Back in Style: Magnetostrictive Sensors*
Mindful of the past, SwRI scientists find fitting solutions to some of today's high-tech problems
By Hegeon Kwun, Ph.D.
A magnetic field produces a small change in the physical dimensions of ferromagnetic materials -- on the order of several parts per million in carbon steel -- and, conversely, a physical deformation or strain (or stress which causes strain) produces a change of magnetization in the material. These phenomena were discovered in the 19th century. The former, known as the magnetostrictive effect, was first reported by Joule in 1847 and the latter, known as the inverse magnetostrictive effect, by Villari in 1864.[1,2]
Until the late 1920s, these phenomena found few practical uses and remained largely of theoretical interest. The first serious engineering application of magnetostrictive effects was the use of magnetostrictive bars to control high frequency oscillators and to produce ultrasonic waves in gases, liquids, and solids.[3,4] Other applications followed, including echo depth-recorders, transducers for sonars during World War II, high power ultrasonic processors, and ultrasonic delay lines for electronic information processing and storage.
In the early 1960s, however, developmental interest in magnetostrictive devices declined because of technical advances in piezoelectric devices, in particular through the production of versatile and efficient ceramic piezoelectric materials such as lead titanate zirconate.
Piezoelectric effects are similar to magnetostrictive effects but occur in dielectric materials, causing physical dimension changes due to electric fields and, conversely, generating electric charges due to strain or stress. For many engineering applications, such as sonars, ultrasonic transducers, electronic filters and resonators, delay lines, and accelerometers, devices based on piezoelectric effects have several key technical advantages over those based on magnetostrictive effects, including reduced size, greater efficiency, and ease of fabrication. As a result, piezoelectric devices have dominated the field for these applications. For almost three decades, research and development activities related to magnetostrictive effects were reduced to relative obscurity and its potential for other engineering applications largely forgotten.
In 1990, scientists in the Nondestructive Evaluation (NDE) Science and Technology Division at Southwest Research Institute (SwRI) began to consider the use of magnetostrictive effects for NDE applications in which existing technology was impractical. They were looking for an efficient means of inspecting steel cables or strands used in traditional suspension bridges and modern cable-stayed bridges, for corrosion damage such as broken wires in the cable. Since these cables carry the load of the bridge, ensuring their structural safety through regular inspection and maintenance is critical. This is particularly true for cable-stayed bridges, where cables are used as the primary load bearing members. If one stay cable should snap, it could trigger progressive collapse that would be devastating and without warning.
A stay cable typically consists of hundreds of individual strands and is grouted into a protective jacket. Because the wires comprising the cable are hidden and not directly accessible, inspection of the cables for corrosion damage is difficult and challenging. SwRI scientists believed that if a means could be developed to allow transmission and detection of elastic waves in individual wires from outside the cable's protective jacket, the inspection problem would be solved. The key was how to devise such a means.
In fact, a means of transmitting and detecting ultrasonic waves without direct physical contact and without a couplant such as water or oil has long been sought in the field of NDE, for engineering applications in which piezoelectric transducers, which require direct physical contact and a couplant, are either difficult or impossible to use, as in the case of bridge cable inspection. Other examples include the internal inspection of gas pipelines using devices such as mechanical 'pigs' that traverse hundreds of miles of pipe, where the use of couplant is logistically difficult or unacceptable; inspection of materials at high temperatures, such as mill products during fabrication or plant equipment under operation; and inspection of structures whose surfaces are not directly accessible, such as those with thick paint or coating and piping under thermal insulation.
To meet these inspection needs, sensors called electromagnetic acoustic transducers (EMAT), based on an electromagnetic force called the Lorentz force, were developed in the 1960s. Although EMATs have enjoyed some limited success, they are plagued with low efficiency and high sensitivity to lift-off variations. Also, because their efficiency rapidly decreases with increasing lift-off, their use is limited to a relatively narrow gap (typically about 0.04 inch) between the sensor and the material surface. They are therefore not the answer to cable inspection and similar problems where the space between the sensor and the surface may exceed one inch.
