Abstract: A microcomputer or PC based non-destructive coated-object testing system and method combines thermoelectrically measured surface coating thickness data with flaw/crack depth data determined from eddy current measurements of the surface of the object to locate and selectively identify faults or cracks which penetrate the surface coating. The microcomputer controls scanning of eddy-current and thermoelectric probes over the object surface and stores measured data along with surface coordinate information to provide a color-keyed graphics display of surface conditions for visual analysis.

Abstract: Eddy-current response voltages, generated during inspection of metallic structures for anomalies, are often replete with noise. Therefore, analysis of the inspection data and results is difficult or near impossible, resulting in inconsistent or unreliable evaluation of the structure. This invention processes the eddy-current response voltage, removing the effect of random noise, to allow proper identification of anomalies within and associated with the structure.

Abstract: A method for nondestructive examination of an object of type having localized permeability variations therein and varying surface conditions and grain structures comprises the steps of AC flux saturating a localized surface area of the object so as to magnetically saturate the surface area to quiet localized permeability variations therein and to set up a stabilized high level primary field on the surface substantially unalterable by the varying surface conditions and grain structures in the object and detecting eddy currents in the surface area. Coil structure and apparatus for implementing the method are also defined. Further, the method is disclosed as being implemented in part by the use of components of a pre-existing object evaluation device.

Abstract: A system and method is provided for aligning a scanning surface of an inspection probe relative to a workpiece surface under inspection. The probe preferably includes at least three alignment eddy current elements, each producing a respective spacing indication electrical signal in accordance with a spacing between each alignment eddy current element and the workpiece. The system further comprises processing means which receives each spacing-indication electrical signal so as to generate data indicative of the relative alignment between the scanning surface of the probe and the workpiece surface. A controller is responsive to the alignment data for aligning the probe such that in operation the scanning surface thereof is substantially parallel relative to the workpiece surface. The probe can include inspection eddy current elements, in which case the alignment and the inspection eddy current elements can be fabricated to form an integral eddy current inspection probe.

Abstract: A magnetic inspection device for detecting structural faults in elongated magnetically permeable objects has two opposite magnetic poles, at least one pole being an end pole, that induce a first magnetic flux axially in a first section of an object between the poles, and an oppositely directed magnetic flux in another section adjacent the end pole. As the device and object move relative to one another, eddy currents induced in the object by the changing longitudinal fluxes at the end pole are detected to locate structural faults in the object.

Abstract: A device for the nondestructive testing of riveted junction sections or the like contains at least one electric type detection probe movable along the junction sections and a memory containing, for each junction section, its specific structure and, for each specific junction section structure, the operational setting to be applied to the probe. A microprocessor uses the contents of the memory for controlling the probe as a function of the specific structure of the junction section being examined.

Abstract: A scan head includes an eddy current probe and a thermal print head. A sheet of thermal paper is mounted to the top of a laminated part being inspected and the scan head is moved across the surface of the part. A map of defect markings will appear on the thermal paper in a 1-to-1 relationship with the actual sub-surface defects. This greatly expedites the inspection process since the mapped defects are in spatial correspondence with the actual part.

Abstract: An eddy current probe for detecting material flaws by inducing and measuring eddy currents in the surface of a material. The probe is held at a constant lift-off distance from the surface by a probe carriage that slides across the surface in a straight scan line. The carriage moves relative to a motorized bracket that slides the carriage across the surface. This relative movement allows the carriage to move across an uneven surface without deflecting the probe from a straight scan line. The probe has a split core so that an adjacent pair of coils project and detect electromagnetic fields on the surface being measured. Both coils have opposing flat sides that allow the coils to be closely packed together within the probe body.

Abstract: Previously, only webs between connectors in reactor pressure vessel lids have been examined for incipient cracks by using an ultrasonic scanning head. In order to be able to test the connector penetrated by a bush from an annular gap formed between the connector and the bush, a frame is provided which carries a housing through lockable floating bearings for receiving a rotary plate in the housing. The rotary plate is concentrically associated with a centering piece which is adjustable in the vertical direction and is eccentrically associated with a lifting cylinder which is likewise vertically adjustable. A free end of the lifting cylinder carries a probe drive for inserting a probe into the annular gap. The annular gap is scanned by the probe due to an oscillating movement of the rotary plate.

