NON-DESTRUCTIVE TESTING AND

THE PRODUCTION ENGINEER

by W. G. COOK, B.Sc.

Presented to the Derby Section of the Institution on 15th October, 1956.

Mr. Cook is a Quality Engineer at Rolls-Royce, Ltd., Derby, in charge ofa section engaged in evaluation and development of non-destructive testingmethods.

At one time a science master, he was in the Aeronautical InspectionDepartment stationed at Rolls-Royce, Ltd., from 1940- 1945, and joined thatCompany in 1950 as a member of the Process Development Department,transferring to the Quality Department in 1955.

Mr. Cook is on the Committee of the Non-Destructive Testing Group ofthe Institute of Physics and is also a member of the Penetrant and MagneticPanel of the British Welding Research Association.

IN the last decade, the functions of the productionengineer have been widened and deepened to anextent which presents a formidable challenge to allmembers of that profession.

The successful production engineer of today is theleader of a team which embodies a wide range oftalents, knowledge and skills. He has at his disposalthe advice of numerous experts and specialists,technical and otherwise, whose knowledge rangesfrom the technical means of production to the estima-tion of the production potential and the complexproblems of capital expenditure likely to be justifiedby a problematic scale of production. A sound know-ledge of industrial phychology and high skill inhuman relations is also required, since the mostdifficult problems of all are likely to be thoseassociated with human beings, both at a higher leveland in subordinate positions.

The specialists involved thus cover a large numberof activities, dealing with machine tools, jigs andfixtures, cutting media, forges, presses, welding equip-ment, foundry techniques, sand casting, die casting,lost wax casting, shell moulding, copying machines,transfer machines, automatics, machining research,operation planning, machine loading, pre-productionplanning, statistics and estimation, inspection, as wellas other activities too numerous to list.

Formidable as this list may appear, it is likely tobe enlarged in the future and, therefore, this is awelcome opportunity to put before a meeting ofproduction engineers an outline of the work beingdone in the field of non-destructive testing. This isnot a new field by any means, but the past few yearshave seen a tremendous quickening of interest in thiswork. This has been brought about automatically bythe increasingly critical nature of defects in highly

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stressed aircraft components of today, highlighted bythe Comet disasters, and by the obvious commercialaspects involved.

The Americans have coined a phrase "Non-destructive testing doesn't cost it pays", andthere is no doubt that in its proper application non-destructive testing can act as an invaluable guide tothe production engineer by telling him how effectivelyhe is succeeding in his efforts and, in the ultimatecircumstance, may be the only means of keeping himin business. Production of itself is meaningless unlessthe product is suitable for its purpose, is of sufficientlyhigh quality to maintain its position in a competitivefield, and unless the production engineer hasdeveloped a technique whereby rejectable componentsare kept to an economic minimum. Here we havethe crux of what can be a very thorny and difficultfield of contention, where it is vital to consider allaspects of the situation in order to ensure that theflaw detection processes are adequate and realistic.

Before examining this problem in any detail, how-ever, it is necessary to indicate briefly the scope andlimitations of the various techniques at present avail-able for non-destructive testing and to stress that inthe main it is proposed to deal with flaw detection.At the same time it is proposed to deal with a numberof projects of special interest and then to return tothe broader problem of how this work is of greatvalue to the production engineer and how, conversely,the production engineer can help by a constructiveapproach and assist in establishing realistic methodsof application.

Available Techniques1. Visual Inspection

The first and perhaps most important techniqueto be considered is visual inspection. Many of thetechniques subsequently mentioned have visualinspection as an integral part of their procedure andin some cases this is the most critical aspect of them,so that some comments on visual inspection areappropriate.

Visual inspection is by far the oldest techniqueknown and can achieve adequate results if properlyapplied. Low magnification lenses or binoculars (upto 5 or 8X) are frequently used and conditions mustbe selected so that there is good contrast betweendefects and the background. Good, glare-freeillumination is required, free from dazzle and specularreflections. Operators must be able to work free fromphysical strain and jigs and supports provided wherethese ease the problem of handling components.

Careful balance must be maintained between themagnification used and the area an operator isrequired to examine. Higher magnifications aresometimes used with the assumption that sensitivityis being increased. Any gain is likely to be paid forin decreased reliability and it must be rememberedthat binocular work on an eight-hour day basis istedious and exacting. High power binocular micro-scopes are basically for looking at something alreadyfound, not for finding something to look at. Fatigueand boredom are serious problems and considerableskill of diagnosis is called for.

INERT POWDER

CRACK

I. PENETRANT 2. SURFACEENTERS CLEANED.CRACK. PENETRANT

IN CRACK.

