In-Situ Neutron Diffraction Study of the Behavior of AL6XN Stainless Steel Under Biaxial Loading
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In-Situ Neutron Diffraction Study of the Behavior of AL6XN Stainless Steel Under Biaxial Loading Michael A Gharghouri1, Tito Marin2, Ronald B Rogge1, and Paul R Dawson2 1 Canadian Neutron Beam Centre, National Research Council, Chalk River Laboratories, Bldg. 459, Stn. 18, Chalk River, K0J 1J0, Canada 2 Mechanical and Aerospace Engineering, Cornell University, 196 Rhodes Hall, Ithaca, NY, 14853 ABSTRACT In–situ neutron diffraction has been used to measure lattice strains parallel to two principal stress directions in biaxially-loaded AL6XN stainless steel. A new fixture was developed for loading thin-walled tubular specimens through combinations of internal pressure and axial loading. Under these conditions, the principal directions (σzz and σθθ in a cylindrical r, θ, z coordinate system) remain constant with respect to the initial crystallographic texture regardless of the level of biaxiality, a distinct advantage for diffraction experiments over the traditional tension/torsion tests for which this condition does not hold. Specimens were first pressurized to the level required to obtain a chosen value of σθθ. The axial load was then increased to reach the yield surface at different σθθ/σzz ratios, ranging from uniaxial to balanced biaxial loading (0, 0.4, 0.7, 1 according to Tresca). The {200}, {220}, {222}, and {311} reflections were measured in the axial and hoop directions as a function of axial load. A sequence of axial loading/unloading episodes was applied for different levels of plastic deformation. Under uniaxial tension, the {200} reflection showed the highest axial strains, followed by the {311}, and {220}/{222} reflections. With increasing internal pressure (biaxiality), the axial lattice strains corresponding to a given axial stress tended to decrease, and the responses of the various reflections tended to merge.
INTRODUCTION In-situ neutron diffraction measurement of lattice strain during uniaxial loading is a wellestablished technique that has provided a wealth of information for the validation and development of self-consistent and finite element models of polycrystal plasticity in BCC, FCC, and HCP metals and alloys [1-4]. The technique probes the mechanical response of different families of grains, defined by the crystallographic direction parallel to the scattering vector. In this study, high strength AL6XN stainless steel tubular specimens were subjected to a combination of axial loading and internal pressure. The goal was to develop a new and unique data set that quantifies the elastoplastic transition at the crystal scale at different levels of stress biaxiality ranging from uniaxial to balanced biaxial. In this manuscript, we describe the testing method, and present experimental measurements of lattice strain as a function of applied axial load.
EXPERIMENT A combination of tension and torsion in thin-walled specimens is often used to produce biaxial loading conditions, because tension/torsion load frames are readily available commercially, and specimen geometry and mounting are
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