Device for indicating if a fastening element at fastening in a piece has reached a tensile yield limit load, wherein the fastening element comprises a threaded shaft, which shaft comprises a magnostrictive material, the device comprises: - means (9, 12) for generating an alternating magnetic field that penetrates the shaft, - means (9, 14) for measuring a physical quantity, - a control unit adapted to receive measured values of said physical quantity, characterized in that the means for measuring the physical quantity (9, 14) 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 a 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 a 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. - means (9, 12) for generating an alternating magnetic field that penetrates the shaft, - means (9, 14) for measuring a physical quantity, - a control unit adapted to receive measured values of said physical quantity, the means for measuring the physical quantity (9, 14) 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 a 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 a 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. 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 a maximal value of the physical quantity in relation to a strain of the shaft, 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 a maximal value,