The Stress-Strain Graph
The Stress-Strain Graph
- A deformation graph gives two things: the gradient (stiffness) and the area (energy).
- The stress-strain graph also shows where the material stops stretching elastically and where it breaks.
The stress-strain graph for a metal
- The graph rises as a straight line through the origin while Hooke's law holds.
- The limit of proportionality is where the line stops being straight.
- The elastic limit (from the earlier lesson) is a little further along: the last point from which the wire still returns to its original length.
- Past the elastic limit the material deforms plastically: the curve bends over and rises slowly until the material breaks.
- The limit of proportionality and the elastic limit are two different points: the first is where proportionality ends, the second is where elastic behaviour ends. The elastic limit is at or just past the limit of proportionality.
Your turn— tap to reveal the worked answer (9702/23/M/J/24 Q3)
On a force-extension graph, mark P (the limit of proportionality) and E (the elastic limit), and estimate the work done to break the sample. (9702/23/M/J/24 Q3(b),(d))
- P is where the line stops being straight; E is the point a little further along the curve.
- The work done to break equals the total area under the graph, about here.
Answer