PHYSICAL PROPERTIES OF GLASS
Density, Thermal Expansion Coefficient, Thermal Conductivity, Emissivity and Selectivity of Coated Glasses
The physical properties of float glass produced in Europe do not vary significantly between manufacturers and can be taken as standard values according to BS EN 572-2. Float glass produced in other parts of the world may vary in colour but the physical properties are very similar. The properties are summarised in Table 1. Rolled plate glass and drawn sheet glass have slightly higher densities because the viscosity required for those processes is higher, but this is unlikely to be significant in design.
MECHANICAL PROPERTIES OF GLASS
Patterns of Breakage of Glass
The three commonly used heat-treatment conditions of soda lime glass are most clearly distinguished by the manner in which they break. Annealed glass, which is the standard condition in which it is manufactured, stocked and cut, has a characteristic bending strength of about 45 MPa (not a design value).
When broken, cracks run as far as they are driven by the applied force, which may be low, such as a thermal stress, or high such as from impact or wind pressure, in which case the cracks branch and propagate to the edges of the pane (Fig. 1).
Heat-strengthened glass, produced to BS EN 1863, has a finely balanced level of residual stress, such that its characteristic bending strength is at least 70 MPa (not a design value), but propagating cracks do not branch so that often fragments or ‘islands’ are produced, which could become displaced from the broken pane.
The residual stress of heat-strengthened glass ensures that any cracks will propagate to the edges, where compressive stress along the edge often causes the crack to branch by 180o and run parallel for a short distance before breaking out.
Toughened glass, also known as ‘fully tempered’, whether heat soaked or not, should break into a large number of roughly cubic fragments (Fig. 2). The size of the fragments is related to the thickness by the standards BS EN 12150 and BS EN 14179. For example, 10 mm glass should break into not less than 40 fragments in a 50 mm square within 5 minutes of breaking.
Strength of Glass
Glass has a high theoretical strength (over 30 GPa) because of strong bonds between its molecules but, the practical strength is determined by brittle fracture originating at surface defects. The absence of crystalline structure prevents plastic flow on a macro scale and so glass exhibits virtually perfect linear elastic behaviour until brittle fracture occurs.
When glass is tested to destruction it is common to obtain results considerably higher than the design stress, or even the characteristic stress, because the surface condition of the test sample is in a better condition than we can assume it will be after many years in service.
On the surface of a glass plate there will be a range of flaws such as scratches or pits, and for the purposes of fracture mechanics, flaws are idealised as semi-elliptical cracks normal to the surface, of depth a, with a radius at the tip σ (Fig. 3). The stress at the tip of the crack is represented by the stress concentration expression:
σtip = 2σn√(a/r)
The radius is usually taken to be ~10-9 mm, and critical depths of flaws in annealed glass are much less than a millimetre, depending on the applied stress. It is useful to combine the severity of a surface flaw with the applied stress when considering the conditions for brittle fracture, and Griffith introduced the stress intensity factor, KI :
KI = Y σn√(πa)
(where Y is a geometrical factor ranging from 0.56 to 1.12 according to the shape of the crack). The fracture toughness of a material can then be represented by a critical stress intensity factor, KIc, and any anticipated service condition compared with that limit (i.e. the Griffith failure criterion):
KI ≥ Kic
The critical stress intensity factor for soda lime glass is around 0.75 MPa m½. By way of comparison, the fracture toughness KIc, for mild steel is of the order of 100 MPa m½.
Soda lime glass is particularly prone to a type of stress corrosion cracking known as ‘static fatigue’ that makes it weaker under continuous loading than under short-term load. Although glass is brittle, it is actually more resistant to a short-term load like the impact of a football than a long-term load like the pressure of water in a fish tank.
Water molecules from the environment can diffuse down a crack in the glass. At the very tip of a crack, if the individual bonds between atoms that are resisting its progress are under enough tension, water molecules can attach themselves and break the bonds, allowing the crack to grow minutely.
This process of slow crack growth can start and stop with variations in loading, and can go undetected for long periods. The strength of glass is found to be highest when measured rapidly because surface flaws under stress will grow, so the strength is usually expressed as the ‘short-term strength’ or ‘sixty-second strength’. Any value of glass strength that is not qualified with the duration of loading should be treated with suspicion.
When the pre-existing flaws grow slowly by static fatigue, their stress intensity increases at an accelerating rate until KI ≥ KIc and the glass cracks visibly, and usually audibly. There is a threshold stress intensity, KI0, below which a flaw will not grow, which is around 0.25 MPa m½. Some glass design methods use a factor of between 2.6 and 3.0 to reduce the short-term strength when considering long-term loads.
The relationship between strengths of glass measured over different time periods of steadily increasing load until failure was represented by Charles (1958), in his classic work on why glass is weak when loaded for long duration, with the following relationship:
σf2 = σf1(t1/t2)1/n where n is a material factor, found to be 16 for float glass in air, and t1 and t2 are the times to failure in seconds.
Post-breakage Characteristics of Laminated Glass Combinations
Laminated glass can consist of any combination of processed glass types with a choice of interlayers with different properties.
The following combinations are all described assuming a typical PVB interlayer. Annealed/annealed This is by far the most common combination and is usually described just by the generic term ‘laminated glass’, and it is the standard material for vehicle front windscreens. If it is broken by a soft body impact, a pattern of cracks like a spider’s web is formed (Fig. 4).
Radial cracks caused by bending stress and membrane stress in the glass panes are crossed by circumferential cracks where the triangular shards are subjected to bending stress. Hard-body impact may create a small star of cracks or a ‘Hertzian cone’ if the projectile is fast moving. The cracks in each layer of the laminate tend to follow similar paths if the breaking force is high, but can deviate when the applied load is less.
Thermal fracture from edge damage to laminated glass will often break both plies from the same location, with the individual cracks following different paths. If the edge is undamaged, thermal stress may generate cracks from different places in the two plies, or only in one ply. Heat strengthened/heat strengthened This combination tends to behave similarly to annealed/annealed because of the similar breakage characteristics of the glass types. This gives the laminated combination a good degree of stability and the capacity to carry small loads once both layers of glass are broken.
It is commonly used for glass floors, particularly for outside applications where thermal shock resistance is required and the breakage pattern similar to that of annealed glass would be preferred to that of toughened glass.
Toughened/toughened This combination offers high ultimate strength but little residual strength after both leaves are broken. When toughened glass fragments it is able to resist compressive loads, but the small particles do nothing to transfer tensile forces. Therefore, a broken panel can only resist bending by virtue of the tensile capacity of the interlayer and tends to fold easily, especially when warm.
The tear resistance of a normal PVB interlayer is rarely adequate to support a broken panel on point fixings. Toughened/heat strengthened This combination is popular in bolted, or ‘point fixed’ glazing systems, where toughened/toughened would be at risk of tearing away from the attachment points, particularly in inclined applications.
The toughened glass provides high bending strength when the panel is intact, and the heat-strengthened glass provides large chunks and unbroken zones to lock onto the fittings after the panel is broken.