Week 2: Mechanical Properties & Viscoelasticity

Question 1

For material property requirements, what is performance based on?

Answer 1

intended use and classifications (I-III)

Question 2

What are the four concepts within safety of a medical device?

Answer 2

• withstands wear
• fatigue
• maximum loads
• structural integrity

Question 3

What are the four concepts within efficacy for a medical device?

Answer 3

• cellular proliferation
• wound healing
• drug delivery
• structural stability of tissue

Question 4

What are the four topics for composition of structure?

Answer 4

• arrangement/alignment of tissue
• crystallinity
• molecular weight
• thickness

Question 5

What does increased molecular weight cause (6)?

Answer 5

• impacts resistance
• less mobility
• absorb more energy
• chemical resistance
• thermal stability
• greater viscosity

Question 6

What is the equation for stress?

Answer 6

force over cross-sectional area

Question 7

What are the two forces for stress?

Answer 7

tension and compression

Question 8

What is the equation for tensile strain?

Answer 8

change in length over initial length

Question 9

What is the equation for lateral strain?

Answer 9

change in diameter over initial diameter

Question 10

What is the equation for poisson’s ratio?

Answer 10

negative lateral (transverse strain) over longitudinal (axis)

Question 11

What is the equation for Hooke’s Law?

Answer 11

stress equals elastic modulus times strain

Question 12

What are two properties of elastic deformation?

Answer 12

nonpermanent and reversible

Question 13

What is the equation for simple tension?

Answer 13

Force times initial length all over elastic modulus times cross-sectional area

Question 14

Materials with a large modulus of elasticity deform ____________

Answer 14

less

Question 15

What is yield strength?

Answer 15

the strength at which noticeable plastic deformation has occured.

Question 16

How do you find yield strength?

Answer 16

Using 0.2% offset

Question 17

What is tensile strength (TS)?

Answer 17

the maximum stress on the stress-strain curves

Question 18

What is ductility?

Answer 18

the amount of plastic deformation at failure

Question 19

What is the equation for percent elongation (%EL)?

Answer 19

change in length divided by initial length multiplied by 100%

Question 20

What is resilience?

Answer 20

the ability of a material to absorb energy during elastic deformation

Question 21

What is toughness?

Answer 21

the amount of energy absorbed before fracture.

Question 22

Rank the toughness of metals, reinforced polymers, and ceramics

Answer 22

ceramics - tall and short curve
reinforced polymers - short but long curve
metals - tall and long curve

Question 23

What is the equation for true stress?

Answer 23

force over instantaneous cross-sectional area

Question 24

What is the equation for true strain?

Answer 24

natural log of instantaneous length over initial length

Question 25

What is hardness?

Answer 25

measure of resistance to surface plastic deformation (dent or scratch).

Question 26

What does a large value of hardness mean?

Answer 26

high resistance to deformation from compressive loads

Question 27

What is Rockwell Hardness?

Answer 27

combination of load magnitude and indenter size

Question 28

What is the importance of design/safety factors?

Answer 28

allowances must be made to protect against unanticipated failure.

Question 29

What is the equation for working stress?

Answer 29

yield strength divided by factor of safety (N)

Question 30

What is the expected value for factor of safety

Answer 30

between 1.2 to 4. the higher the number the greater the safety

Question 31

What does "visco" mean?

Answer 31

resistance demonstrated by a fluid when it is being sheared or stressed.

Question 32

What does "elastic" mean?

Answer 32

ability to return to original shape or form once shear or stress is removed.

Question 33

What are the parameters for a viscoelastic material?

Answer 33

can exhibit elastic behavior ONLY at low temperatures, and viscous behavior at high temperatures. Performs VISCOELASTIC at temperatures between those two regions.

Question 34

What are the three tests to measure viscoelasticity?

Answer 34

1. creep 2. stress-relaxation 3. oscillatory stress

Question 35

What is creep?

Answer 35

when a sample is subjected to constant stress and deformation (strain) is measured as a function of time.

Question 36

What is stress-relaxation?

Answer 36

A material is deformed rapidly and the stress necessary to maintain deformation is measured as a function of time. (constant strain)

Question 37

What is oscillatory stress?

Answer 37

measured with rheometer which records a material's response to oscillatory stress. - rotating plate

Question 38

What is a dashpot?

Answer 38

a damper that resists motion via viscous friction (visco)

Question 39

What is a spring?

Answer 39

compresses to a certain length and returns to it's original state (elastic).

Question 40

What is the purpose of the Maxwell model?

Answer 40

it can model tissue and predict the deformation.