Ultrasonic NDE for Surface Roughness
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ary or rapidly moving, flat or curved, surfaces in process and in (essentially) real time.4 Ultrasonic Technique The ultrasonic technique for monitoring surface roughness uses a piezoelectric transducer which both transmits and receives ultrasound. The transducer is located inside a nozzle which produces a laminar flow of fluid as shown in Figure 1. When the transducer is pulsed with rf, an ultrasonic burst travels down the fluid stream and interacts with the surface of interest. A portion of the energy travels back up the stream for conversion to an electrical pulse. To make the sensor more remote from the surface, a tubular extension can be attached to the nozzle orifice to guide the fluid and ultrasound along a curved path to the surface.4
The technique's basic concept is that the backscattered energy received by the transducer decreases as the surface roughness increases. (The energy also decreases significantly when interacting with discrete surface irregularities such as scratches or dents.) Precise transittime measurements of the ultrasonic pulses to and from the surface can be used to measure the surface-transducer separation and thus the surface contour or profile. The received signal is also routed to a sample-and-hold peak detector whose analog output can be converted to digital data for microprocessing. Microprocessing provides at least three analyses: the average amplitude of a preselected number of pulse echoes, their standard deviation, and the number of outlier echo values. Outlier echo amplitudes, due to extraneous factors such as air bubbles or flying chips in the coupling stream, would be discarded based on average amplitude values and standard deviation of the set. In water-based liquids, ultrasonic frequencies from 5-30 MHz were used, which corresponds to a respective wavelength range of about 300-50 /im. (Air coupling frequencies from 1-5 MHz were also used11 but this requires a dry environment.) 5 Several types of water-based coolant/lubricant toolcutting fluids were tested and compared to water. The signal amplitude using the coolants was reduced, for example, by about 20% at 15 MHz for one coolant, but coupling efficiency was not significantly affected and signal strength was still more than adequate.
Signal Processor
Scope Monitor
Pulser/Receiver and Gate
Liquid Reservoir •
Nozzle Housing
Transducer -
Stream — Coupling
i
Recording Systems
I On-Line \ I Application/
Control Signal T
l I
• Pulse-Echo
Robot etc.
Sample Figure 1. Experimental pulse-echo ultrasonic system for monitoring and recording elastic waves scattered from rough surfaces.
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Ultrasonic NOE for Surface Roughness A laboratory squirter system, a rotator, and cylindrical parts were used for much of the development work. The stream and incident ultrasound can be directed normal to the surface for monitoring, or at an angle for special applications. Only normal incidence is disc u s s e d h e r e . Also, u n d e r t h e s e conditions, the surface sample spot is the area of the stream, nominally 5 mm diameter
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