Structural mechanics - enabling weight reduction and performance enhancement of composite aerospace structures (for redaction)
Submitting InstitutionUniversity of Bristol
Unit of AssessmentGeneral Engineering
Summary Impact TypeTechnological
Research Subject Area(s)
Engineering: Aerospace Engineering, Civil Engineering, Materials Engineering
Summary of the impact
For aerospace vehicles, the development of new materials and structural
configurations are key tools in the relentless drive to reduce weight and
increase performance (in terms of, for example, speed and flight
characteristics). The economic drivers are clear — it is widely recognised
that it is worth approximately $10k to save one pound of weight in a
spacecraft per launch and $500 per pound for an aircraft over its
lifetime. The environmental drivers (ACARE 2050) are also clear — reduced
aircraft weight leads to lower fuel burn and, in turn, to lower CO2
and NOx emissions. With such high cost-to-weight ratios, there
is intense industrial interest in the development of new structural
configurations/concepts and enhanced structural models that allow better
use of existing or new materials. Analytical structural mechanics models
of novel anisotropic structures, developed at the University's Advanced
Composites Centre for Innovation and Science (ACCIS),
are now used in the industrial design of aircraft and spacecraft. Based on
this research, a new, unique anisotropic composite blade, designed to meet
an Urgent Operational Requirement for the MoD, is now flying on
AgustaWestland EH101 helicopters that are deployed in Theatre. In
addition, the new modelling tools and techniques have been adopted by
Airbus, AgustaWestland, Cassidian and NASA and incorporated into LUSAS's
finite element analysis software. These tools have, for example, been used
to inform Airbus's decision to use a largely aluminium wing design rather
than a hybrid CFRP/aluminium wing for the A380.
The development of new structural configurations made from novel
composite layups, and of the analysis tools to understand them, forms a
key part of research into the performance and optimisation of composite
structures. For this reason, Weaver (UoB since 1998) was recruited to the
University's Composites Group (now ACCIS) to develop new
structural mechanics models for aerospace vehicles. His work, and that of
his team within ACCIS, has centred on the influence of
interrelationships between geometric form and anisotropic material
properties on structural performance in developing models for beams,
plates and shells with physical insight and closed form solutions. The
research has been part funded by Airbus and AgustaWestland and relevant
consultancy work includes NASA Langley (2001-2009) and Cassidian (then,
EADS Military Air Systems) (2006).
In 1999, Miller (UoB PhD student 1999-2002) developed thermoelastic
models for predicting temperature build-up in carbon fibre reinforced
plastic (CFRP) parts, when subjected to transient radiative and convective
heating, typical of an aircraft sitting on a hot runway in a tropical
climate. It was shown that the temperature build-up could exceed 80OC,
which, due to the thermal expansion properties of CFRP and aluminium,
could result in significant fatigue stresses in the aluminium parts .
This fatigue issue contributed significantly to cancelling the hybrid
Al/CFRP early concept wing structure in 2000-2001. Miller won the
prestigious accolade of the Thomas Jefferson award for the best student
paper at the American Institute of Aeronautics and Astronautics (AIAA)
Structures, Dynamics and Materials Conference 2001, for this work.
Modern helicopter rotor blades are typically made from high specific
stiffness materials, including CFRP, honeycombs and specialist polymeric
foams. These allow better design of mass and stiffness distributions,
which are crucial to ensure benign vibration characteristics.
State-of-the-art methods relied on finite element analysis to calculate
stiffness properties of realistic blade sections by estimating torsional,
extensional, flap and lag bending stiffnesses. However, transverse shear
effects and anisotropic coupling effects between bending and torsion,
which are typically significant in anisotropic composite blades, were not
included. From 2000-2002, Hill (UoB RA 1996-2006) and Weaver developed an
alternative finite element based method for calculating full 6x6 stiffness
matrices, including all coupling effects as well as transverse shear, of
fully anisotropic sections of arbitrary shape, composed of multiple
materials. They demonstrated that the technique was highly useful to model
the complex responses of helicopter rotor blades . Building on this
work, Lemanski (UoB PhD student 2000-2004) and Weaver developed an
optimisation technique to minimise the mass of rotor blade sections by
changing composite material properties, adjusting aerofoil skin
thicknesses and altering the position of internal parts such as spars .
