GB2202630A - Stress measurement in a body by detecting magneto-acoustic emission - Google Patents

Abstract

A method and an apparatus are provided for measuring stress in a steel pipe or rail utilizing magneto-acoustic emission (MAE). A piezoelectric transducer (24) detects the ultrasonic MAE in a steel rail (12) subjected to a low amplitude modulating field and to a steady magnetic field. The steady field is either sufficient almost to saturate the rail, or is almost zero; and the ratio between the MAE signals under these two circumstances is dependent on the stress in the rail while being substantially unaffected by its microstructure. Both the steady field and the modulating field are aligned parallel with the direction of the expected stresses. The steady field is supplied by a d.c. fed electromagnet 14 and the modulating field is generated by series-connected air-cored windings 28. The transducer 24 is connected to a signal processor and store 32. The device may be vehicle-mounted, e.g. for traversing the bore of a tube. <IMAGE>

Description

Stress Measurement This invention relates to an apparatus and to a method for measuring stress within a ferromagnetic material, and in particular for measuring stress within the walls of a tube or pipe.

The invention utilizes the phenomenon known as magneto-acoustic emission (MAE), which refers to the generation of stress-waves when a ferromagnetic material is magnetized by a changing magnetic field. The MAE waves are typically ultrasonic, extending over the frequency range from about 20 kHz to above 1 MHz, and are believed to be due to changes in local magnetostriction especially when 90 degree domain walls move discontinuously. The discontinuous movement of 180 degree domain walls gives a large change in magnetic moment, but no resultant change in magnetostrictive strain. Consequently such wall motion does not produce MAE. When materials are elastically strained, the preferred domain orientations are modified; the application of tensile stress will cause domains magnetised closest to the stress direction to grow at the expense of neighbouring domains, to reduce the elastic energy.Thus if the stress is parallel to the direction of magnetisation and the magnetic field varies periodically with time, domains magnetised parallel to the stress, and consequently 180 degree domain wall movements, will tend to become dominant. Thus in general MAE is affected by the tensile stress to which a material is subjected.

MAE was first described by A.E. Lord, Jr., in 1974 (see Lett. Appl. and Eng. Sci., Vol. 2, 1974). Research by M. Shibata and K. Ono (see NDT International, October 1981) indicated how the MAE for several iron/nickel alloys varied with applied magnetic field strength, and how it was affected by applied stress. In these experiments the values of MAE given were the r.m.s. signal, the specimen being subjected to a 60 Hz magnetizing field. The MAE in most cases was found to decrease with applied stress, and also to be affected by the microstructure of the specimen.

Shibata and Ono suggested that stress might be determined by measuring the ratio between the MAE values so defined as obtained by two transducers with centre frequencies of 175 kHz and 500 kHz. The effect of stress on MAE was also studied by G.L. Burkhardt et al. (see Materials Evaluation, Vol. 40, May 1982), in which a comparatively slowly varying magnetizing field was used - a triangular current waveform at 1 or 2 Hz - and the MAE pulses counted in successive time intervals along one branch of the hysteresis loop as the field varied. The MAE pulses were found to occur in two peaks whose shape and location depended upon the dimensions of the specimen, and both peaks tended to decrease with applied stress, whether in compression or tension. The ratio of these two peaks, it was suggested, might enable tension stresses to be distinguished from compression stresses.The MAE pulse distribution also appeared to be related to microstructural characteristics such as dislocations.

Thus although it is known that MAE is affected by stress, it has hitherto not been possible to eliminate the effects on MAE of differences in microstructure.

Consequently accurate measurement of stress has not proved practical.

According to the present invention there is provided a method of measuring stress in a specimen of a ferromagnetic material, comprising subjecting the specimen to a varying magnetic field so as to generate magneto-acoustic emission (MAE), and detecting and measuring the MAE, these operations being performed firstly with a magnetic field strength sufficient almost to achieve saturation, and secondly with a magnetic field strength about zero, and determining the ratio between the two values of MAE so measured.

It has been found, surprisingly, that for many materials this ratio is dependent upon stress but is almost unaffected by differences in microstructure such as grain size. Desirably the specimen is subjected to an alternating low amplitude, modulating magnetic field so as to generate MAE, and also to a steady magnetic field which is either sufficient almost to achieve saturation, or is about zero. By low amplitude is meant a magnetic field which is much smaller than the field required to achieve saturation. The lower the frequency of the modulating field, the greater the thickness of the specimen in which MAE is generated, whereas the higher this frequency generally the better will be the signal to noise ratio.

The frequency might be between 10 and 100 Hz, desirably about 50 Hz.

