elastic deformation of mild steel anti-roll bar

Do some study on Young's Modulus (Modulus of Elasticity).

Thanks for the non-help - since when does Young's Modulus address fatigue factors in typical mild steel - which was my question? Not in my experience.

Brian

You gave no dimensions or otherwise allowing for any help. Your question read as if you were ignorant of the basic physical facts re. your "problem".

Bob Swinney

Your question is incorrect: Since the bar took a tweak, it is NOT elastic deformation. Look at a standard stress/strain curve to failure for a mild steel: elastic region (straingt line upwards) on the left, then inelastic wobbly line to failure on the right. You have used up some of the available space between elastic and failure. How much is hard to judge. If it took a substantial tweak, I don't want to drive it at speed.

Brian wrote:

But you really asked two questions:

1. Is it strong enough? Which everyone here agrees is "no,"

and this one:

which the Young's Modulus answer DOES address. This is because the YM of mild and 4130 is the same, they just fail at different points (the 4130 can take more load before failing but under the same load both respond the same). This means the behavior and handling with the existing bar will be the same as with a 4130 replacement of the same design.

--Glenn Lyford

It is extremely unlikely that your suspension bar is mild steel. Most are heat treated alloy. You have twisted it past the yield point of the material. When you twist it back you will have to exceed the yield again only the second yield likely will be a greater force. If you manage to get it back to the original shape without fracturing it just might fracture when you are putting it in service. Imagine bending a paper clip back and forth only you are doing it with a considerably less ductile steel. Randy

My thought is to replace the ARB with a 4130 heat treated version , but I want to drive the car first to see if the bar rate is close to right. Then, I'll make up one with the correct rate in the correct material.

Thanks, Brian

I'm going to skip the lecture about how the bar has already exceeded its yield strength and may fracture in use, as that has already been covered by earlier posts. Therefore, I'm simply going to focus on the practical applicability of your plan outlined in your last paragraph.

It will be impossible for you to gather any meaningful data based on the drivability of your current (damaged) bar, because you will never know the true spring rate of that bar. Since it will have been subject to plastic deformation (twice: once when damaged, and once when "fixed") and possible heat treatment/tempering in the course of your bending it back to shape, you'll have no clue as to the properties of that (now non-linear) spring. It certainly won't behave anything like an identical, undamaged bar.

And rarely ever does a fracturing paper clip put anyone's life or valuable property at risk.

- Michael

Mild steel has very good, but not infinite fatigue resistance. Constant repetitive bending, like bending baling wire in half will quickly lead to failure. In your automotive application, probably not a problem. Bugs

Good post.

Jim

Step one Think of what might happen if the bar fails while you are driving it to see if the bar is close to right.

Step two would be to twist it back into alignment cold. The bar is almost sure to have been heat treated ( unless you know it was made of mild steel )

Step three would be to twist it within the limits of what you expect it to occur in use and verify it does not take a set ( or break ).

Step four would be to do your test drive if you feel like it is safe.

Step five would be to remove it and measure the spring rate. It may not be the same as it was originally, but it can be measured.

Note Well, I am not advocating doing this. Just how I might approach doing it. I certainly would not try it anywhere except some place like an airport where there is no objects that can be hit.

Dan

It's unlikely that it will show any difference in elastic twisting after being bent back. Young's modulus is a property based on the inherent material property and the geometry of the part, not on residual stresses or temper condition, in steel and most other metals. It probably will be as linear as it was before bending -- up to the point of plastic deformation. If there's a kink in it now, though, it will depend on how the bar is supported when it's loaded.

-- Ed Huntress

Probably its torsional spring rate will be indistinguishable from the bar in its original condition. As for its fatigue life, it will depend on whether any damage was done when it took a set (when you bent it past its elastic limit). With mild steel, probably not. But you couldn't tell without a metalographic examination -- or by running it until it breaks. Keep an eye on it. If you're going to replace it with 4130, do it as soon as you've judged whether you have the right torsion rate.

A rough measure can be had by having the bar Magnafluxed. If there is any intragranular tearing, it should show up as microcracks. There's an almost equal chance, though, that any cracks that show up were there before the bar got bent.

Fatigue usually gives plenty of warning in ductile materials like mild steel. You may have to Magnaflux it again after running the car a few times, if you keep it in the car that long.

-- Ed Huntress

That should read "intergranular tearing." Fatigue is a phenomenon that occurs BELOW the elastic limit, from repetitive cycling. The tearing I'm talking about is a result of strain failure BEYOND the elastic limit, and which could hasten actual fatigue failure.

It could take the form of cracks that start on the surface, and which will show up clearly with Magnafluxing, but the insideous form is tearing that occurs just below the surface. Again, it's unlikely that you have a problem that won't show up with Magnafluxing.

-- Ed Huntress