GLASS STRENGTHENING

Basic Concept of Glass Strengthening
In order to improve the tensile strength characteristics it is possible to "pre-load" the surfaces of the glass with compressive stresses. This can be done either by heat strengthening, heat tempering or chemically strengthening the glass. When a glass plate is bent, the convex surface is placed into tension while the concave surface is compressed. By adding compressive stress to the glass, the compressive force must be overcome before the surface becomes tensile in nature. This can dramatically improve the amount of bending force that may be placed on a plate of glass before it breaks.

Measuring the Strength of Glass
In assessing the level of the strengthening levels of glass, there are two major measurements: Compressive Strength and Modulus of Rupture. In addition, for chemically strengthened glass there is an additional measurement: Depth of Layer.

Compressive Strength includes two elements of compression: surface compression and edge compression. Surface and edge compression is the amount of compressive (squeezing type) stress on the surfaces or edges of glass introduced by processing, heat strengthening, heat tempering, chemical strengthening or in primary glass formation. The opposite of compressive stress is tensile (elongation type) stress. Glass is very low in tensile strength and extremely high in the amount of compressive stress it can withstand. Therefore, most glass breakage is due to tensile strength failure.

Edge compression is equivalent to surface compression except it is defined as measured on the edge surfaces. That is, a rectangular plate of glass actually has 6 surfaces: top edge, bottom edge, right edge, left edge, front surface and back surface. Strengthening processes work on all 6 edges during processing. However, not all surfaces are necessarily strengthened equally.

The Modulus of Rupture (MOR) strength of glass is determined by specific tests conducted in accordance with ASTM C 158. This test measures the bending (flexural) strength of glass.

Depth of Layer (DOL) is a measurement of the compressive strength of glass specific to chemically strengthened glass. It is the depth into the surface of glass to which compressive stress is introduced. It is defined as the distance from the physical surface to the zero stress point internal to the glass.

There are three basic types of glass strengthening:  Abrisa has the ability to comply with many industry quality standards. These include ANSI, ASME, ASTM, NCSL, ISO, MIL, DIS, JAN and SAE. Select this link for more information.

1. Heat Strengthening is appropriate for many applications and materials where a full temper is not possible or desired due to thickness, size, low thermal expansion rate of the base material, or other application requirements. This process does not produce a full "dicing" pattern when the glass is broken and will not meet the requirements of ANSI Z97.1 or CPSC 16CFR1201. Where appropriate, ASTM-C-1048 "Kind HS" may apply.

2. Heat Tempering is appropriate where compliance is required to ANSI Z97.1 or to ASTM-C-1048 (also called "Kind FT"). This standard is appropriate when small sized shards (dicing) are required when the glass is broken. Heat tempering will cause some optical distortion. Where no optical change is acceptable, chemical strengthening may be suitable instead. Heat tempering is typically appropriate for glass from 1/8" (3.2mm) - 3/4 (19mm) thick and with sizes from 1" diameter to 48" x 72". Heat tempered glass may be verified by performing a surface compression test or break pattern test for compliance.