Thermographic Detection o Conducting Contaminants in Composite Materials Using Microwave Excitation

This paper describes microwave-source time-resolved infrared radiometry (MW-TRIR) as a method for the detection and characterization of microwave absorption by conductive fibers and other absorbing regions in dielectric materials. Due to recent technical developments in the speed, detector array size, and sensitivity of infrared focalplane arrays, time-resolved infrared radiometry has evolved into an important NDE tool which allows fast area inspection at high spatial resolution. While much prior work has focused on the detection of structural defects or disbonds in a variety of materials [1,2], the increasing importance of composite materials requires new approaches to inspection which allow characterization of local material properties. Defects in such materials may have little thermal contrast compared to the matrix material and may be invisible using conventional infrared radiometry methods. However, where the embedding material is a weak microwave absorber, localized microwave absorbing regions can be detected easily. There are three different classes of absorption processes: (1) dielectric loss (e.g. water), (2) magnetic loss, and (3) Joule heating (e.g. electromagnetic radiation interaction with conducting fibers)

MEMS Louvers for Thermal Control

Mechanical louvers have frequently been used for spacecraft and instrument thermal control purposes. These devices typically consist of parallel or radial vanes, which can be opened or closed to vary the effective emissivity of the underlying surface. This project demonstrates the feasibility of using Micro-Electromechanical Systems (MEMS) technology to miniaturize louvers for such purposes. This concept offers the possibility of substituting the smaller, lighter weight, more rugged, and less costly MEMS devices for such mechanical louvers. In effect, a smart skin that self adjusts in response to environmental influences could be developed composed of arrays of thousands of miniaturized louvers. Several orders of magnitude size, weight, and volume decreases are potentially achieved using micro-electromechanical techniques. The use of this technology offers substantial benefits in spacecraft/instrument design, integration and testing, and flight operations. It will be particularly beneficial for the emerging smaller spacecraft and instruments of the future. In addition, this MEMS thermal louver technology can form the basis for related spacecraft instrument applications. The specific goal of this effort was to develop a preliminary MEMS device capable of modulating the effective emissivity of radiators on spacecraft. The concept pursued uses hinged panels, or louvers, in a manner such that heat emitted from the radiators is a function of louver angle. An electrostatic comb drive or other such actuator can control the louver position. The initial design calls for the louvers to be gold coated while the underlying surface is of high emissivity. Since, the base MEMS material, silicon, is transparent in the InfraRed (IR) spectrum, the device has a minimum emissivity when closed and a maximum emissivity when open. An initial set of polysilicon louver devices was designed at the Johns Hopkins Applied Physics Laboratory in conjunction with the Thermal Engineering Branch at NASA's Goddard Space Flight Center

Ultrafast Laser-Based Spectroscopy and Sensing: Applications in LIBS, CARS, and THz Spectroscopy

Ultrafast pulsed lasers find application in a range of spectroscopy and sensing techniques including laser induced breakdown spectroscopy (LIBS), coherent Raman spectroscopy, and terahertz (THz) spectroscopy. Whether based on absorption or emission processes, the characteristics of these techniques are heavily influenced by the use of ultrafast pulses in the signal generation process. Depending on the energy of the pulses used, the essential laser interaction process can primarily involve lattice vibrations, molecular rotations, or a combination of excited states produced by laser heating. While some of these techniques are currently confined to sensing at close ranges, others can be implemented for remote spectroscopic sensing owing principally to the laser pulse duration. We present a review of ultrafast laser-based spectroscopy techniques and discuss the use of these techniques to current and potential chemical and environmental sensing applications

Ausbreitung von Diffusionswellen in duennen Filmen und ungeordneten Materialien

Available from TIB Hannover: DW 3152 / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Time Resolved Infrared Radiometry for Subsurface Interface Imaging

While most thermal NDE methods concentrate on defect detection and imaging, time-resolved infrared radiometry (TRIR) [1,2,3] has also been used successfully to determine material parameters such as thermal diffusivity and thickness. This has allowed information about material structure such as presence of corrosion, porosity or voids to be obtained. In the TRIR technique, the surface temperature of a sample is monitored via infrared emission during application of a heating pulse from an optical, microwave or induction source. Smaller heating intensities result with this technique as compared to the more common short pulse methods [4,5]. Furthermore, the early time behavior in the temperature-time response allows a self calibration to be performed for each pixel in the image, thus removing emissivity and heating beam intensity variations. The development of infrared focalplane arrays with their full field imaging capabilities at high speeds and current image processing equipment allows the TRIR algorithms which have so far have been used only on single point measurements to be applied to full images.</p

Microwave induced time-resolved infrared radiometry for subsurface defect detection and characterization

The use of microwaves as a heating source in time-resolved infrared radiometry provides the ability to heat surface and subsurface microwave-absorbing regions of a specimen directly. This can improve the contrast and spatial resolution of such regions and enhance their detectability when compared with conventional laser or flash lamp sources. Applications such as subsurface water detection or the detection of carbon fiber contaminates in epoxy composites are investigated experimentally and analytically

Analysis of Thermal Stressing Tehcniques for Flaw Detection with Shearography

Electronic shearographic interferometry [1,2] and time-resolved infrared radiometry (TRIR) [3,4] are both nondestructive evaluation (NDE) techniques that have been shown to be sensitive to subsurface and surface breaking defects. With TRIR, the location and type of defect can be determined by measuring the development of the surface temperature of an object during heating. Electronic shearography is a full-field optical technique that is sensitive to changes in out-of-plane displacement derivatives of a deforming object. The method is based on the evolution of a speckle fringe pattern formed by laser light scattered off the object surface. Various stressing methods have been employed to produce characteristic deformations which may be monitored shearographically [1]. Most of these techniques including vibration, pressure, mechanical, and thermal loading produce wide-field stressing of the test object. Controlled heating with a laser source described in this work provides several advantages for flaw detection. This noncontact, localized stressing method allows defect information to be obtained while heating. In addition, the beam profile can be tailored to aid in the detection of different defect types. This paper presents results of simultaneous observations of material response to an applied thermal load using both TRIR and shearographic detection methods. Of particular importance is the demonstration that the depth of a defect can be determined accurately by measuring the time-dependence of the shearographic fringe development during heating.</p

Characterization of back surface morphology for corrosion detection using patterned heat sources

Characterization of back surface roughness is investigated for specimens exhibiting corrosion and for prepared samples with milled channels of varying geometry. An area heating source is used initially to provide one-dimensional heating of the specimen which allows plate thinning, disbonding, or presence of corrosion to be rapidly detected. A focused heating source is then used to characterize the suspect regions through the interaction of lateral heat flow with back surface roughness