FLAW DETECTION BY PENETRANT

3. POWDERDRAWS OUTPENETRANT.

Fig. 1. Principles of penetrants.

2. PenetrantsOne of the defects of plain visual inspection is often

the lack of contrast between the defect and the back-ground. Most penetrants represent an attempt toovercome this lack of contrast and at the same timepresent a magnified representation of the crack.

The basic procedure is as follows :-The clean component is covered with the penetrant

and time is allowed for the liquid to enter the crack.The surplus is then removed from the surface and thepenetrant drawn out by application of a fine filmof inert white powder. The colour of the penetrantshows up against the background and the naturalseepage from the crack gives a larger area of signalthan the original crack. This is shown in Fig. 1.

Although there are penetrants working on a slightlydifferent technique, the three principal types are asfollows :-

(a) Hot Oil and ChalkThis is one of the earliest and most primitive

methods. The component is soaked in hot paraffinand lard oil and then cleaned with sawdust or bywashing with detergent solution. The clean, drycomponent is then dusted with fine, dry chalk. Thefilm of chalk is stained by the oil and gives a magni-fied indication of the defect. Sometimes a build-up ofchalk adhering at the point of seepage gives thesame effect.

This process, however, is relatively insensitive totight cracks, since the contrast of defect indication tobackground is very low and although the ingredientsare relatively cheap the inspection is both slow anduncertain, except for large defects.

(b) Dye PenetrantsIn order to increase the contrast, dyes, usually red,

have been added to the penetrants which have alsothemselves been modified to work without the appli-cation of heat. These new penetrants are generallywater emulsifiable, so cleaning is a straightforward

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Fig. 2. Pilot plant for penetrants.

water wash, and sensitivity has been further increasedby using a developer consisting of a suspension offine powder in a volatile carrier. Dry powders maybe used with some loss of sensitivity.

This technique is much more sensitive than oil andchalk and is particularly suited to castings and alsofor local application and field work,(c) Fluorescent Penetrants

Still further contrast can be obtained by the use ofhighly fluorescent additives. These penetrants givebright yellow or green indications against a deeppurple background when the component is viewedunder " black" light. This type of penetrantrepresents the greatest sensitivity at present achievableand, in a proper application, the easiest and speediestinspection of all. The process is illustrated in Figs.2 to 5, which show a pilot plant for processing turbineblades.

The difference between the defect indications andthe background (yellow to purple) is probably suffici-ently marked for the use of photo-electric cells andfilters, in order to make the location of defects auto-matic by signalling the presence of yellow light.Attempts are being made to make such an automaticinspection device, although preliminary tests indicatethat the filtering problems are not easy to overcome,and the system must be capable of detecting a pin-point source of light.

Penetrants will only indicate defects which areopen to the surface. As the sensitivity increases., theproblems of diagnosis increase and the selection of asuitable penetrant involves fairly large scale tests toassess these factors. In general, every possible applica-tion should be treated on its own merits, since factorsother than sensitivity may be involved. These includespeed of operation, cost of materials, general work-shop conditions and labour available.

Fig. 3. Washing under black light.

Fig. 4. Developing.

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Fig. 5. Inspecting under black light.

3. EtchingEtching is used extensively for flaw detection, acid,

alkaline and electrolytically assisted techniques beingused according to the material. The chemical action,of course, removes material with preferential attack inthe region of defects and is not completely non-destructive. For components of critical dimensions,only limited etching is possible, as repeated etchingmay affect components produced to tight limits. Asa laboratory technique it is one of the most stringentmethods available, although incorrectly used it canproduce cracks in otherwise serviceable components.

A good etch is a very searching means of crackdetection and may reveal very tight defects likely tobe missed by penetrants. This is due to the defectsbeing opened up by the chemical action, whereasprior to etch no cavity for penetration existed. Thedefects frequently show well against the etched sur-

face, although instances have been found of the etchcamouflaging the defect in the grain boundary pat-tern. Spurious indications can also be very trouble-some and cause difficulties in diagnosis. Frequentlyalso binocular microscopes have to be used to assistin detecting very fine cracks and this makes the pro-cess very tedious and exacting, as mentioned previous-ly. Visual fatigue and boredom are serious factorsand operators have been known to miss glaring crackswhilst searching for minute indications.

Etching is, of course, restricted to defects whichreach the surface, although in some cases it removessufficient material to show defects which were origin-ally beneath the surface.

Considerable care must also be taken to assess theeffects of a proposed etching technique on the fatigueand corrosion resistance properties of the components.This may involve a protracted series of lengthy tests.

