Read the yield strength from the stress value where elastic behaviour ends and permanent deformation begins. The fracture point is the final point on the stress-strain curve, where the specimen breaks.
A stress-strain graph plots engineering stress on the vertical axis against engineering strain on the horizontal axis:
Here, is applied force, is original cross-sectional area, is extension, and is original length. Stress is commonly measured in , while strain has no unit.
| Graph feature | Interpretation |
|---|---|
| Initial straight section | Elastic region: stress is proportional to strain, and the material returns to its original dimensions after unloading. |
| End of elastic behaviour | Yield point: plastic deformation begins. Read horizontally to the stress axis to obtain yield strength. |
| Smooth transition without a clear yield point | Use the 0.2% proof stress: draw a line parallel to the linear elastic section from . Its intersection with the curve gives the proof stress. |
| Highest point on the curve | Ultimate tensile strength: the maximum engineering stress reached. |
| Final point | Fracture point: read its stress and strain coordinates to find fracture stress and fracture strain. |
For example, if yielding starts at , the yield strength is . If the curve ends at and , fracture occurs at a stress of and a strain of , or .
A common misconception is that the highest point is the fracture point. It is actually the ultimate tensile strength; fracture may occur later at a lower engineering stress because of necking.
For IB Design Technology B3.2, label the axes and identify the elastic region, yield point, plastic region, ultimate tensile strength and fracture point. When selecting materials for structural systems, compare yield strength for resistance to permanent deformation and fracture behaviour for safety.