Device and method for indicating if a fastening element has reached a tensile yield limit load

A device for indicating if a fastening element upon fastening to a piece has reached a tensile yield limit load, wherein the fastening element comprises a threaded shaft, which shaft comprises a magnostrictive material, wherein the device comprises:

• means for generating an alternating magnetic field that penetrates the shaft,
• means for measuring a physical quantity that depends on the momentary magnetic susceptibility of the shaft, wherein the means for measuring the physical quantity is adapted to be exposed to said magnetic field after the magnetic field's penetration of the shaft, and
• a control unit adapted to receive measured values of said physical quantity and detect a significant change of the physical quantity, wherein the control unit is adapted to on basis of the significant change determine and indicate if the shaft of the fastening element has reached the tensile yield limit load.