Abstract:
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.

Description:
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 
   Iron 
   Nickel 
   Ferrite 
   Terfenol-D 
   Metglass 
   Magnetostrictive materials were actually discovered in the 1840&#39;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. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a system diagram of the present invention with a torsional guided waves being shown in pictorial view, which torsional guided waves are being generated in a magnetostrictive strip that is dry coupled or bonded to a pipe wherein the alternating magnetic field pulse will not override the magnetic field bias. 
       FIG. 2  shows two sets of data on the same pipe with Set  1  using traditional torsional guided waves and Set  2  using the present invention for an increased signal to noise ratio. 
       FIG. 3  is a system diagram of the present invention with torsional guided waves being shown in pictorial view, the magnetic field bias being generated by an AC generator. 
       FIG. 4  is one example of wave forms that may be applied to the system shown in  FIG. 3 . 
       FIG. 5  is alternative example voltage wave forms that may be applied to the system shown in  FIG. 3 . 
       FIG. 6  is another alternative example voltage wave forms that may be applied to the system shown in  FIG. 3 . 
       FIG. 7  is a system diagram of the present invention with the torsional guided waves being shown in pictorial form, the magnetic field bias being created by either an AC or DC voltage. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to  FIG. 1  of the drawings, a pipe  10  is almost encircled by a magnetostrictive strip  12  except for a small gap  14  between the ends thereon. Wrapped around the short axis of the magnetostrictive strip  12  is a DC coil  16 . The DC coil  16  receives a regulated DC voltage  18  from a voltage source  20 . 
   Wrapped longitudinally around the magnetostrictive strip  12  is an AC coil  22 . The AC coil  22  is connected to the transmitter/receiver  24 . The transmitter/receiver  24  is connected to a pulse generator  26  that receives energy from the voltage source  20 . 
   The magnetostrictive strip  12  may be pressed against the pipe  10  if the pipe  10  is ferromagnetic. Otherwise, if the pipe is not ferromagnetic, the magnetostrictive strip  12  may be bonded or dry coupled to the pipe  10 . 
   In actual practice the voltage source  20  will supply power to a regulated DC voltage  18  which will then apply a DC voltage to the DC coil  16  wrapped around the short axis of the magnetostrictive strip  12 . The DC voltage will create an induced DC bias in the direction indicated by the arrow on the magnetostrictive strip  12 . After the induced DC bias is created in the magnetostrictive strip  12 , a pulse generator  26  driven by the voltage source  20  will deliver an AC pulse through the transmitter/receiver  24  to the AC coil  22  wound around the longitudinal axis of the magnetostrictive strip  12 . The AC pulse delivered to the AC coil  22  will create a B AC  (an alternating bias) in the directions indicated by the arrows in  FIG. 1 . The pulse of alternating current being delivered to the AC coil  22  creates the B AC , which causes a torsional guided wave in the magnetostrictive strip  12 . Because of the bonding or the dry coupling of the magnetostrictive strip  12  to the pipe  10 , the direction of propagation of the torsional guided wave is in the direction indicated by the arrow shown in pipe  10 . If the pipe  10  is ferromagnetic, bonding or dry coupling may not be required. 
   If the torsional guided wave traveling along the pipe  10  hits a defect  28  in the pipe  10 , the torsional guided wave that hits the defect  28  will be reflected back to the magnetostrictive strip  12 . The reflected torsional guided wave from defect  28  is sensed in the magnetostrictive strip  12  by what is called the Villari Effect. The Villari Effect causes a voltage to be realized in the AC coil  22  and sensed by the transmitter/receiver  24 . The sensed signal is sent to a signal conditioner  30 . After feeding the sensed signal through the signal conditioner  30 , a signal processor  32  processes the signal and the signal analysis  34  analyzes the signal. The final output is recorded in some type of signal recorder  36 . 
   Referring to  FIG. 2 , two sets of data are illustrated. Set  2  at the bottom graph in  FIG. 2  is the data that would be obtained using the system as shown in  FIG. 1 . A weld signal is shown as a result of welding two pipes together at weld  36  shown in  FIG. 1 . The 20% defect signal is the defect  28  in pipe  10 . 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 Set  2 , the regulated DC voltage  18  must continue to supply to the DC coil  16  a continuous induced DC bias. In other words, the pulse created by the pulse generator  26  that generates the alternating bias B AC  cannot 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 Set  1  would be generated. As can be seen from the data, Set  1  to Set  2  is an eight to ten times stronger in signal strength. The reason for the stronger signal is because the regulated DC voltage  18  is continuously applied to the DC coil  16  which keeps an induced DC bias. This prevents the AC pulse from the pulse generator  26  through transmitter/receiver  24  that is delivered through the AC coil  22  and generates bias B AC  from overriding or saturating the DC bias. 
