Measurement of stress in a ferromagnetic material

An apparatus for measuring stress in an object of ferromagnetic material, the apparatus comprising a linear array of probes, and a support structure for the array that is arranged to move relative to the object so that the probes in the array pass in succession over a location on the surface of the object;

• each probe comprising an electromagnet means defining an electromagnetic core and two spaced apart electromagnetic poles, and means to supply an alternating electric current to generate an alternating magnetic field in the electromagnet means and consequently in the object, and a magnetic sensor arranged to sense the reluctance of that part of the magnetic circuit between the poles of the electromagnet means and to generate a corresponding signal that can be resolved into components in phase with the alternating current and in quadrature to it;
• wherein the frequency of the alternating magnetic field, the width of each probe, and the velocity of the array relative to the object are such that the said frequency is significantly greater than that calculated by dividing the said velocity by twice the said width;
• and wherein successive probes in the array are oriented differently so that the corresponding orientations of the magnetic field in the object are different.