Technical diagnostics for a safer journey
by Sahan Hasaranga Wasala
“The whole world steps aside for the man who knows where he is
going.”
Thanks to President Mahinda Rajapaksa, we are witnessing a fast
growth of our economy. Simultaneously we have many investors coming into
our peaceful motherland and starting huge development projects.
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Vibration analysis of
an aircraft gas turbine engine using laser |
Hambantota Harbour and other ports are almost completed. Mattala
Airport, highways, railways and new bridges are being built in every
corner. This means that our transport system is improving. Obviously
there will be many jobs in different fields. We watch this change with a
great pride.
One of the most important properties of a transport system is safety.
It’s said that the human factor is the main reason for many transport
accidents, from aircraft to bicycle.
That means there is more chance of an accident occurring because of a
simple mistake of the operator (driver or a pilot), or because of the
engineer who maintains or repairs the vehicle, or the other people who
involved such as the air traffic controller or the policeman on roads.
This article is focused on the engineering side, more precisely about
identifying problems in mechanical part or a structure. What do
engineers do to minimise the probability of having a failure?
Technical diagnostics, we all have heard this word very often in
medicine. Over there, diagnostics means, identification of the nature of
an illness or other problem by examination of the symptoms. The Buddha
tried to found the reason for sadness before cure, doctors search for
reason of an illness before the cure. The engineers do check and
evaluate machine parts before thinking of a way to remedy it.
Type of Engineer
There are a certain type of engineers who make their living only by
inspecting structures or machines. They are usually called NDT or NDE
engineers. NDT stands for Non Destructive Testing (NDE-Non Destructive
evaluation). NDT is the branch of engineering concerned with all methods
of detecting and evaluating material defects.
A small flaw can grow into a huge crack due to time because of uneven
loads. Flaws above the limit can lead to loss of strength, thereafter to
a major structural failure. (For example, the aircraft wing can have
different loads in different flying conditions. Or wheels can have
different loads because of the road conditions and speed variations).
Therefore, the NDT is important in guaranteeing safety of operation
as well as in quality control.
The engineers’ task is to identify those flaws with specific
equipment. Thus it is very important for engineers to have a wide range
of knowledge about what they should look for.
Sometimes it’s almost like searching a blind alley. Patience is
essential. It’s a kind of a meditation. Aircraft and bridges don’t fall
and ships don’t sink because of these engineers. They are the heroes
behind the scene.
This is one of the most important parts of engineering which is
closely related to aerospace engineering, civil engineering, electrical
engineering, material science and engineering, mechanical engineering,
nuclear engineering, petroleum engineering the physics. It’s often used
in the constructional and service sides.
We would need many talented engineers who can offer their services to
ships and aircraft which are about to touch our motherland.
Technical diagnostics
There are several functions in technical diagnostics. The first is to
determine the present condition using NDT methods.
As the second function, the engineer finds reasons for defects with
the help of his education and knowledge.
The third is the prognosis. As in the weather forecasting, engineers
use statistical data to determine the life or machines and possible
future failures. For example, vibration of an aircraft engine can be
very informative.
In my first research in this field, I was quite surprised to see that
the vibration of a whole engine can represent aerodynamical information
of a single blade inside.
Therefore, by watching statistical data of vibration, the engineer
determine internal problems and can give accurate predictions about the
lifetime of an engine.
The final function of technical diagnostics is genesis. These are
mostly about crashes or an accident. I was lucky to listen to some
lectures of an aircraft crash investigator. He travels around the world
where aircraft catastrophes took place.
It’s the worst job when one sees many dead bodies all around,
although, at the end of the day someone has do this. They usually take
evidence of all aspects. The most famous source of evidence is the black
box (The flight data recorder.) In fact so called “black box” is a
fire/shock/water proof “orange box or a ball” with a recording device
inside. It is usually situated in the tail of an aircraft.
NDT methods
Let’s go back to the first function. What methods are used to detect
the present condition of a machine or machine part? Generally they are
non destructive testing (NDT) methods and functional methods. NDT
methods are the most popular ones to find defects.
They are widely used in the aircraft industry, ships, vehicles,
railway, oil, gas, pipes, reservoirs, bridges, and almost in all kinds
construction to achieve a high quality and safety. Some of our
transportation services had frequent failures, possibly because they
paid less attention to NDT.
When objects are tested according to a plan, there is a much less
chance of having an unexpected failure, accidents and catastrophic
situations which can cost many lives and billions of rupees.
The most common NDT methods are Visual inspection, Liquid penetrant
test, Eddy current test, Magnetic particle test, methods which use
different forms of sound waves (Acoustic methods) and X-ray.
These methods are used to look inside or the surface of a material
part for invisible cracks with the naked eye. Visual examiners simply
look at a part to see if surface imperfections are visible. He can use
the help of a magnifying glass or computer controlled camera (video
baroscope).
Baroscope are frequently used to check inside of an engine without
opening it. In a liquid penetrant test, the Engineer evaluates defects
on the material surface with help of special liquids or spray. Eddy
Current methods are carried out with equipment which are based on
electromagnetic induction to detect flaws on conductive materials.
Another use of the magnetism is magnetic particle crack detection.
This is used to evaluate defects on ferromagnetic materials (a material
which can be converted into a magnet) by applying a magnetic field and
pouring magnetic particles on its surface. Particles gather in those
places which have flaws.
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20 ultra sonic non
destructive inspection at a aircraft manufacturing factory. |
In acoustics methods the Engineer uses different forms of acoustic
waves such as sound, ultrasound to evaluate defects. This is very
similar to sonar of submarines, bats or dolphins. The ultrasonic
sonogram is also used in medicine to define the gender of a baby before
birth.
X-Ray uses electromagnetic radiation to detect defects as same as in
medicine. This is one of the most trusted ways of controlling an
objects, thus dangerous towards health.
The other types of methods can be stated as functional. They consist
of evaluating statistical data for unusual trends.
Long term and short term statistical data such as, the temperature of
the part or machine. Pressure of the hydraulic systems, fuel, gas, air
consumption or vibration, are usually gathered for evaluation.
By analysing such parameters an engineer can predict the economical
use of the part or machine and prognosis for future failures, so that it
will be possible to remove it and put a new part or totally stop using
the machine to avoid catastrophes.
Foreign particles
Another way is the spectral analysis of oil to detect foreign
particles (for example most of the engine has an oil system.
Small foreign particles (metals such as iron or titanium) can be
found in an oil system if a ball bearing is destroyed. This might lead
to a failure of the whole machine). This is done by burning oil at a
certain temperature. The light spectrum shows unusual elements.
Scientists use a similar method to find elements of stars.
This part of the science spreads everywhere in our day to day life.
As the NDT engineer should have a wider range of knowledge from
materials to mechanism of the machines, diagnostics is the heart of
engineering to provide a high quality service with economical
efficiency, reliability and safety.
It has to maintain the momentum continuously and stably for the good
of all citizens. Let us be a part of the high quality transport system
which we would have in the near future.
The writer is an MEng (Honors) St. Petersburg
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