SIST-TP CEN ISO/TR 20659-1:2025 PDF

Overview

CEN ISO/TR 20659-1:2024 - "Rheological test methods - Fundamentals and interlaboratory comparisons - Part 1: Determination of the yield point" is a Technical Report published by CEN (adopting ISO/TR 20659-1:2024). It documents an interlaboratory comparison and provides guidance on determining the yield point (yield stress) - the shear stress τ below which a material does not flow - using common rheological test methods. The report is aimed at improving reproducibility of yield point measurements for materials such as paints, varnishes and related dispersions.

Key topics and technical requirements

• Metrological basis for yield point determination, including dependence on temperature, pressure and material history.
• Description of principal test modes: shear rate-controlled rotational tests (ramp in shear rate) and shear stress-controlled rotational tests (ramp in shear stress).
• Evaluation methods and regression models used to extract yield points from flow curves: Bingham, Casson and Herschel–Bulkley models and related axis-intercept, plateau and reference-value approaches.
• Practical measurement considerations: specification of measuring profiles (ramp periods), rheometer resolution (low rotational speeds), transient/time-dependent effects at low shear rates, and the need for rheometer calibration and uncertainty assessment.
• Scope of the interlaboratory study: samples ranged from very low yield points (