Thermal stress & Chromatics

1. Thermal stress & Chromatics

2. Thermal stress & annealed glass • It is well-known that annealed glass can be vulnerable to thermal breakage when there is a differential in temperature across a sheet • If the temperature difference is too big, stress at the edge of the glass will eventually set off breakage

3. Thermal stress & Chromatics Chromatics products resist thermal breakage in 3 ways • Under extreme stress Chromatics does not break, but may crack remaining glazed and in place • Increased design strength – the temperature difference at which Chromatics becomes at any risk is some 25% higher than for annealed glass • Stress relieving factor – the Chromatics process brings the temperature across a pane into equilibrium much more efficiently than a piece of opacified annealed glass, reducing the maximum difference experienced

4. Safe breakage of Chromatics • Chromatics DOES NOT break! • Under extreme conditions it may crack, but will remain glazed, intact & safe until it can be replaced – even if replacement can not be effected immediately

5. Stress Relieving FactorA characteristic of a material by which the stress induced in it’s surface is reduced when subjected to temperature differentials

6. Stress Relieving Factor - Example • Consider a piece of opacified float glass, which is heated in a frame so that the calculated centre-edge difference is say 60°C • Annealed float glass has a SRF of 1.0 meaning that it does not modify the temperature differential at all • That is: actual temperature difference = 60 x 1.0

7. SRF & Chromatics • Extensive work done at British Glass, in conjunction with NSG Pilkington and verified by independent specialist JB Waldron has established an SRF for Chromatics of 0.62 • This means that Chromatics is MORE efficient than annealed glass at bringing temperature differences into equilibrium

8. Stress Relieving Factor - Example • Consider a piece of Chromatics, which is heated in a frame so that the calculated centre-edge difference is say 60°C • Chromatics has an SRF of 0.62 meaning that it modifies the temperature differential as shown below • Actual observed temperature difference = 60 x 0.62 = 37.2 °C

9. Design strength Defined as that temperature below which 95% of panels of that type will not break in 95% of cases • Wired glass – 25 °C • Cast glass – 30 °C • Clear float glass – 40 °C • Heat-strengthened glass – 100 °C • Toughened glass – 200 °C

10. Design strength & Chromatics • Determined using NSG Pilkington methods and analysis • Verified by independent specialist JB Waldron • Value = 50.5°C

11. Ranking of design strengths • Wired glass – 25 °C • Cast glass – 30 °C • Clear float glass – 40 °C • Chromatics – 50.5°C • Heat-strengthened glass – 100 °C • Toughened glass – 200 °C

12. What does this all mean? When considering whether a piece of glass will be thermally safe or at risk in a given location, orientation and application, several factors must first be considered • Type and thickness of the panel • Its size (larger panels are more at risk because they can build up greater stress levels) • Its aspect – does it face North or South, is it mounted vertically or at an angle? • Where is the panel mounted? What is the diurnal temperature range and the solar power at that location? • Its application – is it in a monolithic panel or double glazed? What type and colour frame is used? If a sealed unit, what gas filling has been used? Etc. • Its colour • Other factors – is the panel subject to a static, moving or no shadow? Are there mullions or canopies to consider? What is the temperature behind the panel? Is it insulated? Etc.

13. What does this all mean? • From these factors, it is possible to calculate a theoretical temperature difference across a panel • This difference is modified using the value of SRF • The result is then compared to the design strength • If this is higher than the design strength, the panel is at risk of thermal damage – otherwise, it can be considered not at risk

14. Example • In a given location and application, a panel of black Chromatics has a calculated temperature difference of 65°C • The SRF for Chromatics is 0.625, so that the actual temperature difference will be 65 x 0.625 = 40.6°C • This is lower than the design strength for Chromatics (50.5°C), so this panel should be considered NOT AT RISK

15. Implications in the real world • OK – final calculated temperature likely to be below 45 °C • Calculation should be carried out – final calculated temperature likely to be between 45 - 48°C • Calculation should be undertaken with particular reference to static shadow – final calculated temperature likely to be 48 - 52 °C

16. Conclusions • Chromatics is a unique product – can be cut, is flat like annealed glass, but is not usually at any risk thermally • Chromatics does not break – in an extreme case it may crack, but remain safely glazed • Not at thermal risk in most European applications

17. Supplement John Brian Waldron • Used to verify the calculations for Chromatics • Former European Technical & Standards Manager, Pilkington PLC • Chairman of various CEN & BSI committees • CV available on request