Project 3Environmental Durability of Adhesive Bonds

Report No 9

Forensic Studies of Adhesive Joints.

Part 2 - Bonded Aircraft Structure.

A Beevers

September 1995

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PART 2 - Bonded Aircraft Structure

Contents

1. INTRODUCTION..........................................................................................1

2. HISTORY OF THE BONDED SAMPLES.....................................................2

3. HISTORICAL DEVELOPMENT OF REDUX ADHESIVES...........................5

3.1 Origins.................................................................................................................................... 5

3.2 Aero Research And De Havilland ......................................................................................... 5

3.3 Other Users ............................................................................................................................ 6

3.4 Redux Film ............................................................................................................................. 6

3.5 Comparison Of Redux Variants ............................................................................................ 7

4. MANUFACTURING PROCEDURES............................................................9

4.1 Surface Treatment ................................................................................................................. 9

4.2 Adhesive Application ........................................................................................................... 10

4.3 Curing Procedure................................................................................................................. 10

4.4 Inspection And Testing ........................................................................................................ 11

5. MANUFACTURERS’ PERFORMANCE DATA..........................................13

6. EXPERIMENTAL ASSESSMENT..............................................................15

6.1 Supply Of Test Materials..................................................................................................... 15

6.2 Preparation Of Test Pieces .................................................................................................. 16

6.3 Experimental Test Schedule ................................................................................................ 16

7. RESULTS ..................................................................................................18

7.1 Lap Shear Tests ................................................................................................................... 18

7.2 Wedge Tests ......................................................................................................................... 19

7.3 Lap Shear Durability Tests.................................................................................................. 22

7.4 Non-Destructive Tests (NDT)............................................................................................... 24

7.5 Material Characterisation And Analysis ............................................................................ 24

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8. DISCUSSION .............................................................................................32

9. CONCLUSIONS.........................................................................................33

10. REFERENCES.........................................................................................33

APPENDIX 1 ABSTRACTS FROM PROCESS SEPCIFICATION

APPENDIX 2 MANUFACTURERS’ PERFORMANCE DATA

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PART 2 - Bonded Aircraft Structure.

1. INTRODUCTION

This report is based on the findings of an extensive analytical study of adhesive bondedjoints recovered from a “scrapped” Nimrod/Comet 4C aircraft which was built in1963.

The project was primarily intended to establish the condition of old bonded joints andto document the history of their manufacture and subsequent service. The investigationforms part of a large-scale forensic study of bonded joints from a wide range of differentapplication areas which in turn is aimed to supplement a programme on measurementof durability.

The approach consisted of a range of experimental strength tests, ultrasonic NDinspection, microanalysis of the joints and the fracture surfaces of test pieces, and furtheraccelerated ageing of old joints.

The adhesive material which forms the basis of the study is one of the Redux group ofadhesives, ‘Redux’ being a generic term describing the CIBA phenol-formaldehyde/polyvinyl-formal structural adhesive systems. Archival and documentarydata was obtained from an engineer who worked on the development and applicationof Redux material during the period of the construction of the Comet. Reduxadhesives have continued to be used in the aerospace industry, and their environmentalperformance has been assessed in long-term projects carried out by Royal AircraftEstablishment. Summary data from this RAE investigation has been provided by DRAFarnborough.

The forensic study was given an additional focus by the interests of BAe in assessing thepotential future long-term life of Comet/Nimrod airframe structures. BAe providedthe original Comet bonded structure material for this analysis and also procured recentRedux 775 bonded joints from a current manufacturing programme for comparison.BAe also supplied comprehensive data on manufacturing process specifications, teststandards and related documentation.

The accumulated evidence presented in this report is therefore derived from manydifferent sources, and acknowledgements are extended to the following providers:

A Bush AIM Aviation (ex CIBA)R A Harborne British Aerospace Defence Ltd (Farnborough)N Blackford British Aerospace Defence Ltd (Farnborough)P Higgins British Aerospace Regional Aircraft (Woodford)B Parker DRA FarnboroughJ G Moggeridge National NDT Centre, AEA Technology, Harwell.

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2. HISTORY OF THE BONDED SAMPLES

The Nimrod airframe is a military development of the De Havilland Comet passengerairliner which dates back to the early 1960s. This was the first transatlantic pressurisedfuselage jet airline to enter commercial service. Accommodation was for between 71-101 passengers and was originally powered by four “buried” Rolls-Royce Avon turbo-jets. The Nimrod aircraft utilised much of the original Comet airframe including thefuselage, wing and empennage. Installed in the cabin in place of the passenger seating isthe necessary detection and navigation equipment, together with its operators, toperform the surveillance role. Added also to the airframe is a fuselage long pannier fortransportation of stores, an air-to-air refuelling boom, a dorsal fin, finlets and numerousaerial installations.

The Nimrod fleet, based at Kinloss in Scotland, performs maritime patrol and searchand rescue duties generally over the North Atlantic. The bonded joint samples analysedin this study were obtained from Nimrod MR Mk2 tail number XV 147. This airframewas converted from a new build Comet 4C to Nimrod MR Mk2 specificationachieving its first flight in 1967. It was one of the two first development Nimrods andwas primarily used for mission systems trials. These trials included typical maritimepatrol sorties involving cruising at altitude to the patrol station, descending to lowaltitude over the ocean and performing the search pattern manoeuvres for extendedperiods. On completion of the operation the aircraft returns to base at cruise altitude.Consequently the airframe experiences ordinary flight loads from the transit phasestogether with higher “g” loading associated with low level manoeuvres and the gustloading from turbulence. The airframe is also exposed to significant amounts of seaspray under these conditions. However, since XV 147 was a development aircraft anddid not see operational service, its exact service history is unknown.

After the development phase this airframe discontinued service and was kept for about10 years at DRA Farnborough. It is believed that none of the usual long-term storagemeasures were conducted and the airframe fell into disrepair, as condensation collected.The level of corrosion is believed to be significantly worse than the in-service fleet. XV147 was scrapped in 1994 with only approximately 15 to 20% of service life used.

