BOJCAS: Bolted Joints in Composite Aircraft Structures

航空

游客，如果您要查看本帖隐藏内容请回复

航空

1 AIR & SPACE EUROPE • VOL. 3 • No 3/4 - 2001
The use of composite materials in
aircraft structural components
has grown steadily with each
generation of aircraft. From initial
applications in non-structural
parts and secondary structures,
composites have increasingly found use
in primary aircraft structures, particularly
in light aircraft, commuter planes,
military fighters and helicopters. To
date, their use in the primary structure
of commercial aircraft has been relatively
limited. However intensive efforts are
currently taking place on future composite
wing and fuselage structures,
and the use of composites in primary
structures is likely to increase substantially
over the coming decade.
Such developments are being driven by
the potential benefits of composites,
chiefly in relation to reduced weight
and operating cost. However realising
the full value of this potential still
involves many technical challenges. To
ensure the future competitiveness of the
European aerospace industry, it is critical
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 overweight structures,
in-service structural problems
and high life-cycle costs. This is even
more pronounced in composite structures,
since maximum joint efficiencies
are at best 40-50%, and at worst considerably
less. This compares with 70-80%
for metals and thus detracts from the
weight advantage of composites over
metals. Hence optimising joint efficiency
is crucial to realising the maximum
potential benefits of composites.
Some of the reasons for lower joint efficiency
in composites are: brittleness
which means little stress relief around
the highest loaded holes, anisotropy
which leads to higher stress concentration
factors, low transverse strength,
susceptibility to delamination, and sensitivity
to environmental conditions. All
of these factors together with the complexity
of composite failure modes,
make the analysis and design of composite
joints far more complex than that
of metallic joints. Much effort has been
put into developing analytical design
methods for composite bolted joints,
using both closed-form analytical methods
and numerical techniques such as
the finite element method. However, the
majority of models to date have been
overly simplistic in nature, and have
had limited success in predicting joint
behaviour. Consequently, the current
design methods used in industry are
largely empirical and heavily reliant on
expensive and time-consuming testing.
Many of the methods have advanced little
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 laminates,
and increased quantities of material
used in each test, is likely to lead to
expensive design cycles and overweight
joint designs. With recent developments
in computational mechanics and continued
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 fundamental
understanding of joint behaviour,
hence ensuring continued safety.
Project objectives
The overall objectives are:
• reliable and user-friendly analysisbased
design methods, with improved
predictive capability which will
enable:
(a) a significant reduction in testing,
and hence time and cost of development,
and
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.
Aircraft Technologies STRUCTURES
2
(b) the incorporation of composites
into the primary structure with
optimal weight savings.
• a fundamental improvement in understanding
of composite bolted joint
behaviour, especially in primary
structures, thus contributing to continued
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 table 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 preliminary
design of complex, multi-fastener
joints;
• detailed design methods for final
design of critical joints;
• methods to couple global and detailed
design methods, i.e. to streamline
the process of producing a detailed
analysis from a preliminary analysis;
• design guidelines for primary composite
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 globallocal
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 analysis
components.
At the global level, a series of ‘benchmark’
structures representative of complex,
primary, multi-fastener joint configurations,
will be designed and tested.
Global design techniques will be used to
design and predict the performance of
these benchmarks. Initially, existing inhouse
methods will be used to provide
a baseline (Tasks 2.1 and 2.2). These
methods include handbook/design
chart methods and two-dimensional
finite element methods. Then new global
methods will be developed mostly
based on two-dimensional finite element
methods with specialised techniques
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 variations
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 benchmark
structure will be representative of
bolted composite repairs, which are
essentially complex multi-fastener
joints. Improved analysis of repairs will
considerably reduce testing needed for
the certification of repair configurations
and procedures given as standards
within the Structural Repair Manual.
Other benchmark structures are aimed
at studying the effects of variable bolt
patterns, as well as damage tolerance.
For comparison with the global methods,
the benchmark structures will also
be modelled using global-local methods.
These methods are being developed
to automatically couple global
models with much more detailed local
models of the bolt regions. More
detailed models are needed because
several effects influencing failure are
three-dimensional in nature, and cannot
BOJCAS
Figure 1. BOJCAS programme structure.
Table I. The BOJCAS Partnership.
Ireland University of Limerick (Coordinator)
United Kingdom Airbus UK, DERA
Germany EADS Airbus
Sweden SAAB AB, FOI, Royal Institute of Technology
Italy CIRA
The Netherlands NLR
Greece ISTRAM
Switzerland SMR
be accounted for by two-dimensional
techniques. For example, non-uniform
through-thickness stress distributions
exist in situations involving countersunk
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 threedimensional
phenomenon, involving
through-thickness cracks and delaminations.
Such comparisons will also enable
the improvement of global methods via
improved spring stiffnesses and correction
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 damage
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 prediction
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 preliminary
design phase. Work in BOJCAS
is aimed at automating the setup process
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 considerably
less experimental tests than
are required at present.
In WP 5, an experimental test programme
will be carried out involving smallerscale
joints than the benchmarks. These
tests will provide further data related to
some of the issues covered by the benchmark
structures (e.g. composite-metal
joints, bolted repairs), as well as providing
data for validation of the 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 will be extensively instrumented
using techniques such as strain
gauging, photoelasticity and intrumented
bolts, and detailed fractographic failure
analysis will be performed (figure 5).
3 AIR & SPACE EUROPE • VOL. 3 • No 3/4 - 2001
Aircraft Technologies STRUCTURES
Figure 2. Example benchmark structure: skin-stringer joint element
for hybrid metal/composite wing. (Doc. Airbus UK)
Figure 3. Three-dimensional stress distributions in single-lap joint. (Doc. ULIM)
Figure 4. Progressive damage propagation at different load levels
(upper surface of a [90/0-45/45]S8 laminate). (Doc. ISTRAM)
4
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 path
towards implementation will be drawn
up. Overall, the results from the tests
and analyses will be used to form
design guidelines for composite bolted
joints.
Current status
In the first year of the project, all the
benchmark structures have been designed
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
are under way and will also be complete
by mid-term. Modelling of the benchmark
structures with global methods
has been performed, and initial globallocal
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
BOJCAS is focused on an enabling technology
for the increased use of composites
in aircraft structural components.
Since joints have such a critical effect on
the safety and efficiency of aircraft
structures, it is vital that the most
advanced design methods are used. A
high potential exists to reduce development
costs, maximise weight savings,
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 shipbuilding,
space, nuclear, chemical, offshore,
automotive, rail and civil engineering.
Several of the partners in BOJCAS have
been involved for many years in national
programmes on composite bolted
joints. BOJCAS will pool this collective
expertise and provide a European perspective
on this important topic. 
BOJCAS
About the authors:
Michael McCarthy is Director, Composites
Research Centre; and Lecturer,
Department of Mechanical
and Aeronautical Engineering, University
of Limerick, Ireland.
Michael.McCarthy@ul.ie
Figure 5. Analysis of damage progression using microscopy (FOI).

tyrone1225

想来了解一下