温度・粘度
Strain Zone
Japanese: 歪域
A temperature range of approximately 450°C or lower. This is a safe range where the glass becomes completely rigid and no permanent residual stress is generated. Once the temperature drops to this range, rapid cooling (AFAP) is possible.
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The strain zone is the temperature range below the “strain point” at a given viscosity of the glass; for the Bullseye COE90 series, this corresponds to temperatures of approximately 450°C or lower (strain point Tst = 442°C) (§1.1). In this temperature range, the glass behaves as a perfectly rigid body, and no deformation due to viscous flow occurs even when an external force is applied. Therefore, this is a “safe zone” where there is virtually no increase in permanent residual stress due to thermal history. The strain point is a viscosity fixed point defined by log η = 14.5 (Pa·s); below this point, stress does not relax and remains fixed.
The practical significance of this in a firing program is that once the temperature has dropped to the strain range (below the strain point), relatively rapid cooling (AFAP: As Fast As Possible) is permissible thereafter (§8.1, H stage). However, since the risk of thermal shock cracking still exists, gradual cooling is recommended for thick pieces even below the strain range. Furthermore, the risk of “fracture below the strain point (delayed fracture),” in which stress concentrates on surface defects below the strain point, may manifest as “spontaneous cracking” several hours to several days later if slow cooling conditions are insufficient (§4.1).
Related Concepts
Slow-cooling zone (annealing zone) (annealing-zone), annealing soak (Stage F) (firing-stage-f), annealing cool (Stage G) (firing-stage-g), polariscope test and the η-T-t framework to gain an understanding of the overall design of the cooling process.
This entry is written in Japanese by Rei Chikaoka and translated for readers outside Japan. If something reads oddly, the original is the source of truth.