The steady field may be created by a permanent magnet or by a constant-current electromagnet. The zero field may be achieved by de-energising the electromagnet, if it is iron-cored, or by moving the magnet away from the specimen and/or introducing low reluctance keepers between the poles of the magnet.

The invention is especially applicable to steels, in particular those with cementite, ferrite and/or pearlite structure. Martensitic steels are not suitable, because in martensite there is only one easy axis of magnetisation so that only 180 degree domain walls can exist, and so no MAE occurs. A more accurate assessment of the stress can be obtained if the method also includes an investigation of Barkhausen emission (BE) during magnetisation, as the Barkhausen emission variation with applied field enables the type of microstructure, i.e. the types of domain wall pinning sites, to be qualitatively determined. This makes possible the choice of the appropriate calibration of the MAE ratio dependence on stress.For example ferrite/pearlite gives rise to an initial and a central peak in the BE, the initial peak being about half the height of the central peak, and gives MAE ratios of about 1.1. Cementite gives a high BE signal with an initial peak dominating the profile, and gives larger MAE ratios, between about 1.5 and 3.5 for example. Provided the microstructural phases present in the steel do not change from place to place, the MAE ratio (as defined above) enables the stress to be determined, as this ratio is not sensitive to changes in grain size or in the proportions of the different phases.

The invention also provides an apparatus for measuring stress in a specimen of ferromagnetic material, comprising an electromagnet for subjecting the specimen to a varying magnetic field so as to generate MAE, a transducer adapted to be coupled to a surface of the specimen for detecting the MAE, a signal analysis means connected to the transducer for measuring the MAE, and means for determining the ratio between the values of MAE so measured with the specimen subjected firstly to a magnetic field sufficient almost to achieve saturation, and secondly to a magnetic field of almost zero.

Desirably the apparatus includes means such as an electromagnet for creating an alternating, low-amplitude, modulating magnetic field (which subjects the specimen to a varying magnetic field), and another means for providing a steady magnetic field sufficient almost to achieve saturation in the specimen, which can be de-energised or moved away and/or have its poles linked by a low-reluctance keeper so as to achieve zero field.

For measuring stress at successive locations along a long specimen such as a steel pipeline or a steel railway rail, the apparatus might comprise a vehicle supporting two identical transducers spaced apart along the specimen, each with a respective modulating electromagnet, but only one of which is adjacent to the means for providing the steady magnetic field to almost achieve saturation in the adjacent region of the specimen. The measurements of MAE from the two transducers are stored in a memory. The apparatus is then moved along the specimen until one transducer is in the location previously occupied by the other transducer, and the measurements are repeated. Hence the value of'the ratio for that location on the specimen can be determined.

Alternatively, if it is desired to avoid any problems due to the unknown acoustic coupling between the transducer and the specimen, the apparatus may comprise a vehicle supporting a transducer and a modulating electromagnet, which stops at a location at which the stress is to be assessed, and the two measurements are made before moving the vehicle on again. Preferably the steady field is in a direction parallel to a longitudinal axis of the specimen, and the modulating field is in the same direction.For a pipe, the vehicle may support a plurality of such transducers, each with a respective modulating magnet and equally spaced around the inside of the pipe's circumference, and at least one steady field generating magnet rotatable about the longitudinal axis of the vehicle so as to subject the pipe wall adjacent to each transducer in turn to the field sufficient almost to achieve saturation. Preferably between adjacent transducers around the circumference are arranged low reluctance keepers, axially aligned.

A comparison of the stresses measured at diametrically opposite locations in the pipe wall, enables the bending stresses in the pipe to be determined. The apparatus of the invention can thus be used to inspect steel or cast-iron pipes, the vehicle travelling along inside the pipe. The speed attainable is governed by the time taken to saturate the material magnetically, and by the distance along the pipe between successive measurement points. It is estimated that a 0.3 m diameter pipe could be inspected every 150 mm at a maximum speed of between 0.2 and 1.0 km/hour.

The invention will now be further described, by way of example only, and with reference to the accompanying drawings in which: Figure 1 shows diagrammatically a stress measuring apparatus; Figure 2 represents graphically the variation of MAE during one half of a hysteresis cycle; Figure 3 shows diagrammatically an alternative stress measuring apparatus; Figure 4 shows diagrammatically another alternative stress measuring apparatus; and Figure 5 shows diagrammatically a sectional view on the line V-V of Figure 4.