4. Anodic InspectionThis technique is used mainly on forged aluminium

alloy components and relies for its effectiveness on theseepage of yellow chromic acid showing against thematt grey background. The process has also a mildetching action and defects which do not seep canalso be detected.

This technique only reveals surface defects.

5. Magnetic Flaw DetectionThis process is applied to magnetisable components

and relies on the fact that the defects have a lowerpermeability than the material itself and consequentlydistort the magnetic field. At or near the surface, thisdistortion causes a leakage field which is strong enoughto attract fine particles of magnetic material.

The accumulation of particles indicates the defectwhich may be a crack, slag inclusion, segregation, or

LEAKAGEFIELD

DEFECT

N fMAGNETICFLOW LINES

PERMANENTOR ELECTROMAGNET

I. MAGNETIC FLOW

HOLLOWCOMPONENT

\ _

DEFECT

COPPERBAR

LOW VOLTAGEHIGH AMPERAGE AC

3. THREADING BAR

CURRENT THROUGHCOMPONENT

DEFECT

nA

I LOW VOLTAGEHIGH AMPERAGE AC J

2. CURRENT FLOWCOMPONENT IN COIL

DEFECT-LOW VOLTAGE

HIGH AMPERAGEAC OR DC

4. SOLENOIDIN EACH CASE DEFECT SHOWN FOR MAX. SENSITIVITY.

Fig. 6. Principles ofmagnetic flaw detection.

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Fig. 7. Magnetic weld inspection.

change of structure. Not all of these are necessarilycauses for rejection. The basic procedure is illustratedin Fig. 6. Leakage fields are also caused at sharpchanges of section, corners, etc. Magnetic crackdetection when properly applied is very sensitive,although diagnosis can present-considerable trouble.It reveals surface and sub-surface defects accordingto the procedure but has the following limitations.It is directional to some extent and flaws in or nearthe direction of the magnetic flow lines are liable tobe missed. To inspect a component for flaws in everydirection can be slow and for complex shapes involvessuch a number of handlings and inspections as tomake the process difficult to control, particularlywhen large numbers of components are involved. Itis also rather messy and it is not always easy to tellwhether the component has been effectivelymagnetised. The indications are easily wiped off andgreat care is needed in handling components.

The magnetic powders in use are generally ironoxide suspended in a paraffin and black powder is themost common, although red, yellow, silver andfluorescent varieties are available. These variantsare used in attempts to increase the contrast between

Fig. 8. Comparison of magnetic and X-ray results onbutt welds.

the flaw indication and the background. Anotherway of doing this is to paint the background white oruse thin, white adhesive tape. The tape techniqueshows very promising results for weld inspection andthe results of tests compare very favourably withX-ray. Fig. 7 shows the procedure and Fig. 8 showstypical results.

A variant of the ink technique is in process ofdevelopment. In essence this is an attempt to pickup the leakage field at a crack by a coil or probe andfeed the signal into electronic equipment which willautomatically signal the defect. Equipment is avail-able to do this in fairly simple cases, although thesensitivity achieved to date does not equal that ofthe ink method. The basic problem is that the leak-age field at a crack is very weak and can only bepicked up very close to the surface.

To make probes capable of scanning so close to asurface is not easy. The ink technique, of course, hasthe advantage that the fluid is in intimate contact andweak, local fields are detectable.

However, insofar as the probe technique offers ameans of introducing automatic inspection withoutthe need of viewing, it is a line of development to bepursued energetically and the moderate sensitivity sofar achieved should not be allowed to discouragefurther attempts to improve on this.

6. Eddy CurrentsIn recent years, considerable progress has been

made in developing electronic equipment which willdetect defects by eddy currents. In principle, the

Fig. 9. Crack detector for light alloy, compressor blades.

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Fig. 10. Measurement of anodic films.

instrument measures the effect on a coil carrying highfrequency currents when a metal is introduced intothe field of the coil. Devices of this nature have beenavailable for a number of years, but suffered fromthe defect that they were affected by a number ofvariables such as dimensions, conductivity, composi-tion and hardness as well as defects, and it wasdifficult or impossible to resolve the effects of these(and possibly other) variables. Recent work has pro-duced instruments capable of resolving these to someconsiderable extent and instruments are now availablefor detecting defects in non-ferrous materials. Theseinstruments are, however, very sensitive to the positionof the probe relative to the component so that theapplication is limited to uniform components, e.g.,ground bar, tubes, etc., and a great deal of theirvalue depends upon the severity of the defects theyare expected to detect.

The limitations due to geometry are thus con-siderable for general use, but for the inspection ofcritical components where considerable expense is

justified, their application is likely to be extended veryconsiderably in the next few years. Here again, wehave a potential method of inspection without visualinspection.

