Mechanical properties of Glass
The density of glass is 2.5, which gives flat glass a mass of 2.5kg per m2 per mm of thickness, or 2500 kg per m3.
The compressive strength of glass is extremely high: 1000 N/mm2 = 1000 MPa. This means that to shatter a 1cm cube of glass, it requires a load of some 10 tonnes.
When glass is deflected, it has one face under compression and the other in tension. Whilst the resistance of glass to compressive stress is extremely high, its resistance to tensile stress is significantly lower.
The resistance to breakage on deflection is in the order of:
- 40 MPa (N/mm2) for annealed glass
- 120 to 200 MPa for toughened glass (depending on thickness, edgework, holes, notches etc).
The increased strength of SGG SECURIT toughened glass is the result of the toughening process putting both faces under high compression. SAINT-GOBAIN GLASS can advise on appropriate working stresses for different glass types and can calculate suitable thicknesses for any architectural application.
Glass is a perfectly elastic material: it does not exhibit permanent deformation, until breakage. However it is fragile, and will break without warning if subjected to excessive stress.
• Young’s modulus, E
This modulus expresses the tensile force that would theoretically have to be applied to a glass sample to stretch it by an amount equal to its original length.
It is expressed as a force per unit area.For glass, in accordance with European standards :
E = 7 x 1010 Pa = 70 GPa
• Poisson’s ratio, μ (lateral contraction coefficient)
When a sample is stretched under mechanical stress a decrease in its cross-section is observed. Poisson’s ratio (μ) is the relation between the unit decrease in the direction perpendicular to the axis of the effort and the unit strain in the direction of the effort.
For glass in buildings, the value of coefficient μ is 0.22.
Linear expansion is expressed by a coefficient measuring the stretch per unit length for a variation of 1°C. This coefficient is generally given for a temperature range of 20 to 300°C.
The coefficient of linear expansion for glass is 9 x 10-6 m/mk.
Due to the low thermal conductivity of glass, see Thermal Insulation Glazing, partially heating or cooling a sheet of glass creates stresses, which may cause thermal breakage. When glass is framed, the edges are encased in the rebate, which protects them from direct solar radiant heat. This can cause temperature differentials sufficient to cause thermal breakage.This risk is increased where heat absorbent solar control glasses are used.
A piece of glass 2 metres in length (expressed in mm) subjected to an increase in temperature of 30°C will lengthen by:
2000 x 9 x 10-6 x 30 = 0.54 mm Theoretically, an increase of 100°C will cause 1 metre of glass to expand by approximately 1mm.
The table below lists the coefficients of linear expansion for other materials.
In applications or systems where there is a risk of creating significant temperature differences in the glass pane, it may be necessary to take special precautions during processing and installation.
Heat strengthening or toughening glass enables it to withstand temperature differences of 150 to 200°C. SAINT-GOBAIN GLASS can perform thermal safety risk analyses on request.