Dry Surface Stress Measurement: Principles, Methods, and Applications for Glass and Coated Materials

Dry surface stress measurement is a non-destructive photoelastic technique that quantifies the residual compressive stress at the surface of heat-treated or chemically strengthened glass, as well as the internal stress developed in dried coating films. Because surface compression directly governs strength, thermal shock resistance, and safe fragmentation behavior, this measurement is a critical acceptance test governed by standards such as ASTM C1279, ASTM C1048, GB 15763.2, EN 12150, and JIS R 3222. This article explains the technical principles, sample requirements, testing items, methods, and typical applications of dry surface stress determination as performed by accredited third-party laboratories.

Technical Overview

Dry surface stress measurement is a non-destructive photoelastic testing technique used to determine the residual compressive stress present at the surface of heat-treated or chemically strengthened glass, as well as the internal stress that develops in coatings and films after they have fully dried and cured. In tempered glass, a controlled air-quenching or ion-exchange process locks the surface into a state of compression that is balanced by tensile stress in the core. The magnitude of this surface compression directly governs bending strength, thermal shock resistance, and safe fragmentation behavior.

The term "dry" refers to the mandatory specimen condition: the measurement surface must be thoroughly cleaned and dried, free of moisture, condensation, oil, fingerprints, dust, and loose residues before testing. Surface contamination alters the optical coupling between the instrument and the specimen, scatters or attenuates the probe beam, and can introduce significant measurement error. Reliable determinations therefore follow standardized procedures such as ASTM C1279 for the non-destructive photoelastic measurement of edge and surface stresses in flat glass, ASTM C1048 for the specification of heat-treated glass, GB 15763.2 for tempered safety glazing used in buildings, EN 12150 for thermally toughened soda-lime silicate safety glass, and JIS R 3222 for chemically strengthened glass.

Testing Samples

Fully Tempered (Toughened) Glass

Flat tempered panes, typically 3 mm to 19 mm in thickness, sourced from curtain wall units, doors, partitions, or finished products. Acceptance criteria commonly require a minimum surface compression of 69 MPa (approximately 10,000 psi) under ASTM C1048 for general applications, or a minimum of 95 MPa under GB 15763.2 for architectural safety glazing. Specimens may be tested as full panels or as representative cut sections, provided the measurement zone is intact and undamaged.

Heat-Strengthened Glass

Heat-strengthened glass exhibits roughly twice the bending strength of annealed glass while retaining a fragmentation pattern similar to annealed glass. Depending on the governing specification, the target surface compression typically falls between 24 MPa and 69 MPa, and measurement confirms that the heat treatment was correctly executed without over- or under-strengthening.

Annealed Float Glass

For annealed products, surface stress measurement verifies that residual stress is near zero and that the annealing process has effectively relieved processing stresses. Elevated edge or surface stress in annealed glass indicates a higher risk of spontaneous cracking during cutting, handling, or service.

Chemically Strengthened and Coated Glass

Ion-exchanged cover glass for displays and touch panels is tested for both surface compression and the depth of the compressive layer (case depth). Dried coating films, paints, and functional layers on rigid substrates are evaluated for internal stress after complete drying and curing, since excessive film stress causes cracking, delamination, or substrate deformation.

For all sample types, the measurement surface is prepared by cleaning with anhydrous alcohol or a suitable solvent, wiping with a lint-free cloth, and allowing the surface to dry completely. Specimens are conditioned at room temperature before testing to avoid condensation and thermal gradients that could distort the readings.

Testing Items

• Surface compressive stress, reported in MPa, including mean, minimum, and maximum values across the measured points
• Depth of the compressive stress layer (case depth) in micrometers, applicable to chemically strengthened glass
• Edge stress in MPa, evaluated to assess the risk of edge-initiated fracture
• Stress uniformity and distribution mapping across the pane, comparing center, corner, and edge zones
• Internal stress of dried coatings and films, reported as tensile or compressive stress in the cured layer

Testing Methods

Grazing-Incidence Surface Polarimetry

A polarized light beam is introduced into the surface layer of the glass at a grazing angle. The stress-induced birefringence produces an optical retardation that is measured with a compensator, and the surface stress is calculated from the retardation using the stress-optical coefficient of the glass, which is approximately 2.7 Brewsters for soda-lime compositions. A small quantity of refractive index matching oil is applied to the cleaned, dry surface to ensure uniform optical coupling, which is why thorough surface drying beforehand is essential.

Scattered-Light Polarimetry

A laser beam is directed into the glass interior, and the intensity of light scattered at depth depends on the local polarization state, which in turn depends on the local stress. This method provides depth-resolved stress profiles, making it particularly suitable for chemically strengthened glass with thin compressive layers and for thick sections where surface methods alone are insufficient.

Differential Surface Refractometry

This technique compares the refractive index of the compressed surface layer with that of the stress-free interior by observing the critical angle of total reflection. Because compressive stress changes the refractive index of glass, the measured difference can be converted directly into surface stress. The method is well suited to ion-exchanged glass with shallow compressive layers.

Cantilever and Beam Deflection Methods for Dried Films

For coatings, the test material is applied to a thin, elastic substrate such as a metal or glass strip. As the film dries and cures, the internal stress causes the substrate to bend, and the measured deflection is converted into film stress through the Stoney equation. For crystalline films, X-ray diffraction using the sine-squared-psi method can also be applied to determine residual stress.

Applications

• Acceptance testing of tempered and heat-strengthened glass for architectural curtain walls, facades, and windows
• Verification of safety glazing for automotive, rail, and marine applications
• Quality control of household appliance glass, including oven doors, refrigerator shelves, and cooktop panels
• Evaluation of furniture glass, shower enclosures, railings, and partitions
• Characterization of cover glass for smartphones, tablets, and other electronic displays
• Tempering furnace qualification, heat-treatment process validation, and troubleshooting of spontaneous breakage incidents
• Development and failure analysis of industrial coatings, paints, and functional thin films

FAQ

Why must the surface be dry and clean before measurement?

Moisture, oil, and particulate residue change the optical coupling conditions between the instrument and the glass, scatter the probe beam, and prevent the refractive index oil from wetting uniformly. Any of these effects introduces systematic error into the retardation reading. A cleaned, fully dried surface is therefore a prerequisite for accurate and repeatable results.

Is dry surface stress measurement destructive?

No. All photoelastic surface stress methods are non-destructive and can be performed on finished, installed, or packaged products without cutting or breaking the specimen. This allows testing of deliverable goods and in-situ inspection of glazing already installed on site.

What stress levels indicate a properly tempered product?

Under ASTM C1048, fully tempered glass generally requires at least 69 MPa of surface compression, rising to higher minimums for certain safety glazing applications, while GB 15763.2 requires a minimum of 95 MPa for architectural tempered glass. Heat-strengthened glass is typically specified in the range of 24 MPa to 69 MPa. Values below the specified minimum indicate insufficient tempering and reduced fragmentation performance.

How many measurement points are taken per specimen?

Typical protocols measure multiple points on each pane, covering the center region and locations near the corners and edges, following the sampling plan agreed with the client or specified in the purchase specification. Reporting normally includes individual values, the mean, the minimum, and the distribution across the panel to demonstrate uniformity.

Can coated, tinted, or printed glass be tested?

In many cases yes, but the feasibility depends on the optical characteristics of the product. Thin transparent coatings usually permit measurement directly on the glass side, while heavily tinted, reflective, opaque, or ceramic-printed surfaces may attenuate or block the probe beam. In such cases, measurement can be performed on the uncoated face, or alternative methods such as scattered-light polarimetry can be evaluated by the laboratory.