Re: Ultrasonic tests during motion Dr. Pau -
Ultrasonics is usually done with either the part or transducer in motion, no practical difference between the cases. Motion during ultrasonics has an obvious potential effect on the test results because the target will move during the sound travel into the part and back out and also because the "sample" taken may not be close enough together to find a small flaw.
The first effect is straight forward since we know the time of flight and the range of thicknesses we need to consider. A 100 mm thick part will require a time of flight of 31 microseconds for longitudinal wave. To determine the speed we can move the part we must make some assumption about how much we can let the part move while the sound is traveling through it. Generally we would not want to have the part move more than one tenth of a wavelength although this is only a rule of thumb and may not apply in many cases. At two megahertz this is 325 microns. At a travel time of 100 microseconds (we may want to see more than one echo) that is a speed of 3.250 meters per second. For many applications this requirement is easily met. The only exception I can think of is tube testing where a 100 mm diameter tube would exceed this speed on the outside at 700 rpm. Although this is fast some systems can go that fast. A similar procedure can be used at other frequencies and times of flight.
If you are measuring the wall thickness then both of these effects also apply but the sampling issue is governed by the rate at which the part thickness can vary. Examples abound of under thickness tubing parts escaping detection because of insufficient sampling. This is more of a problem on wall thickness units because they often do not use one pulse to make a measurement or take a relatively long time to make a measurement. This reduces the pulse repetition frequency (PRF) sometimes to only 200 or 300 pps. This means a 10 mm diameter tube spun at 3,000 rpm will only be sampled 4 times per revolution. It is unlikely that the 4 will reliably adequate define the wall thickness. The pitch will moderate this somewhat but it has to be kept in mind that the two rates are going to describe a spiral which may or may not improve the problem.
The second effect is due not to the time of flight but to the PRF. Every time the transducer is pulsed the system takes a sample of the moving part. These samples are space v*t apart where v is the speed of the part motion and t is the time between pulses (1 over the PRF). We want this interval either smaller than the smallest flaw or smaller than the beam diameter. Some codes regulate beam overlap but that applies to the pitch of the scan rather than the time samples. ASME and some other codes limit scanning speed to 152 mm/second or less to address this issue. If we use a focused transducer we would probably not want to take sample less than the diameter of the beam. This is unlikely to be less than 1 mm. The maximum PRF various with the instrument used but the usual maximum is 10,000 pps. This corresponds to a maximum speed of 10 meters per second. Using out 100 mm tube we would have to spin it 1900 rpm to exceed this speed. This is unlikely in most tubing systems.
If the part is moving away and toward the transducer rather than transverse then the frequency can be doppler shifted due to the motion. The shift in wavelength is proportional to the ratio of the speed of the part to the sound speed. In water sound speed is 1500 meters per second so the part must be moving at 150 meters per second to change the frequency 10%. The shifted frequency is the source frequency divided by the sum of the source velocity & the speed of sound. Rotating machinery and some kinds of machines can achieve this speed, piezoelectrics for example, but these are not usually insonified. Whether a change in frequency is important to the measurement will also depend strongly on the nature of the instrumentation used.
Robert (Rocky) A. Day
904 Cortland Avenue
San Francisco, CA 94110 - 5633
Fax: (415) 641-5502
: I would perform some ultrasonic
: pulse-echo test over mechanical
: parts in movement, so I'm interested in experiences
: about this topic (e.g. what kind of probes are used?
: what kind of techniques??)