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Mechanical behavior of materials used in the fuselage of
commercial aircrafts
Right from the time Wright brothers first flew there ‘box kite’, man has been pursuing his
restless ambition of ‘heavier than air’ human flight
...
First planes to fly were the ones
made of wood laminates, then with the introduction of jet engines, metals and alloys made their
way into the aviation industry
...

Composites are a macroscopic mixture of different materials so combined, that each material
retains its identity
...

Military aircraft and fighter jets with their mammoth budget allocations have always been
pioneering advancements in the aviation technology
...
Boeing and
Airbus being the leading commercial aircraft manufacturers are in a fierce competition over fuel
economy and the overall comfort level of passengers
...


Picture ©Airbus

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Fig 1: Composite Material usage in latest commercial aircrafts
...
The Fuselage
Fuselage is the central part of an aircraft to which wings and tails are attached
...
Fuselage is an
internally-pressurized, humanity-containing tube that is arguably the most safety-critical part of
an aircraft
...


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The fuselage has to carry the payload, and is the main body to which all parts of aircraft are
connected
...
The strength and
stiffness of the fuselage structure should be at par with the requirements, at the same time weight
should be brought to a minimum
...
Semi-monocoque structure has predominantly been adopted as a standard
among travel aircrafts
...
Stringers latch onto hoop shaped fame for strength
...
Closed support structures are called
Bulkheads, while the open or ring shaped support structures are termed as frames
...

The cutouts do not help in carrying the loads that are present on the fuselage skin; rather they
interrupt the direct load paths
...
A cargo plane has much bigger doors compared to a travel aircraft so the
frame and stringer structure need be incorporated in the door to carry the loads
...
During normal operation the floor of aircraft is at the center
of the pressurized fuselage so that pressure forced balance out and no significant load acts on it
...

Frames maintain the cross-sectional shape of the fuselage and prevent it from flexural damage
during landing and takeoffs when fuselage comes under bending loads
...
Torsion and bending
loads incurred by maneuvers in air are managed by stringers having minimal weight
...
Flat disc or curved bowl
type pressure bulkheads close off the pressurized passenger cabin
...

2
...

Fuselage is made in parts for the ease of manufacturing and assembly
...
Fuselage with joined panel claims to offer a maintainability advantage
...

However, the abrupt departure from the time tested metal fuselage structures has given rise to
some very daunting questions that yet require a convincing answers
...
The GAO the General
Accounting Office (GAO), the watchdog over official policy in the USA, report cited:


The difficulty of detecting damage;



Limited standardization of composite materials and repair technologies;



The level of training and awareness of workers handling composites; and



A dearth of information on the longer term behavior of aircraft composites due to limited
in-service experience so far
...
Carbon epoxy composite (CFRP) fuselages are filament wound or tape laid
...

Boeing and Airbus have shown optimism in answering to questions posed against the reliability
of composites
...
The lower sections of fuselage often suffer impact damage at the hands
of baggage loaders, catering carts and other service vehicles
...

Bolted patch repairs were previously used by repair technicians throughout the world who are
accustomed to metals
...

Bonded repairs involving use of uncured epoxy for patching continuous fiber plies over the
damaged surface is the preferred way of repair for composites
...
For the European
Airworthiness Safety Administration (EASA) and Federal Aviation Authority (FAA), the
certification of bonded repairs has been withheld on the grounds that lack of certainty over bond
quality cannot be addressed with Non-destruction inspection methods at hand
...
On the brighter
side composite failures are anticipated to be progressive and not cataclysmic as in metals
...
)

3
...
The fiber resin system for
Boeing 787 was kept the same but automated fiber placement techniques enable a weight saving
of 20%
...

Autoclave curing cures the epoxy resin after which the mandrels are disassembled and removed
...

Composites allow for an appropriate increase of thickness in parts susceptible to high probability
of impact damage such as doors, door surrounds, wing tips, wing leading and trailing edges and
wing-to-body fairings are all prone to ground (service) vehicle impact damage
...
The outstanding
corrosion resistance of composites has allowed Boeing to consider placing a cabin humidifier for
making the passenger cabin environment more comfortable
...
Airbus is driven by similar motivations for incorporating Fiber
metal laminated (FML) composites in its new A350 (Aircraft Technology Engineering
&Maintenance, 2005; Wall, 2005)
...
The single-piece manufacturability has drastically reduced simplified the
structure by eliminating thousands of fasteners previously used in multi segment fuselage
...
Sandwich structures are light
in weight but are susceptible to strength reducing voids inside the material
...


