Ductility
Quick Definition
Ductility is how far a material can be stretched before it breaks. It is measured in a tensile test, as percent elongation or percent reduction of area. Steel, copper and aluminium are ductile; glass and cast iron are not.
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Ductility explained
A ductile bar stretches evenly, then narrows at one spot, called the neck, and breaks there. The test gives two numbers. Elongation A compares the marked gauge length before (L₀) and after (Lu). Reduction of area Z compares the cross-section before (S₀) and at the break (Su). A high reduction of area with modest elongation is normal: most of the stretching happens in the neck. The same test also yields tensile strength and yield strength. The curve behind it is explained in What Is Stress and Strain?
Formula
Elongation A = (Lu − L₀) ÷ L₀ × 100 %
Reduction of area Z = (S₀ − Su) ÷ S₀ × 100 %
Real-World Example
A 10 mm steel bar has S₀ = 78.54 mm², and its gauge length is 50 mm. After fracture the two halves measure 62.4 mm when fitted together, and the neck is 38.5 mm² in area. So A = (62.4 − 50) ÷ 50 = 24.8%, and Z = (78.54 − 38.5) ÷ 78.54 = 51.0%.
Why elongation figures need a gauge length
The neck is the same size whatever the gauge length, so a short gauge reports a larger percentage than a long one. ISO tests usually use L₀ = 5.65√S₀, and ASTM tests often use 2 in (50 mm) on sheet and plate. Compare only figures measured the same way. A datasheet that says "A = 22%" and one that says "22% in 200 mm" do not describe the same steel.
Typical values for metals
A common textbook rule calls a material ductile above 5% elongation and brittle below it. These are typical values at room temperature, not specification minimums.
| Material | Elongation | Behaviour |
|---|---|---|
| Annealed copper | about 45% | very ductile |
| 304 stainless steel | about 40% | very ductile |
| Mild structural steel | about 20–25% | ductile |
| Aluminium 6061-T6 | about 12% | moderately ductile |
| Grey cast iron | under 1% | brittle |
| Glass, most ceramics | near 0% | brittle |
Ductility, malleability and toughness
Gold is both ductile and malleable. Lead is very malleable but not very ductile, because it is too weak to draw into fine wire. Toughness needs both strength and ductility, so a weak ductile metal can still have low toughness.
| Property | Question it answers | Measured by |
|---|---|---|
| Ductility | How far does it stretch in tension? | Elongation, reduction of area |
| Malleability | How far can it be squeezed thin in compression? | Rolling or hammering without cracking |
| Toughness | How much energy does it absorb before breaking? | Impact tests, area under the stress-strain curve |
What changes ductility
Ductility is not fixed. Cold work and quenching reduce it, while annealing restores it. Hardness and ductility usually move in opposite directions. Many steels turn brittle below a transition temperature, and notches, thick sections and fast loading push a part the same way. So a ductile steel can still crack with little warning at a sharp weld defect in cold weather.
Ductility of a whole structure
Structural engineers also use the word for a whole member: displacement ductility μ = Δu ÷ Δy, the deflection at failure over the deflection at first yield. A beam that yields at 12 mm and fails at 60 mm has μ = 5. Earthquake design relies on this capacity. Reinforcing bars are chosen partly for it, as the rebar size chart notes.
Why It Matters
Ductility lets a part yield and spread a local overload rather than crack, and it allows metals to be drawn, rolled and forged. Low ductility signals a risk of sudden failure.
Industry Applications
Sources
- International Organization for Standardization, ISO 6892-1, Metallic materials — Tensile testing
- ASTM International, ASTM E8/E8M, Standard Test Methods for Tension Testing of Metallic Materials
- International Organization for Standardization, ISO 2566-1, Steel — Conversion of elongation values
Related Standards
Frequently Asked Questions
What does ductile mean?
Able to stretch or be drawn out a long way without breaking.
How do you measure ductility?
With a tensile test, as percent elongation over a stated gauge length, or percent reduction of area.
What is a good example of ductility?
Copper being drawn into wire.
What is the difference between ductile and malleable?
Ductile metals stretch in tension; malleable metals flatten in compression.
Is steel ductile?
Mild steel is, at about 20–25% elongation. Hardened steels are much less so.
Is ductility the same as strength?
No. Strength is the stress a material can carry; ductility is how far it deforms before breaking. Stronger steels are usually less ductile.
Why is ductility important?
It lets parts yield and spread a local overload rather than crack, and it allows metals to be formed.
See Also
Related Glossary Terms
Tensile Strength
Tensile strength, or ultimate tensile strength (UTS), is the highest stress a material can carry in tension: the maximum force in a tensile test divided by the original cross-section. Ductile metals reach it before they neck and break.
Yield Strength
Yield strength is the stress at which a material stops springing back and starts to deform permanently. For metals without a sharp yield point it is defined as the stress that leaves 0.2% permanent strain — the 0.2% proof stress, R_p0.2.
Brittleness
Brittleness is a material's tendency to fracture with little or no plastic deformation, absorbing little energy before it breaks. It shows up as low elongation and reduction of area in a tensile test and low absorbed energy in a Charpy impact test.
Hardness
Hardness is a material's resistance to permanent indentation. It is measured by pressing a hard indenter into the surface under a set load, and reported on a scale such as Rockwell (HRC), Brinell (HB) or Vickers (HV). Each scale has its own number, so a hardness value must always name its scale.
Annealing
Annealing is a heat treatment that softens a metal: you heat it, hold it, and cool it slowly. It restores ductility and relieves stress. Hardening does the opposite, using a fast quench to make steel harder and stronger.
Elasticity
Elasticity is a material's ability to return to its original shape after a load is removed. In engineering, it is measured by a modulus such as Young's modulus. In economics, the same word means how strongly demand responds to price.