Page 61 - Plastics Engineering

44 Mechanical Behaviour of Plastics

Fig. 2.2 also illustrates an interesting phenomenon observed in some plastics.
This is cold drawing and it occurs because at low extension rates the molecular
chains in the plastic have time to align themselves under the influence of the
applied stress. Thus the the material is able to flow at the same rate as it is being
strained. Nowadays this phenomena is utilised in some forming methods for
plastics. It creates highly aligned molecular structures which exhibit excellent
strength and stiffness properties in the alignment direction.
Occasionally, materials are tested in tension by applying the loads in incre-
ments. If this method is used for plastics then special caution is needed because
during the delay between applying the load and recording the strain, the material
creeps. Therefore if the delay is not uniform there may appear to be excessive
scatter or non-linearity in the material. In addition, the way in which the loads
are applied constitutes a loading history which can affect the performance of
the material. A test in which the increments are large would quite probably
give results which are different from those obtained from a test in which the
increments were small or variable.
As a result of these special effects in plastics it is not reasonable to quote
properties such as modulus, yield strength, etc as a single value without qual-
ifying these with details of the test method. Standard short-term test methods
for plastics are described in BS 2782, ASTM D638 and ASTM D790. These
relate to both tensile and flexural short-term tests. It should be noted that, as
pointed out above, in uniaxial loading the stress, 6, is simply the force divided
by the cross-sectional area and the strain, E, is the deformation divided by the
original length. The modulus of the material is then the ratio of stresdstrain.