Design Guide - Solvay Plastics

Mechanical Properties
The mechanical properties of a material are of
fundamental importance to engineers when designing
a part. The designer must match the mechanical
properties of various candidate materials to the
performance requirements of each application in order
to determine which material is suitable for a given part
design. Conversely, the designer can use the material
property values to achieve an optimum part design.
To assist the designer, the material properties listed
in this manual have been grouped into short-term
or instantaneous and long-term or time-dependent
properties. The short-term properties generally measure
strength at failure while the long-term properties show
how the material properties are affected by temperature,
continuous loading, or chemical exposure as a function
of time.
Short-Term Mechanical Properties
Short-term mechanical properties typically include
tensile strength and modulus, flexural strength and
modulus, several impact tests, compressive strength,
shear strength, and surface hardness. These properties
are usually reported at room temperature, and other
temperatures as appropriate. Since some polymers
absorb atmospheric moisture which may affect the
properties, the moisture content may also be specified,
often using the Relative Humidity (RH) convention.
The data sheets provided by the material suppliers
typically list short-term properties, and their primary
utility is for comparing similar materials. When using
data sheets to compare materials, it is very important
to insure that the same test methods have been used
and that the data is reported in similar units.
The utility of short-term mechanical properties in design
is limited. Typically, the properties are measured using
molded test specimens that have been specifically
designed to yield reproducible results, under carefully
controlled environmental conditions, using specified
loading rates. These measurements often provide the
highest value obtainable for any property and material.
When parts are fabricated by a molding process they
will likely contain a number of features such as stress
concentrations, weld lines, corners or other aspects
that may reduce strength. The strength of a material in
an actual component may also be reduced, or in some
cases enhanced, by reinforcing fiber orientation, relative
degree of crystallinity, or thermal history (annealing).
In addition, short-term properties do not provide any
insight regarding time-related effects or the influence
of chemical environments.
Tensile Properties
Test Methods
There are two widely accepted methods of testing
tensile properties, ASTM method D638 and ISO method
527. These test methods measure the same property,
but slightly different test specimens and test procedures
are used. If the same material is tested using both
procedures, the results will be similar but not the same.
Therefore, only values obtained using the same method
should be compared. In this document, whenever a
tensile property value is given, the test method is also
given, and in many cases values by both methods
are provided.
Regardless of which test method is used, tensile
properties are determined by clamping each end of
a test specimen in the jaws of a testing machine that
applies a unidirectional axial force to the specimens at
a specified rate. The force required to separate the jaws
divided by the minimum cross-sectional area of the
test specimen is defined as the tensile stress. The test
specimen will elongate as a result of the stress being
applied. The amount of elongation divided by the original
length of the test specimen is the strain.
When the applied stress is plotted against the resulting
strain, a curve similar to that shown in Figure 5 for Amodel
ET-1000 HS resin is obtained. This is known as a "Stress-
Strain" curve and is very useful in determining the short
term behavior of a material when a load is applied. The
curve of a ductile metal would have a similar shape.
Figure 6 shows a typical stress/strain curve for a nonductile
material, Amodel A-1933 PPA. Strain at failure
is much lower than that of the unreinforced grade.
The addition of glass fiber reinforcement improves
the strength and stiffness but reduces the elongation
or strain at failure.
Stress, MPa
Figure 5: Typical Stress/Strain Curve for Amodel
ET-1000
70
60
50
40
30
20
10
0
0
0 5 10 15 20
Strain, %
10
8
6
4
2
Stress, kpsi
Property Data
Amodel ® PPA Design Guide
19