Testing of Laminated Safety Glass

Vincent Sackmann
Christian Schuler
Holger Gräf

Laminated safety glass is a composite of two or more glass panes connected by interlayers of plasticized polyvinyl butyral (PVB). The interlayer transfers shear forces between the glass panes which increases the load bearing capacity of the composite.

For small strains usually found in building applications the viscoelastic nature of the PVB interlayer in laminated safety glass is mainly influenced by load duration, temperature and aging. It is
however independent of the shear angle and the shear rate. Knowledge of how these parameters influence the mechanical behaviour of the interlayer and the composite is important if the shear bond is taken into account for structural design. Until today the established building codes in Germany do not allow to consider the shear bond for design purposes. Taking into account the shear transfer between the glass panes can lead however to a more economic design of glass thickness in
certain applications e.g., vertical glazing and aerodynamic loads.

When laminated safety glass is exposed to outdoor environment the PVB interlayer is subject to weathering by sunshine, temperature, moisture and pollutants. This can cause degradation of the polymer through irreversible chemical aging. Moisture can intrude unprotected edges of laminated glass where it is absorbed by the hygroscopic interlayer and affects the adhesion between glass and interlayer. Ultraviolet radiation close to the visible range of light (UV-A) affects the interlayer by photo-oxidation. The aging of the PVB interlayer can lead to alteration of its material properties such as shear rigidity, shear and tensile strength, ductility or adhesion. Theses alterations may also affect the load bearing capacity and remaining lifetime of broken glass elements if the interlayer has lost its adhesion or tears due to reduced tear strength and ductility.

In a test program the authors investigated the structural behaviour of glass elements subjected to
different load durations, long-term cyclic loading and aging through weathering by humidity and
ultraviolet radiation. The tests were done in a temperature range between 0 °C and 60 °C.

One purpose of the still ongoing research program is to find a single value for the shear modulus which is sufficient to calculate the rigidity of the shear bond at a characteristic time and temperature of a given load case. Another important purpose is to investigate the durability of the shear bond of the interlayer when exposed to weathering.

The main results of the investigation can be summarized as follows:

• The behaviour of the composite proved to be linear viscoelastic in the investigated temperature and deformation range. This will make it possible to superpose load cases.

• Relaxation and creep was observed at both high and low temperatures and the shear modulus showed to be highly dependent on the temperature (see Figure 1)

• For usual load and temperature conditions found in architectural applications a monolithic behaviour of the composite could not be achieved in stiff laminated glass elements (having
short span and thick glass panes). For long-term loading monolithic behaviour can generally not be assumed. This applies also for glass elements near point supports.

• Long-term cyclic loading with up to 50.000 load cycles and with low frequencies does not influence the rigidity of the shear bond in glass elements and does not lead to delamination.

• Weathering influences the shear bond of the composite. Intensive ultraviolet radiation of several weeks has only little affect on the rigidity of the shear bond whereas moisture in the interlayer has a tremendous effect on the shear bond and on the adhesion between interlayer and glass panes. Moisture that enters the composite in the area near edges and holes can lead
to delamination and has to be avoided.

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The Authors