Stiffness of glass/ionomer laminates in structural applications

The increasing demand for transparent constructions leads to an expanded use of glass as a supporting element. To solve safety issues related to the brittle nature of the material, commonly laminated glass is used. The latter consists of several glass sheets which are attached to plastic interlayers over their entire surface. In case of fracture of a glass pane, this polymer interlayer is able to absorb energy, so the fraction of multiple glass sheets is in most cases avoided. Moreover, the glass fragments stick to the interlayer, so the risk of possible injury is diminished. This research specifically focuses on the mechanical behaviour of laminated glass with an ionomer interlayer, namely SentryGlas®, in structural applications. SentryGlas® is a relatively stiff interlayer, of which the mechanical behaviour is less known than e.g. the more traditional interlayer material PVB. Therefore, an extensive test program was executed with both torsional and bending tests, by which the time and temperature dependent stiffness of the laminates was determined experimentally. As a first step in the analysis, the relationship between the test temperature and the load duration was determined. The corresponding time-shift function allows the prediction of the long term behaviour at a certain temperature level based on the experimental results of short tests performed at a higher temperature. Additionally, it becomes possible to analyse together the results of multiple test series, performed between 5 °C and 65 °C. All results were then processed using the equivalent thickness method, as proposed in the European pre-standard prEN 13474. The outcome indicated that the resulting equivalent thickness is highly dependent on the test configuration and therefore not directly useful to predict the laminate stiffness under divergent loading conditions. Therefore, all results of experiments on laminated glass were transformed into the actual material properties of the intermediate layer itself, using both analytical and finite element modelling. Finally, a user-friendly material model was composed, based on the numerically obtained shear modulus of the interlayer which yielded the lowest dispersion and seemed to fit best with the real test configuration. The model allows the calculation of a laminated structural element in an application with realistic loading conditions. This is possible with both simplified elastic models, as with finite element packages for more complex visco-elastic simulations.