Download
1 / 34
350 likes | 566 Views
Materials. Composites. Introduction. Introduction. The major problem in the application of polymers to engineering is their low stiffness and strength Moduli are 100 times lower Strengths are 5 times lower. Introduction. Two methods are used to overcome these deficiencies
E N D
Materials Composites
Introduction • The major problem in the application of polymers to engineering is their low stiffness and strength • Moduliare 100 times lower • Strengths are 5 times lower
Introduction • Two methods are used to overcome these deficiencies • Use of shape (moment of inertia) • Ribs • Gussets • The addition of reinforcing fibers to form a composite material
Introduction • A good reinforcing additive has the following properties • It is stiffer and stronger than the polymer matrix • It has good particle size, shape, and surface character for effective mechanical coupling to the matrix • It preserves the desirable qualities of the polymer matrix
Introduction • The best reinforcement in any application is the one that achieves the designers objective at the lowest cost
Mechanism of Fiber Reinforcement • We have a single reinforcing fiber embedded in a polymer matrix and perfectly bonded to it. • The particle is stiffer than the matrix and deform less, causing the matrix strain to be reduce overall • The strain is much less at the interface
Mechanism of Fiber Reinforcement • The reinforcing fiber achieves its restraining effect on the matrix entirely through the fiber-matrix interface • The strength of the composite depends on the strength of bond between fiber and matrix, and the area of the bond.
Mechanism of Fiber Reinforcement • A useful parameter for characterizing the effectiveness of the reinforcement is the ratio of surface area of the reinforcement to the volume of reinforcement. • We want the area to volume ratio to be as high as possible. • We define the aspect ratio (a) as the ratio of length to diameter
Mechanism of Fiber Reinforcement • The figure on the next slide show a plot of aspect ratio(a) vs area to volume ratio. • It show the optimum shapes for a cylindrical reinforcement to be: • a>>1, a fiber • a<<1, a platelet
Mechanism of Fiber Reinforcement • Two main classes of reinforcement are fibers and platelets. • Examples of fibers: • Glass fibers • Carbon fibers • Carbon nanotubes • Examples of platelets • Mica • Talc
Forming Reinforced Plastics • Reinforced thermoplastics are usually formed using extrusion or injection molding. • Alignment of the fibers is caused by drag on the particle by the flowing viscous polymer. • Usually aligned in the direction of flow. • But the flow field varies greatly and we end up with random fiber alignment. • The damage done to the fiber must also be taken into account.
Forming Reinforced Plastics • Thermoset resins can be formed by compression molding. • The fiber and resin are premixed before being loaded into a heated mold which causes the resin to crosslink. • Many forms of premix are available, making a variety of fiber arrangements possible.
Forming Reinforced Plastics • Many other forming processes: • Pultrusion • Continuous fibers are pulled through a bath of resin, then through a shaping die. • The resin is then crosslinked. • Produces a long fiber with uniaxial alignment.
Forming Reinforced Plastics • Filament winding • Continuous fibers are pulled through a bath of resin, then wound onto a driven mandrel. • Then the resin is crosslinked. • This method is used for making pipe and other shapes
Forming Reinforced Plastics • Pultrusion and Filament winding
Forming Reinforced Plastics • Hand Layup • The fiber is laid down by hand in the required arrangement and shape, then resin is applied with a brush. • The resin then crosslinks. • Hand Spray Layup • Fibers are fed to a spray gun which chops and sprays the fibers at a panel where the reinforcement is needed. • Resin is then applied with a brush. • The resin then crosslinks.
Physical Properties • Density • The density of the composite differs from that of the polymer • A mass (m) of composite occupies a volume (V) • mf of fibers occupies Vf • mm of matrix (polymer) occupies Vm • m = mf + mm • V = Vf +Vm
Physical Properties • The proportion of fibers and matrix in the composite are expressed as fractions of the total volume they occupy.
Physical Properties • The density(ρ) of the composite with no voids is:
Physical Properties • In practice, composite materials contain voids. • A void is a source of weakness • Over 2% voids indicates poor fabrication. • Less than 0.5% voids indicates “aircraft quality” fabrication.
Mechanics of Fiber Reinforcement • Accurately predicting the mechanical properties of a composite material is not easy • The differences between properties of the reinforcing particle and the polymer matrix cause complex distributions of stress and strain at the microscopic level, when loads are applied. • By using simplified assumptions about stress and strain, reasonably accurate predictions can be made
Mechanics of Fiber Reinforcement • Consider the case of the fibers that are so long that the effects of their ends can be ignored.
Mechanics of Fiber Reinforcement • The equation for the Composite Modulus (E) in the 1 direction is: • The equation for the Composite Modulus (E) in the 2 direction is:
Mechanics of Fiber Reinforcement • Poisson’s ratio (ν), the elastic constant of the composite in the 1,2 direction is: • Poisson’s ratio (ν), the elastic constant of the composite in the 2,1 direction is:
Mechanics of Fiber Reinforcement • When a shear stress acts parallel to the fibers, the composite deforms as if the fibers and matrix are coupled is series. • The shear Modulus (G12) is:
