MECH 270 Theory

driving force for recovery?

reducing the overall energy of the system by removing stress filled grains and removing dislocations

Graph comparing final grain size vs cold work strain?

decrease in grain size as a function of cold work, as more cold work = more nucleation sites. Also include a gap at the beginning representing the minimum cold work needed

why do grains grow in grain growth

They grow to reduce the overall surface energy of the system by consuming strain filled grains and reducing grain boundaries

what is the difference in bonding between a brittle and plastic polymer

Brittle polymers include more covalent bonds in crosslinkage which causes their brittle behaviors as these dont allow for much flexibility.

Explain the glass transition temperature

the glass transition temperature is the point at which a polymer move from “rubbery to glassy
as the increase in temperature helps increase polymer flexibility as the atoms are more active

What is the driving force for recovery

reducing the overall energy of the system by reducing dislocation density as dislocations reorganize themselves to avoid stress fields

why is there a ductile-brittle transition for BCC but not FCC

FCC metals are abundunt with slip planes where as BCC have limited slip planes so they must be heated up to recieve enough thermal energy for slip to activate

why is there a change in tensile strength during recrystalization

During recrystalization there is an extreme reduction in dislocation density and grain boundaries both of which contribute to a decrease in UTS

What is the driving force for grain growth

reducing overall energy of the system via reducing surface energy and grain boundaries

What will increase the Glass temperature of polymers

anything that increases the stiffness of a polymer such as increasing molecular weight, adding more crosslinks or more double bonds in the backbone as well as more polar side groups

what is vulcanization

vulcanization is the process of adding crosslinks to a polymer which increases the UTS by incorporating stronger covalent bonds as well as creating a more ordered chain increasing crystallinity and E

what are atactic and isotactic polymers and which are more likely

atactic polymers have a random attachment of monomers where as in isotactic they are more ordered and patterned allowing for better mechanical properties and also being more common to crystalize because of this orderdness

under what stress and temperatures does creep occur in

high temps, low stress

why is a lathe formed component so much worse than a polished component under fatique

surface roughness leads to an increase in stress concentrations which will amplify applied stresses proving detrimental under fatigue

what does the force curve look like

basically the opposite version of energy well

would a deep or shallow energy well have a higher elastic modulus

a deep energy well means stronger bonds and thus a stronger response to deformation so it would have a higher elastic modulus

What are 3 needs for elastomers

amorphous structure, free to rotate, above Tg, moderate degree of crosslinking

Describe the UYP, LYP, region W and the top of the stress strain diagram for steel

upper yield point is when the bonds of solute are broken allowing dislocation motion

lower yield point is once solute has had time to redistrubute leading to an increase in stress

Region W is the age hardening of the material as dislocation density increases and dislocation movement becomes harder

The uts is the ultimate tensile strength where necking occurs

What does pre-deformation by drawing in polymers do and what is it

PDD is the stretching of a polymer prior to use, like warming up a muscle it creates a more uniform structure thus increasing E, UTS and decreasign ductility

state how plastic deformation occurs for a plastic polymer and how they deform above and below TG

molecules start straightening out in he non-cystaline regions before elastic deformation begins, next plastic shearing begins as the crystaline blocks break into segments and then amorphous chains.