MEEN 361 Final Study Set
Correlation between hardness & tensile strength?
Empirical relationship. Tensile strength can have an empirical relationship to hardness of steel.
What is the difference between engineering stress and true stress in a tensile test?
Engineering stress divides the load (force) on the specimen by the original area; while true stress divides the load by the instantaneous area which decreases as the specimen stretches.
Give some examples of commercial products which are laminar composite structures.
Examples given in Table 9.2 are: automotive tires, honeycomb sandwich structures, fiber reinforced polymer structures such as boat hulls, plywood, printed circuit boards, snow skis made from fiber reinforced polymers, and windshield glass.
What is the most common fiber material in fiber-reinforced plastics?
E-glass.
Why do you temper?
Introduce ductility back into the part.
What are the three types of static stresses which materials are subjected?
tensile, compressive, shear
Which of the following is the correct definition of ultimate tensile strength, as derived from the results of a tensile test on a metal specimen? (a) the stress encountered when the stress-strain curve transforms from elastic to plastic behavior (b) the maximum load divided by the final area of the specimen (c) the maximum load divided by the original area of the specimen (d) the stress observed when the specimen finally fails
(c)
Which of the following metals is used as the matrix metal in nearly all WC cemented carbides? (a) aluminum (b) chromium (c) cobalt (d) lead (e) nickel (f) tungsten (g) tungsten carbide
(c)
Which of the following polymer types are most commonly used in polymer matrix composites? (a) elastomers (b) thermoplastics (c) thermosets
(c)
Which one of the following types of stress strain relationship best describes the behavior of brittle materials such as ceramics and thermosetting plastics: (a) elastic and perfectly plastic (b) elastic and strain hardening (c) perfectly elastic (d) none of the above
(c)
Which one of the three polymer types is not normally considered to be a plastic? (a) thermoplastics, (b) thermosets, or (c) elastomers.
(c)
Wood is which one of the following composite types? (a) CMC (b) MMC (c) PMC
(c)
Which answers complete the following sentence correctly (more than one): As the temperature of an amorphous thermoplastic polymer is gradually reduced, the glass transition temperature Tg is indicated when (a) the polymer transforms to a crystalline structure, (b) the coefficient of thermal expansion increases markedly, (c) the slope of specific volume versus temperature changes markedly, (d) the polymer becomes stiff, strong, and elastic, or (e) the polymer solidifies from the molten state.
(c) and (d).
Ceramic matrix composites are designed to overcome which of the following weaknesses of ceramics (more than one)? (a) compressive strength (b) hardness (c) hot hardness (d) modulus of elasticity (e) tensile strength (f) toughness.
(e) and (f)
Which one of the following is the most common reinforcing material in FRPs? (a) Al2O3 (b) boron (c) carbon (d) cobalt (e) graphite (f) Kevlar 49 (g) SiO2
(g)
What is the defining characteristic of a Newtonian fluid?
A Newtonian fluid is one for which viscosity is a constant property at a given temperature. Most liquids (waters, oils) are Newtonian fluids.
What is a cermet?
A cermet is a composite material consisting of a ceramic and a metal. In the text, it is defined as a composite consisting of ceramic grains imbedded in a metallic matrix.
What is a composite material?
A composite material is a materials system consisting of two or more distinct phases whose combination results in properties that differ from those of its constituents.
What is a copolymer?
A copolymer is a polymer made up of two different types of mers, such as ethylene and propylene.
Describe the difference in mechanical properties as a function of temperature between a highly crystalline thermoplastic and an amorphous thermoplastic.
A highly crystalline TP retains rigidity during heating until just before its Tm is reached. An amorphous TP shows a significant drop in deformation resistance at its Tg as temperature is raised; it becomes increasingly like a liquid as temperature continues to increase.
What is a hybrid composite ?
A hybrid composite is a fiber-reinforced PMC in which two or more fibers materials are combined in the FRP.
What is a plasticizer?
A plasticizer is a chemical added to the polymer to make it softer and more flexible. It is often added to improve the polymer's flow characteristics for shaping.
What is a polymer?
A polymer is a compound that consists of long-chain molecules. The molecules consist of repeating units, called mers, connected end to end.
What is a terpolymer?
A terpolymer is a polymer with three different mer types. An example is ABS (acrylonitrile-butadiene-styrene) plastic.
Tensile testing is not appropriate for hard brittle materials such as ceramics. What is the test commonly used to determine the strength properties of such materials?
A three-point bending test is commonly used to test the strength of brittle materials. The test provides a measure called the transverse rupture strength for these materials.
What is a whisker?
A whisker is a thin, hairlike crystal of very high strength.
What does the term advanced composites mean?
An advanced composite is a PMC in which carbon, Kevlar, or boron fibers are used as the reinforcing material.
What happens to an elastomer when it is below its glass transition temperature?
