The physical properties of the resin influences the
behavior of the composites includes the following
Shrinkage during cure.
Modulus of elasticity
Tensile or flexural strength
Ineterlaminar shear strength
Factors influencing the mechanical properties of the composite
Factors influencing the mechanical properties of the composite Critical Fiber Length The minimum length per given fiber diameter essential for high tensile fracture stress.When the length of the fibre is below critical fibre length, the maximum fibre stress may never reach the ultimate fiber strength f l l< l c l = l c l > l c
How to calculate critical fibre length? Consider an infinitesimal length distance dx at a distance x form one of the fibre ends the force equilibrium for this length is ( /4.d 2 f ) ( f + d f ) – ( /4. d 2 f f ) - d f dx. = 0 (1) Which on simplification gives d f /dx = 4 / d f (2) Where f = longitudinal stress in the fibre at a distance x from one of its ends. = Shear stress at the fibre/matrix interface. d f = Fibre diameter. Factors influencing the mechanical properties of the composite
Assuming no stress transfer at the fibre ends, i.e f =0 at x=0, and integrating equation (2) the normal stress distribution in the fibre ends as f For simple analysis, it is assumed that interfacial strength is constant. Then equation becomes f = (4 I / d f ) x Where = interfacial shear stress. The maximum fibre stress that can be achieved at a given load is ( f ) max = 2 I (l t / d f ) Where x= l t /2 = load transfer length at each fibre end . Factors influencing the mechanical properties of the composite 3 4 5
For l f <l c , the maximum fibre stress may never reach the ultimate
fibre length. In this case either the fibre/matrix interfacial bond
or the matrix may fail before fibre achieve their potential strength.
For l f >l c , the maximum fibre stress may reach the ultimate fibre
strength over much of its length. Over a distance of l c /2 the fibre
Factors influencing the mechanical properties of the composite Where l c = minimum fibre length required for the fibre stress to be equal to the fibre ultimate strength f = ultimate fibre strength
Factors influencing the mechanical properties of the composite Fibre content
As fibre content increases mechanical properties such as tensile and
flexural strength,Young’s an flexural modulus for the composite
also increases up to a optimum fibre load beyond that limit decreases.
At lower fibre loading dispersion of fibre is very poor so that stress
transfer will not occur properly. At higher fibre loading there is a
chance for fibre-fibre interaction and poor wetting of fibres and thereby
reducing the effective aspect ratio.
Crack initiation and its propagation will be easier at higher loadings .
How to calculate fibre content and composite density? Factors influencing the mechanical properties of the composite Where R is the resin content in composite, r is the sisal fibre vol%, D is the density of the resin d is the density of sisal fibre. T d =100/(R/D + r/d) V f = W f / f W f / f + (1-W f ) / m Where W f is the fibre weight fraction ( 1-W f ) is the matrix weight fraction m is the resin density f is the fibre density
Interfacial Adhesion How to improve interfacial adhesion? By Chemical Methods By Physical methods By using Coupling agents Factors influencing the mechanical properties of the composite fibre matrix interface
Influence Of Fibre Orientation Longitudinal Transverse
Longitudinally aligned fibrous composites are anisotropic
in that maximum strength is achieved in the direction
Transverse direction fracture usually occurs
at low tensile stress.
Factors influencing the mechanical properties of the composite
Continuous and aligned fibre composites Longitudinal loading There are composites in which the fibers are aligned in the direction of applied stress.Assume that all the filaments are perfectly bonded to the matrix. Where c = composite strain f = fibre strain m = Matrix strain Factors influencing the mechanical properties of the composite f = m = c The total tensile force applied on the composite lamina is hared by the fiber and matrix P =P f +P m Since load = stress x area: then
Rule of mixtures Factors influencing the mechanical properties of the composite c . A c = f . A f + m . A m c . = f (A f /A c ) + m (A m /A c ) Where c . = Average tensile strength A f = Area of the fibre A m = Area of the matrix A c = A f + A m Since V f = A f /A c and V m = Am/A c c . = f V f + m V m This equation is known as rule of mixtures
For transverse loading In this type the load is applied at 90 0 angle. Under this situation Stress to both phases are exposed in the same time E c = E m .E f /V m .E f + V f . E m Factors influencing the mechanical properties of the composite E c = elastic moduli of the composite E f = elastic moduli of the fibre E m = elastic moduli of the matrix V f = elastic moduli of the fibre V M = elastic moduli of the matrix
For randomly oriented fibre composites are composed short and discontinuous fibre. Under these circumstance the expression for the elastic modulus K= Fibre efficiency parameter which value is lees than unity Modified rule of mixture Factors influencing the mechanical properties of the composite E c = KE f V f +E m V m
During the incorporation of fibers into matrix or during the
manufacturing, of laminates, air or other volatiles may be
trapped in the material.
