Alrighty, I'll stop being cryptic and just use an example to lead you to what I think is your desired answer. We are going to ignore tennis strings for a moment and instead talk about wires. Strings are strings as far as material properties go, so this shouldn't make much difference. For this example, we're going to use copper since most people are familiar with it if they've ever wired up a speaker.
We have two wires of 1m in length and 1cm in diameter: one is a solid core conductor, the other is a multi stranded wire like the wires that are used to wire up high voltage electrical components (simply think about a big extension cord). Now, pull tension of 60lbs on both (both ends of the string are fastnened). You'll notice something immediately: the solid core wire hardly changes shape but the multi-stranded wire stretches a bit. Why? The solid core wire is relying solely on stiffness, and yes, it has been deformed permanently albeit slightly. The stranded wire, however, has had the tension divided amongst each and every string which does not exceed their individual elastic limit. Therefore, they can stretch just slightly. Having tensioned them, go to the center of each string and press down until the string is deflected 1cm. Now you'll see that the solid core wire is simply bent in the middle, but the stranded wire was more or less "plucked" and is vibrating. This simple test can be replicated with tennis strings with the same results (it would obviously need to be scaled).
The point is, the solid core string is less elastic even though the material in both has the same stiffness. Stiffness is the ability for a material to resist deformation and elasticity is the ability for it to return to shape after being deformed. The copper in both has the exact same stiffness. The only difference is that since you bundled them together into many many many tiny strands, you've increased the elasticity of a very stiff string. The fact that it's vibrating instead of being permanently bent as a whole reflects this.
The problem is this: the multifilament was still stressed and is still slightly deformed. This means that it will be able to hold its tension greater than the solid core string, but only over a long period of time. It is, however, less durable from our standpoint. The reason has nothing to do with anything talked about above: it's because since the string is going to be moving more, it will experience much more wear. Those individual strands break very easily compared to a very thick one, and as the strands break, the tension is going to be held by fewer and fewer strands which will be able to hold the tension less and less. However, many strands holding a lot of tension will deform much less than one large strand holding a lot of tension. Keep in mind aside from the wire analogy, this durability aspect applies to tennis strings, and nothing else where the strings are not just tensioned, but also abraded by one another.
Last edited by pvaudio; 11-14-2012 at 01:53 PM.