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Biomaterials > Exam 1 (Ch. 3-4) > Flashcards

Exam 1 (Ch. 3-4) Flashcards

1
Q

an _____ dislocation occurs when an extra portion of a plane of atoms in a crystal ends (perpendicular)

A

edge (1D)

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2
Q

the Burger’s vector does what?

A

completes the atomic circuit

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3
Q

a _______ dislocation occurs when shear forces cause a portion of a crystal to move one atomic distance relative to the other portion (parallel)

A

screw (1D)

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4
Q

Burger’s vectors are longer for (ceramics/metals). Why?

A

ceramics, because of the electroneutrality requirement - this is why they exhibit more brittle fracture

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5
Q

dislocations allow _______ deformation of a crystalline material via dislocation glide

A

plastic

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6
Q

dislocations move similarly to what creature?

A

caterpillar (one atomic distance at a time)

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7
Q

dislocation glide occurs more easily on planes with (high/low) atomic density. why?

A

high. the “steps” required to move each atomic distance are smaller, and thus require less energy

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8
Q

plastic deformation in crystals is called ______ and occurs in the _______ plane

A

slip, slip plane

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9
Q

atoms at the surface of a material have (higher/lower) energy than those inside a crystal, leading to surface free energy or surface tension

A

higher

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10
Q

the interface between grains is called the…

A

grain boundary

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11
Q

grain boundaries have (higher/lower) energy

A

higher

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12
Q

the greater the angle of misorientation between two grain boundaries, the (higher/lower) the energy

A

higher

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13
Q

_____ boundaries are formed when edge dislocations align

A

tilt (2D)

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14
Q

______ boundaries are formed when screw dislocations align

A

twist (2D)

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15
Q

_______ boundaries are formed when a mirror image of atomic placement occurs across a boundary

A

twin (2D)

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16
Q

do corrosive attacks in metals begin at the grain boundaries or at the center of the grains? why?

A

grain boundaries, they are higher in energy and want to participate in reactions

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17
Q

__________ are 3D defects formed from clusters of substitutional or interstitial impurities

A

precipitates

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18
Q

_________ are 3D defects formed from clusters of vacancies

A

voids

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19
Q

voids form what 3D defect that is sometimes controlled to alter biological response?

A

pores

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20
Q

__________ are added during implant fabrication to create pores

A

porogens [solid (NaCl or gelatin) or gaseous (N2 or CO2)]

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21
Q

pores are also sometimes formed by weaving ________ together to form a mesh containing large voids

A

fibers

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22
Q

list some advantages of porous materials

A

they allow for exchange of fluids and gases deep within the material, they encourage tissue ingrowth and implant anchoring (downside is greater rate of degradation and corrosion)

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23
Q

(atactic/isotactic/syndiotactic) polymers have a much harder time crystallizing

A

atactic - hard to form ordered structures

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24
Q

the basic unit of crystalline structure is the…

A

lamella (chain-folded model)

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25
Q

_________________ connect lamella together in amorphous regions

A

tie molecules

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26
Q

when polymers are crystallized from a molten state, ____________ form, 3D aggregates of lamellae that are analogous to grains

A

spherulites

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27
Q

(metals/ceramics/polymers) have the longest Burger’s vectors

A

polymers, because their unit cells are so large

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28
Q

what mechanism of deformation occurs in noncrystalline materials?

A

viscous flow

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29
Q

the _______________ is the temperature below which the material acts as a solid, seen in ceramics and amorphous polymers

A

glass transition temperature, Tg

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30
Q

the working point occurs at ______ P

A

10^4 P

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31
Q

higher amounts of branching in a polymer will (increase/decrease) the melting temperature

A

decrease

32
Q

higher molecular weight will (increase/decrease) the melting temperature and glass transition temperature

A

increase

33
Q

a polymer with more flexible chains will have a (higher/lower) glass transition temperature

A

lower, easier to move around energetically

34
Q

polar side groups that promote chain interactions (increase/decrease) glass transition temperature

A

increase

35
Q

cross-linking (increases/decreases) glass transition temperature

A

increases

36
Q

at the _____________ temperature, polymer chains have enough energy to move into a highly ordered state

A

crystallization temperature (Tc)

37
Q

polymers are __________ by raising the temp to the Tc, maintaining this temp for some time, and then slowly cooling

A

annealed

38
Q

DSC, a popular thermal analysis technique that provides a material’s transition temperatures, stands for what?

