Maybe you’ve already thought about how weight affects motion, but what about static structures or fabrications? Weight can be important there too. Some point to consider are:
- Will the fabrication need transporting? Almost certainly yes, unless you’re assembling it on-site. (A steel-framed building, for example.) Lighter will make it easier to move, and perhaps less expensive too.
- Is it self-supporting, like maybe a spiral staircase. (Perhaps it’s going up the side of a silo or in a building.) The heavier the staircase the stronger it must be to support itself. Use a lightweight material and the staircase needn’t be so big and bulky. Same goes for shelving or a mezzanine floor.
Choosing your Material
Let’s start with weight, well more specifically, density. Steel weighs 490lb/cubic foot. (The type of steel – stainless, mild, 4340 – doesn’t make much difference.) Aluminum weighs 169lb/cubic foot and GFRP comes in at 112lb/cubic foot.
(Brief digression here: let’s explain what GFRP is. Sometimes it’s called fiberglass but it’s a bit more complicated than that. GFRP is a matrix of glass fibers embedded in a polymer, typically an epoxy, less often nylon or polycarbonate. The fibers provide strength and impact-resistance while the polymer holds them in place. GFRP is generally strong, lightweight and inexpensive.)
If you’re just looking for sheeting material, maybe to skin a trailer, you’re probably not concerned about strength. In that case both aluminum and GFRP will help you bring the weight down. (That’s one reason the Corvette has a fiberglass body and why aircraft are skinned in aluminum.) However, in many applications strength is a design consideration.
Strength is expressed in many ways. There’s toughness, Young’s Modulus and so on. For simplicity, here we’ll just talk about Ultimate Tensile Strength or UTS. (UTS measures how far a material can be bent before taking on a permanent deformation.)
UTS numbers for the materials discussed here are:
- Mild steel (1020 grade): 380 Mpa
- 304 grade stainless: 590 Mpa
- 7075-T6 aluminum: 570 Mpa
- GFRP: 530 Mpa
Before you start specifying one of these materials in your design please remember that UTS depends a lot on the precise grade of material. However, you can see there’s not a lot to choose between a 7000-series aluminum, stainless and GFRP. (Other grades of aluminum are not as strong as the 7000’s.)
Something else to consider under the heading of strength is fatigue resistance. Aluminum is at a definite disadvantage here as it’s prone to cracking under cyclical loads.
The Cost Dimension
If these materials all cost the same, at this point you’d probably be picking GFRP for your next project. Of course, cost is always a factor, and here’s where things can get real complicated.
First, there’s the material cost itself. Then there’s how much of it you need, and finally there’s the cost of turning it into the shape or fabrication you want.
Material costs fluctuate, so for comparison we’ll look at them relative to the cheapest, mild steel. Roughly speaking, aluminum costs 3.5 times more per pound than steel. GFRP is almost eight times more expensive per pound. Stainless is about five times the cost per pound.
But, and this is a big but, as the more expensive materials, (stainless excepted,) are also less dense you don’t need so much of them. GFRP is about one quarter the density of steel so your cost penalty per cubic foot is a factor of two and not eight. A similar argument applies to aluminum.
Then there are manufacturing costs. Aluminum is readily cut and formed, (and available in a wide range of extruded sections). Steel is harder to work but is also available in a range of sheet, bar, angle and tube forms. GFRP however needs complicated lay-up and molding processes. These are messy, take time and can have environmental and Health & Safety issues.
If you’re not already thoroughly confused let’s throw a few more points at you. Does corrosion-resistance matter? Steel, other than stainless, likes to rust and needs protecting while aluminum won’t rust and can be polished to a bright shine. GFRP won’t rust but neither will it look very pretty: you’ll definitely want it painting.
How about thermal expansion? Fabricate your structure from aluminum and it’ll grow markedly as the temperature rises. Whether or not that’s a problem depends on your application.
What about recycling? Maybe you’re not concerned now but at some point your new fabrication will be broken up and scrapped. At that point you might wish you’d chosen aluminum with it’s healthy scrap metal market. In contrast, GFRP is just going to (expensive) landfill.
Choosing your Lightweight Material
So, having explained why weight should be a consideration in everything you design, let’s talk about the material to use. Obviously, there’s no single “best” material: as with every design decision, it depends on what you’re trying to achieve.
If weight is absolutely paramount, as in racing, then GFRP might be the way to go. And staying with vehicles, if you want to maximize trailer payload an aluminum skin might make sense. Though not as light as GFRP it’s inexpensive, easy to work and doesn’t even need painting.
For static structures rather than something, mobile aluminum could be the right choice. You can make lightweight fabrications from extrusion and sheet that will be transportable but still have a lot of strength. (They’ll probably look good too!)
So what about steel? Well it’s not really a lightweight material, but it is inexpensive. Regular 1020 does need corrosion protection, which adds cost, but you might also consider stainless. As strong as GFRP, though also much heavier, like aluminum it has the advantage of looking good without needing paint.
So what’s the bottom line? Well if you’ve decided it’s time to “add lightness” to your fabrication talk to us. We may have some material suggestions that you haven’t considered. If you don’t ask you won’t find out.