ISO 18488:2025

Overview

ISO 18488:2025 specifies a laboratory test method for determining the strain hardening modulus of polyethylene (PE) materials used in piping systems. The strain hardening modulus, obtained from true stress vs. draw ratio curves on compression‑moulded sheets, is used as an intrinsic measure of a material’s resistance to slow crack growth (SCG). This second edition updates definitions, specimen handling and data‑analysis guidance and applies to all PE types regardless of manufacturing route, comonomer or catalyst.

Key topics and technical requirements

• Test principle: tensile testing of compression‑moulded specimens at 80 ± 1 °C, recording true stress vs. draw ratio beyond the natural draw ratio to extract the strain hardening modulus.
• Specimen preparation: compression moulding per ISO 293 into sheets of 0.30 mm or 1.0 mm thickness, annealing and punching five specimens free of defects.
• Specimen geometry: defined gauge length, narrow parallel section (4.0 mm width) and specified radii and overall lengths to avoid grip failure.
• Test equipment:
  • Tensile machine and speed 20 ± 2 mm/min (conforming to ISO 527‑1).
  • Load cell Class 1 (ISO 7500‑1) and extensometer Class 1 (ISO 9513) capable up to draw ratio λ = 12.
  • Temperature chamber to maintain 80 °C and suitable grips to prevent slippage or premature fracture.
• Measurements and tolerances: thickness measured to 0.005 mm accuracy, width to 0.01 mm; gauge marks and alignment requirements; use of the lowest measured thickness and average width in data analysis.
• Data analysis: slope of a Neo‑Hookean constitutive model fitted to data between draw ratio 8 and up to maximum stress (but not above λ = 12) defines the strain hardening modulus <G>.

Applications and who should use it

• Material suppliers evaluating PE grades for resistance to environmental stress cracking and long‑term SCG performance.
• Pipe and fitting manufacturers for quality control, product development and material selection decisions.
• Testing laboratories performing polymer characterization and comparative assessments of PE formulations.
• R&D teams correlating microstructure, processing and lifetime performance of PE piping materials.

Practical benefits include a fast, reproducible measurement (independent of long SCG test durations) that correlates with established ESCR/PNET/FNCT/CRB performance indicators.

Related standards

• ISO 293 - Compression moulding of thermoplastic test specimens
• ISO 527‑1 - Tensile properties - General principles
• ISO 7500‑1 - Calibration of force‑measuring systems
• ISO 9513 - Calibration of extensometer systems

Keywords: ISO 18488:2025, strain hardening modulus, polyethylene (PE), slow crack growth, PE piping, true stress vs. draw ratio, compression moulded specimens, tensile test, ESCR.