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

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.

Chapter 16 Objectives – Cont. Be able to show why the ratio of force carried by the fiber to the matrix is given by 16.11. 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.

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.

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°

Composite Map

Fiber Reinforcement Influences Concentration Size Shape Distribution Orientation

Particle Reinforcement Rule of Mixtures (Tungsten in Copper)

Continuous Fiber/Longitudinal Isostrain Upper Bound

Continuous Fiber/Transverse Isostress Lower Bound

Influence of Fiber Length

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

Continuous/Aligned Fiber – brittle Matrix - ductile

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

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

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

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

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

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

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

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

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

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

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

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

Discontinuous Fibers Aligned for for Randomly Oriented

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

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

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?

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

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

NTSB - AA587

NTSB - AA587