We use an infrared thermal imaging system in combination with a fluorescence microscope to map the dynamics
of the local surface temperature and fluorescence intensity under cw, UV excitation of laser-modified fused silica
within a damage site. Based on a thermal diffusion model, we estimate the energy deposited via linear absorption
mechanisms and derive the linear absorption coefficient of the modified material. The results indicate that the
damage growth mechanism is not entirely based on linear absorption. Specifically, the absorption cross-section
derived above would prove insuffcient to cause a significant increase in the temperature of the modified material
under nanosecond, pulsed excitation (via linear absorption at ICF laser fluences). In addition, irreversible changes
in the absorption cross-section following extended cw, UV laser exposure were observed.
Current efforts at Lawrence Livermore National Laboratory in the area of vibrothermography (VibroIR or SonicIR) are presented. The primary goals of the efforts of the NDE group at LLNL have been to demonstrate the applicability of vibrothermography to new areas, to examine the degree to which VibroIR may replace existing NDE inspection procedures, and to conduct research on the underlying processes and optimal parameters in its implementation. We report three new applications of VibroIR, in the areas of brazed tube joint inspection, evaluationtion of thick multilayer carbon/carbon composites as used in the NASA Shuttle, and the inspection of soft composite materials. The goal of the brazed joint inspection process is ultimately the replacement of a current dye penetrant inspection procedure. Therefore a direct comparison between VibroIR and dye penetrant inspection is made. Preliminary results of the analysis of a leading edge panel from a NASA Shuttle is also reported as an example of the application of VibroIR to thick composites. Finally, a
comparison betweeen the effectiveness of VibroIR versus a spectrum of other NDE techniques (ultrasonic imaging, radiographic tomography) for the imaging of known ceramic defects is briefly discussed.
Work is presented which allows flaw characteristics to be quantified from the transient IR NDE signature. The goal of this effort was to accurately determine the type, size and depth of flaws revealed with IR NDE, using sonic IR as the example IR NDE technique. Typically an IR NDE experiment will result in a positive qualitative indication of a flaw such as a cold or hot spot in the image, but will not provide quantitative data thereby leaving the practitioner to make educated guesses as to the source of the signal. The technique presented here relies on comparing the transient IR signature to exact heat transfer analytical results for prototypical flaws, using the flaw characteristics as unknown fitting parameters. A nonlinear least squares algorithm is used to evaluate the fitting parameters, which then provide a direct measure of the flaw characteristics that can be mapped to the imaged surface for visual reference. The method uses temperature data for the heat transfer analysis, so radiometric calibration of the IR signal is required. The method provides quantitative data with a single thermal event (e.g. acoustic pulse or flash), as compared to phase-lock techniques that require many events. The work has been tested with numerical data but remains to be validated by experimental data, and that effort is underway. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract W-7405-Eng-48.
Work is presented which begins to define the envelope of applicability for the sonic IR NDE technique. Detection limits define the faintest flaw signal that can be perceived, which is a function of flaw size and depth, excitation strength and duration, and the detector limits (spatial, temporal, thermal). A unique contribution of the present work is a model to predict the dynamic frictional heating of a crack, and this is combined with a transient heat transfer analysis to define the detection limits. Damage limits consider the risk of damage to a part from the application of the dynamic excitation. Experience has shown that the dynamic excitation can damage parts, notably for brittle materials such as ceramics with existing flaws. Since sonic IR is intended to be nondestructive it is important to test parts in a manner consistent with preserving the part integrity. The evaluation of damage limits assumes that additional part damage during testing is a fatigue process that propagates existing cracks. Paris' law for fatigue damage is employed to provide an estimate of fatigue crack propagation during the dynamic forcing. Both detection limits and damage limits are combined to create an envelope of applicability for sonic IR. Further experimental effort is required to tune and validate the analytical tools presented herein. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract W-7405-Eng-48.
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