11. 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
12. 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
13. 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
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16. 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
17. 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
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24. 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
25. 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
26. 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
27. 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
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33. 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
36. 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
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41. 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
53. Factors governing the edge flow? Injection pressure Only a marginal increase for mould filling at the edge with increasing edge pressure
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62. 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
68. Rotational Moulding It is used t o produce small to large hollow items with very uniform wall thickness Heating while rotating Cooling while rotating Part removal
73. 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.
74. 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.
