Sunday Observer Online


Sunday, 11 July 2010





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Technical diagnostics for a safer journey

ďThe whole world steps aside for the man who knows where he is going.Ē

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.

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. Itís 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 engineersí 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 itís almost like searching a blind alley. Patience is essential. Itís a kind of a meditation. Aircraft and bridges donít fall and ships donít 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. Itís 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.

Itís 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 ďblack boxĒ is a fire/shock/water proof ďorange box or a ballĒ with a recording device inside. It is usually situated in the tail of an aircraft.

NDT methods

Letís 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.

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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