Introduction
A crown or bridge can look perfect in the mouth while it bends a little with every bite. The stress-strain diagram records how a material behaves as the load on it rises, from the first small change in shape until it breaks. Most of the mechanical properties in this subject can be read straight from this one curve.
Why deformation matters
Damage often starts long before anything visibly breaks, so the way a material deforms under load can matter more in practice than the load that finally fractures it.
- A bridge pontic that flexes during chewing overloads the periodontium of its abutment teeth.
- A crown or filling that deforms can break its adhesive bond to the tooth and lose its correct contact with the opposing teeth.
How the diagram is built
A test body is loaded steadily, in tension or in compression, and the result is plotted on two axes:
- Vertical axis: stress, in MPa.
- Horizontal axis: strain, as a percentage change in length. This is lengthening in a tensile test and shortening in a compressive test.