Abstract: Previously when testing a connector penetration of a reactor pressure vessel lid, one assumed that an annular gap between the connector and the sleeve was concentric. However, tests have shown that in some lid structures a deflection of the connector took place after cooling to room temperature, which led to an eccentric configuration of the sleeve and the connector. In order to be able to completely test such annular gaps, a method and device for nondestructive testing of a penetration of a reactor pressure vessel lid according to the invention includes displacing the sleeve with the help of a thrust piece, until an accessible annular gap is reached. A sensor then arrives at the annular gap through a recess in the thrust piece.

Abstract: Eddy current based method and apparatus uses a repetitive square wave signal applied to an excitation coil and operates in a pulse echo mode by gating the detected field signal at synchronized intervals to generate one or more sequences of like gated slices obtained from respective detector positions on a circular scan path. Suitable time gating to exclude surface and near-surface reflections overcomes surface clutter and flaws are detected by examination of the sequence of slices for characteristic changes in level. Both method and apparatus have special application for crack investigations at rivet fasteners or similar, e.g. in airframes, and have ability to cope with ferrous fasteners in non-ferrous material. Excitation coil and magnetic detector are combined in a probe and the latter is driven around a circular scan path. A centering display is derived by examination of the near-surface reflection signal to determine the position and level of the peak slice level on the scan path sequence of slices.

Abstract: Method of and apparatus for continuous non-destructive testing of bar-shaped specimens formed of iron-containing material of specific quality characteristics, where the bar-shaped specimens, such as pipes, move on a roll table (2) along an elongated path through two testing blocks (8, 9) spaced apart along the path. The testing blocks (8, 9) are mechanically decoupled from one another and are logically linked to an electronic testing installation (10). As a specimen moves through a first testing block (8) a signal is developed indicating the presence or absence of flaws and their location in the specimen. The location is referred to the length of the specimens. The testing blocks (8, 9) are located between a supply station (1) and a sorting station (3) with the roll table (2) extending between the two stations. Based on the signals generated, the specimens are sorted in the sorting station between an accepted tray (4) and a number of rejected trays (5, 6, 7) for reworkable or scrap specimens.

Abstract: An eddy current probe for nondestructive testing, having a rotatable shoe that holds the eddy current coil. The probe is attached to an arm of automated eddy current test equipment, and has a coil-containing shoe that may be rotated to a desired angle with respect to the surface to be inspected. The probe has a motor and pulley system for providing this rotational motion.

Abstract: A method for determining the thickness of a pipe by inserting a coil into the pipe and applying a voltage to the coil and measuring the time it takes for current in the coil to magnetically saturate the pipe and determining the thickness of the pipe from the measure time.

Abstract: A method and system for accurately inspecting an electroconductive film using an eddy current and a process and apparatus for production of an optical fiber which measures on-line the electrical resistance, which shows the state of formation of the electroconductive hermetic coating of the optical fiber by the electroconductive film inspection method and reflects back the measurement results to the hermetic coating forming conditions are disclosed. Optical fiber is made up of a core, cladding, amorphous carbon film or other electroconductive hermetic coating formed on the outer surface of the cladding, and a protective coating. The electrical resistance of the hermetic coating generates an eddy current at the coating, the eddy current generated is detected, and the phase angle of the complex impedance is detected to enable calculation.

Abstract: A method and apparatus for inspecting the profile of the inner wall of a conduit, such as a heat exchanger in a steam generator by a probe body that includes eddy current sensing coils. The number of eddy current sensing coils is increased for more completely sensing the conduit surface without a corresponding increase in coaxial cables by the use of a bank switch which switches respective banks of coils to existing conduits.

Abstract: Inspection probe for inspecting a tubular member for anomalies, the tubular member having an inside surface that may be irregularly-shaped. The inspection probe includes an enclosure sized to be slidably received along the inside surface of the tubular member. The enclosure has a cavity therein and a housing disposed in the cavity. A spring is disposed in the cavity of the enclosure for outwardly biasing the housing toward the inside surface of the tube. A swivel body is pivotably mounted in the housing. The swivel body has a sensor pivotably mounted thereon for sensing anomalies (i.e., flaws and wall thinning) in the tube wall. Connected to the swivel body and to the sensor is a first pivot pin for pivoting the sensor about a first pivot point laying on a first axis extending transversely to the longitudinal axis of the tubular member.

