Stress–strain graphs, creep and fatigue

Overview

Yield and tensile strength, plasticity, stress–strain graphs for ductile, brittle and rubber materials, the limit of proportionality, the area under the graph, creep, fatigue and hip replacements.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Define yield strength.

   Model answer

   Yield strength is the stress at which a material begins to deform plastically. Beyond the yield point the material will not return to its original length.

2. 2

   Define tensile strength.

   Model answer

   Tensile strength is the maximum stress a material can withstand while being stretched before it breaks. Tensile strength is also called the ultimate tensile strength.

3. 3

   Define plasticity.

   Model answer

   Plasticity is the ability of a material to be permanently deformed without breaking. The change of shape remains after the force is removed.

4. 4

   Describe the stress–strain graph of a ductile metal such as copper.

   Model answer

   The graph is a straight line up to the limit of proportionality and then curves after the yield point. A long plastic region rises to a maximum stress before the wire necks and breaks.

5. 5

   Describe the stress–strain graph of a brittle material such as glass.

   Model answer

   The graph is a straight line right up to the point of fracture. The glass shows no plastic region and breaks suddenly.

6. 6

   Describe the stress–strain graph of a rubber band.

   Model answer

   The graph is curved rather than straight and reaches very large strains at low stress. The unloading curve lies below the loading curve, so energy is lost as heat.

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