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Tensile Strength

Science brain teasers require understanding of the physical or biological world and the laws that govern it.

 

Puzzle ID:#8049
Fun:*** (2.71)
Difficulty:** (1.64)
Category:Science
Submitted By:Bobbrt*****
Corrected By:Jake

 

 

 



Which has a higher tensile strength? A 1" diameter, 6 foot long steel rod, or a 6 foot long steel cable made out of the same amount of steel as the rod? More importantly, why?

If you don't know what tensile strength is, click on "hint".





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Comments

jonko
Nov 04, 2002

I disagree with this answer . Yes tensile strength deals with a tug of war but you must consider a tug of war with a solid 1 inch steel bar and a cable that is made up of lets say 20 wires each 1/20 th of an inch thick wrapped around each other . The cable is more flexible ( which is the purpose of making cable ) but is NOT stronger because each wire is only 1/20 th the diameter. You can pull up on a heavy mass such as an elevator more safely using a solid bar ( less chance of ripping apart ) but it is not flexible enough to wind around the wheel . The diameter of the respective wires is the MOST important consideration for tensile strength .
Bobbrt**
Nov 05, 2002

Sorry, Jonko, but this comes right out of a textbook on fibrous composite structures. Don't you agree that it would be easier to break a bar that is cut 1/4 of the way through than a 20-fiber cable with 15 fibers in perfect condition?
electronjohn*us*
Nov 07, 2002

Bobbrt is right with this one. I worked for about 10 years doing failure analysis. No question the cable has a much greater tensile strength.
jonko
Nov 11, 2002

Sorry boys , but I must disagree with your suggestion that a braided wire is stronger that a solid wire of the same thickness . First of all you must consider that cables are used because they are flexible . Each part of the cable is made of a small diameter wire that has less TENSILE STRENGTH than a larger diameter wire ; and just like in a long chain , the chain will break at it's weakest point . The weaker small wires will RIP APART easier that a big fat solid wire . Perhaps to really clear this up consider this example ...try holding up a 500 lb block of concrete with a cable or a solid steel rod of the same diameter . Obviously the cable will flex and not hold up what the solid rod can ...this not only applies to COMPRESSION of the materials but also the EXPANSION of the same materials which is what TENSIL STRENGTH deals with .
electronjohn*us*
Nov 11, 2002

The important point that Bobbrt has made, and that you are ignoring Jonko, is that steel fails in tensile strength tests from tiny defeats in the grain structure. At some point the worst defeat will propagate across the bar and it will tare apart. (I have looked at many of these fracture surfaces under an SEM.) In the case of a cable of the same grade of steel the worst defeat will be in one wire and when it goes you will still have 99% of the cable left to hold the load. You would have to have that worst defeat, which is often a void or some impurity, in each wire making up the cable before it would snap at the same low load as the bar.

In your classroom Jonko you could give me an F, but in the real world I would be alive with my cable and you would be crushed to death after your bar failed and dropped its load on you.
speedyg1000*
Dec 28, 2002

It seems there is a very heated argument here, but it is going nowhere. Jonko is argueing that each fiber is less strength then a whole bar. therefore, it requires less force to break part of the wire then to break part of the rod. However, although it requires more force to break the rod, it will break in one go. To word this differently, if the force was steadily increased, the first wire in the cable would snap pretty early, and then somemore would break more consistently, but right before the last couple of wires broke, the whole rod would break, leaving the cable victorious. or course, when all of the pressure is applied at once, it would fall pretty close to even.
jimbo*au*
Mar 16, 2003

This looks more and more like a question from a graduate exam in mechanical engineering than a teaser! I'll go find a professor to ask!
curtiss82**
Dec 24, 2003

The cable is stronger. Even though the cable is made up of small wires, there are numerous of them and as a whole act as one solid piece of steel. But like described before, it is less likely that the cable will fail from an impurity in the material.



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