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Laminated glass is a type of safety glass, where multiple layers of glass are bound together using a transparent interlayer material. Laminated glass is an excellent material for windows in the automotive industry as they hold together under impact, rather than shattering. In this article, we look at how laminated glass behaves under an impact.
Laminated glass is a useful material for the automotive industry, because not only is it stronger and less prone to shattering than conventional windscreen glass, it also presents a much higher safety for any passengers in a when their car undergoes an impact. It also useful for security and can stop projectiles from penetrating through the windshield – which is another potential safety hazard nullified.
What is Laminated Glass?
Laminated glass consists of two or more panes of glass bound together using a transparent polymer interlayer material. The most common types of polymer used are soft in nature, such as polyvinyl butyral (PVB), ethyl vinyl acetate (EVA). Laminated glass uses possess two layers of glass, but more can be added, although it is not often required unless it is for the aviation industry. EVA is also a special interlayer material. Once thermoset, the EVA will completely bind to the glass panes through cross-linking mechanisms.
In addition to an increasing strength upon impact, laminated glass also offers an increased fire resistance and sound dampening properties compared to standard windshield glass.
How Laminated Glass Behaves Under an Impact
Laminated glass does not shatter, instead it holds its shape upon impact. The main function of laminated glass is to retain the broken glass fragments upon impact. The interlayer material is key to achieving this and cannot be achieved by purely stacking multiple layers of glass on top of one another. However, the presence of multiple glass panes is what prevents large pieces of glass from being released upon impact.
Upon impact, there are three stages that the laminated glass goes through. The first, is where there is no force on the glass and both panes are still intact. The second stage is when the bottom (or lower) glass pane cracks and propagates towards the interlayer and upper layer of glass. In this stage, the load is carried entirely by the top sheet of glass. The final stage is when the crack appears in the upper layer and the interlayer becomes tense. Even though both sheets are fractured, the pieces in the glass sheets lock together through compressional forces and the tension in the interlayer provides an added post-breakage resistance. However, the effectiveness of this locking is largely dependant on the properties of the interlayer.
So, the quality of the interlayer material determines whether the glass pieces will lock together, or whether they will shatter. If the glass is properly reinforced, the impact pattern will be similar in nature to that of a spider’s web.
However, the final stage can present some potential issues. There is the potential for ‘dropouts’ to occur when the glass plies have broken during the impact and the interlayer is left carrying the load. In these circumstances, the glass sheet can then separate itself from its supports.
The remaining load bearing, and structural capacity, of the laminated glass post-impact is dependent upon the fragments left behind. Because the fragment size is inversely proportional to the energy stored in the glass, larger fragments post-impact tend to possess better post-break behaviour. For smaller fragments, there is the potential that the glass panes will sag/arch. If this is the case, whether or not the fragments shatter is entirely dependent upon the tensile strength and rigidity of the interlayer material.
Because the impact is dependent upon the propagation through the first sheet to the second sheet, the maximum impact force can be determined by the thickness of just one pane of glass. The energy of the impact is reduced by at least double the amount reduced by using a single pane of glass and the impact force can be reduced by up to 40%.
Source:
“Investigation of impact fracture behavior of automobile laminated glass by 3D discrete element method”- Zang M. Y., et al, Comput. Mech. 2007, DOI:10.1007/s00466-007-0170-1
“Blast and Impact Resistance of Laminated Glass Structures”- Hooper P., et al, Proceedings of the IMPLAST 2010 Conference, 2010
Lund University: http://www.byggmek.lth.se/fileadmin/byggnadsmekanik/publications/tvsm5000/web5198.pdf
SUSCOS: http://www.ct.upt.ro/suscos/files/2013-2015/1E05/2E5_Glass_structures_L3_2014_VU.pdf
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