Composites Flashcards
(21 cards)
Composites
A material formed y the combination of two or more phases to achieve superior properties than either constituent acting alone
What are two phases a composite consists of
The continuous phase (the matrix) which is to transfer the applied load to and protect the dispersed phase (the reinforcement ). Often matrix is metal, ceramic or polymer. Reinforcement is often ceramic.
What are the three reinforcement classifications of composites
Particle reinforcement (where usually particular phase is harder and stiffer then matrix, fibre reinforces and structural
What are the three matrix classifications of composites
Metal, ceramic or polymer matrix composites
How do particle reinforced composites work
Matrix transfers some of the applied stress to particles, which bear a fraction of the load. Degree of reinforcement depends on strong bonding at the matrix particle interface.
What are the two types of particle reinforced composites
Dispersion strengthened. Particles normally much smaller. Strengthening mechanism is sismilar to the of precipitation hardening where matrix bears portion of applied load and particles impede motion of dislocation.
Large particle. adding solid particles into a continuous matrix material
What should the particles be line in large particle composites
-roughly same size in all directions
-evenly distributed
-more particles usually mean better mechanical properties
What’s the rule of mixtures
Estimates composite stiffness. Has two equations for:
Upper Bound (Ec₍u₎) — When load is shared in parallel
Lower Bound (Ec₍l₎) — When load is shared in series
Cemented carbide (most common cermet- an example or ceramic metal composites)
Composed of excrement hard particles of a refractory ceramic in a metal matrix. Metal enhances toughness as it isolates the particles preventing crack probation usually used in cutting tools as refractory.
Carbon black
Consists of very small and essentially spherical particles of carbon. Is added to vulcanized Uber to increase tensile strength, toughness and tear and abrasion resistance. Particles must be even.y distruvted and form strong adhesive bonds with rubber matrix
Dispersion strengthened composites
Uniform dispersion of several volume percent of fine particles of a very hard inert material. Strengthening mechanism involves interactions between particles and dislocations within the matrix
Benefit of dispersion strengthening over precipitation hardening even if effect is not as pronounced
Can be retained at elevated temps for extended periods because dispersed psrticles are unreactive with the matrix phase. For precipitation hardened alloys strength may disappear from near treatment due to precipitation growth or dissolution of the precipitate phase
What is mechanical characteristics of fibre reinforced composites dependent on
Properties of fibre
Degree to which an applied load is transmitted to the fibres by the matrix- the fibers only help strengthen the composite if the matrix can grip them well enough to transfer stress into them. This depends on how long the fiber is.
Fibre orientation and concentration
Fibre reinforced composites
the fibers are the main load-carrying elements, and the matrix (e.g., plastic resin) helps to transfer load between them.
Critical fibre length
There’s a minimum length a fiber must have to actually contribute meaningfully to the strength of the composite. This is called the critical length, l_c. If the fiber is too short, the matrix can’t “grab” enough of it to stretch it fully
What Happens at the Fiber Ends?
At the very ends of the fiber, the matrix can’t apply much force — so the load gradually increases from the ends toward the center. If the fiber is short, only the middle part carries meaningful stress. If the fiber is long enough, the whole fiber can be used to carry load.
How are continuous and discontinuous fibres normally aligned and which has best overall properties
Continuous: aligned
Discontinuous: aligned, partially oriented or randomly oriented
Best when fibre distribution is uniform
Describe stress strain stages of fibre composite. Know what graph looks like
At low stress, both matrix and fibers deform elastically
Then matrix starts to yield. Fibres still in elastic range. They carry more and more load
Fibres start to break
Why is fracture not sudden for fibre composite
Not all fibers break at once — there’s variation in strength.
• Broken fibers are still stuck in the matrix and can carry some load.
• The matrix is still there, deforming plastically.
Longitudinal vs transfers strain
Longitudinal is applied aligned to fibre aline,ent and transverse is at 90 degrees
