Materials Composites

Introduction

The major problem in the application of polymers to engineering is their low stiffness and strength compared to steel. – 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

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.

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

Density The density of the composite differs from that of the polymer A mass (m) of composite occupies a volume (V) – m f of fibers occupies V f – m m of matrix (polymer) occupies V m – m = m f + m m – V = V f +V m

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 (G 12 ) is: