All bodies at finite temperature emit electromagnetic radiation. The hotter the object the more energy it radiates and the shorter the wavelength it emits. Room temperature objects emit in the long wavelength infrared (LWIR) spectrum range. The ability to produce a temperature map of an object is very useful in nondestructive evaluation (NDE). Static IR imaging only tells us about the surface condition; operator interpretation is important to understand what happens underneath the surface.
Why are waves are so good for subsurface imaging
When light fails to be useful for animals such as bats in the dark or dolphins in murky water, they usually resort to echolocation. The reason for the success of echolocation in perception of the environment is due to two properties of wave propagation: 1) constant velocity or arrival time being proportional to distance and 2) that the shape of the returned echo being similar to the transmitted one. NDE and medical imaging both make use of straight line wave propagation in techniques such as laser and ultrasound or radar.
Why heat is not so good for subsurface imaging
Heat is different. Propagation time is not proportional to distance, and the energy field shape becomes distorted as it propagates. Early active thermal imaging was based on simple direct observation. For example, when the surface of an object is momentarily heated from one side there is an initial temperature rise on the surface. After a short period of time the surface will cool as the heat begins to propagate through the thickness of the object. By measuring the heat distribution on the surface of the object as it cools, it is possible to identify regions where the thickness is reduced. This method works but lacks sensitivity and resolution. Due to the nature of heat propagation, the initial temperature is always larger than later responses. This differs from straight line wave propagation techniques in which the reflected energy can be nearly as large as the original signal.
Why heat can be used for subsurface imaging
Imaging with wave propagation is done using two properties of the waves: 1) reflection from internal reflectors and 2) distance obtained from propagation time Imaging from heat energy can be similar to that of straight-line wave propagation although extraction of distance from time delay is more complicated, it is still possible.
Equivalent Wave Field Transform (EWFT)
EWFT is a unique mathematical algorithm which simplifies imaging for applications such as thermal propagation. EWFT is a wave solution sharing the same geometry and boundary conditions as the length equivalent. By obtaining the equivalent wave field corresponding to the observed heat pattern, the equivalent wave field can be analyzed instead of the observed heat to take advantage of the simple propagating properties of the wave. This can help provide information regarding flaw length and flaw depth.
Time-elapsed thermal images of crack from initial heat application through cool down.
EWFT is a new capability for EWI in thermography inspection. Currently, EWI is working on a laboratory prototype. Once introduced, the technology will be a reliable inspection tool for many industries, such as oil & gas, automotive, and aerospace.
If you are interested in learning more about EWI’s capabilities in nondestructive testing, please contact Oleg Volf at [email protected].