Abstract: The onset of rolling contact fatigue in an object may be detected by measuring the residual stresses in the surface of the object, and monitoring changes in their magnitude or their direction. The values of residual stresses in the surface are those averaged over a depth that may be in the range say 0.1 mm to 8.0 mm, for example about 2.5 mm. For ferromagnetic objects such as rails, the method may utilize an electromagnetic probe. Prior to rolling contact fatigue the surface stresses are compressive, and the onset of rolling contact fatigue may be characterized by a marked decrease of the stress in the generally transverse direction.

Abstract: Stress in the wall of a pipe (12) is measured using a pig (10) carrying at least one linear array of probes, so that the probes (30) in the array pass in succession over a location on the pipe wall. Each probe (30) comprises an electromagnetic core (32) with two spaced apart electromagnetic poles (34), and a magnetic sensor (36) arranged to sense the reluctance of that part of the magnetic circuit between the poles (34), and an alternating magnetic field is generated in the electromagnet means and consequently in the pipe wall. Successive probes (30) in the array are oriented differently so that the corresponding orientations of the magnetic field in the pipe wall are different. Preferably the probes (30) also include sensors (38) between the two poles (34) to sense magnetic flux perpendicular to the direction of the free space magnetic field between the poles. The signal from the sensor (36) and (38) enable the stress to be determined. Such an array may be used with any long object of ferromagnetic material.

Abstract: The variation in properties of a ferromagnetic material with depth below the surface is assessed in a nondestructive fashion using a probe that incorporates an electromagnet. An alternating magnetic field is generated in the electromagnet and so in the object, and a magnetic sensor is arranged to sense a magnetic field due to the electromagnet. Signals from the magnetic sensor are analysed into an in-phase component and a quadrature component, and these are mapped directly into material property and lift-off components; this analysis enables accurate measurements of material property (such as stress) to be distinguished from changes in lift-off. The measurements are repeated for at least five different frequencies of the alternating magnetic field; and the measurements at different frequencies are deconvolved assuming a functional form for the variation of material property with depth, the function having no more than five unknown constants. The stress at depths for example in the range 0.5 mm to 5.

Abstract: Stress in the wall of a pipe (12) is measured using a pig (10) carrying at least one linear array of probes, so that the probes (30) in the array pass in succession over a location on the pipe wall. Each probe (30) comprises an electromagnetic core (32) with two spaced apart electromagnetic poles (34), and a magnetic sensor (36) arranged to sense the reluctance of that part of the magnetic circuit between the poles (34), and an alternating magnetic field is generated in the electromagnet means and consequently in the pipe wall. Successive probes (30) in the array are oriented differently so that the corresponding orientations of the magnetic field in the pipe wall are different. Preferably the probes (30) also include sensors (38) between the two poles (34) to sense magnetic flux perpendicular to the direction of the free space magnetic field between the poles. The signal from the sensor (36) and (38) enable the stress to be determined. Such an array may be used with any long object of ferromagnetic material.

Abstract: The variation in properties of a ferromagnetic material with depth below the surface is assessed in a nondestructive fashion using a probe that incorporates an electromagnet. An alternating magnetic field is generated in the electromagnet and so in the object, and a magnetic sensor is arranged to sense a magnetic field due to the electromagnet. Signals from the magnetic sensor are analysed into an in-phase component and a quadrature component, and these are mapped directly into material property and lift-off components; this analysis enables accurate measurements of material property (such as stress) to be distinguished from changes in lift-off. The measurements are repeated for at least five different frequencies of the alternating magnetic field; and the measurements at different frequencies are deconvolved assuming a functional form for the variation of material property with depth, the function having no more than five unknown constants. The stress at depths for example in the range 0.5 mm to 5.

Abstract: The onset of rolling contact fatigue in an object may be detected by measuring the residual stresses in the surface of the object, and monitoring changes in their magnitude or their direction. The values of residual stresses in the surface are those averaged over a depth that may be in the range say 0.1 mm to 8.0 mm, for example about 2.5 mm. For ferromagnetic objects such as rails, the method may utilise an electromagnetic probe. Prior to rolling contact fatigue the surface stresses are compressive, and the onset of rolling contact fatigue may be characterised by a marked decrease of the stress in the generally transverse direction.

Abstract: The absolute values of biaxial stresses in a ferromagnetic material (16) are measured using a probe (12) which comprises an electromagnet (26), a sensor (32) for stress-induced magnetic anisotropy (SMA) and a sensor (30) for directional effective permeability (DEP). The DEP sensor (30) enables absolute values of stress to be determined; the SMA sensor (32) enables the directions of the principal stress axes to be accurately determined, and improves the accuracy of the stress measurements.