Dissemination of IT for the Promotion of Materials Science (DoITPoMS)

DoITPoMS Teaching & Learning Packages Introduction to Mechanical Properties of Materials Tensile test: force and extension
Introduction to Mechanical Properties of Materials AimsBefore you startIntroductionTensile test: force and extensionStress, strain and Poisson's ratioElastic deformation: Hooke's law and stiffnessViscoelasticityPlastic deformation: strength and ductilitySlip: resolved shear stress and Schmid factor, Taylor factorHardness and work hardeningCreepFracture: toughnessDuctile-brittle transition temperatureSummaryQuestionsGoing further
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Tensile test: force and extension

One of the most common ways in which the mechanical properties of a material can be assessed is through a tensile test.

'Tensile' means the sample has increased in length, i.e. extension, with a positive displacement. 'Compressive' means the sample has decreased in length, i.e. compression, with a negative displacement.

Here, a specimen is commonly cut into a 'dog bone shape', which gives a better grip at the end and a constant cross sectional area in the middle where it is stretched. The corresponding force and extension of the sample are recorded simultaneously. Such a test can continue until it reaches a predetermined point or until the material is unable to sustain the load and fails.

tensometer image

Figure 1: A tensometer with dog-bone sample loaded

You can investigate the mechanical behaviour of copper below, by adjusting the cross-sectional area and the length, and plotting force as a function of extension.

We can see that the behaviour measured by force and extension depends on the sample dimensions (cross-sectional area and length) of the samples. It is therefore more common to plot stress and strain instead, which translate force and extension to quantities independent of geometry.

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