Modeling creep and anelasticity in particle strengthened alloys with strain gradient crystal plasticity

Modeling creep and anelasticity in particle strengthened alloys with strain gradient crystal plasticity For small material volumes, size effects, e.g. due to the interface constraints or heterogeneous strain ¿elds, may signi¿cantly affect the mechanical behavior of metals such that a deformation mechanism that is less important for the response in bulk form may become decisive for the performance of the material. Such second order effects were observed experimentally in the last two decades and form engineering challenges for the development and production of high-end modern technology. For example, creep and anelasticity observed in metallic thin ¿lm components of capacitive RF-MEMS switches may lead to time dependent deviations from the design speci¿cations of the device. The characterization and understanding of the mechanical behavior of the material is indispensable to overcome the reliability issues of these switches which hinder their full commercialization. In this thesis, a numerical framework is presented for modeling the time dependent mechanical behavior of thin ¿lms made of particle strengthened fcc alloys as an extension of a previously developed strain gradient crystal plasticity (SGCP) model (here referred to as Evers-Bayley type model) for pure fcc metals. A physically based ¿ow rule for crystallographic slip is developed based on the dislocation-dislocation and dislocation-particle interaction mechanisms. The extended SGCP framework is intrinsically able to capture the effect of an inhomogeneous distribution of geometrically necessary dislocation densities on the material behavior via the formulation of a back stress incorporating a material length scale. In chapter 2, the physically based Evers-Bayley type model and a thermodynamically consistent strain gradient theory of crystal plasticity by Gurtin are compared by deriving micro-stresses for the Gurtin type formulation based on the energetic back stresses of the Evers-Bayley type models, incorporating dislocation-dislocation interactions. It is shown that the defect energy function for a micro-stress that con¬tains the physical description of the interaction between dislocations of different slip systems has a more complicated form than those suggested in literature and is possibly non-convex. It is also shown that similar boundary conditions can be de¿ned for the Evers-Bayley type and Gurtin type models despite their differ¬ent additional ¿eld equations within the ¿nite element context. Thereafter, in chapter 3, the SGCP model is employed in electromechanical ¿nite element simulations of bending of polycrystalline thin beams made of a pure metal and a two phase alloy with a focus on the description of anelastic material behavior. Sim¬ulation results obtained with the SGCP model show a macroscopic strain recovery over time following the load removal. However, a detailed analysis demonstrates that the anelastic relaxation time and strength have no solid physical basis. A comparison of the results with experimental data implies that a single deformation mechanism may not be adequate for capturing the material response. Moreover, the slip law falls short in describing the behavior of a particle enhanced material. Subsequently, an extension of the SGCP model for a more realistic description of the time dependent mechanical behavior of two phase alloys, i.e. creep and anelasticity, is given in chapter 4 and its appli¬cation in multiphysical simulations of a capacitive RF-MEMS switch is presented in chapter 5. A new constitutive rule for crystallographic slip is developed by considering dislocation-dislocation interactions and three distinct dislocation-particle interactions: i) the Orowan process, ii) the Friedel process and iii) the climb of edge dislocations over particles. The new constitutive rule is obtained by the combination of separate slip laws for each type of interaction and is built based on the physically well-founded Orowan type rate equation. A ¿ow rule for the slip rate of mobile dislocations governed by dislocation-dislocation interactions is written by taking into account the jerky and continuous glide regimes of dislocations. Slip laws corresponding to the Orowan and Friedel processes are constructed by considering thermally activated dislocation motion. The climb of edge dislocations is described via a thermal detachment model. Results of ¿nite element simulations of bending of a single crystalline thin beam and a micro-clamp experiment with the extended SGCP model show that creep and anelastic behavior of a metallic thin ¿lm can be predicted with the extended SGCP framework. The amounts of the plastic deformation, anelastic recovery strength and associated relaxation times strongly rely on particle properties, the diffusional rate and the magnitude of internal stresses. The results of the simulations of the micro-clamp experiment imply that inhomoge-neous material diffusion may play an important role in the anelastic behavior of polycrystalline thin ¿lms. The results also suggest that the internal stress formulation of the extended SGCP may need to be extended by considering additional sources of internal stresses. The extended SGCP framework is applied to analyse the behavior of a capacitive RF-MEMS switch in multiphysical simulations. The electrodes of the switch are considered to be made of a metal thin ¿lm with incoherent second phases and have a polycrystalline structure with columnar grains through the thickness and passivated surfaces. The variation of the gap between the electrodes over time is analyzed. First, the in¿uences of particle size, volume fraction, surface constraints and ¿lm thickness on the performance of the switch after a loading and unloading cycle are studied. Then, the effects of cyclic loading and the duration of the unloaded state between sequential cy¬cles are investigated. The results show that the residual changes in the gap and the amount and rate of time dependent recovery after the load removal are highly sensitive to the microstructure and the ¿lm thickness. The smallest amounts of permanent deformation and anelastic recovery are obtained with an upper elec¬trode made of a relatively thin ¿lm which has a surface passivation and involves small incoherent particles with a relatively large volume fraction. Furthermore, the simulations revealed that the maximum residual change of the gap measured after completion of the unloading stage of each cycle saturates within a few cycles. A shorter duration of the unloaded state between successive loading-unloading cycles leads to a larger maximum residual gap change. Due to the decreasing gap, the pull-in voltage also decreases within a few cycles and shows a tendency to level off to a certain value. However, the release voltage does not seem to be as sensitive to the residual deformations as the pull-in voltage. Finally, in chapter 6, the conclusions and recommendations for a future work are given

