Method and system for the generation of torsional guided waves using a ferromagnetic strip sensor

A method is shown for magnetostrictive testing of structures using a magnetostrictive strip and magnetostrictive techniques. A torsional guided wave is generated in the magnetostrictive strip which is dry coupled or bonded to the structure. A magnetic field creates an induced bias along the longitudinal axis of the magnetostrictive strip, which magnetic field is sufficiently strong so that when an AC pulse applies an AC bias perpendicular to the induced bias, the AC bias does not override or saturate the induced bias. This allows for an increased signal to noise ratio in a reflected signal indicating a defect in the structure. The induce bias and the AC bias are created by respective windings around the magnetostrictive strip, which windings are perpendicular to each other.

1. BACKGROUND OF THE INVENTION

This invention relates to a method and apparatus for the generation of torsional guided waves in a ferromagnetic strip sensor, more particularly, where the bias magnetic field is not overwridden by the alternating magnetic field pulse.

This is an improvement over U.S. patent application Ser. No. 11/970,378 filed on Jan. 7, 2008 having the same inventor and assignee, which application is hereby incorporated by reference.

2. BRIEF DESCRIPTION OF THE PRIOR ART

As computer technology has advanced, so has the technology related to sensors. Sensors provide the input for the computers to use in signal processing. Just as computers are used in every aspect of our daily life, computers are also used in evaluating structures to determine if the structures are safe, or if the structures have defects that may impair their safety. One of the problems in evaluating structures using sensors and computers is that not all parts of the structure are available for inspection to determine defects. For example, a bridge may have cables or beams that are inaccessible. Another example may be a containment vessel at a nuclear power plant where major portions of the vessel are buried underground and inaccessible. A further example may be pipes buried underground that are inaccessible.

While many other methods of inspection of inaccessible structures has been developed, one method that is still being developed is the use of magnetostrictive sensor technology to locate defects in inaccessible structures. A good article giving the current state of the art of magnetostrictive sensor technology is “Overview of Magnetostrictive Sensor Technology,”Journal of Intelligent Material Systems and Structure, Vol. 18, October 2007, by Frederick T. Calkins, Alison B. Flatau and Marclo J. Daampino. In a nutshell, magnetostrictive materials convert magnetic energy to mechanical energy and visa-versa. If a magnetostrictive material is magnetized, it strains in the direction of magnetization. If an external force produces a strain on the magnetostrictive material, the magnetic state of the magnetostrictive material will change.

A good explanation of the physical changes in the magnetostrictive material in response to rapid magnetization or demagnetization is contained in U.S. Pat. No. 6,917,196 to Kwun, et al, more particularly, the explanation in conjunction with FIGS. 13A through 13C contained therein. Examples of good magnetostrictive materials are as follows:

Cobalt

Nickel

Magnetostrictive materials were actually discovered in the 1840's by James Prescott Joule when he noticed that iron changed length in responses to changes in magnetism and named the phenomena the Joule effect. What occurs is the magnetic domains within the material align with the magnetic field causing the expansion. Similarly, when a magnetostrictive material is strained (stretched or compressed), its magnetic energy changes. This is the opposite of magnetostrictive action and the phenomena is called the Villari effect.

Some of the advantages of using a sensor made of magnetostrictive materials is the sensor can operate at higher temperatures than other types of sensors. Also the magnetostrictive materials can undergo higher strains with lower input voltages than other types of sensors.

Conventional magnetostrictive sensor techniques for generation of torsional guided waves use Wiedmann effect and require both permanent and alternating magnetic fields. The permanent and alternating magnetic fields may be applied directly to the specimen, or to a magnetostrictive strip with enhanced magnetostriction, which magnetostrictive strip is bonded or dry coupled to the specimen. The orientation of the permanent magnetic field determines the orientation of the permanent bias field, which in turn determines the initial orientation of the magnetic domains in the structure and/or strip.

The orientation of the alternating magnetic field is different from the permanent bias field. To elicit movement of the magnetic domain in the conventional manner, an AC current establishes a variable AC magnetic bias. The angle between the variable AC bias and the permanent magnetic field bias is 90°. This provides for magnetic domains to be aligned in such a direction that the torsional guided wave will be propagated in the direction perpendicular to the magnetic field bias. In the traditional way of generating a torsional guided wave, the orientation of the AC coil is coincident with the orientation of the magnetostrictive strip, and perpendicular to the direction of torisonal guided wave propagation.

