ASTM E2948-22

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Designation: E2948 − 16a E2948 − 22
Standard Test 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. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. 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). 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 inch-pound units are to be regarded as standard. The values given in parentheses are mathematical
conversions to SI units that are provided for information only and are not considered standard.
1.3 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
E177 Practice for Use of the Terms Precision and Bias in ASTM Test Methods
E468 Practice for Presentation of Constant Amplitude Fatigue Test Results for Metallic Materials
E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
F562 Specification for Wrought 35Cobalt-35Nickel-20Chromium-10Molybdenum Alloy for Surgical Implant Applications
(UNS R30035)
This test method is under the jurisdiction of ASTM Committee E08 on Fatigue and Fracture and is the direct responsibility of Subcommittee E08.05 on Cyclic
Deformation and Fatigue Crack Formation.
Current edition approved Oct. 1, 2016Feb. 1, 2022. Published November 2016February 2022. Originally approved in 2014. Last previous edition approved in 2016 as
ɛ1
E2948–16–16a. . DOI: 10.1520/E2948-16A10.1520/E2948-22
The boldface numbers in parentheses refer to a list of references at the end of this standard.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E2948 − 22
E739 Practice for Statistical Analysis of Linear or Linearized Stress-Life (S-N) and Strain-Life (ε-N) Fatigue Data
E1823 Terminology Relating to Fatigue and Fracture Testing
2.2 ANSI Standard:
ANSI B4.1 Standard Limits and Fits
3. Terminology
3.1 Definitions:
3.1.1 Terms used in this practice shall be as defined in Terminology E1823.
4. Summary of Test Method
4.1 This test methodology describes a means to characterize the fatigue response of small diameter solid round wire using a
rotating bending test. Small diameter wire, to be consistent with Specification F562 definition of “fine wire”, is less than or equal
to a diameter of 0.063 in. (1.60 mm). The wire is subjected to a constant-amplitude bending strain (stress) while it rotates at a fixed
speed. This creates a fully reversed, R = (minimum strain (stress)/ maximum strain (stress))= –1, bending strain at any point on
the circumference of the wire. The number of revolutions or cycles is counted until a failure (fracture into two or more distinct
pieces) is detected. Surface effects due to environmental factors (for example corrosion or cavitation) can be extremely important
in assessing fatigue performance. Such effects can be assessed in a myriad of environments (air, phosphate buffered saline (PBS),
NaCl, O , N , varying humidity, etc.) using the protocol outlined in the standard.
2 2
5. 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.
6. Methods
6.1 Non-guided or guided rotating bending tests, or both are included in this test method. Typical test frequency ranges from 1
to 37 000 cycles per minute. Test frequency should be selected carefully since it can influence the rate at which fatigue damage
accumulates. In the guided rotating bending test, the guiding mandrel maintains the test specimen geometry and is recommended
for test specimens under high bending strain (stress); test specimens that exhibit strain (stress)-induced phase transformations; test
specimens with asymmetrical tension and compression behavior; test specimens with a non-central neutral axis and test specimens
exhibiting excessive vibration during high speed tests (9, 10).
6.1.1 Non-guided rotating bending fatigue test—The ends of the precut wire are attached to two driven, parallel, counter-rotating,
shafts such as illustrated in Fig. 1. Or, in an alternate method, one end of the wire (precut to a precise length) is attached to a driven
shaft and the other end is inserted into a restraining bushing, Fig. 2. The wire end is free to rotate within the bushing. A cumulative
cycle counter records each revolution of the wire as a fatigue cycle. Cumulative cycles can also be determined from the time to
fracture at a constant rotation rate. The specimen is rotated in the arc geometry until a failure occurs (herein defined as complete
separation or fracture of the wire) tripping the failure sensor, see Fig. 1 and Fig. 2, and terminating the test. Spacing between the
rotating shafts and the specimen length determine the bending strain (stress) through the radius of curvature thereby making the
bending strain (stress) readily adjustable. It is necessary to maintain the shaft spacing and specimen length relations of X1.1 for
a valid test. These relations ensure a zero bending moment at the collets (or collet and bushing) and an axial stress that is negligible
compared to the maximum bending stress at the midpoint of the specimen.
Available from American National Standards Institute (ANSI), 25 W. 43rd St., 4th Floor, New York, NY 10036, http://www.ansi.org.
E2948 − 22
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
6.1.2 Guided rotating bending fatigue test—One end of the precut test wire is attached to a driven shaft, Fig. 3. The wire passes
through a bushing to help reduce vibration and ensure more consistent results. The test wire is then bent around a mandrel (or in
a machined groove) of a low friction material with a fixed radius of curvature. The mandrel radius determines the outer-fiber strain
(stress). The other end of the wire is supported by an idler mandrel in which the wire freely spins. A cumulative cycle counter
records each revolution of the wire as a fatigue cycle. The specimen rotates while bent around the mandrel until a failure occurs
(herein defined as complete separation or fracture of the wire) tripping the failure sensor and terminating the test.
