Technically Speaking

Types of Tests Used to Characterize Springmaking Materials — Part 6: Fatigue Testing of Wire

By C. Richard Gordon

This is the sixth Springs magazine article in the series regarding mechanical and technological tests for springmaking materials, fatigue testing of wire. Previous articles in the series are the tensile test (Winter 20201), the coiling and wrapping tests (Spring 20202), the torsion test (Summer 20203) the hardness test (Fall 20204) and the reverse bend test (Winter 20215). This article includes presentation materials from a Testing and Properties class6 that I have taught for a number of years for the Wire Association International as part of their Fundamentals of Wire Manufacturing program.

Overview

In general, the testing of materials represents an important part of all quality work. It can include the control of incoming raw materials, materials in production, and produced materials or components before delivery.

Many different techniques are used, including chemical analysis; microscopy; nondestructive testing; mechanical tests such as tensile strength, hardness and fatigue; and technological tests such as bending, torsion, coiling, wrap and weldability.

In this series of articles, we have focused on mechanical and technological tests used to characterize springmaking materials. In this article, fatigue testing of round wire will be discussed.

Fatigue Testing

Fatigue testing is a vital component for measuring the strength and long-term performance of products. Fatigue is the fracture of a metal by cyclic stressing or straining7. A fatigue fracture generally occurs in three stages: 1) crack initiation, 2) crack propagation and 3) catastrophic fracture of the remaining cross section. Fatigue damage is caused by the combined action of cyclic stress, tensile stress and plastic strain. If any one of these three is not present, a fatigue crack will not initiate and propagate. The plastic strain resulting from cyclic stress initiates the crack; the tensile stress promotes crack propagation or growth. Studies have shown that microscopic plastic strains can be present at low levels of stress where the strain might otherwise appear to be totally elastic. Spring processing, heat treatment, surface treatment, finishing, and service environment can significantly influence the behavior of a metal under cyclic loading. A detailed description of all of the factors that must be considered for the prevention of fatigue failure is beyond the scope of this article. More information is available for fatigue in general as well as specifically for springs in many references. A good starting point is reference8 .

Figure 1. Fatigue striations in an aluminum alloy subjected to loading at high stress (10 cycles) and low stress (10 cycles) alternately as the fatigue crack progressed across the sample9. (Reprinted with permission of ASM International. All rights reserved. www.asminternational.org)

As wire manufacturers evaluate new rod sources for the production of wire products intended for dynamic, cyclic spring applications such as music spring wire, they often use fatigue testing as a critical tool to assure satisfactory wire performance.

While this is not an article about fatigue failure analysis, Figure 1 shows how a fatigue crack, once initiated, proceeds to grow under alternating high stress and low stress cycling.

Rick Gordon is the technical director for SMI. He is available to help SMI members and non-members with metallurgical challenges such as fatigue life, corrosion, material and process-related problems. He is also available to help manage and oversee processes related to failure analysis. This includes sourcing reputable testing labs throughout North America, forwarding member requests to the appropriate lab and reporting results and recommendations. He can be reached at c.richard.gordon@gmail. com or 574-514-9367.

Figure 2. Schematic of the Kenyon rotating arc fatigue test from the original patent10 .

Figure 3. Schematic representation stress in the wire during rotation in the rotating arc fatigue test where σa is the stress amplitude.

Figure 4. Schematic of the Kenyon rotating arc fatigue test showing wires in a high stress and low stress positions10 . Tension

σ Stress σm = 0

σa

Time t

Compression

The most efficient method used to determine the high cycle fatigue strength of wire samples is the rotating bending fatigue test. In this article, we will focus on two tests: the Kenyon rotating bending fatigue test and Hunter rotating bending fatigue test. With these test methods, a bending strain is imposed on a wire sample, and the sample is rotated, while remaining in the bending plane. Testing is done to establish the fatigue limit or endurance limit of materials. Fatigue testing is important to springmakers as historical work has shown that the fatigue performance of wire is well correlated with spring fatigue performance8 . Other test methods that are used for fatigue testing of wire products include the reverse bend test5 and axial fatigue test.

Kenyon Rotating Arc Fatigue Tester

John Kenyon obtained a patent (US Patent 2,170,64010) in 1939 for the rotating arc fatigue test.

Figure 2 is a schematic of the testing unit from the Kenyon patent. The wire is bent in an arc less than 180 degrees and fixed so one end is free to move at a tangent to the holding device. The wire sample will assume the arc of a circle. The sample is rotated using a constant rpm motor. Thus, the stress in the wire goes from compressive to tensile as the wire is rotated and the mean stress is zero, as shown in Figure 3.

Figure 3 is a schematic representation stress in the wire during rotation in the rotating arc fatigue test where σa is the stress amplitude.

Figure 4 shows wires in the test unit demonstrating a higher stressed sample and a lower stressed sample. Approximately 15 inches of wire undergoes uniform stressing in this test, which is a relatively large sample test length. The liquid in the tank (Figure 2) is usually oil used to dampen vibration. Two test fixtures can be inserted in one tank. Different liquids can be used, for example to study corrosion fatigue.

Stress is calculated using the following equation:

S = d*E/(2R)

Where: d = wire diameter (inches) E = modulus of elasticity (30x106 psi for steel) R = radius of curvature (inches)

The common size range for this type of unit is 0.035 in. to 0.060 in. (0.89 to 1.5 mm) diameter.

