[1][2] The civil designer ensures that the stresses developed in a structure due to service loads do not exceed the elastic limit.
In mechanical engineering design such as design of pressure equipment, the method uses the actual loads predicted to be experienced in practice to calculate stress and deflection.
Such loads may include pressure thrusts and the weight of materials.
The predicted stresses and deflections are compared with allowable values that have a "factor" against various failure mechanisms such as leakage, yield, ultimate load prior to plastic failure, buckling, brittle fracture, fatigue, and vibration/harmonic effects.
However, the predicted stresses almost always assumes the material is linear elastic.