Lecture 8: Material Properties – Solids

Question 1

What shape do gases, liquids, and solids assume?

Answer 1

gases: assumes the shape and volume of its container

liquids:assumes the shape of the part of the container it occupies

solids: retains a fixed volume

Question 2

Are gases, liquids, and solids compressible?

Answer 2

gases: yes – free space between particles

liquids: no – little free space between particles

solids: no – little free space between particles

Question 3

Do gases, liquids, and solids flow easily?

Answer 3

gases: yes

liquids: yes

solids: do not flow

Question 4

Do gases, liquids, and solids resist compression, tension, and/or shear?

Answer 4

gases: compression

liquids: compression, tension

solids: compression, tension, shear

Question 5

What is a fluid?

Answer 5

substances capable of flow – gas and liquid

Question 6

What is compression?

Answer 6

stress generated when an inward force is applied to a material, perpendicular to the surface

Question 7

What is tension?

Answer 7

stress generated when an outward force is applied to a material, perpendicular to the surface

Question 8

What is shear?

Answer 8

stress generated when a force is applied to a material, parallel to the surface/object cross-section

Question 9

What is the hierarchical scheme of solid classification?

Answer 9

(1) materials → (2) structures → (3) systems

Question 10

What is the non-hierarchical scheme of solid classification (some dichotomous ways to classify materials )?

Answer 10

composition:
- simple: accumulations of only 1 material
- composite: combinations of 2 or more simple materials

directional dependence:
- isotropic: mechanical properties are not directionally dependent
- anisotropic: mechanical properties are directionally dependent

Question 11

What are tensile mechanical behaviours?

Answer 11

capable of stretching – ie. tendons, ligaments

Question 12

What are pilant mechanical behaviours?

Answer 12

capable of bending easily

Question 13

What are rigid mechanical behaviours?

Answer 13

unable to be forced out of shape – ie. bones

Question 14

What is a universal tester?

Answer 14

used to test the tensile compressive properties of material samples

• produces stress/strain curves
• performs many other tests

Question 15

What is displacement of a Hookean material (ie. spring) directly proportional to?

Answer 15

the applied load

Question 16

Why do many biological materials show a J-shaped stress/strain curve?

Answer 16

due to the progressive recruitment of stress-bearing members

Question 17

What are on the x and y-axes of stress/strain curves?

Answer 17

x: strain
y: stress

Question 18

Do stress/strain curves show a generalized property of the material?

Answer 18

NO – shows the properties of the particular sample being measured

• thin strip of material would not need as much force to change its length compared to a thick strip – can’t actually see if there’s a difference in properties of the materials because differences are being obscured by differences in size
• if you were to generalize the properties of the material, you need to remove the effect of the sample size from your analysis – then you can say this is a property of the material, not the chunk of the material (sample) you’re dealing with

Question 19

Stress/Strain Curves

What does converting force to stress, and extension to strain do?

Answer 19

material specific (normalized) properties

• dividing force by cross-sectional area (to get stress) removes any effects of CS area
• converting extension to strain (dimensionless ratio (ie. division) of length due to stretching (delta L) to initial unsure the length (L0)) takes away any effect of length on the sample
• stress/strain curve graph tells you properties of the material itself, and has nothing to do with the piece of material

Question 20

What material properties do stress/strain curves give? (5)

Answer 20

• stiffness or modulus
• strength
• extensibility
• toughness
• resilience

Question 21

How is force related to stiffness?

Answer 21

more force needed = more stiff

Question 22

Stress/Strain Curves – Stiffness (Modulus)

What is Young’s modulus of elasticity (E)?

Answer 22

‘stiffness’ or ‘elastic modulus’ of the material under tension/compression – force needed to change its length

Question 23

Stress/Strain Curves – Stiffness (Modulus)

How do you determine stiffness from the graph?

Answer 23

slope = Young’s modulus

• steeper slope = stiffer material

Question 24

Stress/Strain Curves – Strength and Extensibility

What is tensile strength?

Answer 24

stress at failure (breaking stress)

Question 25

Stress/Strain Curves – Strength and Extensibility What is extensibility?

Answer 25

strain at failure (breaking failure)

Question 26

Stress/Strain Curves – Strength and Extensibility How do you determine tensile strength from the graph?

Answer 26

y-value (stress) at the failure point of the curve

Question 27

Stress/Strain Curves – Strength and Extensibility How do you determine extensibility from the graph?

Answer 27

x-value (strain) at the failure point of the curve

Question 28

Stress/Strain Curves – Compressive Strength When does deformation occur?

Answer 28

at the point where the graph crosses the yield point

Question 29

Stress/Strain Curves – Compressive Strength How do you determine compressive strength and compressibility from the graph?

Answer 29

same as tensile strength and extensibility

Question 30

Stress/Strain Curves – Toughness (Work of Extension) What is toughness?

Answer 30

work required to stretch a unit volume of a material to failure

Question 31

Stress/Strain Curves – Toughness (Work of Extension) How do you determine toughness from the graph?

Answer 31

integrated area under stress/strain curve = work per volume absorbed as the material extends

Question 32

Stress/Strain Curves – Resilience How do you determine resilience from the graph?

Answer 32

work of contraction / work of extension the larger the difference between the two areas (work of extension and work of contraction), the less resilient the material is – and more of the energy that is put in will be dissipated from the material as heat

Question 33

What is stress?

Answer 33

force/cross-sectional area (pressure)

Question 34

What is strain?

Answer 34

measure of energy recovered from elastic storage – dimensionless value expressed as % energy recovered

Question 35

Stress/Strain Curves – Resilience Is work of contraction or work of extension greater?

Answer 35

work of contraction is always going to be slightly less than the work of extension ∴ resilience only ever approaches 100%

Question 36

Why are capture threads of spider silk (viscoelastic) low resilience?

Answer 36

because it needs to absorb the energy from the impact of a flying insect – thread is effective in absorbing energy of the insect’s impact, and not re-releasing it and catapulting the insect away - insect collides with the thread and imparts a force on it, which causes thread to strain, and changes the length of the thread - bug slows down, and work of contraction < work of extension - this is useful for the thread because do NOT want thread that captures energy well and also releases it well (would catapult the insect out of the thread net)

Question 37

Why are catgut (tennis racket strings) high resilience?

Answer 37

energy needs to be absorbed and released

Question 38

What is shear stress?

Answer 38

occurs when a force is applied parallel to the surface of an object not causing it to lengthen or compact, but tilting it over

Question 39

What is shear strain?

Answer 39

the deformation – ratio of height to how far it is tilted

Question 40

What is shear modulus?

Answer 40

stiffness of the object under shear