Design Allowable Considerations for use of Laminated Composites in Aircraft Structures

Design Allowable Considerations for use of Laminated Composites in Aircraft Structures
اسم المؤلف
Prakash D. Mangalgiri
التاريخ
7 نوفمبر 2020
المشاهدات
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Design Allowable Considerations for use of Laminated Composites in Aircraft Structures
Prakash D. Mangalgiri
Abstract | Carbon fibre polymer composites have evolved over the years
to become major structural materials for primary structures of the aircraft
today. The paper reviews the work carried out over last four decades
on carbon fiber polymer composites to create an understanding of their
behaviour in order to set up a philosophy of design and arriving at design
allowable values for strength so as to ensure safety and performance of
aircraft with minimum weight penalty. The rationale behind the choice of
allowable values and the process to arrive at them is explained. While
several issues are discussed, three major aspects are emphasised: the
environmental (hygrothermal) effect, the effect of holes and fasteners and
impact damage. The directions of the current improvements and course
of future developments are indicated in relation to their influence on the
design allowables.
Keywords: Carbon Fibre Composites, Laminates, Aircraft Structures, Design Allowable, Structural
Design, Impact Damage, Holes in composites, Fastener joints, Hygrothermal effect.
7 Concluding Remarks
Choice of design allowable strengths for composite materials has an important bearing on the
entire design process of an aircraft and the final
product and its certification and performance.
We have outlined some important considerations while arriving at design allowable values. In
doing so, we have examined the effort that has
gone into developing underlying understanding
of composite behaviour and its implication for
the allowable values. In particular, we looked into
the hot-wet effect, holes and fasteners and impact
damage. The literature on these issues is really vast
and the references quoted here should be taken as
only representative of the entire effort. No claim is
made to being exhaustive, nor is any claim made
towards relative importance of cited literature
over the omitted ones.
At the present design allowable values, the
potential of composites as aircraft structural
materials remains vastly unrealised. Future efforts
need to be and will be directed towards expanding
the current envelope of usage, such as for example, exploring post-buckling strengths, strength
beyond first-ply-failure, use of non-symmetric
lay-ups and through-thickness reinforcements.
For, this to happen, it is important to develop better understanding of the material behaviour in
those regimes and develop better predictive capability. Better material systems would be explored
and toughness and resistance to hot-wet would be
the target directions. Newer developments such
as Structural Health monitoring can bring in new
thinking about allowable damage which also will
hopefully allow higher design allowables and better utilisation of the potential of composites.
Acknowledgements
Much of my understanding and knowledge of
designing with composites is derived from the
work carried out for the LCA programme during
1980’s and 1990’s and this forms the cornerstone of
the discussions in this paper reflecting a view from
a practising engineer rather than a research scientist. Many colleagues and collaborators, too numerous to name, from ADA, HAL, NAL, IISc, IITs and
other collaborating institutions have helped me in
learning about composites and their contribution
is gratefully acknowledged. In particular, thanks are
due to Dr. KN Raju, head of the erstwhile National
Team for Composite Wing, and my colleague at
ADA for long time, Dr. K Vijayaraju. Thanks are also
due to my teacher at IISc, Prof B Dattaguru. The
help and guidance received from Dr. AR Upadhya,
Dr. Kota Harinarayana, Shri DY Katti, Shri TGA
Simha and Prof KP Rao through various discussions
from time to time is gratefully acknowledged.
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