Fracture is the separation or breaking of a material into two or more pieces under the action of stress. It occurs when the material can no longer withstand the applied load due to the growth of cracks or defects, leading to a complete loss of mechanical integrity.
Fracture behavior depends on the material properties and the conditions of loading. There are two main types:
1. Ductile Fracture:
Characterized by significant plastic deformation before failure.
Involves necking (local reduction in cross-sectional area) and the formation of microvoids that grow and coalesce.
Fracture surface appears fibrous or dimpled under a microscope.
Common in metals like steel, copper, and aluminum.
2. Brittle Fracture:
Occurs with little or no plastic deformation and happens suddenly.
Often initiated at pre-existing flaws, such as cracks, inclusions, or surface defects.
Crack propagates rapidly and perpendicular to the applied stress.
Fracture surface appears flat and shiny, often with cleavage patterns.
Common in ceramics, glass, and some hardened metals.
Crack Initiation:
Begins at stress concentrators like notches, voids, inclusions, or surface scratches.
Crack Propagation:
The crack grows with continued loading. Can be:
Stable: grows slowly under increasing load.
Unstable: grows suddenly, leading to catastrophic failure.
Final Fracture:
Complete separation of the material into two or more parts.
Fracture can occur in different modes depending on how the load is applied:
Mode I (Opening Mode): Crack opens perpendicular to the applied tensile stress.
Mode II (Sliding Mode): Crack surfaces slide over one another due to shear stress.
Mode III (Tearing Mode): Crack surfaces move relative to each other in a twisting or tearing motion.
Fracture toughness is a material property that describes a material’s ability to resist fracture in the presence of cracks. It’s critical in design to avoid catastrophic failure, especially in brittle materials.