What is tension?
We call tension the value of the distribution of forces per area around a material point within a continuous body or medium. See example below:
Note that the figure on the right is equivalent to that of the middle figure.
Tensile Strength
σ stress, also known as tensile strength, is defined by the ratio:
where F is the instantaneous force or load applied in an orthogonal direction to the straight section (Ao). When it represents the area of the original section (before the application of force).
After the strain, the stress used to continue the plastic deformation process increases until it reaches its maximum value (point M). See figure below:
From this point on the stress decreases until the material fails or breaks up (point F).
This is due to the rapid decrease of the resistant section when exceeding the tensile strength limit (maximum stress).
Tensile strength limit
The tensile strength limit "TS", is the stress at the "highest " point of the stress/engineering strain curve. In other words it is the maximum stress that can be sustained by a material under tension.
The deformation to point M is uniform along the region of the material under traction.
However, after this maximum tension, a small constriction begins to form, and all deformation after this point becomes "easier"because it reduces the resistant area and creates a stress concentrator in this region.
The name of this phenomenon is necking and disruption occurs in this region.
The tension applied at the time of fracture is called fracture resistance of the material. Note that fracture stress is NOT the same thing as Tensile Strength.
Tensile strength is used by engineers (along with the yield limit) unlike fracture stress that is restricted to research.
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