Composites
Why Study Composites?
With the knowledge of various types of composites,
and the factors (constituents, their relative amounts,
geometry & distribution) that affect their
performance (properties), we can design materials
with desired properties. These desired properties
are better than those of available materials, metals,
polymers and ceramics.
What is a Composite?
1. A composite is a multi-phase material that exhibit a significant
proportion of the properties of both constituent phases such
that a better combination of properties is realized.
Better property combinations are fashioned by judicious combination
of two or more distinct materials.
2. A composite is artificially made, as opposed to one that occurs or
forms naturally. In addition, the constituent phases must be
chemically dissimilar and separated by a distinct interface.
a. Multi-phase alloys (as Pearlite, combination of α Ferrite &
Cementite) are composites?
b. Wood is a natural composite (core is covered by cellulose)
c. Bone is a natural composite (protein collagen and mineral
apatite)
Do a, b, c fit to definition of composites???
Why Need Composites?
Many of our modern technologies require materials with unusual
combinations of properties that can not be met by conventional
metal alloys, ceramics and polymeric materials. This is especially
true for aerospace, underwater and transportation applications.
For example, aircraft engineers are increasingly searching for
structural materials that have low densities, are strong, stiff, and
abrasion and impact resistant, and are not easily corroded.
This is a rather difficult combination of characteristics.
Frequently, strong materials are relatively dense.
Also, increasing the strength or stiffness generally results in a
decrease in impact strength (& ductility).
This is why, we need to engineer new materials, called
composites, by combining various existing ones.
Two Phase Composites
Many of composites are composed of just 02 phases:
1. Matrix phase
Continuous phase and surrounds the other phase
2. Dispersed phase
This phase is dispersed in the matrix phase
• The properties of composites depend on :
- Properties of constituent phases
- Relative amounts of constituent phases
- Geometry of dispersed phases
- shape of particles
- particle size
- distribution of particles
- orientation of particles
Geometry of Dispersed Phase
-
Classification of Composites
-
A. Particle Reinforced Composites
1. Large-Particle Composites
• Particle-Matrix
• Mostly, Particulate phase (dispersed) is harder & stiffer than Matrix
phase
• Matrix transfers some of applied load to Particles
2. Dispersion-strengthened Particles
• Particle-Matrix
• Strengthening mechanism is similar to precipitation hardening
• Particles impede the dislocation motions and restrict plastic
deformation
• Matrix bears the major portion of load
• Particle size is b/w 0.01-o.1μ
1. Large Particle Composites
P-R Composites
• Particles can have a variety of geometries (Ref Prev Fig), but they
should be equi-axed
• For effective re-inforcement the particles should be small and evenly
distributed throughout the matrix
• Mechanical properties are enhanced with increasing particulate
contents
• The elastic modulus of composite can be predicted using 02
expressions, called Rule of Mixture Equations
• The experimental Ec should fall between
these 02 upper and lower limits
Cu-W composite:
W is dispersed phase
Experimental
1. Large Particle Composites: Examples
P-R Composites
Cermet: Ceramic-Metal Composite
• Most common cermet: Cemented Carbide
• Extremely hard particles of refractory carbides (TiC, WC) are
embedded into matrix of Co or Ni metals.
- These hard composites are used as cutting tools for hardened steels
- Hard carbide particles provide the cutting surface , however, being
brittle cannot withstand heavy machining loads/stresses
- Necessary toughness to withstand stresses is achieved by adding Co,
Ni. Matrix also isolates carbide particles and prevents particleparticle crack propagation
- Abrasive action of the composite is increased by keeping vol. fraction
of particles above 90%.
WC-Co cemented carbide
1. Large Particle Composites: Examples
P-R Composites
Tyre: Polymer-Carbon Composite
• Vulcanized Rubber (polymer) is mixed with carbon black
- Carbon black, an inexpensive material, enhances UTS, toughness and
tear & abrasion resistance
- Tyres contain carbon black ranging from 15-30%
- For C black to provide significant reinforcement, the particle size
must be extremely small with diameter b/w 20 & 50nm.