Voids destroy the integrity of the composite and as they grow
and interact with each other, initiate cracks and promote
How to calculate void content? Factors influencing the mechanical properties of the composite where T d is the theoretical composite density, M d is the measured composite density . V= 100(T d -M d )/T d
In this feeding a stream of chopped fibres into a spray of liquid
in a mould cavity
A specialized spray gun is used to apply the chopped fibre and resin
to the tool.
The direction of fiber is random.
Uniformity for the surface occurs.
Void content is lees when compared to handlay up.
COMPRESSION MOULDING Moulding through the force of compression is another very common industrial process. The materials used are melamine, phenol and urea formaldehyde, Polyesters etc. Process Description The mould is held between the heated platens. A 'slug' or piece of the plastic is placed into the mould . The hydraulic press closes with sufficient pressure. The Compound softens and flows to shape. If necessary cooling is done. The press is opened and the moulding removed
Classification of Moulds Positive Mould Semi-positive mould Flash Moul d
Filament Winding is the process of winding resin-impregnated fiber
or tape on a mandrel surface in a precise geometric pattern.
This is accomplished by rotating the mandrel while a delivery head
precisely positions fibers on the mandrel surface.
By winding continuous strands of carbon fiber, fiberglass or other
material in very precise patterns, structures can be built with properties
stronger than steel at much lighter weights.
Filament winding machines operate on the principles of controlling
machine motion through various axes of motion .
The filament winding process was originally invented to produce missile casings, nose cones and fuselage structures, but with the passage of time industries other than defense and aerospace have discovered the strength and versatility of filament winding. Picture
The mandrel rotates more or less continuously while the fibre feed
carriage traverses back and forth at a speed regulated to generate
the desire helical angle.
After the first circuit is applied fibre are not adjacent, additional
circuits must be traversed before the patterns..
The mandrel revolution s per
circuit vary with winding angle
band width and overall length
of the vessel.
Any combination of diameter and
length may wound by trading
off winding angle
Polar The fibre passes tangentially in the polar opening at one end of the chamber.Reverses direction, and passes tangentially to the opposite side of the polar.It is simple and winding speed can be maintained Hoop patterns High angle helical winding that approaches an angle of 90 o. They are generally combined with longitudinal windings to produce a balance structure
The unexpected movement or displacement of reinforcement in the closed mold. It leads to the failure of RTM due to fibre displacement and interrupt the uniformity of predetermined reinforcement distribution
In RTM due to small clearance there exist a path for resin flow during mold filling.This edge flow can create dry spots or spillage of resin.
Is fibre washing is related to injection pressure ? How? Fibre washing increases when pressure is increased
Is fibre washing is related to fibre content? How?
Fibre washing distances reduces with more number layers of fibre and
virtually reduces to zero due to
-Higher clamping forces occurs. It can be increased by prelaying of narrow nonwoven strips along the edge to intimate contact with the mold
In this polymer in a suitable solvent is allows to fall at a
precalculated rate on an endless metallic belt of high finish
moving at a constant speed.
Continuous sheet of polymer solution is formed.
The solvent is evaporated under controlled condition.
The film is removed by stripping.
Eg: Cellophane sheet, photographic films.
INJECTION MOULDING It involves forcing or injecting a fluid plastic material into a closed mould where it solidifies to give the product Two basic categories: Thermoplastic; Thermosetting In former material is melted and force through an orifice or gate into a cool mould . In later a reacting material is injected into a warm mould in which the material further polymerizes into a solid part
A sheet of plastic (for example, compressed polystyrene) is clamped in position above the mould. The heater is then turned on and the plastic slowly becomes soft and pliable as it heats up. The plastic can be seen to 'warp' and 'distort' as the surface expands . After a few minutes the plastic is ready for ‘ forming’ as it becomes very flexible.
The heater is turned off and the mould is moved upwards by lifting the lever until it locks in position. The 'vacuum' is turned on and this pumps out all the air beneath the plastic sheet. Atmospheric pressure above the plastic sheet pushes it down on the mould. At this stage the shape of the mould can be clearly seen through the plastic sheet. When the plastic has cooled sufficiently the vacuum pump is switched off.