Types of Tests to Characterise Mechanical Properties of Polymers: Tension

POLYMER TEST TYPES 

Stable Micro Systems manufactures instruments that measure the tensile and compressional properties of polymers. As with any manufacturing innovation, the end product must go through a quality control process to assess its physical properties. A Texture Analyser is a crucial part of this procedure, giving a reliable way to test the mechanical (and sensorial) properties of items by applying a choice of compression, tension, extrusion, adhesion, bending or cutting tests to measure a product’s physical properties e.g. tensile strength, snap force, brittleness, compressibility, to name but a few. 

Due to the wide-ranging properties of polymers and their vast range of applications, test types are broader than the traditional tensile and bend tests usually used for engineering materials. Testing can help to determine the optimum material for an application, design parts to withstand forces during use in applications, determine strength or longevity of parts under varying stress conditions and provide the best quality control checks for each application. 

As with any sample type, there are industry specifications that must be complied with. Many manufacturers, particularly those in consumer, automotive, medical and aerospace industries, choose to test their samples according to standard methods. 

Product performance is a key concern to manufacturers at every level of the polymer supply chain, from raw material suppliers and manufacturers through to those of the finished product. Polymer raw materials often come in powder form. These can be measured using powder measurements such as Powder Flow Analysis, Unconfined Yield Stress and Powder Vertical Shear. However, this article focusses on the properties of polymer materials in the solid state. 

Tests used to investigate polymer properties include standard configurations such as flexural, compressive and tensile in their pure form, although many tests include a combination of stress states. These include indentation and imitative measurement. Imitative measurements provide the possibility to assess samples in a manner in which they are perhaps subjected to in real-life and are not bound by the restricted form specified in a standard method, thereby providing testing flexibility and potentially more meaningful results. Time-dependent creep and relaxation properties are also of interest to many manufacturers due to the viscoelastic nature of most polymers. These tests are typically carried out using a similar configuration or test setup to standard tests, but rather than a standard load-unload cycle, tests are performed with a hold period at a set stress or strain, or cycles may be performed at a variety of test speeds.  

Tension 

Schematic diagram of a typical
tensile test – dogbone and
film samples

Example standard methods: ASTM D638, ISO 527, ASTM D882, ASTM D412. Tensile testing is widely-used in the materials industry due to its simplicity. Stress and strain can be calculated using the sample’s geometry in a more reliable way than in other tests such as flexure or compression. ‘Tensile testing’ generally refers to the placement of a sample between two Tensile Grips, which are then pulled apart at a constant speed until either a target distance is reached or the sample breaks. However, samples may be put into a tensile state in other tests; the puncture of a thin film sample clamped around its edge puts the material into a state of biaxial tension. This type of testing can still give information on the tensile properties of a material that for practical reasons might not be suited to being clamped between two grips. 

At its most basic, a tensile test can be used to measure breaking force (N) and elongation at break (mm). However, stress-strain graphs can be plotted if accurate dimensional measurements are made. These can give information on yield strength, tensile strength (at break), elastic modulus (often a secant or tangent modulus rather than Young’s modulus due to the typical polymer S-shaped curve), tensile strain at any point, yield strain and strain at break. 

When testing elastomers, test equipment may not extend far enough to break samples in tension. Instead, tensile stress is often recorded at various points, such as 200% strain. An elastic modulus may be measured at the same point. 

Examples of Tensile Grips depending on sample
type and maximum force range

Some brittle polymers have low breaking strains (<5%) with a stress-strain curve that is linear until fracture. These include polymers below their glass transition point and fibrereinforced polymer matrix composites. This often results in a high breaking strength but sudden and dramatic failure.

There is a Texture Analysis test for virtually any physical property. Contact Stable Micro Systems today to learn more about our full range of solutions.

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For more information on how to measure texture, please visit the Texture Analysis Properties section on our website.

The TA.XTplusC texture analyser is part of a family of texture analysis instruments and equipment from Stable Micro Systems. An extensive portfolio of specialist attachments is available to measure and analyse the textural properties of a huge range of food products. Our technical experts can also custom design instrument fixtures according to individual specifications.

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