Fatigue crack initiation identification and quantification with Bragg edge strain tomography

Neutron diffraction is a powerful tool in quantitatively studying fatigue crack initiation in notched samples. So far full-field strain mapping has been impractical due to the high acquisition time associated with neutron scattering measurements. However, with the Bragg edge mapping technique becoming increasingly popular, full-field strain mapping of notched components under cyclic loading is now achievable. In a novel experiment which makes use of the ENGIN-X and IMAT instruments at ISIS neutron source in Oxfordshire, UK, we will study the three-dimensional full-field stress distributions around the apex of notches during fatigue testing. The experiments are based on the theory of critical distances to estimate the number of cycles required to initiate a crack in the medium cycle fatigue regime. This allows for tests can be completed in reasonable time scales, whilst still giving insight into the mechanisms that contribute to failure in notched specimens. Bragg edge tomography will be used to acquire elastic strains, and shall be complemented with total strain measurements obtained by neutron tomography with digital volume correlation. The combination of drops in elastic strain and significant increases in total strain are used as indicators of crack initiation, validating the calculations based on the theory of critical distances.