Overview
ISO 16540:2015 - "Corrosion of metals and alloys - Methodology for determining the resistance of metals to stress corrosion cracking using the four-point bend method" - describes a generic, repeatable four-point bend test procedure to evaluate resistance to stress corrosion cracking (SCC) for metals including carbon steel, low-alloy steels, and corrosion resistant alloys (CRAs). The standard covers testing of both parent plate and welded specimens, guidance for materials with and without a distinct yield point, and the core methodology rather than application-specific acceptance criteria (industry standards should be consulted for service-based limits).
Key topics and technical requirements
- Test principle: Constant-displacement four-point bend test that produces a uniformly stressed tensile face between inner loading rollers; mid-thickness stress is zero and stress gradients through thickness can affect crack propagation.
- Loading jig design: Typical guidance (e.g., inner span 50–60 mm, outer span 100–130 mm, roller diameter 6–10 mm, A ≈ H/4) and requirements for rigidity and SCC resistance of jig materials.
- Roller and electrical isolation: Recommendation to use ceramic rollers to electrically isolate specimens and minimise galvanic/crevice corrosion; minimise friction and enable free rotation where possible.
- Specimen preparation:
- Flat rectangular specimens with width 1.5–5× thickness.
- Avoid hydrogen-generating machining; if used, remove surface material (e.g., ~500 µm for many CRAs) or bake out hydrogen without altering microstructure.
- Surface finish control (commonly Ra ≤ 0.25 µm) unless testing original surface condition is required.
- Strain gauging and calibration: Strain measurement at tensile surface and calibration to set target strain levels; guidance provided for materials lacking distinct yield points.
- Testing conditions: Procedures for elevated temperature testing, test environment control, and failure appraisal (including special considerations for welds and soft-zone cracking).
- Reporting: Requirements for documenting specimen details, jig geometry, strain, environment, and failure observations.
Applications
- Laboratory evaluation of SCC susceptibility for materials selection, R&D and failure investigation.
- Comparative screening of parent materials, welds, claddings and piping surfaces under controlled corrosive environments.
- Supporting materials qualification programs in oil & gas, chemical processing, power generation and marine industries.
Who should use this standard
- Materials engineers, corrosion specialists, and metallurgists performing SCC testing.
- Test laboratories and manufacturers qualifying steels and CRAs for corrosive service.
- QA/QC and R&D teams involved in weld procedure qualification and materials selection.
Related standards
- ISO 7539 series (general guidance on stress corrosion testing)
- ISO 8407 (removal of corrosion products from test specimens)
Keywords: ISO 16540, four-point bend, stress corrosion cracking, SCC testing, corrosion resistant alloys, weld testing, specimen preparation, loading jig, strain gauging.