Overview
ISO 7884-7:1987 - Glass: Viscosity and viscometric fixed points - Part 7 specifies a standardized beam-bending method to determine the annealing point and strain point of glass. The method measures the midpoint viscous bending rate of a simply loaded glass beam and derives the strain point by extrapolation. Results are used to define cooling programmes in glassware production and to control residual stress relief during manufacture.
Key topics and requirements
- Measurement principle: Rate of midpoint deflection of a loaded beam is recorded during controlled cooling; annealing point is taken from a specific deflection-rate criterion and the strain point obtained by extrapolation.
- Apparatus:
- Electrically heated furnace with controlled temperature distribution (temperature variation along the specimen ≤ 2 °C).
- Low-inertia thermocouples (type S or K) for measurement and control, with calibrated cold junctions.
- Ceramic support stand and loading rod (matching thermal expansion); single-crystal sapphire or vitreous silica often recommended.
- Extensometer (e.g., LVDT) capable of reliably measuring at least 2.5 mm with direct reading to 0.025 mm and resolution ~0.0025 mm.
- Specimens:
- Cylindrical or rectangular beams with non‑uniformity ≤ 2% along length.
- Typical support span ≈ 50 mm; recommended cross‑sectional moment of inertia about 2–10 mm4 for that span (Annex A provides formulae for I).
- Procedure controls:
- Heating and cooling ramps (e.g., heating ~5 °C/min; controlled cooling ≈ (4 ± 1) °C/min during measurement).
- Calibration with reference glasses of known annealing/strain points; graphical plotting of log(deflection rate) vs temperature and calibration plots versus reciprocal moment of inertia (see Annex B for apparatus example).
- Viscosity context: The annealing point corresponds to a viscosity near 10^13 dPa·s; the extrapolated strain-point deflection rate corresponds to ≈10^14.7 dPa·s.
Applications and users
- Practical uses:
- Specifying and validating cooling programmes and annealing cycles for glassware and laboratory glass products.
- Quality control to minimize residual stress and reduce breakage during downstream processing.
- Material characterization during glass development and batch formulation.
- Typical users:
- Glass manufacturers and process engineers, QA laboratories, materials scientists, standards and test houses, and furnace/equipment designers.
Related standards
- ISO 7884 series (other parts cover rotation viscometers, fibre elongation, sinking bar, softening point, etc.)
- IEC 584-1 (thermocouple types and reference tables) - referenced for temperature measurement.
Keywords: ISO 7884-7:1987, annealing point, strain point, beam bending, glass viscosity, viscometric fixed points, extensometer, LVDT, calibration, furnace control, cross-sectional moment of inertia.
Open preview (PDF)Glass — Viscosity and viscometric fixed points — Part 7: Determination of annealing point and strain point by beam bending
