All materials, from steel cables to plastic casings, have a limit to the external load they can endure before failure. Understanding this physical boundary is essential for design and safety, ensuring engineered objects function reliably. When a material is subjected to tension, its internal structure resists the load until total physical separation occurs. This maximum resistance measurement is known as breaking strength, which provides the absolute failure point for the material.
Defining the Limit of a Material
Breaking strength, also called fracture strength or rupture strength, is the maximum mechanical stress a material can tolerate before it physically breaks. This measurement represents the final loss of structural integrity under a tensile load. Standard units for this property are stress units, such as megapascals (MPa) or pounds per square inch (psi). It is important to distinguish between the total force applied and the stress required for failure. Stress is calculated by dividing the failure force by the material’s initial cross-sectional area, providing a normalized value intrinsic to the material that allows engineers to compare components regardless of size.
The Testing Process
Breaking strength is determined through tensile testing using a Universal Testing Machine (UTM). A machined sample is secured by two grips, and a steadily increasing pulling force is applied along its longitudinal axis until it fractures. During the test, the machine records the applied load and resulting elongation, generating a stress-strain curve. This curve maps the material’s behavior under load, from initial elastic stretching to final rupture. Breaking strength is identified as the final data point on this curve, representing the exact stress coordinates where the material snaps and loses all load-bearing capacity.
Breaking Strength Versus Other Measures of Failure
Breaking strength is distinct from other measures like yield strength and ultimate tensile strength (UTS). Yield strength is the point where the material ceases to deform elastically and begins to deform permanently, a limit reached long before the material breaks. UTS is the maximum stress a material endures throughout the test, marking the highest point on the stress-strain curve. For brittle materials, such as ceramics, the UTS and breaking strength are nearly identical because the material fails suddenly with minimal deformation. Conversely, for ductile materials, the breaking strength is often lower than the UTS when calculated using the original cross-sectional area, due to the severe reduction in the cross-section during necking.
Applications in Safety and Design
Knowledge of breaking strength is fundamental to calculating the safety factors required in structural and mechanical design. Engineers do not design components to operate at their breaking strength; instead, they use this value to establish a safe working load limit (WLL). This limit is calculated by dividing the breaking strength by a factor of safety, which is a number greater than one, often ranging from 3 to 10 depending on the application’s risk. For instance, in applications involving lifting equipment, like ropes, chains, and cranes, the breaking strength data directly informs the maximum load that can be safely handled. This built-in margin of safety accounts for unpredictable variables such as material flaws, wear and tear, dynamic loading, and environmental conditions that could reduce the material’s failure threshold over time.