If in a plug-type screw connection the nut is gradually tightened more and more firmly, then the resistance increases steadily until a threshold is reached at which the screw begins to lengthen at which the screw begins to lengthen permanently. At this stage, even a weak force suffices to lengthen it even further.
The tightening torque does not increase further during this phase. When the strain decreases, the screw does not return to its original length, like a rubber band, because its “yield point” was exceeded. The yield point indicates the maximum transferable force in newtons (N) per square millimeter of the cross-sectional area for which a permanent deformation of the screw does not occur yet. If this load is exceeded, the screw lengthens and eventually fractures on the occurrence of even greater strain. The yield point definition is utilized in the case of screws having a discontinuous transition from the elastic to the plastic range.
The image above shows the individual phases of permanent deformation in the case of a tough material. In this context, it can be clearly being seen how the material narrows prior to fracture.