Referring to Figure 1 there is shown a stress measuring apparatus 10 arranged to measure the stress in a long steel rail 12. The apparatus 10 includes a U-shaped electromagnet 14 with a low-remanence soft-iron core 15 defining two poles 16, 17, and a winding 18 connected via a switch 20 to a d.c. power supply 22. The poles 16, 17 of the electromagnet 14 are adjacent to the surface of the rail 12, and the electromagnet 14 is such that when it is energised the portion of the rail 12 between the poles 16, 17 is almost saturated. Midway between the poles 16 and 17 is a piezoelectric ultrasonic transducer 24 coupled to the rail 12 by couplant gel 26. Between the transducer 24 and the poles 16 and 17 are air-cored electromagnet windings 28, each with its axis perpendicular to the surface of the rail 12.The two windings 28 are connected in series, and are wound such that when energised they produce opposite magnetic polarities adjacent to the rail 12, and they are energised by an a.c. supply 30 so as to generate a low amplitude modulating field in the rail 12 in the vicinity of the transducer 24.

The transducer 24 is connected to a signal processing unit and store 32, in which the signals from the transducer 24 are amplified, filtered through a 32-100 kHz band-pass filter, and recorded. The ratio of the recorded values so obtained, with and without energisation of the electromagnet 14, is then calculated. Thus in operation the windings 28 are continuously energised by the a.c.

supply 30 to generate a low amplitude modulating field, and therefore to cause MAE within the steel rail 12 which is detected by the transducer 24 and recorded. The ratio of the recorded values of MAE with and without the strong magnetic field of the electromagnet 14 gives a measure of the stress to which the rail 12 is subjected.

Referring to Figure 2 there is shown graphically the variation of MAE with the current through the electromagnet 14 for an apparatus similar to that of Figure 1 but in which the power supply 22 could be varied in magnitude and direction. The graph shows the MAE from a specimen of Incoloy 904 (51.0% Fe, 33.8% Ni, 14.0% Co, 1.2% Ti by weight), a high strength, low thermal expansivity alloy.

While subjecting the specimen to a 20 Hz modulating field the electromagnet current was gradually and steadily increased from -2A to +2A in a time of about 521 minutes.

The current required to almost achieve saturation was about 0.7A, so the graph represents the MAE occurring in half of a hysteresis loop. The ratio calculated by the processor 32 in the apparatus 10 of Figure 1 thus corresponds to the ratio between the values of MAE marked A and B in Figure 2.

Referring to Figure 3 there is shown an alternative stress measuring apparatus 40 with many similarities to the apparatus 10 of Figure 1, arranged to measure the stress at several locations along a steel pipe wall 42. The apparatus 40 includes a wheeled vehicle (not shown) movable along the length of the pipe. The vehicle supports two identical piezoelectric transducers 44 separated from one another in the direction of movement of the vehicle, and both coupled to the wall 42 by couplant gel 46 (if desired this can be continuously supplied by a pump (not shown) from a reservoir (not shown)). Each transducer 44 is situated between a pair of air-cored electromagnet windings 48 connected in series to a respective a.c. power supply 50 and arranged so as to create a low amplitude modulating magnetic field in the pipe wall 42 in the vicinity of the transducer 44, parallel to the direction of movement. A U-shaped permanent magnet 54 is arranged with its poles near the pipe wall 42 on opposite sides of the right hand (as shown) pair of coils 48 so as to create a magnetic field in the pipe wall 42 near the right hand transducer 44 sufficient almost to saturate the steel, that magnetic field also being parallel to the direction of movement.

Both the transducers 44 are connected to a common signal processing unit and store 52, in which the signals are amplified, filtered by a 32-100 kHz band-pass filter, and recorded. In operation, both power supplies 50 are energised, and the MAE signals from the two transducers 44 are recorded with the vehicle stationary. The vehicle then moves along the pipe a distance equal to the separation between the transducers 44, say to the right, so the left hand transducer 44 is where the right hand transducer 44 had initially been, and measurements of MAE are again taken with the vehicle stationary. The ratio is then calculated between the recorded values of MAE detected by the two transducers 44 when at the same position.

Alternatively the distance moved by the vehicle between successive measurements might be an integral submultiple of the separation between the transducers 44.

The apparatus 10 of Figure 1 has the advantage that only one transducer 24 is used; but it suffers the disadvantage that energy is dissipated in magnetising and demagnetising the electromagnet core 15. In contrast the apparatus 40 of Figure 3 obviates the need for an electromagnet, as a permanent magnet 54 can be used; but suffers the disadvantage that the two measurements whose ratio is to be determined are made by two different transducers 44 whose characteristic and whose coupling to the pipe may not in practice be identical.

