What is the definition of composites?

Composites are artificially produced multiphase materials having desirable combinations of the best properties
of the constituent phases.
How are composites classified?
Composites can be classified as particle-reinforced, fiber-reinforced, and structural.
What are large particle composites?
Large-particle composites are polymeric materials with fillers, improving properties and/or replacing some
polymer volume with a less expensive material.
What is the rule of mixtures?
The behavior of a two-phase composite is influenced by the volume fraction of the constituent phases, with
mechanical properties being enhanced with increasing particulate content.
How does fiber length influence the mechanical properties of a fiber-reinforced composite?
The mechanical properties of a fiber-reinforced composite depend on the fiber's properties and the load
transmitted by the matrix phase. The interfacial bond between the fiber and matrix phases is crucial for load
transmittance.
What is the critical fiber length?
The critical fiber length (lc) is crucial for composite material strengthening and stiffening, influenced by fiber
strength, diameter, and fiber-matrix bond strength/matrix shear yield strength.
How does fiber orientation and concentration impact the strength and properties of fiber-
reinforced composites?
Fiber orientation, concentration, and distribution significantly impact the strength and properties of fiber-
reinforced composites. There can be a parallel alignment of the fibers in a single direction or a totally random
alignment.
What are the mechanical properties of continuous and aligned fiber composites?
The mechanical properties of continuous and aligned fiber composites depend on factors like fiber and matrix
phase stress-strain behaviors, phase volume fractions, and load direction. These composites are highly
anisotropic.
What is the total load sustained by a composite in longitudinal loading?
The total load sustained by a composite in longitudinal loading is equal to the sum of the loads carried by the
matrix phase and the fiber phase.
What is the modulus of elasticity in the longitudinal direction for a continuous and aligned
fiber-reinforced composite?
The modulus of elasticity in the longitudinal direction for a continuous and aligned fiber-reinforced composite is
given by the expression: Ecl = σm Vm + σf Vf
What is the purpose of pultrusion?
Pultrusion is a technique used to produce components with continuous lengths and constant cross-sectional
shape by pulling continuous-fiber rovings impregnated with thermosetting resin through a steel die.
What is the process of prepreg production?
Prepreg refers to continuous-fiber reinforcement pre-impregnated with a partially cured polymer resin. The
process involves collimating spool-wound continuous-fiber tows, sandwiching and pressing between sheets of
release and carrier paper, and coating the release paper sheet with a thin film of heated resin solution.
How does filament winding work?
Filament winding is a process that positions continuous reinforcing fibers in a predetermined pattern. The fibers
are fed through a resin bath and wound onto a mandrel. The applied layers are then cured in an oven or at room
temperature, and the mandrel is removed after curing.
What is a laminar composite?
A laminar composite is composed of two-dimensional sheets or panels that have a preferred high-strength
direction. The layers are stacked and subsequently cemented together, with the orientation of the high-strength
direction varying with each successive layer.
What are sandwich panels?
Sandwich panels are lightweight, high-strength structural composites consisting of two outer sheets separated by
and adhesively bonded to a thicker core. The outer sheets provide stiffness and strength, while the core material
serves multiple functions including continuous support for the faces and resistance to panel buckling.
What are the functions of the faces in a sandwich panel?
The faces of a sandwich panel provide high stiffness and strength to the structure, ensuring it can withstand
tensile and compressive stresses from loading.
What are the functions of the core in a sandwich panel?
The core material in a sandwich panel provides continuous support for the faces, sufficient shear strength to
withstand transverse stresses, and high shear stiffness to resist panel buckling. It also experiences lower tensile
and compressive stresses compared to the faces.
What are the applications of sandwich panels?
Sandwich panels are used in a wide variety of applications including roofs, floors, and walls of buildings, as
well as in aerospace and aircraft for wings, fuselage, and tailplane skins.
How is the modulus of elasticity of a composite material determined?
The modulus of elasticity of a composite material can be determined using the equation Ecl = Em Vm + Ef Vf,
where Ecl is the modulus of elasticity of the composite, Em and Ef are the moduli of elasticity of the matrix and
fiber respectively, and Vm and Vf are the volume fractions of the matrix and fiber respectively.
How is the load carried by each phase of a composite material determined?
The load carried by each phase of a composite material can be determined using the equation Ff = Fm (Ef / Em),
where Ff and Fm are the loads carried by the fiber and matrix phases respectively, and Ef and Em are the moduli
of elasticity of the fiber and matrix respectively.
What is the heat capacity at constant volume?
The heat capacity at constant volume (CV) is the amount of heat required to raise the temperature of a substance
by one degree Celsius while maintaining the volume constant.
What is the heat capacity at constant pressure?
The heat capacity at constant pressure (CP) is the amount of heat required to raise the temperature of a substance
by one degree Celsius while maintaining the pressure constant.
How is thermal energy generated in solid materials?
Solid materials generate thermal energy through the vibrations of their atoms at high frequencies and small
amplitudes, which is known as vibrational thermal energy.
What is a phonon?
A phonon is a quantum of vibrational thermal energy in solid materials, similar to the quantum of
electromagnetic radiation.
What is the Debye temperature?
