Composites and Advacnced Materials
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- Created by: OmololaS15
- Created on: 04-01-21 21:18
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
Composites
- 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
- But want compromise between the two
- Can coat fibre with interphase to reduce adhesion with matix
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Bio-composites: high stiffness and high toughness
Nacre
- 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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