Patent ID: 8910526

Claim:
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 in response to the generating of an alternating magnetic field, a control unit adapted to receive measured values of said physical quantity, characterized in that the means for measuring the physical quantity comprises a sensor element adapted to be exposed to said magnetic field after the magnetic field's penetration of the shaft, wherein said physical quantity is at least one of a resistance and an inductance of the sensor element that depend on the momentary magnetic susceptibility of the shaft, wherein the control unit is adapted to identify: 1a) a mainly constant rate of change of the physical quantity in relation to a strain of the shaft, or 1b) any of an initial value or maximal value of the physical quantity in relation to a strain of the shaft, and detect a significant change of the physical quantity corresponding to: 2a) a deviation from the mainly constant rate of change of the physical quantity exceeds a first certain value, or 2b) an absolute change of the physical quantity from any of an initial value or maximal value and on basis of the significant change determine and indicate if the shaft of the fastening element has reached the tensile yield limit load.