The stress applied to a wire produces a strain in it. Young’s modulus or elastic modulus (Y) is defined as a ratio of tensile stress to the resulting tensile strain. Mathematically,

The SI unit of Young’s modulus is N/m².
Young’s modulus characterizes elastic properties of materials under tensile forces independent of its size or shape. The elastic modulus is the measure of the stiffness of a material. In other words, it is a measure of how easily any material can be bent or stretched.

The strain-stress curve, shown in Figure 6.9, is a graphical way to show the reaction of a material when a load is applied. You can see that the elastic modulus is the slope of stress and strain diagram up to the limit of proportionality. The graph of stress against strain is linear or a straight line only for stresses below the elastic limit labeled as yield strangth.

The elastic limit of a material depends on its internal structure. For example, most metals such as Aluminium and copper have a low elastic limit while most nonmetals such as rubber have a high elastic limit. Based on this property, materials can be categorized as ductile, brittle or malleable.

• A material is said to be ductile if it is drawn to a wire. Metals are dectile.
• There are materials that have no plastic stage at all before they break. Such materials are described as brittle.
• Materials that can fold or roll easily into sheets is called malleable. Metals are malleable.