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Introduction

Non Destructive Testing (NDT) is the name given to any form of testing or inspectionthat can verify the structural integrity of a component without compromising its abilityto perform in service.

It is already, or is rapidly becoming, mandatory for safety critical components inapplications such as aerospace, oil and gas, pressure vessels and nuclear power,for the verification of weld quality or the monitoring of corrosion damage in service.

By definition non destructive tests have to be non damaging and non invasive, andmethods and techniques have been developed to considerable levels ofsophistication. As will be seen they all require operation of equipment andinterpretation of results by skilled, well-trained personnel.

This Profile will discuss the most widely used methods, introduce some othermethods and address some of the aspects of training and accreditation of operators.

Test Methods

Industry utilises many NDT methods. Some are used for surface examination andcondition monitoring

• Visual Inspection• Liquid Penetrant Testing• Magnetic Particle inspection• Others are used to examine the bulk or whole section of a component or structure

• Radiography• Ultrasonic Inspection• Acoustic Emission• Leak Testing

These methods will be described highlighting their application, current status, futuretrends and their strengths and weaknesses.

Graham Phipps, Central Test Centre Ltd

The selection of the most appropriate NDT method for a particular componentdepends on a number of factors

• Material of construction• Thickness of section• Type of defect to be detected• Cost• Inspection equipment available• Operator skill• Level of inspection

Visual Inspection

Visual Inspection is probably the first and most widely practiced method of NDT.

It involves inspecting, directly or indirectly, for surface condition, defects, roughnessor dimensional changes.

Typical applications Include:

Weld Inspection• verification of correct weld preparation• inspection for surface defects, e.g. undercuts and cracks• verification of completed weld profile (cap, root)

Metrology• measurement of component geometry• measurement of surface roughness

Condition monitoring in service• detection of corrosion• detection of defects and cracks

Etch Inspection• detection of grinding defects• detection of incorrect heat treatment (case depth)• metallographic examination (weld profile, inclusions grain size, microstructure)

Visual inspection has many techniques to aid the operator and extend its capability.

Lighting must be at an appropriate level and of the correct type.

Magnifiers, from low power, x2 to x5, hand held lenses to high power microscopes.

Borescopes are devices that permit inspection of internal surfaces in piping,engines, air frames etc. They contain systems of lenses, mirrors, prisms andlighting which illuminates the test area and transmits images to the inspector.

They may be either rigid or flexible. The flexible type uses optical fibres, the smallesthave outside diameters of just a few millimetres and can be steered into positionaround tight bends.

Television cameras combined with borescopes, video recorders and imageprocessing equipment permit the remote collection and storage of visual images.This permits inspection in difficult and hazardous sites, e.g. nuclear installations andunderwater pipelines. Image enhancement by computer is also possible.

Surface replication using strippable films also permits the remote inspection ofsurfaces.

Etching for metallographic examination and to reveal cracks, inclusions andlocalised variations in composition and hardness.

Ultimately, visual testing is dependent upon the eyesight of the inspector. Regulareyesight tests for both visual acuity and colour differentiation are essential. The timefor continuous viewing must be restricted with frequent rests to reduce fatigue.

Liquid Penetrant Testing

Fig.1: Liquid Penetrant Testing

Apart from visual inspection this is probablythe oldest and most widely used of all theNDT methods. It can be used on any non-porous material. Its use is confined to thedetection of surface breaking defects.

A coloured or fluorescent dye, when appliedto a clean surface and allowed to dwell for 10to 20 minutes, will be drawn into thediscontinuity by capillary pressure. Thecapillary pressure is determined by the widthof discontinuity, surface tension and contactangle of the dye on the surface.

The excess dye is then removed from thesurface and a developer applied, Figure1

The developer has two functions• to assist in drawing the dye back out to the

surface, giving an indication.

• to assist in seeing this indication by givinggood visual contrast.

After a time, usually a minimum of 10 minutes, the surfaces are visually inspected.White light is used for red dyes, ultra-violet light for fluorescent dyes.

Although fluorescent dyes do give a high contrast ratio that should make indicationseasier to see they do not always give the maximum chance of discontinuitydetection.

They are not suitable for components with rough or poor surface finishes as it isdifficult to remove the excess dye, causing an overall fluorescence that masksdiscontinuities, preventing their detection.

