Brittleness
Brittleness is a material property characterised by the tendency to fracture suddenly under stress with little or no prior plastic (permanent) deformation, typically showing less than 2–5% elongation at fracture in a tensile test. Brittle materials store elastic energy and release it catastrophically at fracture, giving virtually no warning of impending failure. Materials such as cast iron, glass, ceramics, and concrete exhibit brittle behaviour, and engineering designs using them must account for the absence of ductile redistribution of stress.
Worked Example
Problem
A grey cast iron component fractures in a tensile test. The original gauge length was 50 mm and the fracture length is 50.3 mm. Classify the material as brittle or ductile and calculate the percentage elongation.
Solution
Step 1: Calculate percentage elongation. % Elongation = ((50.3 − 50) / 50) × 100% = (0.3 / 50) × 100% = 0.6% Step 2: Classification. A material with % elongation < 2–5% at fracture is classified as brittle. 0.6% << 5% threshold → the material is brittle.
Answer
% Elongation = 0.6% — the material is classified as brittle (consistent with grey cast iron).
Brittle vs Ductile Material Behaviour — Property Comparison
| Property | Brittle Material | Ductile Material | Example (Brittle / Ductile) |
|---|---|---|---|
| % Elongation at fracture | < 2% | > 5% | Glass 0% / Copper 35% |
| Fracture surface | Flat, granular, smooth | Fibrous, cup-and-cone | Glass / Steel |
| Warning before failure | None | Visible necking | Ceramic / Mild steel |
| Fracture toughness KIc | Low (1–3 MPa√m) | High (50–100 MPa√m) | Alumina / Steel |
| Compressive vs tensile strength | Much stronger in compression | Similar in both | Concrete / Aluminium |
| Sensitivity to notches | Very high | Moderate | Glass / Carbon steel |
Interactive Tools
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Related Terms
Ductility
Ductility is a mechanical property that describes a material's ability to undergo significant plastic (permanent) deformation before fracture under tensile stress. It is quantified as the percentage elongation or percentage reduction in area measured in a tensile test. Ductile materials such as mild steel and copper provide engineers with warning before failure (through visible deformation and "necking"), making them safer choices for structures subjected to overload or impact loading.
Material Fatigue
Material fatigue is the progressive and localised structural damage that occurs in a material subjected to repeated cyclic loading, even when the peak stress is well below the material's static yield or ultimate strength. Fatigue cracks typically initiate at stress concentrations such as notches, holes, or surface defects, and propagate incrementally with each load cycle until sudden fracture occurs. It is responsible for the majority of mechanical failures in practice, including failures in aircraft, bridges, shafts, and biomedical implants.
Material Hardness
Material hardness is the resistance of a material's surface to permanent plastic deformation, typically measured by pressing a standardised indenter into the surface under a controlled load and measuring the size or depth of the resulting indentation. It is a surface property that correlates with wear resistance, machinability, and (for steels) approximate tensile strength. Common hardness scales include Vickers (HV), Brinell (HB), and Rockwell (HR), each suited to different materials and applications.
From Old English "breotan" (to break) and later Latin "brittus" (broken). The modern scientific usage solidified in the 19th century as engineers began systematically comparing materials through tensile testing. Griffith's 1921 fracture mechanics theory provided the first quantitative explanation of why brittle materials fracture at stresses far below theoretical strength.