Post on 03-Jun-2018
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Measurementand Testing
Ultrasonic
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Is the railroad still safe?
Could a small crack grow?
Has the pipeline been checked?
Wherever human or environmental safety is a risk, components are measured and checked ultrasonically. The method is similar
to echo-sounding: Sound pulses of high-frequency are sent through the material and reflected back from edges, cracks or defects.
Sound speed:
in air 330 m/s
in water 1500 m/s
in steel ~6000 m/s
The picture shows measurement of the thickness of a plastic tube wall. The transit time of a
short ultrasonic pulse is measured. Combined with the information about the speed of sound
in the material the wall thickness can be easily calculated.
Ultrasonic Measurement and Testing
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Ultrasound is sound of such high frequencymore than 20 kHz, which means more than20000 oscillations per second - that it is no longer audible.
Technically, ultrasound is produced by means of a vibrating crystal in the
ultrasonic sensor, also called probe or transducer. Using the piezo-electric
effect, electric impulses are converted into mechanical waves - in this case
sound waves. The ultrasonic test equipment used for crack detection is
called ultrasonic flaw detector,normally generates ultrasound with a frequency of several MHz.
In many cases, so-called longitudinal waves are used for crack
detection. They form a pull-push sequence, causing rapid expansion
and compression of the material as the waves pass through it.
Because air is a poor conductor of ultrasonic waves, a film of water or oil is
spread on the material to create direct contact with the ultrasonic sensor.
In the picture this film appears in green. The fluid is called coupling medium.
Ultrasound
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The sensoremits a short ultrasonic pulse which passes through the component being tested and is reflected
from the far end. The sensor acts first as a transmitterand then as a receiver.
A test instrument not only produces the electrical impulses to set the crystal in motion, but also possesses a
monitor screen that displays the ultrasonic signals as they travel through the material.
The ultrasonic pulses are periodically produced. The pulse repitition frequency is so high (around 1 kHz) that
the operator sees a static image.
As the pulse returns from the material, the sensor works as a receiver. The initial pulse and the
reflection from the back-wall are clearly seen on the screen.
The total travel time corresponds to twice the length of the piece.
transmitter andreceiver
This elegantly simple principle is called the
Impuls-echo-method.
Ultrasonic Test Equipment
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This particular type of measurement does
not really require a monitor screen:
all it needs is a digital display to show the reading.
Ultrasonic measurement of the pipe wall thickness in
petrochemical plants not only provides safety, but also saves
time and costs. For instance, pipes carrying hot or agressive
chemicals must be regularly checked for internal corrosion.
A shut-down of the production would call high costs.
Nevertheless ultrasound can be applied since only one side
of the pipe must be accessible.
Wall thickness measurement
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no defect detected
defectdefect
Test
material
initialpulse (IP) back-wallecho (BWE)
defect without BWE
defect without BWE
different
defect depths
defect with BWE
defect with BWE
different
defect depths
different
defect sizes
Ultrasonic reflections from internal cracks and pores arrive earlier than reflections from the back wall of the
component. This phenomenon has been used for the detection and subsequent evaluation of internal defects.
The illustration shows various types of defects and how they are displayed on the screen:
Checking for internal defects
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Weld inspection
initia
lpulse
notv
isible
lack of fusion
crack
Checking welds for defects is a
demanding task for the workers.
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Automatic testing of pipes, bars and rails
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60 Years KARL DEUTSCH
Definitely Tested !
Presented by Instruments, Sensors & Systems for NDT, Germany.
Authors: Dr.(USA) WolframA. Karl Deutsch c/o KARL DEUTSCHProf.Dr.Helmut Richter c/o University of Wuppertal
Websites: www.karldeutsch.de and www.wissensfloater.de