Lori Bassman

Pigeons steer like helicopters and generate down- and upstroke lift during low speed turns

Turning is crucial for animals, particularly during predator–prey interactions and to avoid obstacles. For flying animals, turning consists of changes in (i) flight trajectory, or path of travel, and (ii) body orientation, or 3D angular position. Changes in flight trajectory can only be achieved by modulating aerodynamic forces relative to gravity. How birds coordinate aerodynamic force production relative to changes in body orientation during turns is key to understanding the control strategies used in avian maneuvering flight. We hypothesized that pigeons produce aerodynamic forces in a uniform direction relative to their bodies, requiring changes in body orientation to redirect those forces to turn. Using detailed 3D kinematics and body mass distributions, we examined net aerodynamic forces and body orientations in slowly flying pigeons (Columba livia) executing level 90° turns. The net aerodynamic force averaged over the downstroke was maintained in a fixed direction relative to the body throughout the turn, even though the body orientation of the birds varied substantially. Early in the turn, changes in body orientation primarily redirected the downstroke aerodynamic force, affecting the bird’s flight trajectory. Subsequently, the pigeon mainly reacquired the body orientation used in forward flight without affecting its flight trajectory. Surprisingly, the pigeon’s upstroke generated aerodynamic forces that were approximately 50% of those generated during the downstroke, nearly matching the relative upstroke forces produced by hummingbirds. Thus, pigeons achieve low speed turns much like helicopters, by using whole-body rotations to alter the direction of aerodynamic force production to change their flight trajectory.

A lattice-based micromechanical continuum formulation for stress-driven mass transport in polycrystalline solids

Abstract This work represents a first step toward the development of a continuum field formulation for the coupled phenomena of diffusion and mechanics in polycrystalline solids. The basis of the formulation lies in lattice-level mechanisms, from which a continuum thermodynamic description of processes at micron length scales is developed. Considering self-diffusion, the composition problem is posed in terms of a binary vacancy–atom mixture. For mechanics, isotropic linear elasticity and isothermal conditions are assumed. The coupled constitutive relations for composition and mechanics are formally derived from the underlying thermodynamic principles. When applied to governing partial differential equations for each subproblem, the coupled nature is realized. Under applied tractions or intrinsic stress, the atoms diffuse — in general from surfaces with compressive normal traction to those with relatively tensile normal traction. The flow is mediated by electric fields via the mechanism of electromigration. In the case of metal interconnect lines in integrated circuit devices, the results of these microscopic processes are manifested in phenomena such as diffusional creep, hillock formation, grain growth, grain boundary motion, void formation and void evolution. These phenomena have a significant impact on the function, performance and failure of interconnect lines. A computational framework based on the finite element method has been developed to solve the coupled equations. Several numerical examples are presented and comparisons with analytical results are provided where the latter are available.

Three-wavelength electronic speckle pattern interferometry with the Fourier-transform method for simultaneous measurement of microstructure-scale deformations in three dimensions

We present the simultaneous measurement of three-dimensional deformations by electronic speckle pattern interferometry using five object beams and three colors. Each color, corresponding to an orthogonal direction of displacement, is separated through dichroic filtering before being recorded by a separate CCD camera. Carrier fringes are introduced by tilting the beam path in one arm of each of the three interferometers. The measured deformation modulates these carrier fringes and is extracted using the Fourier-transform method to achieve high displacement sensitivity. The field of view is on the order of a millimeter, making the system suitable for study of microstructural deformations. We compare experimental results with calculated values to validate out-of-plane and in-plane deformation measurements and demonstrate sensitivity on the order of 10 nm.

Boundary identification in EBSD data with a generalization of fast multiscale clustering

Electron backscatter diffraction (EBSD) studies of cellular or subgrain microstructures present problems beyond those in the study of coarse-grained polycrystalline aggregates. In particular, identification of boundaries delineating some subgrain structures, such as microbands, cannot be accomplished simply with pixel-to-pixel misorientation thresholding because many of the boundaries are gradual transitions in crystallographic orientation. Fast multiscale clustering (FMC) is an established data segmentation technique that is combined here with quaternion representation of orientation to segment EBSD data with gradual transitions. This implementation of FMC addresses a common problem with segmentation algorithms, handling data sets with both high and low magnitude boundaries, by using a novel distance function that is a modification of Mahalanobis distance. It accommodates data representations, such as quaternions, whose features are not necessarily linearly correlated but have known distance functions. To maintain the linear run time of FMC with such data, the method requires a novel variance update rule. Although FMC was originally an algorithm for two-dimensional data segmentation, it can be generalized to analyze three-dimensional data sets. As examples, several segmentations of quaternion EBSD data sets are presented.

