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

0
0
0
0
0
5
0
0
Fig. 2
Stress-strain
curve PA 66
(conditioned)
TECAMID 66
0 1 2 3 4
• Stress [MPa]
120
100
80
60
40
20
17
0
0 1 2 3 4
• Strain [%]
4. Influence 20 of temperature
Let us assume that the machine 10 100 1000 being 10000 supported heats up
15
15
to a temperature of appr. 60 °C in use. As the strength and
rigidity of a material reduce at elevated temperatures while
10
10
toughness increases, this circumstance must also be taken
into account in the configuration of the underlays.
5
5
For this, isochronous stress-strain diagrams can be used
2.5
which were determined at a corresponding temperature.
0
0 1 2 3 4 5
20
0
0 1 2 4
10 100 1
As a result of the moisture absorption, the mechanical
strength 20 drops tangibly. Surface compression of 204 MPa at
1 % admissible deformation 10 100 1000 is 10000 still possible, but under
15
15
these conditions a load of appr. 17 MPa can no longer be
supported.
10
10
Fig. 10 4 100 1000 10000
Isochronous
stress-strain
curve PA 66 (60 °C)
20
15
10
10 100 1000 10000
0
0
0
0
0
0
7
0
4
3. Influence of time
5
5
The machine is required to remain standing over a long
2.5
period on the underlay. The maximum underlay deforma-
0
0
TECAMID 66
0 1 2 3 4 5
0 1 2 4 6 8
tion of 1 % should not be exceeded. Here, we assume a value
for time of 10,000 h. This corresponds roughly to one
year. For this type of estimate, isochronous stress-strain
curves can be used. These show the stress-strain progression
for different stress periods in a single diagram.
Fig. 3
Isochronous stress-strain
curve PA 66
(23 °C, conditioned)
TECAMID 66
0 1 2 3 4
• Stress [MPa]
20
15
10
5
0
10 100 1000 10000
0 1 2 3 4 5
• Strain [%]
The diagram above indicates an admissible surface area
compression of appr. 5 MPa for a load duration of 10,000 h
and an admissible deformation of 1 %. The underlay would
still fulfil its purpose under these conditions. However, the
influencing factors of humidity and time already substantially
reduce the admissible surface area compression.
With a load duration of 10,000 h and an admissible deformation
of 1 % at an elevated temperature of 60 °C, an admissible
surface area compression of appr. 2.5 MPa is determined.
This is lower than the actual effective surface
area compression. In this case, suitable measures must be
taken to counteract this effect. The actual surface area compression
20can be reduced, 20
10
for
100 1000
instance,
10000
by making design
5
• Stress [MPa]
5
2.5
0
0 1 2 4 6 8
• Strain [%]
adjustments such as increasing the support surface area.
15
15
Another possibility is to improve the material characteristics,
for example by means of glass fibre reinforcement, or
10
10
by changing to a different material.
This calculation has been simplified in a number of ways,
2.5
2.5
and is intended only to demonstrate the extent to which the
0
0
0 1 2 4 6 8
0 1 2 4 6
plastic characteristics depend on the ambient conditions.
The more data is available, the more effectively it is possible
to make a correct material choice.
In many cases, extensive material data is not available.
However, estimated calculations can be made by interpolating
or extrapolating the existing data.
5
10 100 10
73