Determination of prestress profiles in chemically toughened glass by means of photoelasticity

Chemical toughening is an option to substantially increase the mechanical strength and temperature resistance of structural glass. Although thermal tempering is still the common used procedure in construction industry, the advantages of chemical toughening are significant. While prestresses of thermally toughened glasses can reach values of up to 100 MPa, values of 200 MPa or higher are possible in chemically strengthened glasses. This amplification creates new structural opportunities. Hence this promising technique needs to be assessed in detail, since the defined profile of the stresses can be produced with current methods in production technology, but a reliable reproducibility has not been ensured yet. Therefore, as part of the German research project ‘Verfahren zur Glashärtung mit definierten und messbaren Vorspannungsprofilen’ (‘Methods of glass toughening with defined and measurable prestress profiles’), a measuring method was developed to reliably determine the prestress profiles and the depth of the compressive zone. Glasses, prestressed due to ion exchange, were studied in this series of experiments. Those were submitted to a four-point flexural test setup. The occurring stresses were monitored during this test using two different photoelastic measuring devices. One enabling scattered light measuring and the other one being a refractometer. Finally a procedure is recommended and prospects on pending investigations are given.

Maximilian Laurs

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