The extent of use of adhesive bonding in the Comet aircraft is shown in Figure 1. Thesamples supplied to Oxford Brookes University are from the top wing skin between ribs6 and 7, about a third of the way outboard. The under-side of the skin is stiffened byspanwise stringers which are attached by Redux adhesive bonding as shown in Figure 2.The thickness of the materials and dimensions of the stringers vary slightly betweensamples depending on the wing position, thinner sections generally being associatedwith areas closer to the wing tips.

The top skin is primarily subjected to plain compression and the bonded joints wouldsee a combination of shear and tensile peel forces. These are restricted by the presenceof fasteners at intervals of approximately 300mm along the length of the flange. Theskin forms part of the integral fuel tank and is hence subjected to some fuel pressureloading and attack from fuel. However the whole of the inside surface was coated witha flexible sealant skin which was apparently deposited from a solvent carrier at the timeof manufacture. This would have provided some protection of the bonded joints. Alsothe external surfaces were primed and painted, and there was no evidence of corrosionor bare metal on the samples provided.

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3. HISTORICAL DEVELOPMENT OF REDUXADHESIVES

From its inception, ‘Redux’ has been a generic term describing the CIBA phenol-formaldehyde/polyvinyl-formal structural adhesive system, used successfully for over 50years in aerospace and general industries.

While Redux 775 is generally regarded as the aerospace grade, it was not the type usedin Comet (and Nimrod) airframes; these, and other De Havilland aircraft of the time,used an early version called Redux Liquid E and Formvar. De Havilland used thissystem from 1943 until the early 1960s. From the mid-1940s other aircraft makers usedthe Redux Liquid 775 and Powder 775 system and, after 1954, Redux Film 775 joinedthe Redux list. The differences between Redux Liquid E/Formvar and Redux 775 (Land P, and film) are substantial, both chemically (liquid component) and physically(powder component). Confusingly, all systems were termed ‘Redux’ with little or nodifferentiation in published literature. The following notes summarise the key featuresand nomenclature of the different Redux systems.

3.1 Origins

Redux (Research at DUXford) was developed in 1941 by Aero Research Ltd (ARL) inDuxford, following experiments by de Bruyne using phenolic resins as a matrix of oneof the first composite materials (Gordon Aerolite).

The adhesion of phenol-formaldehyde (PF) resins to metal was found to beconsiderably improved by using a thermoplastic material as a “toughening agent”.Polyvinyl-formal (PVF) was tried, first in film form, and then powder form by Newellwho carried out many of the early experiments under de Bruyne. This led to a stronginterest by De Havilland Aircraft Co Ltd at Hatfield (with whom ARL had a long-termresearch association), resulting in the bonding of aluminium to wood for the Hornetaircraft.

Immediately after the second world war, De Havilland used Redux to bond stiffeners toskins in the Dove (and Heron) passenger aircraft. Over 500 Doves were eventuallyproduced. The good experience with Dove structural bonding led to its adoption inthe Comet airframe, in increasing amounts, from Comet 1 through to Comet 4. Thefirst Comet flew in July 1949, and Redux continued to be used after the Comet crashinvestigations (1955) absolved Redux from any blame.

3.2 Aero Research And De Havilland

The Redux system used by De Havilland was, surprisingly, hardly supported by ARL.De Havilland developed manufacturing techniques using the ‘first generation’ PF liquidcalled Redux Liquid E, and a coarse (8/10 BS mesh) PVF powder, sold under the tradename ‘Formvar’ by the (Canadian) Shawinigen Company. The powder particle sizewas not considered very practical by ARL, and by the mid-1940s, ARL had not onlychosen a more practical 30/52 BS mesh Formvar powder but had developed a new PFliquid component called Redux K6.

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De Havilland carried out considerable development work with Redux Liquid E andcoarse Formvar, believing that the system gave optimum mechanical properties(particularly peel strength) and consistency, especially when used with gritblasting aspart of the adherend pretreatment system. While De Havilland carried on with ReduxLiquid E and Formvar, ARL marketed the Redux Liquid K6 and 30/52 Powder system(later termed ‘Redux 775 Liquid and Powder’) to the aerospace industry in general.ARL supported De Havilland only to the extent of Liquid E manufacture. DeHavilland procured the coarse Formvar direct from Shawinigen (later Monsanto) in theUSA.

In 1955, ARL qualified Redux Liquid K6 and Powder (30/52 mesh) to DTD 775, aspecification for aerospace structural adhesives. The Redux nomenclature was changedto include ‘775’ when sold in compliance with DTD 775, but remained as Liquid K6and Powder C for industrial use. In some literature, occasional references (aerospace)can be found to Redux K6 and Powder, but this can safely be assumed to be Redux 775Liquid and Powder prior to 1955.

As far as can be determined, Redux Liquid E/Formvar was never qualified to DTD775, De Havilland relying on in-house specifications for product and process control.Those working at ARL at the time cannot recall any research or development work onthe Liquid E/Formvar system. The De Havilland use of Redux Liquid E/Formvarcontinued until approximately 1963, and had been used extensively on Dove, Heron,Comet and Nimrod airframes. It was also used on some military aircraft, and to anunknown extent on the Trident aircraft. From 1962/63 Hatfield moved to usingRedux Film 775, and the DH 125 (1962) and 146 used the material for large numbersof components.

3.3 Other Users

Notable among Redux Liquid and Powder 775 system users in the 1940s and 1950swere SAAB (Sweden), Fokker (The Netherlands), Sud Aviation (France), BreguetAviation (France) and others in the UK and Europe. Sales penetration in NorthAmerica was poor (although researched often), where indigenous adhesivespredominated. The one exception was the Fairchild Engine and Airplane Corp whoused Redux Liquid and Powder 775 to licence build Fokker F-27 aircraft (circa 1958).

3.4 Redux Film

The ‘third’ variety of Redux appeared in May 1954 when Redux Film 775(nomenclature post 1955) was produced. This used the basic Liquid 775 and Powder775 in a controlled way in terms of liquid : powder ratio and weight. A variety of filmweights and reinforcements (carriers) was produced.

