Ductile vs Brittle behavior
The behavior of materials under loading can be classified as ductile or brittle depending on if the material undergoes gross plastic deformation. The following figure represents the stress-strain curve for ductile and brittle materials.
As we can see that a brittle material almost fails at elastic limit. But brittleness is not an absolute property of a material. For example, tungsten is brittle at room temperature and is ductile at high temperatures. The three factors contributing to brittle fracture are : triaxial state of stress, low temperature and high strain rate. All these factors need not be present for a material to show brittle cleavage. A triaxial stress which is present at notch along with low temperature may lead to brittle fracture. Also high strain rate assists those two factors.
For most of the engineering materials ductility is an important criteria. Thus, little amount of ductility is needed for most of the material. Also a material being ductile has its own advantage. A ductile material before failure undergoes plastic deformation thus indicating that it is going to fail. Whereas a brittle fracture is sudden without any warning. Thus most of the place ductile materials are preferred over brittle materials.
Usually ductile facture is preceded by gross plastic deformation known as necking which begins from the maximum stress. Necking begins when increase in strength due to strain hardening fails to compensate by the decrease in cross-sectional area. A rupture occurs when a ductile material is drawn down to a line or a point and then fails. The necking introduces a triaxial state of stress in that region. Many cavities are also formed in that region which on continued strain grows into a large crack. This crack grows in a direction perpendicular to the axis of the specimen. It then propagates along 45° i.e. the shear plane. This type of fracture is also known as cup and cone fracture. While the crack growth direction id outwards i.e. transverse to the tensile axis of the specimen, in a microscopic scale the crack actually moves in a zig-zag manner. The preferred sites of void formation are usually in inclusion, secondary phase particles etc. , whereas for pure metal they are grain boundary triple point. Ductility decreases as the void fraction increases, as the strain-hardening exponent n.