Pigeons produce aerodynamic torques through changes in wing trajectory during low speed aerial turns

The complexity of low speed maneuvering flight is apparent from the combination of two critical aspects of this behavior: high power and precise control. To understand how such control is achieved, we examined the underlying kinematics and resulting aerodynamic mechanisms of low speed turning flight in the pigeon (Columba livia). Three birds were trained to perform 90 deg level turns in a stereotypical fashion and detailed three-dimensional (3D) kinematics were recorded at high speeds. Applying the angular momentum principle, we used mechanical modeling based on time-varying 3D inertia properties of individual sections of the pigeons body to separate angular accelerations of the torso based on aerodynamics from those based on inertial effects. Directly measured angular accelerations of the torso were predicted by aerodynamic torques, justifying inferences of aerodynamic torque generation based on inside wing versus outside wing kinematics. Surprisingly, contralateral asymmetries in wing speed did not appear to underlie the 90 deg aerial turns, nor did contralateral differences in wing area, angle of attack, wingbeat amplitude or timing. Instead, torso angular accelerations into the turn were associated with the outside wing sweeping more anteriorly compared with a more laterally directed inside wing. In addition to moving through a relatively more retracted path, the inside wing was also more strongly pronated about its long axis compared with the outside wing, offsetting any difference in aerodynamic angle of attack that might arise from the observed asymmetry in wing trajectories. Therefore, to generate roll and pitch torques into the turn, pigeons simply reorient their wing trajectories toward the desired flight direction. As a result, by acting above the center of mass, the net aerodynamic force produced by the wings is directed inward, generating the necessary torques for turning.

The effect of initial microstructure and processing temperature on microstructure and texture in multilayered Al/Al(Sc) ARB sheets

Abstract A commercial purity Al alloy and an Al-0.3Sc (wt.%) alloy, the latter in either the supersaturated or artificially aged condition, were accumulative roll bonded at either 200 or 350°C to high strain to generate sheet materials consisting of 32 or 64 alternating layers of Al and Al(Sc). The microstructure and texture of the processed materials were investigated mainly using electron backscattered diffraction scanning electron microscopy and transmission electron microscopy. The deformation microstructure and texture of these two alloy combinations were strongly influenced by both the initial heat treatment condition of the Al(Sc) alloy whereby large-scale shear bands were generated during rolling when a dispersion of fine Al3Sc particles is present in the Al(Sc) layers. The effect of initial microstructure and processing temperature affected the subsequent recrystallization microstructure and texture of the Al/Al(Sc) composite during annealing at 350°C. Here, the Al(Sc) layers remain unrecrystallized in all materials with the Al layers undergoing continuous and discontinuous recrystallization after low and high temperature ARB, respectively. The lack of recrystallization in the Al(Sc) layers generated an alternating recrystallized/recovered microstructure in all materials.

Subgrain Boundary Identification in 3D EBSD Data through Fast Multiscale Clustering

Complete and accurate characterization of subgrain microstructural features permits study of the relationships among loading, microstructure and properties in plastically deformed metals. 3D electron backscatter diffraction data can produce reconstructed crystallographic volumes, however low angle subgrain boundaries cannot be determined simply with point-to-point misorientation thresholding because many are gradual transitions in orientation. We demonstrate a novel 3D implementation of the data segmentation technique Fast Multiscale Clustering, which uses a quaternion representation of orientation and a corresponding distance metric. Examples of the 3D segmentation of microbands and morphological analysis from the results are presented for die-compressed nickel single crystal and uniaxially loaded commercially pure aluminum.

Segmentation of 3D EBSD data for subgrain boundary identification and feature characterization

Subgrain structures formed during plastic deformation of metals can be observed by electron backscatter diffraction (EBSD) but are challenging to identify automatically. We have adapted a 2D image segmentation technique, fast multiscale clustering (FMC), to 3D EBSD data using a novel variance function to accommodate quaternion data. This adaptation, which has been incorporated into the free open source texture analysis software package MTEX, is capable of segmenting based on subtle and gradual variation as well as on sharp boundaries within the data. FMC has been further modified to group the resulting closed 3D segment boundaries into distinct coherent surfaces based on local normals of a triangulated surface. We demonstrate the excellent capabilities of this technique with application to 3D EBSD data sets generated from cold rolled aluminum containing well-defined microbands, cold rolled and partly recrystallized extra low carbon steel microstructure containing three magnitudes of boundary misorientations, and channel-die plane strain compressed Goss-oriented nickel crystal containing microbands with very subtle changes in orientation.

Atomically-based Field Formulations for Coupled Problems of Composition and Mechanics

Preliminary ideas are presented on developing coupled, continuum field formulations for composition and mechanics with applications to microelectronic materials. A thermodynamic basis is constructed, reflecting atomic-level transport mechanisms and the associated mechanics. Formal arguments lead to constitutive field relations, and specification of balance laws completes the coupled field description. Early numerical results are also presented.

3D-EBSD Studies of Deformation, Recrystallization and Phase Transformations

A focused ion beam (FIB) coupled with high resolution electron backscatter diffraction (EBSD) has emerged as a useful tool for generating crystallographic information in reasonably large volumes of microstructure. In principle, data generation is reasonably straightforward whereby the FIB is used as a high precision serial sectioning device for generating consecutive milled surfaces suitable for mapping by EBSD. The successive EBSD maps generated by serial sectioning are combined using various post-processing methods to generate crystallographic volumes of the microstructure. This paper provides an overview of the use of 3D-EBSD in the study of various phenomena associated with thermomechanical processing of both crystalline and semi-crystalline alloys and includes investigations on the crystallographic nature of microbands, void formation at particles, phase redistribution during plastic forming, and nucleation of recrystallization within various regions of the deformation microstructure.