Module 4b: Time Dependent Mechanical Properties of Biomaterials

Question 1

GOODNOTES -> complete the practice problem on slide 3

Answer 1

completed

Question 2

T/F: The fatigue limit is the stress below which a material will never fail, even after infinite cycles, and it exists for all materials

Answer 2

This is false; many materials will not last infinite cycles

Question 3

GOODNOTES -> answer iClicker #2 on slide 7, and explain why that answer is correct

Answer 3

Since the line never touches the fatigue limit (dashed line) and demonstrates asymptotic behavior, the specimen won’t fracture under a certain load, regardless of the number of cycles it experiences

Question 4

GOODNOTES -> complete the practice problem on slide 8

Answer 4

completed

Question 5

GOODNOTES -> look over the bending test formulas on slide 9

Answer 5

completed

Question 6

GOODNOTES -> complete the practice problem on slide 10

Answer 6

completed

Question 7

Stress relaxation is most often found in what type of materials

Answer 7

Polymers

Question 8

What is creep?

Answer 8

Plastic deformation of a sample under constant load (stress) over time

Question 9

What symbol is used to represent stress?

Answer 9

Sigma (σ)

Question 10

What symbol is used for strain?

Answer 10

Epsilon (ε)

Question 11

What materials is creep observed in?

Answer 11

ALL materials, but is only a concern at temps above 0.4*Tm for metals (for some polymers, around RT)

Question 12

Describe the characteristics of primary creep

Answer 12

Increase in strain with time

Question 13

Describe the characteristics of secondary creep?

Answer 13

Linear relationship between creep strain and time, longest phase

Question 14

Describe the characteristics of tertiary creep

Answer 14

Elongation until failure, gross defects appear inside material

Question 15

GOODNOTES -> draw and label the graphs for creep on slide 14

Answer 15

completed

Question 16

What is creep in metals related to?

Answer 16

Creep in metals is related to defects (specifically dislocation climb, grain boundary sliding, and voids & cracks)

Question 17

Describe creep in polymers

(Susceptibility to creep ______ with increasing crystallinity bc of fewer amorphous regions)

Creep ONLY @ T>Tg

Answer 17

Less distinct stages, results in movement of amorphous regions via viscous flow

Susceptibility to creep decreases with increasing crystallinity

Question 18

What is stress relaxation?

Answer 18

Decrease in stress observed over time under constant strain

Question 19

What materials is stress relaxation mostly found?

Answer 19

Most often found in polymeric materials (think rubber band around stack of papers relaxing over time)

Question 20

Relaxation requires molecular motion, which mostly occurs in the ________ regions of polymer structures

Answer 20

Amorphous

Question 21

How does more crystalline polymers affect relaxation? Why?

Answer 21

More crystalline polymers result in less relaxation because the structure is more rigid and there’s less movement

Question 22

Stress relaxation only occurs in polymers when the temperature is above Tg. What is Tg?

Answer 22

Glass transition temperature

Question 23

GOODNOTES -> draw and label the stress relaxation graphs on slide 16

Answer 23

completed

Question 24

What properties are involved in viscoelastic state? Tg < T < Tm

Answer 24

Both viscous and elastic properties

Question 25

GOODNOTES -> draw the heat flow vs. temp graph on slide 18

Answer 25

completed

Question 26

GOODNOTES -> based on the graphs on slide 19, demonstrate how viscoelastic materials retain some behaviors of elastic and viscous materials

Answer 26

completed

Question 27

A common example of viscoelastic material is Silly Putty; at high strain rates, the _____ component dominates, while at low strain rates, the _____ component dominates

Answer 27

High strain rate -> elastic dominates Low strain rate -> viscous dominates

Question 28

Elastic deformation and viscous deformation are modeled by what laws?

Answer 28

Elastic deformation is modeled by Hooke's Law (spring) while Viscous deformation is modeled by Newton's Law of Viscosity (dashpots)

Question 29

What is Newton's Law of Viscosity?

Answer 29

The applied shear stress is directly proportional to the rate of viscous deformation

Question 30

Model that demonstrates a spring and dashpot in series with applied stress

Answer 30

Maxwell model

Question 31

Model that demonstrates a spring and dashpot in parallel with applied stress

Answer 31

Kevin-Voigt model

Question 32

GOODNOTES -> using slide 23, draw examples of Maxwell and Kevin-Voigt models

Answer 32

completed

Question 33

GOODNOTES -> using slide 27, derive the equation for the Maxwell model (NEED TO KNOW FOR EXAM)

Answer 33

completed

Question 34

Maxwell Model is good for modeling?

Answer 34

Stress relaxation

Question 35

Maxwell model does NOT accurately model?

Answer 35

Creep

Question 36

GOODNOTES -> Using slide 30, derive the equation for the Kevin-Voigt model

Answer 36

completed

Question 37

Kevin Voigt model is good for modeling?

Answer 37

Creep

Question 38

Kevin Voigt model does NOT accurately model?

Answer 38

Stress relaxation

Question 39

GOODNOTES -> MAKE SURE YOU CAN DERIVE THE EQUATIONS LISTED ON SLIDE 33

Answer 39

completed

Question 40

What is the main difference between Maxwell and Voigt model in terms of creep?

Answer 40

Maxwell model is not useful for predicting creep while Voigt model is!

Question 41

GOODNOTES -> complete the practice problem on slide 35

Answer 41

completed

Question 42

What model has equal stress for the spring and dashpot

Answer 42

Maxwell

Question 43

At which temperature are polymers more likely to have viscoelastic properties?

Answer 43

Tg < T < Tm

Question 44

What is the simplest viscoelastic model that captures both creep and stress relaxation?

Answer 44

Standard Linear Solid (SLS) model