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

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Discussion Overview

The discussion revolves around the design of a rod intended to support a 40 lb car hood, focusing on the choice of material and the appropriate diameter for the rod. It includes considerations of stress, buckling, and safety factors in a practical application context.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Participants discuss the use of yield strength in calculations, questioning whether compressive yield strength is equivalent to tensile yield strength.
  • One participant suggests applying a safety factor, recommending that calculated stress should not exceed 40% of the yield strength.
  • Another participant introduces the concept of an "impact factor" to account for dynamic loads, suggesting typical values between 1.5 and 1.8.
  • There is a query about the advantages of different materials, specifically steel versus aluminum, with one participant mentioning low carbon steels like ASTM A108 Type 1006 as a starting point.
  • Concerns are raised about the cost-effectiveness of aluminum compared to low carbon steel, with a suggestion that steel may be both cheaper and more suitable for the application.

Areas of Agreement / Disagreement

Participants generally agree on the importance of applying a safety factor and considering impact factors, but there is no consensus on the best material choice, as opinions vary between steel and aluminum.

Contextual Notes

Participants have not resolved the question of whether compressive yield strength is the same as tensile yield strength, nor have they established a definitive recommendation for the material choice based on cost and performance.

Who May Find This Useful

This discussion may be useful for individuals interested in mechanical design, materials science, or those working on practical engineering applications involving load-bearing structures.

GreenGrass777
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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?
 
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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.
 
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...
 
GreenGrass777 said:
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.
 
Ok thanks. Do you think Aluminum is a good choice here or is low carbon steel the cheaper option?
 
GreenGrass777 said:
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.
 

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