ISO 7685:2019

Overview

ISO 7685:2019 specifies methods to determine the initial ring stiffness of glass‑reinforced thermosetting plastics (GRP) pipes. The standard defines two equivalent test methods - Method A (constant load) and Method B (constant deflection) - for measuring how a pipe resists diametral compression. Within the specified deflection limits each method is equally valid and can be applied to any pipe diameter. ISO 7685:2019 replaces the 1998 edition and is intended for use where a referring standard sets the exact test parameters.

Key topics and technical requirements

• Test principle: A pipe ring is supported horizontally and compressed vertically; vertical deflection is measured and used to derive ring stiffness.
• Two test methods:
  • Method A (constant load): apply a load to achieve an initial relative deflection (e.g., around 3 %) and hold the load; measure final deflection after a specified period.
  • Method B (constant deflection): impose a set relative deflection and hold it; measure the final load required.
• Apparatus:
  • Compressive‑loading machine with ±1 % loading accuracy.
  • Load application via parallel plates (min width 100 mm) or beam bars (width 20 mm ±2 mm for DN ≤ 300; 50 mm ±5 mm for DN > 300).
  • Recommended plate/beam parallelism: within 1 mm/m (1:1 000).
• Measurements and accuracy:
  • Dimensional instruments calibrated to ±0.1 mm.
  • Deflection measurement accuracy ±1.0 % of maximum.
• Test pieces:
  • Complete rings cut from the pipe, ends smooth and perpendicular.
  • Default length 300 mm ±15 mm if referring standard does not specify; reference lines marked at 60° intervals.
  • Default number: one per pipe size unless otherwise specified.
• Conditioning:
  • Store test pieces at test temperature for at least 0.5 h; in disputes condition for 24 h at 23 °C ±5 °C.
• Preload:
  • DN 100–450: 15 N ±5 N; DN ≥ 500: 100 N ±10 N.
• Calculation:
  • Ring stiffness is calculated from the apparent modulus and the second moment of area (I = e^3/12 per metre length); results expressed in N/m².

Applications and users

ISO 7685:2019 is used by:

• GRP pipe manufacturers for product development and quality control.
• Independent testing laboratories performing compliance and classification tests.
• Design and specification engineers verifying pipe structural performance.
• Procurement and certification bodies requiring objective stiffness data.

Practical uses include verifying resistance to buckling and deformation under external loads, confirming manufacturer claims, and supporting material selection and pipe class assignment.

Related standards

• ISO 3126 - referenced for dimensional measurement methods.
• Prepared by ISO/TC 138 (Plastics pipes).

Keywords: ISO 7685:2019, GRP pipes, glass‑reinforced thermosetting plastics, initial ring stiffness, ring stiffness test, constant load, constant deflection, pipe testing, preload, beam bars, plates.