They also observed that the then state-of-the-art closed-form solutions
did not quantify the effects of bend/twist coupling because they neglected
the effects of vertical wall stiffness and foam, resulting in inaccurate
and unusable results. Hence, they developed an anisotropic box model that
quantified these effects resulting in much improved analytical modelling
From 2000, Weaver developed a series of buckling and postbuckling
solutions for anisotropic plates and cylindrical shells subject to
compression, shear and combined loading. Based on early work on designing
composite cylindrical shells under axial compression, Weaver was invited,
in 2001, to be a consultant at NASA Langley. This consultancy continued
until 2010 (grants totalling over $450k), where his insight, ideas and
methods influenced high profile developments such as the ongoing Shell
Buckling Knockdown Factor (SBKF) project. This project was proposed to
address the innate conservatism in the design of cylindrical shell
structures used in launch vehicles such as rockets and the space shuttle
where, due to inherent imperfection sensitivity in the shell structures,
high knockdown factors (safety factors) are traditionally used.
Scientifically demonstrating that lower knockdown factors are justifiable
allows a reduction in the vehicle's weight. Taking the widely acknowledged
$10k cost-saving per pound of payload reduction, the estimated 10% weight
saving in a structure weighing 1000 lbs leads to both a cost saving of
£10M per launch. Particular insight shown by Bristol's work into
anisotropic effects in cylindrical shells proved most valuable to the
project. The work was summarised in the 46 page book chapter  having
previously been published in AIAA journals, the International Journal of
Solids and Structures and AIAA conference proceedings.
Cosentino (UoB PhD student 2006-10 and Airbus employee since 2006) and
Weaver developed analytical structural models for the prediction of
specific failure mechanisms of stiffened CFRP plates, representative of
aircraft wing structures. They showed comparable accuracy to finite
element models for predicting failure by stiffener debonding but with two
orders of magnitude improvement in computational speed that allowed rapid
sizing studies during the design process. This culminated in the
conception of a new joint design, the compound joint , which
using a combination of bolting and adhesives to separately facilitate the
transfer of stresses by normal (Mode I) and shear (Mode II), respectively.
References to the research
 *J. Miller and P.M. Weaver, 2003. Temperature distribution in a
composite box structure subject to Transient Heat Fluxes. Journal of
Thermophysics and Heat Transfer, 17(2), 269-277,
 G.F.J. Hill and P.M. Weaver, 2004. Analysis of anisotropic
prismatic sections. The Aeronautical Journal, 108(1082), 197-205,
(can be supplied upon request).
 S.L. Lemanski, P.M. Weaver and G.F.J. Hill, 2005. Design of
composite helicopter rotor blades to meet given cross-sectional
properties. The Aeronautical Journal 109(1100), 471-475, (can be
supplied upon request).
 *S.L. Lemanski and P.M. Weaver, 2005. Flap-torsion coupling in
sandwich beams and filled box-sections. Thin-Walled Structures
43(6), 923-955, dx.doi.org/10.1016/j.tws.2004.12.004.
 P.M. Weaver, 2008. Anisotropic Elastic Tailoring in Laminated
Composite Plates and Shells. Book chapter in Buckling and
Postbuckling Structures: Experimental, Analytical and Numerical Studies,
Ed: B.G. Falzon & M.H. Aliabadi, World Scientific Press, ISBN-13:
 *V.A. Imperiale, E. Cosentino, P.M. Weaver and I.P. Bond, 2010. Compound
joint: A novel design principle to improve strain allowables of FRP
composite stringer run-outs. Composites: Part A, 41(4), 521-531,
* References that best indicate the quality of the underpinning research.
Details of the impact
The impact of the University's work on developing new structural concepts
and structural methods has been the use of the results and the tools by
industry on a variety of aerospace applications. Here, we present six
1) A new structural concept that is now in-flight
Lemanski and Weaver's unique anisotropic composite design  was used in
the development of the BERP IV helicopter rotor blade in 2005-06 for
AgustaWestland's EH101 helicopter. The Principal Rotor Engineer from
AgustaWestland reported that "the design, which uses unbalanced
anisotropic angle plies at a specific fibre orientation, gives
performance enhancement by coupling bending and torsion within the blade
to reduce the loads experienced by the rotor control system. Eighty five
of these blades, each with typical value of £100k, have been purchased
by the UK MoD as part of an Urgent Operational Requirement in order to
enhance the range and capability of the EH101 aircraft in Theatre"
[a]. Note that of the 85 blades, worth approximately £8.5M, 70 were
ordered since 2008. They all continue to be flown and are used in
situations where a heavy lift capability is required.
2) A new analysis tool that influenced Airbus's A380 wing design
Miller and Weaver's thermoelastic analysis  has been developed into a
new structural analysis tool for Airbus. When designing the A3XX (which
subsequently became the A380) aircraft, Airbus considered both aluminium
and hybrid CFRP/aluminium wing designs. The Bristol-based tool was used by
Airbus to inform the decision to use a largely aluminium, rather than
hybrid, wing design [b]. As of July 2013 there have been firm orders for
262 A380s [c] with a market value of over $105B, based on an average list
cost of $404M each [d].
