The focus of our research is vibrothermography. This is a Nondestructive Evaluation (NDE) technique for finding cracks, delaminations, or other flaws. In vibrothermography, high-amplitude sonic or ultrasonic vibration causes frictional heating at cracks or other delaminations, and the heating is measured with an infrared camera.
Major applications of this technology include maintenance inspection and manufacturing quality assurance of jet turbine blades and discs, as an alternative to Fluorescent Penetrant Inspection (FPI). Vibrothermography has also been used for inspection of automobile engine parts and composites.
Vibrothermography originated with the work of Henneke et al.[1,2]. and has more recently been improved and popularized by Favro et al.. It has transitioned from a laboratory curiosity to one of the most exciting new NDE methods. Nevertheless, substantial questions remain about its reliability and the underlying physics of the heat-generation process.
Most vibrothermography laboratories use an ultrasonic welder (designed for welding plastic) as a vibration source. These welders generate 1-3 kW of acoustic power and transmit that power through a very small tip into the specimen under test. The high energy flux creates the risk of damage to the specimen. Moreover, the acoustic impedance mismatch between tip and specimen often causes the tip to break contact during each acoustic cycle creating a hammering effect. This hammering effect is extremely sensitive and often not reproducible ("chaos"), leading to questions about reliability.
The goal of our research program is to improve the reliability of vibrothermography. We have developed next-generation experimental apparatus, and we will use this both to investigate the underlying physics of the heat generation phenomenon and to apply that knowledge to verifying and quantifying the reliability of the measurement.
A key advantage of our vibrothermography system is that it uses a (relatively) low power broadband excitation system instead of the high power single frequency excitation used elsewhere. Broadband excitation, such as a frequency sweep (chirp), can excite the natural resonances of a specimen and therefore can cause enough motion to generate detectable amounts of heat at cracks with comparatively low. In addition, the broadband excitation can be tuned to match the specific resonant frequencies of the specimen, thereby improving the chances of detecting a crack.
- E. G. Henneke II, K. L. Reifsnider, and W. W. Stinchcomb, Thermography – An NDI Method for Damage Detection, J. Metals 31(9) 11–15 (1979)
- K. L. Reifsnider, E. G. Henneke, W. W. Stinchcomb, “The Mechanics of Vibrothermography”, in Mechanics of Nondestructive Testing, ed. W. W. Stinchcomb (Plenum Press, New York) 249–276 (1980)
- L. D. Favro, X. Han, Z. Ouyang, G. Sun, H. Sui, and R. L. Thomas, Infrared imaging of defects heated by a sonic pulse, Rev. of Sci. Inst. 71(6) 2418–2421 (2000)
- S. D. Holland, C. Uhl, Z. Ouyang, T. Bantel, M. Li, W. Q. Meeker, J. Lively, L. Brasche, and D. Eisenmann, Quantifying the Vibrothermographic Effect, NDT&E Intl. (2011).
- S. D. Holland and J. Renshaw, Physics-based image enhancement for infrared thermography, NDT& E International 43(5), 440-445 (2010).
- J. Renshaw, S. D. Holland, and R. B. Thompson, Measurement of crack opening stresses and crack closure stress profiles from heat generation in vibrating cracks, Applied Physics Letters 93, 081914 (2008)
- J. Renshaw and S. D. Holland,Full-field vibration measurement for vibrothermography Review of Quantitative Nondestructive Evaluation 27 498--503 (2008).
- S. D. Holland, C. Uhl, and J. Renshaw, Toward a viable strategy for estimating vibrothermographic probability of detection Review of Quantitative Nondestructive Evaluation 27 491--497 (2008).
- S. D. Holland, J, Renshaw, and R. Roberts Measurement of dynamic full-field internal stresses through surface laser Doppler vibrometry, Applied Physics Letters 91, 134101 (2007)
- S. D. Holland, First measurements from a new broadband vibrothermography measurement system, Review of Quantitative Nondestructive Evaluation 26 (2007) 478-483