Zhong Ouyang

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.

Thermosonics: Detecting Cracks and Adhesion Defects Using Ultrasonic Excitation and Infrared Imaging

Abstract We describe a new nondestructive evaluation technique which makes cracks and adhesion defects visible to infrared cameras. This is accomplished by infusing the sample with a short pulse of low frequency ultrasound. The sound causes the faying surfaces of defects to heat up by friction or clapping and, thus, makes them visible in the infrared. Surface or near-surface defects appear to the camera within milliseconds of the initiation of the ultrasonic pulse, while the appearance of subsurface defects is delayed by the thermal propagation time from the defect to the surface being observed.

Theoretical modeling of thermosonic imaging of cracks

A theoretical model is presented to describe the thermosonic imaging of surface-breaking and sub-subsurface cracks, together with illustrative comparisons with experimental measurements.

Recent developments in thermosonic crack detection

We describe recent developments in thermosonic crack detection. This technique uses a single short pulse of sound to cause cracks to heat up and become visible in the infrared. A low frequency (15 to 40 kHz) ultrasonic transducer fills the sample with sound that causes frictional heating at crack interfaces. We show that the technique can be applied equally well to quite large and irregularly shaped objects, and to small delicate objects using the same apparatus. We present examples of this technology applied to cracks as small as 20 microns and as large as several inches, and to materials ranging from brittle ceramics, to soft metals and composites.

Thermosonic imaging for NDE

Pulsed sonic/ultrasonic excitation, combined with infrared imaging, is used to image the presence of cracks in metals. The technique is rapid (50–280 ms) and is sensitive to both surface-breaking and subsurface cracks.

Progress in thermosonic crack detection

We describe progress in thermosonic crack detection. In this technique, a short single pulse of ultrasound is used to cause cracks to heat up and become visible in the infrared. A low frequency (say 10s of kHz) ultrasonic transducer infuses the sample with sound. Where cracks, disbonds, delaminations or other defects are present, the sound field causes the defect to heat locally. The technique is applicable to large and irregularly shaped objects. We present illustrative applications of this technology to aerospace, and automotive inspections.

Thermal wave reflections of a pulsed stripe heat source from a plane boundary

An experimental and theoretical study is carried out of thermal wave reflections from a plane boundary, such as the back wall of a solid slab, following pulsed heating of the front surface of the slab. The heating pattern is a long, uniform stripe source of finite width. Imaging is carried out using a high spatial resolution, high frame rate focal plane array infrared camera to monitor the surface temperature, through its emission of IR radiation in the 3–5 μm spectral band. We consider the spatial temperature distribution as a function of time, not only on the back surface of the slab, but also on the front surface. The theory is based on the assumption of thermal wave reflections from the two insulating (solid/air) boundaries of the slab, and its predictions are in excellent agreement with experimental data for both surfaces of a pure isotropic slab (Cu) and a highly anisotropic slab (uniaxial carbon-fiber-reinforced polymer composite). The unheated region of the front surface shows two peaks as a funct...

Infrared video lock-in imaging at high frequencies

We describe a high-frequency IR lock-in imaging technique which relies on the use of an externally synchronized, snapshot-mode, infrared focal plane array camera.

Rapid, Contactless Measurement of Thermal Diffusivity

We report a novel method for the rapid determination of the thermal diffusivity of materials. The technique utilizes pulsed heating in a geometrical pattern on the surface of the sample, with infrared imaging of the resultant temperature field on the surface as a function of time. By fitting the observed temperature fields to theoretical models, we extract the thermal diffusivities in three orthogonal directions. Knowledge of these diffusivities is important for the design and manufacture of advanced materials for aerospace structures, because of the high temperatures reached on leading edges of airfoils, etc., in supersonic flight conditions. This technique is applicable to a variety of new space-age materials, including ceramic-matrix composites, three-dimensionally woven polymer composites, and high-temperature alloys.

Quantitative Thermal Wave Imaging of Corrosion on Aircraft

Pulse-echo thermal wave imaging is accomplished using a pulsed heat source (usually high-power flash lamps), an infrared (IR) video camera, and image processing hardware and software, all of which is controlled by a personal computer. The system has been described in detail elsewhere. [1,2] Figure 1 shows the thermal wave imaging system in operation at the FAA’s Aging Aircraft NDI Validation Center (AANC). As can seen from Fig. 1, the imaging head is hand-held. The computer, power supplies, etc., are located some distance away at the end of a fifty-foot long cable, the other end of which can be seen attached to the imaging head in Fig. 1. This same cable also carries the power for the flashlamps and the control signals from the computer. To make an image, the imaging head is held in place for three seconds. During this time, the flashlamps are fired, and a sequence of images is acquired and transferred to the computer’s hard disk. The head can then be moved to the next area to be imaged. An area of approximately a square foot is imaged at each flash by the system, so that wide areas of the aircraft can be covered very rapidly.