Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
  • G01N27/72—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating magnetic variables
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
  • G01L1/00—Measuring force or stress, in general
  • G01L1/12—Measuring force or stress, in general by measuring variations in the magnetic properties of materials resulting from the application of stress
  • G01L1/125—Measuring force or stress, in general by measuring variations in the magnetic properties of materials resulting from the application of stress by using magnetostrictive means
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
  • G01L1/00—Measuring force or stress, in general
  • G01L1/12—Measuring force or stress, in general by measuring variations in the magnetic properties of materials resulting from the application of stress
  • G01L1/127—Measuring force or stress, in general by measuring variations in the magnetic properties of materials resulting from the application of stress by using inductive means

Definitions

• the present invention relates to a method for measuring or detecting a change in the mechanical state such as a change in the mechanical load, yielding, initiation or propagation of cracks, separation within the material or vibration or a change in the mechanical state with respect to time in a body showing magnetostrictive effects, when, under the influence of a driving magnetic force (H), an associated magnetic flux is generated in at least a part of the body, on which flux the change in the mechanical state via the magnetization (M) of the body acts a measurable or detectable disturbance.
• H driving magnetic force
• M magnetization
• the invention relates also to a device for carrying out the method.
• the object of the present invention is to eliminate the above mentioned double dependence of the magnetization so that it only depends on the change in the mechanical state. This is achieved by keeping the magnetizing force (H 0 ) and the average length (l) of the magnetic circuit essentially constant and independent of the change in the mechanical state when measuring or detecting.
• the flux consists partly of a main flux ( ⁇ 0 ) associated with the driving magnetic force (H 0 ) having a definite direction and being of such a magnitude that the magnetic properties of the body depart from the region of irreversibility, and partly of a superposed alternating flux ( ⁇ v ) which gradually vanishes.
• This alternating flux must have such an initial magnitude that saturation is achieved in the two directions of the alternating flux ( ⁇ v ⁇ , i e so that in a way known per se a point on the anhysteretic curve is reached when the alternating flux ( ⁇ v ) has vanished.
• the disturbance which is generated by the change in the mechanical state is registered as a voltage which in turn is induced by the change in the flux corresponding to disturbance.
• Fig 1 shows schematically a device which is operated according to the invention when measuring a change in the mechanical state of a loaded body.
• Fig 2 shows by solid lines one half of the magnetizing curve for a ferromagnetic solid and by dashed lines the corresponding part of the anhysteretic curve of the body.
• Fig 3 is an enlarged part of Fig 2 illustrating how, for a given value H 0 of the driving magnetizing force, the magnetic flux ⁇ can assume a number of values, three of which are specially indicated in the figure.
• Fig 4 shows with three curves how.the magnetic flux in a body varies with time, when the body is subjected to triangular mechanical load pulses.
• the intermediate one of the curves is related to a measuring procedure according to the invention and the others to conventional measuring procedures.
• Fig 5 shows in detail one embodiment of a device according to the invention and intended for measurements on a round specimen.
• Fig 6 shows in a projection a device to the invention for measuring on a notched threepoint bend specimen, which is subjected to bending forces.
• the ferromagnetic body 1 is a test specimen, which is subjected to mechanical stress due to tension,. compression or torsion, as is indicated by the arrows 2, acting individually or in combination as tension-torsion or compression-torsion.
• the stresses give rise to a change in the mechanical state of the specimen which can encompass elastic and plastic deformation, separation within the material, crack initiation , crack propagation and even fracture . It should be mentioned that a change in the mechanical state also can arise from thermal stresses.
• yokes 3 one of which is shown in the figure, carrying a primary coil 4 and a secondary coil 5.
• a current is produced, which in turn generates the magnetizing force.
• This generates a flux through a magnetic circuit, which consists of the yoke 3 and a part of the specimen 1.
• the yoke shall be arranged in such a way in relation to the specimen that the magnetic circuit is essentially free from air gaps and so that the yoke is not affected by the stress. This can be achieved by letting the yoke 3 slide along the specimen 1. It is obvious that the coils may be placed anywhere along the magnetic circuit. The location of the coils is determined by practical considerations only. In order to achieve a symmetrical flux a number of parallel yokes should be used.
• both the magnetizing force, in the following named H 0 , and the average length l of the magnetic circuit shall be kept essentially constant and independent of the change in the mechanical state.
• the average length l is kept constant by allowing at least one of the two shanks of the yoke to slide along the specimen.
• the magnetizing force H 0 is kept constant by feeding the coil 4 with a constant direct current. This current is obtained by a divider of a voltage U by use of a potentiometer 9.
• each value of the magnetizing force H 0 corresponds to a number of values of the flux ⁇ , two of which, a and b, have been marked on the hysteresis curve of the body and one, c, on the anhysteretic curve.
• c is the one which corresponds to the value of the flux ⁇ at which the specimen has reached its lowest electromagnetic energy for the given value H 0 of the driving magnetizing force.
• an alternating flux ⁇ v which gradually vanishes, is therefore superposed.
• This flux is produced by a generator 11 which supplies a voltage u, decreasing with time to the summation point 10 in the unit 6, Fig 1.
• the object of the alternating flux ⁇ v is to bring the tested body to reach an initial state which is stable from an electromagnetic point of view. This will be the case if one starts from the anhysteretic curve.
• This curve relates , as contrasted to the hysteresis curve, the magnetization M unambiguously to the magnetizing force H. Furthermore, from a magnetic point of view, a departure must be made from the irreversible region of the body.
• a triangular-shaped voltage u is fed to the summation point 10 in the unit 6, Fig 1.
• the hysteresis curve is then registered and from it a suitable value of the driving magnetizing force H 0 is selected beyond the above mentioned region.
• the detection can be performed according to the invention.
• the 'point c in Figs 2 and 3 corresponds to the value of the flux ⁇ at which the body assumes its lowest state of electromagnetic energy at the magnetizing force H 0 .
• values of the magnetizing force there are of course corresponding values of the flux, which are to be found from the anhysteresis curve of the body.
• the mean value of the curve is time invariant during one or more cycles. This is obviously not the case for the two earlier described outer curves which illustrate measurements according to known methods.
• the mean value of these curves is not constant but tends towards the mean value of the intermediate curve. This could be explained by the supposition that energy is required for bringing the flux at points a and b to the same level as at c. This energy is taken from the change in the mechanical state which consequently cannot be stably reproduced in time through studies of the mangetic flux.
• the change in flux generated by the change in the mechanical state and detected by the measuring coil 5 as a voltage can be used in a number of ways by the indicator unit 8 as illustrated in Fig 1.
• a signal is obtained at output A proportional to this voltage as well as at output B corresponding to the time integral of the voltage.
• C and D signals are obtained corresponding to the voltage after passage of a high-pass filter and a low-pass filter, respectively, and integration.
• the voltage can be treated in other ways e g being digitized.
• Figs 5 and 6 components having the same or corresponding functioning as those in Fig 1 have been given the same reference numbers as in Fig 1.
• the body 1 is a conventional cylindrical test specimen where the deformations occur mainly in the waisted section, assuming that its diameter is considerably smaller than the other parts of the specimen.
• the primary coil 4 and the secondary coil 5 are wound on a tube-shaped coil core 13 with end walls.
• the central hole of the coil core is of such a size as to allow an easy penetration of the body.
• the coil 4 and 5 are protected by a cover 14.
• the positioning of the coils 4, 5 was chosen to obtain as small a leakage and as homogeneous a flux as possible.
• To obtain a symmetric flux through the body 1 a number of yokes 3 have been placed symmetrically around the coil core. These are U-shaped plates intended to slide in radially arranged slits in the end walls of the coil core 13.
• the body 1 can slide relatively free in the central hole of the coil core 13 which leads to a constant average length of the magnetic circuit independent of the applied mechanical loads.
• the yokes 3 are held in place during the measurement by magnetic forces between the yokes and the body. When measuring is not performed the yokes can be secured by e g elastic strings or hoops closing the yokes.
• the body 1 is a conventional threepoint bend specimen having a rectangular cross section.
• a notch 15 In the center of the specimen is a notch 15.
• the primary and secondary coils 4 and 5 are placed on yokes 3, of which only one is shown in Fig 6.
• the yokes are placed symmetrically with respect to the notch on the longitudinal sides adjoining the upper side.
• the specimen is loaded with forces as is showed by arrows in the figure so that the specimen is mainly subjected to bending stresses.
• the forces P/2 are applied from above symmetrically with respect to the notch 15 and the force P acts from below at the notch 15.
• the stress must not influence the average length of the flux.
• the yokes 3 are slidably arranged relative to the specimen. The measuring should be carried out as is described above in connection with Fig 1.
• the measuring method can, as is stated in the appended patent claims, be used in many measuring applciations .
• an arbitrary magnetizable body e g a stay crutch, a beam, a rod or a plate acted upon by a load
• the change in the mechanical state due to the load can be -measured.
• the presumptions for this is that one starts from a point on the anhysteresis curve of the body and that the average length of the flux is not affected by the load.
• the main magnetic flux has in the applications above been generated by use of. one or more primary coils, but can, as is apparent from a special characteristic of the invention, also be produced by permanent magnets. These are arranged in such a way that the main flux is constant.

