1. ME 330 Engineering Materials
Lecture 22
Composites
Composite types
Failure modes
Damage Mechanisms
Fiber reinforcement
Processing techniques
Processing examples
Machining problems
Please read Chapter 16

2. Chapter 16 Objectives
Cite the three main classifications of composite materials and describe the distinguishing feature of each.
Understand the importance of interfaces in the behavior of composite materials.
Know the equations for and how to use the upper and lower-bound rule of mixtures for elastic modulus of large particle reinforced materials.
Describe the strengthening mechanism for large-particle and dispersion-strengthened particle-reinforced composites and how the two mechanisms differ.
Classify the types of fiber reinforced composites and understand the relative strengths and strengthening mechanisms of each. Consider fiber length and orientation in addition to concentration.
Understand the concept of critical fiber length, the terms involved in equation 16.3, and how to use the equation.
Know the equations for the longitudinal and transverse modulus for continuously reinforced fiber composites, and the modulus for discontinuous and randomly oriented fiber composites.

3. Chapter 16 Objectives – Cont.
Be able to show why the ratio of force carried by the fiber to the matrix is given by
Be able to calculate the longitudinal modulus and longitudinal strength for aligned and continuous fiber composites.
Be able to calculate the longitudinal strengths for discontinuous and aligned fibrous composites.
Cite the three common fiber reinforcements for polymer-matrix composites and the desirable characteristics and limitations for each.
Know the applications for, and the advantages and disadvantages of carbon-carbon composites.
Describe the desirable features for metal-matrix composites.
Note the primary reason for the creation of ceramic matrix composites.
Describe and understand the toughening mechanisms for ceramic matrix composites in detail for those that toughen by phase transformation and by whisker reinforcement.

4. Making the Connection
We have examined strength, deformation, creep, fracture, fatigue properties of metals, polymers, and ceramics
Each have certain advantages/disadvantages
Ceramics - high compressive strength, stiffness, creep resistance, light, good thermal properties, very brittle, poor tensile strength
Metals - good tensile properties, good creep resistance, moderate fracture toughness, generally rather heavy, corrosion can be problem
Polymers - ductile, light, often high fracture toughness, weak, poor creep properties, rate sensitive
So, we want to combine the best of both worlds and create new materials.
Principle of combined action - “better property combinations are fashioned by the judicious combination of two or more distinct materials”
If we don’t do better than before, we don’t do it.

5. Some Common Examples Reinforced concrete Fiberglass composites
Concrete poor in tension
Steel bars carry tensile load
Steel would buckle in compression
Concrete takes much of compressive load
Concrete is very formable, protects steel from corrosion
Fiberglass composites
Epoxy matrix with random glass fibers
Boat hulls
Corvette body
Complex shapes can be made
Lightweight
Excellent corrosion resistance
Also:
Car tire - steel belted rubber
Plywood - laminate with grain oriented at 0/90°

6. Composite Map

7. Fiber Reinforcement Influences
Concentration
Size
Shape
Distribution
Orientation

8. Particle Reinforcement
Rule of Mixtures (Tungsten in Copper)

9. Continuous Fiber/Longitudinal
Isostrain
Upper Bound

10. Continuous Fiber/Transverse
Isostress
Lower Bound

11. Influence of Fiber Length

12. Fiber Orientation & Concentration
Randomly Oriented
Continuous & Aligned
Discontinuous & Aligned

13. Continuous/Aligned
Fiber – brittle
Matrix - ductile

14. Polymer-Matrix Composites
Polymer resin is most common matrix material
Ductile and tough
Bonds well to fibers
Cheap and easy to work with
Glass-fiber reinforcement
Easily manufactured
Cheap and commonly available
Many types of glass to choose from
Carbon-fiber reinforcement
Extremely high strength/stiffness
Relatively cheap with polymer matrix
Good for relatively high temperature applications
Polymer-fiber reinforcement (Aramids, e.g. Kevlar)
High strength-to-weight ratio
Tough, impact resistant

