Development of a composite cargo door for anaircraft

H.G.S.J. Thuis

NLR-TP-99434

Nationaal Lucht- en Ruimtevaartlaboratorium

National Aerospace Laboratory NLR

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This report is based on a presentation to be held at the 10th International Conference onComposite Structures ICCS-10, 15-17 November 1999, Monash.

The contents of this report may be cited on condition that full credit is given to NLR andthe author(s).

Division: Structures and Materials

Issued: October 1999

Classification of title: Unclassified

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Contents

INTRODUCTION 3

DESCRIPTION OF THE DOOR 3

THE FOAM CORE 5

THE METAL INSERTS 5

THE SKIN 5

THE HAT-STIFFENERS 5

THE CORNER REINFORCEMENTS 6

THE MOULD CONCEPT 6

THE FABRICATION CONCEPT 6

TESTING THE DOORS 7

CONCLUSIONS 8

REFERENCES 8

1 Table

10 Figures

(14 pages in total)

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Development of a composite cargo door for an aircraft

H.G.S.J. Thuis

National Aerospace laboratory NLR, Voorsterweg 31, 8316 PR Marknesse, TheNetherlands, Fax: +31 527 248604, E-mail: thuis@nlr.nl

Nowadays, aircraft manufacturers are not only looking for ways to reduce the structural weight of theiraircraft but they are also searching for structural concepts that will lead to a cost reduction. One way torealize a cost reduction is to design a component with a high level of part integration since this will leadto a reduction in labor intensive trimming and assembly costs. By using composites in combination withnew fabrication concepts this part integration becomes feasible. One of these new fabrication concepts isResin Transfer Moulding (RTM) with pre-pregs. In traditional RTM processes, dry fiber pre-forms are

positioned in a mould cavity. After the mould is closed, resin is injected into the mould and the fibers areimpregnated. In the RTM process described in this paper, parts of the dry pre-form are replaced by pre-preg. After closure of the mould, the mould is heated and the resin in the pre-preg starts to melt. Then

RTM resin is injected into the mould. The pressure of the RTM resin is used to pressurize the pre-preg.The main advantage of this fabrication concept is that sub-preforms can be made very easily in pre-pregthat would be very difficult to make with dry fabric due to the lack of tack. Another advantage is that the

RTM process time is reduced, because only a small quantity of resin has to be injected. In order todemonstrate the feasibility of this fabrication concept, a hat stiffened cargo door concept was developed.Two doors were made. The doors were tested by applying a pressure difference to the door of 0.12 MPa.

Both doors did not fail during the tests.

Keywords: Resin Transfer Moulding, Composite Cargo Door.

INTRODUCTION

In the framework of the BRITEEURAM project: Advanced PRImaryCOmposite Structures (APRICOS),structural components for a compositefuselage of an aircraft were developed.The components ranged from discretestiffened fuselage panels, beams, framesand a cargo door (see fig. 1). The maingoal of the program was to achieve acost reduction of at least 20% by usingcomposites instead of metals. TheNational Aerospace Laboratory NLRwas responsible for developing afabrication concept for a compositecargo door. The paper presents a

description of the cargo door. Themould and fabrication conceptdeveloped for producing the door ispresented in detail, followed by a briefdescription of the test program and apresentation of the test results.

DESCRIPTION OF THE DOOR

The door represents an inwards-openinggeneric cargo door for an aircraftcategory like the Airbus A320. Thepurpose of the program was to developa cost effective fabrication concept forthis category of fuselage doors. Sinceonly two doors had to be produced as

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deliverables of the program, it wasdecided not to develop a curved door buta non-curved door in order to limittooling costs. However, the fabricationand tooling concept developed, had to bevalid also for producing curved doors.

During flight conditions the door willonly be loaded by the pressure differencebetween the pressure in the fuselage andthe atmospheric pressure. This meansthat Design Limit Load for the door is0.06 MPa. Design Ultimate Load is 2.0 x0.06 = 0.12 MPa.

The cargo door concept is based on anouter skin that is stiffened by a lattice ofhat stiffeners, integrated pin-locks,door-stops and two hinge points (seefig. 2). Figure 3 shows a cross sectionof a cargo door with a schematicpresentation of the door-stops and thepin-locks (note that the cargo door inthe figure has a curvature, whereas thedoor described in this paper is flat).

Inside the lattice of hat stiffeners a foamcore is present. This foam core is onlypresent for fabrication purposes and wasnot taken into account in the strengthanalyses. Each intersection of two hat-stiffeners is reinforced with four cornerreinforcements (see fig. 2). Tenaluminum inserts are incorporated in thedoor. After curing the door, these metalinserts will be machined to formrespectively door-stops and pin-locks(see fig. 2).

