温度・粘度
Sub-Strain-Point Fracture (Delayed Fracture)
Japanese: 歪点以下破壊
Delayed fracture caused by stress concentration at surface defects in the solid phase. The material may spontaneously crack within a few hours to a few days after firing. This occurs when the cooling rate is insufficient.
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Sub-strain-point fracture (delayed fracture) is a phenomenon in which, when glass is in a solid state below its strain point (approximately 442°C for the Bullseye series) , and is characterized by “spontaneous” cracking occurring several hours to several days after firing is complete (treatise §4.1). Since the glass cracks over time even if there are no abnormalities immediately after production, this phenomenon accounts for many instances where people report, “It was supposed to be annealed, but it cracked later.” The key difference from thermal shock cracking is that a surface Griffith crack (a microscopic scratch) serves as the point of stress concentration.
For prevention, it is important to maintain the material near the slow-cooling point during the annealing (F stage)—with sufficient holding time (set according to thickness based on the Adams-Williamson rule)—followed by gradual cooling (G stage). If the slow-cooling conditions are insufficient, permanent residual stress remains within the glass; when combined with Griffith cracks, this can lead to spontaneous fracture at a later date. By checking the residual stress (retardation value) using a polariscope test, it is possible to verify after firing whether the result is safe. Bullseye’s recommended pass criteria are a retardation value of 10 nm/cm or less; for storage quality, the value should be 5 nm/cm or less.
Related Concepts
熱衝撃割れ（thermal-shock-fracture）・Griffith亀裂（griffith-crack）・Adams-Williamson則（adams-williamson-rule）・偏光板テスト（polariscope-test）・残留応力（retardation）・歪域（strain-zone）
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.