method is limited to the fatigue testing of small diameter solid round wire subjected to a constant amplitude periodic strain (stress). The methodology can be useful in assessing the effects of internal material structure, such as inclusions, in melt technique and cold work processing studies. However, there is a caveat. The strain, due to the radial strain gradient imposed by the test methodology, is a maximum at the surface and zero at the centerline. Thus the test method may not seek out the “weakest link,” largest inclusions, that govern uniaxial high cycle fatigue life where the strain is uniform across the cross section and where fatigue damage initiates at a subsurface location (1-5).2 Also, pre-strain, which can influence fatigue life, is not included in this test method.
Note 1: The following documents, although not specifically mentioned, are considered sufficiently important to be listed in this test method:
ASTM STP 566 Handbook of Fatigue Testing
ASTM STP 588 Manual on Statistical Planning and Analysis for Fatigue Experiments
ASTM STP 731 Tables for Estimating Median Fatigue Limits (6-8)
1.2 The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in each system may not be exact equivalents; therefore, each system shall be used independently of the other. Combining values from the two systems may result in non-conformance with the standard.
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Designation: E2948 − 14
StandardTest Method for
Conducting Rotating Bending Fatigue Tests of Solid Round
Fine Wire
This standard is issued under the fixed designation E2948; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 2. Referenced Documents
1.1 This test method is intended as a procedure for the 2.1 ASTM Standards:
performance of rotating bending fatigue tests of solid round E468Practice for Presentation of Constant Amplitude Fa-
fine wire to obtain the fatigue strength of metallic materials at tigue Test Results for Metallic Materials
aspecifiedlifeinthefatigueregimewherethestrains(stresses) F562 Specification for Wrought 35Cobalt-35Nickel-
are predominately and nominally linear elastic. This test 20Chromium-10Molybdenum Alloy for Surgical Implant
method is limited to the fatigue testing of small diameter solid Applications (UNS R30035)
round wire subjected to a constant amplitude periodic strain E739PracticeforStatisticalAnalysisofLinearorLinearized
(stress).Themethodologycanbeusefulinassessingtheeffects Stress-Life (S-N) and Strain-Life (ε-N) Fatigue Data
of internal material structure, such as inclusions, in melt E1823TerminologyRelatingtoFatigueandFractureTesting
technique and cold work processing studies. However, there is 2.2 ANSI Standard:
a caveat. The strain, due to the radial strain gradient imposed ANSI B4.1Standard Limits and Fits
by the test methodology, is a maximum at the surface and zero
3. Terminology
at the centerline. Thus the test method may not seek out the
3.1 Definitions:
“weakest link,” largest inclusions, that govern uniaxial high
cycle fatigue life where the strain is uniform across the cross 3.1.1 Terms used in this practice shall be as defined in
Terminology E1823.
section and where fatigue damage initiates at a subsurface
location (1-5). Also, pre-strain, which can influence fatigue
4. Summary of Test Method
life, is not included in this test method.
4.1 Thistestmethodologydescribesameanstocharacterize
NOTE 1—The following documents, although not specifically
the fatigue response of small diameter solid round wire using
mentioned, are considered sufficiently important to be listed in this test
a rotating bending test. Small diameter wire, to be consistent
method:
ASTM STP 566 Handbook of Fatigue Testing
withSpecificationF562definitionof“finewire”,islessthanor
ASTM STP 588 Manual on Statistical Planning andAnalysis for Fatigue
equal to a diameter of 0.063 in. (1.60 mm). The wire is
Experiments
subjected to a constant-amplitude bending strain (stress) while
ASTM STP 731 Tables for Estimating Median Fatigue Limits (6-8)
it rotates at a fixed speed. This creates a fully reversed,
1.2 The values stated in either SI units or inch-pound units
R=miniumstrain ~stress!/maximumstrain ~stress!=-1, bending strain
are to be regarded separately as standard. The values stated in
at any point on the circumference of the wire. The number of
each system may not be exact equivalents; therefore, each
revolutions or cycles is counted until a failure (fracture into
system shall be used independently of the other. Combining
two or more distinct pieces) is detected. Surface effects due to
values from the two systems may result in non-conformance
environmentalfactors(forexamplecorrosionorcavitation)can
with the standard.
beextremelyimportantinassessingfatigueperformance.Such
effects can be assessed in a myriad of environments (air,
This test method is under the jurisdiction ofASTM Committee E08 on Fatigue
and Fracture and is the direct responsibility of Subcommittee E08.05 on Cyclic For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Deformation and Fatigue Crack Formation. contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Current edition approved May 1, 2014. Published July 2014. DOI: 10.1520/ Standards volume information, refer to the standard’s Document Summary page on
E2948-14 the ASTM website.
