Design Guide - Solvay Plastics

Figure 50: Flex Fatigue of GR Resins at 23°C
(73°F) and 32 Hz
Figure 51: Flex Fatigue of AS-1145 Resin at
Elevated Temperature
Stress, MPa
140
120
100
80
60
40
20
0
AS-1145 HS
AS-1133 HS
33% GR PA 6,6
10 3 10 4 10 5 10 6 10 7
Cycles to Failure
20
18
16
14
12
10
8
6
4
2
0
Stress, kpsi
Stress, MPa
70
60
50
40
30
20
10
0
110°C (230°F)
170°C (338°F)
10 3 10 4 10 5 10 6 10 7
Cycles to Failure
10
8
6
4
2
0
Stress, kpsi
Fatigue Strength of Amodel Resin
When measuring and/or comparing the fatigue strength
of plastic materials, it is critical to specify the mode
(tensile, compressive, or flexural), the frequency, and
the stress profile.
The fatigue strength of Amodel resins was determined
in accordance with ASTM test method D671. This
method uses a cantilever beam configuration with a
constant deflection. The results shown in Figure 50
were generated at a frequency of 32 Hz. Failure was
defined when the stress level decayed to 90% of its
initial value. The Amodel resins resist higher cyclic
loads longer than PA 6,6.
Figure 51 gives the fatigue behavior of Amodel
AS-1145 HS resin at 110°C and 170°C (230°F and
338°F). As expected, raising the temperature reduces
the amount of cyclic stress that can be endured.
A good example of an application involving cyclical
stress is a gear. As the driving gear causes the driven
gear to rotate, each tooth is subject to stress. This is
then followed by a period of time at low or zero stress.
In designing a gear tooth, the appropriate strength
criteria is the fatigue endurance limit of the material
at the operating conditions of the gear.
After all of the calculations have been made, the part
design can be modified to best utilize the properties
of the selected material. Conversely, the material
grade can be modified to best suit the application
requirements. An extremely useful tool to verify a design
is to perform Finite Element Analysis (FEA). In FEA, the
part is electronically modeled and a computer program
is used to simulate the anticipated loads and stresses.
The resulting strain levels can be evaluated and, if
necessary, part design or material changes can be
made. It is much easier to make changes to the FEA
model than to a fabricated part or tooling.
Even after calculations and FEA’s have been performed,
the best way of evaluating a material for a component
subject to dynamic stresses is actual prototype testing.
Prototype parts can be machined from available stock
shapes and subject to performance testing under the
requirements of the application. Stock shapes are usually
available as extruded slabs or rods and generally exhibit
slightly lower mechanical properties than the injection
molded part. Additionally, the machining operations
usually induce stresses that would not normally be
present in an injection molded part. As a result of this,
a successful prototype test imparts an extremely high
confidence level in the reliability of the design.
Property Data
Amodel ® PPA Design Guide
37