This offers high speed automatic, foolproof inspec-tion and justifies the expenditure of considerableeffort despite the limitations and difficulties en-countered. Fig. 9 shows an eddy current machinefor detecting cracks in the bores of compressor blades.

By suitable phase analysis it is possible to devisemachines based on eddy currents which can measurethe thickness of anodic films on light alloy com-ponents (see Fig. 10) and a number of measuringdevices on the same principle have been developed.

Direct measurement of conductivity is also feasiblewith this technique and the sorting of mixed materialsis often possible by this means (see Fig. 11).

7. UltrasonicsUltrasonic inspection is a tool of great importance

to the production engineer and recent years have seena great increase in its use. This has been due to theimprovement in instruments and the rather slowerrealisation of its potential by engineers generally.Although attempts are being made to deal withfairly complex sections, it is mainly in the field oflarge simple shapes that ultrasonics promises to bemost useful. The principle of contact scanning isshown in Fig. 12 and Figs. 13 and 14 show applica-tions of this simple principle. The extrusion defectsfound in the rectangular bar are shown in Fig. 15,the bar having 0.150" machined off one edge to re-veal the defects which are completely internal. De-fects found in the round bar are shown up bypenetrant in Fig. 16. Fig. 17 shows a fractured barand Fig. 18 shows pipe located by ultrasonics. Analternative method of finding these defects by ultra-sonics is shown in Fig. 19, using an angle probe anda beam of ultrasonic waves echoing up and downalong the bar.

Fig. 11. Sorting materials by eddy currents.

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TOP SURFACEFLAW ECHO

BOTTOM SURFACE

-SCREEN TRACEPROBE

OIL FILMCOUPLING

ULTRASONICUNIT COMPONENT ^-DEFECT

CONTACT SCANNING

Fig. 12, Principles of contact scanning.

Fig. 15. Internal defects found by ultrasonics in rectangularbar (x 2 / 3 ) .

13. Ultrasonic inspection ofrectangular bar.

Fig. 14. Ultrasonic inspection of round bar.

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By finding defects in material at the earliest pos-sible stage considerable sums of money are savedand the resources of a modern production unit are notdissipated in efforts to produce components from un-sound stock.

This point is further emphasised by Figs. 20 and 21.These show internal flakes due to hydrogen embrittle-ment in a 6' 5 | " billet of S.82 steel. The materialwas rejected in the billet form.

Another^gain from the consumer's point of view isthat the metal producers become actively interested inthe techniques and results obtained, and this can leadto a big improvement in quality.

Ultrasonic inspection by contact scanning has anumber of disadvantages. Apart from' the question ofawkward shapes already mentioned, it is inclined tobe messy and unreliable when scanning large areas.The probes tend to wear away, but the most importantfeature is the inability to be sure that the operatorhas fully covered the component. Most modern setshave single probes combining the duties of trans-mitter and receiver, although some sets have doubleprobes, but even with single probes difficulties areencountered which can be best described as " fingertrouble ".

In an attempt to overcome these difficulties, themethod of immersion scanning has been developed.The principle is basically the same, but the couplingbetween the transmitter and the component is effectedby a layer of water instead of a film of oil. Thebasic idea is shown in Fig. 22 and a simple tankworking on this principle is shown in Figs. 23 and 24.This technique ensures complete scanning of suitableareas and this can be made fully automatic withmonitoring to indicate defects. A great deal of workhas been done in the States on this type of equipment,chiefly on simple shapes such as light alloy slabs foraircraft spars, etc. Steel discs and circular forgings arealso being examined in much the same way.

Figs. 16 - 19 (reading from top down).

Fig. 16. Cracks in round bar found byultrasonics and shown by penetrant.

Fig. 17. Fracture of round bar showingdefect (x 1) .

Fig. 18. Internal pipe found by ultra-sonics (x | )Fig. 19. Ultrasonic inspection of bar byangle probe.

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Fig. 20. Axial flake in S.82 steel found by ultrasonics(x i ) .

The tank illustrated in Fig. 23 has produced somevaluable results. Typical defects located inside steelforgings are shown in Figs. 25 to 28. The ability toassess the depth of these flaws as well as their radiusenabled predictions to be made as to whether theywould fall across or in the finished section.

Only in this case can the forging be rejected, sincedefects outside the finished section will machineaway without being noticed.

In addition to the large saving in machining timeeffected in these cases, some of these componentswere in short supply and their early rejection enabledreplacements to be progressed and thus reduce delaysin the engine programme.