2 4
The boldface numbers in parentheses refer to a list of references at the end of Available fromAmerican National Standards Institute (ANSI), 25 W. 43rd St.,
this standard. 4th Floor, New York, NY 10036, http://www.ansi.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E2948 − 14
phosphate buffered saline (PBS), NaCl, O,N , varying test,theguidingmandrelmaintainsthetestspecimengeometry
2 2
humidity, etc.) using the protocol outlined in the standard. and is recommended for test specimens under high bending
strain (stress); test specimens that exhibit strain (stress)-
5. Significance and Use
inducedphasetransformations;testspecimenswithasymmetri-
cal tension and compression behavior; test specimens with a
5.1 A method for obtaining fatigue strain (stress) at a
specificlifeisofinteresttothewiremanufacturer,designerand non-central neutral axis and test specimens exhibiting exces-
sive vibration during high speed tests (9, 10).
consumer.Themethodisusefulinproductioncontrol,material
acceptance and determination of the fatigue strain (stress) of 6.1.1 Non-guided rotating bending fatigue test—Theendsof
the wire at a specific fatigue life, that is, fatigue strength. the precut wire are attached to two driven, parallel, counter-
Rotating bending fatigue testing of small diameter solid round rotating, shafts such as illustrated in Fig. 1. Or, in an alternate
wireispossiblebyloopingaspecimenofpredeterminedlength method, one end of the wire (precut to a precise length) is
through an arc of 90° to 180°. The bending strain (stress) is attached to a driven shaft and the other end is inserted into a
determined from the geometry of the loop thusly formed. The restrainingbushing,Fig.2.Thewireendisfreetorotatewithin
methodology is capable of high frequency testing provided the the bushing. A cumulative cycle counter records each revolu-
temperature of the test article is constant and there is no tion of the wire as a fatigue cycle. Cumulative cycles can also
adiabatic heating of the wire. A constant temperature can be be determined from the time to fracture at a constant rotation
maintained by immersing the specimen in a constant tempera- rate.Thespecimenisrotatedinthearcgeometryuntilafailure
ture fluid bath or test media. This makes it practical to quickly occurs(hereindefinedascompleteseparationorfractureofthe
test a sufficient number of specimens to provide a statistical wire) tripping the failure sensor, see Fig. 1 and Fig. 2, and
frequency distribution or survival probability distribution of terminating the test. Spacing between the rotating shafts and
fatigue life at a given strain (stress). Fatigue life information is the specimen length determine the bending strain (stress)
useful to ascertain wire in-service durability and to assess, for through the radius of curvature thereby making the bending
example,theeffectsofmeltpracticeandcoldworkprocessing. strain (stress) readily adjustable. It is necessary to maintain the
shaft spacing and specimen length relations of X1.1 for a valid
6. Methods
test. These relations ensure a zero bending moment at the
collets (or collet and bushing) and an axial stress that is
6.1 Non-guidedorguidedrotatingbendingtests,orbothare
negligible compared to the maximum bending stress at the
includedinthistestmethod.Typicaltestfrequencyrangesfrom
midpoint of the specimen.
1 to 37000 cycles per minute. Test frequency should be
selected carefully since it can influence the rate at which 6.1.2 Guided rotating bending fatigue test—One end of the
fatigue damage accumulates. In the guided rotating bending precut test wire is attached to a driven shaft, Fig. 3. The wire
A) Dual driven collets: Both wire ends are held in driven collets. An environmental chamber may be placed on the platform and tests can be performed in a temperature
controlled liquid medium. Loss of electrical continuity from one collet through the wire to the other collet indicates wire fracture and test termination. B) Wire supports: The
wire passes through slits in the supports to maintain in-plane motion of the wire during the test. The supports should be placed such that they do not impose any additional
force or torque on the wire. Preferred placement for the supports is just off the apex of the wire loop perpendicular to a tangent to the loop. The support material should
be a low friction material and support placement should be chosen to minimize friction.
FIG. 1 Non-Guided Rotating Bending Apparatus with Counter-Rotating Shafts
E2948 − 14
A) L-bracket: Contains support bushing and allows for adjustment of driven collet to bushing spacing. B) Bushing: In this apparatus, there is a single driven collet. The
wire is free to rotate in the bushing. Clearance between the wire and inside diameter of the bushing is important in order to minimize the tendency of the wire to “fly out”
of the bushing. Too great a clearance and the wire may not remain in place and too small a clearance may prevent rotation. C) Collet: The spacing of a single driven collet
to bushing fixes the strain amplitude. D) Wire supports: The wire passes through small slits in the supports so that it can be held in-plane during the test. Preferred
placement for the supports is just off the apex of the wire loop perpendicular to a tangent to the loop. A test setup with a collet to bushing spacing (that is, center distance
as defined in X1.1) greater than 4-5 inches (10.2-12.7 cm) would benefit from an extra set of supports (not shown) to help minimize possible wire out-of-plane oscillation.
