Aircraft Technologies STRUCTURES

BOJCAS: Bolted Joints in Composite Aircraft Structures Michael McCARTHY

The objective of BOJCAS is to develop advanced numerical design methods for bolted joints in composite aircraft structures, This is a critical technology supporting the introduction of composites into the primary structure of large commercial aircraft. The methods developed have the potential to significantly reduce testing, and hence time~cost of development, as well as aircraft weight with consequent increase in efficiency, They will also help to ensure continued safety, This article provides an overview of activities within the project.

he use of composite materials in aircraft structural components has grown steadily with each generation of aircraft. From ini- tial applications in non-struc-

tural parts and secondary structures, composites have increasingly found use in primary aircraft structures, particu- larly in light aircraft, commuter planes, military fighters and helicopters. To date, tl~eir use in the primary structure of commercial aircraft has been relative- ly limited. However intensive efforts are currently taking place on future con> posite wing and fuselage structures, and the use of composites in primary structures is likely to increase substan- tially' over the conning decade. Such developments are being driven bv the potential benefits of composites, chiefly in relation to reduced weight and operating cost. However realising tlne full value of this potential still involves many technical challenges. To ensure the future competitiveness of the European aerospace industr}5 it is criti- cal that the maximum possible benefits are obtained to meet the challenge from similar developments elsewhere. It is also essential to maintain and improve current levels of safety. To achieve these goals the knowledge base of composite structures behaviour needs to be extended and advanced design tools need to be developed. BOJCAS addresses a critical aspect of this challenge, namely composite bolted

joints. Because joints represent potential weak points in the structure, the design of the overall structure tends to follow from, and be significantly limited by', the design of the joint. Non-optimal design of joints can lead to overweiglnt struc- tures, in-service structural problems and higln life-cvcle costs. This is even more pronounced in composite struc- tures, since maximum joint efficiencies are at best 40-50%, and at worst consid- erably less. This compares with 70-80% for metals and thus detracts from the weight ad\antage of composites oxer metals. Hence optimising joint efficien- cy is crucial to realising the maximum potential benefits of composites. Some of the reasons for lower joint effi- ciency in composites are: brittleness which means little stress relief around the highest loaded holes, anisotropy which leads to higher stress concentra- tion factors, low transverse strength, susceptibility to delamination, and sen- sitivitv to environmental conditkms. All of these factors togetlner with the con> plexity of composite failure modes, make the analysis and design of com- posite joints far more complex than tlnat of metallic joints. Much effort has been put into developing analytical design methods for composite bolted joints, using both closed-form analytical meth- ods and numerical techniques such as the finite element method. Howeveb the majority of models to date have been overly simplistic in nature, and lna\,e

had limited success in predicting joint behaviour. Consequentl), tile current design methods used in industry are largely empirical and heavih." reliant on expensive and time-consuming testing. Many of the methods have advanced lit- tle from those developed during an intense period of testing in the USA in the 70s and 80s. Their application to new, primary structures of commercial aircraft, with increased uncertainties due to new materials and thicker lami- nates, and increased quantities of mate- rial used in each test, is likely to lead to expensive design cycles and overweight joint designs. With recent developments in computational mechanics and contin- ued increase in processing power, there is the potential to develop more advanced analysis tools which could be used to optimise joint design, reduce the quantity of experimental tests required in development, and improve funda- mental understanding of joint behav- iour, hence ensuring continued safety.

Project objectives The overall objectives are: • reliable and user-friendly analysis-

based design methods, with improv- ed predictive capability which will enable: (a) a significant reduction in testing,

and hence time and cost of devel- opment, and

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(b) the incorporation of composites into the primary structure witln optimal weight savings;

• a fundamental improvement in un- derstanding of composite bolted joint behaviour, especially in primary structures, tiros contributing to con- tinued safety.

The partnership The consortium consists of three aircraft manufacturers, four national aerospace laboratories, two universities, and two research companies. Eight countries are represented as shown in tabh' I. The start date was February 2000 and the duration of the project is 36 months.

Programme content The programme structure is illustrated in figure 1. Bearing in mind the needs of industry for preliminary and detailed design tools, the following outputs are planned: • global design methods, for prelimi-

nary design of complex, multi-fasten- er joints;

• detailed design methods for final design of critical joints;

• methods to couple global and detail- ed design methods, i.e. to streamline the process of producing a detailed analysis from a preliminary analysis;

• design guidelines for primary com- posite bolted joints based on analyses and tests.

The project is divided into a global strand (WP 1,2 and 3) and a local strand (WP 4 and 5) with the coupled global- local methods bridging the two strands. Interaction takes place between the strands by using the knowledge gained from the detailed local methods to improve the global methods. Each strand contains major testing and analy- sis components. At the global level, a series of 'bench- mark' structures representative of com- plex, primary, multi-fastener joint con- figurations, will be designed and tested. Global design techniques will be used to design and predict the performance of these benchmarks. Initially, existing in- house methods will be used to provide

Ireland United Kingdom Germany Sweden Italy The Netherlands Greece Switzerland

Table I. The BOJCAS Partnership.

