Computation of Burgers vectors from elastic strain and lattice rotation data.

A theoretical framework for computation of Burgers vectors from strain and lattice rotation data in materials with low dislocation density is presented, as well as implementation into a computer program to automate the process. The efficacy of the method is verified using simulated data of dislocations with known results. A three-dimensional dataset retrieved from Bragg coherent diffraction imaging (BCDI) and a two-dimensional dataset from high-resolution transmission Kikuchi diffraction (HR-TKD) are used as inputs to demonstrate the reliable identification of dislocation positions and accurate determination of Burgers vectors from experimental data. For BCDI data, the results found using our approach show very close agreement to those expected from empirical methods. For the HR-TKD data, the predicted dislocation position and the computed Burgers vector showed fair agreement with the expected result, which is promising considering the substantial experimental uncertainties in this dataset. The method reported in this paper provides a general and robust framework for determining dislocation position and associated Burgers vector, and can be readily applied to data from different experimental techniques.

Malnutrition Risk, Rurality, and Falls among Community-Dwelling Older Adults.

OBJECTIVES: We aimed to evaluate if malnutrition and rurality are associated with fall risk and future falls in community-dwelling older adults. DESIGN: Prospective Cohort. SETTING: Community, Vermont. PARTICIPANTS: Older adults receiving home support services who completed a health risk assessment (n=3,300; Mean age 79.6 years ±8.4, 75% female). Additional analysis was completed with a subset of 2,043 participants with two-years of consecutive health assessments. MEASUREMENTS: Fall Risk Questionnaire, DETERMINE Nutrition Risk Questionnaire, and fall history. RESULTS: Independently, high malnutrition risk and rurality were associated with fall risk (p<0.001) and high malnutrition risk was associated with rurality (p<0.001). After adjusting for age, sex, and physical function, individuals with high nutrition risk had a 66% increase in the odds of falling over the next year, but rurality was not significantly associated with a new fall. CONCLUSION: These findings suggest that falls are associated with malnutrition risk, but the relationship between falls and rurality is less evident. Further research is needed to identify services that may best alleviate malnutrition risk in older adults and aspects of nutrition that are most protective against fall risk.

Dislocation dynamics modelling of the creep behaviour of particle-strengthened materials.

Plastic deformation in crystalline materials occurs through dislocation slip and strengthening is achieved with obstacles that hinder the motion of dislocations. At relatively low temperatures, dislocations bypass the particles by Orowan looping, particle shearing, cross-slip or a combination of these mechanisms. At elevated temperatures, atomic diffusivity becomes appreciable, so that dislocations can bypass the particles by climb processes. Climb plays a crucial role in the long-term durability or creep resistance of many structural materials, particularly under extreme conditions of load, temperature and radiation. Here we systematically examine dislocation-particle interaction mechanisms. The analysis is based on three-dimensional discrete dislocation dynamics simulations incorporating impenetrable particles, elastic interactions, dislocation self-climb, cross-slip and glide. The core diffusion dominated dislocation self-climb process is modelled based on a variational principle for the evolution of microstructures, and is coupled with dislocation glide and cross-slip by an adaptive time-stepping scheme to bridge the time scale separation. The stress field caused by particles is implemented based on the particle-matrix mismatch. This model is helpful for understanding the fundamental particle bypass mechanisms and clarifying the effects of dislocation glide, climb and cross-slip on creep deformation.

Assessing the precision of strain measurements using electron backscatter diffraction--part 1: detector assessment.

We analyse the link between precision of pattern shift measurements and the resolution of the measurement of elastic strain and lattice rotation using high resolution electron backscatter diffraction (HR-EBSD). This study combines analysis of high quality experimentally obtained diffraction patterns from single crystal silicon; high quality dynamical simulations using Bloch wave theory; quantitative measurements of the detector Modulation Transfer Function (MTF) and a numerical model. We have found that increases in exposure time, when 1×1 binning is selected, are the primary reason for the observed increase in sensitivity at greater than 2×2 binning and therefore use of software integration and high bit depth images enables a significant increase in strain resolution. This has been confirmed using simulated diffraction patterns which provide evidence that the ultimate theoretical resolution of the cross correlation based EBSD strain measurement technique with a 1000×1000 pixel image could be as low as 4.2×10(-7) in strain based on a shift precision of 0.001 pixels.

Assessing the precision of strain measurements using electron backscatter diffraction--part 2: experimental demonstration.

The residual impression after performing a microhardness indent in silicon has been mapped with high resolution EBSD to reveal residual elastic strain and lattice rotation fields. Mapping of the same area has been performed with variable pattern binning and exposure times to reveal the qualitative and quantitative differences resulting from reducing the pattern size and exposure time. Two dimension 'image' plots of these fields indicate that qualitative assessment of the shape and size of the fields can be performed with as much as 4×4 binning. However, quantitative assessment using line scans reveals that the smoothest profile can be obtained using minimal pattern binning and long exposure times. To compare and contrast with these experimental maps, finite element analysis has been performed using a continuum damage-plasticity material law which has been independently calibrated to Si [9]. The constitutive law incorporates isotropic hardening in compression, and isotropic hardening and damage in tension. To accurately capture the localised damage which develops during indentation via the nucleation and propagation of cracks around the indentation site cohesive elements were assigned along the interfaces between the planes which experience the maximum traction. The residual strain state around the indenter and the size of the cracks agree very well with the experimentally measured value.