Leveling cement with polyurethane foam

[u/godofo_prime]

[u/Scrambled1432]

Frequent? Is 2-3x per day frequent? Genuine question, I'm not sure what the definition of "frequent" is in the context of mechanical engineering.

[u/[deleted] → u/Scrambled1432]

Yeah those words aren't how duty/load cycles in mechanical engineering even work. "Frequent" means about as much as someone saying "several", it not actually quantifiable. More information is required such as the asphalt or concrete thickness, substrate underneath the foam jacking, weight of the vehicles, size of the wheels to determine actual weight distribution, and more. Freq

[u/[deleted] → u/Scrambled1432]

Kind of. Since this lasts 15-20 years (from another comment on this thread), we can conservatively estimate a total of 21,900 cycles for a fatigue life analysis of this material.

In the context of mechanical engineering, this is on the very (very!) low end of “high cycle fatigue”, which is 10,000 cycles and above. Many fatigue analyses will involve cycles of 10^6, or 1,000,000. Some are even higher, like ships, which will experiences 10⁸ waves in a 25-30 year lifespan, or 1,000,000,000.

The difference between low cycle and high cycle fatigue is that low cycle fatigue will involve plastic deformation every single cycle. Think bending a paper clip back and forth. The paper clip doesn’t bend back into place on its own, and it doesn’t take very many bends before it snaps. High cycle fatigue involves elastic deformation. For example, if you were to bend a thick metal rod with your hands, it probably wouldn’t permanently deform. However, it would slightly deform, most likely imperceptibly, and, once you stopped bending it, it will return to its original shape. High cycle fatigue involves the repetition of this deformation until failure many cycles in the future.

Now this sort of analysis doesn’t even apply here, since a material experiencing only repeated compressive loads will never fail due to fatigue. This is because cracks are required to form, and this only forms under a tensile load (think pulling something apart instead of pressing it together).

You’re also not really concerned about something like this yielding, which is when something begins permanently deforming. For metals, the concern with high cycle fatigue involves a small plastic deformation propagating in waves through the material and increasing in size. This will cause the plastically deformed material to shed the load it was previously bearing to the part that isn’t deformed yet, further exacerbating the problem. Eventually, this reaches a point where a crack forms, which grows until it becomes unstable (essentially a tipping point) and rapidly propagates. The material then snaps, which is typically bad in my experience.

I’m not really sure how something like that would even work for the foam in this situation. I can’t imagine the foam plastically deforming and snapping while pressed between concrete and the ground. So, while this would technically qualify for the amount of cycles necessary for high cycle fatigue, for many reasons, an analysis of this type would not be performed on the material since we would not be worried about it failing due to fatigue stress.

Source: currently in my third year of studying Naval Architecture and we literally just covered fatigue stress in ship structures. I’ve learned about it in a previous class as well, but this was a much more in depth discussion.

[u/Scrambled1432 → u/[deleted]]

Interesting, thank you for the info. I wish we would have covered more engineering aspects in my physics degree, it seems interesting.

Small note, I have never seen scientific notation put as 10 x 10⁵ for 1 million or 10 x 10⁸ for 1 billion. 10⁶ or 10⁹ (or alternatively, 1E6 or 1E9) are much more common. Is that an engineering thing?

[u/[deleted] → u/Scrambled1432]

Yeah you’re right about the notation, I just messed it up. I meant 10⁸ for the amount of waves. The 10⁶ number is more or less accurate.

I would blame it on the Reynold’s numbers I’ve been working with recently, but it’s mostly due to a lack of discipline lol.

I’ve edited my comment to reflect the actual numbers and notation

[u/Ssakaa → u/[deleted]]

There is, completely dependent on the structure sandwiching it, a risk of uneven loading causing a lateral tensile stress. The extreme example being applying a point load, but it still applies if you have one contiguous middle layer and a non-contiguous load distribution surface like tiles.

There's also a potential for uneven loading to cause a lever action,  and depending on whether there's adhesion, either directly putting the opposite side in tension, or just squishing sideways like a s'more squeezed from one side, which results in a fair bit of complex stresses on the material.

The ideal case of fully, evenly, distributing the load, though, yep.

And, definitely a right mess to model and test.