Gradient crystal plasticity modelling of anelastic effects in particle strengthened metallic thin films

It is now a well known phenomenon that thin films are susceptible to size effects, which can be captured adequately by gradient plasticity theories. Besides the scale dependency, metal thin films also exhibit time dependent behavior: anelasticity (deformation recovery over time following elastic spring back upon load removal) and creep (permanent deformation developed over time at constant loads). This work focuses on the extension of a strain gradient crystal plasticity (SGCP) model (Int J Solids Struct 43:7268–7286, 2006; Phil Mag 87:1361–1378, 2007; J Mech Phys Solids 52:2379–2401, 2004; Int J Solids Struct 41:5209–5230, 2004), previously developed for the scale dependent behavior of pure fcc metals, so that it can be exploited for the description of the scale and time dependent mechanical behavior of thin films that are made of metal alloys with second phase particles. For this purpose, an extended physically based slip law is developed for crystallographic slip in fcc metals by considering the deformation mechanisms that are active within the grains. In doing so, the interaction of dislocations with other dislocations and with second phase particles is taken into account. Three types of dislocation–particle interactions are considered: (i) the Orowan mechanism, (ii) the Friedel mechanism, and (iii) dislocation climb. Finite element simulations of the bending of a single crystalline beam show that at low stress levels, the plastic slip rate is controlled by dislocation climb within the presented model. Provided that a considerable lattice diffusion occurs and sufficiently large back stresses exist in the material, the extended SGCP model predicts a noticeable time dependent recovery, reducing the residual deformation after unloading. The magnitude and the characteristic time scale of the anelastic recovery are controlled by dislocation glide limited by climb

Financial citizenship and nation-building in Malaysia: elites' and citizens' perspectives

This article presents a postcolonial analysis of financial citizenship (FC) programmes in Malaysia. Drawing on secondary data and on interviews with elites and citizen investors, the paper explores the spatial and historically specific nature of financialisation in a postcolonial context. Specifically, the paper draws out the significance of FC as part of broader nation building objectives in Malaysia from an elite perspective, while also observing the reluctance of citizen investors who are engaging with the equity market to support the formal objectives of the policy. In doing so, it provides an example of the financialisation of everyday life in a distinctive and complex emerging economy context. Moreover, the paper explores these processes from both elite and citizen perspectives, allowing these layered relations within FC to be analysed. The article, therefore, contributes to the financialisation literature by bringing new understandings of elite–citizen relations in postcolonial nation-building strategies

