Sub Module 7.18: Aircraft Non Destructive Testing (NDT) Techniques
EASA PART 66 / SARI
MODULE 7
NON-DESTRUCTIVE TESTING (NDT):
Non-destructive tests seek to detect defects such as cracks or corrosion either at the surface of below it and a number of methods are presently in use. In each case the part being tested is not harmed in any way.
The selection of the method to be used will depend on the design of the structure, its accessibility and the nature of the suspected defect.
1. VISUAL INSPECTION
Visual inspection is the oldest of the non-destructive methods of testing. It is a quick and economical method of detecting various types of cracks before they progress sufficiently to cause failure. Its reliability depends a great deal on the ability and experience of the inspector who must know how to search for structural failures and how to recognize areas where such defects are likely to occur. Defects not visible to the naked eye can often be detected with the aid of optical devices because they magnify the defects and they allow visual inspection of areas that are not accessible to the unaided eye.
2. PENETRANT DYE FLAW DETECTION PROCESSES
Minute surface cracks which are difficult to detect by visual means may be highlighted by increasing the contrast between the defect and its background. This may be done by using penetrant dyes.
Penetrant dye inspection is a non-destructive test used mainly for the detection of defects open to the surface, in parts made of any non-porous material including metals, ceramics, plastics, molded rubber and glass. The process will detect cracks which may be caused by fatigue or shrinkage, porosity and cold shuts, or any failure of a bond between metals.
Basically, all the processes consist of applying a red penetrant dye to the surface of the part being tested, removing after a time the dye which remains on the surface and then applying a developer which draws to the surface any dye that has entered defects. The resultant stains indicating the positions of the
defects.
3. FLUORESCENT PENETRANT FLAW DETECTION METHOD:
In this technique the penetrant used contains a dye which fluoresces brilliantly in ultra-violet radiation (black light). The penetrant enters any cracks and carries with it the fluorescent dye. After the penetration time the surplus penetrant is removed and a developer is applied. Examination under ultra
violet radiation shows where the developer has drawn penetrant from any surface defects.
Fluorescent penetrant processes are used mainly for the detection of flaws in non-ferrous and non-magnetic ferrous alloys. They may also be used on some non-metallic materials such as plastics and ceramics.
4. MAGNETIC FLAW DETECTION
This method is used in Ferro-magnetic materials (which are easily magnetized) for the detection of surface defects and those just beneath the surface. The technique is to induce a magnetic field in the suspect part and then to brush over it an ink containing a magnetic powder. A crack in the part causes the magnetic field to become distorted and stray outside the part the magnetic particles in the ink are then attracted to this area. Defects are indicated by a buildup of particles much wider
than the actual crack and therefore easier to see. The best indications are given when the defect is at right angles to the lines of magnetic flux because this produces the greatest distortion of the magnetic field. Therefore, during tests, the part is magnetized from different angles and various methods of
magnetization are also used.
It is important that the part being tested is clean as any oil, grease or corrosion will prevent the
magnetic ink from contacting the surface of the part. A part may be magnetized either by passing a current through it, or by placing it in the magnetic circuit of a permanent magnet or an electro-magnet.
The magnetic particles used to indicate the defects are either in the form of dry powder or suspended in a suitable liquid such as paraffin. They are colored either red or black, red being more suitable for rough castings.
5. EDDY CURRENT FLAW DETECTION
The eddy current method is a non-destructive means for detecting cracks, corrosion and heat damage in aircraft structures. It can be used on all electrically conducting materials i.e. most metals. The surface being examined does not require removal of the paint finish or cleaning and the work does not interfere with other work being carried out on the aircraft. It is more sensitive to surface and sub surface defects
than other non-destructive techniques.
The equipment is portable and can be used in relatively inaccessible places in aircraft structures. To initiate the eddy currents in a part an alternating current at a suitable frequency is supplied to a test coil mounted in a probe, which is held close to or in contact with the part. The coil carrying this current then induces a magnetic field of the same frequency in the part and causes eddy currents to flow. The eddy current itself produces an alternating magnetic field which opposes the original field produced by the coil. The flow path of the currents is affected by any defects in the material. The resulting impedance to the flow of alternating current in the coil is measured and indicated usually on a moving coil instrument.
In aircraft work eddy current testing is carried out by comparing the indications from the part being examined with indications from a reference piece made from a similar material and containing an artificial defect. The reference piece would contain defects of known size and shape. The change in coil impedance against the known defect could be used as an acceptance limit on the part being examined. Even better results would be obtained if comparative readings were obtained from an actual defective aircraft component.
