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Unformatted text preview: University at Buffalo Department of Mechanical and Aerospace Engineering MAE 589 Prof. D.D.L. Chung Diffraction, Microscopy and Spectroscopy Techniques 1. Introduction The most basic function of a smart structure is sensing. The sensing of damage is particularly important, since damage is a hazard. Damage sensing is akin to nondestructive evaluation (NDE) or nondestructive testing (NDT), which refers to the evaluation or testing of a component without harming the component, so as to assess the quality (during or after manufacture) or condition (during or after use) of the component [1-6]. Assessment during manufacture enables adjustment of the manufacturing conditions of either the particular component being manufactured or similar components to be manufactured, so as to improve the quality of the product; this assessment relates to smart manufacturing and quality control. Assessment during use enables the monitoring of damage induced by the use (whether damage due to fatigue, heat, corrosion, etc.), so as to mitigate hazards; this assessment relates to smart structures and structural health monitoring. These notes describe sensing methods that involve materials or instrumentation that are external to the structure. These methods are commonly used for inspection because they are non-intrusive and widely applicable, though they are limited to the sensing of damage such as cracks. Strain, residual stress and subtle damage cannot be sensed. Moreover, they may not be amenable to sensing in real time. 2. Liquid penetrant inspection Liquid penetrant inspection is for detecting surface defects. It involves applying a penetrant (a liquid that wets) to the surface to be inspected by dipping, spraying or brushing. The penetrant is then pulled into the surface crack by capillary action (Fig. 1(b)). After allowing sufficient time for the penetrant to be drawn into the surface cracks, the excess penetrant is removed (Fig. 1(c)). After this, a developer (an absorbent material capable of drawing the penetrant from the cracks back to the surface) is applied, so that some penetrant is extracted to the surface (Fig. 1(d)) and visual inspection is possible. In order to make the penetrant more visible, brightly colored dyes or fluorescent materials are often added to the penetrant. Moreover, the developer is usually chosen to provide a contrasting background. After inspection, the developer and remaining penetrant are removed by cleaning. 3. Ultrasonic inspection An ultrasonic wave has higher frequency than audible sound. The frequency typically ranges from 25 to 100,000 MHz. A higher frequency means a shorter wavelength, which allows smaller defects to be detected. Ultrasonic inspection [7-15] involves sending an ultrasonic wave emitted by a pulsed oscillator and transducer (a piezoelectric actuator that changes electrical energy to mechanical vibration) through the material to be inspected and measuring the intensity of the reflected wave Fig. 1 Liquid penetrant inspection. Fig....
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This note was uploaded on 07/09/2011 for the course MAE 438 taught by Professor Chung during the Spring '09 term at SUNY Buffalo.
- Spring '09