ISO 7539-10:2020

Overview - ISO 7539-10:2020 (Reverse U‑bend method)

ISO 7539-10:2020 is part of the ISO 7539 series on corrosion of metals and alloys - stress corrosion testing. It specifies procedures for designing, preparing and using reverse U‑bend (RUB) test specimens to evaluate susceptibility to stress corrosion cracking (SCC). The term “metal” in the standard includes alloys. The RUB test is a severe, accelerated screening method aimed mainly at high‑corrosion‑resistance materials (for example, Ni‑based alloys) and is suitable for tubing, piping, plate, bar and welded products.

Key topics and technical requirements

• Specimen types and preparation
  • RUB specimens are made from half tubes or formed semi‑circular sections of plate/bar; surfaces should be finished (e.g., P600) without overheating.
  • Two specimen styles: half‑tube RUB specimens (constrained cross‑section) and RUB specimens with a gauge section (allowed to flatten at the apex).
  • Welding orientation and heat‑treatment sequence (final heat treatment before bending) are specified where applicable.
• Reverse U‑bending process
  • Two‑stage stressing: bending followed by fastening with a loading bolt; avoid elastic stress relief during fastening.
  • Fastening details: final leg spacing ≈ 1 mm less than minimum during bending; use curved washers, double nuts and bolt materials chosen to avoid galvanic corrosion and thermal mismatch.
  • Pre‑straining may be used for gauge‑section specimens to achieve desired stress levels.
• Experimental procedure
  • Degrease (e.g., acetone), rinse, dry and inspect for pre‑existing cracks before exposure.
  • Test exposure methods: multiple immersion tests (sets removed at different times) and serial immersion tests; use multiple specimens for statistical evaluation.
  • Electrical insulation may be required to prevent galvanic corrosion; ceramic insulators are suitable if compatible.
• Post‑exposure evaluation and reporting
  • Visual and destructive inspections, cleaning per ISO 8407 where needed, and documented test reporting are required.
• Practical notes
  • RUB testing gives lower stress relaxation than conventional U‑bend/C‑ring tests, enabling faster screening, but the stress state is complex and variable - replication is often necessary.

Applications - who uses ISO 7539-10:2020

• Materials engineers, corrosion testing laboratories, metallurgists, quality assurance teams performing SCC screening and acceptance testing.
• Manufacturers of tubing, piping and welded components, and organizations evaluating high‑performance alloys (e.g., Ni‑based) for service in aggressive environments.
• Research & development groups studying material and process effects on SCC susceptibility.

Related standards

• ISO 7539‑1 - General guidance on stress corrosion testing procedures
• ISO 8407 - Removal of corrosion products from corrosion test specimens
• Other parts of the ISO 7539 series for complementary SCC methods

Keywords: ISO 7539-10:2020, reverse U-bend, RUB specimens, stress corrosion testing, stress corrosion cracking, corrosion testing, metals and alloys.