ISO 7539-6:2018

Overview

ISO 7539-6:2018 - "Corrosion of metals and alloys - Stress corrosion testing - Part 6" specifies procedures for the design, preparation and use of precracked specimens for stress corrosion tests conducted under constant load or constant displacement. The standard explains how to produce fatigue pre-cracks from machined notches, control test environments, monitor crack growth, and derive fracture‑mechanics parameters (e.g., threshold stress intensity for stress corrosion cracking, KISCC). It is intended for thicker products (plate, bar, forgings) and welded parts; precracked specimens are generally not suitable for very thin products such as sheet or wire.

Key topics and requirements

• Specimen selection and geometry: Guidance on choosing standard fracture‑mechanics specimen types and on maintaining adequate constraint (plane‑strain conditions) so plasticity at the crack tip is confined.
• Precracking and preparation: Procedures for creating fatigue pre-cracks from machined notches and for specimen identification and handling.
• Loading modes: Use of fixtures/equipment for applying constant load or devices that produce constant displacement at loading points.
• Fracture‑mechanics parameters: Determination and use of stress intensity factor K, initial K (KIi), threshold K for SCC (KISCC), and relationships to specimen dimensions and applied load.
• Crack initiation/arrest and growth measurement: Methods for determining K by crack arrest or initiation, and for measuring instantaneous and average crack velocity using continuous monitoring techniques.
• Environmental control: Requirements for environmental chambers, chemical/aggressive environments, monitoring and control during exposure.
• Reporting and data use: Test report contents and recommended use of empirical data to estimate critical defect sizes and crack growth rates for service-life assessment.
• Annexes: Informative guidance on using notched specimens (Annex A) and determination of crack growth velocity (Annex B).

Practical applications and users

ISO 7539-6 is used to:

• Quantify susceptibility to stress corrosion cracking (SCC) for metals and alloys.
• Estimate critical defect sizes and SCC crack‑propagation rates needed for fitness‑for‑service, inspection planning and life‑prediction. Typical users:
• Corrosion/materials engineers and researchers
• Mechanical and structural designers concerned with SCC risk
• Laboratory testing and accreditation bodies
• Failure analysts, NDT specialists and asset integrity teams

Related standards

• ISO 7539-1 - General guidance on stress corrosion testing procedures
• ISO 7539-9 - Tests under increasing load or increasing displacement (complements Part 6)
• Other parts of the ISO 7539 series covering specific SCC testing methods

Keywords: ISO 7539-6, stress corrosion testing, precracked specimens, stress corrosion cracking, KISCC, constant load testing, constant displacement testing, crack propagation, fracture mechanics.