CME 260 QUESTIONS WITH COMPLETE SOLUTIONS

Explain why MgO can act as a network modifier in a silicate glass CORRECT ANSWERS: MgO has larger cations than silica, so it has higher CN and breaks the links between Si04 groups in a network should you put test bars in the test fixture with ground side up or down? Explain CORRECT ANSWERS: Face bars ground side up, facing single loading point. That side is in compression, so the flaws that were produced during grinding wont affect the failure which will initiate from bottom of bar (which is in tension) if a service load is less that the maximum load allowed for a ceramic, can we be completely confident that failure will not occur? CORRECT ANSWERS: we can't be completely confident because the mechanical properties of ceramics have very large statistical variation. specifically, the standard deviation in large and it is possible another coupon could fail lower that max load Why are thermosetting polymers not usually recycled? CORRECT ANSWERS: they are hardened due to polymerization and can't be remelted, so they cant be easily formed into new products. What is the difference between a thermosetting polymer and a thermoplastic polymer? CORRECT ANSWERS: Thermosetting- connot be remelted, formed during polymerization process thermoplastic- can be remelted and is formed by injection molding what is the meaning of cis and trans symmetry in polymers, and why are polymers with cis symmetry more likely to be elastomers? CORRECT ANSWERS: cis and trans apply to polymers that have double bonds in mer. cis-side groups on same side of chain & double bond, tending to coil up with no tensile stress applied trans- have side groups at alternating sides of the double bond If a polymer is an elastomer, what does this imply about its mechanical properties? CORRECT ANSWERS: they have high capacity for elastic deformation because the molecules coil up after tensile straightening. if one polymer material has more crystallinity than the other, how will the physical & mechanical properties of the two differ? CORRECT ANSWERS: more crystalline= higher density and higher strength if one polymer material has a branched structure and the other has a linear structure, which is more likely to show substantial crytallinity? CORRECT ANSWERS: the