Patent ID: 8316726

Claim:
A method of measuring biaxial stress in an object of a ferromagnetic material, the method comprising: (a) subjecting material in a region ( 10 ) in the vicinity of a surface ( 11 ) of the object to a conditioning method by application of a conditioning magnetic field ( 38 ) that is at least initially at a high field strength; and then (b) measuring values of biaxial stress within the said region by taking measurements with an electromagnetic measuring probe ( 14 ) in at least two different orientations, wherein the electromagnetic measuring probe ( 14 ) uses an alternating measuring magnetic field ( 28 ) that is at a field strength well below saturation; wherein the conditioning method comprises subjecting the region to a low frequency alternating magnetic field ( 38 ) initially at a high field strength, and gradually reducing the alternating magnetic field strength to zero over a decay time period at least equal to the time for many cycles of the alternating magnetic field, wherein the direction of the magnetic field is either substantially orthogonal to the surface ( 11 ), or substantially parallel to the surface ( 11 ) within the region.