Virtually all current Redux 775 bonding uses Film 775, including the last bastion ofLiquid and Powder use, Fokker, who changed only in 1993.

Reading across from Redux Liquid and Powder 775 to Redux Film 775 is not wise;the critical liquid : powder ratio may produce not only different mechanical results, butdifferent ageing behaviour. While establishing differences in mechanical behaviourbetween the two systems is not difficult, ageing behaviour differences presents a farmore difficult task.

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3.5 Comparison Of Redux Variants

Phenol-Formaldehyde (PF) componentsLiquid E was produced using an excess of caustic soda in the reaction process; a darkred/purple liquid resulted. The liquid was probably highly alkaline (no values can befound) and as a result tended to have a shelf-life measured in weeks. This was not adesirable feature, and ARL researched and developed more suitable PF resins.

The result was Redux Liquid K6, an almost neutral (pH8 measured in recent years)liquid produced with 88% less caustic in the reaction. The liquid was stable (life inexcess of 12 months) and light straw in colour; this changed with time, becomingred/brown through oxidation and advancement.

Table 1 shows the general properties of each PF component.

Property Liquid ELiquid 775(Liquid K6)

P : F ratio 1:1.57 1:1.43Mol ratio caustic soda in manufacture 0.097 0.011Solids content, % bw 62 ± 2 70 ± 15Colour (at manufacture) Dark red/purple Light strawSolvent Industrial Methylated Spirit (IMS) IMS

Table 1 Properties of PF Components.

ARL (a CIBA company after 1974) adopted Liquid K6 and actively marketed the typein both aerospace and more significantly (in terms of tonnage), to general industrialmarkets.

Polyvinyl-Formal (PVF) componentDe Bruyne tried a number of materials as ‘tougheners’. In the vinyl family, PVFs andPVBs (Polyvinyl Butyral) were tried, with the most effective being PVFs. From therange available, de Bruyne chose a high molecular weight grade used for electrical wireenamelling; this was the 15/95E grade from Shawinigen. The choice of particle size isnot clear but this may have been the only generally available size at the time. While DeHavilland clearly made use of the large particle size, ARL continued work through1942-1944 to find more practical sizes.

Particle size is critical as far as ‘pick-up’ of liquid coated surfaces is concerned. Finepowder (say >100 BS mesh, 150 micron DIN) ‘blinds’ the surface and low weights ofpowder in the joint result. Large particles (such as 8/10 BS mesh) would probablyresult in higher weights of powder.

ARL (later CIBA (ARL)) carried out much work grinding and sieving raw powder,even dissolving and re-precipitating material to get sufficient quantities of the chosen30/52 BS mesh size. Older employees can recall ‘huge piles’ of Formvar in the plant.By the mid-1950s powder was being brought in ground and sieved to size fromShawinigen/Monsanto.

Table 2 shows the essential physical differences between the powder used by DeHavilland and that marketed by CIBA (ARL).

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REDUX PVF POWDERUsed with Liquid E

(De Havilland) Powder (775)BSS 410 mesh size 8-10 30-52*DIN 4188 mesh size - µm 2000-1670 500-300

* >65% by weight in sample

Table 2 PVF Powder Properties.

A further PVF source was found in the mid/late 1960s, with procurement of materialfrom an Italian source, SIVA. It was qualified by CIBA to DTD 775 and suppliedalmost exclusively to users of Liquid and Powder 775; until recent years it was rarelyused for Redux Film manufacture, where the specific surface topography of theMonsanto powder produced a better quality (in terms of drape and cohesion) film thanSIVA powder.

A second source was necessary because of the dwindling supplies from Monsanto. Asimproved insulation materials became available for electrical wire enamelling, theavailable 30/52 fraction (less than 12% of total production) fell dramatically, with anumber of world producers ceasing PVF production. Monsanto PVF supplies ceased inthe late 1980s, with Duxford stocks running out in 1990. Recent research onevaluation of different powders has shown that, physical differences apart, the chemicalnature of Monsanto and SIVA PVFs were very similar, producing comparablemechanical properties.

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4. MANUFACTURING PROCEDURES

The application of Redux adhesives to aircraft structures required a detailedspecification of the whole bonding process. Both ARL and De Havilland developedand maintained comprehensive procedure documentation with essentially similarcontents, based on surface pretreatment, adhesive application, bonding/curing, andtesting/inspection.

There have been occasional changes to some of the details within each of these stagesduring the life of Redux as a natural evolution of process improvement and refinement.There have also been corporate changes in the Redux manufacturing company and theaircraft constructor which have led to some confusion in identity of some of thespecification numbers.

The best available documentation which may be relevant to the manufacture of theComet details studied in this forensic investigation are given in Appendix 1. Thisassembly of documents includes abstracts from the following services.

CIBA (ARL) Ltd Redux 775 (Liquid and Powder). Instructions for use when bonding aircraft components, Ref No 775/1, May 1961.

British Aerospace Process specification: Bonding with Redux E and(Commercial Aircraft) Ltd Formvar powder, Ref BAEP 4020, Jan 1992

(supersedes DHA 450).

British Aerospace Bonding of starboard side keep panel using Hatfield(Commercial Aircraft) Ltd Press Tool, Ref TS/FI/1093, June 1989.

Hawker Siddely Aviation Process specification: common requirements for metal-metal adhesives, Ref S29 - 62 (now BAEP 9062), Dec 1977.

Hawker Siddely Aviation Process specification: common requirements for Redux bonding for HS 146 and subsequent aircraft, Ref S26 -4396 (now BAEP 4596), Dec 1977.

4.1 Surface Treatment

A summary statement of procedures used on the Comet (supplied by BAe) describes thewing skin and stringer materials and procedures as follows:

Top skin (Panels B, D and E)Material: DTD 5060 Clad PlateProcesses: DHA 404 anodising for Redux bonding

Bottom skinRibs 1-12 (Panels C and F)Material: DHA 80 Clad PlateProcesses: DHA 308 chromic acid anodising

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Ribs 12-20 (Panel A)Material: DHA 100 Clad PlateProcesses: DHA 404 anodising for Redux bonding

All stringersMaterial: DTD 687 Aluminium/zinc Alloy Clad PlateProcesses: DHA 404 anodising for Redux bonding, followed by DHA 565

Titanine Chrometch Primer.

