ISO 8574:2004

Overview

ISO 8574:2004 - Aerospace - Hydraulic system tubing - Qualification tests for bent tubes - defines qualification test procedures for bent hydraulic tubing used in aircraft. The standard specifies pressure impulse and burst (eclatement) test methods to determine and classify the impulse fatigue strength of bent tubes for both high- and low-pressure aerospace hydraulic systems. It is intended for qualification of tubing configurations and evaluation of new materials.

Key topics and requirements

• Scope: Qualification tests for bent hydraulic system tubing used in aircraft (impulse fatigue and burst testing).
• Test types:
  • Pressure impulse test - performed in accordance with ISO 6772. At least six bent tube samples per dimension are required.
  • Burst (eclatement) test - hydraulic pressure increase until burst. At least two samples per dimension are required.
• Sample configuration:
  • Samples are bent "S" shaped with radii typically R = 3 × nominal tube diameter unless specified otherwise.
  • Dimensional tables are provided for metric (DN) and inch sizes.
• Pre-test checks:
  • Measure and record outside diameter, wall thickness, and ovality of the bent portion.
  • Material mechanical properties from straight samples: 0.2% proof (conventional yield), tensile strength, and elongation.
• Test conditions and limits:
  • Tests normally at ambient temperature, unless material fatigue properties require a specified temperature spectrum tied to the aircraft application.
  • Proof (test) pressure prior to impulse testing: normally 2× system nominal operating pressure.
  • Burst pressure ramp rate: (150 000 ± 37 500) kPa/min [ (20 000 ± 5 000) psi/min ] until tube burst. Tubes may exhibit visible dilation before failure.
• Acceptance criteria:
  • Impulse testing must demonstrate a minimum life (example requirement): 200 000 cycles at maximum specified circuit impulses.
  • Burst test must show the tube withstands a burst pressure consistent with operating pressure plus application-specific safety margin.
• Fittings and joints: Failure at tube fittings or connection interfaces is not classed as a tube defect; replace fittings and continue or retest new samples.

Applications and users

ISO 8574:2004 is used by:

• Aircraft manufacturers and airframe integrators qualifying hydraulic tubing systems.
• Tubing and material suppliers validating new alloys or tube processes for aerospace use.
• Test laboratories performing impulse fatigue and burst qualification.
• Certification bodies and maintenance organizations assessing component fitness-for-service.

Keywords: ISO 8574:2004, hydraulic system tubing, bent tubes, pressure impulse test, burst test, impulse fatigue, aerospace tubing qualification.

Related standards

• ISO 6772 - Fluid systems - Impulse testing of flexible hoses, tubes and fittings (referenced by ISO 8574).
• ISO 8575 - Fluid systems - Tubes for hydraulic systems (related requirements for aircraft hydraulic tubing).