Ethical issues in engineering design - 3TU.Centre for Ethics and ...

Design of a lightweight trailer
composites, especially in the steps in which the fibres are woven and combined
with the resins, flaws may be introduced that will weaken the composite.
With composites, moisture and temperature can sometimes degrade the
mechanical properties of the material over time; therefore these influences need
to be accounted for in a conversion factor. Different organisations have
formulated different allowable strains for composites using different conversion
factors. Hans had considerable experience with projects for yacht designs and
there the maximum allowable strains defined by Lloyd’s Register are used. Liz
had just finished a project in which she had used maximum strains defined by
the CLC together with the Rijkswaterstaat and a Research centre for civil
engineering in the Netherlands (CUR). In this project they used values taken
from the literature and from tests designed to define the lower boundaries of
mechanical properties of composites produced using different production
methods. A choice had to be made between the different available maximum
strains, i.e. whether to choose those detailed by the Lloyd’s Register or those
provided by the CLC, Rijkswaterstaat and CUR.
Hans: ‘Lloyd’s indicates that you can allow 0,25% strain.’
Liz: ‘0.25%? Theo said that you could allow 1,2% strain but then you
need some safety factors.’
Hans: ‘You can allow 1% strain under compression and tension. Well,
in fact you can allow a bit more under tension than under
compression but when you take the same strength for compression
then you can allow 1,0 % with a safety factor of 4 so that makes 0,25%
maximum strain.’
Liz: ‘That’s conservative because Theo got to 1,2 % with a safety factor
of 3.’
Liz had no previous experience with the Lloyd’s data. She did not know what was
accounted for in these allowable strains and what was not. She therefore
preferred to use the allowable strains from the CLC, Rijkswaterstaat and CUR. In
one of the presentations for the customer the maximum allowable strain
indicated on the slide was 0,35%.
Load scenarios
Load scenarios are necessary to help engineers calculate minimum material
thickness. Load scenarios are descriptions of what might happen, including the
forces that will be exerted on the trailer, when it is used. If the load scenarios lead
to strains higher than the allowable strains then the material thickness should be
increased or another material must be chosen. Usually there are scenarios for
normal use, for example a trailer making a turn while carrying a load of sand.
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