Analogic Engineering, Inc.
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TECHNICAL SUMMARY

 

The primary research goal was to develop Non-Destructive Evaluation (NDE) techniques to rapidly determine critical aspects of the internal mechanical state and microstructure of metal materials, structures and components. The key innovation resulting from this research is a specially configured acoustic transducer with a novel mode of operation to provide electronic control over the polarization direction of shear waves propagating through the metal. Anisotropic mechanical properties and states, such as microstructure or uniaxial stress, have associated directionally dependant elastic constants. Acoustic wave propagation is also related to these constants, so by measuring directionally dependant acoustic properties, the directional dependence of certain mechanical properties can be inferred.

 

One focus area for applications is longitudinal stress measurement in steel members, with a specific target of providing a stress measurement device for the steel rail used for railroad track. The aim is to allow monitoring and preemptive correction of rail stress extremes to prevent train accidents caused by wintertime “pull aparts” or summertime “heat buckles” (such as led to the Amtrak train derailment in Kensington, MD in July 2002). A similar instrument could measure stress in members of steel structures, such as buildings and bridges, to ensure correct and safe load distribution. Another focus area for applications is the on-line assessment of material properties to enable better control of thermomechanical processing in production and manufacturing lines. For example, tighter monitoring and control to reduce the directional dependence of ductility in aluminum roll and sheet stock would enable thinner material and cheaper alloys to be used to form aluminum cans.   

 

Previous investigations and theoretical studies (see, for example, R.B.Thompson, W.Y. Lu, A.V.Clark Jr., Chapter 7 - Ultrasonic Methods, Handbook of Measurement of Residual Stresses) indicate that shear wave velocity and attenuation rate can be related to the stress state and anisotropic nature of a material. For example, the velocity of shear waves traveling across a steel member in longitudinal stress can vary slightly depending on whether the shear waves are polarized in the same direction as the principle stress axis, or at right angles to this direction. Similar birefringent effects are observed for shear waves propagating in anisotropic materials with a preferred alignment in the material microstructure, according to the relative angle between the shear wave polarization direction and the major alignment axis. Total birefringence is generally a combination of these effects.

 

In recent research (see, for example, US Patent 6,311,558 B1, Clark et al.), differentiating between internal effects (microstructure and residual stress) and external effects (applied stress) has been linked to locating the so-called fast and slow polarization angles. This requires mapping very small changes in shear wave velocity and attenuation over all angles, not just at 0 degrees and 90 degrees to a particular axis. Prior to this Phase I research, this had to be accomplished by mechanically rotating the transducer. The mechanical rotation approach can introduce additional errors due to transducer movement and transducer main lobe eccentricity, and is very time consuming (up to 80 seconds per complete test). This greatly limits the rate at which material can be dynamically tested since birefringent techniques only make sense if essentially the same column of material is being tested at the different polarization angles. Otherwise, the small birefringent changes in velocity, determined from Time-Of-Flight (TOF), are easily lost in larger changes due to any material thickness variations and/or any compositional differences along the material due to slight variations in thermomechanical history. For continuous testing of, say, stress in railroad track or material microstructure on a processing line, a faster measurement technique is required.

 

The Phase I proposal suggested a method that might achieve electronic steering of shear wave polarization angle (the angle of the line perpendicular to propagation direction representing the path of particle motion) in order to provide rapid and precise measurement of the required acoustic parameters. As a consequence of the method proposed, it appeared that more complex shear wave propagation modes, such as elliptical polarization, might also be achieved (i.e., particle motion in the plane perpendicular to propagation direction). This led to a secondary research goal - to introduce a method of generating and measuring “plane” polarized modes of shear waves. Given the utility of elliptically polarized electromagnetic waves for revealing material structure and properties (surface ellipsometry, optical birefringence, etc.), it is reasonable to expect that useful applications in material evaluation might be found by researchers for this new degree of control over sound propagation in solids. Two-dimensional control over the movement of particle layers might also be useful in other fields such as nanotechnology.

 

 

ACKNOWLEDGEMENTS

 

This research supported by NSF SBIR Phase I Award No. DMI-0128698 “Using Variable Polarization Ultrasonic Shear Waves to Isolate and Quantify the Competing Effects of Microstructure and Stress on the Acoustic Properties of Steel”

 

Analogic Engineering, Inc. wishes to thank Dr. Bruce Maxfield, Dr. David Walrath, Mr. Aarne Haas, P.E., the University of Wyoming College of Engineering, and Mid America Manufacturing Technology Center (now known as Manufacturing Works).

 

The ongoing collaboration with Dr. Maxfield is greatly appreciated, who supplied a customized transducer based on a configuration devised by the PI to enable electronic polarization steering. He collaborated on implementation details of the proposed transducer, and then completed the final mechanical design, component selection and construction based on his experience and expertise in this field. Dr. Maxfield also worked with the PI to interface the transducer to the test instrumentation and evaluate the data from preliminary testing.

 

We appreciate Mr. Haas’ collaboration on the design of steel test pieces and mounting methods for tensile testing; his participation in test data collection and coordinating the use of University of Wyoming facilities and equipment with Dr. Walrath.

 

Thanks to Dr. Walrath for facilitating the preparation of steel test samples at the University of Wyoming Machine Shop, for providing access to laboratory facilities with the  400,000 pound-force (1,780 kN) Tinius Olsen testing machine that was used to establish the feasibility of the transducer for stress measurement and for calculating safety margins for tensile tests.

               EMAT RESEARCH PROJECT 2002


Project Title:                    Using Variable Polarization Ultrasonic Shear Waves

                                      to Isolate and Quantify the Competing Effects of

                                      Microstructure and Stress on the Acoustic

                                      Properties of Steel  

Grant Support:                  National Science Foundation SBIR Phase I

Award Number:                 DMI-0128698

Commencement Date:       1/1/02

Completion Date:              6/30/02

Principal Investigator (PI):  Steven J. Turner

Project Consultant:            Dr. Bruce W. Maxfield