Functional methods

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 machine, say where exactly problem is, and propose options to cure that defect.

Figure: An NDT engineer preparing instrumentation for a vibration analysis of a gas turbine engine

Eddy current Theory

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.

Aircraft Visual Inspection

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

Eddy current method (ET)

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,

Technical Diagnostics and NDT

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