6.2 Fracture detection—Multiple forms of fracture detection devices are available. In one method a corrosion resistant metal wire
is connected electrically such that when contact is made with the fractured metal test specimen the test is terminated and the
instrument motor and timer/cycle counter stop. Fracture detection by sensing electrical continuity between the collets should be
-2
limited to less than 1 mA mm . Other possible fracture detection devices are fiber optic or laser sensors that are triggered by the
fracture of the test specimen.
7. Test Procedure
7.1 Non-guided rotating bending fatigue test—The specimen free length and the collet-to-collet or collet-to-bushing shaft spacing
are determined from the desired fatigue strain or subsequent nominal elastic stress amplitude, the wire diameter and the modulus
of elasticity of the material under test. See X1.1 for strain and nominal elastic stress calculations. A cast, or curvature of the wire,
is commonly associated with cold-drawn wire. The wire should be straightened only by hand without the use of any mechanical
straightening operation to prevent any possible changes in material properties. However, if the desired service state includes
mechanical or thermal-mechanical straightening then mechanical or thermal-mechanical straightening is acceptable. The wire is
assumed to be in a zero residual stress state. If this is not the case, an assessment of the residual stress state and its influence on
the results should be made and reported with the test results. It should then be cut-to-length and the collet-to-collet or
collet-to-bushing shaft center distance adjusted and set according to the calculations in X1.1. Clamp the wire in the collet, inserting
the other end in the proper collet or bushing location, and locate the supports and fractured wire sensors. Be cautious at this point
in the test set-up so as not to kink or unduly bend the wire. It is critical that the supports cause the wire to remain in a single vertical
or horizontal plane throughout the test. Out-of-plane displacement or oscillation of the specimen should be less than 5 mm. Low
friction materials, such as polyoxymethylene or polytetrafluoroethylene, are recommended for the support material. Metallic
E2948 − 22
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 4 in.-5 in. (10.2 cm -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
supports such as bronze, with or without lubrication, are not recommended because of higher friction coefficients and possible
corrosion interaction. Placement of wire supports just off the apex of the wire loop will minimize oscillation. Multiple supports
may be used for large collet-to-collet or collet-to-bushing spacing. Friction between the specimen and support may cause fracture
under the support. In this case the test result is considered invalid. If the wire is held by a pin-vise collet, cautiously clamp the
wire so as not to impart distortion or wire breakage at the collet. Carefully set the wire to wire-support clearance and the wire to
bushing clearance. Clearances should be set to conform to an ANSI Standard RC8 Loose Running Fit (ANSI B4.1). In the case
of the wire supports, too little clearance will hinder rotation resulting in a frictional torque on the wire and invalid test result. In
the case of the bushing, too great a clearance may lead to the wire jumping out of the bushing during the test or too little a clearance
may lead to a frictional torque on the wire and invalid test result. Rotate the collet by hand to ensure the specimen is properly
aligned in the collet(s) and supports to prevent excessive vibration or out-of-plane skew or oscillation. When using an
environmental bath, the test specimen should be positioned with bath-in-place and allowed to equilibrate to the bath temperature.
The amount of time required for equilibration will depend on the mass of the specimen as well as the volume, temperature, and
medium of the bath. Start the test and wait for the specimen to fracture or to reach a predetermined number of cycles. If the point
of fracture does not occur at the center of the loop (the point of maximum strain (stress), that is, minimum radius of curvature),
see X1.2 for a fracture strain (stress) correction factor that may be used based on the location of the fracture.
7.1.1 An alternate geometric method to determine the nominal elastic strain is to take an image of the curved specimen while in
the machine’s collets and curve fit the minimum radius of curvature using one of three methods; (1) an enlargement and computer
software; (2) templates with known radii of curvature matched by overlay on the image or; (3) an osculating circle fit to the image.
E2948 − 22
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
Calibration of length in the enlarged image is necessary. These methodologies provide the strain from the radius of curvature. It
is important to report the method used and to be consistent in this methodology to reduce within laboratory and between laboratory
errors.
7.2 Guided rotating bending fatigue test—The fatigue strain amplitude is related directly to the diameter of the tested wire and
the radius of curvature of the mandrel, shown in Fig. 3, around which the wire is bent. See X1.3 for the strain amplitude calculation.
Testing at a specific strain amplitude for a given wire diameter requires a mandrel with a specific radius. As such, a series of
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