How Do Strength, Hardness and Ductility Differ?

• Yield strength
• Ultimate tensile strength
• Fracture

What you will learn

• Tell elastic from plastic deformation, and yield strength from ultimate tensile strength.
• Explain ductility and why a brittle material breaks with little warning.
• Say what hardness measures and why it relates to wear.

Before you start

Knowing that a force can deform an object is enough. The article uses a spring and a paper clip.

What does strength tell you?

Strength tells how much stress a material can take, and it has two key marks. In the University of Cambridge Mechanical Properties package, a spring with a load hung from it extends and returns to its original shape once the load is removed. That is elastic deformation, a shape change that is recoverable once the load is removed.

Past a threshold the material no longer returns. The package calls the stress at which plastic deformation begins the yield strength. Plastic deformation is a permanent shape change, as when a paper clip is bent. The second mark is the ultimate tensile strength, the maximum stress a material can withstand while being stretched or pulled before breaking.

On a stress–strain sketch the first straight part is elastic, the red dot is the yield strength, the peak is the ultimate tensile strength and the cross is fracture.

What is ductility and how does it differ from brittleness?

Ductility tells how much plastic deformation a material has undergone at fracture. The package defines ductility as a measure of the degree of plastic deformation sustained at fracture. A ductile material deforms plastically a good deal, through a narrowing process called necking, before it fractures, and it absorbs more energy.

A brittle material is the opposite. According to the package, brittle materials hardly undergo any plastic deformation before fracture. That is why the brittle line on the sketch stops early and short, while the ductile line runs on.

• Ductile material
• Brittle material

What does hardness measure?

Hardness is a measure of a material's resistance to localised plastic deformation. The familiar test presses a point into the surface under a known force and looks at the dent. The package says hardness matters when considering a material's resistance to wear.

The package identifies a tradeoff in work hardening: plastic deformation increases hardness and wear resistance but can also make the material more brittle. This describes the hardening process under discussion, not a rule that every hard material is more brittle than every softer one.

• Indenter
• Dent
• Applied force

Real-world example

Bend a paper clip a little and let go. If it only bends slightly it may spring back, because it is in the elastic range. Bend it too far and it keeps the new shape, so it passed the yield strength. A ceramic tile behaves differently: it hardly bends and simply cracks, as a brittle material does.

Common misunderstandings

Strong and hard are the same. The package defines them separately: strength concerns the stress a material withstands, hardness concerns resistance to a local dent.

A hard material cannot break. Resistance to a dent does not guarantee resistance to fracture; the package notes that work hardening can make a material more brittle.

Ductile means weak. Ductility only shows the plastic deformation at fracture. A ductile material fractures after much deformation and absorbs more energy.

Frequently asked questions

What is yield strength?

It is the stress at which plastic deformation begins. Below it, the material recovers its shape once the load is removed.

What is a brittle material?

It is a material that hardly undergoes plastic deformation before fracture, so it absorbs little energy before breaking.

Is hardness the same as strength?

No. Hardness is resistance to local plastic deformation, while ultimate tensile strength is the maximum stress while being pulled. They are separate quantities.

Summary

• Below the yield strength a material is elastic; beyond it plastic deformation begins, and the ultimate tensile strength is the maximum stress when pulled.
• Ductility is the plastic deformation at fracture; brittle materials hardly deform plastically before fracture.
• Hardness is resistance to local plastic deformation and relates to wear; work hardening can trade ductility for hardness.