There appears to be a big future for this type ofwork and it is fairly safe to predict that the ultrasonicinspection of all heavy rotating members of aeroengines will soon be mandatory. The equipmentillustrated has been used to anticipate thisrequirement.

The limitations of this type of equipment are,firstly, its inability to cope with complex shapes due todifficulties of interpretation and, secondly, that itcannot readily deal with sections below about 0.500".These two conditions combined make the use of thistype of ultrasonics on finished aircraft componentsrather small. The use of improved crystals and cir-cuitry, however, may help to reduce the section cap-able of being inspected by echo techniques. Inaddition, two other ultrasonic techniques are nowavailable : inspection by surface waves and inspectionby transmission techniques. Under suitable conditionsultrasonic waves can be generated which skim thesurface of the component and detect cracks by echo.Fchoes are also produced by sharp corners and radiiso the possible applications are limited. Fig. 29 showsa surface wave crystal in use.

The transmission technique consists of sending acontinuous beam of sound through a component, withinternal reflections if necessary, and measuring theintensity of the emergent sound beam. The presenceof a defect causes a sharp fall in intensity.

This seems a very good tool for special investiga-tion, the main difficulty being to ensure good, con-sistent coupling at both transmitting and receiving

Fig. 21. Transverse flake in steel found by ultrasonics(x i ) .

probes and to maintain constant geometrical relation-ship by firm jigging. On a production basis thispresents great difficulties and the use of immersiontechniques seems the only answer. This, however,remains to be established. Fig. 30 shows an instru-ment working on this principle.

8. X-RayRadiography is well established as a means of

inspecting the interior of components, although as ameans of crack detection it must be rated very low.Basically, this is because radiography is capable ofdetecting difference in X-ray path, generally of theorder of 2% of the section under examination, al-though much better sensitivities are claimed in specialcircumstances. Cracks very rarely occupy this propor-tion of the section unless orientated exactly in linewith the beam. For example, X-ray is incapable ofrevealing defects similar to those shown in Figs. 25to 28.

-TOP SURFACEFLAW ECHOBOTTOM SURFACE

TRAVERSE OFSOUND IN WATERTUNED OFF SCREEN

PROBE IN.WATERTIGHTHOLDER

WATER-

COMPONENT-FLAW

IMMERSION SCANNING TURNTABLE

Fig. 22. Principles of immersion testing.

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Fig. 23. Ultrasonic immersion tank.

Fig. 24. Ultrasonic immersion tank inuse.

Fig. 25. Defect found by immersion testing (x } ) , Fig. 26. Defects found by immersion scanning (x

106

Fig. 27. Disc cut and fractured to show defect (x

Thus the main function of X-ray is to detect in-ternal unsoundness of a relatively gross order, andinstances can be found of designers calling for X-rayexamination with the mistaken idea that they areinstituting an effective inspection for flaws.

By replacing the photographic plate by a fluores-cent screen, it is possible to inspect components muchmore cheaply and more rapidly. Unfortunately thereis a serious loss of sensitivity and fluoroscopy can onlybe used to detect gross defects. It can, however, bevery effective in certain applications, e.g., checkinginaccessible assemblies such as radio valves, electriccircuits, etc., for completeness.

General ObservationsThe foregoing notes give a brief outline of the

scope of the work going on in the various fields ofnon-destructive testing. The techniques themselvesare very varied, but anyone working on them willencounter difficulties and problems which are com-mon to them all.

StandardsOne problem is that of standards. A great deal of

confusion frequently exists here and the standard ofserviceability is often only vaguely defined. Truth issaid to be at the bottom of a well and this is certainly

Fig. 29. Inspection by ultrasonic surfacewaves.

Fig. 28. Defects in forging found by immersion testing(x i ) .

a deep one. Designers, quality engineers and inspec-tors often err on the side of safety as they are boundto do, especially if they are dealing with componentsabout which little service experience is known. Pro-duction engineers see the problem a little differentlyand have been known to express strong opinionsabout standards which appear too stringent. It is,however, important to realise that the finding ofdefects by a particular technique and the establish-ment of standards are two distinct processes.Admittedly, a new technique of inspection may revealdefects hitherto unseen and so affect the standard ofacceptance, which has always to be assessed in thelight of as full a knowledge as can be obtained andcalls for close liaison between Production, Service andDevelopment Departments. A great deal of work isinvolved in correlating standards by two differenttechniques and improvement in flaw detection tech-niques almost always makes the problem moredifficult; this difficulty is sometimes the reason forrejecting a new technique, perhaps unwisely.

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Attitude to InspectionA secoind problem is a rather broader one and that

is the attrtude of mind towards inspection. Thisattitude is shown at its worst by those who regardinspection as a necessary evil, not to be encouragedunduly.

