ISO 12108:2018

Overview

ISO 12108:2018 - "Metallic materials - Fatigue testing - Fatigue crack growth method" specifies a standardized method to generate fatigue crack growth rate data for metallic materials. The standard covers testing from the fatigue crack growth threshold (ΔKth) up to the onset of rapid, unstable fracture. It is intended for predominantly linear‑elastic, mode I (opening mode) loading with a constant force ratio (R). ISO 12108:2018 defines specimen types, apparatus, precracking and test procedures, crack‑length measurement techniques, calculations and reporting requirements.

Key topics and technical requirements

• Scope and applicability: Focus on isotropic metallic materials under linear‑elastic conditions; tests assume force applied perpendicular to the crack plane (mode I) and constant R.
• Specimens and annexes: Normative annexes provide geometry and calibration for common specimens - Compact Tension (CT), Centre Crack Tension (CCT), Single Edge Notch Tension (SENT), Single Edge Notch Bend (SENB).
• Apparatus and precracking: Requirements for fatigue precracking to produce a representative crack starter, and guidance on specimen orientation and thickness limits to avoid large‑scale yielding, buckling or out‑of‑plane distortion.
• Test procedures: Procedures include constant‑force amplitude (K‑increasing) and K‑decreasing approaches for different crack growth rate regimes, plus guidance on crack‑growth rate data collection (da/dN versus ΔK).
• Crack length measurement: Visual and non‑visual methods (including electric potential difference) are covered, with guidance on measurement resolution, interruption, and dealing with bifurcation or out‑of‑plane cracking.
• Calculations and data analysis: Methods for converting crack length and cycle count into fatigue crack growth rate are specified, including secant and incremental polynomial methods, treatment of crack‑front curvature and determination of ΔK and ΔKth.
• Influencing factors: The standard highlights effects of residual stresses, crack closure, specimen thickness, waveform, frequency and environment on results and requires reporting these factors.
• Reporting: Detailed test report content is prescribed (material, specimen, precracking details, test conditions, analysis and presentation of results).

Applications and users

ISO 12108:2018 is used by:

• Materials and fatigue testing laboratories generating fatigue crack growth rate curves (da/dN vs ΔK)
• Fracture‑mechanics researchers and R&D teams characterizing crack propagation resistance
• Structural and design engineers using fracture mechanics to assess component life and damage tolerance
• Manufacturers and quality managers verifying material performance and compliance

Typical applications include fatigue life prediction, damage‑tolerance assessments, material selection, and validation of finite‑element fracture mechanics models.

Related standards

• Standards on fracture mechanics and stress‑intensity factor calibration (referenced generically within ISO 12108)
• ISO/TC 164 family standards on mechanical testing of metals and fatigue/toughness testing

Keywords: ISO 12108:2018, fatigue crack growth, metallic materials, fatigue testing, ΔK, stress‑intensity factor, da/dN, CT, SENT, SENB, fracture mechanics.