Module 3 Stress, Strain, Young Modulus

Question 1

Define (tensile) stress?

Answer 1

(tensile) force per unit cross sectional area

Question 2

Unit for tensile stress?

Answer 2

pascals Pa

Question 3

symbol for tensile stress?

Answer 3

σ - sigma

Question 4

What is tensile strain?

Answer 4

the extension per unit original length

Question 5

Unit for tensile strain?

Answer 5

no unit

Question 6

Symbol for tensile strain?

Answer 6

ε - epsilon

Question 7

Definition of Young Modulus

Answer 7

Ratio of stress to strain of an object measured in pascals

Question 8

Equation for Young Modulus?

Answer 8

E=σ/ε

Question 9

What is an alternative equation for Young Modulus

Answer 9

E=FL/Ax

Question 10

What does the gradient of a stress strain graph represent?

Answer 10

Young Modulus

Question 11

Describe the apparatus used to find the Young Modulus of a wire

Answer 11

Control wire kept taught by a fixed weight with a reference scale fixed to it

Wire being tested is attached to the same support with a vernier scale opposite reference scale to measure extension

Mass hanger to vary weights

Question 12

Give a suitable value for the length of a wire when finding the Young Modulus?

Answer 12

2 meters or over

Question 13

What is deformation?

Answer 13

the change in size or shape of an object when put under stress

Question 14

What is elastic behaviour?

Answer 14

when an object returns to its original shape when unloaded

Question 15

What is plastic behaviour?

Answer 15

when an objects shape is permanently deformed once unloaded from a stress

Question 16

What kind of forces can deform an object?

Answer 16

compressive and tensile forces

Question 17

What is another name for plastic behaviour?

Answer 17

inelastic

Question 18

Describe the characteristics of a brittle material

Answer 18

Behaves elastically until it reaches its breaking point
Obeys Hooke’s law

Question 19

Describe the characteristics of a ductile material

Answer 19

Has a small elastic region before behaving plastically (deforming) and ultimately breaking

Below the elastic limit it obeys Hooke’s law

Question 20

Describe the characteristics of a polymeric material?

Answer 20

does not obey Hooke’s law - deforms when stress is applied

Question 21

How can stiffness be compared?

Answer 21

comparing gradients of stress strain graphs - higher gradient the higher the stiffness (can be compared by young modulus as well)

Question 22

What is the strength of a material defined by?

Answer 22

it’s ultimate tensile strength (UTS) - it’s maximum stress it can withstand before breaking

Question 23

What is breaking stress?

Answer 23

the stress at the moment of fracture/breaking

can be different from ultimate tensile strength

Question 24

How do you find the toughness from a stress strain graph?

Answer 24

area = energy stored
indicates toughness

Question 25

When plotting an unloading line how should it be done?

Answer 25

straight dashed line parallel to loading curve

Question 26

How can work done to deform a material be found from a force extension graph?

Answer 26

difference between area under loading curve and area under unloading curve

Question 27

Explain why despite the fact rubber does not obey Hooke’s law, it is still considered elastic?

Answer 27

extension is zero when load is removed/returns to original length when load is removed

Question 28

Explain the difference in area between the loading and unloading curves?

Answer 28

Difference in area = difference in energy

When stretched, some of the energy in rubber is stored and some internal energy is retained

In the form of heat (molecules oscillate with greater amplitude)