Material Properties, Stress-Strain and Young's Modulus

How materials behave under load

An engineer choosing a material has to know how it will behave when a force acts on it. Some materials stretch and spring back, some bend and stay bent, some shatter, and some absorb a heavy blow without failing. These behaviours are described by the mechanical properties of a material, such as strength, stiffness, toughness and ductility. To compare materials fairly, engineers measure stress, which is the force spread over an area, and strain, which is how much a material stretches for its length, and they combine the two into a single measure of stiffness called the Young modulus. This module covers the main properties, the difference between elastic and plastic behaviour, stress and strain, and how a material is tested.

Material property words

Learn these five properties before you compare and test materials.

Match each property to what it describes

Elastic against plastic deformation

When a force changes the shape of a material, the change is one of two kinds.

What does that tell you?

Two materials are pulled with the same force, and material A stretches far less than material B. What does this tell you about material A?

• It is stiffer than material B
• It is weaker than material B
• It is more ductile than material B
• It is less dense than material B

Stress, strain and the Young modulus

To compare materials fairly, engineers turn a force into two measured quantities. Stress is the force divided by the area it acts on, so a thin bar feels more stress than a thick one under the same pull. Strain is the extension divided by the original length, so it shows how much a material stretched relative to its size. If you divide the stress by the strain while the material is still springing back, you get the Young modulus, a single number for stiffness: the higher it is, the stiffer the material. Engineers also build in a factor of safety, choosing materials that are stronger than the job strictly needs, so that an unexpected load does not cause failure.

Pick the true facts about materials

Select every statement about materials under load that is true.

• Stress is the force divided by the area it acts on
• Strain is the extension divided by the original length
• A stiffer material has a higher Young modulus
• Elastic deformation is always permanent
• A brittle material bends a great deal before it breaks

Order a tensile test

Put the steps of a tensile test in the correct order.

• Clamp the material sample in the machine
• Apply a steadily increasing pulling force
• Measure the extension as the force rises
• Plot the force against the extension
• Read off where the material passes its elastic limit

Complete the material facts

The ability to return to shape after a force is removed is _____. How much a material resists being stretched or bent is _____. How well a material absorbs energy without breaking is _____. How easily a material cracks with little bending is _____.

A rubber band and a paperclip

Take a rubber band and a metal paperclip and pull on each one. The rubber band stretches a long way and, the moment you let go, springs right back to how it started. Bend the paperclip, though, and it stays bent in its new shape; it does not return at all. The two behave in completely different ways under the same treatment, and that difference is exactly what an engineer studies before choosing a material. Some materials spring back, some hold a new shape, some shatter without warning, and some soak up a heavy blow without failing. Knowing how a material will act under a force is the whole point, because the wrong choice in a bridge, a tool or a machine can fail at the very moment it matters most.

Tap the mechanical properties

Tap every item below that is a mechanical property of a material.

• hardness
• ductility
• toughness
• the colour of the metal

Work out the stress

A pulling force of 200 newtons acts on a bar with a cross-section area of 4 square millimetres. Stress is the force divided by the area. What is the stress in newtons per square millimetre?

Choose the right material

Read each situation and choose the best answer.

• You need a material for a cutting tool that must not scratch or dent easily in use. Which property matters most?
• A metal part is bent past its elastic limit. What has happened to it?
• Two beams must carry the same load, and you want the one that bends the least. Which do you pick?

Build a materials point

Choose the word for each gap to complete one accurate point about materials under load.

Explain material properties and testing

A friend is choosing a material for an engineering project and asks how engineers describe and test materials. Explain material properties, stress and strain, and the Young modulus clearly.

• Explain three mechanical properties, such as strength, stiffness and toughness
• Explain the difference between elastic and plastic deformation
• Explain what stress and strain are
• Explain what the Young modulus tells you
• Explain how a tensile test is carried out