• aditya mehrotra.

reviewing Von Mises stress and stress tensors [general knowledge]

I think this is a really good thing to know, and especially since I'll need to understand how to do real design next year (hopefully), I want to understand this because it's an important criterion that allows us to determine when something will and won't yield. I want to go over my 2.001 notes for this.

Where the T's are the shear stress and the Sigma's are the axial stress in the given directions. From there we calculate the Von Mises stress and compare Von Mises to the axial yield stress to get the safety factor but there are other interesting things Marty Culpepper pointed out when talking about this I want to save, and the most important part is knowing how to write the stress tensor.


I think to learn about these things I want to actually design something, for example, a space pressure vessel or like a suspension for a solar car, we will do/try that at some point. Need to find me 2.001 notebook(s) first.



Woop found them!



Here are some interesting tad-bits form the recitations. Now one good point is in the first picture. WRINKLES HAPPEN WHEN STRESSES GO FROM TENSILE TO COMPRESSIVE IF THAT HAPPENS ANYWHERE WE GET A WRINKLE.


I do want to take more MechE design/structures classes in the future but for now what I want to do is try to hand-analyze a structure like the sky-crane that landed curiosity on Mars. It actually may not be that bad to just analyze the structure if we make some assumptions. Let's design a frame that can do those kind of things. FOR NOW WE'LL IGNORE HEAT AND AIR

RESISTANCE JUST FOR KICKS.


"When the capsule hit the atmosphere it decelerated from about 7.3 km/s to 0.4 km/s (16330 mph to 900 mph) over three minutes." According to google, and that seems legit from NASA's data about the insight probe.


https://mars.nasa.gov/insight/timeline/landing/entry-descent-landing/


Requirements: the craft needs to enter the atmosphere at ~ 7.3km/s it slows to 0.4km/s when it fires its thrusters, and then it needs to finally slow to 8km/hr approximately. https://earthsky.org/space/landing-on-mars-is-still-hard


Let's ignore things like the heat-shield and etc, the atmosphere itself slowing the craft to 0.4 km/s. Let's focus on that second region first we need to slow to land and we need to fire the thrusters and the structure needs to be able to support that kind of force without yielding. We're going to try to do this with minimum number of bars too. (we'll design it as a space-frame).


____why am I doing this?____ to get more familiar with design/FEA, that's it. We're designing a very basic version of this:


Here's the basic info we extracted from this:


So we extracted curiosity is about ~900kgs, and the craft slows to literally stand-still, 0m/s, and starts at about 100m/s! There are 8 total thrusters on the frame and curiosity sits within the frame.


We're going to try to design something like this for CHIP not for curiosity. We'll start with a drawing and explain the functions of the design right now, and then get into the design itself.


Here's the drawing we made so far:


I think Friday is when we'll get in to things like the Free Body Diagrams, the CAD and etc.


#updates #design #new_mini_projects

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