It is not clear what the differences are between DHA 404 anodising for Redux bondingand DHA 308 chromic acid anodising; nor whether either of these relate to BAEP 9062or TS/F1/1093. There are minor variations in the process described in BAEP 9062compared with the instructions for DTD 915/DTD 910 referred to in the CIBA(ARL) document Ref 775/1. However microanalysis carried out within this forensicstudy confirms the existence of some form of chromic acid anodising.

4.2 Adhesive Application

The general technique of Redux liquid/powder adhesive application consists ofapplying a thin film of liquid to the bond surface and then dipping the component intothe powder. Alternatively the powder may be sprinkled on to the liquid. After shakingoff the excess powder there should be a uniform coated layer.

It is reported that considerable variations would be expected in the resultingliquid/powder (L/P) ratio due to different liquid application weights and also todifferent powder particle sizes. In early literature, CIBA (ARL) suggest a L/P ratio of1:2 but in more recent data sheets, rates of 1:1.125 and 1:1.4 are quoted.

In the De Havilland procedure described in BAEP 4020, and presumably used in theassembly of components in this study, a second application of liquid and powder is madeover the initial coating apparently “......to effect an even spread between grains”.Although there is no indication of required L/P ratio the BAEP 4020 specification doesprovide guidance on the choice of powder size for different application“......determined from experience”. This was probably applicable to the construction ofthe Comet 4C in 1965, under the previous specification DHA 450.

It appears that the quantitative effects of different L/P ratios have not been fullyevaluated. However Bush1 reports that higher powder ratios improve peel strength butlower elevated temperature performance and shear strength slightly. It is also reportedthat PVF powders from different sources give similar mechanical properties. The factthat the coating process is not very tightly controlled suggests that the joint performanceis relatively tolerant to different L/P ratios.

4.3 Curing Procedure

The curing reaction of Redux systems is a ‘condensation’ process which releasesmoisture as the reaction proceeds. As the curing temperature is above 100°C it isnecessary to apply pressure to the joint to prevent the formation of vapour porosity.

A CIBA (ARL) data sheet giving introductory notes for Redux states that “the curingcycle for Redux is simple and not critical but variations can be usefully employed toproduce different properties. The normal cure which conforms to that of DTD 775B isof 30 minutes at a temperature of about 145°C under a pressure of about 100 psi (7

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bar). The cure may be extended by using long times or higher curing temperatures, toproduce improved shear and creep strength at temperatures above 60°C. Extension ofthe curing cycle gives improved fatigue strength but causes some reduction in the roomtemperature peel strength. The practical limit is about one hour at 175°C foraluminium alloys due to the possibility of changing the mechanical properties of thealloy.”

The process specification for Redux E and Formvar as used on the Comet under study,is described in BAEP 4020 (see Appendix 1). It differs from the CIBA (ARL)specification for Redux 775 in that much higher pressures are required for ReduxE/Formvar. Bush1 observes that the higher pressures (not less than 24 bar) wererequired to ‘consolidate’ the glue line because of the large Formvar particle size.

All De Havilland bonding for Comet and Nimrod was carried out in a large heatedplatten press. Other companies tended to use autoclaves as well as presses. Both theBAe and CIBA (ARL) documents emphasise the need for the curing temperature andtime to be referenced to the glue line. In CIBA (ARL) instruction sheet Ref No775/1, it is noted that “extra time must be allowed for heat to penetrate to the glueline”. The BAEP 4020 specification requires that “the total heating times be adjustedto allow the parts to heat through to provide the correct time at temperature at the glueline”. The time taken to reach the glue line cure temperature is normally expected notto exceed 15 minutes. Both documents specify that at the end of the curing periodjoints should be cooled to 70°C or lower before the pressure is removed.

The bonding process naturally produces some edge spew on cure and it is reported thatthe Liquid E/Formvar system produced much greater volumes of spew fillets, perhapsreflecting the double coating of the adhesive applications. Where possible this wasremoved mechanically but in inaccessible areas a brittle sponge-like structure formed byvapour, remained. In early applications in passenger aircraft bellies, this spew couldabsorb corrosive fluids and subsequently De Havilland coated the inside of stringers andsurrounding areas with Liquid E.

In issue 3 of BAEP 4020, the specification for removing the spew is described asfollows. “Surplus cured Redux must be cleaned away from the edge of the bondedparts. This shall be done by scraping, using tool J 71915. Surplus cured resin inside thecrown of fuselage stringers may be removed using a nylon rod. These operations mustbe carried out under inspection supervision.” Another clause requires that beforeapplying Redux to stringers reference should be made to the appropriate protectivetreatment or schedules for protection of the enclosed surfaces from corrosion.

The panels provided for this study contained substantial spew fillets on the inside crownof the stringers. Presumably the allowance of this on wing panels reflects the lowercorrosion risk in the areas. It is also noted that the surface treatment details inSection 4.1 refer to the use of a Titanine chrometch primer which was probably appliedas a protection from corrosion.

4.4 Inspection And Testing

All the data sheets cited include references to inspection and testing in various levels ofdetail. Some of these give instructions for inspection at various stages in themanufacturing process as well as post-bond inspection. For example BAEP 9062includes requirements for water break tests after pickling and “inspection for flaws afteranodising” as well as monitoring process parameters, particularly glue line temperature.

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Post-cure inspection was (and still is) based on a combination of visual examination andmechanical testing. BAEP 4596 also includes reference to NDT using the Fokker bondtests but it is believed that this was introduced after 1965 and would not have been usedin early Comet assembly.

