Composites Module 4a

Spring 2001 ISAT 430 Dr. Ken Lewis2 An aside: Stress – Strain  Tension test Used to determine mechanical properties such as Strength Ductility Toughness Elastic modulus Elongation to break stiffness

Relative mechanical Properties StrengthHardnessToughnessStiffnessStrength/Density Glass FibersDiamondDuctile MetalsDiamondReinforced Plas. Graphite FiberCubic BNReinforced Plas.CarbidesTitanium Kevlar FiberCarbidesThermoplasticsTungstenSteel CarbidesHardened steelsWoodSteelAluminum MolybdenumTitaniumThermosetsCopperMagnesium SteelsCast IronsCeramicsTitaniumBeryllium TantalumCopperGlassAluminumCopper TitaniumThermosetsCeramicsTantalum CopperMagnesiumReinforced Plas. Reinforced TSThermoplasticsWood Reinforced TPTinThermosets ThermoplasticsLeadThermoplastics LeadLycra® Spring 2001ISAT 430 Dr. Ken Lewis

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis4 Stress - Strain  There is a standard specimen for each type of material having a known initial gage length (l 0 ) and diameter (cross sectional area (A 0 ) Metals l 0 =~ 50 mm ( 2 in) A 0 dia =~ 12.5 mm (0.5in) Fibers l 0 =~ 25 mm ( 10 in) A 0 dia =~ the fiber

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis5 Stress - Strain  Specimen is mounted in the jaws of a tensile testing machine. Usually can be tested at various rates of extension and temperature.  Produces the important Stress – Strain curve.

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis7 Stress - Strain  In the elastic region, stress and strain are proportional  The ratio or slope of the line in the elastic region is called Modulus of elasticity, E, Young’s modulus  The linear relationship is known as Hooke’s Law T. Young (1773 – 1829) R. Hooke (1635 – 1703)

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis8 Stress - Strain  The higher the E value The higher the load required to stretch a specimen to the same extent The stiffer the material Note Strain is dimensionless therefore E has units of stress F/A.

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis9 Stress - Strain  Brittle Materials Glass Most un-reinforced ceramics  Elastic to the end.

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis10 Stress - Strain  Materials that strain harden Some steel alloys Material is ductile until grain boundaries intersect Movement stops More stress can then be borne.

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis11 Stress - Strain  Elastomers Lycra® Rubbers  Small truly elastic region  Large elongation with little increase in stress  At the end, crystallization occurs and stress is borne until rupture.

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis12 Stress – Strain - Plastics

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis13 Stress - strain &Temperature  Cellulose Acetate Note the large drop in strength The large increase in ductility

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis14 Composites  A way of combining the benefits of different materials to influence the bulk properties Make plastics competitive with steel Allow building a single piece boat hull Increase the elastic modulus and strength of light metals.  An important class of engineered materials.

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis15 Composites  Definition Combination of two or more chemically distinct and insoluble phases Properties and structural performance superior to those of the constituents acting independently.  Common classes Reinforced Plastics Metal Matrix Composites (MMC) Ceramic Matrix Composites (CMC)

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis16 Early examples  Brick reinforced with straw ( 3000 BCE)  Concrete reinforced with iron rods (1800’s) In truth, Concrete is itself a Composite (sand, cement, gravel) In both cases, the reinforcing material (straw and iron) provide needed tensile strength.

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis17 Mechanical Properties MaterialUltimate Tensile Strength (Mpa) Young’s Modulus E (Gpa) Elongation to break (%) Acetal – Acetal Reinforced Epoxy Epoxy Reinforced Polycarbonate Polycarbonate reinforced Polyester Polyester reinforced Nylon Nylon reinforced

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis18 Composite Classification  Matrix The material that surrounds the other component Provides the bulk form of the part Hold the reinforcing phase in place  Reinforcing phase The embedded material may be metal, ceramic, plastic Usually provides the strength  In terms of its effect, the shape of the reinforcing material is of particular importance.

