Engineering plastics – The Manual - F.wood-supply.dk

Calculations
In order to clarify the correlations described under the heading
"Mechanical properties", the described influencing factors
will first be explained in detail using a simple example:
Description:
Let us assume that we have to produce a simple square machine
underlay.
The underlay is placed flush with its surface on a level floor
and is exposed over its whole surface to an even load of 1 ton.
Given:
h = 10 mm
w = 50 mm
l = 50 mm
m = 1,000 kg h
w
F G
l
ˌˌCrack formation (application of load until irreversible
damage in the micro-range, crazing)
ˌˌApplication-specific max. admissible deformation
For the application described here, a maximum admissible
deformation of 1 % is assumed.
With this value, the admissible surface compression may
be determined with the aid of a quasi-statistical stressstrain
curve. Even if the stress involved in the described
case is compression stress, it is possible to revert here to
results from the tensile test, as with only a few exceptions
the tensile strength of a material is smaller than the compressive
strength. Consequently, at the same time a certain
safety allowance is also taken into consideration. As the results
of the tensile test are easily accessible, this also means
that a good data base is available.
1. The following applies:
To be able to calculate the occurring surface compression,
first of all the weight force is calculated as follows:
F G = m × g = 1,000 kg × 10 m/s² = 10,000 N (simplified)
Fig. 1
Stress-strain
curve
PA 66 (dry)
120
100
80
60
120
100
80
60
This force must then be projected onto the contact surface.
For the sake of simplicity, it is assumed that the force is ideally
distributed over the contact surface. Initially, however,
the contact surface in this case must be calculated as follows:
A = w × l = 50 mm × 50 mm = 2,500 mm²
in order to allow the surface compression to be subsequently
calculated as follows:
p = F / A = 10,000N / 2,500 mm² = 4.0 MPa
In the application described above (simplified calculation) a
surface compression of 4.0 MPa is determined.
In order to allow a suitable material to be recommended,
however, the failure criterion still has to be determined. A
distinction can be made here between several criteria:
ˌˌBreakage (application of load until material breakage)
ˌˌStretching (application of load to the yield point)
TECAMID 66
• Stress [MPa]
40
35
20
0
0 1 2 3 4
• Strain [%]
With a load of 4 MPa and admissible deformation of 1%, for
example, the material TECAMID 66 may be used. Under
these conditions, surface compression can be applied up to
appr. 35 MPa
2. Influence of moisture
The data used above was determined on test specimens
which had just been freshly injected. However, their application
takes place under normal climatic conditions. For
this reason, particularly in the case of polyamides, which
generally have relatively high moisture absorption, the actual
strength under normal climatic conditions must be
taken as a basis for assessment of the application:
40
20
17
0
72