Technical Diagnostics and NDT

Eddy current theory is based on electromagnetic induction. That means, a variable magnetic field can induce an electric current on a conductor which is separate to the source of the magnetic field, and also vice versa, which is an electric current generates a magnetic field. Similar theory is used for the magnetic particles inspection as well. Electromagnetic induction is used in many applications such as, Transformers, Electric motors, Generators, etc...   In the field of Non destructive testing (NDT), we manipulate with this theory which I mentioned above, and try to find surface defects or/and shallow internal defects of ferromagnetic materials.

Short research about the history of eddy current The first person to observe current eddies was François Arago (1786-1853), the 25 th president of France, Who was also a mathematician, physicist and astronomer.1824 He discovered what has been called rotatory magnetism, and the fact that most bodies could be magnetized; these discoveries were completed and explained by Michael Faraday (1791-1867). In 1834, Heinrich Lenz stated the principle that defines how the properties of the test objects are communicated back to the test system. Lenz's law states that the direction of current flow in the test object will be such that its magnetic field will oppose the magnetic field that caused the current flow in the test object. This means, in practice, the eddy currents communicate with the test coil by developing secondary flux that cancels a portion of the coil's flux equivalent to the magnitude and phase of the flux developed by the eddy currents.   Léon Foucault (1819-1868) However,

Global leader in digital imaging to exhibit new offerings for NDT at ASNT conference, booth 725, in Houston   WALTHAM, Mass., Nov 15, 2010 (BUSINESS WIRE) -- PerkinElmer, Inc., a global leader focused on the health and safety of people and the environment, today announced it will launch two new Digital X-Ray Flat Panel Detectors (FPDs) for Non Destructive Testing (NDT) applications at the ASNT Fall Conference and Quality Testing Show in Houston, Texas, from November 15-17 at booth 725.

Computed Tomography (CT) has come a long way since its public inception in 1972. The rapid improvement of CT and the increasing capabilities of CT scans have definitely gone hand in hand. CT scans that used to take hours, now take seconds. This increase in capabilities has led to the use of CT scans more often and in more ways than ever before. The use of CT in the industrial nondestructive testing (NDT) field is one segment that has grown tremendously in the past few years. Industrial CT uses a series of 2D images taken at specific intervals around the entire sample. Any type of industrial CT system uses three principal components: an X-ray tube, an X-ray detector, and a rotational stage. Everything is enclosed in a radiation,

Olympus, a world leader in nondestructive testing technologies and pioneer in industrial phased array ultrasound, is pleased to introduce a comprehensive inspection solution for circumferential welds in pipes with a diameter as small as 21.3 mm OD. A key component of the solution is the COBRA, a new manual scanner specifically designed to

From my article "Technical diagnostics for a safer journey" for the journal Spectrum 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?

WHEN they were built, no one thought they would fly for so long. But fitted with new engines and avionics, aircraft can be kept going for a very long time. The average age of the world’s airliners is more than ten years, with some passenger jets 25 years old or more. Military planes are more geriatric: the Sikorsky Black Hawk helicopter entered service 31 years ago and the Lockheed C-5 Galaxy 40 years back. Both are still going strong. Some Boeing KC-135 aerial-refuelling planes, which are based on the venerable 707, have been flying for over 50 years. Engineers reckon they could still be in the air when they are 80.  Figure 1: C-5 Galexy One thing that does ground old aircraft is the impending failure of their aluminium structure from metal fatigue. This begins in parts that are subjected to repeated strains, such as where the wings join the fuselage. Constant flexing of the structure concentrates stress, which leads to microscopic cracks. These cracks become more numerous and eventua

There are several steps in a magnetic particle inspection. Clean the surface of the ferromagnetic material Magnetize the material Spray or pour the magnetic particles   Visual inspection   Demagnetization   Cleaning Demagnetization is done to disorganize the micro atomic arrangement which produce a magnetic field. After demagnetization the material will act as a normal metal. The magnetized components can cause problems such as,

One of the most important things an inspector should do while MPI is to check for defects at least twice in separate 90 degree angles. Let’s understand this by using a small example.  I have a piece of steel, where there suppose to be a defect. Let’s assume that there is a surface crack, which is quite straight and continues parallel to one of the edges. Then we apply a magnetic field as in the picture below. We clearly see that, no change happens to the magnetic flux.   Accordingly there is no change happens to the magnetic particles we are adding. Therefore the crack will be invisible via magnetic particles method.

First, lets check out how this magnetic particles inspection carries out :). It's always easy to understand something practical by watching a youtube video. Theory can be found in the last article . Another video with a more advanced MPI This one is in Spanish, though the MPI shown very well, and the crack is well visible. MPI usually find cracks which are perpendicular to the direction of the flux, so It's necessary to search for defects twice. I'll explain more about that in a future article, but first it's important to see and understand the basic chemistry of this method.

Magnetic particle test is one of the common NDT methods. As you can see, this has to do something with magnets. For a starter, it is better to remember little bit about magnets before understanding this method. Magnet is a material produces a magnetic field. They have two poles which are known as North, and South.   Opposite poles (N+S) attracts but same poles such as N+N or S+S repels. Figure: A magnetic field, created by iron dust Magnetic field is invisible. Any ferromagnetic material or another magnet responds to a magnetic field. Ferromagnetic materials are materials which magnetized either by another magnet or an electric field.

