An Improved Approach for 3D Rolling Contact Fatigue Simulations with Microstructure Topology

Several 2D and 3D numerical models have been developed to investigate rolling contact fatigue (RCF) by employing a continuum damage mechanics approach coupled with an explicit representation of microstructure topology. However, the previous 3D models require significant computational effort compared to 2D models. This work presents a new approach wherein efficient computational strategies are implemented to accelerate the 3D RCF simulation. In order to reduce computational time, only the volume that is critically stressed during a rolling pass is modeled with an explicit representation of microstructure topology. Furthermore, discontinuities in the subsurface stress calculation in the previously developed models for line and circular contact loading are removed. Additionally, by incorporating a new integration algorithm for damage growth, the fatigue damage simulations under line contact are accelerated by a factor of nearly 13. The variation in fatigue lives and progression of simulated fatigue spalling under line contact obtained using the new model were similar to the previous model predictions and consistent with empirical observations. The model was then extended to incorporate elastic–plastic material behavior and used to investigate the effect of material plasticity on subsurface stress distribution and shear stress–strain behavior during repeated rolling Hertzian line contact. It is demonstrated that the computational improvements for reduced solution time and enhanced accuracy are indispensable in order to conduct investigations on the effects of advanced material behavior on RCF, such as plasticity.     

Flow Rate Limitation of Steady Convective Dominated Open Capillary Channel Flows Through a Groove

An open capillary channel is a structure that establishes a liquid flow path when the capillary pressure caused by surface tension forces dominates in comparison to the hydrostatic pressure induced by gravitational or residual accelerations. To maintain a steady flow through the channel the capillary pressure of the free surface has to balance the pressure difference between the liquid and the surrounding constant pressure gas phase. Due to convective and viscous momentum transport the pressure along the flow path of the liquid decreases and causes the free surface to bend inwards. The maximum flow rate through the channel is reached when the free surface collapses and gas ingestion occurs near the outlet. This stability limit depends on the geometry of the channel and the properties of the liquid. In this paper we present an experimental setup which is used in the low-gravity environment of the Bremen Drop Tower. Experiments with convective dominated systems have been performed where the flow rate was increased up to the maximum value. In comparison to this we present a one-dimensional theoretical model to determine important characteristics of the flow, such as the free surface shape and the limiting flow rate. Furthermore we present an explanation for the mechanism of flow rate limitation for these flow conditions which is similar to the choking problem for compressible gas flows.     

Thiol–Ene Clickable Gelatin: A Platform Bioink for Multiple 3D Biofabrication Technologies

Bioprinting can be defined as the art of combining materials and cells to fabricate designed, hierarchical 3D hybrid constructs. Suitable materials, so called bioinks, have to comply with challenging rheological processing demands and rapidly form a stable hydrogel postprinting in a cytocompatible manner. Gelatin is often adopted for this purpose, usually modified with (meth‐)acryloyl functionalities for postfabrication curing by free radical photopolymerization, resulting in a hydrogel that is cross‐linked via nondegradable polymer chains of uncontrolled length. The application of allylated gelatin (GelAGE) as a thiol–ene clickable bioink for distinct biofabrication applications is reported. Curing of this system occurs via dimerization and yields a network with flexible properties that offer a wider biofabrication window than (meth‐)acryloyl chemistry, and without additional nondegradable components. An in‐depth analysis of GelAGE synthesis is conducted, and standard UV‐initiation is further compared with a recently described visible‐light‐initiator system for GelAGE hydrogel formation. It is demonstrated that GelAGE may serve as a platform bioink for several biofabrication technologies by fabricating constructs with high shape fidelity via lithography‐based (digital light processing) 3D printing and extrusion‐based 3D bioprinting, the latter supporting long‐term viability postprinting of encapsulated chondrocytes.     

Engineered Fibrillar Fibronectin Networks as Three‐Dimensional Tissue Scaffolds

Extracellular matrix (ECM) proteins, and most prominently, fibronectin (Fn), are routinely used in the form of adsorbed pre‐coatings in an attempt to create a cell‐supporting environment in both two‐ and three‐dimensional cell culture systems. However, these protein coatings are typically deposited in a form which is structurally and functionally distinct from the ECM‐constituting fibrillar protein networks naturally deposited by cells. Here, the cell‐free and scalable synthesis of freely suspended and mechanically robust three‐dimensional (3D) networks of fibrillar fibronectin (fFn) supported by tessellated polymer scaffolds is reported. Hydrodynamically induced Fn fibrillogenesis at the three‐phase contact line between air, an Fn solution, and a tessellated scaffold microstructure yields extended protein networks. Importantly, engineered fFn networks promote cell invasion and proliferation, enable in vitro expansion of primary cancer cells, and induce an epithelial‐to‐mesenchymal transition in cancer cells. Engineered fFn networks support the formation of multicellular cancer structures cells from plural effusions of cancer patients. With further work, engineered fFn networks can have a transformative impact on fundamental cell studies, precision medicine, pharmaceutical testing, and pre‐clinical diagnostics.     