4
...


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Where

is the stiffness matrix

The stress and strain tensors are

symmetric, and since the stress-strain relations in linear elasticity can be derived from a strain
energy density function, the following symmetries hold for linear elastic materials
for


...
Therefore, only 9 independent
engineering constants are required to construct the material’s stiffness matrix
...


As shown, there is no interaction between normal stresses

and shear strains


...

The stress-strain relationships of orthotropic composite laminates can also be expressed in terms
of the compliance matrix (Gay & Hoa, 2007),

Or,

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(3
...
Analysis of a Composite Lamina
A unit block of a composite laminated structure is a lamina
...
A lamina is very thin
in relation to its transverse dimensions and it is usually considered to be in plane stress state
when subjected to in-plane loadings
...


Under this plane stress state,

is assumed to be zero
...
The stress-strain relationship of an
orthotropic composite lamina in terms of stiffness coefficients C is;
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With,

The stiffness matrices are inverted to get the compliance matrices which relate strain and stress
in an alternate manner
...
Failure of Fiber-reinforced Composite Laminate
Delamination, matrix rupture and fiber rupture are the most common reasons for failure in Fiberreinforced composite laminates
...


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Fig 7: Failure modes of composite laminates (Gay and Hoa 2007)
Different failure criteria have been developed over time to explain the failure in composite
materials
...

Hashin’s failure criterion has been used by many researchers and it is one of the most reliable
methods to predict the strength of laminated composites (Sun & Tao 1998)
...

Even though a three-dimensional failure criterion is available, but it is limited to the scope of
unidirectional laminates (Hashin and Rotem 1973; Hashin 1980)
...


Mechanical Properties of Fiber Metal Laminate

Fig 8: A typical Fiber Metal Laminate
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Understanding the properties of Fiber metal laminates requires a fairly general discussion about
the mechanical properties involved
...

In 1994, Wu et al (Wu et al
...
Based on tension tests on various FML specimens with various
size and geometry, he proved that these parameters do not affect the elastic modulus, yield stress
and ultimate tensile strength of FML
...
They observed that by increasing the volume fraction of composite
will cause the FML to have higher ultimate strength but lower elastic modulus as shown in
following figures
...
Different materials of interest are evaluated for the
aforementioned factor, following figure shows the details
...


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Fig 11: Compressive behavior of composite material studied
Kawai et al (Kawai & Hachinohe 2002) in his study on fatigue property of GLARE observed that
the high strength properties of aluminum alloy enhanced the specific stiffness and strength of
GLARE in the fiber direction
...
The presence of fibre bridging mechanism in GLARE is also
reported by Hagenbeek (Hagenbeek 2005) and previously proven by Marissen (Marissen 1988)
in his study on fatigue crack growth in ARALL
...
2006) discussed the Inter laminar shear strength of FML
which depends on the adhesion between fibres and matrix and adhesion between metal and
composite laminate
...
2005)
...
investigates the low velocity impact properties and impact damage mechanisms in
FML
...
Minimum cracking energy of FML was found to be increased compared to its
constituent aluminum alloy
...
(Vlot
1993; Vlot 1996; Vlot & Krull 1997)
...
Aluminum alloy 2024-T3 is the used in all
GLAREs except in GLARE 1
...
Failure Mechanism so far observed include initial
delamination between aluminum and fiber reinforced laminate followed by a crack in
undamaged aluminum
...
Combined bending under impact loads causes
damage to spread globally, i
...
the non-impacted sights suffer more damage than the actual
impact locations
...
(Cortés & Cantwell 2005; Nakatani et al
...

Response of Fiber Metal Laminate: Stacking sequence of fiber reinforced laminate
Absorption of less impact energy results in smaller span of impact fault leading to better damage
resistance
...

