Thermal stress & Chromatics

Thermal stress & Chromatics

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

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

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

Stress Relieving Factor

A characteristic of a material by which the stress induced in it’s

surface is reduced when subjected to

temperature differentials

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

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

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

Design strength

• Wired glass – 25 °C

• Cast glass – 30 °C

• Clear float glass – 40 °C•

• Heat-strengthened glass – 100 °C

• Toughened glass – 200 °C

Defined as that temperature below which 95% of panels of that type will not

break in 95% of cases

Design strength & Chromatics

• Determined using NSG Pilkington methods and analysis

• Verified by independent specialist JB Waldron

• Value = 50.5°C

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

What does this all mean?

• 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.

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

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

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

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

Location Light Colours

Dark Colours

London

Frankfurt

Rome

Athens

Corfu

Prague

Moscow

Paris

Stockholm

Reykjavik

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

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