Diet and Parkinson's disease. II: A possible role for the past intake of specific nutrients. Results from a self-administered food-frequency questionnaire in a case-control study

In a case-control study, we compared the past dietary habits of 342 Parkinson's disease (PD) patients recruited from nine German clinics with those of 342 controls from the same neighborhood or region. Data were gathered with a structured interview and a self-administered food-frequency questionnaire. Nutrient intakes were calculated from the reported food intakes through linkage with the German Federal Food Code and analyzed using multivariate conditional logistic regression to control for total energy intake, educational status, and cigarette smoking. At the macronutrient level, patients reported higher carbohydrate intake than controls after adjustment for total energy intake, smoking, and educational status (OR = 2.74, 95% confidence interval [CI]: 1.30-6.07, for the highest versus lowest quartile, p trend = 0.02). This was reflected in higher monosaccharide and disaccharide intakes at the nutrient level. There was no difference between patients and controls in protein and fat intake after adjustment for energy intake. We found an inverse association between the intakes of beta-carotene (OR = 0.67, 95% CI: 0.37-1.19, p trend = 0.06) and ascorbic acid (OR = 0.60, 95% CI: 0.33-1.09, p trend = 0.04) by patients, although only the trend for ascorbic acid intake reached statistical significance. There was no difference between groups for alpha-tocopherol intake after adjustment for energy intake. We also found that patients reported a significantly lower intake of niacin than controls (OR = 0.15, 95% CI: 0.07-0.33, p trend < 0.00005). Our results suggest that if antioxidants play a protective role in this disease, the amounts provided by diet alone are insufficient. Although the interpretation of the inverse association between niacin intake and PD is complicated by the high niacin content in coffee and alcoholic beverages, which were also inversely associated with PD in this study, the strength of this association and its biologic plausibility warrant further investigation.     

The kinked interface crack

The integral equation of the kinked interface crack is solved numerically. The values of K _I, K _II and G for an interface crack with an infinitesimal kink are used to predict the kinking angle for two different material combinations under uniaxial tension.     

Measuring the impact of adversarial errors on packet scheduling strategies

In this paper, we explore the problem of achieving efficient packet transmission over unreliable links with worst-case occurrence of errors. In such a setup, even an omniscient offline scheduling strategy cannot achieve stability of the packet queue, nor is it able to use up all the available bandwidth. Hence, an important first step is to identify an appropriate metric to measure the efficiency of scheduling strategies in such a setting. To this end, we propose an asymptotic throughput metric which corresponds to the long-term competitive ratio of the algorithm with respect to the optimal. We then explore the impact of the error detection mechanism and feedback delay on our measure. We compare instantaneous with deferred error feedback, which requires a faulty packet to be fully received in order to detect the error. We propose algorithms for worst-case adversarial and stochastic packet arrival models, and formally analyze their performance. The asymptotic throughput achieved by these algorithms is shown to be close to optimal by deriving lower bounds on the metric and almost matching upper bounds for any algorithm in the considered settings. Our collection of results demonstrate the potential of using instantaneous feedback to improve the performance of communication systems in adverse environments.     

Microfluidic chips for cells capture using 3-D hydrodynamic structure array

Microfluidic chips were designed and fabricated to capture cells in a relative small volume to generate the desired concentration needed for analysis. The microfluidic chips comprise three-dimensional (3-D) cell capture structures array fabricated in PDMS. The capture structure includes two layers. The first layer consists of spacers to create small gap between the upper layer and glass. The second layer is a sharp corner U-shaped compartment with sharp corners at the fore-end. And another type capture structure with Y-shaped fluidic guide has been designed. It was demonstrated that the structures can capture cells in theory, using Darcy–Weisbach equation and COMSOL Multiphysics. Then yeast cell was chosen to test the performance of the chips. The chip without fluid guides captured ~1.44 × 10⁵ cells and the capture efficiency was up to 71 %. And the chip with fluid guides captured ~5.0 × 10⁴ cells and the capture efficiency was ~25 %. The chip without fluid guides can capture more cells because the yeast cells in the chip without fluid guides are subject to larger hydrodynamic drag force.     

Plastic limit loads of defective pipes under combined internal pressure and axial tension

In this paper, a finite element mathematical programming formulation is presented for the statical limit analysis of 3-D perfectly plastic structures. A direct iterative algorithm is employed in solving the above optimization formulation. The numerical procedure has been applied to carry out the plastic collapse analysis of defective pipes under combined internal pressure and axial tension. The engineering situation considered has a practical importance in the pipeline industry. The effects of four kinds of typical part-through slots on the collapse loads of pipes are investigated and evaluated.