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Each leg of Ingenuity has what I'll call a knee. After just a few flights, on 2021 May 10, NASA wrote:

By plastically deforming and fatiguing as it absorbs energy, this flexure acts much like the crumple zone structure of a car chassis. However, unlike a car or the crumple-cushioned landing gear of the Apollo moon landers, Ingenuity’s titanium springs rebound after each impact to pull these aluminum dampers back into shape for the next landing. The aluminum damper gets a little bit weaker with each cycle as cracks and creases develop. While it would eventually break after a few hundred hard landings, with only a few flights scheduled for this demonstration, that’s a problem we could only dream of having.

Almost a year later, it's had 23 flights. Have any of its sensors, such as its IMU or altimeter, yet been able to detect how quickly this expected gradual cracking is developing? Has some threshold been chosen, beyond which Ingenuity should stick to only gentle landings?

BrendanLuke15's comment's linked paper says the damper is "1100-series annealed aluminum" whose damping is "largely independent of temperature or atmospheric pressure." But how does one apply fatigue datasheets for 1100 to this particular shape (1 x 2 cm? 0.5 mm thick? deflecting how much and how fast)? How differently would a partially fatigued damper behave (as if it were half as thick because of cracks)? Could that behavior be measured by Ingenuity?

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    $\begingroup$ Do we have any indication that there were any "hard landings" so far? $\endgroup$
    – asdfex
    Mar 25 at 10:31
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    $\begingroup$ I'm thinking "a few hundred" is still a low-ball estimate. This sort of tech is nothing revolutionary, pretty common in mid-tier commercial drones, and the durability goes easily into tens of thousands of landings. $\endgroup$
    – SF.
    Mar 25 at 11:42
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    $\begingroup$ Pipenberg, B. et al. "Design and Fabrication of the Mars Helicopter Rotor, Airframe, and Landing Gear Systems," AIAA 2019-0620. AIAA Scitech 2019 Forum. January 2019. would help but it's paywalled. Here is a annotated figure close up view of the "knee" from that paper $\endgroup$ Mar 25 at 12:37
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    $\begingroup$ @asdfex Good question. I would also ask "what are the definitions of hard vs gentle landings?" $\endgroup$
    – CGCampbell
    Mar 25 at 14:26
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    $\begingroup$ @SF there is a very large difference between a simple spring loaded leg and a DAMPED TRAVEL leg. Imagine a pickup truck with a heavy load in its box. With just springs on the axle the truck would bounce and bounce and bounce until the energy was dissipated as heat in the springs. The damping element works against the system 'ringing' so the helicopter can minimize landing bounce. I'd guess the carbon fiber legs are pretty stiff and not flexing (springing) much. $\endgroup$
    – BradV
    Apr 4 at 17:14

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Let me open by saying I do not have an answer.

Thanks BrendanLuke15 for linking to the knee detail image!!

While question is more "design/engineering" than "space exploration"... still very fun to consider!

I used a 1000 series aluminum in a design about 30 years ago. I do not recall the specific alloy, but I selected it because it was almost dead soft. This 1100 alloy damper is very similar in properties. It is not heat treatable, nor would one want higher properties.

I suspect Camille Goudeseune recognises that Ingenuity will not be significantly compromised if the dampers break, but my concern would be uneven bounce when some dampers break and others don't.

The dampers are not load bearing elements and the legs will still hold Ingenuity when dampers break.

my comments in red

enter image description here

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