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4 Designing Plastics Parts
The deformational behavior of polymeric materials depends mainly on the type and
magnitude of loading, time of application of the load, and temperature. The deformation
is related to these factors in a complex manner, so that the mathematical treatment of
deformation requires a great computational effort [I]. However, in recent times computational
procedures for designing plastic parts have been developed using stress-strain data,
which were carefully measured by employing computer-aided testing of polymers [2].
4.1 Strength of Polymers
The basic equation for calculating the design stress of a part under load is given by [ 1 ]
where
K = material strength as a mechanical property
av = maximum stress occurring in the part
°zui ~ allowable stress
S = factor of safety
A = material reduction factor
(4.1)
The relation between allowable stress and the polymer-dependent reduction factors can
be written as [1]
(4.2)
The factor A0 considers the influence of temperature on the strength of the material and
can be calculated from [1]
(4.3)
where 0 = temperature. The limits of applicability of Equation 4.3 are 20 < 0 < 100 0 C.
The value k based on the reference temperature of 20 0 C is given for the following materials
as[l]
PA66 =0.0112
PA6 = 0.0125
PBT = 0.0095