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Файл:A Brief Description оf NDT Techniques. Учебное пособие по английскому языку
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test piece surface the magnetic particles will run off at defect free areas.
In places of leakage fields the magnetic particles are attracted and clustered together, thus, indicating the place of the defect.
The method works best using a pigment which covers the magnetic
particles and shows fluorescence under ultraviolet light. Even cracks can
be detected that are not open to the surface but just below it, as long as
they are not too far below the surface. But one precondition must always
be fulfilled. The magnetic field lines must be perpendicular or at least at
a certain angle to the defect. Otherwise, there will be no leakage field and
the defects remain undetected.
Our test sample has been magnetized in horizontal direction. The magnetic field lines run horizontally. This is the reason why cracks in vertical
Direction are visible particularly well. To make sure to detect all near
surface cracks one has to magnetize the sample once more and best perpendicularly to the first time. To do this the material tester switches the
machine over to the current flow method. A high electric current flows
through the sample generating a circular magnetic field. Now the cracks
in horizontal Direction can be seen very well. The tested sample is a hardened sliding guide. The cracks have been formed during the hardening
process and the subsequent grinding operation.
Still another test piece has to face the magnetic particle inspection a
section of a large gear wheel. The same procedure again. Placing the gear
wheel between the magnetic pole pieces magnetizing and applying magnetic particle suspension. Already in jail it there are indications of
cracks and again they are best visible under ultraviolet light while darkening the daylight. The visible cracks are so-called fatigue cracks. They
have been generated during the operation of the gear drive due to a mismatch of the gear wheels.
LESSON V
Dye Penetrant Testing
https://www.youtube.com/watch?v=xEK-c1pkTUI&t=36s
Dye penetrant examination is an important method of non-destructive
testing of materials. This is the basic principle. Often the surface of a
workpiece is covered with rust or paint which masks the material defects
as shown in the sectional view. Therefore, the first step is always the precleaning of the test piece surface. This must be done very thoroughly so
that any defects present here it is a crack are open to the surface.
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In the second step that penetrant is applied to the test piece usually a
red colored low viscosity oil. Due to the penetrance, high surface wetting
capability it penetrates into the defects and cracks. After several minutes
of dwell time, the third step is to rinse the path surface cautiously with
water. The water removes the penetrance completely from the surface but
leaves it in the crack.
Now the fourth step after drying the test piece a developer supplied on
to the surface. The developer is a fine-grained white powder suspended
in a liquid it forms an even coating on the surface. After drying, it draws
penetrant from the crack out onto the surface. The location of the crack is
indicated clearly on the surface. And this is how it is carried out in practice.
We wish to inspect two samples a segment of a worm gear shaft and a
broken bicycle crank arm. Both test pieces have already been cleaned.
First in line is the worm gear shaft. The material tester applies the penetrant. After a dwell time of about 10 minutes she removes the excess penetrant from the surface by rinsing the test piece gently with water, she
takes care not to rinse for an unnecessary long time because the penetrant
is supposed to stay in the defects. Now she dries the surface of the sample
with a cloth to absorb the excess water. As the final step, she applies the
developer and lets it dry. That's it! Several cracks are indicated on the
surface in deep red colour.
Next, the broken bicycle crank arm has to undergo the die penetration
test: spraying with penetrant, allowing to soak, rinsing, drying, and applying developer. Clearly additional cracks are indicated on the fragments.
Finally, the pros and cons at a glance.
Advantages of this test method almost all materials can be tested
and the method is simple and low-cost for a single inspection.
Disadvantages only suitable for surface defects, no information
about the depth of flaws can be gained, and rough surfaces are difficult to
test.
LESSON VI
NDT Ultrasonic Testing
https://www.youtube.com/watch?v=DgeQTw7q_xQ
It is possible to investigate whether there are discontinuities under the
surface using ultrasonic sound waves. Ultrasound has such a high frequency people cannot hear it. The vibrations can, however, be measured
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with sensitive measuring equipment. Using this storage vessel our weld
is checked for internal discontinuities.
The object being examined must be cleaned and free of any material
that may interfere with the exam. Simply said, it works as follows: sound
waves are transmitted into the material with the transducer and an echo is
sent back from the rear wall or discontinuity. By registering this Echo we
can detect the thickness of the material or if a discontinuity is present. In
case of a discontinuity sound is echoed back faster. Thus, we can see if
the material wall is thinning for instance, by corrosion.
The Examiner first applies a contact fluid to enable the ultrasonic filler
to make proper contact with the material. This bonding layer serves to
conduct the sound waves into the material. Without the contact fluid they
would immediately bounce back from the surface.
A transducer converts an electronic signal into ultrasonic sound
waves. They can both send and receive. The ultrasound equipment captures the reflected waves and converts this into a visual signal. This enables The Examiner to see whether there are any discontinuities. If there is
an indication the exact location can be determined using calculations.
