Effects of Missile Size and Glass Type on Impact Resistance of “Sacrificial Ply” Laminated Glass

Experiments were performed as part of a larger study to develop the “sacrificial ply” design concept for laminated architectural glass. This concept allows windborne debris impacts in severe windstorm environments to break the outer laminated architectural glass ply while the inner ply is preserved in order to carry the design wind pressure. Steel ball size and inner/outer glass ply type (level of thermal tempering) were varied to determine their effects on the impact resistance of the inner glass ply of laminated architectural glass when impacted on the outer glass ply. A mean minimum breakage velocity (MMBV) was determined for each variation in steel ball size and glass temper level, which defines the mean debris impact velocity on the outer glass ply that causes breakage in the inner glass ply. A 46% reduction in MMBV was observed for an increase in steel ball size from 2 g (7.9 mm diameter) to 8.4 g (12.7 mm diameter), and a 65% reduction in MMBV was observed for an increase in steel ball size from 2 g (7.9 mm diameter) to 28.2 g (19.1 mm diameter). Laminated architectural glass constructed with heat-strengthened or fully tempered inner glass plies, regardless of outer-glass-ply type, was found to have a significantly higher MMBV than laminated architectural glass constructed with annealed glass plies. In contrast, changing the outer glass ply from annealed to fully tempered glass was found to reduce the MMBV, regardless of the inner glass ply type. Relating these results to those in a previous impact study by Kaiser et al. suggests that the order of importance for design variables that most influence the inner glass ply impact resistance of sacrificial ply laminated architectural glass is the following, starting with the most important: (1) inner glass ply type; (2) inner glass ply thickness; (3) polyvinyl butyral (PVB) interlayer thickness; and (4) outer glass ply thickness.