MSE200 Lecture 20(CH. 12.1-12.3) Composite Materials Instructor: Yuntian Zhu Objectives/outcomes: You will learn the following: •particle and fiber reinforcement. • Various types of fibers • Unidirectional • Rule of mixture. •Strength of fiber composites Introduction • A composite material is • Properties of composite materials can be superior to its individual components. • Examples: Two types of composite materials • Classified according to the reinforcements – Particle reinforced composites • Examples: – Fiber reinforced composites • Examples: Glass Fibers for Reinforced Plastic Composite Materials • Glass fiber reinforced plastic composite materials have high strength-weight ratio, good dimensional stability, good temperature and corrosion resistance and low cost. ‘E’ Glass : 52-56% SiO2, + 12-16% Al2O3, 16-25% CaO + 8-13% B2O3 Tensile strength = 3.44 GPa, E = 72.3 GPa ‘S” Glass : Used for military and aerospace application. 65% SiO2 + 25% Al2O3 + 10% MgO Tensile strength = 4.48 GPa, E = 85.4 GPa Production of Glass Fibers • Produced by drawing monofilaments from a furnace and gathering them to form a strand. Low cost and hence commonly used. http://www.google.com/search?q=glass+fiber+drawing&tbo=p&tbs=vid%3A1&source=vgc&hl=en&aq=f Glass fiber products http://video.google.com/videosearch?hl=en&q=glass%20fiber%20composite&gbv=2&ie=UTF-8&sa=N&tab=iv#hl=en&q=glass+fiber+composite&gbv=2&ie=UTF-8&sa=N&tab=iv&start=0 Carbon Fibers for Reinforced Plastics • Light weight, very high strength and high stiffness. • 7-10 micrometer in diameter. • Produced from polyacrylonitrile (PAN) and pitch. • Steps: Stabilization: PAN fibers are stretched and oxidised in air at about 2000C. Carbonization: Stabilized carbon fibers are heated in inert atmosphere at 1000-15000C which results in elimination of O,H and N resulting in increase of strength. Graphitization: Carried out at 18000C and increases modulus of elasticity at the expense of strength • Tensile strength = 3.1-4.45 GPa, E = 193-241 GPa, density = 1.7-2.1 g/cc. The highest strength: 6.9 GPa, by Toray Carbon fiber products • http://video.google.com/videosearch?q=carbon+fiber&num=10&so=0&hl=en&start=0 Boeing 787 Dreamliner 50% structure carbon fiber composite Aramid Fibers (Kevlar) for Reinforcing Plastic Resins • Aramid = aromatic polyamide fibers. • Trade name is Kevlar Kevlar 29:- Low density, high strength, and used for ropes and cables. Kevlar 49:- Low density, high strength and modulus and used for aerospace and auto applications. • Hydrogen bonds bond fiber together. • Used where resistance to fatigue, high strength and light weight is important. Kevlar fiber products http://video.google.com/videosearch?q=kevlar&hl=en&emb=0&aq=f# Comparison of Mechanical Properties • Glass fibers are cheap, for cheap civilian products • Carbon fibers are strong but brittle, high strength structure • Kevlar fibers are toughest, for body armor. Fiber frontier: Carbon nanotube (CNT) fiber Commercial LANL CNT fiber CNT fiber Example of spun CNT fiber Ribbons being pulled from array Matrix Materials • Polyester and epoxy resins are the two important matrix materials. • Polyester resins: Cheaper than epoxy resins. Applications: Boat hulls, auto and aircraft applications. • Epoxy resins: Good strength, low shrinkage. Commonly used matrix materials for carbon and aramid-fiber composite. Table 11.2 Fiberglass-polyester Fiber Reinforced-Plastic Composite Materials • Fiberglass-reinforced polyester resins: Higher the wt% of glass, stronger the reinforced plastic is. Nonparallel alignment of glass fibers reduces strength. • Carbon fiber reinforced epoxy resins: Carbon fiber contributes to rigidity and strength while epoxy matrix contributes to impact strength. Polyimides, polyphenylene sulfides are also used. Exceptional fatigue properties. Carbon fiber epoxy material is laminated to meet strength requirements. Properties of Fiber Reinforced Plastics Fiberglass polyester (Carbon fibers and epoxy) Rule of Mixture (isostrain condition) • Stress on composite causes uniform strain on all composite layers. Pc = Pf + Pm = P/A Pc = Load on composite Pf = Load on fibers cAc = fAf + mAm Pm = load on matrix Rule of mixture of binary composites Ec = EfVf + EmVm c = fVf + mVm c = fVf + mVm Loads on Fiber and Matrix Regions • Since = E and f = m P A E A E A E V f = f f = f f f = f f = f f Pm m Am Em m Am Em Am EmVm Pc = Pf + Pm • From above two equations, load on each of fiber and matrix regions can be determined if values of Ef, Em, Vf, Vm and Pc are known. Isostress Condition • Stress on the composite structure produces an equal stress condition on all the layers. c = f = m Assuming no change in area and assuming unit length of the composite c = fVf + mVm Therefore c = , f = , m = Ec E f Em V f V 1 V f V = + m = + m E c E f E m E c E f E m Toughening Mechanisms in Composite Materials • Toughening is due to fibers interfering with crack propagation. Crack deflection: Up on encountering reinforcement, crack is deflected making propagation more meandering. Crack bridging: Fibers bridge the crack and help to keep the cracks together. Fiber pullout: Friction caused by pulling out the fiber from matrix results in higher toughness. HW • Examples problems in Chapter 12: 12.1, 12.2, 12.3 .