If the fastening shown in Fig. 1. 1 is overstressed by applying excess torque to the nut(for example, by extending the length of the spanner with a tube), the bolt will be stressed beyond its elastic range. Once the yield stress for the bolt has been exceeded it exhibits plastic properties and takes a permanent set that is, it becomes permanently lengthened, the spring back "is seriously reduced and the joint faces are no longer held firmly together. Thus for critical assemblies, the designer will seek to control the stress in the fastening and associated components by specifying the torque to be applied to the nut as it is tightened by the use of a torque spanner"set to a specific valueIf the fastening shown in Fig. 1.1 is overstressed by applying excess torque to the nut (for example，by extending the length of the spanner with a tube)，the bolt will be stressed beyond its elastic range．Once the yield stress for the bolt has been exceeded it exhibits plastic properties and takes a permanent set that is，it becomes permanently lengthened，the “spring back” is seriously reduced and the joint faces are no longer held firmly together. Thus for critical assemblies，the designer will seek to control the stress in the fastening and associated components by specifying the torque to be applied to the nut as it is tightened by the use of a “torque spanner” set to a specific value