The Debye temperature (θD) is the temperature above which the heat capacity at constant volume levels off and
becomes essentially independent of temperature.
How does the heat capacity at constant volume change with temperature?
The heat capacity at constant volume (CV) is zero at 0 K and rises rapidly with temperature. Above the Debye
temperature, CV becomes essentially independent of temperature.
What is the linear coefficient of thermal expansion?
The linear coefficient of thermal expansion (αι) is a material property that indicates the degree to which a
material expands upon heating. It has units of reciprocal temperature [(oC)-1 or (oF)-1].
How can volume changes with temperature be computed?
Volume changes with temperature can be computed using the volume coefficient of thermal expansion (αυ),
which is approximately 3 times the linear coefficient of thermal expansion (αι) for isotropic materials.
What is thermal conductivity?
Thermal conductivity is the ability of a material to transfer heat. It is defined as the heat flux per unit time per
unit area per unit temperature gradient.
What are the mechanisms of heat conduction in solid materials?
Heat in solid materials is transported through lattice vibration waves (phonons) and free electrons, resulting in
thermal conductivity.
What is the macroscopic deformation of semicrystalline polymers characterized by?
A tensile stress-strain curve
What are the characteristics of the tensile stress-strain curve for semicrystalline polymers?
It shows both upper and lower yield points followed by a near horizontal region
What happens at the upper yield point in the tensile stress-strain curve for semicrystalline
polymers?
A small neck forms within the gauge section, leading to localized strengthening and resistance to continued
deformation
How does the tensile behavior of semicrystalline polymers contrast with that of ductile
metals?
In ductile metals, subsequent deformation is confined to the neck region, while in semicrystalline polymers, the
specimen elongation proceeds by propagating the neck region along the gauge length
What is the viscoelastic behavior of amorphous polymers at low temperatures?
They may be elastic, conforming to Hooke's law
What is the viscoelastic behavior of amorphous polymers at high temperatures?
They become viscous or liquid-like
What is the viscoelastic behavior of amorphous polymers at intermediate temperatures?
They behave like rubbery solids
What is the difference between elastic deformation and viscous behavior in polymers?
Elastic deformation is instantaneous and completely recovered upon release, while viscous behavior has
deformation delayed or dependent on time and is not reversible or completely recovered after stress is released
What is viscoelastic behavior a combination of?
Viscoelastic behavior is a combination of instantaneous elastic strain followed by a viscous, time-dependent
strain
What is the relaxation modulus for viscoelastic polymers?
The relaxation modulus is a time-dependent elastic modulus for viscoelastic polymers
What is the process of elastic deformation in semicrystalline polymers?
The process of elastic deformation in semicrystalline polymers involves the elongation of amorphous tie chains
and the bending and stretching of chains in crystallite regions.
What are the stages in the elastic deformation of a semicrystalline polymer?
The stages in the elastic deformation of a semicrystalline polymer are: (a) Two adjacent chain-folded lamellae
and interlamellar amorphous material before deformation. (b) Elongation of amorphous tie chains during the
first stage of deformation. (c) Increase in lamellar crystallite thickness (which is reversible) due to bending and
stretching of chains in crystallite regions.
What is the mechanism of plastic deformation in semicrystalline polymers?
The mechanism of plastic deformation in semicrystalline polymers involves the sliding of adjacent chains in
lamellae, causing tilting and aligning of the chain folds with the tensile axis. Crystalline block segments separate
from lamellae and become oriented in the tensile axis, resulting in a highly oriented structure.
What is the process of drawing in semicrystalline polymers?
Drawing is a process used to enhance the mechanical properties of polymer fibers and films. It involves the
separation of crystalline block segments and the orientation of block segments and tie chains with the tensile
axis.
What happens to a semicrystalline polymer when deformation is terminated and the specimen
is heated to its melting point?
If deformation is terminated and the specimen is heated to its melting point, it recrystallizes to form a spherulitic
structure again. The specimen shrinks back to its original dimensions, with the extent of this recovery influenced
by annealing temperature and elongation degree.
How does molecular weight affect the tensile strength of polymers?
Tensile strength increases with increasing molecular weight in many polymers, with the tensile strength at
infinite molecular weight being a function of the number-average molecular weight.
What is the influence of degree of crystallinity on the mechanical properties of polymers?
The degree of crystallinity significantly impacts the mechanical properties of polymers. Increasing crystallinity
generally enhances strength and makes the material more brittle.
What is the purpose of predeformation by drawing in polymers?
Predeformation by drawing is a technique used to improve the mechanical strength and tensile modulus of
polymers. It involves permanently deforming polymers in tension, resulting in highly oriented molecular chains.
How does heat-treating affect the mechanical properties of semicrystalline polymers?
Heat-treating (or annealing) semicrystalline polymers increases crystallinity, crystallite size and perfection, as
well as modifications of the spherulite structure. The effects of heat-treating on tensile modulus, yield strength,
and ductility depend on the material and its processing history.
What are the characteristics of elastomeric materials?
Elastomeric materials exhibit rubber-like elasticity, allowing them to deform to large deformations and then
return to their original form. They have small moduli of elasticity, a nonlinear stress-strain curve, and
amorphous molecular chains that can partially uncoil, untwist, and straighten under tensile load.