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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
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