Mechanics unit 3 - mechanical properties of materials & structures - deck 3

Question 1

• So far in this unit we have considered the behaviour of materials under steady loading conditions but not a steady load over time
• In this section we will consider how materials behave when they are loaded over a period of time

Question 2

Describe the characterisitics of a material which displays viscous behaviour

Answer 2

This is when a material does not deform instantaneously when a load is applied. Its strain (stretching) is prolonged in a slow, non-linear manner. Once the load is removed, the material will not return to its original size and shape.

Question 3

Because a material displaying viscous behaviour will not return to its original size and shape once the load is removed, what does this mean is happening to the energy input to deform the material ?

Answer 3

It means that the energy input is lost as heat and no energy is stored in the material

Question 4

Give some examples of materials which display viscous behaviour ?

Answer 4

• Mainly fluids e.g. water, air, blood plasma
• But some materials display a combination of viscous and elastic behaviour e.g. articular cartilage and cortical bone

Question 5

Why can the behaviours of viscous materials not be represented by Hookes law and youngs modulus unlike in elastic materials ?

Answer 5

Because the stresses arising in viscous materials are dependent upon the strain rate, in contrast to the elastic materials in which the stresses are dependent upon the the strain.

Question 6

What is the factor used to represent the relationship between stress and strain rate in viscous materials ?

Answer 6

The coefficient of viscosity

Question 7

What are the SI units for the coefficient of visocsity?

Answer 7

Nm^-2 .s (Newton per metre squared second) or Pa .s (pascal per seconds)

Question 8

In mathematical terminology what does the dot positioned above a symbol indicate ?

Answer 8

It indicates that it is the rate of change

Question 9

Do SAQ 7 pg. 13 mechanical properties unit 3

Answer 9

Ans in workbook

Question 10

Describe what viscoelastic behaviour is

Answer 10

• This is when a material displays a combination of viscous and elastic behaviour.
• The material responds to the rate of loading by deforming slowly and in a non-linear manner (viscous behaviour) and then displays elastic behaviour as it returns to its original size and shape after loading is removed in a slow and non-linear manner

Question 11

Describe the phenomenon of creep behaviour that some materials display

Answer 11

This is when a material continues to lengthen (or contract) over time when a constant load is applied

Question 12

What materials are particularly vulnerable to creep behaviour ?

Answer 12

• Materials which are exposed to high temperatures, they may even fracture because of it.
• Note many materials display creep behaviour but usually the change is too small

Question 13

Describe the characteristics of a creep curve

Answer 13

1. Initially the bar will undergo an elastic strain, which is equivalent to thaton the bar’s stress-strain curve for the applied stress.
2. The bar may continue to lengthen, passing through the 3 stages due to creep (the different stages are of consequences not needed to be known)
3. The second stage is characterised by a constant creep rate which is equal to the gradient of the creep curve
4. In the third stage the material begins to neck and the deformation accelerates until fracture

Question 14

Describe the time-dependant (like creep) phenomenom of stress relaxation that some materials display

Answer 14

Some materials display this if kept under constant strain then the stress in the material will gradually diminish over time. This is due to the change in the ordering of the atoms within the material

Question 15

Do SAQ 8 pg. 14 mechanical properties unit 3

Answer 15

Ans in workbook