A tensometer is a device used to evaluate the tensile properties of materials such as their Young's modulus (i.e. the degree to which they stretch under stress) and tensile strength. It is usually a universal testing machine loaded with a sample between two grips that are either adjusted manually or automatically to apply force to the specimen. The machine works either by driving a screw or by hydraulic ram. The latter have the great advantage of being able to create much more complex loading patterns, such as the cyclical loads needed for measurement of fatigue strength. Machines can also be equipped with environmental chambers for testing at different temperatures or variable humidity, for example.
Material to be tested must be cut to a specific shape so as to fit the grips, most usually in the form of a dog-bone shape when flat sheet is being tested. The sheet is cut or machined to shape, and great care is needed to create a smooth edge. If defects are left, the result may be premature failure from the defect, thus underestimating tensile strength. Different shaped samples need different grip designs to achieve their objective. Fibres for example, require rod grips around which fibre can be wound prior to straining. Product tests may even need special grips to be designed owing to the shape variability of complex products.
The measurements taken are load and extension and a tensometer gives directly the load extension curve, and not the stress-strain curve. However, the stress-strain curve can be calculated from the cross-sectional area and the original length of the specimen. Such a curve can be labelled with specific identifiable point such as elastic limit, yield point and fracture. A tensometer can be either electronic or manual, the latter being with a handle to apply force.