NDE of Composite Aircraft Structures : Case Studies

M. Ramesh Kumar, V. Srinivasa, Suresh Jangir and H.N. SudheendraAdvanced Composites Division, National Aerospace Laboratories, Bangalore – 560017

E-mail: rameshrk@nal.res.in

AbstractComposite materials are being increasingly used for fabrication of aircraft components for military and civil aircraft programmes. Conventionalmanufacturing employs mechanical fastening or adhesive joining to realize structural assemblies. There is an increasing demand for the designerto reduce the manufacturing cost, process time, weight and the assembly of the part. To achieve this many processes are available and onesuch process is the co-curing, co-bonding technique in composites. Advanced Composites Division (ACD) is vigorously aiming at developinga cost effective and advanced method of composite fabrication. In the context of low speed civil aircraft development in mind our divisionhas further looked into the new trend of manufacturing process leading to low cost and low weight for realizing the net component. Onesuch area is Resin Transfer Mould (RTM) to produce a quality assembled part. RTM process is further refined to a method called Resin InfusionTechnology under vacuum which is being adopted for civil aircraft programmes. A consistent product repeatability and acceptable quality isa criteria, there is a need to establish the required NDE parameters to stabilize the process of fabrication.

Keywords : Resin transfer mould, Composites, co-curing, co-bonding, Resin infusion technology

development of these aircraft structures. Inspection of suchcomplex structures is a challenging task and stringent qualitymeasures are followed to ensure compliance to standards forairworthiness certification. On the other hand the alternatemethod and the latest method of fabrication process is ResinTransfer Moulding (RTM). ACD has adopted a finer versionof RTM process which is Resin Infusion Technology. Thisfabrication process has been stabilized and now in the stageof fabricating the net moulded component. NDE role is toinspect, analysis and evaluate the results of the abovemethods.

2. Prepreg System

Case 1 : The fabrication of composites aircraft structuresstarted with autoclave moulding process. It involves a largenumber of infrastructures and in turn the fabrication processbecomes expensive. The complicated contured part can bemanufactured using the process to an acceptable standard.One such part is the co-cured fin of an aircraft shown inFig. 1. The construction of this fin has two skins connectedthrough an ‘I’ sectional ribs. The Torsional box is a fully co-cured structure, this does not have mechanical fastener [1].Inspecting such a part becomes a challenge for NDE. Themost commonly used NDE technique is Ultrasonic methodand we adopt water squirter method for inspection. Sinceboth the skins are connected through ribs and this being abox structure it is not possible to inspect in throughtransmission mode. To overcome this problem we arrived ata novel method by filling water inside the box to remove thepoor acoustical impedance between the skins. The computercontrolled automated Ultrasonic C-scan inspection is carried

1. Introduction

The polymeric composites for aircraft application havebeen an evolutionary process. The growing demand oncomposites, applications have been rising from secondarystructures to primary load bearing and control surfaces of anaircraft. The enhanced performance of composites has beentaken into advantage as weight saving, high strength andstiffness, damage tolerant and durability. Significant driversfor wide spread of composites in aircraft programmes areadvances in design, innovation in processes and fabricationof structures. Qualification of these process/product demandsan efficient and reliable quality assurance.

Non Destructive Evaluation (NDE) is a non-invasivemeans of measurement technologies and analysis techniquesfor quantitative characterization of quality of materials andstructures. Many NDE methods like sonic, thermal, opticaland electromagnetic employs to ensure the quality, structuralintegrity and safety by probing the material to characterizethe sub-surface flaws. The NDE is not limited formanufactured part and certification requirements alone, butextends to the entire service life of an aircraft. From the viewof aircraft maintenance, the need for reliable inspection duringservice life of aircraft is an issue of paramount importance.With the changing pace of technology, there is a constantneed for steady change and extensions in NDE requirementsto overcome the challenges.

Advanced Composites Division (ACD) of NAL has hadconsiderable exposure to composites technology throughthe development of composite aircraft structures & has beensuccessful in realizing airworthy structures for both civil andmilitary aircraft. Innovative concepts of composite fabricationlike co-curing/co-bonding have been employed in the

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manufacturing cost and net moulded processes to form anyshape are some of the major advantages. ACD is adoptingthe novel and innovative method of RTM process in resininfusion technology [3,4]. This resin infusion process hasmany advantages like controlled flow of resin during theperiod of infusion etc. NDE played a crucial role in stabilizingthe process in the laminates level itself, NDE studies on testlaminates could reveal the basic quality of process and furtherit was improved to arrive at a standard process to producea very good laminate having uniform consolidation with afibre volume fraction of <1%. And the generated mechanicaltest results were meeting the acceptable standards. The Fig.4 shows c-scan image of the infused laminate (initial stage)having uniform thickness with large variation in attenuationlevels.