- The particles must be evenly distributed
- The particles must form strong adhesive bond with rubber matrix
Carbon black reinforced
Rubber: A composite
-Water marks show air pockets
1. Large Particle Composites: Examples
Concrete: Ceramic-Ceramic Composite
Dispersed phase: ceramic particles & Matrix phase: cement
a. Plain Concrete:
- Dispersed phase: stones and Silica
- Matric phase: Cement
b.
Reinforced Concrete:
- Dispersed phase: stones, Silica, Steel bars
- Matric phase: Cement
c. Pre-stressed Concrete:
- Dispersed phase: stones, Silica, Pre-stressed Steel bars
- Matric phase: Cement
P-R Composites
2. Dispersion Strengthened Composites: Examples P-R Composites
• Metal & metal alloys may be strengthened and hardened by the
uniform dispersion of several volume percent of fine particles of a very
hard and inert material.
Examples:
a.
The high-temperature strength of nickel alloys may be enhanced significantly
by the addition of about 3 vol% of thoria (ThO2) as finely dispersed particles
b. The same effect is produced in the aluminum–aluminum oxide system.
Flakes of Al2O3 (0.1 to 0.2 m thick)are dispersed within an aluminum metal
matrix; this material is termed sintered aluminum powder (SAP).
B. Fiber Re-inforced Composites
• In these composites, the dispersed phase is in the form of fiber
• Design goals of FRC often include high strength/stiffness on weight
basis, called as specific strength and specific modulus
• FRC are technologically the most important composites.
Types:
Continuous (long fiber) FRC
Discontinuous (short fiber) FRC
Factors Affecting Mechanical Properties of FRC
FR Composites
Mechanical characteristics of FRC depend on:
a. Properties of fiber
b. Degree to which an applied load is transferred to fibers by matrix
phase
- Load transmittance depends on the magnitude of interfacial bond
b/w the fiber-matrix phase
- Under an applied load (Fig), the fiber-matrix bond ceases at the
fiber ends, yielding a matrix deformation pattern as shown.
- The matrix does not transmit any load to each fiber extremity
c. Fiber size (length/dia)
d. Orientation
e. Concentration
Deformation pattern
of matrix in FRC
a. Influence of Fiber Length
FR Composites
• Some critical fiber length is necessary for effective strengthening
NOTE: For PRC, smaller the dispersed particles,
and stiffening of FRC:
greater the strength of composite
lc= Critical length
For FRC, larger the dispersed fiber, higher will be the
 f *  Tensile strength of fiber
composite strength
d= diameter of fiber
  Shear yield strength matrix
c
For a no of glass and carbon fiber combinations, lc is of the order of 1mm. This ranges b/w 20 and 150
times the fiber diameter
1. Continuous Fibers: l >>lc (l > 15lc)
2. Discontinuous Fibers: (l < 15lc)
OR Short Fibers
If l < < lc, the matrix deforms around the fiber such that there is virtually no stress transference and
little reinforcement by the fiber
b. Influence of Fiber Orientation& Concentration
FR Composites
With respect to orientation, 02 extremes are possible:
a. Parallel alignment
b. Random alignment
- Continuous fibers always have parallel alignment
- Discontinuous fibers could have parallel or random alignment
Better the fiber distribution,
better will be overall
properties of composite
Schematic representations of
(a) continuous and aligned
(b) Discontinuous and aligned
(c) discontinuous and randomly
oriented fiberreinforced
composites.
