Patent Description:
Non-destructive evaluation (NDE) is widely used to inspect objects, such as ships, offshore structures, renewable energy devices, e.g. windmills, bridges, buildings, cranes, railway, cars, machinery, and pipelines, for defects and damage due to corrosion and wear. Defects include cracks, inclusions, lack of fusion, incomplete penetration and porosities. One of the NDE methods is based on the so-called magnetic flux leakage (MFL) in the vicinity of a defect.

A well-known method that is based on MFL is magnetic particle inspection (MPI) for the detection of defects. This method, however, needs the presence and subjective judgment of a surveyor. Therefore, it is preferred to carry out MFL measurements using magnetometers, such as those based on the Hall effect.

As explained in <NPL>. ", regular inspection is needed in order to confirm adequate structural integrity throughout the entire service life of a structure. Detected fatigue cracks that are too long for safe operation need to be repaired. Detected cracks of acceptable length need to be at least inspected more frequently. These inspections are costly, time consuming, and hazardous, so additional inspections on top of the periodical regulatory surveys are to be avoided if possible.

In Van der Horst, it is mentioned that "The envisaged crack monitoring system would consist of arrays of Hall effect sensors that are placed on detected cracks, so that their propagation can be monitored without additional inspections. The Hall effect sensors need to be placed at an optimal altitude above the metal surface, balancing between the sensor sensitivity and accompanied costs, and the spatial density of sensors required for accurate crack length characterization. In principle, a crack can be monitored as long as it is underneath the sensor array. However, crack length dependency of the SMFL creates opportunities to estimate the crack length even when it has propagated outside the sensor array.

It is an object of the present invention to provide an improved method of detection and/or growth monitoring of defects.

To this end, the method according to the invention is characterized as defined in claim <NUM>.

The reading of values, e.g. a current or most recent reading, typically contains little or insufficient information at least in that it does not provide an accurate location of a singular point, such as the tip of a crack, at which the defect propagates or forms. It was found that processing the values of MFL of a current reading with the values of MFL of an earlier reading, such as an initial or reference reading or the preceding reading, significantly improves the accuracy of the data relating to the location of a singular point, in particular the tip of the crack, in the current reading and in many cases even enables locating the singular point where this was impossible with existing methods.

Defects can be surface breaking defects, also known as surface defects or surface flaws, and/or non-surface breaking defects, also known as subsurface defects or embedded flaws. Examples of defects include cracks, plastic deformation (which often is an indication or a precursor of a crack), corrosion, and pitting. Suspected defects include indications that a defect is present and include likely or expected defects, such as critical locations in structures that could contain a defect or where a defect is likely to form or originate.

To enable a more accurate distinction between global effects, such as changes in temperature, expansion of materials, and variations in magnetic (background) field, on the one hand and local (smaller) changes, such as defect growth, on the other, it is preferred that the system comprises at least <NUM> magnetometers, preferably at least <NUM> magnetometers, preferably at least <NUM> magnetometers, preferably at least <NUM> magnetometers, and/or that the distance between most, preferably all, of the magnetometers is equal.

The frequency of the reading of values depends on the circumstances, e.g. the expected rate of propagation or the nature of the structure, i.e. a higher expected rate and more critical structures could necessitate more frequent readings. In general, more frequent readings, e.g. one or more readings per day or per hour, generate more data and are generally preferred. As batteries improve, a higher frequencies will be simpler to implement. With low expected rates and/or non-critical structures readings can be less frequent, e.g. weekly, monthly, or even quarterly.

As mentioned, the magnetometers are placed over the (suspected) defect or near the (suspected) defect, e.g. less than <NUM>, preferably less than <NUM>, preferably less than <NUM>, e.g. within <NUM> from the (suspected) defect.

In an embodiment, the values comprise data of MFL in two or more directions, preferably at least in a direction along the defect (provided of course that the direction can be determined) and in a direction perpendicular to the surface of the ferromagnetic structure on which the magnetometers are placed.

In an embodiment, the method comprises the steps of comparing sets of values of MFL in different directions, selecting at least one set, and determining the location of a singular point of the (suspected) defect from the selected set.

If values are read in two or more, e.g. three directions, in case or surface breaking defects, such as cracks, preferably at least in a direction along the defect (Y-direction), perpendicular to the surface of the ferromagnetic structure (X-direction), and perpendicular to the other two (Z-direction), and processed, at least one signal, i.e. set of values read/measured with a plurality of magnetometers in a particular direction, e.g. along the defect, will typically provide a clearer change in sign, in particular from positive to negative, or direction (peak) than the other(s). This signal is then selected to determine the location of a singular point.