An elastomer is hard and brittle below its Tg.
What does the term anisotropic mean?
Anisotropic means that the properties of a material vary depending on the direction in which they are measured.
Rank metal from lowest to highest hardness.
Annealed, Quenched, Tempered, As Received
Which microstructures are formed when tempering steel?
Bainite & Pearlite
What is difference between a microstructure or phase morphology? Is Bainite a microstructure or phase morphology.
Bainite is a morphology. However, you can have two samples with different microstructures.
What is the complicating factor that occurs in a compression test?
Barreling of the test specimen due to friction at the interfaces with the testing machine platens.
Why cannot a direct conversion be made between the ductility measures of elongation and reduction in the area using the assumption of constant volume?
Because of necking that occurs in the test specimen.
What is the chemical formula of ethylene, the monomer for polyethylene?
C2H4
Correlation between wear, fracture toughness and ductility?
Case hardening hardens the surface and prevents cracks from growing.
What is unique about the polymer cellulose?
Cellulose is a polymer that grows in nature. Wood fiber contains about 50% cellulose and cotton fiber is about 95% cellulose.
What is cross-linking in a polymer and what is its significance?
Cross-linking is the formation of connections between the long-chain molecules in a polymer. It causes the polymer structure to be permanently altered. If the amount of cross-linking is low, the polymer is transformed into an elastomer; if cross-linking is significant, the polymer is transformed into a thermosetting polymer.
How are a polymer's properties affected when it takes on a crystalline structure?
Density, stiffness, and melting temperature increase.
Why are different hardness tests and scales required?
Different hardness tests and scales are required because different materials possess widely different hardnesses. A test whose measuring range is suited to very hard materials is not sensitive for testing very soft materials.
Elastomers and thermosetting polymers are both cross- linked. Why are their properties so different?
Elastomers are lightly cross-linked, whereas thermosets are highly cross-linked. Light cross-linking allows extensibility; a highly cross-linked structure makes the polymer rigid.
Name some of the important applications of FRPs.
FRPs are used in modern aircraft as skin parts, automobile body panels, printed circuit boards, tennis rackets, boat hulls, and a variety of other items.
What are some of the factors that influence a polymer's tendency to crystallize?
Factors are: (1) only linear polymers can form crystals; (2) copolymers do not form crystals; (3) stereoregularity - isotactic polymers always form crystals, atactic polymers never form crystals, and syndiotactic polymers sometimes form crystals; (4) slow cooling from the molten states promotes crystal formation; (5) plasticizers inhibit crystal formation; and (6) stretching the polymer tends to promote crystallization.
Why are fillers added to a polymer?
Fillers are added to increase strength or simply to reduce the cost of the polymer.
How is the shear modulus of elasticity G related to the tensile modulus of elasticity E, on average?
G = 0.4E
What is hardness and how is it generally tested?
Hardness is defined as the resistance to indentation of a material. Its is tested by pressing a hard object (sphere, diamond point) into the test material and measuring the size (depth, area) of the indentation.
State Hooke's Law
Hooke's Law defines the stress-strain relationship for an elastic material: sigma = E*epsilon; where E is the modulus of elasticity.
How does the change in cross-sectional area of a test specimen in a compression test differ from its counterpart in a tensile test specimen?
In a compression test, the specimen cross-sectional area increases as the test progresses; while in a tensile test, the cross-sectional area decreases.
How do the properties of polymers compare with those of metals?
In general, polymers have lower strength, hardness, stiffness, density, and temperature resistance compared to metals. In addition, polymers are low in electrical and thermal conductivity.
What is the basic difference between low density and high density polyethylene?
LDPE has a branched structure and is amorphous. HDPE is linear and highly crystalline. These differences account for HDPE higher density, stiffness, and melting point.
Name the three basic categories of composite materials.
Metal matrix composites (MMCs), ceramic matrix composites (CMCs), and polymer matrix composites (PMCs).
Are mechanical properties independent from microstructure?
NO! Mechanical properties are directly related to the microstructure of the material. Example: Same steel but different amount of coldworking (Higher tensile strength for coldworked material.)
Does any polymer ever become 100% crystalline?
No.
Is heat treating reversible?
Once you start forming Pearlite, Bainite or Martensite, you are committed to the process.
In addition to fillers and plasticizers, what are some other additives used with polymers?
Other additives include: lubricants - to reduce friction and improve flow; flame retardents; colorants; cross-linking agents, antioxidants, and ultraviolet light absorbers.
The nylons are members of which polymer group?
Polyamides.
What is the primary polymer ingredient in natural rubber?
Polyisoprene.
Identify some of the important properties of fiber- reinforced plastic composite materials.
Properties include: high strength-to-weight ratio, high modulus-to-weight ratio, low density, good fatigue strength, good corrosion resistance, and low thermal expansion for many FRPs.
