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In this video, I'm going to demonstrate how to use Truss Me! to design the structure of
a moon lander.
Using the button for adding joints, we first insert two joints with the purpose of creating
a protective cage around the payload.
Using the button for adding bars, we then insert the corresponding structural members
between joints.
We must make sure that the payload is connected to the protective cage by adding all necessary
bars.
To see how the structure behaves up to this point, we hit the play button and start the
simulation. As you can see, the landing gear is not properly attached to the protective
cage, so we add two more bars to fix this problem.
We now hit play again to repeat the simulation. As you can see, the two bars that we just
added fail. In this case they fail due to a compressive instability called "buckling".
Let's take a look at this again.
There are two ways of addressing buckling problems on bars. One way is to make the bars
shorter. We can do that by redesigning the geometry of the structure. The other approach
involves increasing the moment of inertia of the bars. in practical terms, this means
making the bars wider.
To do so, let's push the "make wider" button on Truss Me and then touch the bars we wish
to make wider.
As you can see, the overall weight of the structure increases accordingly. This is a
penalty we pay when increasing the cross sectional area of our structural members. If we run
the simulation again, we can see that the structure now fails at a different point.
This is a very important lesson of structural mechanics: when we fix a structure by making
it stiffer somewhere, it most likely will fail somewhere else as loads redistribute.
In our case, the structure now fails at the landing gear, again by buckling. So we are
going to fix it the same way we did before.
If we run the simulation one more time, we now see that failure is occurring on the top
bar of the protective cage.
As you can see, buckling is the problem again. We fix the problem by making that bar wider
as well.
We run the simulation again, hoping that this time everything works fine.
As you can see, the spacecraft landed safely without breaking any of its structural components
and the total mass of our first successful design is 53 kilograms.
You can repeat the simulation as many times as you wish to better understand how the structure
behaves. Colors during the simulation reflect the state of the bars. Blue means that a bar
is under compression, that is, that the overall deformation of the structure is trying to
make that bar shorter. Red means that a bar is under tension, that is, that the overall
deformation of the structure is trying to make that bar longer.
By looking at the structure we realize all bars at the top are under compression while
all bars at the bottom are under tension.
This can be understood intuitively by looking at the form of our structure. It is like if
we built a compression arch at the top, so that bar over there is not helping very much
because it is making that arch a lot stiffer. The structure would be much more compliant
if we allow it to deform without having that arch structure. So, we could remove that bar
and see what happens. So let's keep in mind that our weight of the structure at this point
is roughly 53 kg. So let's se what we can get by optimizing a little bit our structure.
Let's start by removing the top bar to give the overall structure more flexibility. Also,
let's resize all bars to their original cross section. By doing only these minor modifications
we can see that the overall weight of the structure was reduced by over 30 percent,
with a new overall weight of 36 kg.
We hit play and keep our fingers crossed... and the structure passes the test!
Lots of deformation, sure, but it did pass the test.
Basically, all the energy in the structure was dissipated on that lower bar under tension.
Lots of plastic deformation on that bar but the structure survived.
Let's take a look at this again.
The structure drops, hits the ground, bars deform plastically, and they damp the energy
out.
So let's keep looking how we can keep improving our structure.
Those two nodes over there were originally placed to support the protective cage. But
now we no longer have a bar there so there is no need of having those nodes up there.
Having that in mind, let's move those nodes down a little bit because now we don't need
them there. We can save even more weight by making those bars shorter.
Now we're at 31 kg. Let'*** play and see how the structure deforms... and voila! We
passed the test again!
Let's try to make the structure even lighter. We see all those bars at the bottom that are
under tension. Let's try to make them narrower to see what happens. Now, we hit the narrowing
tool and touch those bars, hit play... and now the structure deforms a lot and that bar
over there yielded. So that bar failed by yielding. This is different than the failure
we had before under compression. Now the bar is initially elastic, then plastifies up to
point where the bar just gets cut in two.
Let's put that bar again to its original size and run the simulation again. Now we see that
we passed the test. We reduced the weight of our original structure by 50 percent.
I hope you liked this video. This just shows a couple of capabilities of Truss Me! and
how you can use Truss Me! to design and optimize structures.