Parallel Continuous
Parallel Discontinuous
Oriented Discontinuous
b. Influence of Fiber Orientation
Continuous & Aligned Fiber Composites
-
-
-
-
-
FR Composites
Tensile Stress-Strain Behavior- Longitudinal Loading
Depends on:
a. Stress-strain behavior of fiber phase & matrix phase
b. Phase volume fraction
c. Direction of load/stress: aligned fiber composites are highly anisotropic, so the
properties depend on the measuring direction
Stress–strain
Fiber phase is brittle (say), has fracture
strength but lower fracture strain than
matrix phase
Composite properties are in b/w fiber &
matrix phases
Stage 1 is elastic region
Composite enter into stage (plastic) when
composite strain equals to yield strain of
matrix phase
In stage 2, major part of load is borne by
fiber phase
Failure in composite occurs when strain in
composite equals to fracture strain of
fiber phase
Failure in this kind of composite is not
catastrophic. Because, 1. there is always a
variation in fiber strength , 2. even all
fiber are broken, they remain in tact with
matrix, as matrix has higher fracture
Schematic tensile
stress–strain
curves for brittle
fiber and ductile
matrix materials.
curve for
composite with
fiber-matrix
superimposed
b. Influence of Fiber Orientation
Continuous & Aligned Fiber Composites
FR Composites
Tensile Strength- Transverse
- The strength of continuous and aligned fiber is highly anisotropic
- These composites are designed for longitudinal loading, however, during
service some transverse loading also act
- Transverse tensile strength of such composites is very low, sometimes
even lower than tensile strength of matrix
- Traverse strength is affected by :
-
Properties of fiber & matrix
Fiber-matrix bond strength
Presence of voids
- Transverse strength can be improved by modifying
matrix properties
Longitudinal and Transverse Tensile Strengths. The
Fiber Content for Each Is approximately 50 Vol%
b. Influence of Fiber Orientation
Dis-Continuous & Randomly Oriented Fiber Composites
FR Composites
- Aligned Fibrous Composites are inherently anisotropic in that the
maximum strength and reinforcement are achieved along the
alignment direction. In Transverse direction, virtually zero enforcement
exists
- The fiber aligned in other directions will demonstrate strength b/w
these 02 extremes
• When multidirectional stresses are imposed within a single plane, aligned layers that
are fastened together one on top of another at different orientations are frequently
utilized. These are termed laminar composites
• When multidirectional stresses are imposed in 3D, composites with fibers randomly
oriented & uniformly distributed in 3D space are used.
b. Influence of Fiber Orientation
Selection of Fiber Length & Orientation
Depends on:
•
FR Composites
Level of stress
Nature of applied stress (unidirectional/Random)
Fabrication cost
Production rates for short-fiber composites (both aligned/randomly
oriented) are rapid
• Intricate shapes can be easily made with short fibers
• Fabrication techniques applied to short fibers include compression,
extrusion and injection molding, similar to those applied for POLYMERS
Fiber Phase
FR Composites
• Small diameter fiber is much stronger than bulk material, especially for
brittle materials
• The presence of critical flaws dangerous, that could cause failure,
reduces with reducing volume of material
• Composites make use of these 02 facts
Types of Fibers:
1. Whisker:
Common whisker materials are:
- Single crystals
Graphite, silicon carbide, silicon
nitride, and aluminum oxide
- Thin
- Extremely large length/diameter ratio
- As a consequence of their small size, they have a high degree of
crystalline perfection and are virtually flaw free, which accounts for their
exceptionally high strengths
- They are among the strongest known materials.
- Whiskers are not utilized extensively as a reinforcement medium because
they are extremely expensive. Moreover, it is difficult and often
impractical to incorporate whiskers into a matrix
Fiber Phase
- Whiskers
have the
highest
properties
Fiber and
metallic wires
have almost
same order to
properties
Both whisker
and Fibers
are non
metals
Only Metallic
wire uses
metal fibers
FR Composites
Fiber Phase
FR Composites
Types of Fibers:
2. Fiber:
- Fibers are either polycrystalline or amorphous
- Fiber materials are polymers and ceramics, not metals
- The polymer aramids, glass, carbon, boron, aluminum oxide, and silicon
carbide
3. Wire:
- Fine wires have relatively large diameter
- Typical materials include steel, molybdenum, and tungsten
- Wires are utilized as a radial steel reinforcement in automobile tires, in
filament-wound rocket casings, and in wire-wound high-pressure hoses
In general, the elastic modulus of the fiber should be much higher than
that of the matrix.