In search of a solution, SwRI scientists examined the early method used for magnetostrictive delay lines, in which an electrical input signal is converted to an elastic (ultrasonic) wave in a thin (on the order of 0.01 inch) ferromagnetic wire or rod, typically nickel. The elastic wave propagates along the wire and is then reconverted into an electrical output at the other end. The conversion of electrical to mechanical energy or vice versa is achieved by using an encircling coil and the magnetostrictive effects in the ferromagnetic wire. Because the elastic wave propagates at a much lower velocity than the electromagnetic wave, a time delay of the electrical signal of up to a few milli-seconds is readily obtained via the magnetostrictive delay line. This signal delay cannot be achieved using purely electrical methods.
Institute scientists recognized that the method used for magnetostrictive delay lines might supply the solution they sought for the noncontact transmission and detection of elastic waves needed for bridge cable inspection, prompting an internally sponsored magnetostrictive sensor (MsS) project in 1992. The success of this first project resulted in a series of research and development programs for other engineering applications, such as pipe inspection, with support from not only SwRI but outside sponsors as well, including the Federal Highway Administration and the Gas Technology Institute.
Based on the promising results obtained by employing the sensors in active inspections of structures such as pipes, tubes, strands, and rods, SwRI scientists have identified wider areas of engineering applications for MsS technology that have important cost-saving implications. These include monitoring structural health, diagnosing combustion engine problems, measuring the impact response of structures, performing dynamic stress measurements such as dynamic torques on rotating shafts, and monitoring composite curing.
MsS Principles, Operation, and Instrumentation
The illustration below shows the arrangement and instrumentation used for active inspection of structures, in this case a steel pipe, using MsS. The sensor consists of two parts. One is a means for applying a time varying magnetic field or detecting a magnetization change in the material. This is most conveniently achieved by using an inductive coil that encircles a component under inspection, as shown in the illustration, or is placed near the surface of the component. The other part is a means for providing DC bias magnetic fields to the component. This is achieved by using a permanent magnet, as illustrated, or by using an electromagnetic or residual magnetization in the component material. The DC bias magnetic fields are used to enhance the efficiency of the energy transduction between electric and mechanical energies and to make the frequency of the elastic wave follow that of the electrical signals and vice versa.
When a pulse of electrical current is applied to the coil in the transmitting MsS, a time varying magnetic field is applied to the component under inspection. This field in turn generates a pulse of elastic waves in the component via the magnetostrictive or Joule effect. The generated elastic waves propagate in both directions along the length of the component. When the propagating elastic pulse reaches the coil in the receiving MsS, it causes a change in the magnetic induction of the material via the inverse-magnetostrictive or Villari effect. This change induces an electric voltage in the receiving coil that is subsequently amplified, conditioned, and processed.
For application in the area of passive detection, as with acoustic emission monitoring, engine diagnosis, and impact and vibration testing, only the receiving MsS is used. The sensor can be readily configured to fit specific applications.
Technical Features and Capabilities
In the past, magnetostrictive effects were used chiefly to produce a separate device for a specific function such as sonar radiators or ultrasonic delay lines. Much like the many piezoelectric devices, these devices contain magnetostrictive materials in themselves.
The MsS technology developed at SwRI is different from earlier magnetostrictive devices in the sense that the sensors themselves do not contain magnetostrictive materials, but make use of the magnetostrictive properties of the material under inspection. Therefore, basic MsS technology works most effectively where ferromagnetic materials, such as ferrous steel, are present. However, MsS technology can also be made to work on nonferromagnetic metals, such as aluminum or Inconel, and nonmetals, such as composites or plastics, by providing a ferromagnetic material at areas where sensors are to be placed. This is achieved, for example, by plating with a thin coat of ferromagnetic material such as nickel or bonding a ferromagnetic medium such as wire or ribbon on a surface of the component. It can also be achieved by embedding a ferromagnetic medium in the component during fabrication, as in the case of composites and concrete structures.
The operating frequency of the MsS ranges from a few Hz to several hundred kHz. The sensor has a broad frequency response and can be used over the entire operating frequency range. It is applicable up to the Curie temperature of a material (1,335 degrees Fahrenheit in steel) at which the material loses its ferromagnetism. The sensor can also be operated with a lift-off space of more than a few inches, and the sensing or inspection range from a single sensor location exceeds several hundred feet on bare metals. The sensor is rugged and can be embedded in concrete or other materials for long-term monitoring.
