Acoustoelastic constants (Lij) for longitudinal and shear waves in engineering materials
- posted 12/28/12

Ultrasonic Characterization of Hydrogen Induced Stress in 4140 Steel
- posted 1/25/10

Ultrasonic Stress Measurement Using the LCR Wave - posted 2/8/09

LCR StressMapTM Summary - posted 2/8/09

Dual Axis Version of LCR Stress Measurement Probe - posted 2/8/09

LCR Demonstrations 2007 - posted 2/8/09

Evaluation of Load Damage in Steel Using The LCR Ultrasonic Technique
- posted 11/17/07

:: LCR Stress Measurement Around the Globe

         Nondestructive mapping of stress fields is available with StessMapTM, based on the LCR ultrasonic technique. The LCR technique uses critically refracted ultrasonic waves propagating below and parallel to the surface of engineering specimens. It uses the acoustoelastic coefficient with the greatest slope, giving more sensitivity to stress than any other ultrasonic method. Moreover, the LCR technique is least affected by material texture. 

       Applying the LCR method is comparatively simple. Normally, little surface preparation is required, and the probe can usually be moved freely around a test surface. And, because of the higher sensitivity of the LCR wave, stress measurements can in some instances be made with commercial, portable,  ultrasonic flaw detectors. Special instrumentation may be required for low stress fields.

     The unique characteristic of the STRESSMAPTM system is the use of hydraulic pressure and freely rotating probes  to reduce couplant-induced travel time variations. With the hydraulic pressure, and other special features of the LCR probe system, stress fields have been mapped in welded steel plates, rolled aluminum plate,  pressure vessels and pressure vessel welds,  and turbine disks and blades. 

     One patent has been granted and two other patents are pending on the LCR probe system.

     For the pressure vessel application, the probe shown on the left below was moved around the vessel and travel time measurements were made as the vessel was pressurized. The LCR travel-times tracked the pressurization very well, particularly at the higher levels. Near to the weld line at one  end, the technique tracked stress changes that were very similar to results obtained by other researchers earlier, using the blind hole drilling technique.  Note in the LCR results are shown on the right, below that the change of 20 ns from the first (assumed zero stress) point to the final point gives a stress change of –29 ksi (-198 MPa).  Ref: Bray, D. E., "Ultrasonic Stress Measurement in Pressure Vessels, Piping and Welds," To appear August 2002 issue of Journal of Pressure Vessel Technology. 

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       Tests on titanium turbine blades used in aircraft engine manufacture showed that the LCR technique could track the effects of a special method for adding residual compressive stresses to the leading edge of the blade, for fatigue life enhancement. The fixture shown on the left below clamped around the leading edge of the blade. It contained four send receive LCR probes, two on the top and two on the bottom. One set was at the leading edge, and the other set was adjacent to it. Comparative travel-times showed that there were statistically significant differences in treated and untreated blades (right, below). Ref: Bray, Don E., Suh, Ui and Hough, Clarence L.,  "Ultrasonic Evaluation of the Effects of Treatment and High Cycle Fatigue in Aircraft Engine Turbine Blades," Review of Progress in Quantitative Nondestructive Evaluation, Brunswick, Maine, 29 July – 3 August 2001, Vol. 21B, American Institute of Physics, 615, pp. 1643-1650.
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Fig. 8 Absolute travel-time (T) from blade group 2
(Set 1 - untreated, Set 2 - treated)

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     The LCR technique was used in a turbine overhaul shop to evaluate the stress state in a compressor rotor. The rotor (see figure) showed a bow to one side, and the question of residual stress state arose. Using a specially designed LCR probe (about 2 inches in length, and shown in the photo) the travel-time state was measured at 45 degree intervals around the rotor. The travel time results indicated compressive stresses on the bowed side of the rotor, which is expected. Ref: Bray, D. E., Tang, W. and Grewal, D., “Ultrasonic Stress Evaluation in a Turbine/Compressor Rotor,” Journal of Testing and Evaluation, Vol. 25, No. 5, Sept. 1997, pp. 503-509. 

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      Rolled in residual stress in aluminum plate can cause warpage during machining. This, of course, can render the final part unusable. Tests using the LCR technique on three aluminum plates with different stress states showed clearly the difference in the non stress relieved and stress relieved plates. More interestingly, though, the LCR results showed an O temper anneal plate to contain significant residual stresses. The stress state was confirmed by a section saw cut from the plate, as shown on the right. Ref: .Bray, Don E., Kim, S-J., and Fernandes, M., “Ultrasonic Evaluation of Residual Stresses in Rolled Aluminum Plates,” Proceedings Ninth International Symposium on Nondestructive Characterization of Materials, Robert E. Green, Ed., Sydney Australia, June 28-July 2, American Institute of Physics, Melville, NY, 1999, pp. 443-448. 
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  More details on the principles of the  STRESSMAPTM system are furnished in the links below.