The finite element code B2000 (ref. 1)was used to design the door. B2000 isNLR’s testbed for ComputationalStructural Mechanics. The finite elementmodel comprised half a door and wascomposed of 753 nine noded QUADelements (see fig. 4). The optimization

module B2OPT (ref. 2) within B2000was used to optimize the door forminimal weight with maximum valuesfor the stresses as constraints. In order tofacilitate the optimization, the followingnine laminate design variables weredefined (see fig. 2):

1. The thickness of the 00 plies in theskin

2. The thickness of the 900 ply in theskin

3. The thickness of the ±450 plies inthe skin

4. The thickness of the 00 ply in the hatstiffeners

5. The thickness of the ±450 plies inthe hat stiffeners

6. The thickness of the ±450 ply in thecorner reinforcements

Not only design variables but also thefollowing side constraint was defined:The thickness of both the optimized skinand optimized hat-stiffeners had to be atleast 1.4 mm each, in order to allow arepair with counter sunk fasteners incase the door will be damaged.

The Tsai-Hill criterion was used as afailure criterion. The optimum designwas found after 13 optimization cycles.

In order to minimize milling, drillingand assembly costs it was decided tofabricate the door (including the metalinserts) in a single fabrication cycle. Dueto the complexity of the door, the ResinTransfer Moulding (RTM) fabricationconcept was selected, since making thedoor with pre-preg in the autoclavewould become very cumbersome.However, injection of a component ofthis size and this complexity wouldbecome very risky with the traditional

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RTM as fabrication process:• The injection time would probably

expand the pot life of the RTM resinat injection temperarture.

• Reproducibility in impregnatingevery spot of the pre-form duringthe RTM process would be difficultto guarantee.

Therefore it was decided to make thehat-stiffeners and corner reinforcementsfrom pre-preg and use the pressure ofthe RTM resin during the RTM processto pressurize the pre-preg. In this wayinjection time is dramatically reducedbecause the hat-stiffeners and cornerreinforcements are already impregnatedand the risk of dry spots in the cureddoor is nearly eliminated.

The next paragraphs describe thedifferent elements that compose thedoor: the foam core, the metal inserts,the skin, the lattice of hat stiffeners, andthe corner reinforcements.

THE FOAM CORE

Figures 5 presents an overview of thefoam core for which Rohacell 71 WFwas used. This foam was selectedbecause it has a low density and is ableto withstand a pressure of 0.4 MPa. Thisis very important since during the RTMprocess a resin injection pressure of 0.4MPa was used. The total weight of thefoam core was 1400 gram.

THE METAL INSERTS

Figure 5 presents the location of thealuminum inserts in the assembled foamcore. After curing the door, these metalinserts were machined to form door

stops and pin locks. In order to reducethe weight of the aluminum inserts,holes were drilled into these inserts.After drilling, these holes were filledwith Rohacell 71 WF. The weight of themetal insets was 5750 gram.

THE SKIN

The skin was composed of dry carbonfabric Lyvertex G808 220 gram/m2. Thisfabric has 90% of its fibers in the warpdirection and 10% of its fibers in theweft direction. The lay-up of the skinpre-form was [+45 –45, 0, 90, 0, -45,+45] with a total thickness of 1.4 mm.The size of the skin pre-form was 900mm x 900 mm. A pre-form of dry fabricof this size is very difficult to handle dueto the lack of tack of the single layers.Therefore the skin pre-form wasstabilized by stitching the edges of theskin pre-form with a 2 x 40 tex Kevlarstitching fiber. The RTM-6 epoxy resinwas used to impregnate the skin pre-form.

THE HAT-STIFFENERS

The hat stiffeners were composed ofFiberite HMF carbon 977-2A-35-6KHTA-5H-370-T2 pre-preg fabric.This pre-preg was selected because testson interlaminar shear specimens, madeof RTM-6 resin and carbon fabric andthe 977-2A-35-6KHTA-5H-370-T2 pre-preg, demonstrated compatibilitybetween these two systems. The lay-upof the hat-stiffeners was: [+45, 0, 90, -45]. The thickness of the hat stiffenerswas 1.4 mm.

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THE CORNERREINFORCEMENTS

The corner reinforcements werecomposed of Fiberite HMF carbon 977-2A-35-6KHTA-5H-370-T2 pre-pregfabric. The lay-up of the cornerreinforcements was: [+45, 0, +45]. Thethickness of the corner reinforcementswas 1.0 mm.

THE MOULD CONCEPT

As mentioned before the RTMfabrication concept was used to fabricatethe door. The goal was to develop astand alone tooling concept (with noneed for an oven or an autoclave) inwhich the door could be produced withtight (reproducible) outer dimensionaltolerances. For example, the tolerance onthe thickness of the different elements(skin, hat stiffeners and cornerreinforcements) was 0.1 mm. In order tomake these tight tolerances feasible astiff two-side mould had to be developedsince the pressure used during the RTMprocess was 0.4 MPa. By incorporatingelectrical heating elements into themould, no oven or autoclave wasneeded. This resulted in a mould conceptwith the following elements:• A steel bottom frame.• An aluminum bottom plate (see fig.

6). The bottom plate had an injectioncircuit at the edges of the plate andone vent located at the center of theplate. This radial injection strategywas selected to minimize injectiontime. Holes were drilled in the mouldin which electrical heating elementswere inserted before the RTM cycle.

