The uniaxial tensile test is the most commonly-used mechanical testing procedure. However, while it is simple in principle, there are several practical challenges, as well as a number of points to be noted when examining outcomes. The uniaxial compressive test is also used occasionally, particularly when large pieces are unavailable or when the machining of the relatively complex shapes needed for tensile testing is difficult.
A central issue concerns the specimen shape. For a compressive test, a simple cylinder can be used, but a more complex shape is needed for tensile testing. The behaviour is monitored in a central section (the “gauge length”), in which a uniform stress is created. The grips lie outside of this section, where the sample has a larger sectional area, so that stresses are lower. If this is not done, then stress concentration effects near the grips are likely to result in premature deformation and failure in that area. Several different geometries are possible.
All testing systems have some sort of “loading train”, of which the sample forms a part. This “train” can be relatively complex – for example, it might involve a rotating worm drive (screw thread) somewhere, with the force transmitted to a cross-head and thence via a gripping system to the sample and then to a base-plate of some sort. It does, of course, need to be arranged that, apart from the sample, all of the components loaded in this way experience only elastic deformation. The same force (load) is being transmitted along the complete length of the loading train. Measurement of this load is thus fairly straightforward. For example, a load cell can be located anywhere in the train, possibly just above the gripping system.
Measurement of the displacement (in the gauge length) is more of a challenge. Sometimes, a measuring device is built into the set-up – for example, it could measure the amount of rotation of a worm drive. In such cases, however, measured displacements include a contribution (elastic) from various elements of the loading train, and this could be quite significant. It may therefore be important to apply a compliance calibration. This involves subtracting from the measured displacement the contribution due to the compliance (inverse of stiffness) of the loading train. This can be measured using a sample of known stiffness (ensuring that it remains elastic).
Several types of device can be used to measure displacement, including Linear Variable Displacement Transducers (LVDTs), eddy current gauges and scanning laser extensometers. These have resolutions of the order of 1 µm. More specialised (and accurate) devices include parallel plate capacitors and interferometric optical set-ups, although they often have more limited measurement ranges.
Alternatively, displacement can be measured directly on the gauge length, eliminating concerns about the system compliance. Devices of this type include clip-gauges (knife edges pushing lightly into the sample) and strain gauges (stuck on the sample with adhesive). The latter have good accuracy (±0.1% of the reading), but are limited in range (~1-2% strain). They are useful for measurement of the sample stiffness (Young’s modulus), but not for plastic deformation.
There are potential sources of complication with both tensile and compressive testing. A major issue with tensile testing is the onset and development of “necks” (regions of strain concentration). This is covered as a separate topic (“Necking & the Ultimate Tensile Strength”). There are no concerns of this type with compression testing. However, there are other potential difficulties. One of these is the danger of (plastic) buckling, particularly if relatively large strains (>~10%) are to be created. In order to avoid this, the aspect ratio (height / diameter) must be kept relatively low – probably not much more than unity. Since a very large sectional area might lead to excessive load requirements, this often means that the height (gauge length) of the sample is limited. This in turn leads to relatively low displacements, placing a premium on measurement accuracy (with the points made above about compliance calibration applying equally to tension and compression).
There are also concerns about the effect of friction. This is potentially important, since one outcome of friction is that the stress and strain fields become non-uniform. In practice, it is common to apply lubricant to the contact surfaces of the sample and to assume that any effect of friction will be small. With no friction (coefficient of friction, μ = 0), there will be unhindered sliding at the interface, the sectional area will remain uniform along the sample length during deformation and there will be no “barrelling”. However, the high contact pressure tends to force lubricant out of the region between platen and sample, so this assumption is often not valid. A compression test that leads to noticeable barrelling is likely to give unreliable results (except possibly if it is modelled using FEM, with a reliable value of μ, although this is very rarely done).
© 2020 Plastometrex. All rights reserved.