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis19 Matrix Function  To support the fibers in place Transfer stresses to them Let them carry the tensile load  To protect the fibers Physical damage Environment  Reduce crack propagation Greater matrix ductility (sort of, usually)

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis20 Reinforcing phase  Fibers Continuous Very long Discontinuous L/D about 100 Whiskers  Hair like single crystals with diameters down to about 40 x in.  Very strong.

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis21 Fibers of choice  Glass – cheapest and most widely used Glass fiber reinforced plastic (GFRP) Made by drawing molten glass through a platinum spinneret. Types E – glass (calcium aluminosilicate glass) S – glass (magnesia aluminosilicate glass)

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis22 Fibers of choice  Graphite – more expensive High strength, stiffness and low density Carbon fiber reinforced plastic (CFRP) Made by pyrolysis of organic precursors – usually polyacrylonitrile (PAN) or pitch Two kinds of fibers Carbon (80 – 95% carbon)  Lower modulus and strength Graphite (> 99% carbon)  Crystalline and very high modulus and strength

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis23 Fibers of choice  Aramids (among the toughest fiber) Kevlar® is the best example Have some elongation before rupture (~3%) so are very tough. Mechanism used in the bullet proof vest! Absorb moisture which can degrade properties.

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis24 Fibers of choice  Boron Formed by chemical vapor deposition onto tungsten fibers. Very strong and stiff both in tension and compression However, very heavy because of the tungsten (  = 19.3 g/cm 3 ) Tungsten Boron

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis25 Particles and Flakes  Orientation in the matrix is usually quite random Properties therefore are isotropic Called fillers  Crack stoppers

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis26 Effect of Fiber Type on Properties  Mechanical and physical properties depend on the reinforcing mediums Kind Shape Orientation  Short fibers are less effective than long fibers

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis27 Fiber Matrix Bond - Plastics  Strength of the fiber matrix bond is critical The load is transmitted through the fiber – matrix interface  Weak bonding can cause Fiber pullout delamination

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis28 Strength as a function of fiber direction and content  In general… The highest stiffness and strength is obtained when the fibers are aligned in the direction of the tension force. Cool…. But, there is a caveat  This makes the composite very anisotropic.

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis29 Strength as a function of fiber direction and content  Result of anisotropy… Other properties are anisotropic Stiffness Creep Thermal & electrical conductivity Thermal expansion Example – fiber reinforced packaging tape Strong in the fiber direction Easily pulled apart in the width direction

Reinforcing Fibers FiberDensity g/cm3)Young’s Modulus (GPa) Tensile Strength (GPa) Steel Tungsten Beryllium E-Glass S-Glass Alumina SiC Boron High Modulus C High Strength C Kevlar® Kevlar®

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis31 The Rule of Mixtures c = composite r = reinforcing phase m = matrix The mass of the composite body is : Equivalently: If we let: since

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis32 The Rule of Mixtures We get: This is the Rule of Mixtures Each component contributes to the properties Of the composite in proportion To its VOLUME fraction

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis33 Fiber reinforcement  When the filler is in the form of thin fibers strongly bonded to the matrix Properties depend on the fiber and the amount present There is a critical minimum volume fraction There is a critical length  Fiber should be continuous of  If chopped long enough to accept stresses transferred from the matrix

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis34 Fiber reinforcement  If we use the criteria It takes more force to shear the matrix at the fiber boundary (pull out the fiber) than to break the fiber  We get an approximate critical length l cr. D = fiber diameter T = fiber tensile strength  i = interfacial shear strength

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis35 Tensile Strength  Unidirectional Composites  Complex Upon loading a single fiber breaks first The aim is to neutralize the effect of local failure. Matrix should be ductile enough to not propagate a crack Matrix should be able to carry the shear stress on the fiber matrix interface. The rule of mixtures is helpful but actual design is made using actual statistical properties.

