Strength versus weight…. the common belief is that you can drill holes in practically anything to make it lighter, without compromising strength. We drill holes in everything, and seldom worry about the strength of what is left behind. A lot of the time it doesn’t matter.
Sometimes strength and stiffness is critical however. In that case, is it true that drilling holes is the best way to add lightness? I recently made some simple bolt together bracing for a car with a very flimsy body being asked to hold up a wing with the potential to make a lot of downforce. The braces consisted of some frame and body side mounting brackets and with aluminum bar connecting them. Simple.
And keeping it simple, what is the best way to keep things light while maintaining enough stiffness to prevent the area surrounding the trunk from caving in on itself at speed?
The easiest thing to do is to cut the bars out of solid 3/16″ aluminum. Pulling out my nameless auto cad program and doing some testing, we approximate about 1mm of displacement under full load, and a weight of 195grams, which came in a little bit lighter than the actual piece I constructed for test fitment on the car.
The obvious thing to do is drill some holes. Note that my testing showed that hole size has a pretty big impact on the result. A single column of larger holes does a better job of shedding weight while maintaining strength, than does multiple columns of smaller holes. Of course, making the holes too large seriously compromises structure. The best balance I found was 1.25″ diameter holes through the 2″ bar. The weight savings were 33%. Stiffness and strength drop 27% and 8% relative to our solid bar.
That seems like a reasonable step forward, but if weight savings are important, what about starting with thinner bar? Shaving 1/16″ off the solid bar lends very similar weight savings (35%), but a much larger 34% drop in stiffness and 32% drop in strength. As you might expect, considering “strength versus weight” there are negligible gains. But if the structural requirements allow, it’s a much simpler way to save weight than starting with something big and cutting holes in it. Obviously.
If this was a race car that was professionally built and engineered 20 years ago, you might see something like this. Weight savings are 46%. Strength falls only 20%, but stiffness is a disappointing 48% that of the solid 3/16″ bar. Stiffness is what is critical to this application, making this design an obvious dump. However, it is also not nearly as easy for a hack in his garage to create. Possible… but, not as simple as just drilling a few holes.
I played with a number of different designs, and was somewhat disappointed (although not surprised) that none of them provided better results than a single line of holes. That’s not to say its the ultimate: 3D structures can easily provide superior stiffness versus weight. When we have limited machining tools/skills however… and hole saws are cheap… making swiss cheese of everything gets really tempting.
What about holes in the thinner material with dimples? Does dimpling turn the 1/8″ material into the 3/16″ material strength or is it more complex?
Really cool stuff. Are you not operating out of an old wooden shack anymore lol?
What do the holes do to lateral stiffness (I can guess) I know a wing doesn’t get a lot of lateral force, but I am visualizing one with large end plates receiving a Fort Mackead 100 kph wind gust.
Murray, the brace here goes inside the rear trunk area to support the wing mount itself, and I’m counting on the body itself to provide lateral stiffness, which hopefully it can do in spades! 🙂
When I was modelling I wanted to keep it all 2D, but I confess I don’t know a great way to model a dimple. I’m also curious though, and would like to figure it out. I’m not confident my press could handle 1/8″ aluminum, have you tried anything so thick?
The wooden shack still stands, but my tools are in various places.
I bet you could do 1/8″ alum. Try it.
Modelled it… I used a piece of 1/8″ steel to get some measurements. That’s thicker than anything I’ve done yet, and it went easy, so you’re probably right.
But, I should have realized what the model result would be. Because the load is exactly vertical, adding a dimple reduces strength and stiffness. In real world where the force might be off by a degree or two it might not be so bad. With 1/8″ plate and 1″ holes, max displacement is 1.902mm. Add the dimples and displacement jumps to 4.3mm.
Dimples over rated much?
Some nerd,
“A dimple die increases the moment of inertia of the plate, Ix , over a plate with no dimple. An increase in I increases the section modulus, Sx , section modulus is the other component in determining bending strength. As an example tell him to hold one end of a piece of paper, its going to flop over, fold the paper so it makes a V, you have increased the I of the paper and now it is harder to bend. A dimple die plate will have greater compression capacity because you have increased the radius of gyration, rx , of the plate. This increase the KL/r of the plate increasing the buckling load, determined by Euler buckling. Tension is based on area only, the dimple die might have a lower strength because bending it could cause cracks in the edges of the plate. Do not say anything about rigidity since it is not a value that determines strength but is a measure of strength. ”
Just good for buckling.
Take a 4×4″ piece of sheet metal and it’s easy to bend. Put a 1″ hole in the middle and it’s still easy (easier) to bend. Dimple that hole and you won’t be able to bend it at all…
Take a 4×8″ piece with 2 dimples. It’s going to easily bend right down the middle between the holes. Like what you’ve modeled.
img.every roll cage ever.jpg
There has to be more to it because it’s supported by weld along the edges.
If you zig zag the dimples to get full coverage in the direction of bending it’s going to be rock solid.
http://img.tradeindia.com/fp/0/149/265.jpg
Dimples look really cool, so fuck it. Aerospace and F1 use them everywhere.
Ohhh, perfect example of overlapped holes.
http://image.4wheeloffroad.com/f/9603667/131_0702_07_z+4x4_fun_buggy_chassis_build+nonflared_panel.jpg
http://image.4wheeloffroad.com/f/9603673/131_0702_08_z+4x4_fun_buggy_chassis_build+flared_panel.jpg
Cool. Pretty rad looking results in that example.
“DA dimple die plate will have greater compression capacity because you have increased the radius of gyration, rx , of the plate. This increase the KL/r of the plate increasing the buckling load, determined by Euler buckling. Tension is based on area only, the dimple die might have a lower strength because bending it could cause cracks in the edges of the plate. ”
Heh!?!
In this case, working purely in compression, I still don’t think it best. However, introducing a lateral force in the model, the dimpled part performs better and better the larger that force becomes.