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Re: Re: Re: Steel vs. Aluminum

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Posted By<" ">Dave Schenken on April 04, 2002 at 23:49:13:

In Reply to: Re: Re: Steel vs. Aluminum posted byNeil Moran on March 15, 2002 at 04:45:42:

: Hi,

: I am a structural/mechanical design engineer and get involved in all types of structures for the Theatre and Stage industries. While I would agree with you all, that a structural grade of aluminium can be as strong, or even stronger than steel, this is only one consideration to cater for. What about the deflection of the structure?

: The modulus of elasticity of aluminium (whatever grade) is only about 1/3 that of steel. So the deflection of an aluminium structure will be 3x that of a similar steel structure. If deflection is important you will need 3x more `second moment of area` (approx 3x more weight) of aluminium in order to achieve the same deflection as the steel structure. Aluminium is also 1/3 the weight of steel so both structures would now weigh the same.

: There is only an advantage in using aluminium where deflection is un-important. Steel is also easier to weld than aluminium.

: Regards,
: Neil Moran

In tensile applications where deflection is a consideration there is no advantage of aluminum over steel. Aluminum is 1/3 the density but is 1/3 the modulos so PL/AE is the same for the same weight.

In bending the advantage is usually to aluminum. For a given weight aluminum will be 3 times thicker and therefore have 27 times the 'I'**. That aluminum is only 1/3 the modulos reduces the advantage to 3:1 over steel. Its why there are few steel skinned airplanes. The stength and deflection of a thin steel skin would be much greater than that of aluminum (9X!)

In compression it is also better. The aluminum part will, for the same weight, be much more resistant to Euler buckling - where the stress due to load is much lower than the compressive ultimate load for the material but the part folds - stamp a soda can lengthwise or push on the end of a soda straw. It also prevents the wing skins on airplanes from wrinkling and failing when carrying compressive loads.

** I = (1/12)*width*thickness^3 for a rectangular section being bent the thin way. I is the moment of inertia.

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