Metals used in fastener(shear stud, bolt, etc) manufacture are elastic materials which will stretch (elongate) under applied loads and return to their original shape when the load is removed. However, if sufficient load is applied, the material will stretch beyond its yield point and enter a plastic zone, losing its elasticity and becoming permanently stretched. Further increased loading on the material will stretch it to its ultimate tensile strength at which point the material will fracture, which will be used to test mechanical properties of shear stud.
The major factor in determining the load a material can carry is its tensile strength, which is related to its hardness. The terms used to describe the strength and load bearing properties of a metal fastener are:
Tensile Strength – is an expression of the maximum capacity of a particular material to stretch under tension load prior to failure.
Yield Point – is an expression of the theoretical point of stress (pressure) beyond which the material loses its elasticity and becomes permanently stretched (realistically, a range rather than a single point).
Proof Load Stress – is an expression of the minimum stress a material must achieve, prior to permanent elongation and, the stress which would be applied to test and remeasure a specific fastener to prove it had not permanently stretched and that it will carry the required load. These terms will also include a unit of area, are approximately between 80% and 90% of the theoretical yield stress and are expressed in the same terms. Proof load stresses also apply to nuts and are the point at which the nut is deemed to have failed.