Question 1

What is the difference between stiffness and strength in materials?

Accepted Answer

Stiffness (quantified by Young Modulus) is a material's resistance to elastic deformation; how much force is needed to produce a small extension. Strength refers to the material's ability to withstand a large force without fracturing. A material can be stiff but not strong (e.g., cast iron is stiff but brittle, so not strong against bending).

Question 2

Why is strain dimensionless?

Accepted Answer

Strain is the ratio of two lengths (extension/original length). Since both numerator and denominator have the same unit (e.g., metres), they cancel out, leaving strain as a pure number without any units.

Question 3

How does temperature affect elastic and plastic behaviour?

Accepted Answer

Generally, increasing temperature tends to make materials less stiff (lower Young Modulus) and more ductile (more prone to plastic deformation before fracturing). This is because higher thermal energy allows atoms to move more easily within the material's structure.

Question 4

What is the difference between a force-extension graph and a stress-strain graph?

Accepted Answer

A force-extension graph depends on the specific dimensions of the object being tested (its length and area). A stress-strain graph is independent of dimensions and describes the fundamental properties of the material itself. The gradient of the linear part of a stress-strain graph is the Young Modulus, a material constant.

Elastic and plastic behaviour

What you need to know

Elastic and plastic behaviour