ISO 7257:2016

Overview

ISO 7257:2016 specifies a rotary flexure test procedure to determine and classify the fatigue strengths of reconnectable or permanent aircraft hydraulic tubing joints and fittings. The standard covers flexure testing of high‑strength hydraulic tubes made from alloys such as corrosion‑resistant steel, Nimonic (trademark), titanium and aluminium, using a rotary bending (rotary flexure) test machine with system pressure applied to produce mean stress. It is intended primarily for comparative evaluation of fitting/tube combinations and for generating S/N and D/N data for fatigue classification.

Key topics and technical requirements

• Test scope: Flexure fatigue tests for reconnectable or permanent hydraulic tube joints and fittings used on commercial and military aircraft.
• Specimens: Typical specimen = adapter fitting (headstock), straight tubing section, and test fitting (tailstock). Specimen lengths are specified by tube nominal diameters (DN) and available in metric and imperial tables.
• Test device: Rotary flexure machine capable of maintaining system pressure, automatic shutdown on pressure loss, and headstock/tailstock alignment. Headstock should permit up to 25 mm (1 in) total deflection and rotation at 1 500–3 600 min−1.
• Loading: Mean stress applied via internal pressure (water or system fluid) while cyclic rotary bending produces alternating bending stress.
• Instrumentation: Use of strain gauges (specified sizes and locations) to establish bending stress; dial indicators may be used for deflection measurement once correlation is established.
• Data output & classification:
  • S/N curves (bending stress vs. cycles to failure) for fatigue classification.
  • D/N curves (deflection vs. cycles to failure) to assess deflection/fatigue strength and relative rigidity.
  • Minimum sampling for S/N testing: at least four sets of two specimens per size.
• Procedure essentials: Specimen centring, free‑state microstrain measurements, bend setting verification at rpm, pressurization, controlled temperature where specified.

Applications and users

• Who uses ISO 7257:2016: aerospace design and stress engineers, qualification/test laboratories, hydraulic‑fittings manufacturers, procurement and quality assurance teams, certification authorities, and MRO (maintenance, repair & overhaul) organisations.
• Practical uses:
  • Comparative evaluation of fitting designs and material combinations for fatigue performance.
  • Producing characteristic S/N and D/N curves used for component selection, design validation, and life‑assessment studies.
  • Supporting qualification programs (complementary to ISO 7169, which covers separable tube fittings procurement requirements).

Related standards

• ISO 2964 - Tubing outside diameters and thicknesses (metric dimensions)
• ISO 7169 - Separable tube fittings for aerospace fluid systems (qualification/procurement)

Keywords: ISO 7257:2016, rotary flexure test, hydraulic tubing joints, hydraulic tube fittings, fatigue strength, S/N curves, D/N curves, strain gauges, aircraft hydraulic systems.