Strength Ductility Toughness
Strength, ductility and toughness — three different things people call “strong”
A material can be very strong and still shatter. Another can be weaker and almost impossible to break. These are separate properties, and they all come off the same graph — you just read it in different directions.
How to read the curve
| Property | What it is | Where it is on the curve | Units |
|---|---|---|---|
| Stiffness | Resistance to stretching while still elastic | The slope at the start | GPa |
| Yield strength | Stress at which it stops springing back | Height at the knee | MPa |
| Ultimate strength | Highest stress it ever carries | Height at the peak | MPa |
| Ductility | How far it deforms before breaking | Length along the strain axis | % elongation |
| Toughness | Energy absorbed before fracture | Area under the whole curve | J/m³ |
Typical values
| Material | Yield (MPa) | Elongation | Character |
|---|---|---|---|
| Mild steel S235 | ~235 | ~26% | Moderate strength, very ductile, tough. |
| Stainless 304 | ~215 | 40 – 55% | Lower yield than you expect, exceptionally ductile and tough. |
| Aluminium 5052-H32 | ~195 | 10 – 14% | Reasonable all-rounder for forming. |
| Aluminium 6061-T6 | ~275 | 8 – 12% | Stronger than 5052, noticeably less ductile. |
| Grey cast iron | — | < 1% | Strong in compression, brittle, low toughness. |
| Glass | — | ~0% | High strength, effectively zero ductility, no toughness. |
Where it decides what you can make
Ductility is bendability. A tight bend asks the outside of the material to stretch a long way. If the material runs out of elongation before the bend is finished, it cracks. That is the whole reason a minimum bend radius exists.
It also explains a comparison that surprises people. 6061-T6 is stronger than 5052, and much worse at bending. Higher yield, lower elongation. If a part cracks on the brake, reaching for a stronger alloy usually makes it worse, not better.
Toughness is what survives an impact. A machine guard, a skid, a bracket that gets knocked — those want toughness, not peak strength. A high-strength part that fractures on the first knock has failed, however impressive its yield figure was.
The honest limits
These are room-temperature figures from a slow tensile test. Cold makes most steels more brittle. A sudden impact behaves differently from a steady pull — which is why impact toughness gets its own separate test rather than being read off this curve.
Elongation figures also depend on the gauge length used in the test, so numbers from different standards are not always directly comparable. Treat them as a guide to character, not a specification.