Manufacturing Processes Wk. 3 Ch. 9
Fibers in PMCs
Various forms: discontinuous (chopped), continuous, or woven as a fabric Principal fiber materials in FRPs are glass, carbon, and Kevlar 49 -Less common fibers include boron, SiC, and Al2O3, and steel Glass (in particular E‑glass) is the most common fiber material in today's FRPs -Its use to reinforce plastics dates from around 1920
Three Factors that Determine Properties
1. Materials used as component phases in the composite 2. Geometric shapes of the constituents and resulting structure of the composite system 3. How the phases interact with one another
Classification of Composite Materials
1. Metal Matrix Composites (MMCs) ‑ mixtures of ceramics and metals, such as cemented carbides and other cermets 2. Ceramic Matrix Composites (CMCs) ‑ Al2O3 and SiC imbedded with fibers to improve properties 3. Polymer Matrix Composites (PMCs) ‑ polymer resins imbedded with filler or reinforcing agent -Examples: epoxy and polyester with fiber reinforcement, phenolic with powders
COMPOSITE MATERIALS
1. Technology and Classification of Composite Materials 2. Metal Matrix Composites 3. Ceramic Matrix Composites 4. Polymer Matrix Composites
One Possible Classification of Composites
1. Traditional composites - composite materials that occur in nature or have been produced by civilizations for many years -Examples: wood, concrete, asphalt 2. Synthetic composites - modern material systems normally associated with manufacturing -Components are first produced separately and then combined to achieve the desired structure, properties, and part geometry
Particles and Flakes
A second common shape of imbedded phase is particulate, ranging in size from microscopic to macroscopic -Flakes are basically two‑dimensional particles ‑ small flat platelets Distribution of particles in the matrix is random -Strength and other properties of the composite material are usually isotropic
FRP Applications
Aerospace - much of the structural weight of today's airplanes and helicopters consist of advanced FRPs -Example: Boeing 787 Dreamliner Automotive - some body panels for cars and truck cabs -Low-carbon sheet steel still widely used due to its low cost and ease of processing Sports and recreation -FRPs used for boat hulls since 1940s -Fishing rods, tennis rackets, golf club shafts, helmets, skis, bows and arrows
Ceramic Matrix Composites (CMCs)
Ceramic primary phase imbedded with a secondary phase, usually consisting of fibers -Attractive properties of ceramics: high stiffness, hardness, hot hardness, and compressive strength; and relatively low density -Weaknesses of ceramics: low toughness and bulk tensile strength, susceptibility to thermal cracking -CMCs represent an attempt to retain the desirable properties of ceramics while compensating for their weaknesses
Why Composites are Important
Composites can be strong and stiff, yet light in weight -Strength‑to‑weight and stiffness‑to‑weight ratios can be several times greater than steel or aluminum Fatigue properties 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
Continuous Fibers vs. Discontinuous Fibers
Continuous fibers - very long -In theory, they offer a continuous path by which a load can be carried by the composite part Discontinuous fibers (chopped sections of continuous fibers) - short lengths (L/D = roughly 100) -Special Case: Whiskers = discontinuous fibers of hair-like single crystals with diameters down to about 0.001 mm (0.00004 in) and very high strength
Laminar Composite Structure
Conventional laminar structure - two or more layers bonded together in an integral piece Example: plywood, in which layers are the same wood, but grains oriented differently to increase overall strength
Other Laminar Composite Structures
FRPs - multi‑layered, fiber‑reinforced plastic panels for aircraft, boat hulls, other products Printed circuit boards - layers of reinforced copper and plastic for electrical conductivity and insulation, respectively Snow skis - layers of metals, particle board, and phenolic plastic Windshield glass - two layers of glass on either side of a sheet of tough plastic
Materials for Fibers
Fiber materials in fiber‑reinforced composites -Glass - most widely used 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
Fiber Orientation
Fiber orientation in composite materials: (a) one‑dimensional, continuous fibers; (b) planar, continuous fibers in the form of a woven fabric; and (c) random, discontinuous fibers
Fibers
Filaments of reinforcing material, usually circular in cross section -Diameters from ~ 0.0025 mm (0.0001 in) to about 0.13 mm (0.005 in) - Filaments provide greatest opportunity for increasing strength of composite -Filament form of most materials is significantly stronger than the bulk form
Sandwich Structure: Honeycomb Core
Foam or honeycomb achieve high ratios of strength‑to‑weight and stiffness‑to‑weight
Interface between Constituent Phases in Composite Material
For the composite to function, the phases must bond where they join at the interface -Direct bonding between primary and secondary phases
Alternative Interphase Form
Formation of an interphase consisting of a solution of primary and secondary phases at their boundary
Reinforcing Phase
Function is to reinforce the primary phase Reinforcing phase (imbedded in the matrix) is most commonly one of the following shapes: fibers, particles, or flakes Also, secondary phase can take the form of an infiltrated phase in a skeletal or porous matrix -Example: a powder metallurgy part infiltrated with polymer
FRP Properties
High strength‑to‑weight and modulus‑to‑weight ratios -FRP weighs only about 1/5 as much as steel -Yet strength and modulus are comparable in fiber direction Good fatigue strength Good corrosion resistance, although polymers are soluble in various chemicals Low thermal expansion for many FRPs
Properties of Composite Materials
In selecting a composite material, an optimum combination of properties is often sought, rather than one particular property -Example: fuselage and wings of an aircraft must be lightweight, strong, stiff, and tough -Several fiber‑reinforced polymers possess these properties -Example: natural rubber alone is relatively weak Adding carbon black increases its strength