A

differential scanning calorimetry

39
Q

more area under the Tm peak implies (higher/lower) crystallinity

A

higher

40
Q

on a DSC plot, peaks are (exothermic/endothermic) and valleys are (exothermic/endothermic)

A

peaks are endothermic, valleys are exothermic

41
Q

large grains (less boundary area) or small grains (more boundary area) are stronger

A

small grains - the slip plane is very rough, hard for dislocation motion, HOWEVER, will degrade faster

42
Q

percent crystallinity formula

A

c(s-a)/s(c-a) x 100 (CSA-SCA)

43
Q

higher % plasticizer (increases/decreases) glass transition temperature

A

decreases

44
Q

testing samples are shaped like what for tensile testing

A

dog bones (easy for the frame to hold on to the ends)

45
Q

engineering stress (σ) is calculated by…

A

F/A (where A is initial A)

46
Q

engineering strain (ε) is calculated by…

A

ΔL/L (where L is initial L)

47
Q

testing samples are shaped like what for compressive testing

A

cylinders

48
Q

shear testing applies force where?

A

parallel to the top and bottom faces of the sample

49
Q

shear strain is calculated how?

A

tan(θ) where θ is the angle of sample deformation

50
Q

Hooke’s law

A

σ = Eε (E = modulus of elasticity, or the slope of the stress-strain curve)

51
Q

Poisson’s ratio

A

v = - εt/εa

52
Q

a high E (Young’s modulus) implies a material is very…

A

stiff

53
Q

plastic deformation begins at what point on a stress-strain curve

A

when the linear portion ends

54
Q

the yield strength occurs where on the stress-strain curve

A

the stress at the end of the elastic (linear) region

55
Q

the ultimate tensile strength occurs where on the stress-strain curve

A

at the maximum of plastic deformation

56
Q

ductility is the ability of a material to do what?

A

deform plastically before fracture

57
Q

true stress is calculated how

A

F/A (where A is A instantaneous)

58
Q

true strain is calculated how

A

ln(li/l0)

59
Q

polycrystalline materials are (stronger/weaker) compared to single-crystal materials

A

stronger due to constraints by neighboring grains (harder for dislocation motion)

60
Q

________ is plastic deformation of a sample under constant stress over time

A

creep

61
Q

________ is the decrease in stress seen when under constant strain over time (think old rubber band)

A

stress relaxation (most common in polymers)

62
Q

what’s an example of a viscoelastic material?

A

silly putty

63
Q

reminder to review Maxwell and Voigt models!

A

reminder to review Maxwell and Voigt models!

64
Q

(ductile/brittle) fracture is preferred. why?

A

ductile, because there is warning (plastic deformation)

65
Q

________, similar to cracks, occur in amorphous plastic polymers and can support a load but weaken the material

A

crazes

66
Q

define and describe stress raisers (aka stress concentrators)

A

small flaws can lead to large cracks or crazes because they amplify applied stress at the tip of the flaw

67
Q

notches, sharp corners, and pores are examples of…

A

stress raisers

68
Q

___________ is the cause of up to 90% of metallic fractures

A

fatigue

69
Q

what are the three stages of fatigue failure?

A

crack initiation –> crack propagation –> final failure

70
Q

fatigue life N = what?

A

Ni (initiation) + Np (propagation)

71
Q

__________ ________ is the stress level that will cause failure after a given number of cycles

A

fatigue strength

72
Q

failure that occurs because of the presence of cyclic stresses in combination with chemical attack is called…

A

corrosion fatigue

73
Q

what is the key principle behind strengthening biomaterials?

A

reducing the movement of dislocations

74
Q

similar to alloys for metals, ______ can be added to polymers to add crosslinks and reduce chain motion (strengthening)

A

fillers (carbon nanotubules are an example)

75
Q

describe grain growth at high temperatures

A

larger grains grow while smaller ones shrink, increasing the overall grain size and decreasing grain boundary area

76
Q

why does very rapid cooling result in a low percent crystallinity?

A

the chains need time to align and form crystals

Biomaterials (6 decks)

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