They do however make seeing indications in internal bores, cored holes, etc. easier.Liquid Penetrant testing systems are usually classified by the method used forexcess dye removal. Table 1.

Table 1: Comparison of Liquid Penetrant Testing systems

There are three main types• Solvent: usually supplied in aerosol cans and applied to the surface, excess dye

is wiped away with a lint free cloth.• Water washable: some dyes contain an additive which emulsifies the oil in the

dye making it soluble in water.• Post emulsification: the emulsifier is applied just prior to the water wash. The

emulsification stage is critical, too short a time means that all excess dye will notbe removed causing contrast reduction, too long a time and the dye will beremoved from the discontinuities making detection more difficult.

The developer is normally a dry powder carried in a non aqueous solvent.

Magnetic Particle Inspection

Fig.2: Magnetic Particle Inspection

The orientation of the flaw with respect to the direction of the flow of magnetic fluxgreatly influences the likelihood of creating a leakage field. Ideally the flaw should beat 90¡ to the flux flow, but detection may still be possible at 45¡. Components shouldbe tested in at least two directions to ensure maximum detection of flaws.

Magnetic flux can be induced in a test piece by

• using a permanent or electro-magnet• passing an electrical current through the test piece• passing an electric current through a coil which surrounds the test piece

For site conditions, or when dealing with large components, the simplest means ofinducing a magnetic field is with a permanent or electro-magnet. With suchequipment it is normal practice to paint the test piece white and to use blackmagnetic ink particles suspended in paraffin for the greatest contrast, again two testsare required at 90 to each other for maximum certainty of detection of flaws.

A very efficient method for the detection ofsurface and near surface discontinuities inany ferromagnetic material.

It relies on the principle that a magnetic fieldwill spread itself uniformly through acomponent unless disturbed by the presenceof a flaw. The disturbance generates a locallystronger field, known as a leakage field. Thiswill strongly attract any finely divided magnetic’ particles, either as an ink or dry powder,which are applied to the component. Thisproduces an indication, making the flawvisible, Figure 2.

A number of factors can influence theproduction and quality of indications

• defect size• depth of defect below surface• magnetising current• level of magnetisation• relative orientation of defect and direction

of magnetisation• magnetic properties of test piece• size and magnetic properties of detecting

media

The other methods are better suited to routine testing in laboratories or inspectiondepartments. Modern equipment is much smaller and lighter than in former times.The limiting factor is now the ’bed size’, i.e. the length between the head stockswhich hold a component and allow the magnetising current to flow.

Advantages• Simple and easy to conduct• Will detect surface and near surface flaws• Can detect flaws filled with contaminants e.g. oxide or non metallic inclusions• Sensitivity of testing can be specified and checked

Disadvantages• Can only be applied to ferromagnetic materials• Will not detect deep internal flaws• High currents applied to component may cause damage• Components usually have to be demagnetised

Radiography

Fig.3: Radiography

Radiography is a most usefulindustrial NDT method forthe detection of internaldefects. The principle is thatshort wavelengthelectromagnetic radiationsuch as X and gamma raysboth penetrate and areabsorbed by matter. Wheredefects are present lessradiation is absorbed and somore can pass through thecomponent into a detectingmedium, usually film, leavinga visible indication, Figure 3.

The essential properties of X and gamma rays may be summarised as:

• travel in straight lines• obey the inverse square law• cause ionisation• penetrate and are absorbed by matter• cannot be detected by any of the human senses• affect photographic film• cause materials to fluoresce

There is no difference in principle between medical and industrial radiography simplythe excitation energy levels needed for appropriate penetrating ability. Denser,thicker materials require higher energy radiation to penetrate them.

The energy levels are classified according to the voltage necessary to produce agiven wavelength and hence penetrating ability, Table 2.

Table 2: Influence of activation energy on the penetrating ability of ionising radiation

It can be readily seen that the energy levels used in industrial radiography are muchhigher than those in the medical field. Ionising radiation destroys human cells andcan cause death. This is a major health and safety issue and must be addressed byrigid adherence to well designed, safe working procedures and appropriate shieldingto absorb the radiation and protect the operators.