Vast sums of money have been spent on the meansof production ; multi-spindle machines, transfermachines, bigger and better forges.^and presses, auto-matic multiple copying machines, high speed tools,automatic finishing machines and so on have beenproduced to speed up production. Whilst the meansof production have vastly increased, very little con-sideration or effort has been made in the past to speedup the means of flaw detection. It is therefore nowonder that management suddenly find that it iscosting more to inspect some components than toproduce them. The checking of dimensions has beenmechanised in some cases. Admittedly the problemsare of a different order of difficulty. To make, say,a multiple head copying milling cutter is simple,compared with the problem of designing an instru-ment capable of snowing the discrimination andsensitivity of the human eye over the wide range ofindications by which defects can show themselves.Nevertheless, unless determined efforts are made tosolve this problem the inspection of components islikely to take an increasing proportion of the overalltime to produce a component. In order to reduce theadverse ratio of production to inspection time, it isevident that considerable time and money must bespent to apply the resources of science and electronicsto produce instruments capable of replacing orsupplementing the human eye.

The Problem of NumbersThe third problem facing the worker in this field

is that of numbers. This is encountered in threerather different ways.

The first is fairly simple, although it is surprisinghow many workers in this field have failed to realise

it. On any production line flaws and defects occurunder very variable conditions and the causes mayrange from individual batch or component troublesto complete changes of technique. It is difficult to besure beforehand that a given batch is even typical,so it is inevitable that considerable numbers of com-ponents need to be processed to arrive at a correctoverall picture and so form a reliable assessment.

Some workers in this field. arC prone to producingodd specimens (usually with a freak defect) and in-viting the unwary enthusiast to try his hand. Theresults are usually misleading and it is imperative towork on as large a number as possible, to use a fairlylarge number in a trial run to learn the pitfalls andestablish a working standard and then to follow thiswith a sizeable pre-production run. The findingsshould be assessed by all means available, using othernon-destructive techniques as well as destructive tests,to ensure that realistic conclusions are obtained. Thenumbers involved will vary according to the type ofcomponent. To give a concrete illustration, a recenttest on a certain critical component involved over8.000 pieces before production inspection wasinstituted. In an exercise on precision castings some300 were inspected to verify the technique and over3,000 for trial purposes. On large castings, smallernumbers may be sufficient to give an answer, sinceeach casting can form a number of exercises in itself.

The Problem of High Standards of ReliabilityThe second problem of numbers is more difficult,

particularly on aero engine work. This problem isbasically that of improving on a process which hasalready achieved a reliability of, say, 99.9 + %. Againtake a concrete illustration. A component is beinginspected at a rate of 50,000 a week with a reliabilityof, say, one defective component missed per 10,000inspected. This is a high order of performance byany standard, but nevertheless represents five com-ponents per week which must be rejected. Thedefects missed are found on a second inspection, but itis obviously desirable to find a foolproof first inspec-

Fig. 30. Inspection by ultrasonictransmission.

108

tion technique. To do this entails, a very large scaleeffort indeed and may necessitate running two inspec-tion processes side by side on a production line, withalarming results on output. Of course, before thiswas embarked upon, smaller trials to assesspotentialities would have to be successfully ^carriedout, but these figures illustrate the size of the problemfacing the aeronautical engineer today. Whether anyprocess involving human perception and skills cansubstantially improve on 99.9% reliability is itselfdoubtful, and the field for reliable automatic inspec-tion devices is wide open.

Detection of Rare Defects in CriticalComponents

The third problem is that of finding an inspectiontechnique for a defect which is rare although it iscritical. In this case, also, one is faced with a veryprolonged programme of testing and evaluation, andconsiderable patience is needed in recording andanalysing results. Frequently much of the evidenceis negative. To take a concrete example ; say 500forgings are inspected by a new ultrasonic techniqueand passed as free from defects. These have to befollowed through and out of these, 490 are finallypassed as satisfactory, and 10 are scrapped for reasonsother than defects. The results of this test are largelynegative. It is only when a defective forging ispredicted and confirmed, or a forging passed by ultra-sonics is rejected as cracked at final inspection, thatpositive evidence can be obtained. The infrequencyof internal cracks in some forgings leads to verylong tests to attempt to prove a point. The diversionof such a large quantity of forgings inevitably hassome effect on the progress of production items, andit is clearly essential .that co-operation with productionpersonnel is maintained. For an exercise of this scaleto be effectively carried out and follow-throughchecks to be made, it is unavoidable that many peopleare involved and the problem is made much easier ifthe production engineers concerned appreciate thebroad outlines and purpose of the effort.