The major disadvantage of the method described hereinabove is with establishing a permanent magnetic bias field where the magnetostrictive strip has significant length. In a majority of the cases, only residual magnetism of the magnetostrictive strip can be used for initial orientation of magnetic domains in the direction of the permanent magnetic field bias. Due to interference from the AC magnetic field with the residual magnetic fields, the strength of the residual fields tend to decrease, which reduces the signal-to-noise ratio of the whole system.

Another major disadvantage of the method described hereinabove is that the AC magnetic field overrides or saturates the permanent bias field. If the AC magnetic field overrides the permanent bias field, the full signal strength of the AC magnetic bias cannot be used in creating the AC magnetic field. To prevent the permanent bias field from being saturated and overridden, something in addition needs to be provided to maintain the permanent bias field. In the present invention, a DC voltage source is used to establish and maintain the permanent bias field to prevent it from being overridden by the AC magnetic bias. Also, an AC voltage source can be used if it has (1) a slower frequency from the AC magnetic bias, (2) a phase shift from the AC magnetic bias, or (3) been rectified.

OBJECTS OF THE INVENTION

It is an object of the present invention to provide a method and apparatus for the generation of torsional guided waves using a ferromagnetic strip sensor.

It is another object of the present invention to use magnetostrictive techniques that has a bias magnetic field of sufficient strength not to be overridden by an alternating magnetic field pulse when generating torsional guided waves.

It is another object of the present invention to generate torsional guided waves in a magnetostrictive strip that is bonded or dry coupled to a structure to create torsional guided waves in the structure.

It is yet anther object of the present invention to use a voltage to create a magnetic field bias in a magnetostrictive strip with the bias direction being along the longitudinal axis and, thereafter, to apply an alternating magnetic field pulse perpendicular to the magnetic field bias to create a torsional magnetic wave in the magnetostrictive strip.

It still another object of the present to use a single voltage source to create both a magnetic field bias along a longitudinal axis of a magnetostrictive strip and an alternating magnetic field pulse perpendicular thereto create a torsional guided wave in the magnetostrictive strip.

In the present invention, a magnetic field bias is created in a magnetostrictive strip by a voltage being applied to a coil wound around a short axis of the magnetostrictive strip. By applying a voltage to the short axis coil, a strong magnetic field bias is created.

In a second coil wound around the longitudinal axis of the magnetostrictive strip, a pulse of AC current is applied. This pulse of AC current generates an alternating magnetic field pulse in the magnetostrictive strip from the alternating current, which alternating magnetic field pulse is parallel to the short axis and perpendicular to the longitudinal axis. The applying of the alternating magnetic field pulse to the bias magnetic field generates a torsional guided wave within the magnetostrictive strip. The direction of propagation of the torsional guided wave is along the short axis of the magnetostrictive strip and perpendicular to the bias magnetic field.

The torsional guided wave is transferred from the magnetostrictive strip by either dry coupling or bonding to a structure under test. If there are defects in the structure, the torsional guided wave will be reflected off the defects. The reflected waves will be received by the magnetostrictive strip, sensed, and processed to determine size and location of the defects. The voltage being applied to generating the magnetic field bias is of sufficient strength to prevent the alternating magnetic field pulse from overriding the magnetic field bias.

While the magnetic field bias can be created by a DC voltage, the magnetic field bias can also be created by (1) a phase shifted alternating voltage, (2) a rectified AC voltage or (3) an AC voltage of a much slower frequency. In this manner, it is possible to use one power supply to provide both the magnetic field bias and the alternating magnetic field pulse.

One of the problems that has occurred in the past is that the alternating magnetic field pulse will override or saturate the magnetic field bias. If that is the case, then only a portion of the signal strength of the alternating magnetic field pulse, and hence the torsional guided wave, can be used. By providing a magnetic field bias that is not easily saturated or overridden, a much stronger strength signal can be obtained.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now toFIG. 1of the drawings, a pipe10is almost encircled by a magnetostrictive strip12except for a small gap14between the ends thereon. Wrapped around the short axis of the magnetostrictive strip12is a DC coil16. The DC coil16receives a regulated DC voltage18from a voltage source20.

