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Mats TF Exam 1
yardy
| Question | Answer |
|---|---|
| Glasses are often grouped with ceramics because they have similar crystal structures. | False (Glasses are not crystalline by definition) |
| The stored elastic energy per unit volume is given by 0.5σ^2/E, where σ is stress and E is the elastic modulus. | True (This also is equal to 0.5σε) |
| The density of a metal is mostly determined by the atomic weight of the atoms. | True |
| The rule of mixtures can be used to calculate the lower bound for composite stiffness for a given volume fraction of reinforcement. | False (Its for the upper bound) |
| The rule of mixtures can be used to calculate the density of a composite for a given volume fraction of reinforcement. | True |
| The stored elastic energy per unit volume is given by 0.5σE, where σ is stress and E is the elastic modulus. | False (The stored elastic energy per unit volume is given by 0.5σε) |
| The density of metals is mostly determined by the crystal structure. | False (It is mostly determined by atomic weight) |
| The density of a composite can be estimated by using the rule of mixtures. | True |
| The stress is always proportional to the strain for any material. | False (This is true only in the elastic range) |
| If a rod of an elastomer is loaded in uniaxial compression below the elastic limit, the elastic modulus of the elastomer is defined as the force applied to the material divided by its change in length. | False (If elastic modulus and applied stress are known, then strain is also known and the stored elastic energy per unit volume can be calculated) |
| Poisson's ratio is positive because generally stretching in one direction leads to contraction in the other two. | True (The Poisson ratio is defined as -εsubt/ε) |
| The elastic modulus is a property that is independent of the applied stress or strain of the material. | True |
| The elastic modulus of a foam can be estimated by using the rule of mixtures. | False (There is only one solid phase, so the rule of mixtures cannot be used. There is a separate formula for elastic modulus for foams) |
| The stored elastic energy per unit volume of a material is proportional to the elastic strain applied. | False (Elastic energy is proportional to ε^2.) |
| The elastic modulus of a composite containing continuous uniaxially aligned fibers depends on fiber orientation relative to the direction of an applied uniaxial tensile stress | True |
| Poisson’s ratio of a material is usually a negative number. | False (pretty much always positive) |
| n elastic deformation of metals, stress is proportional to strain. | True |
| The BCC unit cell has 2 atoms, and the FCC unit cell has 4 atoms. | True |
| Thermoplastic and thermoset polymers are often grouped together because they have similar crystal structures. | False (They are generally not crystalline) |
| Thermoplastic polymers deform permanently by stretching the intermolecular bonds. | False (They permanently deform by molecular sliding, not by stretching bonds) |
| Elastomers show large reversible deformations below the glass transition temperature. | False (Elastomers become stiff below the glass transition temperature) |
| The coordination number of iron atoms (BCC) is 8. | True |
| The elastic modulus of a thermoplastic polymer is directly related to the stiffness of carbon-carbon bonds in the polymer chains. | False (It is related to intermolecular bond strength) |
| Most polymers are amorphous or semicrystalline. | True |
| The density of a pure element can be determined knowing only the atomic mass and the radius of the atom. | False (The crystal structure must also be known) |
| FCC and HCP crystals have the highest packing efficiency, with 100% packing of the unit cell. | False (Packing is only 74% in close-packed structures) |
| Crosslinks in thermoset polymers are due to weak hydrogen-hydrogen bonds that can be easily removed by heating. | False (Crosslinks are strong bonds that are not easily removed) |
| There are three directions along which atoms, represented by hard spheres, touch in a close-packed plane of the crystal structure of copper. | True |
| Most polymers are crystalline solids below their glass transition temperature. | False (Most polymers are not crystalline) |
| Thermoplastic polymers are commonly amorphous. | True |
| Elastomers are lightly crosslinked to give elastic moduli that are greater than the elastic moduli of thermoplastic polymers. | False (Elastomers have the lowest moduli among true solids) |
| Cubic close-packed metals have crystal structures with higher packing factors than the packing factors of hexagonal close-packed metals. | False (They are equal) |
| Elastomers is the material class with the lowest elastic moduli of any true solids (without porosity). | True |
| The coordination number of atoms in aluminum (FCC) is 12. | True |
| Face-centered cubic metals have crystal structures with the same packing factor as hexagonal close-packed metals | True |
| The coordination number of atoms in zinc (HCP) is 12. | True |
| The FCC crystal structure contains 8 octahedral interstices per unit cell. | False (There are 4 octahedral and 8 tetrahedral interstices in the unit cell) |
| The FCC crystal structure contains 8 tetrahedral interstices per unit cell. | True |
| The coordination number of carbon in diamond is 6 | False (Its 4) |
| The coordination number for silicon in silicon carbide is 4 | True |
| Silicon has the diamond cubic structure, so there are 4 silicon atoms per unit cell. | False (Diamond cubic has 8 atoms per unit cell) |
| The coordination number for carbon in silicon carbide is 4. | True |
| The crystal structure of zirconium oxide (ZrO2) can be described using an FCC unit cell and there are 4 oxygen atoms per unit cell. | False (There are 8 oxygen atoms per unit cell, twice as many as the number of zirconium atoms) |
| There are 8 zirconium ions per unit cell in the crystal structure of ZrO2. | False (There are 4) |
| The coordination number of oxygen ions in magnesium oxide (MgO) is 4. | False (It is 6. The coordination number is this case refers to the magnesium ions around the O ion) |
Created by:
brett.barker