Abstract: A method for improving eddy current flaw detection by simultaneously exciting a select plurality of eddy current probe elements with a multiplicity of select frequencies in a simultaneous parallel or serial manner to form a corresponding multiplicity of direct and/or differential images which can be processed together for improved eddy current image resolution. Select eddy current probe elements when driven at select multiple frequencies further provide a capability for simultaneous flaw detection and characterization by dual resolution scanning. Dual flaw resolution is accomplished by first locating a flaw using low resolution frequencies; and thereupon, switching to higher resolution frequencies to characterize the flaw.

Abstract: In a magnetic inspection apparatus for a thin steel strip of the invention, a pole-to-pole distance of a magnetizer, which is housed in a hollow roller contacting a traveling thin steel strip to oppose the thin steel strip, is set between twice and eight times a distance between magnetic poles and the thin steel strip. A position of a magnetic sensor in the travel direction of the thin steel strip is shifted from the central position of the magnetic poles to a travel direction side by a small distance. In this invention, a pair of hollow rollers are provided to sandwich upper and lower surfaces of a traveling thin steel strip, and magnetic sensors are respectively housed in the hollow rollers. As a result, a position of a defect in the direction of the thickness of the thin steel strip, and a defect size can be accurately detected.

Abstract: An apparatus for rotating a spherical object, which includes suspension means for supporting a spherical object in a fixed position relative to its center. The suspension means holds the sphere by simultaneously contacting its surface with contacting elements at at least four points spatially independent of each other on the surface. Each of the contact elements is of regular shapes, such as, spheres, cylinders, cones or other shapes having symmetry about an axis of rotation. At least one of the contact elements is rotationally driven by a rotating drive means. The rotating drive means is rotated at a fixed or time-varying speed to impart controlled movement to the sphere about an axis through its center. The speeds or time-varying functions thereof cause the resultant controlled rotational motion of the sphere by manipulating an axis of rotation through its center.

Abstract: A system for processing signals produced by magnetic flaw detection apparatus having facility for examining objects in the course of transport thereof at speeds varying from a prescribed slow speed to a prescribed high speed, with transport frequency thus being within a first frequency range. The system comprises first circuitry for detecting the speed of transport of an object under examination and generating output signals indicative of the detected speed and extending over a given characteristic range correspondingly with the first frequency range. Second circuitry is provided for receiving the first circuitry output signals and transforming the same into a second frequency range substantially expanded as compared to the first frequency range.

Abstract: A non-destructive eddy current test device using additive flux/substractive flux switching includes a generator supplying a sinusoidal signal, two power amplifiers of equal and opposite gain, a changeover relay, two impedances and two coils forming a Wheatstone bridge, an amplifier controlled by a binary signal to add or subtract and two synchronous detectors for supplying signals representing the real part and the imaginary part of the difference between the complex impedances of the coils. A control device supplies to the changeover relay and to the amplifier a control signal for obtaining additive or subtractive fluxes in the two coils and for respectively subtracting or adding the two output voltages of the Wheatstone bridge. The device is applicable to the non-destructive testing of metal parts.

Abstract: An eddy-current defect test instrument with a rotary head (30) and test probes (32, 34, 36, 38) offset-mounted along a circular track (66) with only one distance probe (40) to achieve compensation for variations in spacing between the probes and the workpiece which only requires one distance signal processing device (60) and only one rotary transmitter (100). This is accomplished by storing the control signals obtained from the distance probe during one revolution of the rotary head (30) and retrieving those control signals associated with the probe position and appropriately modifying a defect signal.

Abstract: Apparatus for measuring flaws in an object, such as a tube, having both an erratic motion, e.g. a back and forth oscillation, and a net axial motion has an idler wheel engaging the tube, and a digital rotary shaft encoder attached to the wheel. A POP circuit is coupled to the encoder. A pair of coils disposed around the object make up the active elements in an eddy current bridge. An ADC is coupled to both the bridge and POP circuit and corrects the signal from the bridge for motion complications. A method for doing same comprises sensing only the net motion of the object, detecting flaws in the object, providing an erratic motion complicated flaw signal, and correcting the flaw signal for the erratic motion.

Abstract: An embodiment comprises a generator constituted by a quartz crystal oscillator and programmable frequency dividers; a power amplifier providing a periodic excitation signal, of squarewave form; two coils electromagnetically coupled to a metal part to be tested; and measuring means comprising two synchronous detectors for measuring the real part and the imaginary part of the difference between the complex impedances of the coils. The frequency stability of the generator effects an improvement in the accuracy of the impedance measurements, when these measurements are made at different frequencies. For application to non-destructive testing of metal parts by eddy currents, at different frequencies.