E) Break detector: When the wire fractures, contact will be made with one of the strategically placed break detectors. The break detector is a corrosion resistant metal wire,
electrically connected such that when contact is made with the metal test specimen the test is terminated and the instrument motor and timer/cycle counter stop. It is
recommended to place one break detector near the apex of the wire loop and a second detector between the support and the collet. Detectors should be placed within
5 – 10 mm of the rotating wire. Adequate detector to wire clearance is necessary to prevent premature shut down.
FIG. 2 Non-Guided Rotary Bending Apparatus with Bushing and Rotating Shaft
passes through a bushing to help reduce vibration and ensure 7. Test Procedure
more consistent results. The test wire is then bent around a
7.1 Non-guided rotating bending fatigue test—The speci-
mandrel (or in a machined groove) of a low friction material
men free length and the collet-to-collet or collet-to-bushing
withafixedradiusofcurvature.Themandrelradiusdetermines
shaft spacing are determined from the desired fatigue strain or
the outer-fiber strain (stress). The other end of the wire is
subsequent nominal elastic stress amplitude, the wire diameter
supportedbyanidlermandrelinwhichthewirefreelyspins.A
and the modulus of elasticity of the material under test. See
cumulativecyclecounterrecordseachrevolutionofthewireas
X1.1 for strain and nominal elastic stress calculations. A cast,
a fatigue cycle. The specimen rotates while bent around the
or curvature of the wire, is commonly associated with cold-
mandrel until a failure occurs (herein defined as complete
drawn wire. The wire should be straightened only by hand
separation or fracture of the wire) tripping the failure sensor
without the use of any mechanical straightening operation to
and terminating the test.
prevent any possible changes in material properties. However,
6.2 Fracture detection—Multiple forms of fracture detec- if the desired service state includes mechanical or thermal-
tion devices are available. In one method a corrosion resistant mechanical straightening then mechanical or thermal-
metal wire is connected electrically such that when contact is mechanical straightening is acceptable.The wire is assumed to
made with the fractured metal test specimen the test is be in a zero residual stress state. If this is not the case, an
terminated and the instrument motor and timer/cycle counter assessment of the residual stress state and its influence on the
stop. Fracture detection by sensing electrical continuity be- results should be made and reported with the test results. It
-2
tween the collets should be limited to less than 1 mA mm . should then be cut-to-length and the collet-to-collet or collet-
Otherpossiblefracturedetectiondevicesarefiberopticorlaser to-bushing shaft center distance adjusted and set according to
sensors that are triggered by the fracture of the test specimen. thecalculationsinX1.1.Clampthewireinthecollet,inserting
E2948 − 14
The wire specimen is bent around a mandrel with a fixed radius, ρ. The optical break detector shown senses a closed or open (failed specimen) optical path through
the mandrel and specimen.
FIG. 3 Guided Rotating Bending Fatigue Apparatus
the other end in the proper collet or bushing location, and wait for the specimen to fracture or to reach a predetermined
locate the supports and fractured wire sensors. Be cautious at number of cycles. If the point of fracture does not occur at the
thispointinthetestset-upsoasnottokinkorundulybendthe
centeroftheloop(thepointofmaximumstrain(stress),thatis,
wire. It is critical that the supports cause the wire to remain in minimum radius of curvature), see X1.2 for a fracture strain
a single vertical or horizontal plane throughout the test.
(stress)correctionfactorthatmaybeusedbasedonthelocation
Out-of-plane displacement or oscillation of the specimen
of the fracture.
should be less than 5 mm. Low friction materials, such as
7.1.1 Analternategeometricmethodtodeterminethenomi-
polyoxymethylene or polytetrafluoroethylene, are recom-
nal elastic strain is to take an image of the curved specimen
mended for the support material. Metallic supports such as
whileinthemachine’scolletsandcurvefittheminimumradius
bronze, with or without lubrication, are not recommended
of curvature using one of three methods; (1) an enlargement
because of higher friction coefficients and possible corrosion
and computer software; (2) templates with known radii of
interaction. Placement of wire supports just off the apex of the
curvature matched by overlay on the image or; (3) an osculat-
wire loop will minimize oscillation. Multiple supports may be
ing circle fit to the image. Calibration of length in the enlarged
used for large collet-to-collet or collet-to-bushing spacing.
image is necessary. These
...
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