Matrix Phase
FR Composites
- Matrix phase of fibrous composites can be metal/polymer/ceramic
- Generally, metals and polymers are used as matrix materials when
ductility is desirable
- However, if matrix is ceramic, fiber phase must be relatively ductile to
provide higher facture toughness
- Matrix serves several purposes:
- binds the fibers together
- acts as the medium by which an externally applied stress is
transmitted and distributed to the fibers
- very small proportion of an applied load is sustained by the matrix
phase
- matrix protects the individual fibers from surface damage as a result
of mechanical abrasion or chemical reactions with environment
- the matrix separates the fibers and, by virtue of its relative softness
and plasticity, prevents the propagation of brittle cracks from fiber to
fiber, which could result in catastrophic failure. In other words, the
matrix phase serves as a barrier to crack propagation.
Matrix Phase
FR Composites
- Even though some of the individual fibers fail, total composite fracture
will not occur until large numbers of adjacent fibers, once having failed,
form a cluster of critical size.
Fiber- Matrix Bonding
- It is essential that adhesive bonding forces between fiber and matrix be
high to minimize fiber pull-out.
- In fact, bonding strength is an important consideration in the choice of
the matrix–fiber combination.
- The ultimate strength of the composite depends to a large degree on the
magnitude of this bond
- Adequate bonding is essential to maximize the stress transmittance from
the weak matrix to the strong fibers.
Polymer-Matrix Composites
FR Composites
- Matrix phase: Polymer resin
- Dispersed phase: Fiber (used as reinforcement medium)
-
Extensively used in a variety of applications due to:
- Their good room-temperature properties,
- ease of fabrication,
- and cost
- Common Types of PMCs
-
Glass Fiber Reinforced Polymer Composites
Carbon Fiber Reinforced Polymer Composites
Aramid Fiber Reinforced Polymer Composites
Type of Fiber used in these
PMC’s is called Fiber
Polymer-Matrix Composites
FR Composites
1. Glass-Fiber Reinforced Polymer Composites
- Long/short fibers of glass are used as fibers.
- Fiber diameters normally range between 3 and 20 μm.
- Glass is popular as reinforcement medium because:
1. It is easily drawn into high-strength fibers from the molten state.
2. It is readily available and may be fabricated into a glass-reinforced
plastic economically using a wide variety of composite-manufacturing
techniques.
3. As a fiber it is relatively strong, and when embedded in a plastic
matrix, it produces a composite having a very high specific strength.
4. When coupled with the various plastics, it possesses a chemical
inertness that renders the composite useful in a variety of corrosive
environments.
Polymer-Matrix Composites
FR Composites
1. Glass-Fiber Reinforced Polymer Composites
Limitations of GFRPC:
• In spite of having high strengths, they are not very stiff and do not display
the rigidity that is necessary for some applications (e.g., as structural
members for airplanes and bridges).
• Most fiberglass materials are limited to service temperatures below at
higher temperatures, most polymers begin to flow or to deteriorate.
• Service temperatures may be extended to approximately by using highpurity fused silica for the fibers and high-temperature polymers such as
the polyimide resins.
Polymer-Matrix Composites
FR Composites
1. Glass-Fiber Reinforced Polymer Composites
Applications of GFRPC:
Many fiberglass applications are familiar:
- Automotive and marine bodies
- Plastic pipes,
- Storage containers,
- Industrial floorings.
- The transportation industries are utilizing increasing amounts of glass
fiber-reinforced plastics in an effort to decrease vehicle weight and boost
fuel efficiencies.
