What is a tensile test?
A tensile test pulls or stretches a sample and as a result the extensibility/elongation and tensile strength properties are measured in terms of force required to stretch and distance something can be stretched to.
The simplest method of tensile testing is when the specimen comes in an elongated shape (either rectangular or circular in cross-section) or from which reproducible test beams can be cut. The specimen is gripped at either end and stretched until it breaks. This is uniaxial tension. One problem with many foods is that of holding the sample so that the break occurs within the sample and not at the jaws that hold the sample. Cutting out dumbbell-shaped test pieces and holding the sample at the wide ends often solve this problem. The sample is then more likely to break in the narrow centre of the test piece. Another method is to encase the ends of the sample between e.g. two pieces of perspex, or freezing the ends before clamping to avoid cutting of the sample at the grip point. Another approach is to perform a bi-axial tensile test where the sample is held like a circular drum skin and stretched in all directions by forcing a ball probe through the centre. This type of testing is popular for testing films and certain foods e.g. tortillas where the measurement of burst strength is required. Whilst clamping a specimen for tensile tests may present difficulties, these can often be overcome by careful choice of the clamping fixture.
Why perform a tensile test?
Many materials are subject to tensile situations where these tensile properties then become important to measure and control in order to be suitable or to maintain the required functionality of a product. For certain materials the ability to withstand tensile stresses can impact on product damage and consumer safety.
Many foods and pharmaceuticals are not normally subject to tensile forces in their manufacture and consumption which is understandable because the process of mastication involves compression, not tension, of the food between the molars. There are however exceptions, e.g. films, dough, gels, spaghetti, confectionery and adhesives where tensile properties are very important and a necessary or expected characteristic of the product. In addition, packaging in all industries can be tested in this way and the force to pull apart or damage the sample in this way is a valuable assessment.
The advantage of tensile testing over compression is that the start of fracture can be observed easily because it is nearly always at the outside of the sample, while with uniaxial compression the start of fracture is often inside the test-piece.
Tensile tests are also used to measure the adhesion of a food to a surface. In this type of test the sample of food has a probe pressed onto it after which the force required to pull it off is measured (see Adhesion for further information).
Properties that can be measured with a tensile test
Tensile tests are typically chosen to measure:
Tensile strength, tensile ‘break point’, extensibility, stretchiness, elongation, burst point, tensile modulus, yield stress and strain, strength and strain to fracture, fracture toughness etc.
To understand how these properties are measured visit the Textural Properties page.
Typical probes and fixtures used for tensile tests
Many different fixtures are available to provide a range of possibilities for successful gripping of the sample in order to stretch it and avoid breakage where it is being held in order to measure the desired tensile properties:
Code: A/TG • A/MTG • A/HDT • A/TGP
Allows measurement of alginate raft strength.
To understand how these fixtures are designed and manufactured visit the Texture Analysis Attachments page.
Items with codes prefixed 'HDP/' must be used with the HDP/90 Heavy Duty Platform.
Items tagged * are Community Registered Designs.
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Tensile testing is less common than compression testing, partly because it is more difficult to grip the sample in such a manner that a tensile load can be applied.
Find out how Stable Micro Systems have overcome some of the challenges of this type of test, and read how a wide variety of samples may be tested by utilising one of the wide range of tensile grip sets or specialist devices. Request our article Tips and tricks for successful Tensile Testing.