Confirmation of principal residual stress directions in rectilinear components by neutron diffraction

Large magnitude residual stresses in precipitation hardened aluminium alloys are a consequence of inhomogeneous plastic deformation arising from thermal strains. Thermal gradients develop because of the metallurgical requirement to quench the material rapidly from the solution treatment temperature. Characterisation of the resulting residual stresses can be usefully performed by neutron diffraction. For rectilinear shapes, it is usually assumed that the orthogonal directions of the sample are coincident with the principal stress directions. To test this assumption, residual strains were measured in a rectilinear block of the aluminium alloy 7449 using the SALSA neutron strain analyser. The strains at a single point in the forging were measured as a function of orientation; that is the forging was rotated around the three orthogonal axes of a coordinate system with its origin at the measurement point. Analysis of the neutron diffraction data allowed the determination of the full three-dimensional strain and stress tensors which confirmed that the orthogonal directions of the forging did coincide with the principal stress directions.