ME 350 – Lecture 12 – Chapter 11 COMPOSITE MATERIALS • Advantages & Disadvantages • Components in Composite materials • Composite Examples • Fiber orientation • Secondary phase examples • Interface & Interphase • Composite Strength
Composite Advantages • Strength‑to‑weight ratio and stiffness‑to‑weight ratios are several times greater than for steel or aluminum • Fatigueproperties are generally better than for common engineering metals • Toughness is often greater • Possible to achieve combinations of properties not attainable with metals, ceramics, or polymers alone.
Disadvantages • Properties are generally anisotropic (they differ depending on direction) • May be an advantage or a disadvantage • Can be weakened by chemicals or solvents • Just as the polymers themselves are susceptible • Generally expensive • Manufacturing methods often slow
Components in a Composite Material Most composite materials consist of two phases: • The primary phase within which the other phase is imbedded: matrix • Shares the load, and in some cases deforms so that the stress is essentially born by the reinforcing agent • The secondary phase or imbedded phase that strengthens the composite: reinforcing agent • Generally in the form of fibers, particles, flakes or various other geometries
Examples of Composite Materials • Metal Matrix Composites (MMCs) ‑ mixtures of ceramics and metals, such as cemented carbides and other cermets • Ceramic Matrix Composites (CMCs) ‑ Al2O3 and SiC imbedded with fibers to improve properties • Polymer Matrix Composites (PMCs) ‑ polymer resins imbedded with filler or reinforcing agent • Examples: epoxy and polyester with fiber reinforcement, and phenolic with powders
Fiber Orientation – Three Cases • One‑dimensional reinforcement – maximum strength and stiffness obtained in the direction of the fiber • Planar reinforcement, i.e. two‑dimensional woven fabric • Random or three‑dimensional in which the composite material tends to possess isotropic properties
Materials for Fibers • Fiber materials in fiber‑reinforced composites • Glass – most widely used fiber filament • Carbon – high elastic modulus • Boron – very high elastic modulus • Polymers - Kevlar • Ceramics – SiC and Al2O3 • Metals - steel • Most important commercial use of fibers is in polymer composites
Carbon Fiber Manufacture - Automotive Click for Video
Particles and Flakes • Sizes ranging from microscopic to macroscopic • Flakes are basically two‑dimensional particles ‑ small flat platelets • Distribution of particles is random • Strength and other properties of the composite material are usually isotropic Carbide with 85% WC and 15% Co
The Interface & Interphase • Interface is between the primary and secondary phases in a composite material • Strength failure typical occurs here • An interphase is like an adhesive layer that is added between the primary and secondary phases • Its purpose is to improve the adhesion of the mattrix to the reinforcing phase.
Strength of Fiber Reinforced Polymer Elastic modulus and density can be estimated by the rule of mixtures (Eq 9.5 & 9.6): Ec= fm·Em+ fr·Erdensity: ρ = fm·ρm + fr·ρr Ec′ = Em·Er /( fm·Er+ fr·Em) f – volume fraction
Anisotropy in Strength and Stiffness Variation in elastic modulus and tensile strength as a function of direction relative to the longitudinal axis of the carbon fiber.
Other Composite Structures • Laminar: two or more layers bonded together. Example: plywood. • Sandwich: thick core of foam bonded on both faces to thin sheets of a different material • Honeycomb: Alternative to foam core Foam or honeycomb achieve highest ratios of strength‑to‑weight