15. Metal-Matrix Composites
Mechanical properties
Ductile and tough like polymers, but better for high temperatures
More chemical, thermal, and creep resistant than polymers
More expensive, heavy, and difficult to shape than polymers
Superalloys, aluminum, titanium, copper
Continuous ceramic-fiber reinforcement:
Long filaments  not easily formable
Discontinuous ceramic-fiber reinforcement:
Whiskers or very fine fibers  more formable
Particulate ceramic-fiber reinforcement:
Dispersion hardened metals
Cermets

16. Ceramic-Matrix Composites
Ceramics have excellent
High temperature properties
Compressive strengths
But extremely brittle and low fracture toughness
CMC’s significantly improve fracture toughness
Transformation toughening
Fibers stunt crack growth
Deflect crack tip
Bridge crack wake
Pull out of matrix and absorb energy as friction
Gradual failure

17. Ceramic-Matrix Composites
Transformation toughening:
Crack tip stress field induces transformation to stable monoclinic phase with larger volume - establish compressive field at crack tip and wake
Weak interface crack deflection:
Crack tip stress field induces fiber/matrix debonding

18. Ductile Fiber / Ductile Matrix
Local fiber necking creates multiaxial stresses
Leads to voids caused by debonding
Fiber
stress x x
Composite x
Matrix
strain

19. Brittle Fiber /Ductile Matrix
Most common case
Fracture in composite due to fiber rupture
Strengthening occurs only when strength of composite exceeds strength of matrix:
:matrix stress at the fiber fracture strain
Fiber
controlled x
stress
strain
Fiber
Composite
Matrix
Matrix
controlled
Vcrit Vf

20. Brittle Fiber/Brittle Matrix
Strength of the composite is limited by the fracture strain in the fiber:
: matrix stress at the fiber fracture strain
Fiber x
stress x
Composite x
Matrix
strain

21. Brittle Fiber/Brittle Matrix
Strength of the composite is limited by the fracture strain in the matrix (at low Vf):
: fiber stress at the matrix fracture strain
Fiber x
stress
x high vf
Composite
x low vf x
Matrix
strain

22. Fracture f* < m*, matrix can’t support extra load
Common in systems with many brittle fibers (high Vf) in ductile matrix
Criterion: matrix doesn’t have enough additional load-bearing capacity
Fibers
fracture
Matrix quickly
fails

23. Multiple Fiber Fracture
f* < m*, matrix can support extra load
Common in systems with few brittle fibers (low Vf) in ductile matrix
Criterion: matrix has enough additional load-bearing capacity to make up for fibers
Some fibers
fracture
Eventually
matrix
fails
More
fibers
fracture

24. Multiple Matrix Fracture
*f > *m, fiber can support extra load
Common in systems with many brittle fibers (high Vf) in brittle matrix
Common in systems with many ductile fibers (high Vf) in brittle matrix
Criterion: fiber has enough additional load-bearing capacity to make up for matrix
Some matrix
fractures
More
matrix
fractures
Eventually
fibers
fail

25. Damage Mechanisms Tensile Loading Compressive Loading
From: Anderson, p. 332

26. Discontinuous Fibers
Aligned
for
for
Randomly Oriented

27. Debonding & Fiber Pullout
One way that fiber reinforcement increases fracture toughness:
Work must be done to debond fibers and then to pull them out lc
debonding
fracture
Fiber pullout
work
Fiber length
If fiber tip is within lc of crack plane, debonding occurs
If fiber tip is more than lc from crack plane, it fractures
…want fiber length close to lc to maximize work done

29. New Concepts Types of composites Strength of composites
Varies with fiber & matrix failure strength and strain
Failure modes and strength predictions
Damage mechanisms
Improve fracture toughness of composites

30. Air Disaster - AA587 November 12, 2001 Airbus A300-600
Took off from JFK
Crashed off Long Island
265 People Killed
Who’s at Fault?
Terrorist?
Airbus? Structural Failure?
Pilot?

31. Terrorist? Two months after 9/11 No evidence of bomb
No evidence of highjackers

32. Airbus A300-600 – tail section
Carbon fiber/epoxy – failed attachment

33. NTSB - AA587

34. NTSB - AA587