Unit 1 - Mechanical properties of materials & structures

Question 1

What is the difference between a material & a structure?

Answer 1

Structure - arrangement of 1/more materials in a way designed to sustain loads

Material - things used to construct a structure

Question 2

How to calculate stress?

Answer 2

σ = F/A

Unit = Pa (N/m^2)

Question 3

Equation for strain?

Answer 3

ε = Δl/l

No units because it’s a ratio

Question 4

What is the proportional limit on a stress/strain curve?

Answer 4

Between origin & proportional limit (P) the strain-strain curve is a straight line

Question 5

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

Answer 5

(E) is the greatest stress that may be applied to the material without causing permanent deformation

Question 6

What is elastic behaviour?

Answer 6

When a material deforms instantaneously on loading & returns immediately to original shape & size when load removed

Question 7

What is plastic behaviour?

Answer 7

Material deforms instantaneously under applied load however retains a new size & shape when load is removed

Question 8

What is the yield point on a stress-stain curve?

Answer 8

Once (Y) passed the material will undergo considerable elongation without a corresponding increase in stress

Stress at yield point = yield strenght

Question 9

What is the strain hardening region of a stress-strain curve?

Answer 9

After material undergoes large strain that occurs during yielding the material begins to strain/work harden - undergoes changes in atomic & crystalline structure resulting in increased resistance to further deformation

Question 10

What is the ultimate strength point on a stress-strain curve?

Answer 10

(U) occurs at highest point of stress-strain curve

In region after ultimate strength point stretching occurs with reduction in stress due to necking of the material whereby CSA decreases

Question 11

What is the rupture point on a stress strain curve?

Answer 11

(R) is the point at which the material breaks (rupture strength)

Question 12

What is Hooke’s Law?

Answer 12

Up to a certain level of stress, the strain produced is proportional to the applied stress

Question 13

What is the equation for Young’s modulus & what is it a measure of?

Answer 13

E = σ/ε (only up to proportional limit)

Is a measure of how stiff a material is:

• small value for E means material is flexible
• large value for E mean material is stiff

Question 14

How do you calculate rigidity?

Answer 14

Rigidity = E/A

This is an indication of a materials ability to resist axial deformation

Units = N

Question 15

How do you calculate stiffness (k)?

Answer 15

k = F/Δl = EA/l (from equation for E)

The force required to produce a unit of deflection (elongate/shorten)

Question 16

What is the equation for flexibility?

Answer 16

f = Δl/F = l/EA = 1/k

Increase in length of a bar will reduce stiffness but increase flexibility

Question 17

Describe viscous behaviour

Answer 17

Does not deform instantaneously when load applied, strain is prolonged

Once load removed, material will not return to original shape & size

Means no energy is stored in material and all energy required to deform it is dissipated as heat

Question 18

What is the equation for the coefficient of viscosity?

Answer 18

η = stress/strain rate

Units = N/m^2 .s or Pa .s

Question 19

What is viscoelastic behaviour?

Answer 19

Exhibits both:

• Viscous: responds to rate of loading
• Elastic: returns to original size & shape after loading

Question 20

What is the difference between creep & stress relaxation?

Answer 20

Creep - increase in strain in a material which is constantly loaded over time

Stress relaxation - reduction is stress in a material which is constantly deformed over time