   In  FIG. 3  an alternative embodiment of the present invention is shown. The pipe  10  with the weld  36  and defect  28  is again checked using a magnetostrictive strip  12 . Again, a pulse generator  26  receives voltage from voltage source  20  to transmit a pulse through the transmitter/receiver  24  to the AC coil  40 . However, rather than having a DC bias, as was explained in conjunction with  FIG. 1 , an AC generator  38  creates 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 generator  26  and delivered via transmitter/receiver  24  to the AC coil  22 . 
     FIG. 5  shows one way the two AC voltages can be utilized, one from the AC generator  38  and another from the pulse generator  26 . In  FIG. 5  the AC pulse from the pulse generator  26  is indicated AC 1 . AC 1  generates pulse bias B AC  in the magnetostrictive strip  12 . The induced AC bias from the AC generator  38  through the AC coil  40  is represented by the signal AC 2  shown in  FIG. 5 . As can be seen in  FIG. 5 , a significant induced AC bias (AC 2 ) is generated in the magnetostrictive strip at the time the pulse AC 1  is delivered to AC coil  22 . However, AC 2  has a different frequency from AC 1 , 
   Using the system as shown in  FIG. 3 , the voltage source  20  is turned on. AC generator  38  continuously generates the bias signal AC 2  to create an induced AC bias through AC coil  40 . Whenever a measurement is to be taken, pulse generator  26  generates a pulse signal AC 1  which causes bias B AC  in AC coil  22 . This results in a torsional guided wave being released in the direction shown in  FIG. 3 . The torsional guided waves will reflect off of weld  36  or defect  28 . The reflected signal will be received back at the magnetostrictive strip  12  and sensed in AC coil  22  by transmitter/receiver  24 . Again, the reflected signal will be sent to a signal conditioner  30 , signal processors  32  and signal analysis  34 . The results of the signal analysis  34  can be read directly or sent to a signal recorder  36 . The signal received would be similar to the signal received in Set  2  of the data of  FIG. 2 . 
   In another embodiment of the invention, the system as shown in  FIG. 3  could generate a pulse signal AC 1  as shown in  FIG. 4 . However, the signal generated by the AC generator  38  to be delivered to the AC coil  40  would be phase shifted 90° as shown by the signal AC 2  of  FIG. 4 . In this manner, an induced AC bias would always be present at the time the pulse generates a bias B AC  in the magnetostrictive strip  12 . 
   A third type of wave form could be utilized in the embodiment shown in  FIG. 3 . The signal from the pulse generator  26  through the transmitter/receiver  24  delivered to the AC coil  22  could be AC 1  as shown in  FIG. 6 . AC 1  as shown in  FIG. 6  is basically the same as AC 1  shown in  FIGS. 4 and 5 . However, the signal being delivered by the AC generator  38  to the AC coil  40  would be AC 2  as shown in  FIG. 6 . The signal AC 2  as shown in  FIG. 6  is essentially a rectified form of the signal AC 1 . By full wave rectification of the AC1 signal, the induced AC bias is generated in the magnetostrictive strip  12  through AC coil  40 . Again, the wave forms as shown in  FIG. 6  would work with the system as shown in  FIG. 3 . 
   Referring to  FIG. 7 , a different configuration of the present invention is shown. The magnetostrictive strip  12  is the same with the small gap  14 . Again, AC/DC coil  42  is wound about the short axis of the magnetostrictive strip  12 . The voltage generator  20  through an AC/DC generator  44  will generate either an AC or DC bias in the magnetostrictive strip  12 . 
   However, the AC coil  46  is wound around the longitudinal axis of the magnetostrictive strip  12  prior to it being applied to the pipe  10 . Particularly note, the AC coil  46  is wound around the longitudinal axis in the magnetostrictive strip  12  prior to bending around pipe  10 . Thereafter, the magnetostrictive strip  12  is bent in a circular manner to encircle pipe  10  with a small gap  14  between the ends thereof. The magnetostrictive strip  12  may be bounded or dry coupled to the pipe  10 . 
   The pulse generator  26  will receive power from the voltage source  20 . Whenever a measurement is to be taken, the pulse generator  26  will generate a pulse signal that is transmitted through transmitter/receiver  24  to the AC coil  46 . This causes a torsional guided wave in the direction as indicated in  FIG. 7 . When a reflected signal is received off defect  28  or weld  36 , the signal being received will be similar to Set  2  of the data shown in  FIG. 2 . The reflected signals from weld  36  or defect  28  are felt in the magnetostrictive strip  12  via AC coil  46 . The signal in AC coil  46  is detected by transmitter/receiver  24  and conditioned in signal conditioner  30 . From signal conditioner  30 , the signal is processed by signal processor  32  and analyzed by signal analysis  34 . Thereafter, the signal can be read directly or sent to signal recorder  36 . 
   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 B AC  generated by the pulse generator  26 . 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 coil  22  or  46 . 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.