By using the appropriate probes, the eddy current tester can be used on ferrous or non-ferrous metals. Special probes are available for detecting cracks which form on the surface of holes. A shaped jig is being used to keep the probe at right angles to the surface of the wheel rim. This is necessary to avoid misleading indications which could be caused by inadvertent angulations of the probe.
6. ULTRASONIC FLAW DETECTION
Ultrasonic inspection may be used for detecting surface flaws and internal flaws which are some distance from the surface and is normally used to complement other methods of flaw detection. Ultrasonic inspection may be used on all materials. It is often used to detect flaws in extrusions, forgings and rolled materials at manufacture and can also be used to detect fatigue cracks and other defects occurring during service.
Thickness measurement by ultrasonic methods is also applied to aircraft parts; for example, the outer panels of aircraft windows are machine polished to remove surface scratches. Afterwards the thickness of the panel can be measured using an ultrasonic technique. Delamination of bonded assemblies
can also be checked by similar methods. Ultrasonic inspection is a fast, reliable non-destructive testing method which employs electronically produced, high frequency sound waves that will penetrate metals, liquids and many other materials at thousands of meters per second. The high frequency sound waves are of short wave length and relatively low energy levels to prevent damage to the structure. No special safety precautions are required for operating personnel under normal conditions.
The principle of ultrasonic testing is that a probe containing a quartz crystal, which can both transmit and receive high frequency sound waves is moved over the surface of the part to be tested. The probe is connected to an oscilloscope which displays the result on a cathode ray tube. Normally the sound waves will pass from the probe through the part and will be deflected from the bottom surface back to the probe which also acts as a receiver. Both the transmitted pulse and the returning pulse are recorded on the cathode ray tube of the oscilloscope. The distance between the ‘blips’ is proportional to the thickness of the part being tested. If a crack is encountered then the pulse is interrupted and reflected back as shown. Since the reflected pulse returns to the probe in a shorter time, an intermediate ‘blip’ appears on the cathode ray tube. The position of this ‘blip’ relative to the other blips indicates the distance of the fault below the surface. In order to inject the sound waves into a part at an angle normal
to the expected defect it may be necessary to mount the probe on a suitably shaped plastic block.
Sometimes separate transmitting and receiving probes are used. To ensure good transfer of the high frequency sound waves it is essential that there is good contact between the probe and the surface of the part. This is done by covering the search area with a film of oil; the rougher the surface the thicker the oil needs to be to maintain contact.
Ultrasonic testing is widely used on parts removed from aircraft but can also be used in situ where other types of inspection would require extensive dismantling. The results of any test must be compared with those obtained from an identical part of known condition. If necessary artificial defects can be introduced into specimen parts. The indications obtained from these artificial defects can then be compared with those obtained from the parts being examined.
7. RADIOLOGICAL EXAMINATION
Radiographic inspection of the aircraft structure is recommended if the suspected structural area is hidden or not easily accessible. This type of inspection is able to reveal fatigue cracks, corrosion, internal damage, the presence of loose articles and mercury spillage without the need to dismantle the aircraft. Radiographic inspection may be used on all materials except lead. It is also a method which allows the examination of a faulty component in the condition in which it failed. This can prove very useful since often when a component is being dismantled it falls apart and the cause of failure cannot be determined.
Usually when radiographic techniques are used the suspected location and orientation of the failure will be known from previous experience. Due to the hazardous nature of radiographic radiation it is very important that operators should be properly trained and aware of the regulations concerned with safety. The aircraft being examined will be isolated and unauthorized personnel kept at a safe distance. The aircraft will be defueled and properly marked with warning signs and roped off. In most instances no dismantling of the aircraft will be required.
There are two forms of electro-magnetic radiations which can be used in radiography, X-rays and gamma rays.
8. BOROSCOPIC INSPECTION
Boroscope inspections involve looking at components within an engine using an optical probe. The probes are inserted in to the engine through ports in the engine casings, and can be rigid or flexible, the choice being dependant on the difficulty at obtaining a satisfactory view of the required features. Some of these inspection ports are the attachment points of other functional devices that intrude into the engine (e.g. ignitor plugs or temperature probes) but on more modern engines there are usually several purposes made ports for boroscope inspections.
EASA PART 66 / SARI MODULE 7 ESSAY QUESTION:
United Airlines Boeing777 made an emergency landing after Engine Fire caused due to Fan Blade Breaking apart during flight. Engine blade crack was the main reason behind this failure. How will you inspect the remaining blades of the engine for any surface or subsurface cracks? Give reason for selecting a particular NDT technique.
