Topics to Discuss.. Composite Materials

Transcription

1 A. K. M. B. Rashid Professor, Department of MME BUET, Dhaka 27 Composite Materials Topics to Discuss.. What are composites? Why do we make composite material? Classifications of composite materials Fibre-reinforced composites Particle-reinforced composites Structural composites

2 What are composite materials? Composites materials are artificially prepared solids containing of two or more physically distinct phases on a scale larger than the atomic. The term composite is usually reserved for those materials in which the phases are microscopically or macroscopically distinct, and properties such as the elastic modulus are significantly altered in comparison with those of a homogeneous material THE MATRIX (aluminium) INTERFACE (allows transfer of stress from the matrix to the dispersed phase) REINFORCEMENT (tungsten fibre) tungsten fibre reinforced aluminium composite 3/37 The matrix Continuous phase, or the bulk material, the property of which is generally reinforced Made from metals, polymers or ceramics Some ductility of the matrix and high bonding strength between matrix and reinforcements are desirable Functions of matrix Binds the reinforcements together Mechanically supports the reinforcements Transfers the applied load to the reinforcements Protects the reinforcements from surface damage due to abrasion or chemical attacks 4/37

3 The Reinforcing Material The dispersed phase in the matrix Made from metals, polymers or ceramics Can be in the form of particles, fibres or various other geometries Functions of reinforcing material: to enhance matrix properties Particle reinforcement Silver, Cobalt; Silica, Carbon black, Rocks, Alumina, Talc, SiC, Si 3 N 4, Glass beads Fibre reinforcement Boron, Steel, Tungsten, Chromium; Carbon, Alumina, SiC, Glass, Kevlar 5/37 6/37

4 Some examples of composite materials (a) plywood is a laminar composite of layers of wood veneer (b) fiberglass is a fiberreinforced composite containing stiff, strong glass fibers in a softer polymer matrix ( 175) (c) concrete is a particulate composite containing coarse sand or gravel in a cement matrix (reduced to 50%). 7/37 Natural Composites: wood and bamboo, shells, bones, muscles Natural fibres: silk, wool, cotton, jute Abalone shell: CaCO 3 + 3% organic material >3000 times stronger than calcite Wood: cellulose-filaments in a matrix of lignin and hemicellulose 8/37

5 The properties of composite materials depend upon their structure (as they do in homogeneous materials). Composites differ from homogeneous materials in that, considerable control can be exerted over the larger scale structure, and hence over the desired properties. The properties of a composite material depend upon 1. the properties of the homogeneous matrix and reinforcing materials, 2. the shape, volume fraction, and orientation of the reinforcing materials, 3. the interface among the constituents, and 4. the adhesion of the reinforcing materials with the matrix. 9/37 Why do we make composites? The combination of phases produces properties that are different from those of its constituents Offset the poor qualities of one phase with the good qualities of another The primary needs for making composites: light weight greater strength and stiffness better corrosion resistance higher operating temperatures higher impact and wear resistance higher reliability and affordability 10/37

6 The best of both worlds Pros electrically, thermally conductive good strength and ductility high toughness magnetic Pros electrically, thermally insulating wear and corrosion resistant high strength and stiffness creep resistant low density Ceramics Cons difficult to form/machine very low toughness Metals Composites Cons dense low creep resistance low/moderate corrosion resistance Pros very ductile easy to form corrosion resistant high strength-to-weight ratio Polymers Cons low stiffness & strength poor high temperature properties 11/37 Classification of composites Composites Metal matrix Ceramic matrix Polymer matrix (based on matrix material) (based on reinforcing material) 12/37

7 Based on Matrix Phase Metal matrix composites Ceramic matrix composites Polymer matrix composites Matrix: Moderately strong, stiff, wear resistant and fatigue resistant Aim: To significantly improve above properties Example: SiC reinforced Al, Precipitation hardened Al, etc. Matrix: Hard and brittle Aim: To make tougher and more reliable Example: Ag reinforced Al 2 O 3, ZrO 2 reinforced TiO 2, steel reinforced concrete Matrix: Weaker and have low melting point Aim: To make stronger and more temperature resistant Example: GFRP, CFRP 13/37 Based on Dispersed Phase Fibrous composites Particulate composites Structural composites continuous vs. discontinuous aligned vs. randomly oriented Large particle vs. dispersion strengthened lamellar vs. sandwich structure vs. honeycomb structure GFRP CFRP WC particle reinforced Co Polymer core sandwiched by Al faces 14/37

