Glossary

Thermal expansion, in the context of engineering and fasteners, is the tendency of any material to change its size—specifically its length, area, or volume—in direct response to a change in temperature. When a component is heated, its constituent atoms vibrate more energetically and move further apart, resulting in physical expansion. Conversely, cooling the component causes the atoms to slow down and move closer together, leading to contraction. This fundamental property is measured by the Coefficient of Thermal Expansion (α), a value unique to each material. The amount a component's length changes is proportional to its original length, the temperature change, and this coefficient.

Calculating Thermal Expansion
The change in length (ΔL) of a component due to a change in temperature (ΔT) can be calculated using a linear expansion formula.

Change in Length=Coefficient of Thermal Expansion×Original Length×Change in Temperature

Impact on Fastened Joints
Understanding thermal expansion is critical in fastener design, particularly when a joint involves multiple materials. This phenomenon, known as Differential Thermal Expansion, occurs when the fastener (e.g., a steel bolt) and the materials it clamps (e.g., aluminum flanges) have different coefficients of thermal expansion (α). If the bolt expands or contracts at a different rate than the clamped material, it can create two significant problems. The first is a loss of clamp load, where the joint loosens because the component expands more than the bolt, a common failure point. The second is the generation of excessive stress, where a highly expansive bolt can over-tighten and crush the clamped components. Engineers mitigate these risks by carefully selecting materials with closely matched α values or by incorporating elastic elements like compression washers to absorb the inevitable dimensional changes.