Ultrasonic inspection refers to some sort of nondestructive screening technique which inspects any work piece and resources by ultrasonic and the help of an ultrasonic designed detector. When these kinds of ultrasonic waves which are inside the materials to be inspected meet the defects, a part of the waves will mirror, then this receptor analyzes all the reflection waves thus finding out the present defects. All the present defects are detected precisely.
Pulse echo is more sensitive to transducer alignment than through transmission. Since there is a large impedance mismatch between air and a solid interface, ultrasound does not propagate well through air; therefore, a couplant is used to more effectively transmit the sound from the transducer to the part. For hand inspection, glycerin compounds are frequently used while all automated systems use water.
The measurement and depth of the defects can also be determined. In addition, its screening pipe, the sole dependable program applied in the industry, is light in weights and thus transportable, and protected to function. This makes it easy to carry out the process in an automatic manner.
Flaws are detectable since they alter the amount of sound returned to the receiver. The test equipment conducts inspection in the frequency range of 1 to 30 MHz, although most composite material inspection is usually tested at 1 to 5 megahertz. High frequencies are more sensitive to small defects, while low frequencies or longer wavelengths can penetrate to greater depths.
There are also special units for cylindrical parts that contain turntables that rotate during the scanning operation. The output from these automated units is displayed as a C-scan, which is a planar map of the part, where light (white) areas indicate less sound attenuation and are of higher quality than darker areas (gray to black) that indicate more sound attenuation and are of lower quality. The darker the area, the more severe sound attenuation is and the poorer the quality of the part.
The transducers are placed close to the part surface (within an inch) and frequencies of 50 kHz to 5 MHz are employed. A relatively new inspection technology is laser ultrasonics. It provides essentially the same information as conventional inspection except that it is faster than conventional methods, especially for highly contoured parts. Two lasers are used. The first laser, generally a carbon dioxide laser, generates ultrasound in the part by causing thermoelastic expansion, while the second laser, normally a neodymium: yttrium-aluminum garnet laser, detects the sound signal as it returns to the top surface.
Laser heating at the surface causes a temperature increase and a resultant local expansion of the material. If the laser pulses are short (10-100 ns), the expansion will create a wave in the 1-10 MHz range. The receiving laser detects light scattered off the surface that is analyzed by a Fabry-Perot interferometer to extract the its signal. In this process, it is important to generate as much ultrasound as possible without causing heat damage to the composite surface.
Baselines and thresholds are determined by conducting effects-of-defects test programs in which known good laminates are compared with laminates of varying porosity levels. Both photomicrographs and mechanical property testing is used to establish the threshold levels. Part zoning can also be used to reduce cost. Areas that are highly stressed would be zoned to lower threshold values than non-critical lower stressed areas. This is enough to show that ultrasonic inspection is there to stay.
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