TENSILE STRENGTH is the measure of the force or stress required to pull something (resistance to lengthwise stress) to the point where it breaks or before permanent deformation results. Usually it is the maximum amount of tensile stress that it can be subjected to before failure, although the definition of failure can vary according to material type and design methodology. Tensile testing is an important materials test to select a specific material for an application, for quality control, and to predict how a material will react to varying forces.
In the fields of material science, mechanical engineering and structural engineering there are three typical definitions of tensile strength:
• Yield strength: the stress at which material strain changes from elastic deformation to plastic deformation, causing it to deform permanently.
• Ultimate strength: the maximum stress a material can withstand.
• Breaking strength: the stress coordinate on the stress-strain curve at the point of rupture.
Tensile Strength can relate to the Toughness of products. The higher the tensile strength a product possesses, the more difficult or tougher it is to stretch. This property can be a positive feature in some products, for example in rope and rubber bands to an extent, but is regarded as unacceptable or negative in chewing gum, mozzarella cheese, noodles or Christmas crackers.
Typical properties that can be obtained from a texture analyser graph:
Tensile Strength, Burst Strength, Distance to Burst, Resistance to Extension/Toughness, Extensibility, Stretch Quality, Elasticity, Tug Force, Tear Strength, Elongation, Breaking Strain, Energy to Failure, Yield Stress/Strain, Resilience, 0.2% proof stress, Young’s Modulus, Resilience
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 in Exponent software. 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 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.
Typical Probe/Fixture used for Measurement:
Tensile Grips and all fixtures that allow successful holding of a product in order to test in a tensile manner >>
Tensile testing involves a sample held in two grips a set distance apart. The loading arm (attached to the top grip) moves up at a constant speed to deform the sample, first deforming it elastically then plastically. If the force required to break the sample is within the limit of the load cell, fracture will occur. This test setup can also provide useful stress-strain data if the sample has a uniform cross-section, providing accurate measurements are made of the sample’s dimensions.
“Dogbone” shaped specimens are often used in tension, with two wide sections tapering to a narrower central section. This central section has a uniform cross-section. These specimens are designed to encourage deformation away from the grips and into the central section in a controlled manner. Dogbone specimens are not mandatory, however. As long as the sample has a uniform cross-section, is long enough to grip properly and does not break off at the grips, it will be suitable for tensile testing.
The above are only typical examples of tensile strength measurement. We can, of course, design and manufacture probes or fixtures that are bespoke to your sample and its specific measurement.
Once your measurement is performed, our expertise in its graphical interpretation is unparalleled – no-one understands texture analysis like we do. Not only can we develop the most suitable and accurate method for the testing of your sample, but we can prepare analysis procedures that obtain the desired parameters from your curve and drop them into a spreadsheet or report designed around your requirements.
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