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• General Physics & Mathematics (AREA)
• Physics & Mathematics (AREA)
• Chemical & Material Sciences (AREA)
• General Health & Medical Sciences (AREA)
• Health & Medical Sciences (AREA)
• Life Sciences & Earth Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Biochemistry (AREA)
• Analytical Chemistry (AREA)
• Electrochemistry (AREA)
• Immunology (AREA)
• Pathology (AREA)
• Investigating Or Analyzing Materials By The Use Of Magnetic Means (AREA)
• Measuring Magnetic Variables (AREA)
• Force Measurement Appropriate To Specific Purposes (AREA)

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• 1977
  • 1977-12-12 SE SE7714052A patent/SE403517B/xx not_active IP Right Cessation
• 1978
  • 1978-11-23 WO PCT/SE1978/000082 patent/WO1979000369A1/en unknown
  • 1978-11-23 DE DE782857236T patent/DE2857236A1/de not_active Withdrawn
  • 1978-11-23 CH CH766479A patent/CH635935A5/de not_active IP Right Cessation
  • 1978-11-23 GB GB7926806A patent/GB2036329B/en not_active Expired
• 1979
  • 1979-07-04 EP EP78900287A patent/EP0007963A1/de not_active Withdrawn
  • 1979-08-10 SU SU792806958A patent/SU973040A3/ru active
  • 1979-08-20 US US06/068,183 patent/US4316146A/en not_active Expired - Lifetime
• 1980
  • 1980-03-13 FR FR8006005A patent/FR2461247A1/fr active Granted

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