Deformable body Mechanics Unit 1: Mechanical properties of materials and structures (class)

Question 1

What is a structure?

Answer 1

an arrangement of one or more materials in a way that is designed to sustain loads

Question 2

What is a material?

Answer 2

a type of matter used to construct structures

Question 3

Are mechanical properties independent of material or structure?

Answer 3

structure

Question 4

Which symbol represents stress?

Answer 4

sigma σ

Question 5

Which symbol represents strain?

Answer 5

epsilon ε

Question 6

Which symbol represents the coefficient of velocity?

Answer 6

eta η

Question 7

What is deformation?

Answer 7

change in shape or size of a structure or any part of it

Question 8

How do we calculate stress?

Answer 8

force (N) / area (m2)

Question 9

What units do we use for stress?

Answer 9

pascal = N/m2

Question 10

How can stress tell us the strength of a material?

Answer 10

it is the same regardless of size or shape of material

a bar with twice the cross-sectional area will require twice to break it but the stress within them will be the same

Question 11

What is strain and how do we calculate it?

Answer 11

a measure of deformation

change in length/ original length

Question 12

What are axial loads?

Answer 12

a load applied a geometric axis of a structure producing pure tension or compression

Question 13

What do stress-strain curves show?

Answer 13

how a material deforms as it is loaded

Question 14

What is the proportional limit in a stress-strain curve?

Answer 14

the point at which the straight line finishes and stress stops being directly proportional to strain

Question 15

What is the elastic limit in a stress-strain curve?

Answer 15

the greatest stress that may be applied to a material without causing any permanent deformation

Question 16

describe a material that exhibits elastic behaviour

Answer 16

it instantly deforms under a load and returns immediately to its original size and shape eg rubber

Question 17

describe a material that exhibits plastic behaviour

Answer 17

it deforms instantly under a load and retains this new shape/size when the load is removed eg putty

Question 18

In structures which region of the stress-strain curve must the material remain within?

Answer 18

elastic region as a material that is plastic is of no use because it will deform greatly under a small increase in load

Question 19

What happens beyond the yield point?

Answer 19

the material will undergo considerable elongation without an increase in stress ie. just maintaining the load will cause continual deformation

Question 20

What type of behaviour is in the region following the yield point?

Answer 20

plastic

Question 21

What happens after the ultimate strength of the material is reached?

Answer 21

stretching occurs with a reduction in stress

results in a reduced cross-sectional area called necking

Question 22

What do we call

a) a material that can sustain only limited strain before breaking?
b) a material that deforms plastically before breaking?

Answer 22

a) brittle

b) ductile

Question 23

What is Hooke’s law?

Answer 23

up to the proportional limit, the strain is proportional to the applied stress

Question 24

What is Young’e modulus a measure of and how do you calculate it?

Answer 24

the stiffness of a material

stress/strain

Question 25

What symbol represents young's modulus?

Answer 25

E

Question 26

What is rigidity?

Answer 26

the ability to resist axial deformation

Question 27

How do we calculate rigidity?

Answer 27

young's modulus x cross-sectional area

Question 28

What are the units of rigidity?

Answer 28

N

Question 29

What is stiffness?

Answer 29

the force required to produce a unit deflection

Question 30

How do we calculate stiffness? give 2 ways

Answer 30

applied force / change in length young's modulus x cross sectional area / original length

Question 31

What is flexibility?

Answer 31

the deflection under a unit load

Question 32

How do we calculate flexibility and what units do we use?

Answer 32

1/stiffness | m/ N

Question 33

Is strength determined by Young's modulus?

Answer 33

no

Question 34

Describe viscous behaviour

Answer 34

material does not deform instantly when a load is applied. The strain is prolonged. once the load is removed the material will not return to its original size and shape. this means that no energy is stored in the material and all the energy is required to deform is dissipated as heat.

Question 35

Give 3 examples of materials that exhibit viscous behaviour

Answer 35

water blood plasma air

Question 36

What are stresses arising in viscous materials dependent on?

Answer 36

strain rate

Question 37

What is the coefficient of viscosity equal to?

Answer 37

η = stress/ strain rate

Question 38

How do we represent strain rate?

Answer 38

epsilon dot

Question 39

What is the strain rate?

Answer 39

change in strain / change in time

Question 40

what are the units for the coefficient of viscosity?

Answer 40

Nm-2.s / Pa.s

Question 41

Describe viscoelastic behaviour

Answer 41

viscous in that it responds to the rate of loading and elastic in that it returns to its original size and shape after the load is removed

Question 42

Give 2 examples of materials that show viscoelastic behaviour

Answer 42

articular cartilage | cortical bone

Question 43

What is creep?

Answer 43

developing strain over time when a constant load is applied

Question 44

Name some examples of creep

Answer 44

soft metals eg lead wood concrete (noticeable after years)

Question 45

What is stress relaxation?

Answer 45

in a material under constant strain the stress diminishes over time

Question 46

what is strain hardening?

Answer 46

material undergoes changes in its atomic and crystalline structure resulting in an increased resistance to further deformation

Question 47

Give the regions of the strain stress curve in order

Answer 47

elastic plastic strain hardening necking

Question 48

what is young's modulus equal to on the stress-strain curve?

Answer 48

gradient upto the proportional limit ie. the elastic region

Question 49

In a stress-strain curve what is the highest stress on the diagram?

Answer 49

ultimate stress