Stress, Strain And Young Modulus

Question 1

What is Hooke’s Law?

Answer 1

The force applied will be directly proportional to the extension produced within the elastic limit (unit of proportionality)

Question 2

What does Hooke’s Law look like on a graph?

Answer 2

• Straight line
• Through the origin
• Scatter is close

Question 3

Equation for the spring constant?

Answer 3

F = kx, where k = force required to produce unit extension

Question 4

Equation for the gradient of a spring constant on a graph

Answer 4

K = 🔼 Force / 🔼 extension

Question 5

Cm^2 ➡️ m^2

Answer 5

x10^-4

Question 6

mm^2 ➡️ m^2

Answer 6

x10^-6

Question 7

cm ➡️ m

Answer 7

x10^-2

Question 8

mm ➡️ m

Answer 8

x10^-3

Question 9

Explain in terms of compression and tension, what happens to concrete

Answer 9

• Weak under tension
• Strong under compression
• Cracks will propagate upwards until failure

Question 10

Explain the process of adding a steel rod to concrete

Answer 10

• Steel rod initially under tension
• Tension released as concrete sets
• Puts lower surface under compression making it as strong as the top

Question 11

What is the area under a F=kx graph equal to?

Answer 11

Energy

Question 12

Why does the elastic limit occur in metals?

Answer 12

Metals have regular atomic structure and long range order, meaning that any force transmitted will be produced equally between each bond

Question 13

Why does the plastic limit occur in a material?

Answer 13

When a material is stretched past the elastic limit, atoms slip past each other

Question 14

When does edge dislocation occur?

Answer 14

Edge dislocation occur when half a plane is missed out. The atoms around this point are places of weakness as they are under strain

Question 15

What happens when the forces are;
(A) small
(B) large

Answer 15

(A) - bonds stretch reversibly so they behave elastically (Hooke’s Law)

(B) - strained bonds snap and migrate in the direction the stress is applied

Question 16

When does edge dislocation stop?

Answer 16

The dislocation carries on until it reaches the edge of the crystal

Question 17

What strengthens materials?

Answer 17

• having smaller grains restricts movement

- introducing foreign atoms

Question 18

What is the elastic limit on a crystalline stress-strain graph?

Answer 18

Where no bonds are broken. If the force is removed, the material will return to its original shape

Question 19

What is the yield point?

Answer 19

Where large extensions occur as planes of atoms slide past each other

Question 20

What is the plastic region?

Answer 20

Where bonds are broken so will NOT return to original shape

Question 21

What do ductile materials do at the end of stress-strain graph

Answer 21

They become narrower and extend rapidly until breaking

Question 22

What features does a polymeric material have?

Answer 22

A polymeric material has strong covalent bonds called cross-links which form between molecules allowing the molecules not to slide past each other

Question 23

What does adding impurities do to a polymeric material?

Answer 23

Adding impurities makes the material rigid however these impurities are scarce

Question 24

Explain what happens at the start of a polymeric stress-strain graph

Answer 24

There are weaker bonds formed called vat der waals. These form between molecules to keep the molecules together making it harder to stretch (steep gradient)

Question 25

Explain the second part of the graph

Answer 25

The van der waal forces break easily and rapidly which therefore untangles the molecules. This makes large extensions with little strain (low gradient)

Question 26

Explain the final part of a polymeric stress-strain graph

Answer 26

The molecules have straightened meaning to stretch the material further, you are lengthening cross-links and lengthening strong bonds between the molecules (steeper gradient)

Question 27

Definition of stress?

Answer 27

Stress is the force per unit cross-sectional area when equal opposing forces act on a body

Question 28

Definition of strain?

Answer 28

Strain is the extension per unit length due to an applied stress

Question 29

Definition of young modulus?

Answer 29

Stress per unit strain

Hooke’s Law region