chapter 3 pt.1

Question 1

define

elastic deformation

Answer 1

a temporary distortion where when a force is removed, an object returns to its original shape

Question 2

state two

properties of elastic materials

Answer 2

1. materials are elastic for low stresses
2. work done is stored when loading & then the material releases all stored energy when unloading

Question 3

define

plastic deformation

Answer 3

a deformation that occurs when the load/force is removed & the object does not return to its original shape

Question 4

fill in the gap

a _______ of forces is needs to deform an object

Answer 4

pair

Question 5

what happens when a

tensile force is applied

Answer 5

spring is stretched by pulling

Question 6

what happens when a

compressive force is applied

Answer 6

spring is squished and shortened

Question 7

what happens when a material is

bended

Answer 7

particles on the outside are pulled further apart, while the ones on the inside are pushed together

Question 8

define

ultimate tensile stress

Answer 8

the maximum value of stress that an object can sustain

Question 9

define

breaking stress

Answer 9

stress at which a material breaks

Question 10

define

Hooke’s law

Answer 10

the extension produced in an object is proportional to the force producing it (provided that the limit of proportionality is not exceeded)

Question 11

state the rule

F = ?

Answer 11

k x e

Question 12

define

spring constant

Answer 12

force per unit extension

Question 13

what are the properties of a

stiffness graph (force extension)

Answer 13

1. directly proportional (straight line passing through the origin)
2. gradient = k
3. steeper –> stiffer –> less extension

Question 14

what are the properties of an

elasticity graph (extension force)

Answer 14

1. k can be calculated using the reciprocal of the gradient for only the straight line part up until the limit of proportionality to make sure it obeys Hooke’s law
2. increasing the gradient –> decreases k –> less extension

Question 15

what happens when

springs are connected in parallel

Answer 15

connect them in parallel to increase spring constant because only half the extension happens
–> total e = 1/2 e
–> spring constant = k

Question 16

what happens when

springs are connected in series

Answer 16

connect them in series to decrease spring constant because double the extension happens
–> total e = 2e
–> spring constant = 1/2 k

Question 17

define

limit of proportionality (P)

Answer 17

the extension beyond which Hooke’s law no longer applies (spring is still ok)

Question 18

what happens when an object reaches its

elastic limit (E)

Answer 18

1. object will not return to its original dimensions when the force is removed (spring is not ok)
2. spring is permanently stretched

Question 19

what happens during unloading when the elastic limit of a material is exceeded?

Answer 19

as the force applied on the material is decreased, a new original length (new extension) is formed due to it getting denatured

Question 20

state two

properties of copper

Answer 20

1. it is ductile and malleable
   —> ductile : can drawn into wires
   —> malleable: can be hammered without breaking
2. when stretched beyond its E, its graph retains a new shape

Question 21

state

properties of glass

Answer 21

1. it is brittle
   –> as in it follows Hooke’s law until it snaps

Question 22

state two

properties of rubber

Answer 22

1. does not follow Hooke’s law
2. it remains elastic until it breaks

Question 23

define

strain energy

Answer 23

the form of potential energy which is stored within materials due to change of shape because of strain

Question 24

state the rule

strain energy = ?

Answer 24

area under the graph
= work done in joules
= 1/2 x F x e
= 1/2 x k x (e)^2

Question 25

# state the rule change in energy = ?

Answer 25

E2 - E1 = (1/2 x F2 x e2) - (1/2 x F1 x e1) = (1/2 x k2 x (e2)^2) - (1/2 x k1 x (e1)^2) = 1/2 x k ( (e2)^2 - (e1)^2 )

Question 26

what does doubling F1 do to e1 and to the energy ?

Answer 26

1. e1 is doubled 2. energy is quadrupled 3. e2 has 3 times the energy of e1

Question 27

what are they keys to understanding any deformation?

Answer 27

stress which is independent, and strain which is dependent

Question 28

# state the rule strain (ε) = ?

Answer 28

extension (e) / original length (Lo)

Question 29

# state the rule stress (σ) = ?

Answer 29

force (F) / cross sectional area (A)

Question 30

# state the rule Young's modulus (E) = ?

Answer 30

stress (σ) / strain (ε) = (F x Lo ) / (e x A) = (k x Lo) / (A) = the gradient of a stress strain graoh

Question 31

# define stress (tensile stress)

Answer 31

the force per unit area required to stretch a material

Question 32

# define strain

Answer 32

1. the extension per unit length --> it is a ratio so it does not have a unit

Question 33

# define Young's modulus

Answer 33

1. the ratio of stress to linear strain 2. the modulus of elasticity

Question 34

# state the rule strain energy per unit volume = ? (from a strain stress graph)

Answer 34

area under the graph = 1/2 x stress x strain = 1/2 x ( (F x e) / (A x Lo) ) = 1/2 x ( (F x e) / V )

Question 35

why do most metals not experience brittle fractures?

