The Chartership Forum

Structural Analysis Fundamentals / module 8

Engineering materials

What a tensile test actually tells you, how the elastic constants connect, and what happens when load is held for years or repeated for millions of cycles.

What you will understand

  • Read a stress–strain curve and name what each region means
  • Distinguish ductile from brittle behaviour, and nominal from true stress
  • Use proof stress for materials with no clear yield point
  • Relate E, G, ν and K, and know which two are independent
  • Explain creep, relaxation and fatigue, and when each governs
  • Calculate thermal strain, restrained thermal stress and impact effects

Lessons

  1. The tensile test and the stress–strain curve

    Where E, yield, ultimate strength and ductility actually come from.

  2. Elastic constants and how they connect

    E, G, ν and K — why only two of the four are independent.

  3. Time and repetition: creep, relaxation and fatigue

    What happens when load is held for years, or applied a million times.

  4. Temperature, restraint and impact

    Why a free member does not care about temperature and a restrained one does.

Module summary
  • The tensile test gives E from the elastic slope, and strength from the yield and ultimate points
  • Nominal stress uses the original area; true stress uses the current one, which is why the nominal curve appears to fall after necking
  • Ductility is measured by percentage elongation and reduction in area, and it is what gives warning before collapse
  • For an isotropic material only two elastic constants are independent: G = E/2(1 + ν) and K = E/3(1 − 2ν)
  • Creep is deformation growing at constant stress; relaxation is stress falling at constant strain
  • Fatigue cracks grow under stress ranges far below yield, and stress concentrations dominate the life
  • Thermal strain is free if the member can move, and becomes stress the moment it cannot: σ = EαΔT
Teaching basis and review status

These are educational models and exercises. Check their assumptions before using the results. The full technical content and every diagram have not yet been independently verified.

Concrete and steel calculations retain their stated first-generation Eurocode teaching basis. Use the standards and amendments specified for your project; do not mix generations without an appropriate design basis.

Read the lesson register and outstanding review notes

Check your understanding

Check yourself

On a nominal stress–strain curve for mild steel, what does the drop after the ultimate point represent?

Practice

A material has E = 70 000 N/mm² and ν = 0.33. What is its shear modulus, in N/mm²?

Practice

A fully restrained steel member (E = 205 000 N/mm², α = 12 × 10⁻⁶ per °C) is cooled by 40 °C. What is the magnitude of the resulting stress, in N/mm²?

Check yourself

Which quantity most directly governs the fatigue life of a welded detail?