A tensile test is a relatively straightforward process for the most part. The purpose of the test is to apply a tensile, or pulling, load to a specimen of the material and observe both the load at which the material breaks and the elongation of the specimen at the point of breaking. A slightly more advanced testing equipment will also give you the Young’s Modulus – also known as the Modulus of Elasticity – which takes into account the extent to which incremental loads create incremental elongations in the material.
Most mechanical labs will have the equipment needed to test for tensile properties. However, while materials such as steel and rubber have well documented and easily accessed standards on how the test must be performed, polymers – most specifically high-performance plastics – can be a different story altogether.
We have observed, on more than one occasion, that the values obtained from a lab differ significantly from those on the material datasheet. Since the materials we buy are from reputed suppliers with their own in-house labs, this creates some confusion as to whether the properties are indeed not meeting the specifications, or whether there is a way to test the materials that the external labs may not always be aware of.
A few factors are at play here and need to be properly understood before sending any polymer to a lab for testing:
1. Shape to be tested
Most standards will specify a certain set of shapes that need to be made for testing. Usually, these are dumbbell in shape, with a thickness of 3mm to 5mm. The dumbbell shape is wider at the ends, where the test sample will be gripped by the machine. The cross-section area of the thin section in the middle is where the load is applied when the dumbbell is stretched lengthwise.
While most standards offer a general suggestion on how to make the dumbbell shape, a lot of thought needs to be put on the exact shape as well as the thickness. Care also needs to be given on deciding how the part should be cut to make the dumbbell, as we shall see below
2. Method of processing – polymers such as PEEK, PTFE, Polyimide, and even PVDF can be processed in a number of ways. PTFE, for example, lends itself to compression moulding, ram extrusion, isostatic moulding, and paste extrusion. Similarly, PEEK can be compression moulded, extruded, or injection moulded. The varying process methods would accordingly cause the internal grain structure of the polymer to be similarly varied.
In the case of PTFE, a tensile test can be performed on a compression moulded part that has then been skived into a 3mm thick tape. A dumbbell can then be punched out of this tape. However, because of the nature of compression moulding, the tensile strength will vary depending on whether the dumbbell is cut along the length of the tape or across it. For this reason, many datasheets for PTFE will give different values for LD (longitudinal direction) and TD (transverse direction). If a lab offers to punch the material into the dumbbell, the manufacturer will still need to specify which the longitudinal direction is so that the best result can be obtained. Similarly, ram extruded tubes would generally exhibit lower tensile properties when compared with paste extruded tubes. This is because ram extrusion is an additive process, whereas paste extrusion is continuous. Hence, there are weak spots in a ram extruded tube that will bring the value of the tensile strength down.
With PEEK, extruded rods would similarly exhibit better tensile properties than compression moulded materials. An added issue with PEEK is that machining the dumbbell shape places a lot of internal stress on the material and the dumbbells themselves need to be properly annealed before any tests are done.
3. Testing speeds
Even if the above precautions are taken, polymers are extremely sensitive to testing speeds. Again, standards will prescribe a range of anywhere from 1mm per minute to 50mm per minute for the speed of the testing, but it is the manufacturer that needs to often take trials to determine which speeds offer values that match the materials base properties and then use this as the benchmark. PTFE generally benefits from being pulled faster, so we test PTFE at 5mm/minute. PEEK, on the other hand, does not do well under high speeds and will break very easily if tested at 5mm/minute. Hence, 1mm/minute is the maximum speed at which PEEK should be tested.
Beyond testing, it is also useful to note that some parts simply cannot be tested. Machined components, for example, cannot be subjected to a tensile test as prescribed in the standards, since the components are shaped in such a way that a dumbbell shape cannot be extracted from it. In such cases, it is up to the manufacture and OEM to decide between them what tests can be done and what tests – for example a test on a moulded part that acts as a proxy to the component – would suffice as representative of the final component.
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