Abstract: An eddy current flaw detection apparatus, having: a pair of detecting coils disposed in a separated manner along a transit path of a conductor and coaxial with the conductor, magnetic fields generated by said detecting coils being in anti-phase; an alternating current bridge circuit, two sides of a bridge of which are comprising the detecting coils, and which, when a relative change occurs in inductances of the detecting coils and equilibrium of the bridge is thus disturbed, outputs an abnormality detection signal; a pair of resonance coils, which are disposed between the detecting coils, and coaxial with the detecting coils, and magnetic fields of which are in phase, and which are so set that in a state in which the bridge is in equilibrium, induction currents generated by said resonance coils cancel each other out; and a capacity circuit for the purpose of bringing oscillation frequencies of the detecting coils into resonance, which is connected to the resonance coils.

Abstract: An AC magnetic flux leakage type flaw detecting apparatus for use in flat surface flaw detecting comprises AC exciting means having a pair of magnetic poles for exciting a steel material to be flaw detected, a leakage magnetic flux detecting element, and rotatably scanning means for rotatably scanning the magnetic flux leakage detecting element in a surface substantially parallel to the flat surface oppositely to the flat surface to be flaw detected of the steel material in a plane of an exciting magnetic field between the pair of magnetic poles.

Abstract: A device for simulating a moving test object sensing transducer uses a plurality of individual transducers which are stationary relative to each other (for example eddy-current based surface transducers), and a selector unit. The individual transducers are, for example, mounted in a plane parallel to the surface of the test object to be sensed and scan the test object with respect to a characteristic, for example a crack. The resultant simulated transducer signal is signal processed by means of, for example, automatic compensation and vector transformation techniques.

Abstract: A hot flaw detector is equipped with a sensor holding member that has a circular opening through which the material to be inspected will pass and that holds a flaw detecting sensor. Provision is made of an annular cooling liquid injection port that is open at an end of the sensor holding member adjacent to the inner peripheral wall of the circular opening, and provision is further made of an annular cooling liquid introduction port that is open at the other end of the sensor holding member adjacent to the inner peripheral wall of the circular opening. The cooling liquid is injected from the annular cooling liquid injection port, and forms a liquid film along the inner peripheral wall of the circular opening, and is recovered through the annular cooling liquid introduction port. The inner peripheral wall of the sensor holding member is effectively cooled by the liquid film which drips little onto the material to be inspected.

Abstract: A method for operating a computer to control the manufacture process of zirconium tubes in a pilger mill operation, the computer including an electronic memory and being coupled to the pilger mill to receive, as input, data related to the operation of the mill. The method comprising, in one embodiment, the steps of storing in the computer memory respective test signals, each test signal corresponding to a signal obtained by inspecting a tube manufactured by the mill with a respective, known defective operation condition, inspecting a zirconium tube finished by the manufacture process and generating a signal representative of the physical dimensions and material configuration of the tube, comparing the generated representative signal of the zirconium tube with the stored test signals, alerting an operator if the generated representative signal of the zirconium tube correlates to a stored test signal, and identifying the defective operation condition which corresponds to the correlated stored test signal.

Abstract: A tool dullness indicator and method comprising the measurement of eddy currents at a known, controlled frequency directly in the tool by measuring the phase angle shift in a coil when a sample is put into or next to the coil. The phase angle change caused by presence of a workpiece is measured when the tool is sharp to provide a reference reading taking into account the surface hardness and other bulk properties of the workpiece, and arriving at a parameter that is dependent on eddy current changes, such as phase angle shift or other measurable changes related to eddy currents in the part, and then analyzing the workpiece, either while working, or at a set programmed check time to determine when the tool is dull as a function of the change in eddy current characteristics caused by work hardening.

Abstract: A detachable, articulated rotating eddy current inspection tool includes a body member having a connector at one end for mechanically and electrically attaching the body member to a rotating drive member while at the other an eddy current probe is supported for radial movement with respect to the axis of rotation of the body member. A motor located within the body moves a slide on which the probe is mounted while at the same time a counterbalancing slide is moved radially in the opposite direction to maintain said tool in dynamic balance. An encoder housed within said body member senses the location of said eddy current probe with respect to the axis of rotation of said body member. A rotary transformer is used to transfer signals which control the position of the probe and to receive information from the eddy current probe and the encoder. The encoder output signals are modulated by the eddy current probe exciter signal to enable probe position information to be sent through a rotary transformer.