Case 1 : The stabilized fabrication process is furtheradopted to fabricated the part of aircraft components. Theskin having few stringer with varying thickness of anoutboard cover of the aircraft structure, has been inspectedusing ultrasonic c-scan system. NDE result shows a goodconsolidation over the part thickness and the results isshown in Fig. 5. The blue region is having thickness of5.1mm and red and pink regions are having 8.16 & 13.23mmrespectively. The quality of the part is very much comparablewith the prepreg fabrication method.

out in the Fin Torsional box to get the complete C-scan imageof the component in one scan, this comprises of two skinsand the ribs flange regions in Fig. 2.

Case 2 : The centre fuselage part of an aircraft is semicircular in shape, it is fabricated in one shot including boththe longitudinal and transverse integral ribs. Inspecting thispart becomes very critical because the water squirter shouldbe always perpendicular to the circular part. Here the watersquirter jet moves in the linear direction and the part is madeto rotate in the circular direction ( / Rotate) with a specialfixture to mount the part. The skin and rib flange regions areinspected in the automated C-scan system. Due to in-accessible to rib web region it is not possible to inspect inthe automated system and a manual contact pulse echotechnique is adapted to inspection these region. The c-scanimage of the circular duct is represented as a flat panelbecause of the inspection methodology all the variations iswell defined including the flanges and the skin region.(Fig. 3). These results are well with in the acceptancestandards [2].

3. RTM Process

The recent improvement in the fabrication method is ResinTransfer Moulding (RTM). In this process of fabrication less

Fig. 2 : C-Scan plot of fin Torsional box

Fig. 1 : C-scan Inspection of Vertical Fin

Fig. 3 : C-scan plot of Circular Duct Top

Fig. 4 : Infused Laminate

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Case 2 : The segment of wing (Figure 6) is quite largerin size and it measures 1.5m. It has been fabricated withnumber of infusion ports. There are many number of stringerin this part and are very close to each other. Fabrication ofthis kind of structures is quite involved. Similarly the NDEanalysis of these results plays a key role in qualifying thepart. The outer section of wing has a large thickness (10.5mm)and the middle segment has two different thicknesses (6.5 &4.5mm). The attenuation distribution noticed in the colourplot represents the thickness variations within the part andthe variations noticed in a particular thickness also shows agood consolidation which is within the acceptable range.The horizontal white strips are the foam stringer which is astiffener for the wing skin.

4. Conclusion

In composite component fabrication NDE role helped thefabrication team to improvise the process and arrive at thestabilized procedure of fabrication. NDE analysis proves thatthe RTM process is comparable to the fabrication process ofprepreg system [5,6].

Fig. 5 : Segment of the skin

Fig. 6 : Outboard Skin of the Wing

The graph shows that the attenuation levels are linearin both the cases. Due the change in the raw material thereis variation in the levels. The NDE results prove that theprepreg and resin infusion process is comparable in all thethickness range.

Acknowledgment

Thanks are due to Mr. MC Devaiah, Mr. S. Sanjeev Kumarand Mr. S. Kali Kiran, for the extensive support during thework. We are also grateful to Dr. Ramesh Sundaram andMr. Kundan Kumar Verma for many useful discussions.

References

1. Quality Assurance of Co-cured Composite Fin for LCA, NAL PDST 9508, (1995).

2. Quality Assurance (QA) document for Composite Componentsof LCA, NAL SP 9810, (1998).

3. Ramesh Sundram and Subba Rao M, Fabrication of highperformance composites by Vacuum Enhanced Resin InfusionTechnology. INCCOM-3 & 3rd ISAMPE National Conferenceon Composites, (2004)

4. Ramesh Sundram, Kotresh M, Gaddikeri, Kundan Kumar Verma,Kailash Singh, Dinesh B L and Subba Rao M, Tooling conceptsfor the VERITy process. INCCOM-5 & 5th ISAMPE NationalConference on Composites (2006).

5. Non-Destructive Evaluation of Composite Components forSARAS : Acceptance Criteria, TB-01, 38 (2000)

6. Acceptance standard for carbon fibre composite components,ADS, 32 (1998)