Bending load limits, aluminium extrusions?

[ken]

Hi

I want to determine the bending load limit to elastic failure of extruded aluminium square section, 50 mm sq, warious wall thickness, 1, 2, 3, 4, 5 mm

I thought I could get a good estimate from the tensile strength. I found a value of 90 Mpa for commercially pure aluminium but doubt that is the right value to use for normal garden variety extruded aluminium from the local hardware.

Also, I'm just going on what I remember from high school mechanics.
Some advice on general approach would be appreciated, just remember, with adding safety factor, it needent be to the nth degree. The application is that I'm converting a sailing dinghy cradle into a beach launching trolly and I want to keep the weight down and find what I need to order for the job.


Regards,
Ken

 

[Q_Goest]

Hi Ken. Yes, the "commercially pure" aluminum value you mention is pretty far off the mark. That material has no alloying elements in it and is relatively soft and weak.

Aluminum alloy comes in a wide variety of different compositions and strengths and the guy at the hardware store isn't likely to know what it is, but you could ask. My guess would be its 6063-T6 or roughly equivalent in terms of strength. http://www.physics.purdue.edu/cmsfpix/mechanics/panel_support/materials/aluminum-alloy-properties.html that gives properties for a few different alloys. As you can see, strength is all over the place.

From your description it sounds like you're looking for a few different characteristics, which may include more than just strength. Does this need to be weldable? Bendable? Corrosion resistant? If so, 6063 is a pretty good choice. If you want to know what you're getting though, I'd suggest ordering structural aluminum either off the internet or check your phone book for aluminum suppliers in your area.

When determining what stress you don't want to exceed, there are a number of considerations. You need to estimate how well you know the forces involved. If something's going to be bouncing around across a beach, can you be sure of how much stress the aluminum is exposed to? Another consideration is fatigue. If this is something you're trailering from site to site, is it going to be exposed to cyclic stress? If this is an on-road application such as a trailer you're towing behind a vehicle, I'd suggest using lower stress values. If this is only used off road and only for moving something across a beach and it isn't going to hurt anyone if it breaks, you can use a higher stress value. The real question is what safety factor you can accept, and no one here can really tell you that without knowing what you're going to do with it and what risks you can accept. There may also be regulations and laws set up that govern things if this is used as a trailer. I just mention all that so you consider carefully what you're using this for.

It sounds like this is for something strictly used on the beach, maybe once a day or less, and it won't hurt anyone if it breaks. It sounds like you may be able to determine the stresses involved fairly accurately and they won't be cyclic (ie: you won't have to worry about fatigue). If that's the case, if this is something fairly innocuous, I'd suggest a safety factor of 1.5 to 2 to yeild. If you don't know what kind of aluminum extrusion you have, I'd suggest not exceeding a stress of 100 Mpa (assuming you can do the calculations accurately).

 

[FredGarvin]

Just as another note, make sure you choose an alloy that has good welding characteristics. Not all aluminum alloys appreciate being welded. A good, alla round alloy is 6061-T6. However, without knowing all of the details of what you are doing and how you plan to assemble this trailer, I'll let you decide on the material.

 

[Cliff_J]

http://8020.net/

They are one of a few places to get extrusions but they also include nice simple formulas in their catalogs to figure deflection of the beams and tables listing moments of inertia.

You may want to try to use the formulas for the worst case scenarios of the load being concentrated in one spot instead of distributed along the length of the beam unless you can properly add gussets and so on to achieve that scenario.

How you apply the formula is part of the trick though, a quick diagram of what you're going to build and weight of the craft (and where that weight is supported) and someone should be able to assist you.