Abstract: An all digital eddy current measurement system is described and illustrated in which an eddy current probe is driven by a driving signal and voltage signals representing the current through and voltage across the probe coil are used to calculate the magnitude and phase angle of a complex probe impedance. Digitization of the voltage signals is controlled by a control logic system which is run separately from but initiated by a microprocessor, the latter of which functions to analyze the acquired data and calculate impedance magnitude and phase angle values therefrom.

Abstract: A measuring device for contactless control of a design of structural elements composed of an electrically conductive material comprises at least one coil through which an alternating current flows and whose measuring voltage is influenced by a structural element, and electronic evaluating unit arranged to compare the measuring voltage of the coil influenced by the structural element with a nominal value, the coil being formed as a cylindrical coil.

Abstract: A device for the direct visualization of surface and near surface cracks, voids, flows, discontinuities, etc. in a target material. A magnetic garnet epitaxial film is deposited on either side, or both sides, of a non-magnetic substrate. A reflective material is provided adjacent to the epitaxial film, and the substrate with its associated layers is placed over a sheet of current carrying material and this sheet is placed over the target material. A bias magnetic field is then applied to the substrate together with its epitaxial film. Polarized light is transmitted onto the substrate with its associated layers and is reflected through the epitaxial layer and back out of the substrate. The existing magnetization, within the epitaxial film, interacts with nearby magnetic fields associated with eddy currents flowing adjacent to flaws in the target material, such that the domain structure of the epitaxial film is altered.

Abstract: A measuring device for contactless determination of condition of screw shaped structural elements comprises two oppositely located coils supplied with an alternating current spaced from one another so that an electrically conductive structural element passes therebetween, and transporting elements formed so that the structural element is guided on an edge so that a distance between the structural element and at least one of the coils remains constant during a measuring process.

Abstract: A method of nondestructively inspecting a magnetic substance such as metal pipe and bar stocks, wherein the method provides a detection coil having a diameter which is specified based on the length of existent peripheral surface flaws such that the directional sensitivity of the detection coil relative to those flaws is improved. The magnetic substance is moved along an axis thereof and has an eddy current induced therein. Relative rotation of the detection coil around the magnetic substance is effected to permit detection of surface flaws.

Abstract: Several phenomena which occur commonly in graphite composites, as well as other materials, and which affect the impedance of an eddy-current probe have been characterized on the complex impedance plane. An eddy-current method is disclosed by which a target material is stimulated at several frequencies, the impedance of the probe is measured and compared to the impedance of reference material, and the condition of the target material is determined. The method described is capable of finding the signatures of a number of conditions commonly found by eddy-currents, and can be programmed to find others, such as conductive and non-conductive plating thicknesses, and inclusions of material with relative ferrous materials. Because the method determines faults on the normalized impedance plane, it is capable of being used on other conductive materials in addition to graphite composites.

Abstract: A compound eddy current probe comprising a nest of concentric ferrite cup re probes. Both the inner and outer radii of the activated coil can be selectively altered by operator or automatic program to correspond with the size and thickness of the material under test.

Abstract: Method, according to the eddy current method, for checking damage on the surface of round, e.g., ball-shaped, metal objects, whereby a similar object is supported and made to rotate around at least one axis while an electronic contact-free scanner or sensor is aimed at the surface of the object, characterized by the fact that the round object is placed on a fixed support and a gaseous medium, e.g., air is blown against the object in such manner that between the support and the object a gas or air layer is formed which carries the object, and the object is made to rotate by the gas or air flow.

Abstract: A phase-difference eddy current imaging system has a bridge circuit including a pair of coils with one or both coils disposed proximate an object that may contain one or more flaws. An oscillator is coupled to the bridge circuit and also to a pair of phase detectors that are coupled to the coils. A differential amplifier is coupled to the phase detectors for providing a comparison signal. A method for eddy current imaging using phase difference detection comprises providing relative movement between an object and at least one of a pair of coils, measuring the phase of the effective reactance in each of the coils, comparing the measured values, and formatting the compared values as a function of spatial position.

Abstract: The present invention is an inspection instrument of the type which reacts to induced eddy currents in a workpiece to be inspected and includes a probe having at least a first probe coil. The instrument further includes a bridge circuit having the first probe coil connected in a first bridge arm. The bridge circuit also has input and output terminals. Connecting cable connects the instrument to a remote signal interpretation unit, and individual buffers connected between each of the output terminals and the cable isolate the capacitive of the cable from the bridge circuit.