Advantages• Can penetrate most commercial materials• Not dependent on surface conditions• Produces a permanent record• Very high sensitivity by selecting correct energy/film combination• Can detect most types of defects under appropriate conditions• Some equipment produces results within minutes

Disadvantages• Safety considerations• Cost of equipment• Heavy shielding needed• Most commercial units, including gamma sources, can only penetrate up to

150mm section thickness• Operator training, qualification and certification

Real Time Radiography

This is a major advance in commercial radiography. The photographic film isreplaced with an image processor, a fluorescent screen and video equipment toproduce an image on a TV monitor in real time. The image can therefore beobserved remotely whilst the radiation source is still working.

Digital signal processing and enhancement can be used to amplify and improveimage sharpness. Images can be stored on video tape or, for more rapid retrieval, onlaser disc. A 12 inch laser disc can store more than 600 images reducing the storageand archival space compared with normal film.

Advantages• Speed of testing• Potential low cost per unit item with large volumes• Image manipulation• Recording of images without processing• Ease of storage and archiving

Disadvantages• Higher capital costs because of mechanical handling

Potential safety problems from mechanical handlingChange over down times can be longLimited flexibilityImage retrieval times

Ultrasonic Testing

Ultrasound is very high frequency sound, typically I to 5 MHz, inaudible to the humanear. It propagates readily through any solid material with a uniform fine grain size,e.g. rolled steel, at a velocity determined by its frequency and wavelength. It isattenuated by coarse, irregular grain sizes or cast structures. This reduction in soundintensity is caused by absorption and scatter from grain boundaries. The extent ofattenuation depends on the grain size to wavelength ratio. Generally testing ofcastings requires the use of lower frequency probes, I to 2-5 MHz, whichproduce longer wavelength ultrasound.

Ultrasound does not propagate through low density materials, e.g. air, and isreflected from any boundary between high and low density materials. Both defectsand the back surface of the component will reflect and any unexpected signal isan indication of a defect.

Ultrasound is generated by electrically exciting a piezo-electric crystal within a probe.Although the probe is placed on the component surface there may be a small airgap, and, as ultrasound will be reflected by an air gap, a liquid couplant is used. Theprobe and component are frequently placed in a water filled tank. The probe isconnected to equipment which activates the crystal, collects, processes and displaysreturning signals on a screen which gives information on flaw size depth and, fromprobe position, location, Figure 4.

Fig.4: Ultrasonic Testing

Modern flaw detectors now have liquid crystal displays rather than cathode ray tubeswhich are more difficult to read in bright sun and artificial light. There are two probeconfigurations, pulse-echo, where a single probe both transmits and receives pulses

of ultrasound, and through transmission which uses separate transmit and receiveprobe.

Other Methods of NDT

Acoustic EmissionMany methods of NDT use a form of external energy, e.g. X rays or ultrasound, todetect discontinuities. By contrast acoustic emission inspection observes the strainenergy released from the stress field around a discontinuity.When a discontinuity in a material is unstable and is affected by loading it will emitacoustic energy which reveals its presence. An analysis of this energy can determinethe integrity of the structure or material.

If the discontinuity is not loaded it is not an energy emitter, i.e. it is in a stablecondition and cannot affect the integrity of the material.Acoustic emission is a real time test and is limited to the detection of ’active’flaws. It is also non-directional and most flaws act as point source emitters whenstress is applied in proof testing, bend loading etc.

Often a single sensor located anywhere on the structure can detect emissions.

A number of methods are used for data collection including plotting summations,comparison of raw signals and visually observing the emission signal on anoscilloscope.

Leak Testing

Leak testing determines the rate at which a liquid or gas can, as a result of pressuredifferentials, penetrate in or out of a tight component or assembly. The term leak’normally refers to a discontinuity or passage through which the fluid permeates.

There are two basic types of leaks, real and virtual.

A real leak is a discrete passage or defect though which fluid can flow or transfer bypermeation. In a permeation leak gas can flow through a solid which has noapparent holes by processes such as absorption, migration or desorption

Virtual leaks are the gradual desorption of gases from the surface within a vacuumsystem, however vacuum systems may also have real leaks.

The methods of leak testing in common use are:

For gas systems at pressure• Acoustic methods• Bubble testing• Flow• Smell• Infra Red

For liquid systems at pressure• Visual examination• Surface wetting• Weight loss

Vacuum systems• Manometers• Gas leak• Mass Spectrometry

In all cases the result is expressed as a leak rate which is measured as the mass offluid transferred per unit of time.

