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

Design of a lightweight trailer
tonnes and the truck 6.5 tonnes, this leaves 44 - 5.5 -6.5 = 32 tonnes for the load
to be transported.
The load scenarios changed during the design process, and the trailer and the
load scenarios appeared to be designed simultaneously. The load scenarios that
were assumed in the first finite element calculations were the following.
Load on the floor: 32 tonnes, dynamic factor 2 and safety factor 1,5 makes 32
tonnes x 3g
Load on the floor and pressure on the side panels: floor 32 tonnes x 3g plus a
hydrostatic pressure on the side panels of “density of the sand” x “the height”
x g
Turning a corner: as a trailer takes a corner the pile of sand is subjected to a
gravitational field of 32 tonnes x 3g on floor and 32 tonnes x 3g on the side
panel
Using these load scenarios, calculations were made using the finite element
model. These calculations led to surprising results: the pressure on the side
panels would lead to a 40 cm of displacement if carrying sand! Turning a corner
would lead to a displacement of 5 m in the side panels. Liz and Hans concluded
that the load scenarios were not realistic and too severe.
Hans: ‘The design problem will succeed or fail with these load
scenarios. Could we not use the same load scenarios as the customer
used when designing an aluminium trailer?’
Liz: ‘That’s the problem. They are not able to deliver load scenarios.
We will use the ones we have used in other projects.’
Hans: ‘There have been load scenarios made for other projects, but we
have to know what the customer does with his aluminium trailer. If
you want to be smart you have to change the load scenarios, otherwise
we will not get the 10% mass reduction.’
Liz: ‘But the customer cannot give me any load scenarios, not even for
the aluminium trailer they produce now.’
Hans: ‘The risk is that the customer built the aluminium trailer based
on experience. He has learnt that he can reduce the material thickness
because it doesn’t fail. This also happened in a yacht building project.
The load scenarios that we have used for our calculations are much too
severe, this will lead to extra mass in the design. We can say that the
material is necessary because we have calculated that there is a need,
but probably having used too severe load scenarios.’
Liz: ‘We could find out what our concept does in comparison with the
strength and stiffness of the existing aluminium trailer.’
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