  This paper presents a proof-of-concept demonstration of triaxial strain
tomography from Bragg-edge neutron imaging within a three-dimensional sample.
Bragg-edge neutron transmission can provide high-resolution images of the
average through thickness strain within a polycrystalline material. This poses
an associated rich tomography problem which seeks to reconstruct the full
triaxial strain field from these images. The presented demonstration is an
important step towards solving this problem, and towards a technique capable of
studying the residual strain and stress within engineering components. A
Gaussian process based approach is used that ensures the reconstruction
satisfies equilibrium and known boundary conditions. This approach is
demonstrated experimentally on a non-trivial steel sample with use of the RADEN
instrument at the Japan Proton Accelerator Research Complex. Validation of the
reconstruction is provided by comparison with conventional strain scans from
the KOWARI constant-wavelength strain diffractometer at the Australian Nuclear
Science and Technology Organisation and simulations via finite element
analysis.