- A host of new applications are being used or currently investigated by the
automotive industry.
Polymer-Matrix Composites
FR Composites
2. Carbon-Fiber Reinforced Polymer Composites
Carbon is a high-performance fiber material that is the most commonly
used reinforcement in advanced polymer-matrix composites.
It is used because:
1. Carbon fibers have the highest specific modulus and specific strength of
all reinforcing fiber materials.
2. They retain their high tensile modulus and high strength at elevated
temperatures; high-temperature oxidation, however, may be a problem.
3. At room temperature, carbon fibers are not affected by moisture or a
wide variety of solvents, acids, and bases.
4. These fibers exhibit a diversity of physical and mechanical characteristics,
allowing composites incorporating these fibers to have specific engineered
properties.
5. Fiber and composite manufacturing processes have been developed that
are relatively inexpensive and cost effective.
Polymer-Matrix Composites
FR Composites
2. Carbon-Fiber Reinforced Polymer Composites
- The properties of C fiber depends on the type of pre-cursor used
- Based on tensile modulus, C fibers are classified as standard,
intermediate, high, and ultrahigh moduli.
- Both continuous and chopped forms are available.
-
•
•
•
•
Applications of CFRPC:
Sports and recreational equipment (fishing rods, golf clubs)
Filament-wound rocket motor cases,
Pressure vessels
Aircraft structural components—both military and commercial, fixed
wing and helicopters (e.g., as wing, body, stabilizer)
• Rudder components
Polymer-Matrix Composites
3.
FR Composites
Aramid-Fiber Reinforced Polymer Composites
• The aramid fibers are most often used in composites having polymer
matrices; common matrix materials are the epoxies and polyesters.
• Since the fibers are relatively flexible and somewhat ductile, they may be
processed by most common textile operations.
Applications:
- Typical applications of these aramid composites are in ballistic products
(bullet proof vests and armor)
- Sporting goods,
- Tires, Ropes,
- Missile cases,
- Pressure vessels
- And replacement for asbestos in automotive brake and clutch linings, and
gaskets
Polymer-Matrix Composites
Comparison of FR-Polymer Composites
Fiber is Continuous and Aligned
Matrix: Epoxy
All Cases the Fiber Volume Fraction is 0.60
CFRPC shows the best properties followed by Aramid and Glass
FR Composites
Metal-Matrix Composites
FR Composites
- Ductile metal is employed as a Matrix
- These materials (MMCs)may be utilized at higher service temperatures than
their base metal counterparts
- The reinforcement may improve specific stiffness, specific strength, abrasion
resistance, creep resistance, thermal conductivity, and dimensional stability
- Some of the advantages of these materials over the polymer-matrix
composites include higher operating temperatures, non-flammability, and
greater resistance to degradation by organic fluids
- Metal-matrix composites are much more expensive than PMCs, and,
therefore, their (MMC) use is somewhat restricted
- The super-alloys, as well as alloys of aluminum, magnesium, titanium, and
copper are employed as matrix materials
- The reinforcement may be in the form of particulates, both continuous and
discontinuous fibers, and whiskers
- Reinforcement concentrations normally range between 10 and 60 vol%.
- Continuous fiber materials include carbon, silicon carbide, boron, aluminum
oxide, and the refractory metals.
- Discontinuous reinforcements consist primarily of silicon carbide whiskers,
chopped fibers of aluminum oxide and carbon, and particulates of silicon
carbide and aluminum oxide.
Metal-Matrix Composites
FR Composites
Normally the processing of MMCs involves at least two steps:
- Consolidation or synthesis (i.e., introduction of reinforcement into the
matrix), followed by a shaping operation. A host of consolidation techniques
are available, some of which are relatively sophisticated
- Discontinuous fiber MMCs are amenable to shaping by standard metalforming operations (e.g., forging, extrusion, rolling)
Applications
- Some engine components have been recently introduced consisting of an
aluminum-alloy matrix that is reinforced with aluminum oxide and carbon
fibers. This MMC is light in weight and resists wear and thermal distortion.