As already described, the sensor can transmit and detect elastic waves in a ferromagnetic material and thus perform the functions of a piezoelectric-type ultrasonic transducer. In addition, because the signal in the receiving sensor is caused by time varying strain or stress, the MsS can perform the functions of a strain gauge, vibration sensor, accelerometer, and piezoelectric acoustic emission sensor. It performs all these functions without direct physical contact with the material and without the use of a couplant.
Active Nondestructive Inspection
In this application, elastic waves are launched and reflected echoes of the waves from defects such as corrosion or cracks are detected using MsS.[11,12] A good example of this is the inspection of pipes and tubes, the primary structural members used in various industries -- such as oil, gas, petrochemical, and electric power -- to transport gaseous or liquid products. Inspecting these structures for defects, particularly those caused by corrosion, is important, not only to ensure uninterrupted operation of equipment but to ensure personal and environmental safety as well.
Conventional NDE techniques such as X-ray, ultrasonics, and eddy current are useful for piping and tubing inspections, but it is expensive to use these techniques to inspect vast lengths of piping. In practice, they are used for inspection of selected areas of piping only.
The MsS offers a cost-effective and efficient means of inspecting piping and tubing for both inside and outside diameter defects. Because the MsS technique inspects the entire cross section of the pipe over a long length, inspection time is greatly shortened and, thus, inspection of the whole length of piping becomes economically viable.
One well-known piping inspection problem is related to pipes that are encased in thermal insulation or lagging. These pipes are widely used in petrochemical and chemical processing plants. An efficient means of detecting corrosion damage to the pipes without costly removal of the insulation has been lacking. The MsS offers an excellent solution to this problem, and SwRI is promoting the first phase of a joint industry program aimed at developing a field MsS instrument for this application.
The MsS can be used to passively detect transient elastic (stress) waves or time varying (dynamic) stresses in a component. Examples include combustion engine diagnosis, described in Diagnosing Engine Problems with Magnetostrictive Sensors and the following:
Acoustic emission monitoring -- Detection of transient stress waves, called acoustic emission signals, can help monitor crack propagation or fracture in a component; for instance, the method can be employed to monitor wire fractures in a steel strand or wire rope for safety purposes. Conventional piezoelectric-type acoustic emission sensors are difficult to use for this application.
Dynamic stress measurements -- An example of this application is measurement of dynamic torque on a rotating shaft. The conventional measurement method is a strain gauge combined with a telemetry system to communicate the data to a detection instrument. This method is time-consuming and expensive and requires interruption of equipment operation to allow bonding of strain gauges on the shaft.
Studies of Wave Dispersion Characteristics
Because of its broad frequency response characteristics and ability to transmit and detect elastic waves without direct physical contact, MsS technology offers a superb means of studying dispersion characteristics of elastic waves in bounded media such as rods or pipes.
The method developed by SwRI scientists involves: (a) transmitting a short elastic wave pulse into the material using an MsS, (b) detecting transient waveforms of the transmitted wave using another MsS, and (c) performing a time-frequency analysis on the detected signals using the short-time Fourier transform.
This method offers an efficient means of testing engineering structural components to help develop ways to control vibration, noise, and dynamic impact response. It can also be used to check the results of various numerical and analytical methods currently employed to predict structural response.
Application to Nonferromagnetic Metals and Nonmetallic Materials
The application of MsS technology can be extended to nonferromagnetic metals such as aluminum and nonmetallic materials such as composites. Examples include inspecting nonferrous structures such as Inconel steam generator tubes in nuclear power plants, monitoring aircraft structures for foreign object impacts and inservice dynamic stresses, and monitoring composite curing states by observing changes in wave propagation along an embedded small diameter wave guide such as nickel wire.
Institute scientists involved in MsS research and development believe that the technology will have a significant impact in many industrial areas, including engine, vibration, and dynamic testing. They also expect a revival of developmental interests in magnetostrictive effects and engineering applications.
MsS Pipe Corrosion Detection Device
Diagnosing Engine Problems with Magnetostrictive Sensors
Published in the Fall 1991 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.