Non-Destructive Testing and Inspection

METHODSVISUAL OR ETCH INSPECTIONFLUORESCENT PENETRANT - OP210MAGNETIC PARTICLE INSPECTION - OP201RADIOGRAPHIC INSPECTION - OP221EDDY CURRENT INSPECTIONULTRASONIC INSPECTIONTAP TESTINGLEAK TESTINGPRESSURE TESTINGHARDNESS TESTING

VISUAL & ETCH INSPECTIONInspection under white light for surface defects.

Can use magnification as an aid - X3 or X10 typical.

Etching can be used to reveal defects.the surface of the part is attacked by chemicals to reveal the grain of the material.

PENETRANT INSPECTIONPRINCIPLEComponent is coated with a penetrating liquid containing a coloured dye.Penetrating liquid enters into surface flaws through capillary actionExcess penetrant removed and developer applied.Penetrant drawn to the surface by absorption, staining the developer.

Basic sequence of operations.Surface preparation and cleaning, including surface protection removal e.g. paint.Application of penetrant.Removal of excess penetrant - can use emulsifier.Dry components.Application of developer.Removal of surplus developer.Inspection, using Ultra-Violet light.Post-inspection cleaning.Re-application of surface protection.

PENETRANTAdvantagesRelatively simple application.Inexpensive.Wide range of material applications.Not limited by component geometry / sizeNot limited by flaw orientation.Effective for tight/hairline surface cracks.Flaws readily visible.Processing can be automated.

PENETRANT LimitationsRelatively time consuming.Only applicable for surface discontinuities.Surface must be free of contamination.Only suitable for non-porous surfaces.Limited use for shallow or wide open flaws.Gives only approximation of depth and size of flaw.

MAGNETIC PARTICLE INSPECTIONPRINCIPLE OF DEFECT INDICATIONThe material is magnetised to a known degree and with a known direction of field.

This creates lines of flux in the material.

Defects lying across the lines of flux distort them and produce leakage fields.

The leakage fields attract iron oxide particles and thus reveal the flaw.

Flux LeakageMagnetic Force LinesNo discontinuitiesInternal defect parallel to fieldSurface discontinuityDeep discontinuity 90 to fieldRound discontinuity sub-surfaceShallow sharp discontinuity 90 to field

Basic sequence of operations.Clean and degrease to ensure absence of contaminants prior to magnetisation.Demagnetisation to ensure that the part being inspected is free from any residual magnetic fields.Magnetisation of the component - at the same time applying the iron oxide particles suspended in a carrier fluidVisual inspection for defect indications (flux leakage).Demagnetisation of the component.Post-inspection cleaning.

+-Fault IndicationsCurrent FlowLines of ForceCircular magnetism will detect faults that are 90o + 45o to the lines of forceDirect Induction

Fault IndicationsCoilMagnetism in a coil will detect faults that are parallel + 45o to the direction of Current FlowLongitudinal Magnetisation

MAGNETIC PARTICLE INSPECTIONAdvantagesQuick resultsTesting parameters are pre-determinedRelatively inexpensive.No elaborate surface pre-cleaning necessarySensitive for fine surface cracksEffective on components with thin non-magnetic coatings. (Maximum thickness - 0.025mm)

MAGNETIC PARTICLE INSPECTIONLimitationsCan only be used on Ferro-magnetic materials.Requires good accessibility & surface contact (Arcing)Surface/immediate sub-surface defects/inclusions.Limited effectiveness for complex parts.Post-cleaning and de-magnetisation is required.Reveals only defects transverse to magnetic field - multiple shots in different planes may be required.

RADIOLOGICAL INSPECTION Electromagnetic radiation is comprised of X-rays and Gamma rays capable of penetrating all solid matter. As the rays pass through matter, they are absorbed. The rate of absorption being dependant on the density of the material. The electromagnetic radiation can create an image on photographic film, or illuminate a flourescent screen. X-rays are produced by the impact of electrons on a target in a vacuum x-ray tube. Gamma rays are produced by the natural decay of radioactive elements.

RADIOLOGICAL INSPECTION

RADIOLOGICAL INSPECTIONDiscontinuities are revealed by a change in density on the radiograph.Pores or lack of fusion appear dark on the X-Ray film.Tungsten inclusions appear light.Cracks appear dark, but only if they are in the correct orientation.

X-RAYAdvantagesProvides a permanent record.Can be used for internal evaluation of most materials.No surface contact required.Internal mechanisms can be examined without dismantling.

X-RAYLimitationsRadiation Hazard.High equipment and testing costs.Defect depth not readily apparent.Access to opposite sides of object required.Complex geometrical shapes can cause problems.Waiting time for results.Material thickness and density restrictive.