(Note: write your answers in the comment section below)
Non-destructive testing is used to detect the cracks and corrosion inside and outside the surface with the help of some special tools.
ReplyDeletePROCEDURE FOR FINDONG CRACKS ON BLADES:
Cracks can ocuur on blades but we cannot find them easily. For this purpose we will use eddy currents.In this process we will use a coil which will infuce magnetic field and send eddy currents to flow in the blades.Eddy currents will also induce their own magmetic fields due to moving current which will opposes the original magmetic field that is produced by the coil.The impedance that is being given is then matched by the correct blade that has no cracks. If impedance matches with the blade that have no cracks then the blade is save it will have no cracks but if the impedance is high than the blade that have no cracks then there will be a crack. The main advantage of using eddy currents is that we will not clean any surface and oil from that surface. We will also use eddy currents on metals.But if we use this process on non-ferrous or ferrous materials than there will be extra probes that are use to detect cracks on these materials. During the whole process it will not disturb any other work that other persons are doing on the aircraft.
We will also consult AMM for the correct type of detection of cracks through eddy currents.
Hammad Malik.
When engine fire is caused by blades breaking apart many unforeseen problems can occur so we need inspection of the aircraft engine.
ReplyDeleteFirst we do visual inspection but many defects cannot be seen by the naked eye so we use eddy current as a means of inspection. Eddy current is a non destructive means of checking cracks. Eddy current does not need cleaning and removal of paint during inspection and it does not affect any other work being done on the aircraft. It is more sensitive to the surface and subsurface cracks than any other non destructive techniques.
The coil carrying current induces a magnetic field of the same frequency in the part and causes the eddy current to flow. The magnetic field of the eddy current opposes the original magnetic field produced by the coil. The flow of the current is affected by the defects in the material. The resulting impedance is measured and shown on the coil instrument. The indication of the part is usually compared with a reference piece made with the same material containing same defects.
These inspections should be done according to the relevant maintenance manual.
Non-destructive tests are used to detect defects such as cracks or corrosion at the surface or below it.
ReplyDeleteMost of the inspections in the aircraft operation to detect the defects are carried out visually sometimes visual inspections aren’t required to be done because the defects maybe below the surface that’s why we should use about her type of NDT method to inspect the defects.
For the inspections of the blades for surface or sub-surface defects we should used the Eddy cureent method as it is more sensitive to surface or non-surface detection of the crack and it is the Non-destructive method used for detecting cracks corrosion and heat damage As it is used in all electrically conducting materials. It doesn’t require cleaning and removal of any paint. Special probes are their for the detections of the crack on surface. As the Eddy currents have proved in which we have to mount a coil providing it a alternating current at a suitable frequency. The coil induces magnetic field isn’t he samne frequency as same on the part cause the Eddy currents to pass. As the Eddy currents itself has the magnetic field as it opposes the original produced by the coil.the indications we got from the part we are examining for cracks is compaired with the referenced piece to know actual defect in the part. As the reference piece would show us the defects of known sizes and shapes.
In order to ensure the safety of aircraft we need to concern the relevant maintenance manual.
Almost all the time the inspection on the structure of the Aircraft is done visually at the arrived intervals of Relevent Aircraft Maintenance schedule. But in some instances the Visual Inspection is not practicable. So we use NDT (non destructive test) which seeks for the defects such Corrosion and cracks at the surface of the part and below it.
ReplyDeleteThere are number of methods to carryout NDT.
In the Situation given we can use Eddy current which is a Non destructive technique for any flaw detection such as cracks, Corrosion and heat damage on the surface.
The reason for using the Eddy current NDT is that is does not require any removal of paint finish and coating, its work don't effect the other works being carried on the Aircraft. This technique is more sensitive for surface and sub-surface damage then other NDT and the equipment that we use is portable and suitable for the place we access is not available.
To carry out Eddy current NDT the AC current of specific frequency is applied to the coil mounted in a probe which is held close or in contact with the part to be examined.The coil carrying this current induces a magnetic field of same frequency in the part which results in the flow of eddy current. This eddy current itself induces a alternating Magnetic field which opposes the original field produced by the coil. The flow of current is effected by the defects in the part.
The Indication obtained from the part being measured is compared with the indication of reference piece of the same material with an artificial defect. This will give us the information about the position and size and type of defect.
All the Procedures for carrying out any kind of NDT should be according to the Relevant AMM.
Most of the time the inspection carried out on the structure of the aircraft is visual but sometimes visual inspection is not enough due to this reason, we use for detecting cracks on fan blades by non-destructive testing techniques(NDT).
