Stress-strain behavior and geometrical properties of packings of elongated particles

We present a numerical analysis of the effect of particle elongation on the quasistatic behavior of sheared granular media by means of the Contact Dynamics method. The particle shapes are rounded-cap rectangles characterized by their elongation. The macroscopic and microstructural properties of several packings subjected to biaxial compression are analyzed as a function of particle elongation. We find that the shear strength is an increasing linear function of elongation. Performing an additive decomposition of the stress tensor based on a harmonic approximation of the angular dependence of branch vectors, contact normals and forces, we show that the increasing mobilization of friction force and the associated anisotropy are key effects of particle elongation. These effects are correlated with partial nematic ordering of the particles which tend to be oriented perpendicular to the major principal stress direction and form side-to-side contacts. However, the force transmission is found to be mainly guided by cap-to-side contacts, which represent the largest fraction of contacts for the most elongated particles. Another interesting finding is that, in contrast to shear strength, the solid fraction first increases with particle elongation, but declines as the particles become more elongated. It is also remarkable that the coordination number does not follow this trend so that the packings of more elongated particles are looser but more strongly connected.Comment: Submited to Physical Review

Large deformation analysis using a quasi-static material point method.

The Finite Element Method (FEM) has become the standard tool for the analysis of a wide range of solid mechanics problems. However, the underlying structure of a classical updated Lagrangian FEM is not well suited for the treatment of large deformation problems, since excessive mesh distortions can lead to numerical difficulties. The Material Point Method (MPM) represents an approach in which material points moving through a fixed finite element grid are used to simulate large deformations. As the method makes use of moving material points, it can also be classifed as a point-based or meshless method. With no mesh distortions, it is an ideal tool for the analysis of large deformation problems. MPM has its origin in fluid mechanics and has only recently been applied to solid mechanics problems. It has been used successfully for impact analyses where bodies penetrate each other and for silo discharging problems. All existing MPM codes found in literature are dynamic codes with explicit time integration and only recently implicit time integration. In this study a quasi-static MPM formulation and implementation are presented. The paper starts with the description of the quasi-static governing equations and the numerical discretisation. Afterwards, the calculation process of the quasi-static MPM is explained, followed by the presentation of some geotechnical boundary value problems which have been solved with the newly developed quasi-static MPM code. The benchmark problems consist of an oedometer test and a slope. For validation, the results are compared with analytical solutions and FEM results, respectively

Batch fabrication of scanning microscopy probes for thermal and magnetic imaging using standard micromachining

We present a process for batch fabrication of a novel scanning microscopy probe for thermal and magnetic imaging using standard micromachining and conventional optical contact lithography. The probe features an AFM-type cantilever with a sharp pyramidal tip composed of four freestanding silicon nitride nanowires coated by conductive material. The nanowires form an electrical cross junction at the apex of the tip, addressable through the electrodes integrated on the cantilever. The cross junction on the tip apex can be utilized to produce heat and detect local temperature changes or to serve as a miniaturized Hall magnetometer enabling, in principle, thermal and magnetic imaging by scanning the probe tip over a surface. We have successfully fabricated a first probe prototype with a nanowire tip composed of 140 nm thick and 11 μ m long silicon nitride wires metallized by 6 nm titan and 30 nm gold layers. We have experimentally characterized electrical and thermal properties of the probe demonstrating its proper functioning. ©2010 IEEE

Assessment of Contraceptive Counseling and Contraceptive Use in Women After Bariatric Surgery

BACKGROUND: Reproductive-aged women are, according to American and European guidelines, recommended to avoid pregnancy for 12-24 months after bariatric surgery. Oral contraceptives may have suboptimal efficacy after malabsorptive bariatric procedures. AIM: The aim of this study was to assess contraceptive use pre- and postoperatively in women who underwent bariatric surgery in two obesity clinics in The Netherlands. Also, the recall of contraceptive and pregnancy counseling was investigated. METHODS: A validated questionnaire was performed among women aged 18-45 years who underwent bariatric surgery from October 2017 through August 2018. RESULTS: In total, 230 women were eligible for final analysis. Postoperatively, 60% used safe contraception, 16.1% unsafe contraception, and 23.9% no contraception. In this study, 43.7% of women using a potential unsafe contraceptive method preoperatively switched to a safe method of contraception postoperatively (p < 0.0001). Only 62.6% of women confirmed to have received contraceptive counseling, mainly preoperatively. The odds ratio for receiving contraceptive counseling and using safe contraceptive methods compared with not receiving contraceptive counseling was 2.20 (95% CI, 1.27-3.79; p = 0.005). Eighty-three percent confirmed that they have received counseling regarding delaying a pregnancy, and 52.6% were familiar with the recommendation to avoid a pregnancy for 24 months postoperatively. CONCLUSIONS: In our study, 60% of women are using safe contraception postoperatively. Contraceptive counseling is suboptimal as 62.6% recall receiving counseling. Those who confirmed receiving counseling were more likely to use safe contraception after bariatric surgery. More counseling and monitoring in the postoperative and in the outpatient setting is recommended

A quadrature filter approach for registration accuracy assessment of fundus images

This paper presents a method to automatically assess the accuracy of image registration. It is applicable to images in which vessels are the main landmarks such as fundus images and angiography. The method simultaneously exploits not only the position, but also the intensity profile across the vasculatures. The accuracy measure is defined as the energy of the odd component of the 1D vessel profile in the difference image divided by the total energy of the corresponding vessels in the constituting images. Scale and orientation-selective quadrature filter banks have been employed to analyze the 1D signal profiles. Subsequently, the relative energy measure has been calibrated such that the measure translates to a spatial misalignment in pixels. The method was validated on a fundus image dataset from a diabetic retinopathy screening program at the Rotterdam Eye Hospital. An evaluation showed that the proposed measure assesses the registration accuracy with a bias of -0.1 pixels and a precision (standard deviation) of 0.9 pixels. The small Fourier footprint of the orientation selective quadrature filters makes the method robust against noise