FLEXURE
Example standard methods: ASTM D790, ISO 178
Flexural testing, commonly referred to as ‘three-point bend testing’, involves a long sample placed on two supports (or “rollers” due to their curved surface) a set distance apart, with a third roller moving down to contact the sample halfway between these supports. In some situations, two rollers are lowered onto the sample to instigate fourpoint bend testing. The loading arm moves down at a constant speed to bend the sample. If the sample is sufficiently brittle and the force required within the limit of the load cell, fracture will occur.
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| Typical three point bend test |
It is a very useful test for monitoring quality of irregular objects, such as the snap of a biscuit or the toughness of a dog chew. However, this test setup can also provide useful stress-strain data if the sample has a uniform rectangular or circular cross-section, providing accurate measurements are made of the sample dimensions.
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| Schematic diagram of a typical three point bend test |
During the test, the force applied, the distance moved by the probe and the time are all recorded in Exponent Connect software. Different samples will give different load-distance responses – stronger and stiffer samples show higher forces, brittle samples break before any plastic deformation occurs and tough samples show a large area under the curve corresponding to a large amount of energy required for deformation.
Since the physical properties of many polymers (especially thermoplastics) vary with temperature, it is sometimes suitable to test materials at the temperature at which they will be used in practice.
Coefficient of Friction Rig
SLIDING FRICTION
Example standard method: ISO 8295, ASTM D1894
Sliding friction is an important measurement for many polymeric materials, particularly those in film form used in packaging, but any semi-rigid to rigid polymer may be measured in this way.
Measurements of frictional properties may be made on a film or sheeting specimen when sliding over itself or over another substance. The coefficients of friction are related to the slip properties of plastic films that are of wide interest in packaging applications. The coefficient of friction is defined as the ratio of the frictional force to the force, usually gravitational, acting perpendicular to the two surfaces in contact. This coefficient is a measure of the relative difficulty with which the surface of one material will slide over an adjoining surface of itself, or of another material. The static or starting coefficient of friction is related to the force measured to begin movement of the surfaces relative to each other. The kinetic or sliding coefficient of friction is related to the force measured in sustaining this movement.
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| Schematic diagram of a typical sliding friction measurement |
Slip properties are generated by additives in some plastic films, for example, polyethylene. These additives have varying degrees of compatibility with the film matrix. Some of them bloom or exude to the surface, lubricating it and making it more slippery. Because this blooming action may not always be uniform on all areas of the film surface, values from these tests may be limited in reproducibility.
Stable Micro Systems are able to assist with many polymer applications well beyond the realms of the standard methods outlined in this article, including imitative, empirical and applied methods, so your product can be tested in the way in which it was Coefficient of Friction Rig intended to be used.
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.
For more information on how to measure texture, please visit the Texture Analysis Properties section on our website.
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