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“Strong”, “hard” and “tough” are often used as if they meant the same thing. They don’t. Glass is hard but not tough. Rubber is tough but neither strong nor hard. A good spring steel is all three. Each property answers a different design question, and each is measured with its own test.
The tensile test and the stress–strain curve
Most mechanical properties come from one test. A standard round or flat specimen is pulled slowly in a testing machine until it breaks, while the force and the stretch are recorded. Dividing force by the original area gives stress, and dividing the stretch by the original gauge length gives strain.
Reading the curve from left to right:
- Elastic region. Stress is proportional to strain, and the specimen returns to its original length when unloaded. The slope is Young’s modulus, E: the material’s stiffness. All steels have about the same E (about 200 GPa), whatever their strength.
- Yield strength (σy or Rp0.2). The stress at which permanent stretching begins. Where there is no clear yield point, the 0.2 % proof stress is used: the stress that leaves 0.2 % permanent strain. Most parts are designed so they never reach it.
- Ultimate tensile strength (UTS or Rm). The highest stress on the curve: the most load per original area the material can carry.
- Necking. After the UTS, the deformation concentrates in one spot, the section thins, and the force falls.
- Fracture. The specimen breaks. The permanent stretch at this point is the elongation at break, a measure of ductility.
Strength
Strength is resistance to breaking or permanently deforming under a load. Designers usually work with yield strength and divide it by a factor of safety; tensile strength matters for brittle materials and for checking how much margin is left beyond yield.
Ductility
Ductility is how far a material stretches before breaking, given as elongation at break (%) or reduction of area. Ductile metals warn you before they fail: they bend, stretch and neck. Mild steel stretches 20–30 %; grey cast iron less than 1 %.
Hardness
Hardness is resistance to local indentation, scratching and wear. It is measured by pressing a hard indenter into the surface with a known force and measuring the dent.
| Test | Indenter | Scale | Used for |
|---|---|---|---|
| Brinell (HB / HBW) | 10 mm hard metal ball | Up to about 650 HBW | Castings, forgings, softer steels, non-ferrous metals |
| Rockwell C (HRC) | Diamond cone | About 20–70 HRC | Hardened steels: shafts, gears, tools |
| Rockwell B (HRB) | Steel ball | Softer metals | Mild steel, brass, aluminium |
| Vickers (HV) | Diamond pyramid | Whole range, also thin layers | Case-hardened surfaces, small parts, coatings |
| Shore A / D | Spring-loaded pin | Elastomers and plastics | Seals, rubber, plastic parts |
Hardness is quick and nearly non-destructive, so it is used to check heat treatment on finished parts. For steels it also gives a rough estimate of tensile strength: UTS (MPa) ≈ 3.45 × HB. A gear tooth, a cam or a bearing race needs a hard surface; the drawing states it, for example “Case harden 58–62 HRC, case depth 0.8–1.2 mm”.
Toughness
Toughness is the energy a material absorbs before it breaks. On the stress–strain curve it is the area under the line, so a tough material needs both strength and ductility. Toughness matters for parts that see impact, shock loads or cracks.
The usual test is the Charpy impact test: a pendulum swings down and breaks a notched bar, and the energy it loses is the impact energy in joules. Many steels are tough at room temperature but become brittle below a transition temperature, which is why steels for cold climates are specified with an impact test at a low temperature, such as S355J2 (27 J at −20 °C).
Fatigue strength
Parts that are loaded and unloaded millions of times, such as shafts, springs and connecting rods, can break at stresses well below their yield strength. The fatigue (endurance) limit of many steels is about half their tensile strength for polished specimens, and lower for real surfaces with notches. Aluminium alloys have no true endurance limit. Smooth fillets, good surface finish and shot peening all raise fatigue life.
Worked example: reading a tensile test
A steel specimen with a 10 mm diameter and a 50 mm gauge length gives these results: 15 kN produces an extension of 0.048 mm (elastic); yielding starts at 22 kN; the maximum load is 34 kN; after fracture the gauge marks are 62 mm apart.
Which property matters for your part?
| If the part must… | Look at | Example |
|---|---|---|
| Carry a load without bending permanently | Yield strength | Bracket, frame, bolt |
| Not deflect too much | Young’s modulus and the section shape | Machine bed, beam, long shaft |
| Resist wear and indentation | Hardness | Gear teeth, cams, guide rails, cutting tools |
| Survive impact or work in the cold | Toughness (Charpy energy at temperature) | Hammer, crane hook, vehicle frame in winter |
| Survive millions of load cycles | Fatigue strength | Shaft, spring, connecting rod |
| Bend or form during manufacture | Ductility (elongation) | Sheet-metal part, crimped fitting |
Hardness and toughness pull against each other: heat-treating steel harder usually makes it less tough. Many parts solve this with a hard surface on a tough core (case hardening, induction hardening or nitriding).
Key points
- Strength resists breaking, stiffness resists bending, hardness resists dents and wear, toughness absorbs energy before breaking.
- Design to the yield strength with a factor of safety; check UTS for margin.
- Stiffness (E) hardly changes between grades of the same metal; strength can change a lot.
- Specify hardness and impact tests on the drawing where the part depends on them.