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CHAPTER 11 Composites

Materials for Civil and Construction Engineers

Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.

Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.

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Introduction • Composite materials are made of two or more distinct materials or phases

• This is done to exploit the best properties of each and reduce the effect of the weak properties

• We can improve strength, stiffness, fracture resistance, corrosion resistance, attractiveness, temperature susceptibility, thermal properties, etc.

• Have been used throughout history for CE applications: straw fibers in mud bricks, plain concrete, reinforced concrete, etc.

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• Recently: fiber-reinforced polymers

• Generally, the constituent materials have significantly different properties

• Properties of composite material are significantly different than constituents

• Auto and aero industries use high strength composite metals to build lightweight vehicles

• Wood is a natural composite of cellulose fibers (cell walls) and lignin (glue)

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• Microscopic composites include fibers or particles in sizes up to a few hundred microns

• Macroscopic composites have constituents of much larger size, such as aggregate particles and rebars in concrete

Composites

Macroscopic Microscopic

Particle-Reinforced Fiber-Reinforced

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11.1 Microscopic Composites

Aligned Fibers Random Particles Random Fibers

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Microscopic Composites (Cont.) • Consist of:

Ø continuous phase or matrix – usually polymer (plastic) Ø dispersed or reinforcing phase

• The matrix phase Ø surrounds, suspends, and binds fibers or particles Ø transfers load to them Ø protects them against environmental attack and damage due to handling

• The dispersed phase Ø generally harder and stiffer than the matrix phase

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Fiber-Reinforced Microscopic Composites • Fibers dispersed in a matrix such as polymer (plastic) • Fibers have near crystal-sized diameters

Ø fewer internal defects Ø much stronger than the bulk material (e.g., glass fibers are about 300x stronger that glass plates)

• Fibers Ø Whiskers (very thin single crystals, high cost, poor bond) Ø Fibers (glass, carbon and graphite, boron, ceramic, etc.) Ø wires

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Particle-Reinforced Microscopic Composites

• Particles dispersed in a matrix phase

• Strengthening mechanism

Ø For small particles, particles hinder or impede the motion of dislocations

Ø For particles >1 micron, particles act as fillers to improve the properties of the matrix or replace some of its volume (less expensive)

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Microscopic Composites CE Applications • Have been used in the last several decades • Common applications Ø Structural shapes replacing steel and aluminum Ø Fiber-reinforced polymer rebars Ø Strengthen and wrap partially damaged columns and bridge supports Ø Fiber-reinforced concrete Ø Entrained air in concrete

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Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.

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Fiber-Reinforced

Concrete

Flexible FRP Rebars

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Plain PCC

Reinforced PCC

11.2 Macroscopic Composites

Engineered Wood

Asphalt Concrete

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11.3 Properties of Composites Loading Parallel to Fibers

Ec = νmEm +ν f E f

Ff

Fc=σ f Af

σc Ac=

E f εAf

EcεAc=

E f

Ecν f

  Xc = νmXm +ν f X f

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Properties of Composites (Cont.)

Ec =

EmE f

νmE f + ν f Em

 

Xc =

Xm X f

νm X f + ν f Xm

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Properties of Composites (Cont.) Randomly Oriented Fiber Composites

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Properties of Composites (Cont.) Randomly Oriented Fiber Composites

Ec = νmEm + Kν f E f

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Properties of Composites (Cont.)