Mechanical stress is the force (weight of load) divided by the area over which it is applied. The weight (measured in newtons) is calculated by multiplying its mass (in kilograms) by the Earth’s gravitational field strength (=9.81 N/kg). Stress has the unit N/m² which is dimensionally equivalent to the pascal (Pa), the SI unit for pressure.

Strain is the extention (length under stress minus length without stress) for every unit length of material subject to stress (strain = extension ÷ original length). Because both extention and original length are measured in metres, strain is a dimentionless quantity.

The stress caused by the load can either be tensile or compressive. Tensile stress occurs whenever the test material is being stretched; compressive stress when the load’s weight pushes down on the material. This is important because some substances can behave differently depending on how it is being deformed. Concrete, for example, is very strong under compression but very weak under tension.

A typical method of obtaining a tensile stress v. strain curve for a metal wire is by attaching a load and stretching it from a clamped stand. The length of the wire should be carefully measured both before and after the load weights are added, one at a time, to it and results noted down each time. The gauge (thickness) of the wire should be measured using a micrometer.

Hooke’s Law suggests that the stress applied to the wire is directly proportional to the strain that is observed. This does happen for small extensions but this relationship breaks down at a point called the elastic limit. Once this point is crossed, not only is Hooke’s Law disobeyed but the wire will be permanently deformed and some of the extension will remain even after the load is removed. If further load is applied after this point, the wire will experience more strain per unit stress than was the case when Hooke’s Law was obeyed. Eventually the wire will break. This point is called the ultimate tensile stress and is the maximum value of tensile stress per unit strain when drawn on a graph.

The graph is drawn in an unorthodox manner of having the strain on the x axis and stress on the y axis, probably so it’s evident how the wire deforms over time.

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