Material testing is designed so the spring manufacturer and/or end user can have confidence that the material they are receiving is as expected for that grade. Therefore, the tests we can perform are based predominantly on the requirements of national and international standards for spring material manufacture.
There are a wide range of techniques and tests required in these standards, but we have the capabilities and expertise to preform them all. Examples of the tests stipulated in these standards, which we can performed are summarised below.
This test applies a tensile load to the material (usually in the form of wire) until it fractures. This can then be used to calculate the ultimate tensile strength (UTS or Rm) for the material, and the reduction of area prior to failure. Both of which are typically specified in spring wire standards and are required for a wire to meet the specification.
This test can also be used to determine the Young’s modulus and yield strength, if required.
This test involves using specialist equipment to uniformly grip a specified length of wire at either end, keeping the wire as straight as possible. The wire will then be rotated at one end while counting the number of turns prior to failure, determining the ductility of the wire during twisting.
This is important, not only because it is typically stipulated in the majority of standards, but also indicates the ductility of the wire under a torsional load. Both compression and extension springs apply a torsional load to the wire during operation.
This test wraps a wire around a mandrel a specific number of times to form a tightly wound helix. The number of turns required, and the diameter of the mandrel is stipulated by the wire standard. During this test the wire must not fracture, and the surface of the wire must remain free from cracks. This determines the coilability and ductility of the wire and can be used to test the adhesion of the zinc coating on galvanised wire.
Comprises bending a wire, which can move freely, over a mandrel of specified diameter, through a specified angle at ambient temperature. Like the wrapping test, this should result in an unbroken and unblemished wire and tests the ductility and toughness of the wire.
This test requires a sample of wire, of specific length, to be tightly coiled around a mandrel, the diameter of which is stipulated by the relevant standard. This helix is then extended so that the coils are separated once the extending force is removed. The pitch of the coils is then examined to find any abnormalities in the helix. Highlighting any deflects which could affect the coiling of springs.
This involves sectioning a small section of wire and mounting it both in the longitudinal and transverse directions. This is then ground and polished to a 1-micron finish. This sample can then be examined under an optical microscope to determine the type and size of defects on the surface of the wire, as well as coating thickness. This is an especially important test for wires which are manufactured to dynamic grades as surface defects can cause fatigue failures and therefore have size limitations within dynamic material standards.
These samples can then be etched, and the microstructure of the wire examined. Checking it aligns will with the expected microstructure for that specific grade and does not contain any defects, such as decarburisation or induced martensite, which could affect the performance of the material. Decarburisation depth and amount of induced martensite, as well as microstructure, are all requirements for spring wire in a large number of international standard.
VICKERS HARDNESS TESTING
This test uses a specified force to indent a polished sample with a pyramid shaped diamond. The indent left on the surface of the sample is then measured and converted into a hardness. The result is typically used as a measure of the strength of the material when the size of the material is too large to be widely tested using a tensile tester