Referring now to Figures 4 and 5 there is shown another stress measuring apparatus 60 similar to those of Figures 1 and 3 for measuring stress at four equally spaced positions around a steel pipe wall 62. The apparatus 60 includes a vehicle (not shown) movable along the length of the pipe. Fixed to the vehicle are four piezoelectric transducers 64, equally spaced around the inner circumference of the pipe wall 62 to which they are coupled by a couplant 66. As in the apparatus 40 of Figure 3, each transducer 64 is between a pair of air-cored electromagnet windings 68 arranged to generate a low-amplitude axial modulating field in the pipe wall 62 in the vicinity of the respective transducer 64; each pair of windings 68 is connected in series to a respective a.c. supply (not shown).Four low reluctance keepers 70 are also fixed to the vehicle, each midway between two adjacent transducers 64. Magnetisation of the pipe walls 62 is brought about by an assembly of four U-shaped permanent magnets 74 with their poles adjacent the pipe wall 62 and equally spaced around the circumference, neighbouring magnets 74 having opposite polarities, and supported on the vehicle so as to be rotatable about the longitudinal axis of the pipe. Each transducer 64 is connected to a respective signal analyser and store (not shown), whose operation is as that in the apparatus 10 of Figure 1.

With the vehicle stationary at a position where it is desired to measure the stresses in the pipe wall 62, the modulating electromagnet windings 68 are energised to cause MAE, and the magnet assembly is caused to rotate. As the poles of the magnets 74 pass through the position shown, midway between the keepers 70, the pipe wall 62 is almost saturated in the vicinity of each transducer 64. Rotation of the magnet assembly through 450 brings the poles in line with the keepers 70, so that most of the magnetic field is within the keepers 70 and the field in the vicinity of each transducer 64 is almost zero. Thus as the magnet assembly rotates, the magnetic field in the vicinity of each transducer 64 varies from near saturation, to zero, to near saturation in the other direction, to zero, and so on.

Each signal processor is hence able to determine the desired MAE ratio and hence the stress in the pipe wall 62 adjacent to the transducer 64 to which it is connected.

Claims (9)

Claims

1. A method of measuring stress in a specimen of a ferromagnetic material, comprising subjecting the specimen to a varying magnetic field so as to generate magnetoacoustic emission (MAE), and detecting and measuring the MAE, these operations being performed firstly with a magnetic field strength sufficient almost to achieve saturation, and secondly with a magnetic field strength about zero, and determining the ratio between the two values of MAE so measured.

2. A method as claimed in Claim 1 wherein the varying magnetic field comprises an alternating, low amplitude, modulating field of frequency between 10 and 100 Hz.

3. A method as claimed in Claim 1 or Claim 2 also including detecting Barkhausen emission during magnetisation of the specimen, in order to identify the type of microstructure present.

4. An apparatus for measuring stress in a specimen of ferromagnetic material, comprising an electromagnet for subjecting the specimen to a varying magnetic field so as to generate MAE, a transducer adapted to be coupled to a surface of the specimen for detecting the MAE, a signal analysis means connected to the transducer for measuring the MAE, and means for determining the ratio between the values of MAE so measured with the specimen subjected firstly to a magnetic field sufficient almost to achieve saturation, and secondly to a magnetic field of almost zero.

5. An apparatus as claimed in Claim 4 comprising a vehicle supporting two identical transducers spaced apart along the specimen, each with a respective modulating electromagnet, but only one of which is adjacent to the means for providing the steady magnetic field to almost achieve saturation in the adjacent region of the specimen, means for moving the vehicle along the specimen, and a memory for storing the measurements of MAE for the two transducers.

6. An apparatus as claimed in Claim 4 suitable for measuring stress in the walls of a pipe comprising a vehicle movable along the pipe, supporting a plurality of said transducers equally spaced apart around the circumference of the pipe, and a respective modulating electromagnet for each transducer, and at least one steady field generating magnet rotatable about the longitudinal axis of the vehicle so as to subject the pipe wall adjacent to each transducer in turn to the field sufficient almost to achieve saturation.

7. An apparatus as claimed in Claim 6 also comprising axially aligned, low reluctance, keepers between adjacent transducers.

8. A method of measuring stress in a specimen of a ferromagnetic material, substantially as hereinbefore described with reference to, and as shown in, Figure 2, and Figure 1 or Figure 3 or Figures 4 and 5 of the accompanying drawings.

9. An apparatus for measuring stress in a specimen of a ferromagnetic material, substantially as hereinbefore described with reference to, and as shown in, Figure 1 or Figure 3 or Figures 4 and 5 of the accompanying drawings.