Why does yielding occur

Ductile materials like metals have higher yield strength values than plastics.It occurs specially in mild steel or in low carbon steel.Why do both yielding and brittle fracture occur at stresses far below the theoretical tensile (cohesive) strength material?The dislocations are locked in due to presence of carbon in case.The unexpected load such as earthquake forces, increase in occupancy loads, heavy wind loads etc.

In materials that exhibit a yield point, the yield point elongation (ype) is the difference between the elongation of the specimen at the start and at the finish of discontinuous yielding (the area in which an increase in strain occurs without an increase in stress).Stack overflow public questions & answers;The yield sign is a regulatory sign.Yield point phenomenon is understood to occur when stress drops down drastically because the locked in dislocations are set free.Once the yield point is passed, some fraction of the deformation will be permanent and.

Why do yield problem occur?