Non-Destructive Testing (NDT)

>> Friday, May 7, 2010

Nondestructive testing (NDT) are noninvasive techniques to determine the integrity of a material, component or structure or quantitatively measure some characteristic of an object. In contrast to destructive testing, NDT is an assessment without doing harm, stress or destroying the test object. The destruction of the test object usually makes destructive testing more costly and it is also inappropriate in many circumstances.

NDT is used in a wide range of industrial areas and is used at almost any stage in the production or life cycle of many components.
The mainstream applications are used in below:
1) aerospace,
2) power generation,
3) automotive, railway,
4) petrochemical and pipeline markets

NDT of welds is one of the most used applications. It is very difficult to weld or mold a solid object that has no risk of breaking in service, so testing at manufacture and during use is often essential.

Personnel Qualification
Personnel Qualification is an important aspect of non-destructive evaluation. NDT techniques rely heavily on human skill and knowledge for the correct assessment and interpretation of test results. Proper and adequate training and certification of NDT personnel is therefore a must to ensure that the capabilities of the techniques are fully exploited. There are a number of published international and regional standards covering the certification of competence of personnel. The EN 473 (Qualification and certification of NDT personnel - General Principles) was developed specifically for the European Union for which the SNT-TC-1A is the American equivalent.

NDT Methods

The main NDT methods are shown below:
1)Ultrasonic Testing (UT),
2)Radiographic Testing (RT),
3)Electromagnetic Testing (ET) in which Eddy Current Testing (ECT) is well know and
4)Acoustic Emission (AE or AET).

Besides the main NDT methods a lot of other NDT techniques are available, such as Shearography Holography, Microwave and many more and new methods are being constantly researched and developed.

NDT Applications in Commercial Aircraft Maintenance

During aircraft maintenance 'NONDESTRUCTWE TESTING' (NDT) is the most economical way of performing inspection and this is the only way of discovering defects. In simply we can say, NDT can detect cracks or any other irregularities in the airframe structure and engine components which are obviously not visible to the naked eye.

Structures & different assemblies of aircraft are made from various materials, such as aluminium alloy, steel, titanium and composite materials. To dismantle the aircraft in pieces and then examine each component would take a long time, so the NDT method and equipment selection must be fast and effective.
In the present trend of NDT application on aircraft 70-80% of NDT is performed on the airframe, structure, landing gears and the rest carried out on engine & related components.

In order to maintain the aircraft defects free and ensure a high degree of quality & reliability and as a part of inspection programme, usually following NDT methods are applied;

1) Liquid Penetrant
Liquid penetrant testing is one of the oldest of modern nondestructive testing methods & widely used in aircraft maintenance. Liquid penetrant testing can be defined as a physical & chemical nondestructive procedure designed to detect & expose surface connected discontinuities in 'nonporous' engineering materials.

Detection of surface detects or structural damage in all materials of aircraft. Fluorescent penetrants are used in critical areas for more sensitive evaluation.

2) Magnetic particle,
Magnetic particle testing is a sensetive method of nondestructive testing for surface breaking and some sub-surface discontinuation in 'ferro-magnetic' materials.
The testing method is based on the principle that magnetic flux in a magnetised object is locally distorted by the presence of discontinuity. This distortion causes some of the magnetic field to exit & re-enter the test object at the discontinuity. This phenomenon is called magnetic flux leakage. Flux leakage is capable of attracting finely divided particles of magnetic materials that in turn form an 'indication' of the discontinuity. Therefore, the test basically consists of three operations : a) Establish a suitable magnetic flux in the test object by circular or longitudinal magnetisation. b) Apply magnetic particles in dry powder of a liquid suspension; and c) Examine the test object under suitable lighting conditions for interpreting & evaluating the indications.

Fluorescent or black oxide particles in the aerosol cans are used during critical areas of aircraft structure/components inspection when using either permanent or electromagnets. Fluorescent particle inspection method is evaluated by black light (Black light consists of a 100 watt mercury vapour projection spot lamp equipped with a filter to transmit wave length between 3200 to 3800 Angstrom unit and absorb substantially all visible white light).

Simple in principle, easily portable. Fast and effective for surface & subsurface defects in ferromagnetic materials of any shape, removed from engines, pumps, landing gear, gear boxes, shafts, shock struts etc. Widely used for bolts inspection.

3) Eddy current
Eddy current tests are important test & widely used method within the broad field of Nondestructive materials & evaluation. This method is particularly well suited for the detection of service induced cracks usually caused either by fatigue or by stress corrosion. Eddy current inspection can be performed with a minimum of part preparation and a high degree of sensitivity.

Eddy current test is used to detect surface & subsurface defects, corrosion in aircraft structures, fastener holes and bolt holes. Surface detects and conductivity testing by high frequency and sub-surface detects by low frequency methods.

Routine eddy current inspection is carried out on aircraft under carriage wheel hubs for cracks also used to detect cracks in different tubes, tublar components of aircraft & engine.

4) Ultrasonic
Sound with a frequency above the limit of audibility is called 'ultrasonic'. It ranges with a frequency of 0.2 MHz to 800 MHz.
Ultrasonic inspection provides a sensitive method of nondestructive testing in most materials, metallic, nonmetallic, magnetic or nonmagnetic. It permits the detection of small flaws with only single surface accessibility and is capable of estimating location & size of the defect Providing both surfaces are parallel, ultrasonics may be used for thickness measurement, where only one surface is accessible. The effective result of an ultrasonic test is heavily dependent on subject surface condition, grain size & direction and acoustic impedance. Ultrasonic techniques are very widely used for the detection of internal defects in materials.

