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

A method 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 magnetostrictive material, wherein the method comprises the steps of:

• generating an alternating magnetic field that penetrates the shaft,
• measuring a physical quantity that comprising at least one of a resistance and an inductance that depend on the momentary magnetic susceptibility of the shaft,
• detecting a significant change of the physical quantity by identifying:
• 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,
• detecting 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,
• determining if the shaft of the fastening element has reached the tensile yield limit load based on the significant change of the physical quantity, and
• indicating that the fastening element has reached the tensile yield limit load in dependency of said determination.

A method 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 magnetostrictive material, wherein the method comprises the steps of:

• generating an alternating magnetic field that penetrates the shaft,
• determining the strain of the shaft,
• identifying a mainly constant rate of change of the physical quantity in relation to an increased strain of the shaft,
• establishing a line in a graph representing the strain on a first axis versus the physical quantity on a second axis so that the line originates from a certain offset strain value at the first axis and extends with said identified constant rate of change,
• detecting said significant change of the physical quantity when the measured physical quantity intersects the established line with a certain error margin, and
• indicating that the fastening element has reached the offset tensile yield limit load if the measured physical quantity intersects the established line with the certain error margin.