LESSON VIII
Acoustic emission testing
https://www.youtube.com/watch?v=1aUOF787c3w
The infrastructure that supports our modern way of life is vulnerable to movements in the ground beneath our feet. We've developed a
novel approach for monitoring performance and safety of infrastructure
by listening to soil as it deforms.
Acoustic emission are high frequency noise that are generated by
soil grains as they deform against each other and when they interact with
pipelines, piled foundations, or by seepage and erosion in earth dams, and
by listening to acoustic emission we can detect problems and provide
early warning of failure.
Our know-how has been used by RST Instruments to develop the
world's first commercially available acoustic emission sensors for geotechnical applications, and they've called the sensors Geo Acoustic
Aware.
Acoustic emission monitoring has many benefits over existing
techniques: it's lower cost, it's easy to install and use and sensors can be
retrofitted to detect deterioration of existing infrastructure.
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The system can be used to monitor long lengths of buried infrastructure, for example, a series of sensors can be attached to a pipe and
the acoustic emission monitoring used to detect areas of deformation
which can provide information on failure.
Our acoustic emission monitoring expertise has been used to design an award-winning landslide early warning system. Community
Slope Safe helps to protect vulnerable people living on slopes. Continuous monitoring of these slopes can provide a real-time early warning of
landslides. Trials totally 130 years of slope monitoring in several countries has proven that the system works.
LESSON IX
Thermography
https://www.youtube.com/watch?v=OWkMUtjSuKE
Most bridges are inspected once every two years. Today, Eric Evens
from the MnDOT Bridge Office is showing me a new inspection method
using an infrared camera.
A common problem for bridges is that concrete can develop subsurface cracking known as delamination. It's still safe to drive on, but delaminations can spall out and create potholes on the bridge deck.
The most common inspection method for locating delaminated areas
is chain-dragging.
Here's a solid piece of deck. Now you hear the difference?
Oh, yeah.
That's delaminated, yeah.
It's easy to hear areas that sound hollow and outline them with
paint. But chain-dragging is not very practical where there is a lot of traffic noise, and inspectors sometimes have to work dangerously close to
moving traffic. The infrared camera solves some of those problems.
We go along, and we're looking for areas that show, like in this
case you see that white. Not the white there, because that's what's being
picked up by this difference in color. But from more like that white
straight out there that you're seeing. There, there's that patch (заплатка),
stuff like that. That's what we're looking for. We constantly adjust
(выравнивать) this temperature range.
That gives you enough contrast that, whatever the temperature is.
Yeah. See, right here is what I saw, Bruce.
That's where you saw the white spots, the temperature difference.
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Yeah. Look at...
Oh, yeah.
So that's what we're looking for. And there's one over there. How
we envision using this is to go along like this from the shoulder of the
road like this, and we would just scan across a bridge deck like this and
then mark those spots. I can take pictures of this, like this right here. I'll
show you how that works. Like let's say we sit back and we take a picture
of this. It gives you two pictures: it gives you the thermogram, and it also
gives you a digital picture of it.
The technology is fairly simplistic, I think. You know, and these cameras are easy to run. There's very few things that, you know, you have to
adjust. And what you have to adjust is fairly easy.
So one of the applications would be to estimate how much of a
bridge deck needs to be repaired.
That's about the main application I see this used. We could come
along here, walk along the shoulder of a bridge like this, spot out these
spots, take pictures of them. There's two things you could do: you could
estimate the total delaminated area; or along with that, you could also take
pictures of what you're seeing. And say several years later you come back,
and you compare if the delaminations are getting bigger from your records of what you had. And that would be another powerful tool for this I think, anyway.
So then you could try to program when your bridge crew should come
out here and either fix these spots or you mill and overlay this deck and
carry on from there.
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Список литературы
1. Mark Willcox, George Downes A Brief Description of NDT
Techniques.
2. Iowa State University Center for Nondestructive Evaluation –
NDE-Ed.org. https://www.nde-ed.org/NDETechniques/EddyCurrent/ProbesCoilDesign/ProbesModeOp.xhtml.
3. Research and technology organization TWI https://www.twi-
global.com/.
4. British Council https://learnenglish.britishcoun-
cil.org/skills/speaking/b1-speaking/agreeing-and-disagreeing.
5. WWW.SGS.COM/NDT.
6. AZoM, the leading online publication for the Materials Science
community. [Официальный сайт] https://www.azom.com/article.aspx?ArticleID=8021.
7. Microwave Testing (μT): An Overview. Johann Hinken, FI Test-
und Messtechnik GmbH. Magdeburg, Germany, July 2016. eJournal of Nondestructive Testing (NDT) ISSN 1435-4934
(NDT.net Journal).
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