ISO 18488:2015

Overview

ISO 18488:2015 specifies a laboratory test method to determine the strain hardening modulus of polyethylene (PE) materials used in piping systems. The strain hardening modulus is measured from stress–strain curves of compression‑moulded samples tested in tension at 80 °C and is used as an intrinsic indicator of a PE material’s resistance to slow crack growth (SCG). The method is valid for all types of polyethylene regardless of manufacturing process, comonomer or catalyst, for pipes and fittings applications.

Key topics and technical requirements

• Scope: Measurement of strain hardening modulus as a predictor for slow crack growth behaviour of PE.
• Specimen preparation: Compression‑moulded sheets (recommended thicknesses 0.30 mm or 1.0 mm), followed by annealing (1 h at 120 °C) and slow cooling. Specimen geometry and punching required to avoid edge defects.
• Dimensions: Typical gauge length l0 = 12.5 mm; narrow width b = 4.0 mm; clamp and overall dimensions specified to prevent slippage.
• Test conditions: Tensile test at (80 ± 1) °C, traverse speed (20 ± 2) mm/min, pre‑stress 0.4 MPa, condition specimens at test temperature for ≥30 min.
• Instrumentation: Tensile machine complying with ISO 527‑1, load cell Class 1 (ISO 7500‑1), extensometer Class 1 (ISO 9513) - non‑contact extensometer preferred for thin samples.
• Data and analysis: True stress vs draw ratio λ calculated assuming volume conservation. Strain hardening modulus <G>p is derived from a Neo‑Hookean constitutive fit between draw ratios λ ≈ 8 and up to λ = 12 (or maximum stress), with an accuracy of fit R² > 0.90.
• Repeatability: Minimum of five specimens; reject samples that break before λ = 8.5 or show clamp slippage.

Applications and users

ISO 18488:2015 is primarily used to:

• Support material selection for PE pipes and fittings with improved SCG resistance.
• Provide quality control and batch comparison for manufacturers and suppliers.
• Inform R&D and compound formulation for improved long‑term performance.
• Supply test data for failure analysis, product certification and regulatory compliance.

Typical users include polymer testing laboratories, pipe and fitting manufacturers, materials scientists, standards bodies and infrastructure project engineers.

Related standards

• ISO 527‑1 - Tensile properties of plastics (general principles)
• ISO 7500‑1 - Verification of force‑measuring systems for testing machines
• ISO 9513 - Calibration of extensometer systems

ISO 18488:2015 complements environmental stress cracking and long‑term performance tests and can be adapted for other PE applications beyond piping.