Sometimes, smallest of the defects can be hardly visible for human eyes. That is when engineers use the Liquid Penetrant (PT) or Dye Penetrant test. Liquid Penetrant Examination is a Non-Destructive Testing method used to detect surface breaking defects, such as cracks, laps, porosity etc., which are unperceivable with the bare eye. The method is rest upon the ability of a fluid to be drawn into a "clean" surface-breaking flaw by smooth action. After a certain time period, unnecessary surface penetrant is displaced and a developer is applied that functions as a "blotter". Color contrast penetrants demand adequate white light whereas fluorescent penetrants are suggested to be used in dark environment and require ultraviolet "black light" where penetrant glows with a luminous color which can be easily seen from our eyes.

Some days ago I heard that Sri Lanka's ministry of tourism is going to develop a village called Kalpitiya in the western cost of the island, as a new tourist destination. I tried to find more information. It is quite a nice place with many small islands, white sandy beaches and yea! dolphins. Looking forward to travel there and see how it looks like.

Pre-flight Walk Around Inspection of a Light Sport Aircraft Walk around inspection of a MD-83

Figure: Boeing 777 aircraft Commercial aircrafts are frequently inspected visually. There is a primary inspection called "walk around check" both in the line and the base, transit checks, daily check, weekly check, A check, B check etc… Line is the more about the quick checks while aircraft is in the park or in between flights. Base is mostly the hanger, where hard checks, repairs take place. A qualified engineer walk around the aircraft and inspect structural items visually. Engine is also checked during the walk around checks. There engineer watches for oil leaks, structural damages in the engine, inlet of the engine for composite parts fixtures to the structure, fan/first stages of the compressor, last stages of the turbine for deterioration or damages , or any unusual changes of the engine. In this example shows how, engineer/Pilot carry on a walk around inspection of a Boeing 777 aircraft. Figure: Walk around inspection of a Boeing 777 1. Left Mid Fuselage

Gas turbine engines are widely used in different fields for generate energy. They are commonly used in aircrafts. If you watch a huge passenger aircraft, you can see the gas turbine engines are either hanging on the wing section or tail section. Those power plants gives enough energy to generate thrust which finally result the aircraft fly. Figure: An Airbus A340's 4 engines are situated on the wing, 2 in each side. Figure: A Tupolev 154 Aircraft where engines situated on the empennage  How does a gas turbine engine work? There are several sections inside of an engine. First one is the air intake and fan which direct the air to inside of the engine, then the compressor, which compress the air to a very high pressure while reducing its speed. Then air meets the combustion chamber, where kerosene (fuel) is spread as small particles and burned in a high temperature.  

Visual Inspection is one of the most common NDT methods used to evaluate the condition and provide a better quality of an item. Visual method is easily carried out, inexpensive and usually does not require special equipment. It entails reflected or transmitted light from a test object being imaged with a light-sensitive device, such as the human eye. Such a testing method requires proper vision, good lighting conditions and the knowledge of what exactly to look for.

As we saw in the beginning, 2nd Section of technical diagnostics is about functional methods. They consist of: • Evaluation of long term and short term statistical data such as, the temperature of the part/machine, Pressure of the hydraulic systems, fuel/air consumption etc… By analyzing such parameters, engineer can predict the economical use of the part/machine and prognosis future failures, so that it will be possible to remove it and put a new part/ or totally stop using the machine before a catastrophe. • By analyzing the physical and chemical parameters such as the spectral analysis of oil in order to detect foreign particles (for example, small particles of metals can be found in oil system when a ball bearing is destroyed, and that can lead to a failure of the whole machine) • Vibration diagnostics: where engineer analyses the vibration of machine and evaluate faults and trends. An experienced engineer watches the frequency spectrum of the vibration without even touching the ma

First of all let's have a brief look at some of the most common NDT/NDE methods, •Visual inspection: Visual examiners follow procedures that range from simply looking at a part to see if surface imperfections are visible. He can use the help of a magnifying glass or computer controlled camera. It is the most basic method of all. •Liquid penetrant Testing: Engineer evaluates defects on the material surface with help of liquids or sprays. •Eddy Current Testing: Engineer uses equipment with electromagnetic induction to detect flaws in or on conductive materials. •Acoustics methods (mainly ultrasonic, Acoustic emission, Impedance etc…): Engineer uses different forms of acoustic waves (such as sound, ultrasound and infrasound) in order to evaluate defects. You probably have heard of the Ultrasonic sonogram they use in medicine to see the gender of a baby before birth. It certainly is another use of NDT. Figure: Ultrasonic Scanning of a T beam •Magnetic particle crack detection:

Diagnostics! Well, yes, we all heard this word very many times in the medicine. There, diagnostics means, identification of the nature of an illness or other problem by examination of the symptoms. As same as Buddha tried to find reason for sadness before cure, doctors try to find reason for illness before cure, Engineers do test and evaluate before thinking a cure for those mechanical objects. It is one of the most important parts of engineering which is closely linked to aerospace engineering, civil engineering, electrical engineering, material science and engineering, mechanical engineering, nuclear engineering, petroleum engineering, physics etc… It’s often used in both constructional and service sides.