8 Fibre-reinforced composites Technologically, the most important composites are those in which the dispersed phase is in the form of a fibre. Design goals of fibre-reinforced composites often include higher strength and/or stiffness on a weight basis. These characteristics are expressed in terms of specific strength (tensile strength / specific gravity) and specific modulus (modulus of elasticity / specific gravity) parameters. Fibre-reinforced composites with exceptionally high specific strengths and moduli have been produced that use low-density fibre and matrix materials. 15/37 Scanning electron micrograph of the fracture surface of a silver-copper alloy reinforced with carbon fibers. Poor bonding causes much of the fracture surface to follow the interface between the metal matrix and the carbon tows ( 3000). 16/37

9 Fibre materials for reinforcement Whiskers thin single crystals - large length to diameter ratio high crystal perfection extremely strong, strongest known very expensive example: graphite, SiN, SiC Fibers polycrystalline or amorphous generally polymers or ceramics example: Al 2 O 3, Aramid, E-glass, Boron Wires metal steel, Mo, W 17/37 Controlling properties of fibre-reinforced composites 1. Aspect Ratio (the length of a fiber divided by its diameter) Fibres can be short, long, or continuous Properties of composite improved when the aspect ratio is large 2. Volume Fraction of Fibre A greater volume fraction of fibre increases strength and stiffness of composite For over 80 vol % fibre, the matrix can no longer completely surrounds the fibres undesirable! 4. Properties of Fibre and Matrix fibre stiff, strong, light matrix tough, ductile 5. Bonding and Failure Good bonding between fibre and matrix is essential to transmit load from matrix to the fibre If bonding is poor, fibre pull out occurs Special coating on fibre may be used to improve bonding 3. Orientation of Fibre Unidirectionally aligned fibre optimum properties along fibre direction; poor properties across fibre direction (anisotropic properties) Randomly oriented fibre isotropic properties; properties not optimum 18/37

10 Increasing the length of chopped E-glass fibers in an epoxy matrix increases the strength of the composite. In this example, the volume fraction of glass fibers is about 0.5. Effect of fiber orientation on the tensile strength of E-glass fiber-reinforced epoxy composites. Mechanics of composites Mechanical properties in many composite materials depend on structure in a complex way. However, for some structures the prediction of properties is relatively simple. stress parallel to fibers (iso-strain condition) stress perpendicular to fibers (iso-stress condition) Voigt (a, laminar; b, fibrous) and Reuss (c) composite models, subjected to tension force indicated by arrows. 20/37

11 Iso-strain Condition The total force acting on the composite is the sum of the forces carried by each constituent: F c = F m + F f σ c A c = σ m A m + σ f A f ( F = σ A) σ c = σ m A m A c + σ f A f A c If the fibers have a uniform cross-section, the area fraction equals the volume fraction: V c = V m + V f σ c = σ m V m + σ f V f 21/37 σ c = σ m V m + σ f V f Using Hooke s law, σ = Eε E c ε c = E m ε m V m + E f ε f V f If the fibers are rigidly bonded to the matrix, both the fibers and the matrix must stretch equal amounts (iso-strain conditions): ε c = ε m = ε f E c = E m V m + E f V f ρ c = ρ m V m + ρ f V f Upper Bound Value of E c (Voigt model) Rule of Mixture property of composite is weighted sum of its constituent materials 22/37

12 Iso-stress Condition stress is applied perpendicular to the axis of the fiber The stresses in each component are equal But, the strains are no longer equal Instead, the weighted sum of the strains in each component equals the total strain in the composite ε c = ε m V m + ε f V f σ c E c = 1 E c = σ m E m V m + V m E m + V f E f σ f E f V f Lower Bound Value of E c (Reuss model) ( σ c = σ m = σ f ) E C = E m E f E f V m + E m V f 23/37 Problem 1 SiC coated boron (a,k,a, Borsic) fibre reinforced aluminum containing 40 vol% fibres is an important high-temperature, lightweight composite material. The fibres are aligned parallel to the loading direction. Estimate the density, modulus of elasticity, and tensile strength parallel to the fibre axis. Also estimate the modulus of elasticity perpendicular to the fibres. Material Density Young s Modulus Tensile Strength (g/cc) (psi) (psi) Fibre ,000, ,000 Aluminium ,000,000 5,000 24/37