How is the shear strength S related to tensile strength TS, on average?
S = 0.7TS
What is work hardening?
Strain hardening is the increase in strength that occurs in metals when they are strained.
How are thermoplastic elastomers different from conventional rubbers?
TPEs are different in two basic ways: (1) they exhibit thermoplastic properties, and (2) their extensibility derives from physical connections between different phases in the polymer.
Define the term tacticity as it applies to polymers.
Tacticity refers to the way the atoms or atom groups replacing H atoms in the molecule are arranged.
What does the degree of polymerization indicate?
The degree of polymerization indicates the average number of mers or repeating units in the polymer molecule.
What are the common forms of the reinforcing phase in composite materials?
The forms are: (1) fibers, (2) particles and flakes, and (3) an infiltrated phase in skeletal structures.
What is meant by the term interface in the context of composite materials?
The interface is the boundary between the component phases in a composite material.
Define the recrystallization temperature for a metal.
The recrystallization temperature is the temperature at which a metal recrystallizes (forms new grains) rather than work hardens when deformed.
What is the rule of mixtures?
The rule of mixtures applies to certain properties of composite materials; it states that the property value is a weighted average of the property values of the components, the weighting being by proportions of the components in the composite.
Define tensile strength of a material.
The tensile strength is the maximum load experienced during the tensile test divided by the original area.
Cross-linking (curing) of thermosetting plastics is accomplished by one of three ways. Name the three ways.
The three ways are: (1) temperature-activated systems, in which elevated temperatures accomplish curing; (2) catalyst-activated systems, in which small amounts of a catalyst cause cross-linking; and (3) mixing-activated systems, in which two reactive components are mixed and curing occurs by their chemical reaction.
What are the two forms of sandwich structure among laminar composite structures? Briefly describe each.
The two forms are: (1) foamed-core sandwich, in which the core is polymer foam between two solid skins; and (2) honeycomb, in which the core is a honeycomb structure sandwiched between two solid skins.
What are the two methods by which polymerization occurs? Briefly describe the two methods.
The two types of polymerization are: (1) addition or chain polymerization and (2) step polymerization, also known as condensation polymerization.
Define yield strength of a material.
The yield strength is the stress at which the material begins to plastically deform. It is usually measured as the 0.2% offset value - the point at which the stress-strain for the material intersects a line which is offset from the elastic region of the stress-strain curve by 0.2%.
The arrangement of repeating units in a copolymer can vary. What are some of the possible arrangements?
There are four possible arrangements of the mers along the chain: (1) alternating, (2) random, (3) block, and (4) graft.
How do the properties of thermosetting polymers differ from those of thermoplastics?
Thermosets are more rigid, brittle, capable of higher service temperatures, and cannot be remelted.
What are the three general factors that determine the properties of a composite material?
Three factors are given in the text: (1) the component materials; (2) the geometric shapes of the constituents - the reinforcing phase in particular - and the resulting structure of the material; and (3) the interaction of the phases.
How can you achieve different microstructures in the same morphology?
Time and temperature are the deciding factors (Rate of cooling). Fast quenching produces smaller grains.
What is the dilemma between design and manufacturing in terms of mechanical properties?
To achieve design function and quality, the material must be strong; for ease of manufacturing, the material should not be strong, in general.
How are traditional composites distinguished from synthetic composites?
Traditional composites have been used for decades or centuries; some of them are obtained from sources in nature, such as wood. Synthetic composites are manufactured.
Identify some of the characteristic properties of composite materials.
Typical properties include: (1) high strength-to- weight and stiffness-to-weight ratios; (2) good fatigue properties and toughness; (3) anisotropic properties in many cases; and (4) other properties and features that are difficult or impossible to obtain with metals, ceramics, or polymers alone.
What is viscoelasticity, as a material property?
Viscoelasticity refers to the property most commonly exhibited by polymers that defines the strain of the material as a function of stress and temperature over time. It is a combination of viscosity and elasticity.
Define viscosity of a fluid.
Viscosity is the resistance to flow of a fluid material; the thicker the fluid, the greater the viscosity.
What are some of the weaknesses of ceramics that might be corrected in fiber-reinforced ceramic matrix composites?
Weaknesses of ceramics include: low tensile strength, poor toughness, and susceptibility to thermal cracking.
In what case does the strength coefficient have the same value as the yield strength?
When the material does not strain harden.
Cemented carbides are what class of composites?
Yes; although the cemented carbide industry does not generally think of cemented carbides as cermets, they fit within the definition.