Neutron Radiography

This form of radiography uses neutrons rather than X rays to form an image of thetest piece. X rays are attenuated by elements of a high atomic number and densitywhereas the attenuation of neutrons generally decreases as the atomic numberincreases. This allows the radiography of materials with low atomic number, evenwhen they are present in assemblies of high atomic number materials.

Typical applications are• the evaluation of adhesive bonded metal assemblies• metal jacketed explosives• the correct placement of plastic components, 0 rings or seals

Neutrons do not affect the photographic medium used in conventional radiography.To observe an indication it is necessary for the beam of neutrons to strike a material,such as indium, which absorbs the neutrons and becomes very briefly radioactive.This produces secondary radiation which affects the detection medium.

Infra Red Thermography

This test method observes differences in the infra red radiation emitted from hot orheated structures to detect variations in their structure e.g. defects, lack of bonding,impact damage or corrosion.

The structure may be heated in service, e.g. furnace walls, pipe work or vesselscontaining hot fluids, electronic circuitry, or by an external heat source such as alaser or high intensity lamp. Infra Red detectors, linked to television cameras, areused to generate and record images which can be enhanced to produce maps oftemperature distribution in the component. For example a scan made on an aircraftstructure when it has just landed can reveal corrosion sites. Any moisture within thestructure will be at corrosion sites and show up on the infra red thermograph cameraas colder areas by a colour change.

Operator Training and Accreditation

It will be obvious from the earlier sections that any NDT examination seeks eitherindications which are indirect evidence of a defect or flaw, or to monitor the conditionof the part in service.

Considerable operator skill is required to conduct the tests, interpret, evaluate andreport results to a standard, code or specification. This places a great responsibilityon both the testing authority and the operator. NDT operators can now beindependently accredited by the 'Personal Certification in Non Destructive Testing'(PCN) scheme which operates in the UK under the guidance of The British Instituteof Non Destructive Testing. PCN satisfies the requirements of EN 473 and ISO9712.

The American Society for Non Destructive Testing (ASNT) also has an operatortraining and accreditation scheme. Companies seeking to utilise NDT should install aquality system and be accredited to ISO 9000.

Conclusion

Growing demands are being made on manufacturers, from their existing customers,to supply components at ever higher quality levels. To meet these needs, and tostrive for business growth in other areas, every available tool including NonDestructive Testing should be employed.

NDT will require initial investment in both equipment and operator trainingand accreditation, but need not be too expensive to install. It will quickly make avery positive contribution to cost reductions in prototyping, product and processdevelopment and in scrap analysis.

An NDT system operated by well trained and qualified staff who enjoy the confidenceof management and its customers, backed by a quality systemapproved to ISO 9000 is a considerable asset to any company.

Where to get advice

The British Institute of Non Destructive TestingI Spencer ParadeNorthampton NNI 5AATel 01604 259056Fax 01604 231489is the learned society for NDT in the UK and is active in all aspects including advice,training and accreditation of operators.

In addition many subcontract testing companies are willing to offer advice andassistance.

The following are all active but the list is not, and cannot be, exhaustive. Youshould always satisfy yourself that a subcontractor is competent.

Central Test Centre LtdUnit 4, Prospect RoadHalesowenWest Midlands B62 8DZTel 0121585 7616Fax 0121550 5535

OIS plcPO Box 45Bowesfield LaneStockton on Tees TS18 3YHTel 01642 604661Fax 01642 670300

Bodycote Midland ResearchHalesowen RoadNethertonDudleyWest Midlands DY2 9PLTel 01384 236661Fax 01384 236635

Bl Commercial X Rays LtdMucklow HillHalesowenWest Midlands B62 8DLTel 0121550 3071Fax 0121585 5614

Aerospace Inspection TrainingHanger ANorwich AirportNorwich NR6 6EGTel 01603 426165Fax 01603 424019

Sources of further Information

Insight' is the monthly journal of the British Institute of Non DestructiveTesting and covers all aspects of the subject. Their address is given above.

ASM Handbook Volume 17 Non Destructive Evaluation and Quality ControlISBN: 0-87170-023-9Published by ASM International,Materials Park, OH 44073 USA