Composites and Advacnced Materials


Intro to Ceramics

  • Structural materials: load/stress bearing (under extreme evironments)
  • Engineering ceramics split into: functional and structural
    • properties can be linked
  • Bond:covalent + ionic 
  • give positive properties: high stiffness and creep resistance
  • high melting temperature, high chemical stability, etc. , low toughness


  • Reinforcement and matrix
  • Improve properties
  • Can have different kinds of reinforcement
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Intro to Ceramic Matrix Composites (CMCs)

CMCs: increases toughness of ceramic

  • Surpasses temeprature limit of aerospace materials with good strength to weight ratio

Performance is affected by:

  • crystal structure
  • componets
  • processing
  • microstructure and defects
  • interface 

Mechanical properties for design: fracture toughness, strenth, crack growth, creep

  • defects act as a stress concentrators 
    • maximum stress at defect tip
    • defects weaken brittle materials 
    • they reduce stress elevation in ductile materials
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Intro to Weibull statistics

Strength at fracture --> determined by toughness and defect size

  • Strength determined by largest defect size
  • Origin of defects : sintering, corrosion, machine damage
  • Ductile materials : stress blunts crack tip
  • Ceramics : no blunting , no dislocation motion
    • Powder processing: organic inclusions, after sintering they leave stable voids
    • Contact damage: contact stress can nucleate crack
      • Propagates in a ring around contact (cone crack)

Weibull statistics

  • If weakest part fails, the entire ceramic fails
  • The probability of survival in 3D is  investigated using volume
  • Weibull statistics uses 3 parameters to investigate the prbability of failure 
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Weibull modulus

Weibull Modulus

  • defines shape of distribution
  • m=0, distribution is independent of stress 
  • m=1, it forms an exponential curve
  • m= infinity, it is a step like function
  • large m --> narrow, more reliable, small spread
  • small m --> wide , large variation
  • Sigma_0 , characteristic strength
  • for ceramic , m~10

Probability of failure

  • for relevant statistics , the number of samples N > = 30
  • Can only compare predictions for the same specimen size in the same load configuration
  • Larger volume = lower failure stress  = higher probability of failure
  • Loading configuration dertermines if the specimen is effectively stressed 
    • tension stresses volume more than 3 point bending 
    • fail at lower stress
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Introduction to Toughening Mechanisms

  • Low dislocation motion in ceramic due to rigid bonds. Thus, crack growth occurs unstably
  • Can have a tough composite if there is extensive fibre pull out
  • There is controlled fracture behaviour of the CMC
  • Two types of mechanisms
    • Process zone: ahead of the crack tip
    • Crack bridging: behind crack tip, many mechanisms can gappen at the same time so it is hard to know which aoone is dominant
  • It is dependnet on matrix and reinforcement and interfacial bond

Crack tip pertubations

  • Crack bowing
  • Crack deflection
  • They toughen composites as they impede crack motion
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Crack bowing and deflection

Crack bowing

  • Non-linear crackfront due to reinforcement's resistance to fracture
  • Stress field in matrix reduced due to perturbation of stress field of crack tip by reinforcement
  • K (fracture toughness) in reinforcement increases and crack front increases bowing until K=K_mat of reinforcemnt then crack breaks through reinforcement
  • toughness increases with increase in the volume fraction of the reinforcement

Crack deflection

  • Crack front deflected(tilting / twisting motion) by reinforcement and becomes non-planar
  • Tilting: mode 1(opening) and mode 2 (shearing)
  • Twisting: mode 1 and mode 2 (tearing)
  • Hgh aspect ratio reinforcement = tougher 
  • High volume farction/reinforcemennt concentration means higher tougheness
    • there is a limiting concentration that this effect takes place 
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Wake toughening / crack bridging

Wake Toughnening

  • Bridge faces of crack
  • stress transfered to fibre (deform elastically)
  • stress acts as crack closure tractions and reduces K at crack tip
  • hinders crack propagation
  • wake/bridge region increases in size as crack grows --> increase in toughness with extension until the strady state toughness value

Comparison of Process zone and Crack bridging 

  • Process zone:as crack grows , the damage zone remains ~ constant 
    • contribution to toughness ~ constant 
  • Crack bridging: material becomes more resistant to crack growth as the crack propagates 
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Microcrack and Transformation toughneing

Microcrack toughnening

  • strain energy of crack lowered as microcrack enters the stress field at the crack tip (stress fields interact)
  • Caused by thermal expansion of matrix and reinforcemnt
  • E.g. Zirconia , transition from t--> m on cooling from hign T  = 3% volume increase = formation of microcracks
  • Toughness only increases with ZrO % untill certain point as too many microcracks leads to larger flaws --> decreasing the strength
  • and the density and size of microcracks

Transformation toughening

  • YSZ: t--> m transition due to stress field at the crack tip
  • Toughness increases as energy absorbed is used to transform
  • Transformation causes compressive stress on the crack faces (behind the crack tip)
  • t is in the metastable state 
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Debonding and fibre pull out

Debonding and Fibre pull out

  • debonding has to occur before pullout
  • interface between matrix and fibre fails under shear
  • Weak interface CMCs
    • But want compromise between the two
      • strong intercae = strength increases 
      • weak interfcae = toughness increases; due to fibre pull out 
  • Can coat fibre with interphase to reduce adhesion with matix 
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Bio-composites: high stiffness and high toughness


  • layered brick like structure 
  • high toughness 
  • Toughneing mechanisms
  • Shearing between nanoasperities
  • Organic layer between brick layers act as viscoelastic glue
  • Breaking of bridge between materials
  • wave structure of bricks = causes lock hardening and spread of non linear deformation around cracks and effects
    • under stress
    • increase toughness

Bio-inspired materials 

  • Lamellar structure
  • Layer of ceramic then polymer
  • Made via freeze casting
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