Review by NDTnet Laser Based Ultrasound Within the Material .... ... It's just ultrasound. Subjected to: refelection, scattering, attenuation. Detection .... ... is border line. But improves with: light gathering ability signal processing. Summary ... Technology Development and Application Engineering in the niches Consider application beyond conventional UT Don't forget hybrids like laser/air and laser/EMAT Research Source efficient Laser arrays - time and space Fundamentals, esp. anisotropic materials Receiver detectability Ligth collection strategies Speckle-intensive detectors Nonlinear optical elements Signal processing - structured signals and data arrays George Alers shows the Principles EMAT Detection of Ultrasound Related Information: Project: MICROSTRUCTURE SENSING FOR PROCESS MONITORING Most material processing for commercial applications is designed to develop specific microstructures that are known to give the final product its desired properties. Therefore, the development of process-control algorithms and nondestructive evaluation procedures should be focused on developing techniques and sensors that provide information on the microstructure instead of detecting localized inhomogenities that may or may not be flaws Project Leader: G.A. Alers S.R. Schaps

INTRODUCTION

In its usual application, ultrasonic nondestructive testing focuses on locating inhomogeneities or cracks in a part and then applies fracture mechanics to predict the likelihood of failure under certain load conditions. In the future, it appears that more emphasis will be placed on using ultrasonic waves to assure that the material being employed for a specific task will have the microstructure needed to meet the material's service needs. Thus, the establishment of quantitative relationships between microstructures and the values of the velocity and attenuation of ultrasonic waves (i.e., Ultrasonic Materials Characterization) will become an important part of the nondestructive testing profession. Examples of how ultrasonics is already being used for materials as characterization can be found in: (1) predicting the hardness or yield strength of a steel from empirical correlations with the velocity of certain ultrasonic waves; (2) predicting the formability of sheet aluminum and steel from the velocity of Lamb waves propagating in the plane of the sheet; (3) detecting the state of cure of polymers or the recovery and recrystallization of worked metals from ultrasonic wave attenuation measurements as a function of time at temperature; (4) inferring magnetic properties from the response of ultrasonic measurements to an applied magnetic field; and (5) using acoustic birefringence to measure the stress level in a structural member or to act as the sensor in a load cell.

II. Why EMATs?

A critical examination of the materials characterization potential of ultrasonics shows that the effects of microstructure on the velocity of sound, for example, are often rather small and require measurement techniques that are reliable to precisions equal to or better than + 0.1%. Furthermore, measurements of the ultrasonic attenuation coefficients that are sensitive to microstructure often include coupling inefficiencies that dominate the measured quantities and make interpretation difficult. Electromagnetic Acoustic Transducers (EMATs) avoid sources of error associated with the coupling of piezoelectric transducers to the material because they excite and detect ultrasonic vibrations in metals across a small air gap. This non-contact feature is also very valuable for production line applications because liquid coupling cannot be tolerated at high temperatures and on fast moving production lines. This means that EMATs are excellent for process monitoring where it is important to maintain a desired microstructure throughout an entire production run. Because they are inefficient, they can produce useful signals without extracting much vibrational energy from the system and, thus, are well suited for measuring the intrinsic damping capacity of a material as it vibrates in a resonant mode.

Not fully appreciated by the nondestructive materials characterization community is the ability of EMATs to excite and detect a very wide range of ultrasonic wave types. This makes it possible to optimize the direction of acoustic wave propagation and polarization to enhance the sensitivity to a particular type of microstructure. Likewise, it allows excitation and detection of very specific resonant modes in a vibrating system so that the measured resonant frequency can be very accurately related to particular elastic constants and the damping capacity to specific stress distributions. In order to emphasize this particular advantage of EMATs, the examples of materials characterization to be discussed in this paper have been organized by the type of ultrasonic wave used.

II. EAMPLES

A. Shear Waves

An EMAT that excites and detects shear waves that propagate perpendicular to the surface under the transtucer consists of a pancake shaped coil placed under a permanent magnet. If the metal under this EMAT is a single crystal or a polycrystal with texture or a metal supporting a stress, the shear wave thus produced will break up into two waves with polarization directions parallel to the principal symmetry axes of the texture or of the stress tensor. These two waves have different propagation velocities and a quantitative measure of this difference can be used to determine the degree of texture present and/or the magnitude of the stress acting on the material. . The absolute value of the shear wave velocity (after taking texture and/or stress into account) provides a measure of the shear or torsion modulus of the material which, in turn, directly reflects the microstructure of the material. Measurements on hardenable steels show that both the shear and extensional moduli are linear functions of the hardness level so it is possible to imagine developing an ultrasonic instrument that measures hardness in a completely nondestructive (and noncontact) manner. Since the change in shear modulus with hardness in a 4340 steel amounts to only about 2%, this instrument must be capable of making absolute wave velocity measurements with a precision exceeding +/-0.1% in order to be of any practical value. Such a precision is difficult to achieve if piezoelectric transducers and coupling media are involved.

B. Shear Horizontal (SH) Waves.

An EMAT wich the same pancake coil as used for the shear waves described above but wich the single magnet replaced by a row of magnets with alternating polarities will produce a shear wave whose polarization lies in the plane of the surface and which can be made to propagate at any angle relative to the surface. Such waves have many useful properties such as easily calculated scattering coefficients from certain flaws and closed form mathematical descriptions of their propagation characteristics in thin sheet metals. When excited in magnetic materials, a measurement of their transduction efficiency can be used to measure the magnetstrietive properties of the material.

C. Lamb Waves.

By using a single permanent magnet as in the shear wave case (A) above but wich the wires in the coil bent into a meander shape, the acoustic wave generated will have a wave length related to the spacing between the elements of the meander and a frequency determined by the electronic transmitters and receivers connected to the coil. Such a configuration will produce Rayleigh waves on the surface of a half space or Lamb waves in a plate or sAeet if the frequency is chosen to equal the quotient of the phase velocity of the desired ultrasonic wave and twice the spacing between the wires of the meander coil. Lamb waves produced and detected with this configuration of coil and magnet have been used to monitor the microstructure that gives sheet steel and aluminum the formability properties that are required for automotive and beverage can applications.

D.Resonant Vibrations.

Products whose shapes are cylinders or spheres have closed form equations that relate the frequencies of their resonant vibrational modes and their dimensions to their elastic moduli. These equations also define the surface displacements associated with each of these modes. By properly placing permanent magnets and coils of wire around the shapes, it is possible to make the electromagnetic forces applied to the body match the displacements of a particular mode so that only that mode is excited. Under these conditions, the resonant frequency and the quality factor, Q, of the resonance can be measured with high accuracy and the elastic moduli determined wich a precision limited by the dimensional tolerances of the body. EMATs used in this way have allowed the measurement of recovery and recrystallization phenomena in aluminum alloys, the depth of hardness in induction hardened automotive drive shafts, fatigue damage in bridge steels and the level of stress in an ultrasonic load cell.

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