Drawing No. EH–MT–014 // Engineering Materials
Glass Properties
Reviewed September 2026
Annealed soda-lime-silicate float glass is used as the reference. Its elastic modulus is well defined, but practical strength is governed by surface flaws, edge condition, size, load duration and strengthening treatment.
Representative properties
Switch units without changing the underlying material reference.
What changes these properties?
Engineering variability
- Annealed, heat-strengthened and fully tempered glass have very different usable strengths while retaining similar elastic modulus.
- Surface scratches and edge damage can dominate fracture strength.
- Pilkington ATS-129 gives the reference physical-property basis used here; thermal shock still depends on gradients, restraint, thickness, edge condition and flaw state.
- Soda-lime glass is amorphous; softening/annealing/working points are more meaningful than a single melting temperature.
Selection note
For glazing and structural glass, use the applicable glass standard and strength model rather than a generic tensile-strength number.
Do not combine strength from one grade/condition with modulus, thermal data or temperature limits from another without checking compatibility. Where the overview shows a range, it is intended to expose variability—not establish allowable bounds.
Glass in practice
A material whose practical strength has almost nothing to do with its theoretical strength, and everything to do with its surface.
Where it is specified
Architectural glazing, containers, optical components and lenses, laboratory ware, display cover glass, fibre optics, and cookware. The composition follows the duty: soda-lime for windows and bottles (cheap, easy to melt), borosilicate for laboratory and cookware (low expansion, thermal shock resistant), aluminosilicate for chemically strengthened display cover glass, and fused silica for optics and semiconductor use.
Chosen over what
Against polycarbonate for glazing: glass is far more scratch resistant, dimensionally stable, UV stable and fire resistant. PC is vastly more impact resistant and lighter, which is why the two are laminated together in security glazing.
Against borosilicate specifically: soda-lime has roughly three times the thermal expansion, so a borosilicate beaker survives a flame and a soda-lime one cracks. That single property difference, not chemical resistance, is the usual reason for the specification.
How it fails
Surface flaws govern everything. Theoretical strength is several GPa; practical design strength is tens of MPa, because microscopic surface cracks concentrate stress. A pristine drawn fibre is enormously strong; the same glass after handling is not. This is why glass is designed by probability of breakage rather than by allowable stress.
Static fatigue. Under sustained tensile load in humid air, water at the crack tip drives subcritical crack growth, so glass fails after hours or years at a stress it survived instantaneously. Long-term design stress is a fraction of short-term strength.
Thermal shock, particularly in soda-lime. Localised heating or cooling generates surface tension and fracture.
Nickel sulphide inclusions in toughened glass — a rare, microscopic inclusion that slowly transforms and expands, causing spontaneous shattering of a pane months or years after installation. Heat-soak testing to EN 14179 exists specifically to force these failures before glazing is installed.
Fabrication notes
Strengthening is by putting the surface into compression. Thermal toughening quenches the hot surface to leave it in compression — roughly four to five times the strength of annealed glass, and it fragments into blunt dice. Chemical strengthening exchanges sodium for larger potassium ions in a salt bath, giving a thinner but deeper-compressed case and much higher strength — the basis of modern phone cover glass.
Toughened glass cannot be cut or drilled after treatment. Every hole, notch and edge finish must be completed before toughening, and this constraint drives glazing design.
Laminated glass bonds two panes with a PVB or ionoplast interlayer: it retains fragments on breakage and provides post-breakage capacity, which is why overhead and balustrade glazing is laminated rather than merely toughened.
Edge quality is structural. A ground and polished edge is markedly stronger than an as-cut one, since edges are where flaws concentrate.
Specifications and standards
EN 572 for basic soda-lime products, EN 12150 for thermally toughened glass, EN 14179 for heat-soaked toughened glass, EN 14449 for laminated glass; ASTM C1036 (flat glass), C1048 (heat-treated), C1172 (laminated); ASTM E1300 and EN 16612 for determining glazing load resistance; ISO 3585 for borosilicate 3.3.
References
Haldimann, Luible and Overend, Structural Use of Glass; ASM Engineered Materials Handbook Vol. 4, Ceramics and Glasses; Schott and Corning technical data sheets.
Property interpretation
Stiffness vs strength
Young’s modulus controls elastic deformation. Yield or fracture strength controls how far the material can be stressed. These are separate material characteristics.
Thermal response
Thermal conductivity controls temperature gradients, specific heat contributes to thermal inertia, and expansion controls thermally induced strain when movement is restrained.
Temperature transition
The final property card is deliberately labeled “melting / transition.” Metals often have a solidus–liquidus range; amorphous polymers and glass use softening or glass-transition concepts; concrete and wood decompose instead of exhibiting one clean melt point.
Technical sources
Source hierarchy favors manufacturers, industry associations, standards bodies and government engineering references. Strength data must still be reconciled with the exact procurement specification.