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis36 Long Fiber Composites  Rule of mixtures works in the longitudinal direction.  Perpendicular to the longitudinal direction

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis37 Example  Suppose we are reinforcing an epoxy matrix whose elastic modulus is 2.7 GPa with 26% by volume of E-glass whose elastic modulus is 75 GPa What is the Elastic modulus of the composite in the longitudinal direction?

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis38 Example  Suppose we are reinforcing an epoxy matrix whose elastic modulus is 2.7 GPa with 26% by volume of E-glass whose elastic modulus is 75 GPa What is the Elastic modulus of the composite in the transverse direction? Recall

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis39 Other Composite Types  A) conventional laminar structure  B) sandwich with a foam core  C) sandwich structure using a honeycomb  B and C gain stiffness using an increase in the moment of inertia

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis40 Applications of Reinforced Plastics  First application was for an acid resistant tank made of a phenolic resin and asbestos.  1920’s – Formica, used for counter tops  1930’s – Advent of epoxy as a reinforcing material  1940’s – fiberglass/epoxy boats, some aircraft, sporting goods  1970’s – beginning of “Advanced Composites” using hybrid plastics and carbon fibers.

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis41 Applications of Reinforced Plastics  Aircraft (DC-10, L-1011, 727, 757, 767, 777) The Boeing 777 is about 9% (weight) composites Floor beams and panels Most of the vertical and horizontal tail The Lear Fan 2100 passenger aircraft structure is almost all graphite/epoxy. 90% of the world circling Voyager is plastic composites The stealth bomber is made of carbon and glass fibers, epoxy resin matrices, high temperature polyimides (and other neat stuff).

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis42 Composite Sailboard K. Easterling, Tomorrow’s Materials, p.133, Institute of Metals, 1990

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis43 Metal Matrix Composites  The matrix is usually a low density metal, primarily Aluminum Aluminum - lithium Magnesium Copper titanium  The reinforcement is often SiC, Al 2 O 3, or carbon

MMCs FiberMatrixApplications GraphiteAluminum Magnesium Lead Copper Satellite, missile, and helicopter structures Space and satellite structures Storage- battery plates Electrical contacts and bearings BoronAluminum Magnesium Titanium Compressor blades and structural supports Antenna structures Jet-engine fan blades AluminaAluminum Lead Magnesium Superconductor restraints in fission power reactors Storage-battery plates Helicopter transmission structures Silicon CarbideAluminum Super alloy High-temperature structures High-temperature engine components Molybdenum Tungsten SuperalloyHigh-temperature engine components

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis45 Ceramic Matrix Composites  In polymer matrix composites the reinforcement is always stronger and of much higher elastic modulus than the matrix Thus a significant increase in strength can be had by transferring stresses from the matrix to the fiber through a strong interface.  This is also true of MMC (for low elastic moduli material such as Al, Mg)  Ceramic already have a high elastic modulus (except glasses) so the purpose of a CMC is to increase toughness.

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis46 Monolithic Ceramics Fail completely In Brittle Catastrophic mode

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis47 Ceramic Matrix Composites Matrix materials Silicon carbide Silicon nitride Aluminum oxide Mullite (aluminum, silicon oxides)  Ceramics are strong and stiff, retained at high temperatures but are brittle.

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis48 CMC Toughness  (A) Crack Deflection A crack meeting the reinforcement is deflected along the interface where energy is used to effect separation  (B) Crack Propagation Barrier Reinforcement can force the crack to bow out, increasing the stress necessary for propagation

Module 4a Spring 2001 ISAT 430 Dr. Ken Lewis49 CMC Toughness  (C) Fiber Bridging Sometimes fibers bear the load across the crack, putting the crack in compression  (D) Fiber Pull-out Most important, energy is used in pulling the fiber out. The interfacial bond strength must not be so high that the fiber breaks rather than pulling out.