Quasi-isotropic orientation (0⁰/45⁰/90⁰) offer better impact damage resistance with higher
stiffness being harnessed with these ply orientations
...
(Seyed
Yaghoubi et al
...
Modes of failure like: transverse shearing, lamina bending and local buckling undergo a
combination of the aforementioned failure mechanisms
...
Composite
laminates do not undergo appreciable plastic deformation because the failure of brittle fiber as
the main load bearing element renders the composite laminate incapable of offering further
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resistance to deformation loads
...
Crashworthiness of composite laminates is governed by the materials of composites,
structural geometry and loading conditions
...
1997)
Failure of Fiber-Reinforced Composite Laminate: Delamination
When the shear stresses separate the laminas (layers) of a laminated composite, this defect is
called delamination
...
Matrix
cracking close to the surface of the laminas or close to a matrix rich area between two plies
causes inter-laminar cracks
...

Interface layers may form close to outer side of laminate subjected to bending
...
Failure mechanisms that spring into action
once a laminate is axially crushed involve buckling as a key player
...

Failure of composite laminates: Impact response
The impact damage in woven laminated composites is known to be a function of impact energy
alone
...
The
aforementioned conclusions were drawn from lateral low velocity impact tests carried out by
Robinson and Davies (Robinson and Davies, 1992)
...
The elastic energy at maximum
impact force subtracted from the incident impact energy gives the energy absorbed by the
specimen
...

Delamination under low velocity impact can be predicted using static interlaminar fracture
toughness (Jih & Sun 1993)
...
It was observed that the
maximum stress in fiber direction is larger than in its in-plane orthogonal direction because the
flexural wave moves faster in the fiber direction
...
2003)
...
The delaminated area is also wide under the same impact energy
...


References
1
...
& Liaw, B
...
Effects of Constituents and Lay-up Configuration on Drop-Weight Tests
of Fiber-Metal Laminates
...
43–62
...
Jih, C
...
& Sun, C
...
, 1993
...
Journal of Composite Materials, 27(7), pp
...

3
...
F
...
, 1994
...
Journal of Materials Science, 29(17),
pp
...

4
...
V & Cantwell, W
...
, 2000
...
, 60, pp
...

5
...
& Hachinohe, A
...
Two-stress level fatigue of unidirectional fiber – metal hybrid
composite : GLARE 2
...
567–580
...
Hagenbeek, M
...
Characterisation of Fibre Metal Laminates under Thermomechanical Loadings
7
...
, 1988
...


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8
...
B
...
, 2006
...
The Production of Metal /
laminate Hybrid Composites
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247–256
...
Khalili, S
...
R
...
K
...
G
...
A study of the mechanical properties of
steel/aluminium/GRP laminates
...
137–
140
...
Remmers, J
...
& de Borst, R
...
Delamination buckling of fibre–metal laminates
...
2207–2213
...
Sinmazçelik, T
...
, 2011
...
Materials & Design, 32(7), pp
...

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...
B
...
, 2014
...
Composite Structures, 107, pp
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13
...
D
...
C
...
, 2013
...
Mechanics of Materials, 66, pp
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14
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, Poon, C
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V, 2002
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, pp
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15
...
& Cantwell, W
...
, 2005
...
Composites Part B: Engineering, 37(2-3), pp
...

16
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, 3, pp
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17
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G
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M
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E
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G
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...

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...
[ 5 ],
...
291–307
...
Vlot, A
...
, 1997
...
, 7(1
997)
...
Lalibert, J
...
, 2005
...
, 43(11)
...
Seyed Yaghoubi, a
...
& Liaw, B
...
Stacking Sequence and Geometrical Effects on LowVelocity Impact Behaviors of GLARE 5 (3/2) Fiber-Metal Laminates
...
223–247
...
Sadighi, M
...
C
...
, 2012
...
International Journal of Impact Engineering, 49, pp
...

23
...
, Karakuzu, R
...
, 2003
...
Composite Structures, 59(1), pp
...

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24
...
J
...
S
...
W
...
THE EFFECT OF CURVATURE ON THE DYNAMIC RESPONSE
AND IMPACT-INDUCED DAMAGE IN COMPOSITE LAMINATES
...
763–773
...
Farley, G
...
& Jones, R
...
, 1992
...
Journal of Composite Materials, 26(1), pp
...

26
...
& Rotem, a
...
A Fatigue Failure Criterion for Fiber Reinforced Materials
...
448–464
...
Gay, D
...
V
...
Composite Materials: Design and Applications Second Edi
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