Engineering applications of neutron scattering at the high flux isotope reactor

The neutron residual stress measurement is based on converting measured lattice strain to residual stress and has been well described in earlier articles in Nerrtron News [1,2]. Strain is obtained from a measurement of the shift in the Bragg diffraction peak position arising from the change in the lattice d-spacing due to residual stress in the scattering sample. The principal advantage in the neutron method derives from the large penetration depth of neutrons which permits a direct and nondestructive examination of engineering materials and structures. The method requires knowledge of the zerostress lattice d-spacing against which the lattice strain is determined. Because residual stress is a tensor, measurements of strain must be made in several directions to properly characterize the residual stress state at a given test point. The neutron scattering arrangement employs a penetrating beam defined by a small aperture and a neutron detector collecting the scattered beam at a scattering angle near 90” through a second small aperture. This arrangement defines a sampling volume, or gage volume, which is formed by the intersection of the incident and scattered beams. The success of the method is dependent on the size and orientation of the polycrystalline grains in the scattering sample as well as the number of phases, which constitute the test material. The international Versailles Project on Advanced Materials and Standards (VAMAS) is currently working on recommendations for procedures and standards for neutron residual stress methods, which are expected to render the method of greater interest to industrial users.