L. D. Favro

Subsurface flaw detection in metals by photoacoustic microscopya

The scanning photoacoustic microscope (SPAM) is used in both the conventional and phase‐contrast modes to detect a well‐characterized subsurface flaw in Al. The physical mechanism is that of thermal diffusion, with a subsurface probe depth and flaw resolution length of approximately one thermal‐diffusion length. Comparison of the dependences of the photoacoustic signal upon chopping frequency from the different regions of the sample confirm that the differential signal from the flaw corresponds to a transition from thermally thick to thermally thin boundary conditions. Experimental results are in good agreement with calculations based upon a three‐dimensional thermal‐diffusion model.

Infrared imaging of defects heated by a sonic pulse

High-frequency pulsed sonic excitation is combined with an infrared camera to image surface and subsurface defects. Irreversible temperature increases on the surface of the object, resulting from localized heating in the vicinity of cracks, disbonds, or delaminations, are imaged as a function of time prior to, during, and following the application of a short pulse of sound. Pulse durations of 50 ms are sufficient to image such defects, and result in surface temperatures variations of ∼2 °C above the defect. As an example, sonic infrared images are presented for two fatigue cracks in Al and of interply delamination impact damage in a graphite–fiber-reinforced polymer composite. The shorter of the two fatigue cracks is ∼0.7 mm in length, and is tightly closed. Thus, this new technique is sensitive, and capable of rapid imaging of defects under wide surface areas of an object.

Thermal wave imaging of closed cracks in opaque solids

Thermal wave scattering from closed, slanted cracks is investigated both theoretically and experimentally. A Born approximation calculation is carried out for the particular case of gas‐cell detection for three crack angles. Good agreement is found with experimental images of both the magnitude and phase of the gas‐cell signal variation for cracks fabricated in an aluminum alloy at these same angles. Good agreement is also found between theory and experiment for the frequency dependence of the thermal wave scattering from a 45° crack. It is shown theoretically and confirmed experimentally that a strictly vertical, closed crack is not observable by gas‐cell detection, but is easily seen by mirage‐effect (optical beam probe) detection. The results for model cracks are applied to the case of brittle fractures in solids.

Acoustic chaos and sonic infrared imaging

We describe a quasichaotic mechanism for the generation of complex vibrations from the application of a 450 ms, 40 kHz excitation coupled nonlinearly to a sample under inspection by means of sonic infrared (SIR) imaging. Monitoring the sample vibration by means of a laser vibrometer, we find that the vibration switches from the fundamental and harmonics to several series of frequencies that are rational fractions of the fundamental driving frequency. The transition to this quasichaotic behavior is important in obtaining high-quality SIR images.

Acoustic chaos for enhanced detectability of cracks by sonic infrared imaging

The technique of sonic infrared imaging (SIR) consists of the excitation of an object with a short pulse of 15 to 40 kHz sound, in combination with IR imaging of the object’s surface temperature as a function of time. Sonic infrared imaging is effective for detecting surface and subsurface cracks. The recent discovery of acoustic chaos has provided a means of greatly enhancing the effectiveness of SIR. We describe the properties of chaotic sound in the context of SIR crack detection, and show examples of the enhancement of the detectability of cracks through the use of chaotic sound.

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.

Imaging the early time behavior of reflected thermal wave pulses

We describe the early time behavior of reflected thermal wave pulses, and relate that behavior to schemes for making depth images.

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.

Inverse scattering algorithm applied to infrared thermal wave images

We present an inverse scattering algorithm that makes possible the inversion of experimental thermal wave images of planar subsurface defects. The method is based on a Green’s function technique. It significantly improves the spatial resolution and removes the blurring which is otherwise characteristic of thermal wave images. The method has been applied to materials ranging from plastics (diffusivity ∼0.001 cm2/s) to aluminum alloys (diffusivity ∼0.5 cm2/s), and for depths ranging up to a few mm.