Table of Contents ECNDT '98
Non-destructive Testing Applications in Commercial Aircraft Maintenance.Md. Alahi Uddin Khan - Biman Bangladesh Airlines, Bangladesh.
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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 2) Magnetic particle, 3) Eddy current 4) Ultrasonic 5) Radiography (x-ray/gama ray) 6) Visual/Optical 7) Sonic/Resonance 8) Infrared Thermography.
The fundamental purpose of penetrant testing is to increase the visible contrast between a discontinuity & its background. This is achieved by treating the area with an appropriately formulated liquid of high mobility & penetrating power (which enters the surface cavities), and then encouraging the liquid to emerge from the developer, to reveal the flaw pattern under white light (when visible dye penetrants are used) or under ultraviolet light (when fluorescent penetrants are used). Evaluation also conduct with the aid of 3X to 5X magnification. The objective of liquid penetrant testing is to provide visual evidence cracks, porosity, laps, seams of other surface discontinuities rapidly & economically with high degree of reliability.
Equipment : Various types of penetrant test units are used in aircraft maintenance
ii) Stationary Test Equipment : This type of equipment is most frequently used in fixed installations, consists of a series of modular work stations. Typical stations are as follows: a) deep tanks for penetrant b) emulsifier & developer c) a number of drain or dwell areas d) a wash area with appropriate lighting e) drying oven and f) an inspection booth.
iii) Small Parts Test Unit : These inspection units designed for processing aircraft small parts. The units are smaller than the stationary system. Small parts are loaded into wire baskets & then processed through each of the stations.
iv) Automated Test System : In this penetrant testing process penetrant application, washing, and drying are automatic, but developer application, the ultraviolet light inspection & interpretation are manually performed by an inspector. Large aircraft components are inspected in this automatic system.
Applications : Detection of surface detects or structural damage in all materials of aircraft. Fluorescent penetrants are used in critical areas for more sensitive evaluation.
Key Points : Fast & simple to use, inexpensive and easily transportable. Can detect very small surface discontinuity. Can be used on aircraft or in the workshop. Frequently used to confirm suspected defects. Area to be cleaned before and after check.
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).
Equipment : Following types of equipments are used for magnetic particle inspection :
ii) Mobile portable magneticflux machine : Hand carried or dolly transported with limited of current facility.
iii)Electromagnet yokes(adjustable) : Suitable for inspecting irregular shaped parts for surface defects.
iv) Permanent magnet : It is used in isolated critical area of small & large parts in aircraft.
Applications : 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.
Key Points : Only suitable for ferro-magnetic materials. Demagnetisation procedure is required. Positional limitations - a magnetic field is directional & best results must be oriented perpendicular to the discontinuity.
Eddy currents are electrical currents induced in a conductor of electricity by reaction with alternating magnetic field. Eddy currents are circular & oriented perpendicular to the direction of the applied magnetic field. The a) electrical conductivity b) magnetic permeability c) geometry and d) homogeneity of the test object, all affects the induced currents.
The electrical conductivity & magnetic permeability of a material is influenced by its chemistry & heat treat condition. Mixed lots of materials or parts subjected to fire or excessive heat damage can be quickly & easily separated (conductivity testing). Changes in the geometry & homogeneity of the test object will change the magnitude & distribution of the eddy currents. By monitoring these changes, the presence of cracks & other flaws can be detected.
The eddy current inspection system basically consists of five functions : a) Oscillator b) Test coil absolute or differential c) Bridge circuit d) Signal processing circuits e) Read out or display.
Equipment : Usually for aircraft eddy current inspection following test instruments are used
2) Impedance plane display instrument - It features a 'flying dot' on a CRT, LCD or video display. The position of flying dot indicates the impedance of the test circuit, but also displays effect of both resistance & reactance presenting both phase and amplitude information.
3) Linear time base display instrument - It is usually used with rotating open hole probe scanners. The 'horizontal position' of the signal on the display indicates sensor clock pPosition in the hole & the 'vertical peak' of the signal indicates amplitude of response.
4) Bargraph display instrument - It features on LCD read out bar scale graduated in voltage sensitive increments. The position of the display indication is adjustable from one bar to full scale.
Compatibility with the instrument & material selection different types of probes are used Such are i) High frequency surface & bolt hole probes ii) High frequency special probes (counter sink plug & shaped) iii) Low frequency probe (spot encircling & shaped) iv) Sliding probe (driver/receiver).
Applications : 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.
Key Points : Only applicable to conductive materials (ferrous, non ferrous & austenitic components). Calibration standards & trained operator required. Fast & portable. Spacial probes required for variation of materials and accessibility.
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.
Ultrasonic inspection is usually performed with two techniques (i) Reflection (Pulse echo) technique (ii) Through transmission technique. 'Pulse echo' technique is most widely used in aircraft maintenance inspection.