13 Answer: From the rule of mixtures: ρ c = (2.36 g/cc) (0.40) + (2.70 g/cc) (0.60) = 2.56 g/cc E c = (55*10 6 psi) (0.40) + (10*10 6 psi) (0.60) = 28*10 6 psi TS c = (400*10 3 psi) (0.40) + (5*10 3 psi) (0.60) = 163*10 3 psi Perpendicular to the fibers: 1 E c = psi psi = psi 1 E c = psi Problem 2 A continuous and aligned glass fibre-reinforced composite consists of 40 vol.% glass fibres having a modulus elasticity of 69 GPa and 60 vol.% polyester resin that, when hardened, displays a modulus of 3.4 GPa. (a) Compute the modulus of elasticity of this composite in the longitudinal direction. (b) If the cross-sectional area is 250 mm 2 and a stress of 50 MPa is applied in the longitudinal direction, compute the magnitude of the load carried by each of the fibre and matrix phases. (c) Determine the strain that is sustained by each phase when the stress in part b is applied. 26/37

14 Answer: E C = E f V f + E m V m = (69 GPa).(0.40) + (3.4 GPa).(0.60) = 30 GPa (a) Given data: E f = 69 GPa E m = 3.4 GPa V f = 0.40 V m = 0.60 Manipulating Hooks law for longitudinal directions, one may find the ratio of forces on the fibres and the matrix F f F m = = E f V f E m V m (69 GPa).(0.40) (3.4 GPa).(0.60) F f = 13.5 F m [1] Again, forces on the composite F C = s C A C = (50 MPa).(250 mm 2 ) = N F C = F f + F m = N [2] Given data: s C = 50 MPa A C = 250 mm 2 Using these two equations, one may find F f = N and F m = 860 N (b) 27/37 For an unit length of composite A m = V m A C = (0.6).(250 mm 2 ) = 150 mm 2 and A f = 100 mm 2 s f = F f / A f = (11640 N) / (100 mm 2 ) = MPa s m = F m / A m = (860 N) / (150 mm 2 ) = 5.73 MPa Then individual strain in each phase e f = s f / E f = ( MPa) / (69 GPa) = 1.69x10-3 (c) e m = s m / E m = (5.73 MPa) / (3.4 GPa) = 1.69x10-3 (c) Thus, as they should be, strains for both fibre and matrix phases are identical 28/37

15 Particle-reinforced composites It is often convenient to stiffen or harden a material by the incorporation of particulate inclusions. Particles used can be ranging in size from microscopic (dispersionstrengthened composites) to macroscopic (large-particle composites) Dispersion strengthening Similar to precipitation hardening Strengthening occurs in atomic/molecular level by making it harder for dislocation to move Large-particle strengthening Harder and stiffer reinforcing particles tend to restrain movement of the matrix phase in the vicinity of each particle 29/37 SiC reinforced Al casting (ductile) (brittle, hard) (compliant) (stiffer) Large-particle composites Dispersion-strengthened composites 30/37

16 The shape of the particles is important. It may be of any shape ranging from irregular to spherical, plate-like to needle-like. In isotropic systems, stiff platelet (or flake) inclusions are the most effective in creating a stiff composite, followed by fibers; the least effective geometry for stiff inclusions is the spherical particle, The distribution of particles in the composite matrix is random, and therefore strength and other properties of the composite material are usually isotropic Particulate strengthening is much less efficient than fibre-reinforcing. Properties follows the rule of mixture: E c(u) = E m V m + E f V f (upper bound) 1 E c(l) = V m E m + V f E f (lower bound) 31/37 The effect of clay on the properties of polyethylene Modulus of elasticity versus volume percent tungsten for a composite of tungsten particles dispersed within a copper matrix 32/37

17 Problem 3 A cemented carbide cutting tool used for machining contains 75 wt% WC, 15 wt% TiC, 5 wt% TaC, and 5 wt% Co. Estimate the density of the composite. Answer First, we must convert the weight percentages to volume fractions. The densities of the components of the composite are: 33/37 From the rule of mixtures, the density of the composite is 34/37

18 Structural composites A structural composite is a multi-layered and normally low-density composite used in applications requiring structural integrity, ordinarily high tensile, compressive, and torsional strengths and stiffnesses. The properties of these composites depend not only on the properties of the constituent materials, but also on the geometrical design of the structural elements. Laminar composites and sandwich panels are two of the most common structural composites. 35/37 (a) (b) (c) Laminar composite structure (2D sheets or panels) Sandwich structure (consists of two strong and stiff outer sheets, faces, or skins that are separated by and adhesively bonded to a thicker lightweight core) Honeycomb sandwich structure (thin foils shaped into interlocking cells (hexagonal or other configurations) with axes oriented perpendicular to the face planes) 36/37

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