Abstract: An eddy current testing device for inspecting test objects, for example billets, with respect to surface defects, for example surface cracks, comprises at least one transducer/sensor, for example a surface transducer or surface transducer arrangement, which is made to scan the surface of the test object. The device is characterized in that the effect of harmless surface blemishes on the testing device is suppressed, at least partially, by compensating for the sensitivity characteristic, in the scan direction of the transducer/sensor, with respect to the surface defect.

Abstract: An eddy current flaw detecting apparatus comprises a pair of detection coils each connected in adjoining arms of flaw detecting bridge circuit, the planes containing the pair of coils are arranged in parallel and inclined to the axis of a fuel clad pipe being examined at an inclination angle between 75.degree. and 85.degree. and an arrangement for rotating the fuel clad pipe relative to the pair of detection coils, when a flaw is detected while moving the fuel clad pipe relative to the pair of detection coils, to obtain a characteristic curve between a flaw signal output and turning angle in order to determine direction of the flaw, such as circumferential and axial, in comparison with characteristic curves between a flaw signal output and turning angle obtained in advance with respect to standard flaw testing pieces.

Abstract: A transducer for object examination comprises a conductive member of generally disc-shape having an opening of generally triangular configuration extending from the periphery of the member toward its center with opposed sidewalls of the opening in nonparallel relation to one another. A winding is applied to the perimeter of the conductive member and spans the opening. In certain instances, the transducer winding defines successive courses in each turn thereof which are of respective different diameter.

Abstract: A device for uncovering imperfections in electrical conducting wire, particularly superconducting wire, by detecting variations in eddy currents. Eddy currents effect the magnetic field in a gap of an inductor, contained in a modified commercial ferrite core, through which the wire being tested is passed. A small increase or decrease in the amount of conductive material, such as copper, in a fixed cross section of wire will unbalance a bridge used to measure the impedance of the inductor, tripping a detector and sounding an alarm.

Abstract: An improved eddy current probe system and method for simultaneously detecting different types of flaws at different depths within a metallic wall, such as a section of Inconel tubing, is disclosed herein. The system comprises a current generator for generating alternating currents of substantially different frequencies, a probe head including first, second and third concentrically arranged coils in separate communication with the current generator, shielding material disposed between the coils for preventing cross talk between each coil and the pulsating magnetic field of the coils adjacent to it, and a detector circuit which may include an inductive bridge for providing an electrical output representative of the impedance changes in the respective coils. In operation, each of the coils conduct currents having substantially different frequencies, the highest frequency being conducted by the smallest-diametered coil and the lowest frequency being conducted by the largest-diametered coil.

Abstract: A system is disclosed for nondestructive object testing comprising a magnetic probe responsive to an excitation signal for generating a magnetic field and thereby inducing eddy-currents in the object. The probe further provides an output signal indicative of the presence of a flaw in the object. A distance sensor generates an output signal indicative of the spacing of the probe from the object. Circuitry is provided for receiving the distance sensor output signal and is responsive thereto to supply such excitation signal to the probe. The circuitry employs the distance indication in the distance sensor output signal as an exponent in generating the excitation signal.

Abstract: A system for verified detection of flaws in an object, comprises a synchronizing sensor for generating an output signal once per mutual rotational revolution as between itself and the object, a flaw sensor for generating output signals responsive to detection of flaws in the object and extraneous influences, a switch unit responsive to operator input for providing output signals respectively indicative of a preselected number of flaws for such flaw verification and of a preselected number of object revolutions for such flaw verification, and a processor responsible to the output signals of the sync sensor, the flaw sensor and the switch unit for providing verified flaw output indication.

Abstract: A method and an apparatus are provided for detecting magnetic discontinuities, for example corrosion pits, in a specimen of magnetizable material. A horseshoe shaped magnet (32) is supported with its poles just clear of a surface of the specimen (12) by a wheeled frame (20), and is moved over the surface. An array of sensors (48) between the poles detects the leakage flux near the surface. The signal from any one sensor at a particular location is corrected by subtracting from it the average of the signals from the same sensor at a plurality of other locations preceding and following that location. If the corrected signal exceeds a threshold value, this indicates abnormally large localized leakage flux, and so indicates metal loss from the specimen.