Message Board Thread - "Stress Corrosion Cracking "

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Stress Corrosion Cracking IR Newbie 9/5/2000
Are there any examples where Stress corrosion cracking has been detected by IR?
 
RE:Stress Corrosion Cracking Gary Orlove 9/5/2000
There hasn't been a great deal written on stress corrosion cracking detection with thermography. A search through the web has come up with the following:

Various components of a large rotary dryer started failing soon after initial installation. Early failure analysis was initiated to identify and rectify some of the problems. The frequency of some problems diminished somewhat. Failures due to other causes increased in frequency and severity, and there was concern that a catastrophic failure could occur. The challenge in preventing a catastrophic failure was to convince management that such an event would happen. A comprehensive failure analysis was eventually approved, the goal of which was to identify the cause for failure in each of the components of the dryer. The comprehensive analysis included finite element modeling of both the mechanical and thermal stresses in each component, infrared thermographic imaging, ultrasonic shear wave inspection of weldments, microstructural evaluation, hardness testing, and fractographic analysis of several components. Additionally, vibration waveform analysis was used to examine some components subjected to shock loading. To conclude that the failure analysis was successful, we required that all methods of analysis provided self-consistent results. Any proposed solution then became the basis for the decision to repair the unit or to install a new unit.


Some books on the subject:
Infrared Methodology and Technology, Edited by Xavier P.V. Maldague. This book focuses on the growth and potential uses of IR thermography as a NDE and monitoring technique. Part 1 is an introduction to current IR NDE theory and technology. The second part describes the wide range of IR NDE and monitoring applications. Also included is an extensive bibliography of IR and thermal NDE. Hard cover. List price $115.00. Gordon & Breach Science Publishers, P.O. Box 41010, Newark, NJ 07101-8398. Telephone: (800) 545-8398, FAX: (215) 750-6343.

Nondestructive Evaluation of Materials by Infrared Thermography, Xavier P. V. Maldague. This is an up-to-date text focusing on infrared thermography and its uses in automated inspection and control in industrial settings. It is intended for advanced material engineering students, materials science technicians, and researchers. Contents includes: history of infrared testing, theoretical aspects, experimental apparatus, internal and external thermal stimulation, quantitative analysis of delaminations, inspection with low emissivity by thermal transfer imaging, thermal diffusivity measurements, thermal tomography, thermal NDE of nonplanar surfaces, and applications of infrared thermography to high temperatures. Hard cover. List price $275.95. Springer-Verlag New York, Inc., 175 Fifth Avenue, New York, NY 10010. Telephone: (212) 460-1500, FAX: (212) 473-6272.

Thermography
The effect of flaws on the thermal conductivity and emissivity of test materials is analyzed by the thermographic NDE method [Jones & Berger, 1992]. Its attractive features are the capability to cover large areas in a single frame and it does not require coupling. Unfortunately, this method was found unreliable when testing bonded joints with a narrow gap between the unbonded surfaces. In the early stages, liquid crystals were used to map the surface distribution of the21 temperature. The improvement of infrared systems made such tools highly sensitive and effective for mapping the cooling or heating profiles to rapidly indicate flaws. Examining the temporal gradient of the thermal maps, i.e., thermal flux, significantly improved the detectability of flaws.

Thermal Wave Imaging
Thermal waves transmitted through test parts can be received and used to produce an image of the internal uniformity. In addition to imaging the pattern of subsurface flaws and corrosion, the technique can rapidly (a few seconds) make quantitative measurements of less than 1% material loss for various regions in the image [Favro & Thomas, 1995 and Hans, et al, 1996]. It can use as a heat source pulses from photographic flash-lamps. The heat source box traps and funnels the light uniformly onto the test structure, and an infrared (IR) focal plane array camera, aimed and focused at the surface through an opening in the rear of the hand-held shroud, monitors the rapid cooling of the surface. The system operates by sending a heat pulse from the surface into the material, where it undergoes thermal wave reflection at either the rear surface or at any interior surface at which the thermal impedance changes, e.g., at disbonds, delaminations, etc. The effect of these thermal wave reflections is to modify the local cooling rate of the surface. The cooling rate, in turn, is monitored through its effect on the IR radiation from the surface, which is detected by the camera, and processed as a sequence of images by the control computer. The contrast in the processed images reveals the presence of defects in the interior or variations in the thickness of the material. In Figure 26, Thermal Wave Image an example is shown for corrosion and unbond near a tear strap/stringer of a Boeing 737 See http://www.ndt.net/article/... and http://www.thermalwave.com/
 


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