ISO 7685:2019

Glass-reinforced thermosetting plastics (GRP) pipes - Determination of initial ring stiffness

This document specifies methods for determining the initial ring stiffness of glass-reinforced thermosetting plastics (GRP) pipes. Two methods are given (constant load and constant deflection), and within the specified deflection limits, each is equally valid and can be used for any diameter.

Tubes en plastiques thermodurcissables renforcés de verre (PRV) — Détermination de la rigidité annulaire initiale

General Information

• Status: Published
• Publication Date: 10-Jul-2019
• ICS: 23.040.20 - Plastics pipes
• Technical Committee: ISO/TC 138/SC 6 - Reinforced plastics pipes and fittings for all applications
• Drafting Committee: ISO/TC 138/SC 6/WG 1 - Methods of test
• Current Stage: 9092 - International Standard to be revised
• Start Date: 15-May-2024
• Completion Date: 13-Dec-2025

Relations

• Revised: ISO 7685 - Glass-reinforced thermosetting plastics (GRP) pipes - Determination of initial ring stiffness
• Effective Date: 18-May-2024
• Revises: ISO 7685:1998 - Plastics piping systems - Glass-reinforced thermosetting plastics (GRP) pipes - Determination of initial specific ring stiffness
• Effective Date: 23-Apr-2016

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.

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 7685
Second edition
2019-07
Glass-reinforced thermosetting
plastics (GRP) pipes — Determination
of initial ring stiffness
Tubes en plastiques thermodurcissables renforcés de verre (PRV) —
Détermination de la rigidité annulaire initiale
Reference number
©
ISO 2019
© ISO 2019
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
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Published in Switzerland
ii © ISO 2019 – All rights reserved

Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 2
5 Apparatus . 3
5.1 Compressive-loading machine . 3
5.2 Load application surfaces . 3
5.2.1 General arrangement . 3
5.2.2 Plates . 3
5.2.3 Beam bars . 3
5.3 Dimension-measuring instruments . 3
6 Test pieces . 4
6.1 Preparation . 4
6.2 Number . 4
6.3 Determination of dimensions . 4
6.3.1 General. 4
6.3.2 Length. 4
6.3.3 Wall thickness . 5
6.3.4 Mean diameter . 5
7 Conditioning . 5
8 Procedure. 5
8.1 Test temperature . 5
8.2 Positioning of the test piece . 5
8.3 Application of load and measurement of deflection . 5
8.3.1 General. 5
8.3.2 Method A: Using constant load . 6
8.3.3 Method B: Using constant deflection . . 6
9 Calculation . 7
10 Test report . 8
Foreword
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described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
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.org/iso/foreword .html.
This document was prepared by Technical Committee ISO/TC 138, Plastics pipes, fittings and valves for
the transport of fluids, Subcommittee SC 6, Reinforced plastics pipes and fittings for all applications.
This second edition cancels and replaces the first edition (ISO 7685:1998), which has been technically
revised. The main changes compared to the previous edition are as follows:
— added recommendations for the parallelism of the plates/bars;
— added preload dependent on DN sizes;
— taring requirements of load and deflection after preload have been added and before start of testing.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/members .html.
iv © ISO 2019 – All rights reserved

INTERNATIONAL STANDARD ISO 7685:2019(E)
Glass-reinforced thermosetting plastics (GRP) pipes —
Determination of initial ring stiffness
1 Scope
This document specifies methods for determining the initial ring stiffness of glass-reinforced
thermosetting plastics (GRP) pipes. Two methods are given (constant load and constant deflection), and
within the specified deflection limits, each is equally valid and can be used for any diameter.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 3126, Plastics piping systems — Plastics components — Determination of dimensions
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https: //www .iso .org/obp
— IEC Electropedia: available at http: //www .electropedia .org/
3.1
compressive load
F
load applied to a pipe to cause a diametric deflection
Note 1 to entry: Compressive load is expressed in newtons.
3.2
vertical deflection
y
vertical change in diameter of a pipe in a horizontal position in response to a vertical compressive
load (3.1)
Note 1 to entry: Vertical deflection is expressed in metres.
3.3
relative vertical deflection
y/d
m
ratio of the vertical deflection, y (3.2) to the mean diameter, d (3.4) of the pipe
m
3.4
mean diameter
d
m
diameter of the circle c
...