Chemically Strengthened Glass Stress Testing: Surface Compressive Stress, Depth of Layer, and Centra

Chemically strengthened glass stress testing measures the surface compressive stress (CS), depth of layer (DOL), and central tensile stress (CT) of ion-exchange strengthened glass used in smartphone cover glass, aircraft windows, rail transit glazing, and industrial flat glass. Accredited third-party laboratories apply nondestructive photoelastic and optical methods—including surface stress meters, scattered-light polarimetry, and ellipsometry—in accordance with standards such as ASTM C1422, ASTM C1279, EN 12337-1, and JIS R 3222 to verify strengthening quality, optimize the ion-exchange process, and support product qualification.

Technical Overview

Chemically strengthened glass, also referred to as chemically tempered or ion-exchange strengthened glass, is produced by immersing cut-to-size glass in a molten potassium nitrate (KNO3) bath at a temperature below the strain point of the glass, typically between 380 °C and 460 °C. During the ion-exchange process, smaller sodium ions in the glass surface are replaced by larger potassium ions from the salt bath. The difference in ionic radius crowds the surface and generates a compressive stress layer balanced by tensile stress in the interior. Because the treatment occurs well below the softening point of the glass, the process introduces virtually no optical distortion, waviness, or warp, and it can be applied to very thin glass, down to sub-millimeter thicknesses.

The stress architecture of chemically strengthened glass differs fundamentally from that of thermally tempered glass. Chemical strengthening produces a very high surface compressive stress, commonly in the range of 400 MPa to 900 MPa, but within a shallow layer typically between 5 μm and 100 μm deep. Thermal tempering, by contrast, produces lower compressive stress of roughly 90 MPa to 120 MPa distributed across approximately 20% of the thickness on each surface. Since the mechanical strength and fracture behavior of chemically strengthened glass are governed jointly by the magnitude of the compressive stress and the depth of the compressive layer, accurate stress measurement is essential for the following purposes:

• Process quality control of the ion-exchange bath, including time, temperature, and salt chemistry verification
• Receiving inspection and outgoing inspection of strengthened glass sheets and finished components
• Product qualification against ASTM C1422, EN 12337-1, JIS R 3222, and customer technical specifications
• Research and development of new glass compositions and deep-strengthening processes
• Failure analysis and fracture mechanics assessment of broken components

Testing Samples

Consumer Electronics Cover Glass

High-alumina aluminosilicate thin glass with thicknesses from approximately 0.3 mm to 1.5 mm, used as cover glass for smartphones, tablets, wearables, and touch displays. These products demand the highest compressive stress levels and precise depth-of-layer control, making them the most common category submitted for stress testing. Test specimens are usually small coupons or finished glass panels with polished surfaces.

Soda-Lime and Borosilicate Chemically Strengthened Glass

Chemically strengthened soda-lime silicate glass for building applications, covered by EN 12337-1, as well as borosilicate glass used in appliance panels, display cases, and specialty flat glass. These glasses generally receive moderate ion-exchange treatments and are tested to confirm that the declared stress class and depth of layer are achieved.

Transportation Glazing Components

Chemically strengthened plies used in aircraft windows, high-speed rail windshields, and marine glazing. These plies are frequently laminated with interlayers before installation. Stress measurement is performed on individual plies before lamination and, where the structure permits optical access, on finished laminates to confirm that the strengthening was not degraded by downstream processing.

Industrial and Optical Flat Glass

Copier platens, scanner beds, instrument windows, sensor covers, and touch panels. These applications combine thin sections with high surface quality, so both stress parameters and surface condition are evaluated together.

Specimen preparation requirements include a flat measurement area larger than the instrument probe footprint, typically at least 10 mm by 10 mm; clean, defect-free surfaces free of coatings or ink at the measurement points; and conditioning at a laboratory temperature of 23 °C ± 2 °C before measurement. Multiple measurement points, usually covering the center and four corners of the part, are recommended to assess stress uniformity.

Testing Items

The core stress parameters of chemically strengthened glass are evaluated as follows:

• Surface compressive stress (CS), expressed in MPa: the maximum compressive stress at the glass surface, which directly governs scratch resistance and flexural strength
• Depth of layer (DOL), also called depth of compression, expressed in μm: the depth from the surface at which the compressive stress decays to zero, which governs strength retention after surface damage and abrasion
• Central tensile stress (CT), expressed in MPa: the balancing tensile stress at the mid-plane of the glass, which controls fracture energy and fragment size
• Stress uniformity: the point-to-point variation of CS and DOL across the surface and the difference between the two faces of the sheet
• Through-thickness stress profile: the complete stress distribution from surface to interior, used for advanced product development and finite-element modeling

In addition to stress measurement, related performance verification is frequently performed to confirm that the measured stress translates into mechanical performance. These supporting tests include fragmentation testing, ball drop impact testing, four-point bending flexural strength, abraded ring-on-ring strength in accordance with ASTM C158, and thermal shock resistance. Typical commercial specifications for consumer electronics cover glass require CS values of 600 MPa to 900 MPa with DOL values of 7 μm to 15 μm for thin glass or 30 μm to 60 μm for deep-strengthened products, while building glass is qualified against the stress classes declared in EN 12337-1.

Testing Methods

Surface Stress Meter Based on Optical Waveguide and Photoelastic Retardation

The most widely used production method measures surface stress with a grazing-angle surface stress meter. Polarized light is coupled into the ion-exchanged surface layer at an oblique incidence. Because the compressed layer has a modified refractive index, it behaves as an optical waveguide, and the photoelastic retardation of the guided light is proportional to the surface compressive stress. The retardation and coupling angle additionally yield the depth of layer, so CS and DOL are obtained simultaneously within seconds. The method is nondestructive and follows the photoelastic measurement principles described in ASTM C1279. Reliable waveguide coupling generally requires a depth of layer of at least 10 μm to 12 μm.

