Tensile Test Strength
The term "tensile test strength", as it is applied to braided speargun line, is not as straight forward as it sounds. Properly testing the tensile strength of braided line requires highly specialized equipment because terminating the line without reducing its strength is difficult and because the load must be increased very gradually to determine the actual breaking point. Attempts to measure the tensile strength of line without the proper equipment will significantly understate its strength. For instance, if you were able to terminate the line without reducing its strength and you were to attach it to the ceiling in your garage. Then you were to add weight to the bottom end of the line very slowly by say, dribbling sand into a bucket suspended from the line, there would still be a problem with motion. Any motion of the rig would increase the instantaneous load above the weight suspended from it.
Each batch of Spearit line is tested by the manufacturer to insure that it meets the minimum test strength that we specify. The word minimum is what differentiates our ratings from many other makers and retailers of speargun line. The exact break strength varies by batch and even within the same batch of line. We feel more comfortable using a minimum strength specification which often signifigantly understates the line strength as opposed to other manufacturers and retailers that rate their line by the highest or maximum observed break strength for a particular type of line. We included a sample tensile test report below. This particular test is for a batch of our 400lb Kevlar. The test results for this batch reveal failure at 465lbs, well above our minimum 400lb specification.
It is also important to understand that tensile tests are performed under ideal conditions with new line. Normal wear and tear, acute bends around other objects and simply tying a knot in a braided line, will signifigantly weaken it.
The test is performed with a straight segment of line and all fibers in the line share the load evenly. If the same line is bent at an acute angle in a knot or around another object, fibers on the outside of the bends carry more of the load. Remember, Kevlar and Spectra do not stretch very much. Because less of the fibers are carrying the load, the failure of the most stressed fibers occurs at a lower tensile load. The load is then transferred to other fibers in the braid and the entire line fails in a split second chain reaction. You may be asking "Why use fiber with such low stretch for speargun line?". The answer is that superior strength, combined with excellent abrasion and cut resistance, more than offset any disadvantage. The characteristic of low ultra stretch, is precisely the reason we make our constrictor cord from Kevlar fiber and the absence of the characteristic is why other commonly used synthetic fibers make inferior constrictors.