University of Limerick (Coordinator) Airbus UK, DERA EADS Airbus SAAB AB, FOI, Royal Institute of Technology CIRA NLR ISTRAM SMR

a baseline (Tasks 2.1 and 2.2). These methods include handbook/design chart methods and two-dimensional finite element methods. Then new glob- al methods will be developed mostly based on two-dimensional finite ele- ment methods with specialised tech- niques to model bolt-hole interaction, and validated on the benchmarks (Task 2.3). Figure 2 illustrates one of the benchmark structures. This benchmark structure will consist of several varia- tions on a skin-stringer joint element for a potential hybrid metal/composite wing. The structures will be relevant to the design of the lateral wingbox in the EU project TANGO and will also address the issue of metal/composite joints in a generic way. Another bench- mark structure will be representative of

bolted composite repairs, which are essentially complex multi-fastener joints, hnproved analysis, of repairs will considerably reduce testing needed for the certification ot repair configurations and procedures given as standard~ within the Structural Repair Manual. Other benchmark structures are aimed at studying the effects of xariable bolt patterns, as well as damage tolerance. For comparison with the global meth- ods, the benchmark structures will also be modelled using global-local meth- ods. These methods are being de\el- oped to automatically couple global models with much more detailed local models of the bolt regions, klore detailed models are needed because several effects influencing failure arc three-dimensional in nature, and cannot

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Aircraft Technologies STRUCTURES

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be accounted for by two-dimensional techniques. For example, non-uniform through-thickness stress distributions exist in situations involving counter- stink bolts, non-symmetrical loading, bolt bending, or bolt tilting in holes with clearance, and lead to significant stress concentrations. This will particularly be the case with thick primary structures. The 'bearing' mode of failure (in which the laminate is locally crushed at the hole) has been shown to be a three- dimensional phenomenon, invoMng through-thickness cracks and delamina- tions. Such comparisons will also enable the improvement of global methods \ia improved spring stiffnesses and correc- tion factors for three-dimensional effects. At the local level, work is focusing on the development and validation of detailed joint models incorporating new means of determining failure. Figure 3 illustrates the localised nature of the stress distributions in single-lap joints, which cannot be accounted for with two-dimensional methods. Figure 4 illustrates the use of progressive dam- age modelling to track the progression of failure in each ply until final failure of the joint. Such techniques possess the potential for more accurate failure pre- diction than the essentially empirical failure criteria currently in use. Such detailed models take considerable time to set up and run, and as such are

not currently suitable for use in the pre- liminary design phase. Work in BOJCAS is aimed at automating the setup pro- cess as far as possible, so that the only barrier to exploiting these methods fully

will be processor speed, which can be expected to be removed within just a few years. Once validated these models can also be used to generate design data for use in preliminary design, with con- siderably less experimental tests than are required at present. In WP 5, an experimental test programme will be carried out involving smaller- scale joints than the benchmarks. These tests will provide further data related to some of the issues covered by the bench- mark structures (e.g. composite-metal joints, bolted repairs), as well as provid- ing data for validation of tile detailed models. An extensive list of parameters is being examined, including variations in geometry, loading, materials, lay-ups, bolt-types, environmental conditions, bolt-hole clearances, clamping force and others. Tests vxill be extensively instru- mented using techniques such as strain gauging, photoelasticity and intrument- ed bolts, and detailed fractographic fail- ure analysis will be performed (fi~(,ure 5).

Figure 3. Three-dimensional stress distributions in single-lap joint. (Doc, ULIM)

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Figure 4. Progressive damage propagation at different load levels (upper surface of a [90/0/-45/45]ss laminate). (Doc. ISTRAM)

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Figure 5. Analysis of damage progression using microscopy (Doc, FO0.

ites in aircraft structural canlponentb. Since joints have suctn a critical effect on the safet;' and efficiency of aircraft structures, it is vital tl'~at the mo>t advanced design methods are used. :k high pntential exists to reduce de\'elap- merit costs, maximise weight sa\rillgS increase manufacturer and operator confidence in composites, and ensure safety of future primary composite structures. The spin-off potential of the developed technology, is very high in several other fields such as shipbuild- ing, space, nuclear, chemical, offshore, automotive, rail and civil engineering. Several of fine partners iri BOJCAS ha\e been in\'ol\ ed for many \cars in nation- al programmes on composite bolted joints. BOJCAS will pool finis collective expertise and provide a European per- spective on this important tapic. •

Finally evaluation and summary tasks will take place. The global methods are aimed at immediate exploitation, and as such will be implemented into the industrial partners' codes of choice and assessed by those partners. The detailed methods will be assessed for their exploitation potential and a patln towards implementation will be drawn tip. Overall, the results from the tests and analyses will be used to form design guidelines for composite belted joints.

Current status In the i:irst \.'ear ot the project, all the benchmark structures ha\'e been desig-

ned and predictions have been made regarding load distributions and failure loads. Fabrication has begun and tests will be complete by the mid-term assessment. The specimen tests in WP 5 arc under way and will also be complete b\' mid-term. Modelling of the bench- niark structures with global methods has been performed, and initial global- local models will shortly be ready for comparison. Interim reports on the detailed models of the specimen tests will be supplied at the end of Month 12.

Conclusions BOICAS is fecusec] on an enabling tech- nology for the increased use of cnmpos-

About the author:

Michael McCarthy is Director, Composites Research Centre; and Lecturer,

Department of Mechanical and Aeronautical Engineering, University

of Limerick, Ireland, MichaeI,McCarthy@ul,ie

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