ISO 7685:2026

International
Standard
ISO 7685
Third edition
Glass-reinforced thermosetting
2026-01
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 2026
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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 .5
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.6
8.3.1 General .6
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
Bibliography . 9

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Foreword
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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 third edition cancels and replaces the second edition (ISO 7685:2019), which has been technically
revised.
The main changes are as follows:
— the formula for the definition of the mean diameter has been corrected;
— Notes have been added in 8.3.2 and 8.3.3.
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.

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International Standard ISO 7685:2026(en)
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 terminology 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 (N).
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 (m).
3.3
relative vertical deflection
y/d
m
ratio of the vertical deflection, y (3.2), to the mean diame
...