Scattered-Light Polarimetry

Laser scattered-light polarimetry injects a laser beam into the glass edge or surface and records the polarization state of light scattered from points along the beam path. The stress-induced birefringence at each depth is reconstructed, producing a complete through-thickness stress profile. This method measures the central tensile stress directly rather than by calculation, accommodates laminated and asymmetric structures, and is valuable for thick sections, bent parts, and research applications.

Ellipsometry

Spectroscopic ellipsometry detects the refractive index gradient introduced by the ion exchange near the surface. By fitting the measured optical response to a modeled index profile, the depth of layer can be determined with high resolution. Ellipsometry is the preferred technique for very shallow layers below approximately 10 μm, where waveguide coupling in a surface stress meter becomes unreliable, and for newly developed shallow-exchange products.

Differential Surface Refractometry

Differential surface refractometry (DSR) compares the refractive index of the ion-exchanged surface layer with that of the unmodified bulk glass. The magnitude of the index difference correlates with the ion concentration and is used to determine the depth of layer. DSR is frequently applied as an independent cross-check of DOL values obtained by other optical methods.

Derived Calculation of Central Tensile Stress

For a symmetrical product, the compressive force resultant in the two surface layers must equal the tensile force resultant in the core. Using the measured CS, DOL, and total thickness, the central tensile stress can be estimated on the order of CS multiplied by DOL divided by the remaining half-thickness, with the exact factor depending on the shape of the stress profile. For contractual or certification purposes, direct measurement by scattered-light polarimetry is recommended, since CT governs fragmentation behavior and overall fracture safety.

Destructive and Mechanical Verification

Chemically strengthened glass does not exhibit the fine dicing pattern characteristic of thermally tempered glass, because its stored elastic energy is concentrated in a shallow surface layer. Fragmentation tests evaluate particle size, count, and edge retention in accordance with the fragmentation provisions of ASTM C1422 and EN 12337-1. Flexural strength by four-point bending, abraded ring-on-ring strength per ASTM C158, and ball drop impact testing validate the strength benefit delivered by the measured stress parameters, including after surface abrasion that simulates service damage.

Measurement Quality Assurance

Reliable results depend on instrument calibration against certified reference stress standards, correct stress-optic coefficients for the glass composition, stable laboratory temperature, and a defined measurement grid. Accredited third-party laboratories typically report the mean, minimum, and maximum of at least five measurement points per specimen, together with measurement uncertainty, instrument identification, and the applicable test method.

Applications

• Consumer electronics: qualification and incoming inspection of cover glass for smartphones, tablets, notebooks, and wearables, where CS and DOL determine drop performance and scratch resistance
• Aerospace and rail transit: verification of chemically strengthened plies in aircraft windows, high-speed train windshields, and mass transit glazing, including stress confirmation after lamination and edge finishing
• Architecture and interior design: security glazing, display cases, museum vitrines, and thin decorative panels qualified under EN 12337-1
• Industrial equipment: copier platens, scanner surfaces, instrument windows, touch panels, and appliance control panels requiring high strength with excellent flatness
• Process development: optimization of exchange time, bath temperature, and salt composition, as well as studies of bath aging and glass composition effects on stress build-up
• Failure analysis: correlation of measured stress with fracture surface features such as mirror, mist, and hackle zones to reconstruct the cause of breakage

FAQ

Is chemically strengthened glass stress testing destructive?

No. The principal methods, including the grazing-angle surface stress meter, scattered-light polarimetry, ellipsometry, and differential surface refractometry, are nondestructive. Only supplementary verification tests such as fragmentation, flexural strength, and impact testing consume the specimen.

What are typical acceptance values for CS and DOL?

Acceptance values are set by the product specification. Common commercial requirements for consumer electronics cover glass range from 600 MPa to 900 MPa for CS with DOL between 7 μm and 60 μm, depending on thickness and design philosophy. Building glass is accepted against the stress classes declared under EN 12337-1, while ASTM C1422 and JIS R 3222 provide the corresponding classification frameworks.

How does stress testing of chemical glass differ from thermal tempered glass?

Thermally tempered glass has a deep compression layer and is commonly assessed with edge-stress and surface-stress photoelastic instruments together with a fragmentation test. Chemically strengthened glass has much higher but far shallower compressive stress, so dedicated waveguide-type surface stress meters, scattered-light polarimetry, and ellipsometry are used, and fragmentation patterns are interpreted differently.

What is the minimum depth of layer that can be measured?

Surface stress meters based on optical waveguide coupling generally require a DOL of roughly 10 μm to 12 μm or greater. For shallower exchange layers, ellipsometry and differential surface refractometry provide accurate depth determination.

Can stress be measured on laminated or coated glass?

Yes, with limitations. Scattered-light polarimetry can profile stress in laminated plies when optical access is available, and surface stress measurement requires a transparent, uncoated measurement window. Opaque coatings, printing, or inks must be avoided at the measurement points or locally removed.

Which standards govern the testing?

The main references are ASTM C1422 for the specification of chemically strengthened flat glass, ASTM C1279 for nondestructive photoelastic measurement of surface stresses, EN 12337-1 for chemically strengthened soda-lime silicate glass in building applications, and JIS R 3222 for chemically strengthened glass. Accredited third-party laboratories select the applicable method based on product type, thickness, and depth of layer.