Alan2smiley
I'm in the shop
I'm building a cross member wondering what would be better to use 1" .120 wall DOM or 1" .250 wall DOM. I'm using a 6" die.
CK5
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Whats stronger DOM .250 wall or.120 wall?
- Thread starter Alan2smiley
- Start date
.250 wall is WAY overkill for that. Plus bending it will bring the suck.
Alan2smiley
I'm in the shop
Yeah it probably is but I see stuff like this and it makes me think there is no such thing as over kill.
Alan2smiley
I'm in the shop
And up travel is not a thing.
rampage
3/4 ton status
Of course 0.250 is stronger.
The questions should be, is it necessary for what I am building? Can I afford it?
The questions should be, is it necessary for what I am building? Can I afford it?
Alan2smiley
I'm in the shop
Different types and size of metals crack when bent that makes them weak, This piece cracked when trying to tweak after it was bent. I'm thinking the fab shop I hired to make these welded them after they cracked. The question isn't for anyone specific.
Thicker wall needs a larger radius than thinner stuff.
Also looks like they were done with a pipe bender, like Harbor freight bender
Also looks like they were done with a pipe bender, like Harbor freight bender
Alan2smiley
I'm in the shop
After bent with 6-in die Is solid 1-in round stock stronger then .120 DOM tubing?
From what I remember, tube is stronger than solid. But I guess not after googlefu haha.
In all respects, a solid bar will be stronger (and stiffer) than a hollow bar of equal diameter according standard beam theory (which assumes linear elastic behavior). This is not arguable. I'll work out the math for examples of tensile strength/stiffness, compressive strength/stiffness, and bending strength/stiffness.
***Tension***
(stress) = F/A
The cross-sectional area of a solid bar of equal diameter is larger, thus the critical force F required to reach the yield stress of a material is higher for a solid bar. This means more tensile strength.(strain) = (stress)/(young's modulus)
Rearranging to get stiffness,(deflection) = F / (stiffness)
where (stiffness) = (young's modulus)*(area) / (length)Again, larger area corresponds to greater stiffness.
***Compression***
Compressive stiffness is the same as tensile stiffness.
For compressive strength, there are 2 primary failure modes: Euler buckling and Squash (yield) buckling. Johnson's rule determines where and how to transition between these two modes, but as I will show, a solid bar is stronger for both and there is no need to apply this (it's somewhat intuitive to me and the math will be too difficult to type without a proper typesetter so trust that this is the case)
For Euler,
Pcr = 
where Pcr is the failure load, E is young's modulus, I is the area moment of inertia, K is a constant depending on the loading conditions, and L is the length of the rod/pipe. E, K, and L are the same for both because those depend on the conditions in which the pipe is set up. I is larger for a solid pipe (google "2nd moment of area" for the equations) .For Squash buckling, the load corresponding to the yield stress is determined the same way as in tension.
I'll make a note here that Euler buckling is catastrophic and is more likely to occur for beams with smaller area moment of inertia (a result of Johnson's rule which predicts departure from Euler's at half of Euler's yield stress) and therefore pipes are more likely to experience this failure mode than squash buckling. I think someone in the linked thread was trying to get at this but didn't back it up with evidence or equations but rather by intuition.
***Bending***
For critical load, we use the equation
[tex]\sigma_{max} = \frac{Mc}{I}[\tex]
where [tex]\sigma_{max}[\tex] is the maximum stress in the cross-section of the rod, M is the moment (aka torque) at a location in the pipe/rod, c is the radius of the rod, and I is the area moment of inertia. Yielding occurs when [tex]\sigma_{max}[\tex] exceeds yield stress. For the same loading conditions and equal diameter, a solid rod has larger I thus it will have less stress in the beam. Thus, the critical load which causes yielding (which influences M) is higher for a solid rod.For stiffness, there are various different loading cases/configurations for a beam, but I'll use the example of a cantilever because the equations are simpler.
