Transcript Materials
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 – 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 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 fibermatrix 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 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 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. How Molecular Orientation Occurs 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 Physical Properties 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. f vf v vm m v m f 1 Physical Properties • The density(ρ) of the composite with no voids is: f f (1 f ) * m 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 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: E1 f * E f (1 f ) * Em • The equation for the Composite Modulus (E) in the 2 direction is: E2 E f * Em (1 f ) * E f f * Em Mechanics of Fiber Reinforcement • Poisson’s ratio (ν), the elastic constant of the composite in the 1,2 direction is: 12 f *12 (1 f ) *12 f m • Poisson’s ratio (ν), the elastic constant of the composite in the 2,1 direction is: 21 12 * E2 E1 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: G12 G f * Gm (1 f ) * G f f * Gm