Returning to the 500 forgings, there is a majorpitfall to avoid. The conclusions on the ultrasonictests assume that checks have been made to provethe validity of the procedure. But it must be bornein mind that 500 good forgings could have beenpassed good by any technique, for that matter withthe ultrasonic set switched off. This point is not soridiculous as it seems. Inspection techniques of thissort have been known and since they are equallyeffective in passing good and bad components, solong as the components are good everyone is happy intheir ignorance. It may be only when defects arefound by some other way that the inadequacy of thetechnique is realised. Where defects are very rare theproblem has always to be posed as to whether theabsence of defects is genuine, or whether they arepresent but not found by the technique in use.

This brings us to the final arbiter of all thingsgood or bad the engine itself. All standards areultimately set by the engine a very hard taskmaster.It shows neither discretion nor consideration, althoughit frequently shows disconcerting unpredictabilities.

As powers and performances advance and stresses rise,new standards are needed and old standards, builtup over years, may be scrapped overnight. Theproduction engineer is faced with making new andmore complicated complexities and tlie inspector andquality engineer have to find ways and means oftesting them to reasonable standards. It is only by afull understanding of each other's problems thatprogress can be achieved and the future assured.

By way of recapitulation, it is worthwhile to lookat the flaw detection processes in a functional capacityand in this respect they serve in three distinct fields.

The first is the inspection of raw materials andpart processed components. Here the main objectiveis to prevent the production engineer applying hisefforts to unsound stock. In many cases, thetraditional laboratory metallurgical procedures arevery inefficient for flaw detection and they areincreasingly being supplemented by the use of ultra-sonics, penetrants, eddy current and other techniques.Inspection at this stage offers great saving in wastedmachining time and also provides a very valuablecontrol in that it prevents the pipe line from beingfilled with unsound material. This latter aspect issometimes not appreciated until it is too late, andproduction schedules and programmes are disruptedwhen components are rejected in the finished state.

The second function is that of detecting defects infinished components. Here, by far the greatest effortis expended and the defects thrown up may be eitherdue to unsoundness of the original stock, or due topoor manufacturing technique. Here again, a valuableguide is available to the alert production engineer toenable him to find the cause of the trouble and toremedy it insofar as it is within his power to do so.Other non-destructive tests are frequently carried outat this stage such as pressure testing, rig testing andthe like but these are outside the scope of this Paper.

The third important function is to test the com-ponents during repair. This is a field with specialproblems of its own, since the defects encounteredare caused by running and are consequently verydifferent from manufacturing defects in new com-ponents. Repair components are often difficult todeal with owing to oxidation, the need for removingpaints and enamels, and cleaning generally. Finefatigue cracks which may be under compression whenthe component is static can also be very troublesomeand call for the most stringent inspection techniques.Whilst the defects found can be valuable to theproduction engineer, the majority prove to be a guideto the designer and development engineer.Nevertheless, the production engineer has to studyany defects found at repair with the greatest care,since the discovery of defects due to faulty manu-facture invariably has very serious repercussions andmay call for remedial action on a very expensivescale, perhaps necessitating the withdrawal fromservice of suspect items.

The main conclusion to be reached from all thiswork is that non-destructive testing is an essentialguide to the production engineer, and that determined

(Concluded on page 96)

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troubled subject of technical education, or would it be moreappropriate to refer to the subject as education for atechnological age ?

There are so . many points one would like to commentupon or enlarge upon, mostly by way of agreement orreinforcement, that quite obviously only a selection can bemade.

The opening paragraphs are a most interesting survey ofthe development of our educational system on the appliedside. It is not fundamental to Dr. Bowden's main thesis,but perhaps a little more notice could have been taken ofthe enormous part that part-time education has played inthe past. Without National Certificate types British industryjust could not have carried on. This is an historical factwhich ought to be recorded. No defence of the system isimplied ; on the contrary, the time has come when wecannot rely upon part-time study and the goodwill ofstudents to attend in what is virtually their own leisuretime. The Germans have long operated a system wherebynobody above the level of craftsman is trained on a part-timebasis. The Berufschulen, at which attendance is compulsoryon one day per week for all young people between schoolleaving age and 18 years, unless they are in some otherform of education, provides training at craft levels. TheFachschulen, which train technicians, are organised on atwo or three-year full-time basis and, of course, the Tech-nische Hochschulen are four or five years of full-time study.Much the same is true in Russia, France and Holland.Only we are content to put part-time training against full-time training in international industrial competition.