Visual checks comprised amount of spew and its colour; although not a definitive test,colour was used as check on the cure state. Redux E/Formvar produced amagenta/purple colour on 145°C cure which is clearly visible in Comet components.In the CIBA (ARL) instruction sheet Ref No 775/1 it is stated that “a buff to lightreddish process indicates correct curing”. This document also refers to the requirementto “process sample lap and peeling specimens which can be tested to destruction. Thesetests are intended as process control tests on the pretreatment and curing processes.”

The description of mechanical tests in BAe documentation is considerably moredetailed and extensive. In addition to the post-bond inspection tests covered in BAEP9062, a separate BAe document BAER 3213 provides a specification for the supply andinspection requirements for structural adhesives.

BAEP 9062 describes procedures for peel tests, shear tests and ‘pull’ tests, but noacceptance values are given. Document TS/F1/1093 does list minimum valuerequirements for peel shear and pull tests but these are applicable to more recentconstruction on BAe 146 aircraft using Redux 775 adhesives. These specificationsgenerally relate to post-bond testing and include descriptions of methods for cutting testpieces out of actual production components. The procedures also provide for ‘witness’samples in which smaller material coupons are subjected to the same process cycles asthe production components.

The specifications for supply and inspection requirements for structural adhesives arebased on industry standard test methods. Typically BAER 3213 refers to AECMA, ENpr 2243 and Redux performance data provided by CIBA (ARL) are derived fromstandards such as DTD 775B, MIL-A-5090B and ASTM D 1781-60T.

Mechanical strength data derived from such tests are described in the following section.

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5. MANUFACTURERS’ PERFORMANCE DATA

As noted in Section 3 the Redux Liquid E/Formvar adhesive was superseded by CIBA(ARL) materials which became identified as Redux 775. Most of the manufacturers’performance data relate to this later material. CIBA (ARL) Ltd issued a series of eightRedux data sheets in May 1961, covering static (shear) strengths, and ageing and fluidresistance. Abstracts from these data sheets are given in Appendix 2. It should beemphasised that the properties of Redux 775 are different from Redux E/Formvar butthe data may be used to give a general indication of characteristics.

There is very little published data for Redux Liquid E/Formvar which De Havillandcontinued to use for Comet construction. Strength values in more recent BAedocuments relate to Redux 775. However a paper reproduced by CIBA (ARL) on‘Criteria for Designing Adhesive Bonded Joints’ describes an application on the F7U-3Cutlass by Chance-Vought. This article (see Appendix 2) includes experimental dataon Redux E/Formvar and quotes lap shear strengths typically in excess of 4000 psi andup to 7000 psi. This value is in the same order of strength as quoted by CIBA (ARL)data sheet 2 for Redux 775. Therefore despite the substantial differences between theRedux systems, it may be appropriate to use the Redux 775 data as a benchmark againstwhich properties of the old Comet joints may be compared.

A large amount of experimental data on Redux 775 has also been produced andpublished by other laboratories such as Royal Aircraft Establishment (now DRAFarnborough). As well as mechanical strength properties the RAE reports cover effectsof outdoor ageing in a number of climatic environments.

The mechanical strength values quoted for Redux 775 are commonly based on DTD775 with 0.036in (0.9mm) sheet. In Redux data sheet 2 it is noted that the test dataincludes results with a high proportion of metal failures. The strengths do not thereforeindicate the true value of the adhesive shear strength. It is surprising that this standardwas also used for an ageing test programme (data sheet 6) which claims to show nodeterioration of joint strength after nearly 5 years of tropical exposure when apparentlythe failures are still in the adherend. A potentially more relevant and meaningfulstandard is MIL-A-5090B which uses 0.064in (1.6mm) sheet material.

Typical values quoted for Redux 775 using MIL-A5090B are:

Lap shear strength at ambient 34.0 MPa (4930 psi)Min. specification requirement 17.6 MPa (2500 psi)

Young’s Modulus 3.35 GPaShear Modulus 1.2 GPa

Extrapolated values for Redux Liquid E/Formvar based on the Chance-Vought dataare:

Lap shear strength at ambient 33.7 MPa (4900 psi)*Shear Modulus 0.3 GPa

* this is a derived value and may not be accurate or reliable.

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The British Aerospace test specification document TS/F1/1093 quotes shear strengthrequirements, depending on location of joint, as:

Min. shear strength 25.3 MPa (3700 psi)

This coincides with minimum specification requirements of DTD 77B.

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6. EXPERIMENTAL ASSESSMENT

6.1 Supply Of Test Materials

Most of the bonded components which were used for the experimental assessment weresupplied by British Aerospace Defence Ltd, Military Aircraft Division. The ‘assemblies’consisted of approximately rectangular panels, about 700mm long cut from aircraft no.XV 147 described in Section 2. Each panel had the stringers attached from the originalbuild.

Common visible features and characteristics on each panel included• a green sealant coating of variable thickness on the inner surface, which also

covered the stringers• grey paint with areas of exposed primer on the centre surface• evidence of spew residues as a reddish brown porous layer in the crowns of the

stringer.

The panels had slightly different sheet thicknesses and stringer configurations, but theywere typically 2mm thick in both the sheet and flange. All the stringer flange widthswere greater than 25mm which allowed the preparation of test pieces as described inRef TS/F1/1093 and shown in Figure 3. In effect this gives a 1in wide lap shear testspecimen having a 0.5in overlap. The materials supplied yielded approximately 50 testpieces to this specification.

A further panel was provided at a later date having flange and sheet thicknesses in excessof 3mm to enable wedge tests to be carried out.

Batches of specimens were also obtained from different sources to provide a comparisonof performance. These include a set of laboratory test pieces from Ciba Polymers whichhad been made in 1963 and a batch of Redux bonded joints from a recent (1995)production of the BAe 146 aircraft.

The substrate material thicknesses, adhesives and surface treatments vary between thesedifferent sources of test pieces and some care is needed in their comparativeinterpretation. A summary tabulation of the test piece details is given below.

