MEPX
Chapter 4 of 12All chapters
  1. 1What engineers do
  2. 2The design process
  3. 3Forces and loads
  4. 4Stress, strain and materials
  5. 5Structures
  6. 6Mechanisms
  7. 7Electrical basics
  8. 8Energy in machines
  9. 9Manufacturing and tolerances
  10. 10Safety factors and failure
  11. 11Testing and standards
  12. 12Responsibility

Chapter 4 of 12

Stress, strain and materials

Why the same shape behaves differently in steel and concrete.

The two quantities

Stress is force divided by the area carrying it, so a thin rod is stressed more than a thick one under the same pull. Strain is how much the material stretches in proportion to its length.

  • Stiffness is the ratio between them, and is a property of the material.
  • Elastic deformation springs back; plastic deformation is permanent.

Choosing a material

Steel is strong in tension and compression. Concrete is strong in compression and weak in tension, which is why it is cast around steel bars. Composites buy strength for weight at a higher price.