The combination of reading values in two or more, e.g. three directions, and arranging the magnetometers in a single dimension, such as a line, in particular a straight line, provides a relatively compact yet effective means for detecting and/or monitoring the forming and/or the growth of defects, in particular cracks or plastic deformation.

The values of magnetic flux leakage (MFL) are read passively, by reading stress induced magnetisation (also known as the magnetoelastic or Villari effect) and/or the Earth magnetic field, which is/are naturally available as a source or origin of magnetism.

In an embodiment, processing the values of the further reading with the values from the earlier reading involves at least one of calculating, preferably for individual magnetometers, preferably for each of the magnetometers individually, a (weighted) moving average of readings at different points in time, a derivative, a difference, a (weighted) mean squared error, a (weighted) least squares error, zero-crossing or similar noise filtering, and signal reduction processing steps, of such readings.

In a preferred example, values from the earlier reading are subtracted from the values of the further reading.

The invention can be carried out e.g. by a system for detecting and/or monitoring the growth of defects, in particular cracks, in ferromagnetic structures, such as civil engineering and marine structures, comprising a plurality of magnetometers and means, such as a controller, for carrying out the method described above.

To enable a more accurate distinction between global effects, such as changes in temperature, expansion of materials, and variations in magnetic (background) field, on the one hand and local (smaller) changes, such as defect growth, on the other, it is preferred that the system comprises at least <NUM> magnetometers, at least <NUM> magnetometers, preferably at least <NUM> magnetometers, preferably at least <NUM> magnetometers, and/or wherein the magnetometers are arranged in an array or in a line, in particular a straight line, or in parallel (straight) lines and/or wherein the distance between most, preferably all, of the magnetometers is equal.

In principle all existing and future magnetometers are suitable. Examples include magnetometers configured to measure magnetism based on Hall effect, on anisotropic magnetoresistance (AMR), or on fluxgate. It is preferred that the magnetometers measure magnetism in at least two directions, preferably in three directions.

The system can comprise a controller connected to the magnetometers and configured to turn the magnetometers on, i.e. activate the magnetometers, initiate the readings/measurements, and turn the magnetometers off, preferably periodically. The system can comprise a battery and/or an energy generating and/or harvesting device.

The controller can comprise a processor, a memory, e.g. an (E)EPROM on a printed circuit board, that is accessible via the controller.

The system can comprise a transmitter to send data read with the magnetometers to a receiver and wherein the transmitter preferably is based on the Global System for Mobile Communications (GSM), Long Range Low Power (LoRa), Wifi, and other Radio Frequency standards.

The magnetometers can be accommodated in a housing and/or embedded in a surrounding material, which housing and/or embedding is water and/or oil resistant or weather-, water- and/or oilproof. The controller and/or the transmitter can be accommodated in a separate housing, and the controller is connected to the magnetometers, wired or wirelessly.

The sensor can comprise an adhesive layer, e.g. in the form of adhesive tape or a coating, mechanical fasteners and/or one or more magnets to mount the sensor on a metal structure.

A computer program can comprise instructions to cause the system described above to execute the steps of the method described above and to a computer-readable medium having the computer program stored thereon.

<CIT> relates to a methodology and system for anomaly recognition and tracking in a variety of components, such as tubular components. The technique comprises sensing for anomalies in the component, and storing detected anomalies in a storage medium. The detected anomalies are matched with stored digital descriptions of anomalies. The stored digital descriptions may comprise anomalies of similar types and/or anomalies previously detected on the specific component being tested. The detected anomalies are ranked according to similarities with respect to the stored digital descriptions.

<CIT> concerns a method and a system to create and use pre-magnetized ferromagnetic structures, where magnetic field direction rotates with depth, and analyze the nature of defects in the structures.

<CIT> discloses a device for discovering, identification and monitoring, of mechanical flaws in metallic structures, based on magneto-graphic/magnetic tomography technique to identify stress-related defects.

<CIT> relates to a method for identifying and locating a defect in a metal rail, which method includes the steps of positioning a first magnetic sensor at a distance above a rail, the first magnetic sensor being configured to measure a magnetic field of the rail; advancing the sensor along a length of the rail; sampling magnetic field measurements; determining multiple magnetic field gradients over different pluralities of samples; identifying a defect in the rail based on a change in one or more of the magnetic field gradients; and determining a position of the defect at a particular distance from the magnetic sensor based on a degree of variation in the magnetic field gradients.

In <CIT> defects in ferromagnetic materials are detected and characterized by analyzing the items' magnetic fields to find portions of the magnetic fields that differ in characteristic ways from residual magnetic fields generated by non-defective portions of the items. The portions of the magnetic fields that differ in the characteristic ways correspond to locations of the defects. The residual magnetic fields correspond to portions of the items distant from the defects. The defect characterization may include volume of material lost due to each defect and/or width and/or depth of each defect.

