Overview
ISO 1288-5:2016 (Glass in building - Determination of the bending strength of glass - Part 5) specifies the coaxial double‑ring test for measuring the comparative bending strength of flat glass specimens with small test surface areas. The method isolates the central tensile stress field (excluding edge effects) so the test is suitable for comparing strength of glass types but not for structural design. Patterned glass is excluded.
Key topics and technical requirements
- Test principle: A flat circular or square specimen rests on a supporting ring and is loaded concentrically by a smaller loading ring to produce a uniform tensile stress area bounded by the loading ring; failure load is used to calculate bending strength.
- Loading device sets: Four ring combinations are standardized (R30, R45, R60, R105) with specified loading/support-ring radii and corresponding specimen diameters (examples: R30 → 66 mm diameter; R45 → 100 mm; R60 → 240 mm; R105 → 420 mm).
- Specimen geometry & tolerances:
- Circular or square specimens with defined diameters/side lengths.
- Local flatness tolerance: 0.1 mm; parallelism tolerance: 2% of specimen thickness within the support contact area.
- Minimum specimen thickness depends on expected bending strength (see ISO 1288‑5 Table 3); if unknown, use values for up to 600 MPa.
- Apparatus & measurement:
- Testing machine with stepless, low‑shock loading and load-measuring accuracy ±2%.
- Loading/support rings with 2.5 mm bearing radius; silicone rubber spacer 3 mm thick (hardness ~40 ±10 IRHD) as required.
- Measuring precision: diameter/side to 1 mm; thickness to 0.01 mm.
- Procedure essentials:
- Remove protective coatings 24 h before test; condition specimens in test environment ≥4 h.
- Use adhesive film on the loaded side to retain fragments and locate fracture origins.
- Evaluate only the central stressed area; follow calculation and reporting rules in the standard.
Practical applications and users
- Who uses it: glass manufacturers, testing laboratories, quality control teams, research & development groups, and conformity assessment bodies involved in material comparison.
- Why use it: to compare bending strength performance across glass types, assess production consistency, support R&D and product development, and provide repeatable laboratory test data for material characterization.
- Not for: direct structural design or load-bearing calculations - designers should consult appropriate design standards.
Related standards
- ISO 1288‑1 (Fundamentals) - essential reading for limitations and interpretation.
- Other parts of ISO 1288 cover large test areas, four‑point bending and channel‑shaped glass.
Keywords: ISO 1288-5, coaxial double‑ring test, bending strength of glass, glass testing standard, glass in building, specimen preparation, comparative glass strength.
Open preview (PDF)Glass in building — Determination of the bending strength of glass — Part 5: Coaxial double ring test on flat specimens with small test surface areas
