As you may recall from some earlier posts, we’re big fans of extruded aluminum. It’s strong for its weight, machines easily, and can be extruded into an almost limitless variety of cross-sections. This makes it ideal for applications ranging from window frames and gutter channels to machine frames and structures. Inevitably though, whenever we propose using extruded aluminum someone will ask how strong it is.
The quick answer is, “surprisingly strong,” but that doesn’t help with your design decisions. So let’s take a deeper dive into the subject of extruded aluminum strength. We’ll talk about:
By “strength” we mean the load a material can take before failing. That’s termed “tensile strength” and it’s expressed in lbf/in², or MN/m2 in the metric system. If you Google “strength of aluminum” you’ll see it relates to material grade, with some grades far stronger than others.
A second aspect of strength is the nature of the load, and this is where shape comes in. Some shapes hold up to applied loads better than others. And last, temperature can also affect material strength.
Pure aluminum is very soft. It gets its strength from the addition of alloying elements. The most commonly used are silicon, iron, copper, magnesium, manganese, and chromium. The widely used 6000 series for example is alloyed with significant silicon and magnesium and some 6000 series alloys also contain appreciable iron. The result is a corrosion-resistant, lightweight material available in medium to high-strength forms. (Typical yield values are around 30,000 lbf/in².)
An interesting point to note about aluminum is that strength increases at very low temperatures. This contrasts with steel, which becomes brittle. So when checking tensile strength values bear in mind the expected service temperatures. Also, while we’re bashing steel, remember that per unit weight, aluminum is the stronger material. (This is the “specific strength” value.) That can be important when low weight is one of the design criteria you’re facing.
Aluminum extrusions are produced by forcing a billet of aluminum through a die. This creates significant work-hardening, proportional to the reduction in area. The effect is uneven, though, being greatest on the outside edges where there’s been more deformation.
Aluminum composition has an effect on “extrude-ability” (if that’s a word.) Silicon in the 6000 series grades means they extrude quickly and easily. In contrast, 7000 series grades are much harder to extrude. This means a 6000-grade aluminum can be extruded faster than a 7000, which helps keep the cost down.
Loads on structural elements are either axial (tensile or compressive), bending, or torsional (twisting.) Shape has little bearing on the material’s ability to resist axial loads but does influence resistance to bending and torsion.
I-beams are widely used for their resistance to bending loads but are only really strong in the direction of the web separating top and bottom. This strength comes from how the lower flat section is compressed while the upper is stretched by a load. However, they are less strong when the load comes from the side.
Square sections provide equal strength in both vertical and horizontal directions, though in neither direction are they quite as strong as an “I” in its strongest direction. Rectangular section extruded aluminum is strongest in the widest direction. So you could design a section for the exact horizontal and vertical strength needed. Where squares don’t work well though is in resisting torsion.
Like squares, circular sections offer resistance to horizontal and vertical bending, but they are not as strong as a square section using the same quantity of metal per linear inch. We’ll spare you the math (because calculus was never our strong suit), but suffice to say, a square section puts more material further from the center of mass.
Where circular sections do score is in resistance to torsion. (Ever seen a square shaft? No, we didn’t think so.) Interestingly though, while strength is proportional to diameter it doesn’t make a lot of difference whether a shaft is hollow or solid. Wall thickness determines tube strength, but only up to a point. The physics is actually much the same as for the I-beam: it’s the material furthest away from the center of mass that provides most of the strength.
Ladders are an interesting example of extruded aluminum in a structural, that is to say weight-bearing, application. The rails are basically I-beams that resist bending as you climb up and down. Rungs however are somewhere between tubes and square or rectangular sections. They need to resist bending in the vertical direction, but they can also handle significant twisting or torsional loads. And fabricating the whole thing from aluminum ensures it’s lightweight and easily transported.
Trailers also incorporate a lot of structural extruded aluminum. This again keeps weight down while using square section to provide lots of resistance to loads in various directions. You could also add things like machine frames, window frames, enclosures, and even furniture to this list.
The point we’re making here is that there’s no simple answer to the question, “How strong is extruded aluminum?” All we can say is, “How strong do you want it to be?” Some grades are much stronger than others, but that often makes them harder to extrude and contributes to a higher price.
As noted, cross section does have a big influence on the strength of extruded aluminum, the exception being when loads are axial. The key to picking the strongest shape is to understand what loads will be placed on your structure. If they’re mostly in one direction, an I-beam is probably strongest. If you can’t say where the greatest loads will come from, go with a square section, and if twisting will be significant you may be best off with a circular tube.
There are many good reasons for using extruded aluminum in your next fabrication project, but we know strength is a concern for some people. Every case is different but it’s our experience that in many applications it works as well or better than steel. In other words, it can be strong enough. If you’d like to discuss your particular project or concerns with us, please call, email, or just stop by. We’re always happy to talk.
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