AdherendSource ofSample Material

Thickness(mm)

SurfaceTreatment

Adhesive

Old Comet DTD 5060 CladDTD 687 Clad

1.3 - 2.61.6 - 2.62.6 - 2.6

Chromic acidanodised

Redux LiquidE/Formvar

Old Ciba Labsspecimens

24ST CladEquivalent to2024-T3

1.64 - 1.64 Chromic acidetch

Redux 775Liquid/Powder

New BAe 146 Clad 2.6 - 52.6 - 2.6

Chromic acidanodised

Redux 775Film

Table 3 Test Piece Details.

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There is some slight uncertainty in the accuracy of some of these details particularlyrelating to the material from the original Comet. The source of the test panels arestated to be from the upper wing skin although it has been suggested that one panel mayhave been from the lower wing surface which had a slightly different alloy. Also thedescription of the stringers being made from clad panels (Section 4.1) is at variance withthe observation that one of the larger sections appears to have been extruded. Acommon feature is that all the sheet materials are clad alloys.

6.2 Preparation Of Test Pieces

All the old Comet panels were machined into the lap shear test piece configuration asshown in Figure 3. The flange/sheet bonded joint was first cut from the panel by band-sawing through the stringer crown on one edge and through the sheet outside the flangearea on the other edge. The specimens were cut to length, avoiding areas whichcontained fasteners, and the edges were milled to give an accurate width. This alsoensured that the bond area extended over the whole section of the test piece. Thetransverse off-set cuts which formed the lap shear area were made with a ‘Junior’hacksaw, care being taken to ensure that the cuts ended in the glue line.

Old laboratory specimens supplied from CIBA’s laboratories had been bonded insimple, single lap configuration. Some were supplied pre-cut to 1in width; theremainder were in wider sheets. These were cut to 1in width by bandsaw. The jointsprovided from recent BAe 146 production were supplied pre-machined.

6.3 Experimental Test Schedule

The analysis was broadly based on a combination of tests which were designed todetermine mechanical properties, durability and material characteristics. The testmethods used consisted of single lap shear for mechanical strength; wedge tests forfracture toughness; accelerated ageing of lap joints by water immersion at 40°C and60°C for durability testing; SEM, XPS, FTIR and micro-hardness for materialcharacterisation. Some ultrasonic non-destructive testing was also carried out. Theresults of the experimental test programme are given in the following section.

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7. RESULTS

7.1 Lap Shear Tests

Lap shear specimens were tested in a Nene Instron universal testing machine at ambienttemperature of approximately 20°C and at a crosshead speed of 2mm/min. The initialtest batch of seven old Comet test pieces were taken from different panel areas andthere were some differences in the specimen sheet thicknesses. This may have hadsome marginal effects on individual joint strengths but the coefficient of variationwithin the batch was within acceptable limits and the results have been ‘lumped’ intobatch averages.

The same observation may be made for the BAe 146 specimens although the meanthickness is slightly higher than the old Comet coupons and this might be expected togive a higher strength.

The mean strengths of the joints are given below.

Batch MEAN LAP SHEAR STRENGTHIdentity kN MPa psi

Old Comet 11.36 ± 1.07 36.35 5272Old Ciba Lab. 12.56 ± 1.27 40.19 5829New BAe 146 12.52 ± 1.43 40.06 5810

Table 4 Lap Shear Test Results.

Most joints gave a predominantly cohesive failure although the CIBA lab specimensexhibited some apparent interfacial failure, perhaps reflecting the absence of anodising.

The fracture surfaces of the old Comet specimens showed a narrow brown ‘stain’ about2-3mm wide on the outside flange edge. This area was subjected to a detailed analysis(described in Section 7.5). A further mechanical lap shear test was carried out on a3mm wide joint cut from the stained area of a standard test piece. This gave strength of5453 psi, indicating that the staining has not adversely affected the adhesive strength.

The slightly higher values of the Ciba laboratory joints and the BAe 146 samples may bedue to their different adhesives (Redux 775 compared with Redux E/Formvar) andalso different sheet properties and thicknesses. They do not necessarily imply deficiencyin the old Comet joints.

Indeed the mean joint strengths of 5272 psi far exceed the minimum requirements of4000 psi in TS/F1/1093, and these initial lap shear strength values give a generalindication that the bonded joint properties have not changed significantly during the lifeof the aircraft.

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7.2 Wedge Tests

The wedge test is now widely used in the aerospace industry to provide a measure ofadhesive joint performance, particularly in terms of fracture toughness and durability.The method was developed by Boeing during the 1960s and would not have been usedon early Redux materials. However results from wedge tests on more recent adhesivesprovide an indication of fracture energy values which might be typically expected.

The test consists of slowly driving a 3mm thick wedge between the sheets of a bondedjoint and measuring the length of the resulting crack in glueline as shown in Figure 4. Aknowledge of the adherend dimensions and properties enables the fracture toughness tobe calculated. A precondition for the test is that the yield strength and sheet thicknessare high enough to prevent yield during the test. The ASTM D3762 specificationrequires 3mm thick sheet specimens in aluminium alloy 2024. The old Comet testpieces were 3.5mm and the fracture energy values were calculated from the equation

=

G

E d h

a h1

2 3

4

3

16 0 6( . )+

Where E is the Young’s Modulus of the aluminium alloy, d is the wedge thickness, his the sheet thickness and a is the crack length.

The test also provides a good indication of joint durability by measuring crack growthrate after exposure to hot wet environments. Typically, rapid crack growth during thefirst 24 hours is indicative of low durability while slow crack growth over long periodsindicates good durability.

A total of six wedge test specimens were prepared and tested from old Comet material.Crack lengths were measured on each side of each specimen by means of a verniertravelling microscope. After the initial measurements three specimens were placed inwater at 40°C and the remaining test pieces were immersed in water at 60°C. Crackgrowth was recorded at appropriate intervals. The results are shown in the followingTable 5 and plotted in Figure 5.

