12769 IUPAC Gold Book - fragility index 10.1351/goldbook.12769 12769 current 1 Quantity characterising the fragility or the strength of a glass at a given temperature \[m = \left (\frac{\rm{d} \log_{10} \tau}{{\rm{d}} (T_{\rm{g}}/T)} \right)_{T = T_{\rm{g}}}\] \(\tau\) = relaxation time [\(\pu{s}\)]; \(T_{\rm{g}}\) = glass-transition temperature [\(\pu{K}\)]; \(T\) = temperature [\(\pu{K}\)]. The value of the fragility index depends on the thermal history of the glass. The values of \(m\) range from \(m \approx 16\) for a very strong glass to \(m \approx 250\) for a very fragile glass. fragile glass https://goldbook.iupac.org//terms/view/12768 glass https://goldbook.iupac.org//terms/view/12773 strong glass https://goldbook.iupac.org//terms/view/12804 PAC, 2013, 85, 1017. 'Glossary of terms relating to thermal and thermomechanical properties of polymers (IUPAC Recommendations 2013)' on page 1028 (https://doi.org/10.1351/PAC-REC-12-03-02) https://goldbook.iupac.org/terms/view/12769/html https://goldbook.iupac.org/terms/view/12769/json https://goldbook.iupac.org/terms/view/12769/plain Citation: 'fragility index' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. 10.1351/goldbook.12769 The IUPAC Gold Book is licensed under Creative Commons Attribution-ShareAlike CC BY-SA 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) for individual terms. If you are interested in licensing the Gold Book for commercial use, please contact the IUPAC Executive Director at executivedirector@iupac.org . The International Union of Pure and Applied Chemistry (IUPAC) is continuously reviewing and, where needed, updating terms in the Compendium of Chemical Terminology (the IUPAC Gold Book). Users of these terms are encouraged to include the version of a term with its use and to check regularly for updates to term definitions that you are using. 2026-05-08T08:03:16+00:00