chichi's version from 2018-03-01 07:50

Section 1

Question Answer
The roles of the matrix in a fiber-reinforced composites are:(1) to keep the fibers in place, (2) to transfer stresses between the fibers, (3) to provide a barrier against an adverse environment, such as chemicals and moisture, and (4) to protect the surface of the fibers from mechanical degradation
A polymer is defined aslong-chain molecule containing one or more repeating units of atoms, joined together by strong covalent bonds
Polymers are divided into two broad categoriesthermoplastics and thermosets
In a thermoplastic polymer, individual molecules are not chemically joined together. They are held in place by weak secondary bonds or intermolecular forces, such as van der Waals bonds and hydrogen bonds. With the application of heat, these secondary bonds in a solid thermoplastic polymer can be temporarily broken and the molecules can be now be moved relative to each other or flow to a new configuration if pressure is applied on them. On cooling, the molecules can be froozen in a new solid shape
In a thermoset polymer, on the other hand,the molecules are chemically joined together by cross-links, forming a rigid, 3D network structure. Once these cross-links are formed during the polymerization reaction, the polymer cannot be melted by the application of heat.

Section 2

Question Answer
In creep tests,a constant stress is maintined on a specimen while its deformation (or strain) increases with time
In stress relaxation tests,a constant deformation (stain) is maintained while the tress on a specimen is monitored as a function of time; stress decreases with time
Both tests are preformed at various ambient temperaturescreep and stress relaxation
In general, thermoset polymers exhibit ______ creep and stress relaxation than thermoplastic polymerslower

Section 3

Question Answer
Heat Deflection Temperatureused for mostly for quality control and materials development purposes
For high-performance composites, the most desirable mechanical properties of a matrix are1. High tensile modulus, which influences the compressive strength of the composite. 2. High tensile strength, which controls the intraply cracking in a composite laminate. 3. High fracture toughness, which controls ply delamination and crack growth
Disadvantages of thermoplastics(1) Limited storage life (before the final shape is molded) at room temperature. (2) Long fabrication time in the mold (where the polymerization reaction, called the curing reaction or simply called curing, is carried out to transform the liquid polymer to a solid polymer). (3) Low strain-to-failure, which also contributes to their low impact strengths
Advantages of thermoplastics(1) Unlimited storage (shelf) life at room temperature, (2) Shorter fabrication time, (3) Postformability (e.g., by thermoforming), (4) Ease of joining and repair by welding, solvent bonding, and so on. (5) Ease of handling (no tackiness). (6) Can be processed and recycled
Acceleratorssometimes added to the liquid mix to speed up a slow reaction and shorten the curing time
Advantages of epoxy matrix over other thermoset matrices:(1) Wide variety of properties, since a large number of staring materials, curing agents, and modifiers are available. (2) Absence of volatile matters during cure. (3) Low shrinkage during cure. (4) Excellent resistance to chemicals and solvents. (5) Excellent adhesion to a wide variety of fillers, fibers, and other substrates
Disadvantages of epoxy matrix over other thermoset matrices:its relatively high cost and long cure time
Promotersfacilitates reactions
Retardersslows rate of reactions