Crack Length (mm)

Ageing Ageing Time (hours)Environment 0 1 4 8 24 48 72 168 72040°C water 33.7

±1.91636.2

±2.47838.48

±2.97738.65

±3.11738.22

±3.42338.45±3.65

--

--

42.37±5.101

60°C water 33.82±4.935

38.72±6.01

42.95±6.128

43.13±6.193

44.13±6.081

46.0±7.459

48.71±2.475

50.15±4.313

51.53±4.483

Fracture Energy (kJm²)

Ageing Ageing Time (hours)Environment 0 1 4 8 24 48 72 168 72040°C water 2.01 1.53 1.21 1.19 1.24 1.21 - - 0.8460°C water 1.98 1.18 0.80 0.78 0.72 0.61 0.48 0.44 0.39

Table 5 Boeing Wedge Test Results.

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The initial fracture energy value of 2kJ/m² is consistent with aerospace adhesivematerials and exceeds most typical toughened epoxy adhesives. The measurement ofthe crack growth also showed characteristics of a slow stable system with gooddurability, the end of the crack tending to form stringy plastic deformation rather than asharp crack tip. This feature is also indicative of a tough material. The slightly fastercrack growth rate at the higher temperature of 60°C would be expected with mostadhesive materials.

The normal expectation of polymeric ageing and degradation, particularly inthermosetting resins such as phenolics, would be of increasing brittleness. Theobservation of high fracture energy and toughness shown by the wedge test suggests thatthere has been very little change in fracture energy properties of the Redux E/Formvaradhesive on the Comet joints during its 32-year life.

7.3 Lap Shear Durability Tests

It was suggested that if changes had occurred in the adhesive in the Comet structureduring its lifetime, the joints might exhibit reduced resistance to degradation. This testseries was therefore carried out to compare the degradation characteristics of the Comettest pieces with the properties of joint from other sources. These included samples ofRedux 775 joints which had been kept in a laboratory environment since 1963, and testpieces from production components from a 1995 BAe 146.

Exposure of test joints to ‘extreme’ accelerated ageing environments is often used toprovide a comparative measure of durability. The selected ageing conditions for thesetests were water immersion at 40°C and water immersion at 60°C. Batches of lap shearpieces from each source were placed in each of these environments and groups werewithdrawn for testing after ageing for 6, 12 and 24 weeks. The results are given inTable 6 and shown graphically in Figure 6 for normalised values of percent strengthretention relative to the control strength.

AcceleratedAgeing

EnvironmentJoint Origin

Control(kN)

6 WeeksAgeing

(kN)

12 WeeksAgeing

(kN)

24 WeeksAgeing

(kN)

40°C WaterCIBA 1963laboratory stored

12.56 ± 1.27 12.05 ± 0.16 11.68 ± 0.27 9.35

Immersion New 146 12.52 ± 1.43 11.14 ± 0.80 10.66 ± 0.43 9.71 ±0.42

Old Comet 11.36 ± 1.07 10.47 ± 1.51 9.47 ± 1.56 8.02 ±0.26

60°C WaterCIBA 1963laboratory stored

12.56 ± 1.27 9.00 ± 0.11 7.95 ± 0.08 6.25

Immersion New 146 12.52 ± 1.43 10.00 ± 0.80 9.21 ± 0.52 8.82 ±0.46

Old Comet 11.36 ± 1.07 6.37 ± 0.69 5.60 ± 1.30 5.13 ±0.83

Table6 Results of accelerated ageing tests

The normalised values of strength retention illustrated in the graphs provide a directcomparison of the degradation characteristics of the three systems. There is very littledifference between the different batches in the 40°C immersion tests; at 60°C the old

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Comet specimens show a slightly greater strength loss. However the 60°C waterimmersion test is considered to be extremely demanding and the results do not implythat the Comet joints have deteriorated in service. The new BAe 146 and CIBAlaboratory were bonded with Redux 775 and the finer powder specimens were madewith Redux 775. The changes in formulation and the fine powder, compared withRedux E/Formvar, may have contributed to the slightly better durability of thesematerials at the higher environmental test temperatures.

Perhaps more surprisingly in the results is the good performance of the CIBA laboratorytest pieces which were reported to have had etched rather than anodised surfacetreatment.

7.4 Non-Destructive Tests (NDT)

Six off-cuts from the flange section of the old Comet were subjected to ultrasonicexamination using a pulse-echo technique. The ultrasonic transducer probe wasscanned transversely at a spacing of approximately 0.75mm over the specimen whichwas immersed in a bath of water. The output signal was recorded by a plotter whichgave a continuous line for a good bond but defects and flaws caused an interruption.The generated images thus show a coarse grey-lined background with white areascoinciding with ‘defects’. A typical image from a Comet bonded flange joint is shownin Figure 7.

Unfortunately the resolution of the system is insufficient to define sharp edge details dueto refraction effects and the outlines appear blurred. However from the images of allthe test pieces, the following observations can be made.

• The large circular areas as seen in Figure 7 coincide with the positions of fastenersthrough the flange and are not flaws.

• Occasional rectangular white areas were caused by specimen identity labels.• Some incidence of ‘flaws’ were subsequently found to coincide with delaminated

areas of the green sealant coating on the external surface of the specimens.• The fuzziness of the edges masked possible interpretation of the effects of the brown

stained areas. In some specimens the stained edge coincided with the clearercontinuous image suggesting that the stained material was not being identified as aflaw or disbond.

• Generally the scans showed a good continuous coverage over the main flange areaswhich indicated good bonding.

7.5 Material Characterisation And Analysis

Visual examinationSome indication of the properties of adhesives can be gained from an examination ofthe fracture surfaces of tested joints. A simple visual inspection of the locus of failureprovides a good initial assessment of joint quality. A cohesive failure, in which thefracture occurs through the adhesive, is generally indicative of good adhesion andsurface treatment. Interfacial failures in which the adhesive separates from the substratesare sometimes associated with poorer bonds and lower durability.

From the visual examination of the old Comet test joints it appeared that all thefractures were cohesive. The fracture surfaces exhibited a fairly random distribution ofrough and smooth regions over the joint area suggesting a mix of ductile and brittle

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failure. The colour of the adhesive material was predominantly beige/white in colourbut most specimens in the initial test batch showed a brown stain about 1 to 3mm widealong the outer edge of the flange.

