Glass fibers are widely applied as reinforcements in composites for applications ranging from lightweight and damage tolerant structures to protective materials providing high energy absorption levels during ballistic impact. Understanding the atomistic mechanism of mechanical response and damage modes under various strain rate loading conditions is important to improve the properties of glass fibers. We performed ReaxFF molecular dynamics (MD) simulations for the mechanical properties and progressive damage mechanisms of S-glass fiber. MD simulations generated the stress-strain response of S-glass fiber for the strain rate range from 1e7/s to 1e15/s. Modulus and strength data exhibit a characteristic S-shaped curve on a semi-log plot indicating strain rate dependent properties followed by a steep rise in properties at approximately 1e12/s to a strain rate-independent plateau. Detailed analysis of the atomic structure during high strain rate tensile loading provides insight into the dependency of properties on the rate of atomistic reconstruction, suggesting that the characteristic time-to-reconstruct is an important factor governing strain rate-dependent progressive damage. The origin of the higher modulus of S-glass over silica arises from electrostatic interactions associated with Al-O and Mg-O bonds in S-glass. Investigation on Al-O-Mg, Mg-BO/NBO interaction, and Al/Si-O ring indicates that the reconstruction of local structure leads to a more ductile-type response and progressive damage evolution in S-glass fiber than silica glass. In addition, we introduce a methodology to construct stress-strain response for low strain rates (1E-3/s) from a series of high strain rate loading to prescribed strain levels followed by stress-relaxation to an equilibrium stress level. The method is computationally efficient, and the results agree with the quasi-static modulus of both S-glass and silica.