Modern Plastics Worldwide - October 2007 - dae uptlax

MODERN EXECUTIVE
Long-term testing critical for material
selection
By Chris O’Connor
More than 5000 plastic product failures
have been the subject of study
at Smithers Rapra Technology. These
have been classified on the basis of primary
failure mode as shown in Figure 1.
A further breakdown of plastics product
failure due to human causes is given in
Figure 2, with 45% due to poor material
selection and poor specification.
Failures due to poor material selection
appear related to a lack of awareness
and understanding of plastics properties.
When considering the design and
development of a plastic component, it is
imperative that designers and engineers
fully understand the fundamental nature
of plastics. They should know that plastics
are non-linear, visco-elastic materials
with temperature/time-dependant properties.
Plastics physically and chemically
age, and are susceptible to chemical
attack, environmental stress cracking
(ESC), and weathering.
Plastics eventually fail under tensile
stress, with the time to failure diminishing
as the stress or temperature (or both)
increases, in the presence of certain environments,
or under cyclic loading.
Designers need to know that weld lines
result in planes of weakness, particularly
in fiber-reinforced materials. In fact, the
addition of any form of filler will always
have some form of detrimental effect on
a material. Plastics can show mechanical
anisotropy due to the alignment of fiber
reinforcement.
Breaking it down
To logically approach material selection,
the basics of polymer structure and polymer
properties must be considered. The
three main polymer groups are thermoset
plastics, thermoplastics, and elastomers.
Correct material selection is a critical factor in eliminating warranty and recall
risk in plastic products and components. Here is a primer for some, and a
reminder for others, on some of the basic and not-so-basic points of plastics
selection.
There are also two
basic types of thermoplastic:amorphous
and semicrystalline.
Amorphous plastics
have no ordered
structure, whereas
semicrystalline plastics
have areas of
order, which form
crystallites. In general,
amorphous plastics
exhibit better
creep resistance (constant
static load over
time) whereas semicrystalline
materials
offer better fatigue,
chemical, and environmental
stress
cracking (ESC)
resistance.
Amorphous plastics
are best used
where transparency
is important and low
levels of mechanical
abuse and chemical
contact are expected.
Semi-crystalline materials are a good bet
when resistance to chemicals/ESC,
mechanical abuse and cyclic loading is
required.
Designers and engineers historically
have worked with metals and other
materials, which exhibit a predictable,
linear elastic stress-strain relationship.
But polymers’ visco-elasticity means they
respond to stress as if they were a combination
of elastic solids and viscous fluids,
so that they exhibit a non-linear stressstrain
relationship, and their properties
Environmental stress cracking
Notched static rupture
Chemical attack
Thermal degradation
Dynamic fatigue
Creep/relaxation
UV attack
Others
Figure 1
Material/phenomenological
causes of failure
depend on the time under load, temperature,
environment and the stress or strain
applied.
When using Finite Element Analysis
(FEA) for elastic materials such as metals,
the software requests the modulus
(stiffness) of the material. For metals the
tensile modulus of the material is simply
entered (depending on the loading scenario)
and the FEA software calculates
the result. When designing with elastic
materials, designers and engineers can
rely on instantaneous stress-strain prop-
22 OCTOBER 2007 • MODERN PLASTICS WORLDWIDE modplas.com