Glass fibres exhibit greatly enhanced properties over glass in bulk form. The very small diameter of the fibres results in a much smaller likelihood that any fibre contains a flaw that is beyond the critical crack length required for fast fracture.
The two most important types of glass fibre used are E-glass (electrical) and S-glass (high strength). While E-glass does not exhibit the mechanical properties of S-glass, it is still the most commonly employed due to its low cost. E-glass is composed of roughly 54% SiO2, 14% Al2O3, 22% CaO, and 10% B2O3. S-glass is more expensive than E-glass but has a higher strength-to-weight ratio and is therefore used primarily in aerospace applications. S-glass is composed of roughly 65% SiO2, 25% Al2O3, and 10% MgO.