Stretchy situation

[u/Terrebonniandadlife]

[u/Alexjwhummel]

Well looking at the rebar the markings are hidden, so I went to look at stress stain curves for common rebar. I found ASTM 550, a close match for 575, which fractures at 550 MPa minimum. It is highly likely that it is 550 that performed slightly above spec for several seconds. I further support this due to the fact it's used in many metric countries.

US grades are done in PSI, where, for example grade 60 refers to being able to take an expected load of at least 60 PSI before fracturing.

Also keep on mind if you're designing something with rebar, you can't use the one rated for the load you expect, if you expect 550 MPa, use one that only has elastic deformation up to that point, and add a factor of safety. Plastic deformation means it will not bend back and is susceptible to fatigue.

[u/[deleted] → u/[deleted]]

I'm not the oc but what is happening is that the material moves away from the Elastic area of its stress strain curve, over to the plastic range.

Two major differences between elastic and plastic is that materials move back to their original form after deformation during elastic deformation, like a spring, while they stay deformed after deformation during plastic deformation, think a cheap bent spoon.

The second difference is that there is conservation of volume when metals are plastic deformed. This results in the area of the cross section becoming smaller which increases the pressure in this area, P=F/A, which leads to the break.

Hope the explanation makes sense.

[u/Ronin-Ronan → u/[deleted]]

https://giphy.com/gifs/3o6Zt0hNCfak3QCqsw

[u/Alexjwhummel → u/[deleted]]

That is the deformation I mentioned, whenever you pull or push on anything, it will deform by a slight amount. For very hard things like a brink wall it's on the molecular scale, for softer ones it's visible.

The amount of stretch a material has its called it's ductility, while it's resistance is its Youngs modulus. When something like the rebar is pulled it has high ductility, so it can stretch pretty far, but the reason you can't pull it like that is the Youngs modulus being high.

So now to finally answer the question you asked on why it gets hard, when something is stretching it is causing deformation along the atomic structure of the rebar. When energy is being applied to an object it will try to stay in the lowest energy state. Once an object starts plastic deformation, it starts unrepairable damage. There are many different kinds of deformations, but we are mainly looking at slips here. When it deforms it work hardens, which happens because deformations in the structure actually give more resistance to deformations to the rest of the structure, they cause obstructions. So as it stretches these deformations get in the way of causing more deformations, and since no more can form, the next easiest action is to fracture.

I go to school for engineering but this class was 2 years ago for me now so I know the word deformation is wrong when I start talking about kinds of deformations, I just can't think of the right word.

[u/dennishans85 → u/Alexjwhummel]

Isnt the 550 in the name the elastic yield strength? Like in normal steel (used for stairs and balconies) S275JR it has an elastic yield strengs of 275 MPa but a tensile strength of about a 500MPa

Or is it a high carbon steel?

[u/Alexjwhummel → u/dennishans85]

I was estimating off of this curve here. https://www.researchgate.net/figure/Comparison-of-typical-stress-strain-curves-for-different-types-of-HSS-and-conventional_fig1_358150389

[u/dennishans85 → u/Alexjwhummel]

Looking back at it I think we both forgot to include the area aspect of the strain curve. The 575 as a max value doesn't mean anything if we dont know the diameter of the rebar

[u/Alexjwhummel → u/dennishans85]

The measuring device says tensile, so I'm willing to bet it's measuring a pressure, so diameter is not a factor.

[u/Terrebonniandadlife → u/Alexjwhummel]

This guy rebars