Technically Speaking
Types of Tests Used to Characterize Springmaking Materials — Part 3: The Torsional Ductility Test By C. Richard Gordon
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n the Winter 2020 issue of Springs1, I discussed the tensile test as an important mechanical test for springmaking materials. In the Spring 2020 issue of Springs2, I discussed the coiling and wrapping tests as important technological tests for springmaking materials. This is the third article in the series on the subject, which will discuss another important test used to characterize springmaking materials, the torsional ductility test, or torsion test. This article includes materials from the presentation that I made at the 2019 SMI Metal Engineering eXpo in Pittsburgh3.
The type of torsion testing varies from product to product but can be characterized as failure, proof or product operation testing. • Failure testing – Twisting the product, component or specimen until failure as an indication of ductility. • Proof testing – Applying a torsional load and holding this torque load for a fixed period of time. • Operational testing – Testing complete assemblies or products to verify that the product performs as expected under torsional load.
Overview
Specimens suitable for torsion testing can take many forms. Torsion properties can be determined from a machined metal sample, wire/rod/bar/cable/tube, or from complete products and components. This article will focus on failure testing of round wire and rod. The torsion test will be described along with the different assessment parameters, including number of twists to failure, fracture appearance, and torque as a function of the number of twists. The torsion test has been standardized and details can be found in 1) “ASTM A938 – 18 Standard Test Method for Torsion Testing of Wire4” and 2) “ISO 7800:2012 – Metallic Materials – Wire – Simple Torsion Test5.” Torsional ductility in high carbon steel products is fascinating in that it can be a function of a number of factors, including steel wire rod characteristics, wire drawing practice, heat treatment and test condition. The torsion test has evolved over time. The test was originally developed as a ductility test where wire samples were
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, the torsional ductility (torsion) test will be discussed. The uniformity of tensile strength and ductility of the wires used to produce springs are critical to successful spring production and end use. The torsion test can be good indicator of these properties, but primarily as a measure of ductility.
Torsion Testing Torsional ductility is an important quality parameter for assessing the suitability of wire for many wire products, including springs, because many products and components are subject to torsional forces during operation. It is included as a specification parameter for many sophisticated product standards worldwide because the test can identify suspect material which may result in a spring failure. Torsion tests can be performed by applying only a rotational motion, or by applying both axial (tension or compression) and rotational forces to the sample of interest.
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 2020 / 17
