Light, Heat and Materials
Thermal Imaging and Nondestructive Evaluation
Applied Physics Laboratory, Johns Hopkins University
President Coates called the 2057th meeting to order at 8:22 p.m. on March 15, 1996. The Recording Secretary read the minutes of the 2056th meeting and they were approved with one correction. The President invited recommendations and suggestions for speakers and topics in the 1996–1997 lecture series.
The President introduced Mr. John Murphy of the Applied Physics Laboratory, Johns Hopkins University, to discuss “Light, Heat and Materials: Thermal Imaging and Nondestructive Evaluation”.
The value of methods for nondestructive evaluation of materials can be assessed by considering their applications in aerospace and civil engineering. The strength of reinforced concrete is largely determined by the absence of air gaps and the integrity of the metal reinforcement bars within. The aluminum skin of airplanes is attached to the aluminum honeycomb underneath by rivets and bonding material. How can it be determined if these structures are ready to fail without destroying them or testing them to failure?
Dynamic thermal imaging is a method for nondestructive evaluation that has been described as “listening to the light”. It can be traced to Alexander Graham Bell's patents for photoacoustics, the transduction of sound to modulated light and back to sound. Visible, infrared and microwave light can be used in thermal imaging, and provides, as x-rays do, three dimensional information about subsurface features hidden inside opaque objects. In dynamic thermal imaging spectroscopic absorption measurements are made using light transmitted from the surface of the object being tested. A family of thermal characterization methods can be used depending of which model for depth dependence of heating is most applicable, whether heating occurs at the surface, with uniform negative exponential absorbance, or with nonuniform subsurface heating.
In cases where thermal absorption occurs predominantly at the surface two techniques that can be used are photothermal imaging and time resolved infrared radiometry. With photothermal imaging, infrared light is measured to determine how the surface temperature varies with the visible light absorbed by an object. This infrared method has been used for checking insulation of buildings and electrical transformers. With time resolved infrared radiometry, the surface image at infrared wavelengths is measured over time after visible laser light exposure to assess spectral and thermal responses characterizing different materials or revealing structural inhomogeneities beneath the surface.
For the problem of determining the strength of reinforced concrete, microwave rather than infrared wavelengths are to make reflectance measurements of the temperature and its dependence on light exposure and time delay. The microwave reflectance method is capable of revealing one of the principle causes for weakness in reinforced concrete, air gap defects. For the problem of determining the bonding strength of the aluminum skin of aircraft thermal transit times to measure remaining skin thickness and reveal any “disbonding” of the skin from the aluminum honeycomb beneath.
Another important nondestructive technique microwave imaging can be compared with the use of ultrasound because the 3 cm wavelength provides images on the same scale of resolution. By combining the use of microwave irradiation with thermal imaging the resolution can be improved by a factor of 1000. However, some environmental problems may be created by the increased use and intensity of microwave radiation for communication, collision avoidance, and other technologies, so biological systems are now being studied to determine the risks that may be involved with this technique.
Mr. Murphy kindly answered questions from the audience. The President thanked the speaker on behalf of the Society, announced the speaker for the next meeting, restated the parking policy, and adjourned the 2057th meeting at 9:38 p.m.
Weather: partly cloudy
John S. Garavelli