Types of Tests Used to Characterize Springmaking Materials — Part 6: Fatigue Testing of Wire
Technically Speaking
By C. Richard Gordon
T
his 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.
SPRINGS / Summer 2021 / 21
