Effects of Fatigue on the Integrity of a Friction Stir Welded Lap Joint Containing Residual Stresses

This research uses a non-destructive method of neutron diffraction to measure the tri-axial residual stresses in a friction stir welded aerospace fuselage component: a stringer-to-skin lap joint. Two different specimens were examined. Fatigue testing was performed on both specimens to determine their fatigue lives. Effects of the different components of residual stresses were examines and related to fatigue performance. A combination of fractography, hardness testing, and residual stress measurement was used to predict areas of high probability of structural failure in the friction stir welded lap joints

A Taylor Model Based Description of the proof stress of magnesium AZ31 during hot working

A series of hot-compression tests and Taylor-model simulations were carried out with the intention of developing a simple expression for the proof stress of magnesium alloy AZ31 during hot working. A crude approximation of wrought textures as a mixture of a single ideal texture component and a random background was employed. The shears carried by each deformation system were calculated using a full-constraint Taylor model for a selection of ideal orientations as well as for random textures. These shears, in combination with the measured proof stresses, were employed to estimate the critical resolved shear stresses for basal slip, prismatic slip, ⟨c+a⟩ second-order pyramidal slip, and { } twinning. The model thus established provides a semianalytical estimation of the proof stress (a one-off Taylor simulation is required) and also indicates whether or not twinning is expected. The approach is valid for temperatures between ∼150 °C and ∼450 °C, depending on the texture, strain rate, and strain path

Measurement of the residual stress tensor in a compact tension weld specimen

Neutron diffraction measurements have been performed to determine the full residual stress tensor along the expected crack path in an austenitic stainless steel (Esshete 1250) compact tension weld specimen. A destructive slitting method was then implemented on the same specimen to measure the stress intensity factor profile associated with the residual stress field as a function of crack length. Finally deformations of the cut surfaces were measured to determine a contour map of the residual stresses in the specimen prior to the cut. The distributions of transverse residual stress measured by the three techniques are in close agreement. A peak tensile stress in excess of 600 MPa was found to be associated with an electron beam weld used to attach an extension piece to the test sample, which had been extracted from a pipe manual metal arc butt weld. The neutron diffraction measurements show that exceptionally high residual stress triaxiality is present at crack depths likely to be used for creep crack growth testing and where a peak stress intensity factor of 35 MPa√m was measured (crack depth of 21 mm). The neutron diffraction measurements identified maximum values of shear stress in the order of 50 MPa and showed that the principal stress directions were aligned to within ~20° of the specimen orthogonal axes. Furthermore it was confirmed that measurement of strains by neutron diffraction in just the three specimen orthogonal directions would have been sufficient to provide a reasonably accurate characterisation of the stress state in welded CT specimens

Neutron diffraction techniques for alloy characterization and development

Important activities in the aluminum industry are the development of new alloys, and the optimization of thermo-mechanical treatments to obtain desired performance. The strength and formability of aluminum alloys depend on the distribution and scale of precipitating phases, on the grain size and grain orientation distribution, on the distribution and scale of flaws, and on the presence of residual stresses. Thus it is useful to have detailed quantitative data on the crystal structures and volume fractions of phases that form during thermomechanical treatment, on the kinetics of solid state reactions, on the distribution of grain orientations, and on the stresses that develop during mechanical testing and forming. Neutron scattering is a powerful tool that can provide unique data to guide the development of improved materials and processes. Of particular interest are in-situ experiments: such experiments are uniquely suited to neutron diffraction because of the high penetrating power of neutrons, which allows data to be collected from materials subjected to realistic conditions (load, temperature, atmosphere) in specialized sample environments. In this presentation, we discuss several examples of neutron scattering studies, including residual strain mapping, in-situ loading experiments, texture analysis, powder diffraction, and tomography.NRC publication: Ye

Neutron Scattering Methods Provide Insights into the Effects of Thermomechanical Processing on the Properties and Structure of Steel

The strength and formability of materials depend on the distribution and scale of precipitating phases, on the grain size and grain orientation distribution, on the distribution and scale of flaws and on the presence of residual stresses. Neutron scattering methods provide unique data to guide the development of improved materials and processes. Of particular interest are in situ experiments; such experiments are uniquely suited to neutron diffraction because of the high penetrating power of neutrons, which allows data to be collected from materials subjected to realistic conditions in specialized sample environments. In this paper, we illustrate the application of neutron diffraction to study 1) the behaviour of a TRIP steel under uniaxial loading and 2) the evolution of the microstructure of a plain carbon steel during gas tungsten arc welding.La r\ue9sistance et la formabilit\ue9 des mat\ue9riaux d\ue9pendent de la distribution et de l\u2019\ue9chelle des phases qui se pr\ue9cipitent, de la taille de grain et de la distribution de l\u2019orientation de grain, de la distribution et de l\u2019\ue9chelle des discontinuit\ue9s et de la pr\ue9sence de contraintes r\ue9siduelles. Les m\ue9thodes de diffusion de neutron fournissent des donn\ue9es uniques pour guider le d\ue9veloppement de mat\ue9riaux et de proc\ue9d\ue9s am\ue9lior\ue9s. Les exp\ue9riences in situ sont d\u2019un int\ue9r\ueat particulier; de telles exp\ue9riences sont uniquement appropri\ue9es \ue0 la diffraction de neutron gr\ue2ce au pouvoir \ue9lev\ue9 de p\ue9n\ue9tration des neutrons, ce qui permet de collectionner des donn\ue9es \ue0 partir des mat\ue9riaux soumis \ue0 des conditions r\ue9alistes dans un \ue9chantillon d\u2019environnements sp\ue9cialis\ue9s. Dans ce document, on illustre l\u2019application de la diffraction de neutron \ue0 l\u2019\ue9tude 1) du comportement d\u2019un acier TRIP sous charge uniaxe et 2) de l\u2019\ue9volution de la microstructure d\u2019un acier au carbone ordinaire lors du soudage \ue0 l\u2019arc avec \ue9lectrode de tungst\ue8ne en atmosph\ue8re de gaz (tig).Peer reviewed: YesNRC publication: Ye

Pseudoelastic behavior of magnesium alloy during twinning-dominated cyclic deformation

In- situ neutron diffraction has been used to examine the pseudoelastic behavior of an extruded Mg-8.5. wt.% Al alloy during twinning-dominated cyclic deformation in both tension and compression. Twinning activity is effectively tracked through the intensity variations of the diffraction peaks for some grain orientations. The results suggest that a fundamental difference in the pseudoelastic behavior between tension and compression cyclic loadings might be due to reversible detwinning-retwinning observed only during cyclic compression. \ua9 2013 Elsevier B.V.Peer reviewed: YesNRC publication: Ye