Tensile strength is a measurement of the force required to pull something such as rope, wire, or a structural beam to the point where it breaks.
There are three typical definitions of tensile strength:
- Yield strength - The stress a material can withstand without permanent deformation. This is not a sharply defined point. Yield strength is the stress which will cause a permanent deformation of 0.2% of the original dimension.
- Ultimate strength - The maximum stress a material can withstand.
- Breaking strength - The stress coordinate on the stress-strain curve at the point of rupture
What Is Yield Stress?
Stress and strain are directly related to each other: as one increases, the other increases as well. So, the more stress that an object experiences, the more it deforms until the object fails.
All objects will begin experiencing elastic deformation at first, but once the stress on the object exceeds a certain amount, it will experience plastic deformation. When that switch happens, the object has reached its yield stress.
Typically, every material has the same stress-strain relationship, though the size of each portion may be different. Elastic deformation is linear. The slope of the line is dependent on the material the object is made out of. Plastic deformation is not linear, making it more difficult to model.
Take a look at this graph. The elastic deformation is in red and you can see that it's linear, while plastic deformation, which is in blue, is not.
Some material also has a yield point, a point where there is a sharp increase in the object's strain that does not correlate with an increase in stress. The yield point happens after an object has reached its yield stress
Fatigue Strengh and Fatigue Life
Fatigue strength is defined as the the amplitude of cyclic stress that can be applied to a material without causing fatigue failure. In other words, how much force you can put on an object before it breaks. Fatigue life is the number of cycles the material survives under a given stress level.
(ultimate tensile strength) - the maximum stress that a material can withstand before failing. Tensile strength is not the same as compressive strength and the values can be quite different.
(fatigue strength: tensile or compressive) - the maximum stress under which a material will fail after it has undergone the range of cyclic stress for a specified number of cycles.
(fatigue shear strength) - the maximum shear stress under which a material will fail after it has undergone the range of cyclic shear stress for a specified number of cycles.
(fatigue limit) - the maximum tensile or compressive stress for which the material will never fail.
(endurance limit | tensile/compressive) the maximum stress from rotating the test object
(endurance limit | shear) the maximum stress from rotating the test object