Why does twisting/braiding strands of string into a rope make it stronger? I am currently doing an experiment where I am measuring the force required to extend two parallel pieces of string to a set distance. The independent variable I'm changing is the number of times the two pieces of strings twists with each other. 
I got a pretty good positive linear graph but I don't know how to mathematically explain it with Physics. I know it has something to do with the string becoming shorter and normal forces but I can't seem to find an equation to express the graph.

 A: I looked for an equation related to your problem but was unable to find one. There are a lot of variables involved, not simply the ones you mentioned. Braiding strings together to make the complete assembly stronger is dependent on keeping the binding force, due to the twisting, very consistent, otherwise you can introduce weak areas. 
Also you need to allow for the method of manufacture of the string, is it composed of complete fibres running the entire length of the string? This is how synthetic ropes / strings are made, so braiding them together will not increase their existing strength. The string that appears in your picture looks like it is composed of natural fibers, and each natural fibre will be short, say less than 15 cm, so the string you have in your picture is already braided, even before you start, to knit together those short length.
If you still need  an equation, I guess it would relate the force that the string can withstand to the force, imposed by you, in creating the helical  pattern to bind the strings together. You would then need to resolve this helical force into two components, one perpendicular to the string length and the other parallel to the long axis of the string.  
So, to sum up, you can't automatically assume braiding will increase strenght, it may instead decrease it. 