Strengthening mechanisms in metals:
grain size control, alloying (solid solution hardening), precipitation hardening, work hardening
For ceramics, the ductile-to-brittle transition occurs at much _________ temperatures than for metals.
higher
Alloying usually ______ the ductile-to-brittle transition temperature.
increases
As the percentage of carbon increases, the hardenability in steel ________; cobalt is the only known to _____ hardenability.
increases; decrease
Fine-grained steels have ________ hardenability.
lower
The three stages of the annealing process are:
recovery, recrystallization, and grain growth
Presence of impurities tends to ___________ the hardenability.
reduce
What are the three basic categories of polymers?
thermoplastics, thermosetting polymers, and elastomers
Which one of the following types of stress strain relationship best describes the behavior of most metals at room temperature: (a) elastic and perfectly plastic (b) elastic and strain hardening (c) perfectly elastic (d) none of the above
(b)
Anisotropic means which one of the following: (a) composite materials with composition consisting of more than two materials (b) properties are the same in every direction (c) properties vary depending on the direction in which they are measured (d) strength and other properties as a function of curing temperature
(c)
The plastic region of the stress-strain curve for a metal is characterized by a proportional relationship between stress and strain: (a) true (b) false
(b) It is the elastic region that is characterized by a proportional relationship between stress and strain. The plastic region is characterized by a power function - the flow curve.
As the degree of crystallinity in a given polymer increases, the polymer becomes denser and stiffer, and its melting temperature decreases: (a) true or (b) false.
(b) Melting temperature increases with higher degree of crystallinity.
Viscosity can be defined as the ease with which a fluid flows: (a) true or (b) false.
(b) Viscosity is the resistance to flow.
Which of the following plastics has the highest market share? (a) phenolics (b) polyethylene (c) polypropylene (d) polystyrene (e) polyvinylchloride
(b)
Which one of the following materials has the highest modulus of elasticity? (a) aluminum (b) diamond (c) steel (d) titanium (e) tungsten
(b)
A branched molecular structure is stronger in the solid state and more viscous in the molten state than a linear structure for the same polymer: (a) true or (b) false.
(a)
Degree of polymerization is which one of the following? (a) average number of mers in the molecule chain (b) proportion of the monomer that has been polymerized (c) sum of the molecule weights of the mers in the molecule (d) none of the above
(a)
If strain measurements were made during a tensile test, which of the following would have the higher value? (a) engineering stain (b) true strain
(a)
Most hardness tests involve pressing a hard object into the surface of a test specimen and measuring the indentation (or its effect) that results: (a) true or (b) false.
(a)
Of the three polymer types, which one is the most important commercially? (a) thermoplastics, (b) thermosets, or (c) elastomers.
(a)
Polystyrene (without plasticizers) is amorphous, transparent, and brittle: (a) true or (b) false.
(a)
The basic difference between low density polyethylene and high density polyethylene is that the latter has a much higher degree of crystallinity: (a) true or (b) false.
(a)
Which of the following types of stress strain relationship best describes the behavior of metals at temperatures above their respective recrystallization points: (a) elastic and perfectly plastic (b) elastic and strain hardening (c) perfectly elastic (d) none of the above
(a)
Which one of the following materials has the highest hardness? (a) alumina ceramic (b) gray cast iron (c) hardened tool steel (d) high carbon steel (e) polystyrene
(a)
Which one of the following reinforcing geometries offers the greatest potential for strength and stiffness improvement in the resulting composite material? (a) fibers (b) flakes (c) particles (d) infiltrated phase
(a)
Which one of the three polymer types does not involve cross- linking? (a) thermoplastics, (b) thermosets, or (c) elastomers.
(a)
Which of the following materials are used as fibers in fiber-reinforced plastics (more than one): (a) aluminum oxide (b) boron (c) carbon/graphite (d) epoxy (e) Kevlar 49 (f) S-glass (g) unsaturated polyester
(a), (b), (c), (e), and (f)
Identify which of the following materials are composites (more than one)? (a) cemented carbide (b) phenolic molding compound (c) plywood (d) Portland cement (e) rubber in automobile tires (f) wood (g) 1020 steel
(a), (b), (c), (e), and (f).
Which of the following metals are most commonly used as the matrix material in fiber-reinforced MMCs (name three)? (a) aluminum (b) copper (c) iron (d) magnesium (e) titanium (f) zinc.
(a), (d), and (e).
A copolymer is a mixture consisting of macromolecules of two different homopolymers: (a) true or (b) false.
(b)
As temperature of a polymer increases, its density (a) increases, (b) decreases, or (c) remains fairly constant.
(b)
If stress values were measured during a tensile test, which of the following would have the higher value? (a) engineering stress (b) true stress.
(b)
The reinforcing phase is the matrix within which the secondary phase is imbedded: (a) true (b) false
(b)
The shear strength of a metal is usually (a) greater than, or (b) less than its tensile strength.
(b)
Which of the following is the chemical formula for the repeating unit in polyethylene? (a) CH2 (b) C2H4 (c) C3H6 (d) C5H8 (e) C8H8
(b)