ReplyDelete.For current situation we use Eddy current flaw detection because it is more sensitive to surface and sub surface defects.It is non-destructive Eddy is used to detect cracks, corrosion or heat damage on the aircraft structure.To start the eddy current process we give alternating current to test coil mounted on a probe.which is close to or in contact with the part.when alternating current supplied to coil it induced magnetic field the frequency of the field is same as to the eddy current flow.At this point eddy current produces magnetic field itself which oppose the magnetic field of the coil.If any defects occur in the material it defects the flow path of the current.Measuring the resulting impedance of the coil is measured by moving coil instrument.After eddy current testing the indication examined by defect material compare with artificial defect.This reference piece give a size and shape of the original defect.
All procedure should be follow by AMM schedule.
Non-destructive test seek to detect defects such as cracks or corossion either on surface or below it and a number of methods are presently in use.
ReplyDeleteTo inspect the remaining blades for internal cracks heat damage or any sort of corrosion we might use the eddy current flaw detection method.
I will use the eddy current method for quite obvious reasons which are, its sensitivity to surface and sub surface defects, can be used on all electrically conducting materials, and can be used in relatively inaccessible places. To detect the cracks and other defects we'll use a test coil to which we'll provide an alternating current of a suitable frequency which is mounted in a probe, which might be held close or in contact with the test blade. The coil would induce a magnetic field in the blade of the fan of the same frequency and causes eddy currents to flow. The eddy currents produce an alternating magnetic field that would oppose the original magnetic field. The current that is flowing the the blade will be affected by any defects in the material. The impedance can be measured by a moving coil instrument. We'll use a refrence piece that will have defects of known size so we can compare the defects of original piece and the refrence piece. Eddy currents can be used on ferrous and non ferrous materials as well by use of appropriate probes.
We will follow the appropriate procedures layed in the relevant AMM.
NDT ( non destructive test ) is used to detect any crack, corrosion or heat damage caused by any event. This method is preferred because the component being tested is not damaged or harmed.
ReplyDeleteI will use eddy current non destructive test to inspect remaining blades. This type of NDT is preferred because when using this the component dose not need to be removed neither dose the paint finish need to be removed. When carrying out eddy current NDT, other work preformed meanwhile won’t be affected by it. It’s the most sensitive to any damage on the surface or subsurface than other NDT.
This test uses a probe that contain a coil which will carry an alternating current. This probe in held near the surface or comes in contact with the surface. This alternating current produces a magnetic field, this magnetic field induces alternating current in the metal surface. This alternating current in the metal will produce a magnetic field in it and will also produce eddy current in it. This eddy current will produce another magnetic field that will oppose its cause. When there is a crack or defective surface in the metal then it will provide impedance to the alternating current flowing in it. This impedance offered by the coil to the current in the coil will be measured and shown on an indicator.
Values obtained from the surface being examined should be compared with those obtained from an artificial damaged surface. This artificial damaged surface contain cracks of known length and shape. We can obtain better results if we had an actual damaged airframe component.
Appropriate probes should be used for ferrous and non ferrous material. There are special probes given for holes.
They are mounted on a jig to reduces the error obtained from unintentional angular movement of the probes. That’s why they are mounted on a wheel to remain at specific angel and to reduce the errors obtained from irregular angular movement of probes.
All the details procedures are written in the relevant AMM and must be consulted.
Normaly most of the inspections on the aircraft are carried out visually but at some instant another method is used called nondistructive examination it can ditect cracks corrosion and small errors accuratly.
ReplyDeleteFor the inspection of the of the fan blades we use eddy current flaw detection so for this method first of all we mount a coil on the prop and provide alternating current of specific frequency then we move prop close or in contact with the surface that is being checked. The magnetic field in the part produced by eddy current will oppose the original magnetic field in the prop if there is any falt in the blades it will affect the flow path of current in the material . Now we compare the values of eddy current with the reference piece which is already tested then we can easily find out which type of diffect is and size of diffect .
A jig is used to maintain the prop in straight line during testing .it is necessary to avoid misleading indication .
Zohaib
Muhammad Hamza Ahmed
ReplyDeleteWhen some engine problem occurs on an aircraft blades during flight or on ground we, must inspect it by doing several inspections.
Firstly we will do visual inspection but by visual inspection we cannot find the actual fault in the engine and on the blades so we will do eddy current inspection to find out the exact place of crack on the blade.
It is a non-destructive method in which we do not have to remove the paint or any layers of metals
We will inspect the blades of the engine for any surface or subsurface cracks by using the eddy current method , while using this method we use a coil which will carry current and induces a magnetic field of the same frequency in the part and causes the eddy current to flow.