- Drive shafts (that have higher rotational speeds and reduced vibrational
noise levels), extruded stabilizer bars, and forged suspension and
transmission components.
- Aerospace structural applications include advanced aluminum alloy metalmatrix composites; boron fibers are used as the reinforcement for the Space
Shuttle Orbiter, and continuous graphite fibers for the Hubble Telescope.
Metal-Matrix Composites
FR Composites
Applications
- The high-temperature creep and rupture properties of some of the superalloys (Ni- and Co-based alloys) may be enhanced by fiber reinforcement
using refractory metals such as tungsten.
- Excellent high-temperature oxidation resistance and impact strength are also
maintained. Designs incorporating these composites permit higher operating
temperatures and better efficiencies for turbine engines.
Properties of Several Metal-Matrix Composites
Reinforced with Continuous and Aligned Fibers
Ceramic-Matrix Composites
FR Composites
• Ceramics are noted for high strength, hot hardness and for sustaining high
temperature under high stress. Resistance to oxidation and degradation at
high temeperatures
• However, they lack fracture toughness, which can be improved in the form
of ceramic-matrix composite (about to 5 times of ceramics)
- In essence, this improvement in the fracture properties results from
interactions between advancing cracks and dispersed phase particles.
- Crack initiation normally occurs with the matrix phase, whereas crack
propagation is impeded or hindered by the particles, fibers, or whiskers.
Ceramic-Matrix Composites
FR Composites
Processing of CMC:
Ceramic-matrix composites may be fabricated using:
Hot pressing,
Hot iso-static pressing,
Liquid phase sintering techniques.
Applications:
SiC whisker-reinforced aluminas are being utilized as cutting tool inserts for
machining hard metal alloys.
Tool lives for these materials are greater than for cemented carbide tools
(Metal-Matrix Composites; Particles of TiC/WC in matrix of Co/Ni metal matrix)
Processing of Continuous FRC
FR Composites
To fabricate continuous fiber-reinforced plastics that meet design specifications
- the fibers should be uniformly distributed within the plastic matrix and,
- in most instances, all oriented in virtually the same direction.
- Important manufacturing processes for Continuous FRC are:
- Pultrusion
- Pre-preg production processes
- Filament winding
1. Pultrusion:
• Pultrusion is used for the manufacture of components having continuous
lengths and a constant cross-sectional shape (i.e., rods, tubes, beams, etc.)
Fibers of glass,
carbon & aramid
-
Continuous process
Easily automated
High production rate
Cost effective
Wide variety of shapes
No practical limit on
length of part
Polyester, Vinyl
ester & Epoxy
-Controls
-machined die to
impart final
fiber/resin ratio
- Resin ranges b/w shape on FRC
- Die is heated to
40 & 70% Vol
cure the FRC
Processing of Continuous FRC
FR Composites
2. Pre-preg Production Processes
Prepreg is the composite industry’s term for continuous fiber reinforcement
pre-impregnated with a polymer resin that is only partially cured. This material
is delivered in tape form to the manufacturer, who then directly molds and fully
cures the product without having to add any resin. It is probably the composite
material form most widely used for structural applications
Calendering: The fibers are sandwiched and pressed between sheets of release and carrier
paper using heated rollers
- Typical Pre-peg tape thickness ranges
Used to spread
a thin film of
between 0.08 and 0.25 mm
polymer resin on
- Tape widths range between 25 and
release paper
1525 mm
- Resin content lies between about 35
and 45 vol%
At room temperature the thermoset matrix
undergoes curing reactions; therefore,
the prepreg is stored at 0C or lower.
Also, the time in use at room temperature
(or “out-time”) must be minimized. If properly
handled, thermoset prepregs have a lifetime of
at least six months and usually longer.