The specimens which had been immersed in water at elevated temperatures forextended periods also gave cohesive failure with apparently a greater proportion ofductile fracture. The brown edge stains were less evident in these ‘aged’ specimenspossibly due to the absorbed water plasticising the adhesive and reducing thegranularity.

Electron microscopyThree different electron microscopy techniques were applied to examine the joints.Transmission electron microscopy was used to examine the aluminium/adhesiveinterface by cutting a thin section of the joint at a shallow angle using anultramicrotome as shown in Figure 8(a). The transmission electron micrograph of thissection, Figure 8(b), shows a columnar structure between the aluminium and theadhesive. This surface morphology is typical of the cell formation of the aluminiumoxide during anodising and appears to confirm the use of a chromic anodisingpretreatment prior to bonding.

High voltage, and low voltage scanning electron microscopy (SEM), were used to studydetails of the fracture surfaces. The high voltage SEM images in Figure 9 show thedifferent characteristics in adjoining rough and smooth regions on the adhesive. Thetear patterns which appear as rough regions in the visual examinations are indicative ofductile failure while the shallow, shell-like craters from the smooth regions areassociated with a more brittle type of fracture. It might be suggested that these craterscould also be associated with presence of the Formvar particles used in the bondingprocess. However the crater diameters are an order of magnitude smaller than thequoted particle mesh size so this correlation seems unlikely. Low voltage scanningelectron microscopy allows surface examination of polymers without the need formetallic coating. This technique was used to assess the morphology and characteristic ofthe stained region. The low voltage SEM image in Figure 10 shows a more crumblyrounded granular structure in the brown stained area and although micro-hardness testssuggested significantly higher hardnesses in this region, the information is notconclusive.

X-ray Photoelectron Spectroscopy (XPS)XPS is a valuable tool in the study of surfaces as it allows chemical analyses to be carriedout on very small areas of materials without damage. The exposure of the surface areato an X-ray source in vacuum releases electrons which are analysed in terms of kineticenergy to give a spectrum. The peaks of this spectrum provide the characteristiccomposition of the surface.

General XPS spectra were obtained from both the main adhesive joint fracture area andthe stained region. The traces from these analyses are shown in Figure 11. Both spectraare virtually identical indicating that there is no difference in composition between thetwo regions.

Fourier Transform Infra-red Spectroscopy (FTIR)The reflective spectra from infra-red radiation may also be used to providecharacteristics of polymeric materials and FTIR was applied to the different regions ofthe adhesive surface of a fractured test joint. The spot diameter of the infra-red beam in

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the FTIR instrument was 6mm. Thus information collected from the brown stain areawill include an overlap into the normal region.

The fingerprint spectra of the two different adhesive areas are given in Figure 12.There is evidence of some differences between these traces which can be visualised byplotting a trace in which the normal sample spectrum is subtracted from the discolouredarea spectrum. If the materials are identical the plot would be a straight line at 0. Themost significant different is the increase of intensity at 3400cm-1 in the stained area.This spectrum band is associated with -OH and may indicate either the presence ofalcohol groups or absorbed water (or both).

Other changes within the spectrum may be attributed to some sort of hydrolysisreaction but the analysis is not conclusive. The specimens had been exposed to theatmosphere for some weeks after fracture and absorbed atmospheric moisture may havecontributed to the changes.

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8. DISCUSSION

The main objective of the experimental analysis in this forensic study was to establishthe condition of the adhesive in a 30 years old bonded structure.

From the strength tests on old joints under quasi-static loading, the joint performancestill exceeds the manufacturer’s original minimum specification requirements andstrengths are about the same as typical values quoted from early data sheets. It istherefore evident that there has been little change in the mechanical characteristics ofthe adhesive.

Direct comparisons with other aerospace bonded joints have to be qualified with theobservation that the Redux adhesives used for Comet structures are slightly different tomost other Redux adhesives. However the similarity in overall performance suggeststhat general comparisons are reasonably valid.

The fracture toughness of the adhesives derived from the Comet structure is still highand the wedge test provides a further indication that the durability of the joints is good.The more detailed durability study using accelerated ageing on groups of Reduxbonded joints derived from different sources confirms the relative stability and resistanceto degradation on the old Comet test pieces. The observation of the locus of failure inthese tests showed consistently cohesive failure indicating the soundness of the chromicanodising of the surfaces.

More detailed and extensive durability studies have been reported on Redux 775joints2. These showed that Redux 775 was virtually unaffected by natural weatheringwhen the joints were unstressed. However at stresses of 20% proof load in 50°C wetconditions creep failure was observed. It is likely that the Redux E/Formvar adhesiveused in the Comet joints might exhibit similar characteristics.

Analysis of the fracture surfaces and adhesive/metal interface was carried out on testedjoints. Although some staining was evident on the edges of some joints this did notappear to be detrimental to the joint performance. NDT and XPS were unable toshow any differences in physical or chemical properties of the stained area and it is notclear whether the discolouring is a result of the original processing (e.g. slightly lowerpressure at the edge of the flanges) or a natural ageing phenomenon (e.g. diffusion ofmoisture/sealant pigments).

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9. CONCLUSIONS

Mechanical properties of the joints removed from old Comet using panels showed littleor no evidence of loss of performance during their 30 year life.

Durability tests in which old Comet joints were compared with materials from othersources gave strength retention values which were similar to new materials at 40°Cexposure. There is no indication of loss of durability in the old Comet joints.

The analytical studies of the fracture surfaces of the old adhesives do not reveal anysignificant deterioration.

The original Redux Liquid E/Formvar adhesives, in conjunction with chromic acidanodised surface treatments provide high performance bonded joints in aluminium alloyaircraft structures. The later Redux 775 derivatives should exhibit equal or betterperformance.

10. REFERENCES

1 Redux in aircraft structures.A. Bush.Review commissioned by Oxford Brookes University, August 1994.

2 Effects of outdoor exposure on stressed and unstressed adhesive bondedmetal-to-metal joints.Final report on Redux 775.B.M. Parker.RAE Technical Report TR 91010, February 1991.

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