Ultrasonic inspection operates on the principle of 'transmitted' & 'reflected' sound wave. Sound has a constant velocity in a given substance; therefore, a change in the acoustical impedance of the material causes a change in the sound velocity at that point producing an echo. The distance of the acoustical impedance (flaw) can be determined if the velocity of the sound in the test material, and the time taken for the sound to reach & return from the flaw is known.

Used for detection of surface & subsurface detects in welds, forging, casting main structural fittings of landing gear legs & engine attachments. Bolts in critical areas, aircraft structure joints & pylon. Also checks adhesive bond quality of lap joints & composite structure. Used for thickness measurement after damage or corrosion removal

5) Radiography (x-ray/gama ray)
Radiography is one of the oldest and widely used nondestructive testing methods. A radiograph is a photographic record produced by the passage of electromagnetic radiation such as x-rays or gamma rays through an object onto a film. When film is exposed to x-rays, gamma rays or light an invisible change called a 'latent image' is produced in film emulsion. The areas so exposed become darker when the film is immersed in a developing solution. After development the film is rinsed to stop development. The film is next put into a fixing bath and then washed to remove the fixer. Finally dried so that it may handled for interpretation and record.

Considering the penetration and absorption capability of x-radiation, radiography is used to inspect a variety of nonmetallic parts; for porosity, water entrapment, crushed core, cracks and resin rich/straved conditions; and metallic products; such as welds, castings and forging as well as locating discontinuities in fabricated structural assemblies such as cracks, corrosion, inclusions, debris, loose fittings, rivets, out of round holes & thickness variations. Gamma ray radiography is usually used for detection of internal flaws of aircraft structure (steel & titanium) and engine components which require higher energy levels or other assemblies where access is difficult.

6) Visual/Optical
Visual inspection is probably the most widely used of all the nondestructive tests. It is simple, easy to apply, quickly carried out and usually low in cost. The basic principle used in visual inspection is to illuminate the test specimen with light and examine the specimen with the eye. In many instances aids are used to assist in the examination.

This method is mainly used i) to magnify defects which can not be detected by the unaided eye, ii) to assist in the inspection of defects and iii) to permit visual checks of areas not accessible to unaided eye.

Detection of surface defects or structural damage in all materials. Optical instruments are used for visual checks of internal areas and for deep holes and bores of aircraft structure, landing gears etc. Widely used to monitor engine components, such as, turbine wheels and nozzles, compressor vanes and blades combustion cans without opening the engine. 'Borescopes', 'fibrescopes' and 'video imagescopes' are most important optical aids in remote - visual inspection, which area is normally inaccessible.

7) Sonic/Resonance
Sonic and resonance testing methods are used primarily for the detection of separations between layers of laminated structures.
Sonic and Resonance testing is effective for detection of crushed core or debonds in adhesive bonded honeycomb, impact damage and delimitations in composite structures and exfoliation corrosion.

The tap test method has demonstrated the ability to detect cracks, corrosion, impact damage and debonding. The sonic testing instrument operate in the audio or near audio frequency range.

Resonance testing instruments may operate either or both the sonic or ultrasonic frequency range. Different methods of transmitting and receiving energy have been developed. Basically, each technique introduces a pressure wave into the specimen and then detects the resonant, transmitted or reflected wave.

To examine bonding exists between honeycomb, detect delaminations in composite laminates. Large structures such as, fairings, cowl and wing trailing edge, rudder, flaps, ailerons, elevators etc. are made from composites and honeycomb materials.
Tap testing is limited to detection of disbonds or voids between upperfacing sheet and adhesive. It will not detect disbond or voids at 2 nd or 3 rd layer bondlines, such as doubler areas. It is limited to the detection of delaminations, approximately 25 mm (1 inch) in dia or greater, located less than 1.3 mm (0.05 inch) below the surface being examined.

8) Infrared Thermography.
Infrared and thermal methods for nondestructive are based on the principle that heat flow in a material is altered by the presence of some types of anomalies. These changes in heat flow cause localized temperature differences in the material. The imaging or study of such thermal patterns is known as 'thermography'. The terms 'infrared' and 'thermal' are used interchangeably in some contexts. Thermal refers to the physical phenomenon of heat, involving the movement of molecules. Infrared (below the colour red) denotes radiation between the visible and microwave regions of the electromagnetic spectrum.

The intensity and frequency/wavelength of the radiation can be correlated closely with the heat of the radiator. it follows that radiation sensors can be used to tell us about the physical condition of the test object. This is the basis of the technology of 'thermography'.

Used to detect certain voids, inclusions, debonds, liquid ingress or contamination, foreign objects and damaged or broken structural assemblies. Infrared thermography also been chosen for quick operational use and the reliability of defection 'liquid contamination' in the composite sandwich in compared to x-ray method. Detection of
thermal overheating in electrical & hydraulic system.

Specially thermographic inspection on aircraft structures are carried out to detect following defects :
(i) Composite laminate parts - for delamination debonding or foreign objects
(ii) Composite sandwich parts - for debonding and liquid contamination.
(iii) Metallic bonded parts - for debonding of corrosion on.
(iv) Metallic sandwich parts - for liquid contamination, debonding of corrosion.


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