• A composite top mould with integralelectrical heating wires (see fig. 7). Itwas technically not necessary to use

a composite top mould since the topmould could also be made of metal.However, the composite top mouldwas developed in order to gaininsight in the behavior of compositemoulds for RTM with internalelectrical heating wires. Theexperiences gained with thiscomposite mould will be not bepresented in this paper.

• A steel top frame. Figure 6 presents apicture of the metal top frame andthe aluminum bottom plate. Thepicture provides a good insight in thesize of the mould.

THE FABRICATION CONCEPT

Two doors were made. Each door(including its metal inserts) was madeduring a single RTM fabrication cycle.During this fabrication cycle thefollowing steps were distinguished:• Laminating the hat stiffeners with

pre-preg on laminating blocks andpre-compacting the laminates undervacuum for at least 10 minutes.

• Laminating the cornerreinforcements with pre-preg onlaminating blocks and pre-compacting the laminates undervacuum for at least 10 minutes.

• Preparation of the dry fabric skinpre-form by stitching the edges witha Kevlar stitching wire.

• Positioning of the pre-preg cornerreinforcements in the composite topmould.

• Positioning the pre-preg hatstiffeners in the composite topmould.

• Positioning the dry fabric skin pre-form in the aluminum bottom mould.

• Assembly of the top and bottommould.

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• Assembly of the steel bottom andtop frames.

• Applying vacuum to the mould forthree hours.

• Heating the assembled mould to 1200C by heating the electrical heatingwire in the composite top mould andby heating electrical heatingelements inside the aluminumbottom mould.

• Injection of RTM-6 resin into thebottom mould with a pressure of 0.4MPa. The pressure of the RTM-6resin will pressurize the pre-preg.The total injection time wasapproximately 25 minutes.

• Heating the mould to 160 0C afterthe skin was impregnated with resin.

• Curing the pre-preg and the skinlaminate at 160 0C for 4 hours.

• De-moulding the cured product.• Machining the metal inserts and the

corners of the door.

The weight of the machined doorincluding the foam core and the metalinserts was 13.47 kg.

Figure 8 presents one of the two doorsafter being machined. Figure 9 presentsa detail of a machined pin-lock.

At this moment the cost comparisonbetween a metal and the composite cargodoor has not been made yet.

TESTING THE DOORS

The main goal of testing the doors wasto demonstrate sufficient load carryingcapability. In order to facilitate the testson the doors a pressure box wasdesigned and fabricated. Before beingtested, the doors were mounted to thepressure box and all gaps between the

door and the pressure box were sealed.Then the pressure inside the pressurebox was gradually increased. Themaximum pressure difference appliedwas 0.12 MPa. During the tests thedisplacement of the skin was measuredat three locations (see fig. 10) by lineardisplacement transducers.

Both doors were tested according to thefollowing test program:• Test no.1

Test on door no. 1 to DesignUltimate Load (0.12 MPa pressuredifference). The door did not fail atthis load level.

• Test no. 2Test on door no. 2 to DesignUltimate Load (0.12 MPa) pressuredifference).

• Applying two impact damages todoor no. 2: a 30 Joule impact in theskin resulting in a Barely VisibleImpact Damage (dent depth of 0.9mm) and a 30 Joule impact in theskin under a hat stiffener resultingin a puncture.

• Test no. 3Test on door no. 2 to failure. Failureoccurred at 1.2 bar pressuredifference (Design Ultimate Load),hence demonstrating the damagetolerance of the door.

Table 1 presents the measureddisplacements at maximum load of thedoors during these tests.

TestPressureDifference:(Mpa)

Displ. 1(mm)

Displ .2(mm)

Displ. 3(mm)

1 0.119 3.4 7.2 2.32 0.118 3.5 7.4 2.43 0.118 3.4 9.7 2.7

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CONCLUSIONS

A fabrication concept for producing a

composite hat stiffened door was

developed successfully by combining

dry fabrics with pre-preg during the

RTM process.

Two doors were fabricated and tested.

The tests not only demonstrated a

sufficient load carrying capability of

both doors but also damage tolerance of

the doors. The cost evaluation will be

carried out later.

REFERENCES

1. Merazzi, S.; B2000 – A modular

Finite Element Analysis System.

Version 1.0, Memorandum SC-85-

041 U, National Aerospace

Laboratory, Amsterdam, June 1985.

2. Arendsen, P.; The B2000

Optimization Module: B2OPT,

NLR-Report TP 94116 L, National

Aerospace laboratory, Amsterdam,

April 1994.

Fig. 1 Composite fuselage demonstrator

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Fig. 2 Drawing of the door concept

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Fig. 3 Schematic cross section of the hat stiffened door with pin locks and door stops

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• Element type: nine nodes QUAD• Number of elements: 753• Total degrees of freedom: 16563• Material: composite• Load case: Pressure: 2 × 0.597 = 1.194 bar

Fig. 4 Finite element model of the hat stiffened door

Fig. 5 Overview of Rohacell 71WF foam core and metal inserts

B2OPT Model

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Fig. 6 Aluminium bottom plate and steel top frame

Fig. 7 Composite top mould with internal electrical heating wires

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Fig. 8 Composite cargo door after machining

Fig. 9 Detail of a machined pinlock

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Fig. 10 Location of the displacement transducers