ISO 9538:2025

Aerospace series - Hydraulic tubing joints and fittings - Planar flexure test

This document specifies a planar flexure test procedure for reconnectable and permanent hydraulic tube joints and fittings. This procedure is intended for conducting flexure tests on fittings with high-strength hydraulic tubes made of corrosion-resisting steel, titanium and aluminium for use on commercial and military aircraft.

Série aérospatiale — Joints et raccords des tuyauteries hydrauliques — Essai de flexion plane

General Information

• Status: Published
• Publication Date: 18-Jun-2025
• ICS: 49.080 - Aerospace fluid systems and components
• Technical Committee: ISO/TC 20/SC 10 - Aerospace fluid systems and components
• Drafting Committee: ISO/TC 20/SC 10 - Aerospace fluid systems and components
• Current Stage: 6060 - International Standard published
• Start Date: 19-Jun-2025
• Due Date: 21-Nov-2025
• Completion Date: 19-Jun-2025

Relations

• Revises: ISO 9538:2017 - Aerospace series - Hydraulic tubing joints and fittings - Planar flexure test
• Effective Date: 26-Nov-2022

Overview

ISO 9538:2025 - Aerospace series - Hydraulic tubing joints and fittings - Planar flexure test defines a standardized planar flexure test procedure for reconnectable and permanent hydraulic tube joints and fittings used on commercial and military aircraft. Applicable to high‑strength hydraulic tubes made of corrosion‑resisting steel, titanium and aluminium, the standard specifies the test setup, instrumentation, stress determination and the test procedure to assess flexure fatigue performance.

Key topics and requirements

• Scope and purpose: A planar flexure test alternative to the rotary test (see ISO 7257) for qualification and validation of hydraulic fittings used in aerospace systems.
• Test rig and components: Specifies a vibrator, a rigid manifold to mount test specimens, and a hydraulic supply unit that maintains static operating pressure and provides pressure‑drop shutdown monitoring. The vibrator shall allow vibration frequencies up to 300 Hz.
• Specimens: Each specimen comprises the test tube, the fitting under test, and the sealing fitting. The number of specimens is defined by the applicable qualification specification.
• Stress components: Combined stress (σf) = axial tensile stress from internal pressure (σp) + bending stress (σb). Bending stress is demonstrated using strain gauges, with deflection optionally checked by angle gauges.
• Instrumentation & gauge locations: Unidirectional strain gauges mounted on each specimen; location: ~4 mm from tube‑to‑fitting attachment for tube sizes to DN 16, and ~8 mm for DN 20 and above. An accelerometer is used on the header for frequency verification.
• Frequency and natural frequency: Specimen natural frequency should be 110–120 Hz; test frequency performed at roughly 100 Hz (i.e., 10–20% below natural frequency to avoid resonance). Verification uses a sine frequency sweep (5–200 Hz) with ~1 g acceleration peak.
• Calibration and procedure: Calibrate bending stress using a static load (F) then apply nominal pressure and record strain gauge output. During the test set vibrator frequency at 80–90% of measured resonant frequency and increase amplitude until the predetermined stress cycle is achieved.
• Acceptance: Specimens shall withstand 10 cycles with no leakages or failures.

Applications and users

ISO 9538:2025 is intended for:

• Aerospace fluid‑system designers and qualification engineers validating hydraulic fittings and tube assemblies.
• Test laboratories performing flexure fatigue testing and conformity assessment.
• Manufacturers and suppliers of hydraulic fittings and tubes for commercial and military aircraft conducting production or type‑approval testing.
• Maintenance and airworthiness authorities requiring reproducible test evidence for component durability.

Related standards

• ISO 7169 - qualification procedures for separable tube fittings.
• ISO 7257 - rotary flexure test (alternative method).

Keywords: ISO 9538:2025, planar flexure test, hydraulic tubing joints and fittings, aerospace, flexure fatigue, strain gauges, natural frequency, bending stress, hydraulic fittings testing.

Standards Content (Sample)

International
Standard
ISO 9538
Third edition
Aerospace series — Hydraulic
2025-06
tubing joints and fittings — Planar
flexure test
Série aérospatiale — Joints et raccords des tuyauteries
hydrauliques — Essai de flexion plane
Reference number
© ISO 2025
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
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Website: www.iso.org
Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Device for flexure test . 1
5 Flexure test specimens . 3
6 Stress determination . 3
7 Procedure . 4
7.1 Instrumentation and strain gauges .4
7.2 Frequency .5
7.2.1 Test frequency .5
7.2.2 Natural frequency of the specimens .5
7.2.3 Verification of the natural frequency . .5
7.3 Bending stress calibration of test specimen .5
7.4 Test .5
8 Requirements . 5
Bibliography . 7

iii
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This document was prepared by Technical Committee ISO/TC 20, Aircraft and space vehicles, Subcommittee
SC 10, Aerospace fluid systems and components.
This third edition cancels and replaces the second edition (ISO 9538:2017), which has been technically
revised.
The main changes are as follows:
— Clause 4 and Clause 7 were updated.
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