Some test procedures suggest that the initial stress should be set at 60 percent of

the wire ultimate tensile strength (UTS), while other suggest a value of 45 percent of the wire UTS. Additional samples are tested at decreasing levels of stress, in 5 percent increments until the fatigue limit is reached. For initial studies, some suggest testing three specimens at each stress level. A constant rpm motor (3600 rpm) runs 2780 minutes (46.3 hr.) to reach 10,000,000 cycles.

Stress (S) is plotted on the y-axis versus the number of cycles to failure (N) on the x-axis. A logarithmic scale is used for N as shown in Figure 5. This is typically referred to as an S-N curve. This is an interesting graph for mild steel (a ferrous alloy) and an aluminum alloy (nonferrous alloy). The mild steel data shows a plateau known as the fatigue limit or endurance limit where the aluminum alloy shows a continuous increase in cycles with decreasing stress. For material exhibiting behavior like the mild steel sample, 10,000,000 cycles without failure is considered to be the fatigue limit. For the aluminum alloy, the stress at 100,000,000 or 500,000,000 cycles is considered to be the fatigue strength.

Figure 6 shows example data from the Kenyon patent for four materials which show a fatigue limit: Piano wire (sample A), two samples of high carbon wire (samples B and C) and a copperberyllium wire (sample D). Data shown includes the ultimate tensile strength (UTS), fatigue limit (FL) and FL/UTS ratio for each material. Many companies build their own test units based on the design concept from the Kenyon patent. The design is suitable for adoption for testing larger diameter wire samples. With larger wire diameters, vibration is a key point for consideration and test speeds may be lower.

Hunter Rotating Beam Fatigue Tester

Another rotating bending type test used to test smaller diameter wires is the Hunter rotating beam fatigue tester. These testers were originally produced by the Hunter Spring Company, Lansdale, Pennsylvania. The common size range for this type of unit is 0.002 in. to 0.040 in. (0.05 to 1.0 mm) diameter. In this test, the maximum stress occurs at one point at ρmin as shown in the Figure 7 schematic. Parameters are also defined in this figure for the test and the modulus of elasticity E is shown for steel. Figure 8 shows a commercial test unit12 . As in the case for the Kenyon fatigue tester, a synchronous motor running at 3,600 rpm will take 2780 minutes (46.3 hr.) to reach 10,000,000 cycles.

Calculated bending stress (1,000 psi) 60 50 40 30 20 10 0

105 106 107 108 109 Number of cycles to failure (N) Figure 5. Typical fatigue curves for ferrous and nonferrous metals11 .

Mild steel Fatigue limit

Aluminium Alloy

Figure 6. Fatigue test results presented in the Kenyon patent10 . C = 1.198* (Ed/σb) [in.] C = Chuck to bushing distance E = 30,000,000 psi E = Elastic modulus d = Wire diameter (in.) r = Wire radius (d/2) (in.) σb = E(r /ρmin) σb = Bending stress (test stress) psi L = Sample length (+0.75 in. to allow for insertion in the chuck and bushing [in.] h = Distance from the chuck to wire bend height at ρmin

L = 2.19C h = 0.835 C ρmin = 0.417C h

c Driven Idling chuck chuck

ρmin

Figure 7. Hunter rotating beam fatigue test schematic.

Figure 8. Hunter rotating beam fatigue test set up12. Wire guides and a wire break detection fixture are shown in the insert. Photographs are courtesy of Fort Wayne Metals.

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Summary

In this article, two types of rotating bending type fatigue testers for testing round wire products were described. The tests are well established and continue to be used for testing wire products intended for cyclic and/or safety critical applications. n

References

1. Gordon, C.R., Types of Tests Used to Characterize Springmaking

Materials – Part 1: The Tensile Test, Springs, Winter 2020, p.27 2. Gordon, C.R., Types of Tests Used to Characterize Springmaking

Materials – Part 2: The Coiling and Wrapping Tests, Springs,

Spring 2020, p. 27. 3. Gordon, C.R., Types of Tests Used to Characterize Springmaking

Materials – Part 3: The Torsional Ductility Test, Springs, Summer 2020, p.17. 4. Gordon, C.R., Types of Tests Used to Characterize Springmaking

Materials – Part 4: Hardness Testing, Springs, Fall 2020, p.19. 5. Gordon, C.R., Types of Tests Used to Characterize Springmaking

Materials – Part 5: The Reverse Bend Testing, Springs, Winter 2021, p.19. 6. Gordon, C.R., Ferrous Testing & Properties, Fundamentals of

Wire Manufacturing, WAI, Spring 2021. 7. Wright, R.N., Wire Technology: Process Engineering and Metallurgy, Elsevier, p150 (2016). 8. ASM Handbook, Vol. 1, Properties and Selection: Irons, Steels, and High-Performance Alloys, 10th edition, ASM International,

Materials Park, OH, (1990). 9. Chandler, H., Metallurgy for the Non-Metallurgist, (1998), ASM

International, Materials Park, OH 10. Kenyon, J.N., “Fatigue Testing Machine,” US Patent No. 2,170640, August 22, 1939 11. Dieter, G.E., Mechanical Metallurgy, (1976) McGraw-Hill, USA 12. Valley Instruments – Division of Positool Technologies, Inc., www.positool.com