<CIT> relates to a system and method using magnetic sensing to non-intrusively and non-destructively characterize ferromagnetic material within infrastructure. The system includes sensors for measuring magnetic field gradients from a standoff distance adjacent to ferromagnetic material. The method includes using the system to measure magnetic fields, determining magnetic field gradients measured by a sensor array, and comparing measured and modeled or historical magnetic field gradients at the same or similar positions to identify differences caused by a phenomenon in the ferromagnetic material, and, in a particular embodiment, to recognize defects and developing defects.

The invention will now be explained in more detail with reference to the Figures.

<FIG> schematically shows a system <NUM> for detecting and monitoring the growth of defects comprising a plurality of sensor strips <NUM> to be placed over or near a (suspected) defect and a control unit <NUM> for operating and communicating with the sensor strips. Each strip comprises a plurality of triaxial Hall effect magnetometers <NUM>, preferably at least <NUM>, preferably arranged in a line, in particular a straight line, and has an adhesive surface to attach the strip to a steel structure, such as a civil engineering and marine structure.

In this example, the control unit <NUM> comprises a battery pack <NUM>, a permanent magnet <NUM> to attach the control unit to the steel structure, a controller-board and processor <NUM>, such as a plc, a microcontroller, or a single board computer, connector ports <NUM> for cables <NUM> connecting the sensor strips to the control unit, and a radio frequency (RF) module <NUM> to communicate with a central computer (not shown).

The controller is configured to turn the magnetometers on, i.e. activate the magnetometers, read values of magnetic flux leakage (MFL) from the plurality of magnetometers in three directions, and turn the magnetometers off. The transmitter is configured to send data read with the magnetometers to a receiver connected to the central computer, e.g. via GSM, LoRa, Wii, or another Radio Frequency standard.

<FIG> shows a sensor <NUM> for carrying out MFL measurements, having <NUM> triaxial Hall effect magnetometers <NUM> arranged in a straight line at a consistent center to center separation of <NUM>. In <FIG>, the strip is adhered to a steel structure <NUM> alongside and essentially parallel to an existing surface breaking crack <NUM> having a clearly visible singular point, i. the tip <NUM> of the crack <NUM>.

The sensor in <FIG> was used to carry out a first reading of values of MFL in three directions, in this example.

In addition, the total field strength (scalar) at each magnetometer was calculated in the central computer from to the values measured in the three directions.

In this example, second and third readings were carried out one month later and two months later, respectively. The results of these readings are shown in <FIG>, where the first reading is marked "JUL", the second reading "AUG", and the third reading "SEP". In <FIG>, the locations of the actual tip of the crack at the times of the readings are marked with vertical lines.

The individual readings shown in <FIG> contain no or at least insufficient information to accurately located of the tip of the crack.

<FIG> shows values of magnetic flux leakage (MFL) from the second and third readings processed with the reading of the first and second readings, respectively. In this example, the values of the first reading were subtracted from the values of the second reading and the values of the second reading were subtracted from the values of the third reading.

The resulting curves of the processed values for MFL in the Y-direction, along the crack, change from a positive value to a negative value at sensors #<NUM> (AUG) and #<NUM> (SEP) and the resulting curves of the processed values for MFL in the X-direction each show a single, distinct peak at the same sensors, #<NUM> (AUG) and #<NUM> (SEP). The position of these sensors corresponded accurately to the actual location of the tip of the crack at the time of the second reading (AUG) and the third reading (SEP), respectively.

Benefits of defect detection and a defect monitoring system based on such measurements are e.g. that the risk level of measured objects can be assessed, that rational decision making on defect repair can be made, and that less frequent inspections can be made by surveyors what translates into less injuries/accidents, and/or significantly reduced costs.

Claim 1:
Method of detecting and/or monitoring the forming and/or the growth of cracks (<NUM>) or plastic deformation and/or suspected cracks (<NUM>) or plastic deformation in ferromagnetic structures (<NUM>), such as civil engineering and marine structures, comprising
passively reading values of magnetic flux leakage (MFL) by reading stress induced magnetisation and/or the Earth magnetic field from a plurality of magnetometers (<NUM>) arranged in a line and mounted on a structure (<NUM>) and over or near a crack (<NUM>) or plastic deformation and/or suspected crack (<NUM>) or plastic deformation, at a first point in time, and
storing the values of magnetic flux leakage (MFL),
passively reading values of magnetic flux leakage (MFL) by reading stress induced magnetisation and/or the Earth magnetic field from a plurality of magnetometers (<NUM>) at a further point in time,
processing the values of the further reading with the values from the earlier reading, and
determining the location of a singular point (<NUM>) of the crack (<NUM>) or plastic deformation and/or suspected crack (<NUM>) or plastic deformation from the processed further reading.