Wrapped longitudinally around the magnetostrictive strip12is an AC coil22. The AC coil22is connected to the transmitter/receiver24. The transmitter/receiver24is connected to a pulse generator26that receives energy from the voltage source20.

The magnetostrictive strip12may be pressed against the pipe10if the pipe10is ferromagnetic. Otherwise, if the pipe is not ferromagnetic, the magnetostrictive strip12may be bonded or dry coupled to the pipe10.

In actual practice the voltage source20will supply power to a regulated DC voltage18which will then apply a DC voltage to the DC coil16wrapped around the short axis of the magnetostrictive strip12. The DC voltage will create an induced DC bias in the direction indicated by the arrow on the magnetostrictive strip12. After the induced DC bias is created in the magnetostrictive strip12, a pulse generator26driven by the voltage source20will deliver an AC pulse through the transmitter/receiver24to the AC coil22wound around the longitudinal axis of the magnetostrictive strip12. The AC pulse delivered to the AC coil22will create a BAC(an alternating bias) in the directions indicated by the arrows inFIG. 1. The pulse of alternating current being delivered to the AC coil22creates the BAC, which causes a torsional guided wave in the magnetostrictive strip12. Because of the bonding or the dry coupling of the magnetostrictive strip12to the pipe10, the direction of propagation of the torsional guided wave is in the direction indicated by the arrow shown in pipe10. If the pipe10is ferromagnetic, bonding or dry coupling may not be required.

If the torsional guided wave traveling along the pipe10hits a defect28in the pipe10, the torsional guided wave that hits the defect28will be reflected back to the magnetostrictive strip12. The reflected torsional guided wave from defect28is sensed in the magnetostrictive strip12by what is called the Villari Effect. The Villari Effect causes a voltage to be realized in the AC coil22and sensed by the transmitter/receiver24. The sensed signal is sent to a signal conditioner30. After feeding the sensed signal through the signal conditioner30, a signal processor32processes the signal and the signal analysis34analyzes the signal. The final output is recorded in some type of signal recorder36.

Referring toFIG. 2, two sets of data are illustrated. Set2at the bottom graph inFIG. 2is the data that would be obtained using the system as shown inFIG. 1. A weld signal is shown as a result of welding two pipes together at weld36shown inFIG. 1. The 20% defect signal is the defect28in pipe10. Other harmonics of the reflected signals may be received such as the first multiple of the weld signal or a harmonic of the defect signal. To get the type of signals as shown in Set2, the regulated DC voltage18must continue to supply to the DC coil16a continuous induced DC bias. In other words, the pulse created by the pulse generator26that generates the alternating bias BACcannot override or saturate the induced DC bias. This is prevented by providing a continually induced DC bias.

If a traditional method is used of creating the DC bias such as an elongated magnet, with all other things remaining the same, a set of data similar to Set1would be generated. As can be seen from the data, Set1to Set2is an eight to ten times stronger in signal strength. The reason for the stronger signal is because the regulated DC voltage18is continuously applied to the DC coil16which keeps an induced DC bias. This prevents the AC pulse from the pulse generator26through transmitter/receiver24that is delivered through the AC coil22and generates bias BACfrom overriding or saturating the DC bias.

InFIG. 3an alternative embodiment of the present invention is shown. The pipe10with the weld36and defect28is again checked using a magnetostrictive strip12. Again, a pulse generator26receives voltage from voltage source20to transmit a pulse through the transmitter/receiver24to the AC coil40. However, rather than having a DC bias, as was explained in conjunction withFIG. 1, an AC generator38creates an induced AC bias as is indicated by the arrows. For an induced AC bias to work, it must be of a different frequency or phase shifted from the AC pulse generated by the pulse generator26and delivered via transmitter/receiver24to the AC coil22.

