ISO 21432:2019

Overview

ISO 21432:2019 - "Non‑destructive testing - Standard test method for determining residual stresses by neutron diffraction" - specifies a standardized procedure for measuring residual stresses in polycrystalline materials using neutron diffraction. The standard covers principles (Bragg’s law), instrument types (monochromatic and time‑of‑flight (TOF) neutron diffractometers), specimen preparation, alignment, calibration, measurement procedures, data analysis and uncertainty estimation. It applies to both homogeneous and inhomogeneous materials, including multiphase systems, and describes how to define the sampled gauge volume relative to grain size and the expected stress state.

Key Topics and Technical Requirements

• Principle and instrumentation
  • Overview of neutron diffraction principles (Bragg’s law).
  • Distinction between monochromatic and TOF instruments and their measurement implications.
• Measurement planning
  • Selection of appropriate diffracting lattice planes and measurement directions for different materials and phases.
  • Guidance on gauge volume selection and centroid positioning relative to microstructure and grain size.
• Specimen preparation and alignment
  • Procedures for accurate positioning and alignment of test pieces in a neutron beam.
  • Detailed recording and positioning requirements to ensure traceability.
• Calibration and reference
  • Precautions and procedures for calibrating neutron diffraction instruments.
  • Techniques for establishing a stress‑free or reference lattice spacing.
• Data analysis and uncertainty
  • Methods for converting measured strains to stresses (including the sinψ approach), choosing elastic constants, peak fitting and background handling.
  • Statistical analysis, reporting requirements and estimation of measurement uncertainty and reliability.

Practical Applications

ISO 21432 is used where accurate, non‑destructive mapping of internal residual stresses is required, including:

• Research & development and materials characterization (residual stress mapping).
• Failure analysis and root‑cause investigation where internal stress fields affect performance or cracking.
• Quality assurance for heat treatments, welding, surface treatments (peening), and manufacturing processes that introduce residual stress.
• Validation of numerical models (FEA) by providing benchmark experimental stress fields.

Keywords: ISO 21432, residual stresses, neutron diffraction, non‑destructive testing (NDT), neutron diffractometer, gauge volume, strain to stress conversion, uncertainty estimation.

Who should use this standard

• NDT engineers and technicians performing neutron diffraction residual stress measurements.
• Researchers and materials scientists needing standardized procedures for strain/stress evaluation.
• Laboratories operating neutron diffractometers for industrial testing, certification bodies, and design engineers validating component integrity.

Related standards

• Supersedes ISO/TS 21432:2005 and incorporates related corrigenda; prepared by ISO/TC 135 (Non‑destructive testing). Users should consult relevant ISO/IEC guides on uncertainty and technical committee outputs for complementary guidance.