The magnetic field of the eddy current opposes the original magnetic field produced by the coil and it will detect the resistance which will be more or less than the comparable metal that we will use to compare the resistance.
The resulting impedance is measured and shown on the ohmmeter.
These inspections should be done according to the relevant AMM ( Aircraft Maintenance Manual)
Non destructive tests are used to detect the cracks and any type of corrosion which occur during the flight operation. If we want to check the cracks or corrosion occur on the aircraft structure following methods can be used.
ReplyDeleteVisual Inspection
Visual inspection is quick and economical method for detecting the various type of cracks in the aircraft structure but if visual inspection is done by inexperienced person he will miss out the cracks and faults in the structure, if defects are not visible with the naked eye optical devices may be used so that during visual inspection we don’t miss out anything.
Magnetic flaw detection
This method is used in fero magnetic materials for the detection of the defects which are beneath the surface. In this method an ink containing magnetic powder is brush over the suspected part. Crack causes the magnetic field to stray out of that area. Due to which the defected part visible to the engineer.
Eddy current flaw detection
Eddy current flaw detection is a non-destructive means for detecting cracks, corrosion and heat damage in aircraft. It can be used on all electrically conducting material such as metals. It has also many advantages such as it doesn’t require cleaning or removal of the paint and surface on which task been performing. This equipment is also portable and can also be used in relatively inaccessible places in aircraft.For performing this task an alternating current at a suitable frequency is supplied to test a coil mounted in a probe, which is in contact with the part.The coil carrying this current induces the magnetic field of the same frequency and causes eddy current to flow. Then the eddy current produces itself an alternating magnetic field which opposes the original magnetic field produces by the coil.The flow path of the field is affected by the defects in the material. The greater the defects the greater the resistance face by the current to flow through the material. Better results would be obtained if comparative readings were obtained from an actual defective aircraft component.
We will the appropriate procedures by following the relevant AMM.
We can use non-destructive test to detect the corrosion, cracks and heat damage in aircraft structure.
ReplyDeleteWhen cracks can occurred on blades but we can not find the cracks easily. Than now we use eddy current flaw detection method.
In this process we will use a coil which will infuce magnetic field and send eddy currents to flow in the blades.Eddy currents will also induce their own magmetic fields due to moving current which will opposes the original magmetic field that is produced by the coil.The impedance that is being given is then matched by the correct blade that has no cracks.we can also use different probes for the holes.probes can also be used at different angles to check resistance.probes are also used for the ferroo and non-ferromagnetic materials.
We can consult AMM for the correct type of detection.
Aj vekhay ga crowd groove mera TV tay.
Ehtesham from mirpur azad Kashmir
We can use non-destructive test to detect the corrosion, cracks and heat damage in aircraft structure.
ReplyDeleteWhen cracks can occurred on blades but we can not find the cracks easily. Than now we use eddy current flaw detection method.
In this process we will use a coil which will infuce magnetic field and send eddy currents to flow in the blades.Eddy currents will also induce their own magmetic fields due to moving current which will opposes the original magmetic field that is produced by the coil.The impedance that is being given is then matched by the correct blade that has no cracks.we can also use different probes for the holes.probes can also be used at different angles to check resistance.probes are also used for the ferro and non-ferromagnetic materials.
We can consult AMM for the correct type of detection.
Introduction
ReplyDeleteGenerally all the inspections are done visually as according to aircraft maintenance schedule. During visual inspection identify any defect on the surface of the blade. For more detail inspection we will use NDT methods. NDT will use in Thermal fatigue and overheating are two main causes of deterioration in turbine blades NDT extends the service life of a jet engine.
EDDY CURRENT
The most common NDT methods on jet turbines include eddy current testing and ultrasonic testing. Each of these methods is used to inspect turbine blades. There are many types eddy current flaw detection is one of NDT type. Eddy current testing uses the natural phenomena of electromagnetic induction to find defects in conductive materials. Eddy current occurs when an alternating current (AC) is applied on the component. The probe magnetic field is opposed by magnetic field is induced in the test piece, and the flow of eddy current will change if there are defects or discontinuities near the surface. Eddy current testing can be moreover continuous wave or pulsed. Will use an artificial defected piece for comparing with testing piece also apply same method. Then will match the reading of both pieces. Eddy current testing is an effective method used to detect surface or near surface cracks. Eddy current inspection detects changes in material thickness or the presence of any surface level fault or discontinuity. All the procedure should be followed according to the relevant AMM.