I am sure Dr. Bowden is right in attributing our malaiseto early training in the schools. Failure on the part of theteacher to teach such things as mathematics, rather thanfailure on the part of the pupil to learn, is the root of allevil in this connection. Unfortunately for modern Britain,our grammar school system had too early a start. It beganto come into existence 400 years ago mainly for a vocationalpurpose. It succeeded so well that now our whole educationalsystem is geared to it. Even our modern schools are staffedby ex-grammar school pupils and so the tradition goes oninto schools where it is alien, and the nation's children arebrought up in institutions which are more like Lot's wifethan the young man who cried " Excelsior ". They claimthat their business is to prepare for life, but they fail tospecify in which century that life is to be lived.

From our arts specialists since we have so many ofthem I think the nation has a right to expect someattention to the contemporary world, some considerationto the implications for mankind that the splitting of theatom or the advent of much higher rates of productionof consumer goods (commonly known as automation) willhold.

For the rest of Dr. Bowden's Paper, one could get anenormous kick out of underlining his many arresting com-parisons. For instance: " In 1951, M.I.T. had 3,000 under-

graduates and 2,000 post-graduate students. In the sameyear (1951) only 270 post-graduate degrees were awardedin the whole of England in all branches of technology ".Or: " Seventy millions (to be spent in five years) may seemto be an enormous sum of money but we spend nearlylour times as much in advertising every year and at least12 times as much on liquor. The Government is in factproposing to spend the equivalent of . . . 30 cigarettes ayear for every man, woman and child in the country." Dr.Bowden could have quoted the total gambling bill of thecountry as another possible comparison. Can it be that as anation we are more concerned with trivialities and thatdeep down we are dealing with nothing less than moralissues, a lack of desire to put first things first? The ratepayerand taxpayer will gain no satisfaction from Dr. Bowden'sfigures, for to him expenditure is high. But is it going bothnationally and individually on the most important things ?

Just one question: Does Dr. Bowden think the nation hassufficient reserves of mental capacity to provide all thepeople of the necessary quality that we really need ?

Finally, having had a foretaste of Dr. Bowden's thinkingon these things, I for one will look forward with immensepleasure to seeing his book may it have the desired effecton our national thinking.

From : Mr. K. D. Marwaha (Graduate Member).Dr. Bowden's lecture will no doubt serve to instruct many

and stir them out of their complacency. His lucid treatmentof the subject deserves praise and admiration.

One aspect of the problem is that there are industrialfirms, even today, who are averse to employing a universitygraduate. This is particularly true in the field of productiontechnology, where higher academic attainments are viewedas superfluous and unnecessary, and where one with yearsof experience on the shop floor is considered a "better buy".Unless management recognise the importance of reinforcingtheir production staff by men of the highest merit, the*will be depriving industry of the versatility and imaginative-ness which it needs so much today.

Secondly, I would like to suggest the settingup of an institute of production technology on the lines ofthe College of Aeronautics at Cranfield, where fundamentalresearch on metal-cutting, machine tool design and otherallied subjects could take place. The work at P.E.R.A. ismore of an applied nature and does not embrace the funda-mental research I have in mind. This would go a long wayin providing the necessary stimulus to this vital branch oftechnology.

The whole emphasis in the world seems to be on furtheringtechnological education. Let us not deliver a death blowto the studies of Arts and Humanities. If there is not thehistorian to unravel the past, or the novelist to create drama,or the painter to convey the abstract, or the musician toinstil rapture, this life may not be worth living " just fortechnology ".

" NON-DESTRUCTIVE TESTING AND THE PRODUCTION ENGINEER "(concluded from page 109)

and continued efforts must be made by all concernedto obtain full and correct information about com-ponents under consideration. Despite its limitations,non-destructive testing has much to offer and bycontinued and constructive collaboration its frontierswill be extended until testing techniques will beevolved of a type not even envisaged at the presenttime. These techniques will, we hope, be swift,reliable and automatically self checking to such adegree that they can be operated with a minimum ofskilled labour and be free from errors due to thehuman element. Before this is realised, we must faceup to many failures and disappointments. Progressalong this path will be easier and swifter if all con-

cerned have a clear appreciation of its objectives andincentives. It is hoped that this Paper will havehelped in a small way to further this progress.

AcknowledgmentsIn conclusion, it is desired to acknowledge the very

great help received from my colleagues in the Non-Destructive Testing Section of the Quality Depart-ment at Rolls-Royce, Ltd., and to stress that theopinions here expressed are those personally held bythe Writer and do not necessarily represent the officialpolicy of Rolls-Royce, Ltd. Acknowledgments arealso due to the Management of Rolls-Royce, Ltd., bywhose kind permission this Paper has been published.

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