Overview
ISO 11782-2:1998 - "Corrosion of metals and alloys - Corrosion fatigue testing - Part 2: Crack propagation testing using precracked specimens" specifies a fracture‑mechanics method for measuring corrosion‑fatigue crack growth. The standard covers the creation of fatigue pre‑cracks in notched specimens, controlled cyclic loading in defined environments, monitoring crack length versus cycles, and deriving crack growth rates (da/dN) as a function of the stress‑intensity factor range (ΔK). Results support design and life‑assessment of components where corrosion‑fatigue crack growth governs service life.
Key topics and technical requirements
- Fracture‑mechanics approach: Use of precracked specimens to determine crack growth rates (da/dN) and the threshold stress‑intensity factor range (ΔKth) below which growth is insignificant.
- Specimen selection and preparation: Common geometries described include compact tension (CT), single‑edge notch bend (SENB3, SENB4) and centre‑cracked tension (CCT). Specimens should be machined in the relevant heat‑treated condition, degreased, and identified.
- Precracking and testing procedure: Induce a fatigue pre‑crack by cyclic loading, then adjust loading to achieve target ΔK and stress ratio (R). Monitor crack advance vs cycles and convert to da/dN vs ΔK curves.
- Crack length measurement: Surface optical methods and through‑thickness averaging techniques (e.g., electrical resistance) are discussed; Annex A provides measurement information. Avoid perturbing crack‑tip chemistry when removing corrosion products.
- Apparatus and environment control: Environmental chambers must enclose the test section, avoid galvanic artifacts, provide adequate solution volume and circulation, and allow for applied potentials where needed.
- Important test variables: Stress intensity factors (K), stress ratio (R), waveform, frequency, residual stresses, specimen thickness and constraint (plane‑strain considerations), and environmental transport (e.g., hydrogen or through‑thickness solution flow).
- Limitations and interpretation: Results apply directly only when test conditions match service conditions. Residual stresses, short‑crack effects and frequency‑dependent or time‑dependent cracking modes can affect interpretation.
Practical applications
- Corrosion‑fatigue life prediction and fracture‑safe design of components made from plate, bar, forgings and welded parts.
- Material selection and qualification for corrosive service (petrochemical, offshore, power generation, transport).
- Failure analysis and R&D where quantifying crack‑growth rates (da/dN) and ΔKth in specific environments is required.
- Development of inspection intervals and fitness‑for‑service assessments for structures susceptible to corrosion fatigue.
Who should use this standard
- Corrosion and materials engineers, fracture mechanics specialists, test laboratories, quality assurance teams, and design engineers responsible for components exposed to cyclic loading in corrosive environments.
Related standards
- ISO 11782‑1 (Corrosion fatigue - Part 1: Cycles to failure testing)
- ISO 7539‑1 (Stress corrosion testing - general guidance)