Interphase
In some cases, a third ingredient must be added to bond primary and secondary phases Called an interphase, it is like an adhesive
Other Composite Structures
Laminar composite structure - conventional Sandwich structure Honeycomb sandwich structure
Cermets
MMC with ceramic contained in a metallic matrix -The ceramic often dominates the mixture, sometimes up to 96% by volume -Bonding can be enhanced by slight solubility between phases at elevated temperatures used in processing Cermets can be subdivided into 1. Cemented carbides - most common 2. Oxide‑based cermets - less common
Composite Material Defined
Material system composed of two or more distinct phases whose combination produces aggregate properties different from those of its constituents Examples: -Cemented carbides (WC with Co binder) -Plastic molding compounds with fillers -Rubber mixed with carbon black -Wood (a natural composite as distinguished from a synthesized composite)
Metal Matrix Composites (MMCs)
Metal matrix reinforced by a second phase Reinforcing phases: 1. Particles of ceramic -These MMCs are commonly called cermets 2. Fibers of various materials -Other metals, ceramics, carbon, and boron
Example: Fiber Reinforced Polymer
Model of fiber‑reinforced composite material showing direction in which elastic modulus is being estimated by the rule of mixtures
Components in a Composite Material
Most composite materials consist of two phases: 1. Primary phase - forms the matrix within which the secondary phase is imbedded 2. Secondary phase - imbedded phase sometimes referred to as a reinforcing agent, because it usually strengthens the composite material - The reinforcing phase may be in the form of fibers, particles, or various other geometries
Importance of Geometric Shape: Fibers
Most materials have tensile strengths several times greater as fibers than as bulk materials By imbedding the fibers in a polymer matrix, a composite material is obtained that avoids the problems of fibers but utilizes their strengths -Matrix provides the bulk shape to protect the fiber surfaces and resist buckling -When a load is applied, the low‑strength matrix deforms and distributes the stress to the high‑strength fibers
Common FRP Structures
Most widely used form of FRP is a laminar structure -Made by stacking and bonding thin layers of fiber and polymer until desired thickness is obtained -By varying fiber orientation among layers, a specified level of anisotropy in properties can be achieved in the laminate Applications: boat hulls, aircraft wing and fuselage sections, automobile and truck body panels
Cemented Carbides
One or more carbide compounds bonded in a metallic matrix Common cemented carbides are based on tungsten carbide (WC), titanium carbide (TiC), and chromium carbide (Cr3C2) -Tantalum carbide (TaC) and others are less common Metallic binders: usually cobalt (Co) or nickel (Ni)
Fiber Orientation - Three Cases
One‑dimensional reinforcement, in which maximum strength and stiffness are obtained in the direction of the fiber Planar reinforcement, in some cases in the form of a two‑dimensional woven fabric Random or three‑dimensional in which the composite material tends to possess isotropic properties
Other Polymer Matrix Composites
Other PMCs contain particles, flakes, and short fibers -Called fillers when used in molding compounds -Two categories: 1. Reinforcing fillers - used to strengthen or otherwise improve mechanical properties 2. Extenders - used to increase bulk and reduce cost per unit weight, with little or no effect on mechanical properties
Fiber‑Reinforced Polymers (FRPs)
PMC consisting of a polymer matrix imbedded with high‑strength fibers Polymer matrix materials: -Usually a thermosetting plastic such as unsaturated polyester or epoxy -Can also be thermoplastic, such as nylons (polyamides), polycarbonate, polystyrene, and polyvinylchloride -Fiber reinforcement is widely used in rubber products such as tires and conveyor belts
Cemented Carbide
Photomicrograph (about 1500X) of cemented carbide with 85% WC and 15% Co (photo courtesty of Kennametal Inc.)
Ceramic Matrix Composite
Photomicrograph (about 3000X) of fracture surface of SiC whisker-reinforced Al2O3 (photo courtesy of Greenleaf Corp.)
Polymer Matrix Composites (PMCs)
Polymer primary phase in which a secondary phase is imbedded as fibers, particles, or flakes -Commercially, PMCs are more important than MMCs or CMCs -Examples: -Most plastic molding compounds -Rubber reinforced with carbon black -Fiber‑reinforced polymers (FRPs)
Physical Shapes of Imbedded Phase
Possible physical shapes of imbedded phases in composite materials: (a) fiber, (b) particle, and (c) flake
Functions of the Matrix Material
Primary phase that provides the bulk form of the part or product made of the composite material -Holds the imbedded phase in place, usually enclosing and often concealing it -When a load is applied, the matrix shares the load with the secondary phase, in some cases deforming so that the stress is essentially born by the reinforcing agent
Disadvantages and Limitations
Properties of many important composites are anisotropic -May be an advantage or a disadvantage Many polymer‑based composites are subject to attack by chemicals or solvents -Just as the polymers themselves are susceptible Composite materials are generally expensive Manufacturing methods for shaping composite materials are often slow and costly
Sandwich Structure: Foam Core
Relatively thick core of low density foam bonded on both faces to thin sheets of a different material
Example: Fiber Reinforced Polymer (continued)
Stress‑strain relationships for the composite material and its constituents The fiber is stiff but brittle, while the matrix (commonly a polymer) is soft but ductile
Applications of Cemented Carbides
Tungsten carbide cermets (Co binder) - Cutting tools, wire drawing dies, rock drilling bits, powder metal dies, indenters for hardness testers Titanium carbide cermets (Ni binder) -Cutting tools; high temperature applications such as gas‑turbine nozzle vanes Chromium carbide cermets (Ni binder) -Gage blocks, valve liners, spray nozzles
Cemented Carbide Properties
Typical plot of hardness and transverse rupture strength as a function of cobalt content
