ure, the flaw will increase in size until fracture occurs. It can be shown (Allard, 1990) that the time to failure is inversely propor- tional to the nth power of the applied stress or strain, where n lies in the range of 14 to 25, the higher value being in dry conditions and the lower value at 100% humidity. Fibres with the largest flaws can be eliminated by applying a proof test, typically of 0.6% strain, as an online manufacturing process, and ensuring by cable design that the stress on the fibre at all stages in its life remains low. A fibre buffer or primary coating serves the dual purpose of protecting the fibre from rough surfaces which would introduce microbending losses, and, of preserving the pristine strength of the fibre. Hence, the coating is applied on the pulling tower within a second or two of the fibre being drawn in such a way that the fibre is not scratched or marked. Thermally cured polydimethyl silicone (e.g. Sylgard) provides a marked increase in a usable fibre strength and very small tempera- ture dependence. More recently acrylate resin coatings have been widely adopted as these offer advantages in fibre drawing speeds and in more versatile buffering techniques. A typical system em- Figure 15.4 Weibull plot of strain at fibre break (147 ploys a low modulus inner coating with a high modulus outer sambles; 10m gauge length tests). Reproduced with coating, giving a package which provides a tough smooth exterior permission of STC Technology Ltd.) capable of withstanding subsequent handling and abrasion, while the soft inner protects against microbending losses. Glass fibre, unlike steel or copper wire, does not have a well The coated fibre requires further protection before it can be defined or controllable breaking strain because failure is initiated in incorporated into a cable. A common solution is to extrude a loose flaws that are usually at the surface, but may also be within the fibre. tube over the fibre, and if the fibre is made to lie in a long helix These flaws cause local intensific