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.
Worked Example
Problem
In a Vickers hardness test, an indenter leaves a square impression with a diagonal length of 0.42 mm under a load of 30 kgf. Calculate the Vickers Hardness Number (VHN).
Solution
Step 1: Recall the Vickers formula. VHN = 1.854 × F / d² where F = load in kgf, d = mean diagonal in mm Step 2: Substitute values. VHN = 1.854 × 30 / (0.42)² VHN = 55.62 / 0.1764 VHN = 315.3
Answer
Vickers Hardness VHN ≈ 315 HV30 (equivalent to approximately 320 MPa tensile strength for carbon steel)
Hardness Scale Comparison for Common Materials
| Material | Brinell HB | Vickers HV | Rockwell HRC | Condition |
|---|---|---|---|---|
| Annealed mild steel | 120–140 | 120–140 | — | Soft, formable |
| Normalised carbon steel | 170–200 | 170–200 | 5–12 | General use |
| Hardened tool steel | 650–750 | 650–750 | 58–65 | Cutting tools |
| Cast iron (grey) | 180–220 | 180–220 | — | Engine blocks |
| Aluminium alloy 6061-T6 | 95 | 107 | — | Aerospace |
| Diamond | — | ~10 000 | — | Hardest natural material |
Interactive Tools
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Related Terms
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.
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.
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.
The concept of hardness has been used since antiquity. The Brinell scale was introduced by Johan August Brinell in 1900, the Rockwell scale by Stanley Rockwell in 1919, and the Vickers scale by Smith and Sandland of Vickers Ltd in 1921. "Hardness" derives from Old English "heardness" (the state of being hard).