3) A new beam section analysis tool for AgustaWestland and then
Hill and Weaver's beam section analysis , for calculating 6x6 stiffness
matrices for helicopter rotor blade sections, and the subsequent
optimisation method  have been incorporated into AgustaWestland
Helicopter's internal design codes (BSAP) [a]. The inclusion of these
tools allows AgustaWestland to predict the response of rotor blades more
accurately such that flight performance is improved via a reduction in
deleterious vibration response whilst maintaining structural integrity. In
addition to this improvement, Bristol's optimisation technique reduced the
work of an engineer that previously took two weeks, to a matter of minutes
Bristol's beam section analysis, which provides the 6x6 stiffness
properties for finite element beams, was an improvement on the then
current state-of-the-art techniques used in commercial finite element
packages. It was incorporated into LUSAS's finite element software in 2008
[e], giving LUSAS the commercial advantage of being able to model the
structural response of complicated multi-material beam sections.
4) New buckling analysis tool for Cassidian
Weaver's closed-form buckling solutions for plates and shells [5-6] are
now a key part of Cassidian's LAGRANGE multidisciplinary optimisation
software. The work was transferred to Cassidian via a £10k consultancy in
2006, and allows rapid analysis of the buckling capability of thin-walled
sections such as wings and fuselages. As an example application, it has
been used to help design the Airbus A350XWB aircraft structure [f]. The
Airbus A350XWB is the first commercial Airbus aircraft to be mostly
designed with composite materials. It had its maiden flight in July 2013
and first delivery is expected in mid 2014. So far 682 firm orders have
been received [g] with a value of circa $173B, based on an A350-800
average list price of $254M [d]. Cassidian also intends to use these
structural analysis models in the design of a European Unmanned Air
Vehicle in 2014 [f].
5. New structural failure analysis led to new structural concept for
New analytical expressions for predicting the structural failure of
stiffened wing panel structures  have been adopted in Airbus' design
codes. Specifically the Senior Lead Stress for the A350-1000 Midbox states
that "the in depth understanding and methods developed during my PhD
[at Bristol] helped Airbus refine their stringer runout design. The
knowledge has been transferred transnationally throughout Airbus and
Bristol's expertise is acknowledged. Furthermore, the papers published
have been used as a starting point to define methods and allowables,
helping Airbus prepare comprehensive Aircraft Certification Documents
minimising the need for large tests" [b].
These methods have supported the design of wings in commercial aircraft
variants of the A350XWB: A350-900 and A350-1000 as well as the military
aircraft A400M. As of August 2013, when it had its maiden flight, 174
A400M aircraft have been ordered [h]. Current savings of over £700k from
virtual testing are estimated based on a typical cost of £35k per test. In
addition to this cost saving, the new design based on Bristol's methods
reduces the weight of a wing by approximately 10kg per wing [b] giving
operational savings of around £14M based on the orders for A350XWB and
A400M using a £800/kg cost to weight value.
This analysis subsequently allowed a new stringer-to-skin, aircraft wing
joint design to be proposed : "Compound joints have become part of
the certification training material. [Bristol's] 2 and 1/2
dimension model was used for A380 checkstress. The way the Mode 1 and 2
contributions are decomposed in Stringer Run Out is used as a bonded
joint pre-sizing method" [b].
6. New structural analysis for shells exploited by NASA
NASA's Shell Buckling Knockdown Factor (SBKF) project was set up to "help
future heavy-lift launch vehicles weigh less and reduce development
costs" by generating new shell-buckling knockdown factors replacing
the existing ones which "date back to pre-Apollo-era studies — well
before modern composite materials, manufacturing processes and advanced
computer modeling" [i]. New, more accurate and less conservative
methods have now been developed for the design of shell structures for
space launch vehicles. Estimated savings of the order of £10M/launch are
expected with the first launch based upon these new design methods
anticipated from NASA Wallops in 2014.
Weaver was consultant for NASA from 2001-2010 with support funds from
NASA totalling around $450k. [text removed for publication]. [j].
Sources to corroborate the impact
[a] Principal Engineer Rotors (Methods), AgustaWestland Helicopters.
[b] Senior Lead Stress, A350-1000 Midbox, Airbus.
[c] Airbus press release Orders and Deliveries, July 2013.
[d] Airbus press release Airbus Aircraft 2013 Average List Prices,
[e] Director and Founder of Lusas finite element analysis — email
[f] Senior Expert for Multidisciplinary Airvehicle Analysis and Design
Optimization, Cassidian, Germany — email correspondence.
[g] Airbus press release Orders, Deliveries, Operators, July
[h] Airbus Military A400M homepage, downloaded August 2013.
[i] NASA — Marshall Star news article 2011 Marshall Space Flight
Center Year in Review, January 11, 2012.
[j] Senior Research Engineer and Principal Investigator of SBKF project,
NASA Langley Research Center, USA.