FIG. 5shows one way the two AC voltages can be utilized, one from the AC generator38and another from the pulse generator26. InFIG. 5the AC pulse from the pulse generator26is indicated AC1. AC1generates pulse bias BACin the magnetostrictive strip12. The induced AC bias from the AC generator38through the AC coil40is represented by the signal AC2shown inFIG. 5. As can be seen inFIG. 5, a significant induced AC bias (AC2) is generated in the magnetostrictive strip at the time the pulse AC1is delivered to AC coil22. However, AC2has a different frequency from AC1,

Using the system as shown inFIG. 3, the voltage source20is turned on. AC generator38continuously generates the bias signal AC2to create an induced AC bias through AC coil40. Whenever a measurement is to be taken, pulse generator26generates a pulse signal AC1which causes bias BACin AC coil22. This results in a torsional guided wave being released in the direction shown inFIG. 3. The torsional guided waves will reflect off of weld36or defect28. The reflected signal will be received back at the magnetostrictive strip12and sensed in AC coil22by transmitter/receiver24. Again, the reflected signal will be sent to a signal conditioner30, signal processors32and signal analysis34. The results of the signal analysis34can be read directly or sent to a signal recorder36. The signal received would be similar to the signal received in Set2of the data ofFIG. 2.

In another embodiment of the invention, the system as shown inFIG. 3could generate a pulse signal AC1as shown inFIG. 4. However, the signal generated by the AC generator38to be delivered to the AC coil40would be phase shifted 90° as shown by the signal AC2ofFIG. 4. In this manner, an induced AC bias would always be present at the time the pulse generates a bias BACin the magnetostrictive strip12.

A third type of wave form could be utilized in the embodiment shown inFIG. 3. The signal from the pulse generator26through the transmitter/receiver24delivered to the AC coil22could be AC1as shown inFIG. 6. AC1as shown inFIG. 6is basically the same as AC1shown inFIGS. 4 and 5. However, the signal being delivered by the AC generator38to the AC coil40would be AC2as shown inFIG. 6. The signal AC2as shown inFIG. 6is essentially a rectified form of the signal AC1. By full wave rectification of the AC1 signal, the induced AC bias is generated in the magnetostrictive strip12through AC coil40. Again, the wave forms as shown inFIG. 6would work with the system as shown inFIG. 3.

Referring toFIG. 7, a different configuration of the present invention is shown. The magnetostrictive strip12is the same with the small gap14. Again, AC/DC coil42is wound about the short axis of the magnetostrictive strip12. The voltage generator20through an AC/DC generator44will generate either an AC or DC bias in the magnetostrictive strip12.

However, the AC coil46is wound around the longitudinal axis of the magnetostrictive strip12prior to it being applied to the pipe10. Particularly note, the AC coil46is wound around the longitudinal axis in the magnetostrictive strip12prior to bending around pipe10. Thereafter, the magnetostrictive strip12is bent in a circular manner to encircle pipe10with a small gap14between the ends thereof. The magnetostrictive strip12may be bounded or dry coupled to the pipe10.

The pulse generator26will receive power from the voltage source20. Whenever a measurement is to be taken, the pulse generator26will generate a pulse signal that is transmitted through transmitter/receiver24to the AC coil46. This causes a torsional guided wave in the direction as indicated inFIG. 7. When a reflected signal is received off defect28or weld36, the signal being received will be similar to Set2of the data shown inFIG. 2. The reflected signals from weld36or defect28are felt in the magnetostrictive strip12via AC coil46. The signal in AC coil46is detected by transmitter/receiver24and conditioned in signal conditioner30. From signal conditioner30, the signal is processed by signal processor32and analyzed by signal analysis34. Thereafter, the signal can be read directly or sent to signal recorder36.

In the present invention, it is very important that the induced bias, whether it is AC or DC, not be overrun or saturated by the bias BACgenerated by the pulse generator26. By the providing of a continual induced bias signal, the saturation is prevented. Therefore, when a reflected signal is received back, there is an induced bias (whether AC or DC) present in the magnetostrictive strip so that the reflected signal can be sensed by the AC coil22or46. By preventing the saturation of the induced bias, the reflected signal is eight to twelve times larger than it would otherwise be. The signal to noise ration is much greater which allows defects in a structure being monitored or tested to be determined much more readily.