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ROVER WHEELS: Monitoring changes over time, NOTE: Read back through the thread to avoid repeating misconceptions
algorithm
post Aug 29 2014, 07:25 PM
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I read in earlier posts that the wheels suffered catastrophic failure when the treads had basically been damaged across the entire circumference, and that the wheels would basically split into two parts, one third and two thirds the wheel width.
I have noticed in recent sols that the profile of the stiffening ring/flexure rim is becoming more pronounced as seen from the wheel exterior.
It would seem that subject to exterior pressures this area is indeed a point of weakness and potential tearing, and would therefore bear out the testiing results.


Attached Image


Does this seem like a fair assesment?

Edit:When I say increased weakness, I mean increased tensile stress for the outer rim across the wheel width at that point.
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elakdawalla
post Aug 29 2014, 07:31 PM
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The stiffening ring being a nexus of non-damaged wheel has been visible since they first started studying the problem on Mars. I think one of the best pieces of news about the testing program I've heard lately is that they've done such a good job recreating the problem on Earth. That shows they understand the causes, and suggests that mitigation methods that work on Earth to prevent further damage might also work on Mars.


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algorithm
post Aug 29 2014, 07:34 PM
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As I said, this would bear out the testing results.
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algorithm
post Aug 29 2014, 08:00 PM
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QUOTE (elakdawalla @ Aug 29 2014, 08:31 PM) *
and suggests that mitigation methods that work on Earth to prevent further damage might also work on Mars.


Lets hope so, in fact i'm sure that they will.

The thing I wonder is, how far does the rover have to travel across what is damaging terrain, to find a traverse/terrain that consistently allows the results of those methods mentioned to become apparent?

We need a guy to rake out the bunkers! smile.gif

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jmknapp
post Aug 31 2014, 12:31 AM
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Does the ambient temperature affect the strength/ductility/brittleness of the aluminum wheels much? There can be more than a 100°C swing from day to night temperature.

EDIT: found this reference:

QUOTE
Unlike most steel grades, aluminium does not become brittle at low temperatures. Instead, its strength increases. At high temperatures, aluminium’s strength decreases.


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serpens
post Aug 31 2014, 10:48 PM
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QUOTE (jmknapp @ Aug 31 2014, 01:31 AM) *
Does the ambient temperature affect the strength/ductility/brittleness of the aluminum wheels much?

In the temperature range that Curiosity experiences the change in marginal increase in tensile and yield strength is marginal. While various aluminium alloys differ in response it is really above say 50 C that the effect becomes really noticeable. Certainly this would not even be a consideration as a risk benefit for Curiosity.
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jmknapp
post Sep 1 2014, 01:03 AM
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Sounds like you're looking at some data. What alloy are the wheels made of?


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serpens
post Sep 1 2014, 04:21 AM
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I think (relying on memory) that it is 7075-T6 alloy.
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jmknapp
post Sep 1 2014, 06:59 PM
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Evidently that matters somewhat, as well as the fracture direction:

Attached Image


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serpens
post Sep 2 2014, 12:38 AM
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Most of the trials and data for aerospace alloys and tempers has been directed towards aircraft applications where fatigue crack growth rates get a lot of attention. But with Curiosity's wheels we are well outside normal designs and comfort zones and for the thin skin between cleats deterioration will be a function of accumulating penetrative injuries rather than crack growth. The cleats themselves seem to be hanging in there but as I think Paolo mentioned will be subject to repetitive flexing although the frequency of such is anyone's guess. Mars has thrown up an unexpected, damaging terrain that could not possibly have been anticipated from orbital imaging, but overall the mobility system has met all performance expectations. But in the temperature range at Gale there would be no wheel durability advantage in night driving.
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jmknapp
post Sep 2 2014, 12:54 AM
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Just a note: plane strain fracture toughness is not related to fatigue per se.



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serpens
post Sep 2 2014, 02:18 AM
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I really think that we have to differentiate between the deterioration of the thin skin of the wheels and the cleats. The damage to the skin occurs when an encounter with a shard of rock results in the weight on a wheel being taken completely or primarily by the thin skin rather than a cleat. This seems to result in a localised tear or deformation which weakens that particular area and any further encounter will exacerbate the damage. I stand to be corrected but I do not think that fracture growth is of particular concern given the failure mode. The cleats are a different matter in that they can initially withstand such encounters and the probability of failure (fracture break of the cleat) would seem to be a function of fatigue relating to the number of encounter cycles and encounter geometry for each particular cleat. I don't know to what extent the thin skin contributes to the structural integrity of the cleats and whether deterioration of the skin reduces the number of cycles to failure for the cleats. What I am trying to get at I guess is that even the empirical testing to failure conducted once the terrain and wheel problem was recognised is only indicative. It would be interesting to know though whether that testing indicated that the cleat failure was a catastrophic event (sudden break) or growth of an initial partial crack.
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Gerald
post Sep 2 2014, 11:33 AM
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QUOTE (serpens @ Sep 2 2014, 04:18 AM) *
It would be interesting to know though whether that testing indicated that the cleat failure was a catastrophic event (sudden break) or growth of an initial partial crack.

Take a wire and bend it several times. In the beginning you won't feel much difference. After a while the surface gets rough, meaning there form lots of cracks; for thick wires you can even hear it, sometimes. Then the wire gets rapidly weaker at one position, since one or more of the cracks grow through the wire. Ductility gets lost, too. The break then occurs rather suddenly.

The cleats should behave much the same. Once the strength starts to decrease in an obvious way due to fatigue, more or less sudden failure will follow soon due to rapid growth of a crack.

The skin should reduce one degree of freedom of bending of the cleats (parallel to the circumference), and disperse local forces to several cleats.
Therefore loss of larger areas of the skin won't be an advantage for the cleats.
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Gerald
post Sep 10 2014, 11:43 AM
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Tears and dents in right central wheel formed hole between Sol 667 and Sol 744:
Attached Image
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peter.neaum
post May 2 2015, 12:23 AM
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I just stumbled across this pic.
Amazing.


Reason for edit: Replaced huge inline image with thumbnail and link
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