Equipment : The ultrasonic flaw detection equipment comprises with the following basic elements : (i) Cathode ray oscilloscope (ii) Timing Circuit (iii) Rategenerator (iv) RF pulser (v) Amplifier & (vi) Transducer (search unit)
Acoustic energy (transmitted or reflected) are presented, displayed or recorded in four ways.
ii) B-Scan : It displays discontinuity depth and distribution in 'cross sectional view'. Means of presentation recording paper and computer monitor.
iii) C-Scan : It displays discontinuity distribution in 'flat plan view'. Recording paper & computer monitor required for presentation.
iv) Digital Readout : It displays a ultrasonic time of flight information in digital format representing sound velocity thickness readings.
Applications : 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.
Key Points : Fast, dependable & portable. Results are immediately known. Calibration standards & trained operator required. Discontinuity orientation of test object must be known to select wave mode.
Typical x-ray equipment is consists of following features : i) Tube envelope ii) Cathode of the x-ray tube iii) Anode of the x-ray tube iv) Focal spot (size of the radiation focal spot) v) X-ray beam configuration vi) Accelerating potential (the operating voltage - difference in electrical potential between the cathode and anode)
Gamma-ray : Gamma-rays are the emissions from the disintegrating nuclei of radioactive substances. Two most commonly used 'isotopes' for performing industrial inspections are Iridium-192 and Cobalt-60. But in aircraft maintenance during gamma-radiography Iridium- 192 is usually used. Isotopes of Radium-226 and Cesium- 13 7 are available but are not generally used for aircraft radiography. Gamma-ray radiography has the advantages of simplicity of apparatus, compactness of the radioactive sources and independence from outside electrical source.
Applications : 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.
Key Points : Radiation hazard, aircraft must be clean of all personnel. Trained operator, film processing & viewing equipments required. Crack point must be nearly paralleled to X-ray beam. Eliminates many disassembly requirement. Provides permanent records of findings. Accessibility required in both sides of the test specimen.
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.
Equipment : Visual and Optical tests are carried out in aircraft maintenance with following equipment:
ii) Magnifying Mirror - This one is a concave reflective surface, such as a dental mirror may be used to view restricted areas of aircraft not accessible with a magnifying glass.
iii) Microscope - It is a multiple element magnifier, providing very high power magnification, is used for the inspection of parts removed from the aircraft. Some portable units are also used to evaluate suspected indications found on the aircraft.
iv) Borescope - Borescope is a precision optical instrument with builtin illumination. Borescopes sometimes called 'endoscopes' or 'endoprobes', which consists with superior optical systems and high intensity light sources, some broescopes provides magnification option, zoom controls or accessories.
v) Flexible Fibre Optic Borescope - Permits manipulation of the instrument around camers and through passages with several directional changes. Woven stainless steel sheathings protects the image relay bundle during repeated flexing and manoeuvring. The working lengths are normally 60 to 365 cm with diameters from 3 to 12.5 min.
vi) Video Imagescope - The video Imagescope is similar to a Fibrescope with the exception that video camera and its connections have replaced the image bundle and a TV monitor has replaced the eyepiece. This image may be magnified for precise viewing. The field of vision is up to 90 degree and probe tip has four way articulation. Presently the smallest diameter is 9.5 mm with working length up to 100 feet.
Applications : 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.
Key Points : Simple to use in areas where other methods are impractical. Accessibility required. Reliability depends upon the experience of the operator.
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.
Generally following acoustic mechanical principles are used to evaluate the damping characteristics of the specimen.
b.Pitch/catch swept test method : This test is best detecting unbonds and deeper defects.
c.Pitch/catch impulse test method : In this method the joints not testable by swept method, can be tested satisfactorily by this mode.
d.MIA(Mechanical Impedance Analysis) test method : This method works well on unbonds crushed core and defects on the inside of composite structure.
e.Eddy sonic harmonic test method : It is capable of detecting both near side and far side disbond.
f.Tap test : Tap test is a manual method. Tap testing is a common and inexpensive type of inspection. In this procedure the inspector taps the surface of the test structure and evaluate the sound generated. The inspector either listens directly to the sound or uses specially designed receiver to analyse the sound and compare the response with defect free part.
Application : 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.
Key points : Loses sensitivity with increasing material thickness. Electrical source and reference standards required.
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'.
Equipment : A thermal imager basically consists of a detector, a scanning system, an optical system & video display unit. The majority of cameras function like a television camera and their output is a video signal which is proportional to the output signal of the detector. Subsequently, this passes on to a signal treatment and visualization system which assigns to each level a grey tone in an scale or false colour. In this way, an image can be obtained on a TV monitor which represent the distribution of temperatures throughout all the field of viewor printed out as colour graphics.
Applications : 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.
Key points : This method shows temperature changes which can indicate defects. Required trained operator. Transportable & reference standards may be required.
Only with appropriate applications of nondestructive testing techniques can bring the benefits of advanced materials science be fully utilized.