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

Ethical issues in engineering design
i.e. turning a corner. One scenario had been changed, i.e. load on the floor. One
scenario, torsion, was not changed, but this is not really a load scenario. A load
scenario is meant to be used to describe what loads can be expected. To
determine torsional stiffness one unit torsion is placed on a model. When the two
scenarios, mentioned before, were introduced in the finite element model, the
maximum displacement of the floor was 33 mm. 7 Liz and Hans reasoned that this
displacement was calculated using 2g so the displacement would only be 16,5
mm if the trailer is at rest. This was actually within the requirement of maximum
20 mm but Liz preferred a larger margin.
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Hans: ‘In fact, let’s look at the relative displacements. Because I think
that that is what is important, and it is probably a lot less. This is really
the bending of the floor. You see the yellow part is not 0.’
Liz: ‘Yellow is 6.’
Hans: ‘It is 7, the displacement ranges from 7 to 33 mm so the
bending 33-7=26 divided by 2. The bending is 13 mm by 1g.’
From this quote it can be seen that Liz and Hans decided the bending over the
largest span should be less than 20 mm not the total overall displacement. The
bending over the largest span was 13 mm. The bending was therefore well within
the requirement. The stresses and strains within the materials were lower than
the allowable stresses and strains and therefore the concept composite trailer was
stiff and strong enough.
Liz and Hans had also calculated very quickly whether a heavy driver walking
through the trailer would cause the trailer to bend too much. To calculate this
they considered a mass of 200 kg on a surface of 20 cm 2 , this should simulate a
heavy weight driver standing on one foot. The strains in the floor were found to
be below allowable strains and the driver would not have the feeling that the floor
was saggy.
The load scenarios turning a corner and braking were calculated by hand, not
using finite element modelling. 8 Load scenarios were still needed to make the
braking and turning corners calculations. Estimations of the mass and
acceleration during braking and turning a corner were necessary. The engineers
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7 Notice that this is different from the 23 mm that the engineers attained before but the material
thicknesses had also changed. As I indicated before, material thicknesses and properties were
changed simultaneously with the load scenarios.
8 These calculations are called “hand calculations” but for most calculations a computer
program (Mathlab) is used to solve the equations. This computer program is widely used
throughout the engineering world for analytical and numerical calculations. The difference with
the finite element program is that finite element calculations are always numerical solutions,
and that, to perform such calculations a model of the product or part needs to be made. It is not
necessary to make a model when using Mathlab.