Ductile fracture is a mode of material failure characterized by significant plastic deformation before the material ultimately breaks. It is typically a slow and progressive process, giving warning signs before complete failure, making it more desirable in engineering applications compared to brittle fracture.
Noticeable necking in tensile specimens before rupture.
Formation of microvoids that grow, link, and eventually coalesce into a crack.
High energy absorption prior to failure.
Fracture surfaces show a dimpled appearance under a scanning electron microscope (SEM).
Occurs in materials like low-carbon steel, aluminum, copper, and other ductile metals.
Ductile fracture occurs in three distinct stages:
1. Void Nucleation
Microscopic cavities or voids form at inclusions, second-phase particles, or grain boundaries.
Typically initiated at regions of stress concentration like non-metallic inclusions or impurity particles.
2. Void Growth
Under continued loading, these voids grow as plastic deformation increases.
Material around the voids stretches and becomes thinner, causing them to grow larger.
Nearby voids start to interact and link up.
3. Void Coalescence and Crack Propagation
The growing voids begin to coalesce, forming a crack.
The crack propagates through the material, primarily along the regions of linked voids.
The final rupture typically involves a shear lip and a cup-and-cone fracture surface in tensile specimens.
Cup-and-cone fracture:
Central region: cup-shaped, formed by void coalescence.
Outer region: cone-shaped, formed by shear deformation.
Dimples on the fracture surface (seen under SEM) are evidence of void growth and coalescence.
The size and shape of dimples can provide insight into loading conditions and fracture mechanisms.
In a stress-strain curve, ductile fracture is indicated by:
A long plastic region after the yield point.
Necking near the fracture point.
The ultimate tensile strength (UTS) is followed by localized reduction in area before final failure.
Metals and alloys with high toughness and moderate to high ductility:
Mild steel
Aluminum and its alloys
Copper
Nickel
Titanium (depending on temperature and processing)
Temperature: Lower temperatures can reduce ductility, promoting brittle behavior.
Strain rate: Higher strain rates may reduce ductility.
Material purity: Impurities or inclusions promote void nucleation, influencing fracture behavior.
Microstructure: Grain size, phase distribution, and heat treatment all affect fracture mode.
Triaxial stress state: Increases void nucleation and growth, affecting ductile fracture progression.
Predictable and gradual: Gives visible warning before failure.
High energy absorption: Prevents sudden collapse, useful in structural applications.
Toughness: Ductile materials can deform to absorb impact without breaking.