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. A tensile test can be performed on a Materials Testing instrument or Texture Analyser which are force measuring instruments that perform compression and tensile tests and provide an objective quantification of compressive or tensile strength.
Tensile strength is the maximum force (or stress) that can be applied before it breaks or fails or before permanent deformation occurs. This point is also called the yield point of the material. During any tensile test there are other parameters that can be measured such as Young’s Modulus, Tensile Strain and Elongation.
Why perform a tensile test?
Many materials and/or products are subject to tensile situations where their tensile properties then become important to measure and control in order to determine whether they will be suitable or able to maintain the required functionality. For certain materials the ability to withstand tensile stresses can impact on product damage and consumer safety therefore the measurement of materials under tension can be an imperative assessment that engineers and quality managers need to ascertain. A lack of mechanical integrity would otherwise cause a material to fail at some point which could jeopardise safety and ultimately consumer dissatisfaction.
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.
Packaging materials and products in all industries need to be very strong to bear the conditions that they are subject to during storage, handling, stacking and transit. The tensile strength of these materials is one of the most important properties that needs to be determined to ensure they can bear the forces that are exerted on them during their usage. The higher the tensile strength, the stronger the packaging will be and the more able it will be to provide protection to the enclosed products. All packaging materials, such as paperboard and plastics can be tested in this way and the force to pull apart or damage the sample in this way is a valuable assessment.
What types of Tensile Test are there?
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 using Tensile Grips. This is uniaxial tension and is the basis of many Standard ISO or ASTM Test Methods for materials testing. With the development of new materials, there are often instances where a standard method is not available or where the sample cannot be prepared in the manner in which a standard method stipulates. A standard method might also not be available for the tensile property that is required to measure.
One problem with many foods, for example, 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 solves 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.
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
Tensile testing is less common than compression testing, partly because it is more difficult to grip/hold the sample in such a manner that a tensile load can be applied without the sample breaking at the grip/hold point. For this reason, a wide range of fixtures have been designed to overcome sample mounting issues and prevent grip point failures thereby allowing a successful measurement of tensile properties.
Code: A/TG • A/MTG • A/HDT • A/TGP
Allows measurement of alginate raft strength.
Why perform a tensile test?
Axes units (N, g, kg, N/mm, N/mm2 etc.) can be chosen based on a standard method requirement, manufacturer’s material specification requirements, academic literature submission guidelines or operator choice.
Typical Texture Analyser graph with annotated properties of material tension to failure
Stress-strain graph from a tensile test on a tin sheet dogbone sample
As with all tests, the force applied, the distance moved by the probe and the time are all recorded. The force-distance graph usually begins with a straight section that corresponds to elastic (reversible) deformation, then most samples show a curved section that shows plastic (irreversible) deformation. 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. Different materials show very varied graph shapes. Several useful parameters can be calculated from a tensile stress-strain graph, using the standard engineering equations for stress and automatically collected in Exponent software as long as the sample’s stress area has been input into the software. The more accurate this measurement, the more accurate the stress data.
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.
You might also be interested in:
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.