Effective Temperature Conditions in Heat Soak Tests for Heat Strenghtened and Tempered Glass

Masashi Kikuta, Chihiro Sakai

Adapted Heat Soak Test (HST) have applied to increase a reliability of heat strengthened and tempered glass. However, in spite of application its treatment, spontaneous breakage is rarely occurred in market (three cases reported in a last year). And its events were happened in about one month after HST treatment. These accidents causes decreasing a reliability of heat strengthened and tempered glass.
After the investigation of sudden fracture glass samples treated HST, it was thought that the main factor of all actual events might be connected with the location of NiS stones inside the glass pane. To avoid the spontaneous breakage of heat strengthened and tempered glass using HST treatment, the possibility of additional physical external load, which is different from that NiS volumetric expansion and also residual stress of glass, was studied in the heat soak furnace. Assuming the slow crack growth from the NiS inclusion inside the glass, many analyses of crack propagation caused by a thermal distribution in a glass pane was carried out with fracture mechanics by using the computer simulation.
The different effective temperature conditions were obtained for heat strengthened glass and for tempered glass, respectively. And it was thought that these temperature conditions have also effect to other stones (e.g. ZrO2, Al2O3-ZrO2 and so on). Then, we discuss the new idea of HST.

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