EDDY CURRENT INSPECTION.A magnetic field is produced in the test coil by passing AC current through it.When the coil is close to or touchs a conductive matl, eddy currents are induced.The induced eddy currents produce a magnetic field that opposes the field of the coil.This causes a change in the impedance of the test coil.It is this change of impedance that is used to identify change in material characteristics.

EDDY CURRENT INSPECTION.

EDDY CURRENT INSPECTION.AdvantagesVersatile and mobile equipment.Rapid testing with immediate results.Minimum amount of preparation.Permanent record can be madeMaterial sorting capabilitySurface and sub-surface defects can be found.

EDDY CURRENT INSPECTION.LimitationsDifficult to test complex geometrysUse on conductive materials.High level of operator skill required.

ULTRASONICS

Ultrasonic testing can be likened to Echo sounding, in that it employs the use of high frequency sound energy.This energy is reluctant to cross an interface of large sound velocity change. The time taken to reflect a sound beam off the opposite face of a component is measured and displayed on a CRT.Any discontinuity favourably orientated will reflect back some of the sound energy.

GENERATION OF ULTRASONIC ENERGYUltrasonic waves are of the same nature as sound waves, but at a frequency too high to be audible too the human ear.Alternating current is used to excite a crystal to generate the high frequencies required.Most common crystals used are of Quartz or Barium Titanate.The principle is known as the Piezo Electro effect, which is reversible, in that the sound energy received by the crystal can be transformed back into electrical energy.

ABCCBComponentDefectProbeCouplingMediumCRT DisplayA : Contact Surface EchoB : Defect EchoC : Back Surface EchoBASIC ULTRASONICSET UP

ULTRASONICSAdvantages.Versatile and mobile equipment.Rapid testing with immediate results.Minimum amount of preparation.Effective on metallic and non-metallic materials.Not limited by material thickness - can be used on steel ingots up to 30 ft. thick.Ability to detect depth, size and orientation of defect.Sensitive to fine/hairline internal defects, if the orientation is correct.Permanent record.

ULTRASONICSLimitationsRequires a coupling medium.Poor signal (scattering) can be caused by rough surfaces or coarse grained material. Nature of the defect not readily determined.May lose defects close to surface.Limited capability on highly complex geometric shapes.Equipment is relatively expensive.Operators require extensive experience.

Other NDT Inspection methods.Transient Acoustic PropagationLeak Test - OP 222Pressure Test - JES 72

Transient Acoustic Propagation (TAP)Typically can used for crack detection on L.P. Compressor blades by determining the presence of the internal vacuum.PrincipleA probe, coupled to the blade, emits a sound pulse. The rate of decay of the sound pulse is measured by the Tap Test machine, which gives a visible numeric reading. If the reading is within a pre-determined range the test is satisfactory.

TAP TESTER MODEL TH100-BACOUSTICEMISSIONPROBE

L.P. Compressor BladeRootPosition probe in thisarea on convex airfoil

Tap test.A simpler method of tap test is that commonly used on composite structures. It literally involves tapping the structure with a solid object, and listening to the sound it makes.

Leak Test - OP222Used to check integrity of honeycomb seal brazing.Principle Determines braze integrity by filling the cells with a low viscosity liquid. (OMat 1/272 or 2/7).Liquid is stored in a tank, fitted with a circulating pump, capable of delivering a jet of liquid to remove trapped air from the honeycomb cells.

Leak Test - Op 222Method 1. Clean. (Not necessary if being tested immediately after brazing and parts have been kept under a clean plastic sheet).2. Immerse in solution & pass through jets until all air removed from cells.3. Drain surplus liquid.4. Examine for reflected light using asupplementary light source.5. Post-test clean (Air blast to remove all liquid).6. Oven dry.7. Visual overcheck of any suspect cells using fibre optic light source.

Pressure Test - JES72Used as part of inspection procedure for testing tubes after weld repairs.Prior to pressure testing, tubes should be visually, penetrant and X-ray inspected.Tubes should be pressure tested over their entire length.

Pressure Test - JES 72 - General PrinciplesOil feed tubes - Hydraulic & Air pressure.Oil vent & scavenge tubes - Air pressure.Hydraulic test 290 lbf/sq. in. for two minutes.Air pressure 15 lbf/sq. in. for one minute, immersed in water, watch for air bubbles Leakage/pressure drop not acceptable in either test.

NDT SPECIFICATIONS ALIGNED TO TSD OPSRPS 128- Vapour degrease - OP 101RPS 367- Corrosion protection - OP 340 RPS 461- Aqueous degreasing and cleaning - OPs 102,117,118RPS 693- High velocity steam/water cleaning - OP 144RPS 700- Process specification - MPI - OP 201RPS 702- Process specification - penetrant - OPs 210

Specifications ContdRPS 908- Training and approval of personnel outside RR - No alignmentRPS 906- Control of NDT - planning and technique instruction - No alignment. RPS 915- Training and approval of personnel within RR - No alignment