What is a Stress-Strain Curve?
A stress-strain curve is a graphical representation of a material’s mechanical behaviour when subjected to an axial load. It shows how a material deforms (strain) under applied force (stress) from the elastic region through to fracture.
Definitions
Engineering Stress: σ = F / A₀ (applied force divided by original cross-sectional area, in MPa or N/mm²)
Engineering Strain: ε = ΔL / L₀ (change in length divided by original gauge length, dimensionless or %)
Key Points on the Stress-Strain Curve (Mild Steel)
| Point/Zone | Description | Significance |
|---|---|---|
| Proportional Limit (A) | Stress proportional to strain (Hooke’s Law) | Upper limit of elastic Hooke’s Law behaviour |
| Elastic Limit (B) | Maximum stress with no permanent deformation | Material returns to original shape below this |
| Upper Yield Point (C) | Sudden drop in load; crystal structure dislocations move | Specific to mild steel and annealed metals |
| Lower Yield Point (D) | Sustained yielding at nearly constant stress | Design yield stress often taken here |
| Ultimate Tensile Strength (E) | Maximum stress on the curve; necking begins | Maximum load-bearing capacity |
| Fracture Point (F) | Material breaks | Ductility and toughness assessed here |
Young’s Modulus (E)
In the elastic (proportional) region: E = σ / ε = Stress / Strain
Young’s modulus for common materials:
- Mild Steel: 200–210 GPa
- Aluminium: 70 GPa
- Copper: 110–130 GPa
- Cast Iron: 100–170 GPa
Percentage Elongation and Reduction in Area
% Elongation = (L_f – L₀) / L₀ × 100 — measures ductility
% Reduction in Area = (A₀ – A_f) / A₀ × 100 — measures ductility at the neck
Brittle vs Ductile Materials
Ductile materials (mild steel, aluminium): large plastic zone, significant necking before fracture, high energy absorption. % elongation > 5%.
Brittle materials (cast iron, glass): little or no plastic zone, fracture near UTS with minimal necking. % elongation < 2%.
Universal Testing Machine (UTM) Experiment
The stress-strain curve is plotted experimentally using a Universal Testing Machine (UTM). The procedure:
- Prepare the specimen per IS:1608 (gauge length = 5.65√A₀ for circular specimens)
- Mount the specimen in the UTM grips
- Apply tensile load at a controlled crosshead speed (typically 2 mm/min)
- Record load vs extension using a load cell and extensometer
- Plot stress-strain from the load-extension data
- Identify proportional limit, yield point, UTS, and fracture point
Scientico India’s UTM (capacity 40–100 kN) includes data acquisition software to automatically plot the stress-strain curve. Request specifications.
Scientico India manufactures this and the full range of Strength of Materials equipment — CE-certified and supplied to engineering colleges worldwide.
Explore Strength of Materials Equipment → or request a quote