Answer 35

because dislocations tend to stop cracks from growing and spreading

Question 36

- there's a certain property of *ductile* materials when they get stretched, what is it? - and what happens after the ductile material starts to neck?

Answer 36

1. when exceeding the yield point in a ductile material, little extra force is needed to produce a large extension 2. a ductile fracture occurs

Question 37

# state three properties of brittle materials

Answer 37

1. they are stiff 2. they exhibit elastic behaviors up until very small values of strain 3. they hardly change shape in the elastic region, but under great stresses they develop cracks on the surface that open up as the stress increases, they they fracture --> this is called a brittle fracture

Question 38

# properties of rubber and how its stress strain graph looks like rubber and hysteresis loop

Answer 38

1. rubber undergoes high strains without break 2. its stress strain graph has two branches: loading and unloading 3. the loop formed by both branches is called the elastic hysteresis loop

Question 39

# describe hysteresis loop graph (force extension)

Answer 39

1. in one loading and unloading cycle the strain energy (represented by the area bound by the loop) is lost and eventually dissipated as heat 2. the area under the loop is the recoverable energy

Question 40

# describe hysteresis loop graph (stress strain)

Answer 40

the area bound by the loading and unloading branches is the heat energy loss per unit volume

Question 41

- what is meant by hysteresis ? - why does it happen ? - why is rubber a bad material for storing energy ?

Answer 41

1. hysteresis is the percent of energy loss per each deformation 2. it results from friction happening inside the rubber which creates heat build up 3. due to the heat lost during unloading

Question 42

# fill in the gap work is done as a material is stretched, under tension this energy is stroed as ____________ energy

Answer 42

strain

Question 43

# state three possibilities what happens when the load is released

Answer 43

1. if it is elastic, all the energy can be recovered 2. if the material is plastic, less energy is recovered --> as in, energy is consumed & expelled from the system as the microstructure is irreversibly changed 3. shows hysteresis --> rubber

Question 44

# in stress strain curves describe the linear elastic region

Answer 44

1. at relatively low strains, the material obeys Hooke's law, and stress is directly proportional to strain (this section of the curve is a straight line) 2. the constant of proportionality is the Young Modulus of the material

Question 45

# in stress strain curves describe the non-linear elastic region

Answer 45

1. if the material is stretched byeond its elastic region, it will soon reach its elastic limit (this section is not a straight line) 2. despite it not obeyeing Hooke's law, if the force is removed, the material goes back to its original shape (it is still elastic)

Question 46

# in stress strain curves describe the yield region

Answer 46

1. the material suddenly experiences increased deformation (this part is called the upper yield point) 2. the stress begins to decrease with the increasing strain until it reaches the lower yield point

Question 47

# in stress strain curves describe the region beyond the yield point

Answer 47

1. stress increases with increasing strain, however; this increase is not elastic 2. the material's cross-sectional area begins to change uniformly 3. at some point, the value of stress reaches its maximum value (*ultimate tensile stress*)

Question 48

# in a stress strain graph describe what happens when a material is stretched beyond its ultimate tensile stress

Answer 48

1. the material shows necking 2. stress dramatically increases in the necking region, but the overall stress decreases with the increasing strain until a breaking point is reached 3. then the material fractures in the necked region --> the stress at this point is called the breaking stress

Question 49

# define and relate it to energy breaking stress

Answer 49

1. the stress at a material's breaking point is called the breaking stress 2. it is related to the energy required to break internal bonds between the atoms or the molecules of the material

Question 50

why do springs extend a lot for a small load ?

Answer 50

because they are coiled

Question 51

# explain why is Young modulus better ?

Answer 51

1. when using a force constant, the constant is different for each specimen of a material having a different shape 2. it is far more convenient to know a constant for a particular materials, which would then enable us to find extensions --> this constant is known as Young modulus

Question 52

# fill in the gap a strong material has a high ___________________

Answer 52

ultimate tensile stress

Question 53

# fill in the gap a stiff material has a high ____________

Answer 53

Young's modulus

Question 54

# fill in the gap a ductile material be drawn into ________

Answer 54

wires

Question 55

when can a material be called malleable ?

Answer 55

if it can be hammered into different shapes

Question 56

# explain according to energy concepts what happens if a spring is suddenly released ?

Answer 56

the strain energy stored in the spring is suddenly converted into kinetic energy for this spring strain -------> KE

Question 57

why would it not be appropriate to use the Young modulus of a mterial in order to determine its extension when a breaking force is applied ?

Answer 57

because the stress is no longer proportional to the extension

Question 58

# fill in the gaps materials that undergo ________ deformation are said to be _________

Answer 58

plastic ductile