Multi vs Single Strand Strength ```Name: Brad Status: Other Grade: Other Location: KY Country: United States Date: May 2006 ``` Question: Is a cord of 3 strands really much stronger than 3 times a single strand? How about a cord of 2 stands? Replies: The "short" answer is yes, but the mechanisms that are operating are much more subtle than appears at first. A strand will begin to break if it develops some stress cracks which then propagate axially down the strand and radially. The result is the strand fails. For three strands to fail by this mechanism the failure would have to happen in all three more or less at the same time. How a crack propagates in a particular strand depends upon its chemical composition, which can be altered to increase its tensile strength. But things are even more complicated because multiple strands can be braided and/or twisted. These configurations allow the strands to "stretch" and "slip" by one another, which to the outside observer seems to make the composite stronger than just single fibers which can only stretch axially. Other factors play a role also. For example, if the strands are "normal" they get weaker as the temperature increases, but if the strands are "rubbery" they actually get stronger as the temperature increases moderately. So your "simple" question actually is very complicated. Vince Calder There is a whole branch of engineering physics in the literature that can compactly be described as: "the physics of ropes". A Google search on "physics of ropes" turned up these interesting site -- but there are many more. The topic is quite interesting and applies some physics concepts not usually addressed in elementary courses. As you can see from these 'hits' designing a rope is a complicated business. http://www.bstorage.com/speleo/Pubs/rlenergy/Default.htm http://www.physicsclassroom.com/Class/momentum/U4L1c.html Vince Calder First some assumptions, the cords are equal diameter , equal composition, material etc. Wound cords, as 3 are stronger than 3 individual cords. That is the principal behind cables for suspension bridges. The 3 wound have a larger final diameter. The outer fibers are not stressed as much as a smaller diameter, compare to a single diameter, hence stronger final product. Consider a beam, under a vertical load the bottom fibers are in tension. The top in compression. So, it is rather obvious a deeper beam is stronger than a shallower one. Say, a failure in tension, the deeper beam has less stress on the bottom fibers. Basically , it is the same for a wound cable. Similarly, a 3 cable stronger than 2, if all are equal. James Przewoznik Click here to return to the Engineering Archives

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