EASA/GCAA Part-66 · Module 6 — 6.1 Aircraft Materials · B1/B2

Stress, Strain & the Tensile Test

Objective: read a stress–strain curve — elastic region and Young's modulus, yield point, ultimate tensile strength, necking and fracture — and compare common aircraft metals.

Tensile test specimen

Stress–strain curve

Applied strain 0 %

Auto-pull speed 1 ×

Stress

0MPa

Strain

Region

Young's modulus E

0GPa

Stress σ = F/A · Strain ε = ΔL/L · Elastic region: σ = E·ε (Hooke's law)

Pulling a test piece traces a stress–strain curve. At first it is elastic — stress is proportional to strain (Hooke's law) and the slope is Young's modulus E; release the load and it springs back. Past the yield point it deforms plastically and won't fully recover. Stress keeps rising to the ultimate tensile strength (UTS), then the specimen necks and finally fractures. Steel is stiff and strong; aluminium is lighter but yields sooner; titanium balances strength with low weight.

Try this: Pull each material to 0.5% strain, then 'release' (lower the slider). Which springs back?
Try this: Compare the slopes in the elastic region. Which material has the highest Young's modulus?
Try this: Keep pulling past the UTS. What happens to the specimen and the curve?