Strongest material conformation

[trigirl]

Hi,

I am a biomechanist looking into Achilles tendon injuries. I was at a conference the other day and they were talking about using tendons for ACL reconstructions and about tubularising tendons. This got me thinking. Is there a certain conformation of fibres that is stronger than others.

Basically for those who don't know tendons are made up of many bundles of collagen fibres aligned in the direction of tensile force. The material properties that are used to describe tendons is stiffness, strain, stress and Young's modulus. The Achilles tendon has a broad flat area where it originates at the muscle, becomes rounded in the mid portion and then flattens out again where it inserts into the bone. The mid portion is the area most prone to degeneration as a result of overuse. The Achilles tendon primarily must withstand high tensile loading. I thought that carbon fibre constructs are possibly materials that might have similar groupings of multiple fibres.

Does anyone know of any resources that might have investigated the differenc ein material pproperties for different conformations of fibres. I would ideal like to see if the stress/strain etc for a broad flat material is different to that of a rounded bundle.

I hope that makes sense.

Thanks to anyone who can help

 

[dan barr]

ropes vs. straps

i'm no expert. here's my stab at this.

the main difference i see between round and flat fiber arrangement is this.

flat fiber arrangement such as with a nylon cargo strap must remain flat with the exception of a half twist in the strap which is the maximum allowable amount of twist for a 10,000 lb nylon cargo strap (just one example). any more twist than this starts to spread the load unevenly between the individual fibers thus subjecting some of the fibers to more or less load instead of spreading the load evenly across all fibers. twists (of more than half of a twist) in the strap can cause concentration of tensile force on a small portion of the fibers. this occurs because the length of the fibers throughout the strap is uniform. introducing a twist relieves some of the fibers from part of the load while that amount of relief is then placed as additional load on other fibers. the initial point where the fibers are anchored is spread out. however, you might argue that there is a hook with a single point. i point to the wide area where the strap is initially attached to the hook. the wide area transfers the force to the single point in the mouth of the hook.

by contrast, if the fibers are arranged in a cable (round) configuration, the load on all fibers is easily equalized across the width of the cable at any point. However, attaching a single cable type end to any object requires that amount of strength at that point.

the Achilles tendon has the advantages of both. the broad flat binding ends allow small bonds at many many points which give it the overall strength. (instead of one single binding point) the tendon forming into a cable in the middle allows the load to remain spread among all fibers as equally as possible at the other end regardless of twists, turns, etc. introduced while dynamically loading the tendon. if the middle of the tendon were wide and strap-like, then the individual fibers would fail unless the load were always distributed evenly or the individual fibers were each rated for the maximum allowable load in which case all other fibers would be redundant.

to answer your question, the Achilles tendon is perfect as it is. if you were to use either the strap or cable fiber configuration on its own you would be using an inferior configuration.

cheers,

dan

 

[trigirl]

Thanks that is very informative and much appreciated. Do you know of anything I can reference regarding the stresses of straps and cables etc?

Cheers
A