(stiffness) = [tex]\frac{3EI}{L^3}[\tex]
and, again, I is larger for a solid beam resulting in greater stiffness.Now for the reason that I believe this rumor is so widespread:
People are correct in stating that the centers of rods have generally less effect on strength/stiffness. If you look at the area moment of inertia equations, for a circle I is proportional to r^4. That means doubling the radius increases strength/stiffness 16 fold for the equations above which contain I. Since area moment of inertia can be analyzed with superposition, cutting out the center circle makes the new area moment of inertia proportional to (R^4-r^4) where R and r are the outer and inner diameters respectively. Of course, r has a much smaller effect than R.
What does this mean? This means that in any equation above which contains I, the strength to weight ratio of a pipe is much higher than for a rod. For any equation which uses A, cross-sectional area actually increases proportionally to weight so the strength to weight ratios are pretty much the same. Why is strength to weight ratio important? Of course, designing a building, for example, means the weight of the structures at the top of the building are going to be weighing down on the structures at the bottom of the building. In animals/plants, lighter means fewer resources are required to achieve the same strength, as well as allowing for better mobility (recall F=ma ... same muscle force garners greater acceleration if mass is decreased). The Eiffel Tower took inspiration from bone structures and showcased the ability of a mostly empty structure to support so much weight.
In conclusion, saying a pipe of equal MASS is stronger than a rod is generally true (depending on which metric of strength is being used, they may be the same). Saying a pipe of equal diameter is stronger than a rod is flat out false.
I hope I was able to clear up any misconceptions.
For reference, I'm finishing up an undergrad mechanical engineering degree and I've been taking and TAing multiple structural analysis courses every semester for multiple years. For those of you who don't trust theory/equations, yes, I've also seen these tests done in real testing machines for materials like steel, aluminum, titanium, plastic, carbon fiber, etc. and you'd be shocked how closely the results match the equations. The biggest departures from theory generally appear in heat treatment (which generally just changes the stiffness and/or yield point but does not change the fact that the material still follows linear elastic behavior), nonlinear behavior (i.e. loading past yield and some strange materials), and non-isotropic materials (i.e. wood, carbon fiber, etc). For ordinary steel, copper, or aluminum pipes/rods, this theory 100% applies.
Also,
However, also consider different failure characteristics.
½" conduit will collapse when bent sharply... so once it is bent, there remains far less structural support. The ¼" rod can be straightened again and may have a large percent of its initial strength.
However, also consider different failure characteristics.
½" conduit will collapse when bent sharply... so once it is bent, there remains far less structural support. The ¼" rod can be straightened again and may have a large percent of its initial strength.
is this going to be a single tube? i would think 2 at the .120 wall, 4" to 6" apart tied together with some thinnish plate here and there would plenty stout.. think ORD's one is similar...
Alan2smiley
I'm in the shop
I never understood why but I always thought hollow was stronger than soild tube, 1" .250 is almost soild is why I question it's strength after bent. Thanks for the info.
I did catch someone saying that the closer the material is to the center, the less effect it has on the strength. Which makes sense.I never understood why but I always thought hollow was stronger than soild tube, 1" .250 is almost soild is why I question it's strength after bent. Thanks for the info.
Alan2smiley
I'm in the shop
Yeah I was thinking typical cross member is flatter therefore shorter and stronger? Not sure. I'm trying to keep it up tight for more up travel and less lift. The fat diesel oil pan doesn't help much with clearance. I'm thinking a brace from the coilover hoops will help tie everything together as well.
i'm going with Agan Fab's I think...
bp71k5
3/4 ton status
Solid is always stronger.
A trick to catch if your logic is right. Take a solid round bar and lathe a hole in the center all the way through it. Does it become stronger because you drilled a hole in it? No. There is a diminishing return effect that happens as you increase wall thickness though. No need for solid bar if the same diameter tube will work.
