Ductility and Brittleness

The distinction between ductile and brittle behavior is clearly visible on the stress-strain curve. A ductile material exhibits a significant strain after the yield point and before the fracture point. This large area under the curve after yielding indicates that the material can absorb a great deal of energy before it breaks. This property is crucial for safety in many engineering applications, as a ductile failure provides a visible warning (e.g., bending or stretching) before a complete collapse. Key measures of ductility are percent elongation, \((\frac{L_f – L_0}{L_0}) \times 100\), and percent reduction in area, \((\frac{A_0 – A_f}{A_0}) \times 100\), where the ‘f’ subscript denotes the final dimension at fracture.

Conversely, a brittle material shows very little strain after its elastic limit. The fracture stress is often close to the ultimate tensile strength, and failure occurs suddenly and without warning. Ceramics, glasses, and some polymers are classic examples. The behavior of a material can also depend on external conditions. For instance, many steels that are ductile at room temperature undergo a ductile-to-brittle transition at low temperatures, a phenomenon that has led to catastrophic failures, such as in the Liberty ships during World War II.