Page 65 - Plastics Engineering

48 Mechanical Behaviour of Plastics

A variety of other creep-strain equations have been proposed by Sterrett

et al. These take the form

1. &- - u bf Parabolic

I+

+ ct Power

2. E- - a ,+*

3. &=- Hyperbolic

1 .+ bt

1

4. E= Hyperbolic

1 +atb

5. E = a + ln(b + t> Logarithmic

a(b .+ Int)

6. E= Logarithmic
c-tdlrlt

7. Exponential

8. E = sinh ut Hyperbolic sine

9. E = a +. b sinh ct Hyperbolic sine

10. E = a + b sinh ct 1/3 Hyperbolic sine

113
-cf
11. E = a(l + bt )e Pow erkx ponential

The mechanism of creep is not completely understuod but some aspects have

been explained based on the structures described in Appendix A. For example,

in a glassy plastic a particular atom is restricted from changing its position

as a result of attractions and repulsions between it and (a) atoms in the same

chain, (b) atums in adjacent chains. It is generally considered that for an atom

to change its position it must overcome an energy barrier and the probability

of it achieving the necessary energy is improved when a stress is applied. In

a semi-crystalline plastic there is an important structural difference in that the

crystalline regions are set in an amorphous matrix. Movement of atoms can

occur in both regions but in the majority of cases atum mobility is favoured in

the non-crystalline material between the spherulites.

Plastics also have the ability to recover when the applied stress is removed

and to a first approximation this can often be considered as a reversal of creep.

This was illustrated in Fig. 1.8 and will be studied again in Section 2.7. At

present it is proposed to consider the design methods for plastics subjected to

steady forces.

2+5 Design Methods for Plastics using Deformation Data

Isochronous and Isometric Graphs

The most common method of displaying the interdependence of stress, strain

and time is by means of creep curves, However, it should be realised that these