Page 58 - Plastics Engineering

CHAPTER 2 - Mechanical Behaviour of Plastics

2J Introduction

In Chapter 1 the general mechanical properties of plastics were introduced. In

order to facilitate comparisons with the behaviour of other classes of materi-

als the approach taken was tu refer to standard methods of data presentation,

such as stress-strain graphs, etc. However, it is important to note that when

one becomes involved in engineering design with plastics, such graphs are

of limited value. The reason is that they are the results of relatively short-

terrn tests and so heir use is restricted to quality control and, perhaps, the

initial sorting of materials in terms of stiffness, strength etc. Designs based on,

say, the modulus obtained from a short-term test would not predict accurately

the long-term behaviour of plastics because they are viscoelastic materials.

This viscoelasticity means that quantities such as modulus, strength, ductility

and coefficient of fiction are sensitive to straining rate, elapsed time, loading

history, temperature, etc. It will also be shown later that the manufacturing

method used for the plastic product can create changes in the structure of the

material which have a pronounced effect on properties. The behaviour of the

moulded product may therefore be different from the behaviour of a moulded

test-piece of the same material.

The time-dependent change in the dimensions of a plastic article when

subjected to a constant stress is called creep. As a result of this phenomenon

the modulus of a plastic is not a constant, but provided its variation is known

then the creep behaviour of plastics can be allowed for using accurate and well

established design procedures Metals also display time dependent properties

at high temperatures so that designers of turbine blades, for example, have to

allow for creep and guard against creep rupture. At room temperature the creep

behaviour of metals is negligible and so design procedures are simpler in that

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