Full Description ADOPTED_FROM:EN ISO 21432:2020 This document describes the test method for determining residual stresses in polycrystalline materials by neutron diffraction. It is applicable to both homogeneous and inhomogeneous materials including those containing distinct phases. The principles of the neutron diffraction technique are outlined. Suggestions are provided on: — the selection of appropriate diffracting lattice planes on which measurements should be made for different categories of materials, — the specimen directions in which the measurements should be performed, and — the volume of material examined in relation to the material grain size and the envisaged stress state. Procedures are described for accurately positioning and aligning test pieces in a neutron beam and for precisely defining the volume of material sampled for the individual measurements. The precautions needed for calibrating neutron diffraction instruments are described. Techniques for obtaining a stress-free reference are presented. The methods of making individual measurements by neutron diffraction are described in detail. Procedures for analysing the results and for determining their statistical relevance are presented. Advice is provided on how to determine reliable estimates of residual stresses from the strain data and on how to estimate the uncertainty in the results.