Patent ID: 7215117

Claim:
A method for measuring biaxial stresses in an object of ferromagnetic material, said method comprising the steps of using at least one probe, the, or each, probe comprising an electromagnet means and a magnetic sensor arranged for sensing a magnetic field due to said electromagnet means, the method for the, or each, probe comprising arranging the probe adjacent to said object, and activating the electromagnet means for generating an alternating magnetic field in said electromagnet means and consequently in said object, the generated magnetic field alternating at a frequency selected for providing a desired depth of penetration below a surface of said object; detecting signals from said magnetic sensor and resolving said signals into an in-phase component and a quadrature component which are orthogonal in phase, and mapping said in-phase and quadrature components directly into apparent stress and lift-off components by means of a first calibration, using a sample of said ferromagnetic material and subjecting said sample to a range of different uniaxial stresses with one said probe at a range of different values of lift-off for deducing corresponding curved contours in a graph of quadrature against in-phase components; arranging the one said probe adjacent to the ferromagnetic object, turning the one said probe to different orientations and detecting signals from said magnetic sensor when the one said probe is oriented with two principal stress axes thereof, and then from the curved contours deduced in the first calibration deducing two corresponding values of apparent uniaxial stresses along said two principal stress axes; and, by means of a second calibration with a sample of the ferromagnetic material subjected to a range of different biaxial stresses, deducing the true biaxial stress in said object from the two values of apparent uniaxial stress.