mm Flashcards
(15 cards)
Make comparisons of thermoplastic and thermosetting polymers (a) on the basis of mechanical
characteristics upon heating, and (b) according to possible molecular structures.
Solution (a) Thermoplastic polymers soften when heated and harden when cooled, whereas thermosetting
polymers, harden upon heating, while further heating will not lead to softening. (b) Thermoplastic
polymers have linear and branched structures, while for thermosetting polymers, the structures will normally be
network or crosslinked.
13.2 D1
Some modern kitchen cookware is made of ceramic materials.
(a)
List at least three important characteristics required of a material to be used for this application.
(b)
Compare the relative properties and cost of three ceramic materials.
(a)
List at least three important characteristics required of a material to be used for this application.
must have a high resistance to thermal shock, it must be nontoxic, it must be relatively strong and tough
(b)
Compare the relative properties and cost of three ceramic materials.
soda-lime glass, a borosilicate (Pyrex) glass, and a glass ceramic
(c)
For each of edge, screw, and mixed dislocations, cite the relationship between the direction of the applied shear stress and the direction of dislocation line motion.
edge dislocation—parallel
screw dislocation—perpendicular
mixed dislocation–neither parallel nor perpendicular
a) Define a slip system.
(b) Do all metals have the same slip system? Why or why not?
(a) A slip system is a crystallographic plane, and, within that plane, a direction along which dislocation motion (or slip) occurs.
(b) All metals do not have the same slip system because the plane and direction will vary from crystal structure to crystal structure.
7.21 Briefly explain why small-angle grain boundaries are not as effective in interfering with the slip process as are high-angle grain boundaries.
because of the small crystallographic misalignment across the boundary.
Approximately 90% of all metal failures
3 types:
– Reversed stress cycle – sinusoidal over time between tension (σmax) and compression (σmin)
– Repeated stress cycle - σmax and σmin are not equidistant apart from σ = 0
– Random
Discuss various factors that can affect fatigue life of a component.
1- Mean Stress
2- Surface effects
3- sharp corners
Solubility limit
maximum concentration of solute atoms that can dissolve in a solvent to form a solid solution
Name the two stages involved in the formation of particles of a new phase. Briefly describe each.
The two stages involved in the formation of particles of a new phase are nucleation and growth. The nucleation process involves the formation of very small particles of the new phase(s). The growth stage is simply the increase in size of the new phase particles.
a) Briefly describe the phenomena of superheating and supercooling.
(b) Why do these phenomena occur?
Answer
(a) Superheating and supercooling correspond, respectively, to heating or cooling above or below a phase transition temperature without the occurrence of the transformation.
(b) These phenomena occur because right at the phase transition temperature, the driving force is not sufficient to cause the transformation to occur. The driving force is enhanced during superheating or supercooling.
Cite advantages and disadvantages of hot working and cold working.
cold working hot working surface finish High quality Poor mechanical properties May be varied Limited deformations Large deformations are possible Possible but expensive Deformation energy High low
For a ceramic compound, what are the two characteristics of the component ions that determine the crystal structure
(1) the magnitude of the electrical charge on each ion, and (2) the relative sizes of the cations and anions.
Cite one reason why ceramic materials are, in general, harder yet more brittle than metals.
Crystalline ceramics are harder yet more brittle than metals because they (ceramics) have fewer
slip systems, and, therefore, dislocation motion is highly restricted.
15.11
For thermoplastic polymers, cite five factors that favor brittle fracture. Solution For thermoplastic polymers,
five factors that favor brittle fracture are as follows:
: (1) a reduction in temperature, (2) an increase
in strain rate, (3) the presence of a sharp notch, (4) increased specimen thickness, and (5) modifications of
the polymer structure.
(a) List four reasons why glass fibers are most commonly used for reinforcement.
It is easily drawn into high-strength fibers from the molten state.
It is readily available and may be fabricated into a glass-reinforced plastic
economically using a wide variety of composite-manufacturing techniques.
As a fiber it is relatively strong, and when embedded in a plastic matrix, it produces a composite having a very high specific strength.