Processing of Continuous FRC
FR Composites
2. Pre-preg Production Processes
• Both thermoplastic and thermosetting resins are utilized; as matrix
• Carbon, glass, and aramid fibers are the common fiber reinforcements.
Fabrication of Final Product Using Pre-peg
- Actual fabrication begins with the “lay-up”: pre-peg sheets are piled up to
make desired thickness, after removing carrier paper
- The lay-up arrangement can be uni-directional. But often they are stacked in
different orientation to make a cross ply or angle ply
- Final curing is accomplished by simultaneous application of heat and
pressure
- Lay-up is can be done with hand, in which sheets are cut and positioned on
tool by hand. Alternatively ply pattern can be cut with machine also and
positioned with hand
- Lay up Process can be automated also (as filament winding) to reduce
product cost.
Processing of Continuous FRC
FR Composites
3. Filament Winding
- Filament winding is a process by which continuous reinforcing fibers are
accurately positioned in a predetermined pattern to form a hollow (usually
cylindrical) shape.
- The fibers, either as individual strands or as tows, are first fed through a
resin bath and then are continuously wound onto a mandrel, usually using
automated winding equipment.
- After the appropriate number of layers have been applied, curing is carried
out either in an oven or at room temperature, after which the mandrel is
removed.
- As an alternative, narrow and thin prepregs (i.e., tow pregs) 10 mm or less in
width may be filament wound.
Processing of Continuous FRC
3. Filament Winding Techniques
Applications:
- Filament-wound parts have very high
strength-to-weight ratios.
- Rocket motor casings, storage tanks
and pipes, and pressure vessels.
- I-beams.
- This technology is advancing very
rapidly because it is very cost
effective.
FR Composites
Structural Composites
St Composites
A structural composite is normally composed of both homogeneous and
composite materials, the properties of which depend not only on the properties
of the constituent materials but also on the geometrical design of the various
structural elements.
Laminar
- Laminar composites
composites
- Sandwich panels
A laminar composite is composed of two-dimensional sheets or
panels that have a preferred high-strength direction
Laminations may also be constructed using fabric material such
as cotton, paper, or woven glass fibers embedded in a plastic
matrix.
Thus a laminar composite has relatively high strength in a
number of directions in the two-dimensional plane; however,
the strength in any other given direction is, of course, lower
than it would be if all the fibers were oriented in that direction.
One example of a relatively complex laminated structure is the
modern ski (see the chapter-opening illustration for this
chapter).
For example, adjacent
wood sheets in
plywood are aligned
with the grain
direction at right
angles to each other.
Structural Composites
St Composites
Sandwich panels
-
Consists of 02 faces and one core
-
The outer sheets called faces are made of a relatively stiff and strong material,
typically aluminum alloys, fiber-reinforced plastics, titanium, steel, or plywood
They (faces) impart high stiffness and strength to the structure, and must be
thick enough to withstand tensile and compressive stresses that result from
loading.
The core material is lightweight, and normally has a low modulus of elasticity.
Core materials typically fall within three categories: rigid polymeric foams (i.e.,
phenolics, epoxy, polyurethanes), wood (i.e., balsa wood), and honeycombs.
-
-
Structurally, the core serves several functions.
- First of all, it provides continuous support for
the faces.
- In addition, it must have sufficient shear
strength to withstand transverse shear
stresses,
- and also be thick enough to provide high
shear stiffness (to resist buckling of the
panel)
Tensile and compressive stresses on the core
are much lower than on faces
Structural Composites
St Composites
Sandwich panels
Honey comb cores
-
-
The honeycomb material is normally either an aluminum alloy or aramid
polymer.
Strength and stiffness of honeycomb structures depend on cell size, cell wall
thickness, and the material from which the honeycomb is made.
Applications of Sandwich Panels:
- They are stiffer, strong and light wieght
- Roofs, floors, and walls of buildings
- In aerospace and aircraft (i.e., for wings, fuselage, and tail plane skins).