P. K. Kuo

Determination of thermal diffusivity of low‐diffusivity materials using the mirage method with multiparameter fitting

A method to determine anisotropic thermal diffusivities parallel to the surface of a solid sample is presented. The measurement method is based on the mirage effect and the data are analyzed using multiparameter least‐squares regression fitting. The specimens studied were of medium and low diffusivity: rare earths, ceramics, and polymers. Before this study, the low‐diffusivity limit for the applicability of the mirage method was about 0.02 cm2/s, and therefore this technique has not been suitable for polymeric materials. In this work the limit has been reduced to 5×10−4 cm2/s by improving the measurement apparatus and the data analysis method. The thermal diffusivities obtained agree with the values obtained using the flash method within ±20%. The agreement with published values is good, taking into account that for ceramics and polymers the literature gives only the correct order of magnitude due to the differences in the manufacturing process. Even though the sensitivity of the measurement decreases wit...

Parallel Thermal Wave Imaging Using a Vector Lock-In Video Technique

The appearance of the IR video camera has extended the wavelength range of the visible video camera to the thermal IR range (3–12 µm), thus providing a powerful tool to researchers in thermal wave imaging. However, imaging in the thermal IR range has its special handicaps, not shared by its visible counterpart. Most objects in conventional photography reflect rather than emit light of their own. As a result one often has the freedom to choose the intensity, direction and color of illumination to accentuate the aspects of the object to be photographed. In thermal IR imaging the situation is very different in that nearly all objects emit thermal radiation of their own, in addition to reflecting radiation of other objects. What is recorded in a thermograph is always a mixture of emitted and reflected radiation, some of which even comes from components of the camera itself, including lenses and their supporting structures. This problem is particularly severe in the 8–12 µm range, because it corresponds to the peak of blackbody radiation at room temperature. It is this same range of wavelength that is most relevent in non-destructive evaluation. In conventional scanned thermal wave imaging applications this problem is overcome by the use of a lock-in analyzer synchronized to the source of the thermal wave. Without the lock-in technique, the IR video camera is capable of observing only very slow thermal phenomena[1], despite the fact that the intrinsic band width of the camera is very broad. This limitation offsets the main advantage of the IR video camera, namely its high data-acquisition rate. In this paper we report on instrumentation development which combines the lock-in technique with the IR video camera. With this technique the information of each pixel of an image is handled in the manner of a lock-in analyzer, while the object is illuminated (i.e., heated) or stimulated (e.g., joule heating) with a signal which is synchronous with the reference signal of the lock-in detection. This way the unsynchronous background radiation is rejected and the signal-to-noise ratio is enhanced.

Spatial resolution of thermal wave microscopes

It is demonstrated theoretically and confirmed experimentally that the intrinsic spatial resolution of a thermal wave microscope in the extreme near field limit is independent of thermal wavelength and is determined by the depth of the thermal scatterer beneath the surface of the specimen.

Photoacoustic microscopy of an integrated circuit

The in‐phase and quadrature components of a photoacoustic signal have been used to form images of an integrated circuit with ∼6 μm resolution. These photoacoustic images contain information about subsurface properties of the sample.

Photoacoustic phase signatures of closed cracks

The photoacoustic phase signatures of closed cracks are calculated and compared with experimental observations. The differential signals from near subsurface lateral cracks are advanced in phase by 45° with respect to the background signal, in contrast to the 45° lag in phase observed for a near lower surface step.

Early-Time Pulse-Echo Thermal Wave Imaging

We describe the early time behavior of reflected thermal wave pulses from planar subsurface scatterers, and describe methods for making depth images, independently of the lateral size of the scatterer.

A simplified approach to computations of photoacoustic signals in gas‐filled cells

A relationship is derived between photoacoustic signals from localized sources and temperature distributions from extended sources. The result is applied to simple geometries.

PULSE-ECHO THERMAL WAVE IMAGING

We present calculations which describe the three-dimensional reflection of thermal wave pulses from planar sub-surface defects. We also present the results of confirming experiments for the case of subsurface flat-bottomed holes with various depths and lateral dimensions.

Synchronous thermal wave IR video imaging for nondestructive evaluation

We describe a system for real-time processing of infrared video images in synchronism with the time-dependence of the target objects temperature. The system can either be used either with periodic or pulsed heating of the target. With periodic heating, the system operates as if it were a collection of lock-in amplifiers, one for each of the quarter of the million pixels of the image. With pulsed heating, it operates as if it consisted of a similar number of box-car averagers. In both cases, the signal-to-noise ratio and temperature sensitivity of the infrared camera are improved. The technique lends itself to a wide spectrum of NDE applications.

Thermal Wave Imaging of Aircraft Structures

In a previous report [1], we introduced the application of thermal wave imaging to adhesion disbonds and corrosion in aircraft. In the present paper, we describe the application of pulse-echo thermal wave imaging to NDT of aging aircraft. The technique uses high-power photographic flash lamps as a heat source and an IR video camera as a detector. The flash lamps launch pulses of heat into the skin of the aircraft and the IR camera images the returning thermal wave reflections from subsurface defects. The system also includes electronic hardware and software for carrying out the time-gated imaging and real time analysis of the defects. It also has the ability to image large areas in short times. The current inspection speed enables the imaging of over 90 feet of a 16″ strip of aircraft per hour. Here we present some examples of airframe defects, both for metal and composite structures.