What Material and Diameter Should a Rod Be to Support a 40 lb Car Hood?

[GreenGrass777]

Homework Statement

You have to design a rod to hold up the hood of your car. Assume the hood weighs 40 lbs, and the rod must be 3 feet long.
What material would you use to make the rod? And what diameter would you make the rod?

Homework Equations

Stress= Force/Area
Buckling equation for member pinned at both ends

The Attempt at a Solution

1) use yield strength of chosen material in stress formula, and solve for area, and therefore diameter.
2) verify that the solved rod diameter will not buckle under given load

Could you please clarify:
-is the yield strength in compression the same as in tension?
-should a safety factor be applied? Is it worth considering for such a low-risk application?

 

[edgepflow]

For steel, the compressive strength is usually higher than the tensile strength. However, for purposes of calculation, use tensile strength values for compression too.

Yes, apply a safety factor. Limit your calculated stress to no more than 40% of the yield strength.

Also, apply an "impact factor", FI, to the 40 lb hood weight to account for the fact that it will briefly generate more than 40 lb when it is stopped against the rod. Typical values of FI are from 1.5 to 1.8.

 

[GreenGrass777]

Thanks for the quick response. I didn't think of the Impact Factor...interesting.

Would you recommend any particular type of material? I'm not sure if steel or aluminum would have an advantage here. And even then there's so many different kinds...

 

[edgepflow]

Thanks for the quick response. I didn't think of the Impact Factor...interesting.

Would you recommend any particular type of material? I'm not sure if steel or aluminum would have an advantage here. And even then there's so many different kinds...

For material, you might start with the low carbon steels, such as ASTM A108 Type 1006.

 

[GreenGrass777]

Ok thanks. Do you think Aluminum is a good choice here or is low carbon steel the cheaper option?

 

[edgepflow]

Ok thanks. Do you think Aluminum is a good choice here or is low carbon steel the cheaper option?

Steel is likely cheaper and better for the applications also.