Applying Differential Transformation Method to the One-Dimensional Planar Bratu Problem

Abstract This paper is the application of differential transformation method (DTM) to solve the Bratu problem. A considerable research works have been conducte

Whole Brain Vessel Graphs: A Dataset and Benchmark for Graph Learning and Neuroscience (VesselGraph)

Biological neural networks define the brain function and intelligence of humans and other mammals, and form ultra-large, spatial, structured graphs. Their neuronal organization is closely interconnected with the spatial organization of the brain's microvasculature, which supplies oxygen to the neurons and builds a complementary spatial graph. This vasculature (or the vessel structure) plays an important role in neuroscience; for example, the organization of (and changes to) vessel structure can represent early signs of various pathologies, e.g. Alzheimer's disease or stroke. Recently, advances in tissue clearing have enabled whole brain imaging and segmentation of the entirety of the mouse brain's vasculature. Building on these advances in imaging, we are presenting an extendable dataset of whole-brain vessel graphs based on specific imaging protocols. Specifically, we extract vascular graphs using a refined graph extraction scheme leveraging the volume rendering engine Voreen and provide them in an accessible and adaptable form through the OGB and PyTorch Geometric dataloaders. Moreover, we benchmark numerous state-of-the-art graph learning algorithms on the biologically relevant tasks of vessel prediction and vessel classification using the introduced vessel graph dataset. Our work paves a path towards advancing graph learning research into the field of neuroscience. Complementarily, the presented dataset raises challenging graph learning research questions for the machine learning community, in terms of incorporating biological priors into learning algorithms, or in scaling these algorithms to handle sparse,spatial graphs with millions of nodes and edges. All datasets and code are available for download at this https UR

The role of the family in attributing meaning to living with HIV and its stigma in Turkey

Stigma attached to HIV/AIDS remains a global problem, with severe negative consequences for people living with HIV (PLHIV). Family support is fundamental for PLHIV’s psychological and physical well-being. HIV-related stigma is high in Turkey, where HIV/AIDS prevalence is low and the epidemic is not considered a priority. Based on qualitative data generated with HIV-positive women and men, this article explores the process of stigmatization, as experienced and perceived by PLHIV in Turkey, focusing on the institution of the family. Results indicated that enacted stigma from family members is lower than anticipated. While most participants’ narratives showed patterns of support rather than rejection from families, the strong expectations around the cultural value attributed to “the family” are found to be the main facilitators of internalized stigma. The article critically discusses the meaning and implications of family support, addressing the role of patriarchal values attributed to womanhood, manhood, and sexuality in Turkey

Neuroinflammation, Mast Cells, and Glia: Dangerous Liaisons

The perspective of neuroinflammation as an epiphenomenon following neuron damage is being replaced by the awareness of glia and their importance in neural functions and disorders. Systemic inflammation generates signals that communicate with the brain and leads to changes in metabolism and behavior, with microglia assuming a pro-inflammatory phenotype. Identification of potential peripheral-to-central cellular links is thus a critical step in designing effective therapeutics. Mast cells may fulfill such a role. These resident immune cells are found close to and within peripheral nerves and in brain parenchyma/meninges, where they exercise a key role in orchestrating the inflammatory process from initiation through chronic activation. Mast cells and glia engage in crosstalk that contributes to accelerate disease progression; such interactions become exaggerated with aging and increased cell sensitivity to stress. Emerging evidence for oligodendrocytes, independent of myelin and support of axonal integrity, points to their having strong immune functions, innate immune receptor expression, and production/response to chemokines and cytokines that modulate immune responses in the central nervous system while engaging in crosstalk with microglia and astrocytes. In this review, we summarize the findings related to our understanding of the biology and cellular signaling mechanisms of neuroinflammation, with emphasis on mast cell-glia interactions

Clonal diversity and detection of carbapenem resistance encoding genes among multidrug-resistant Acinetobacter baumannii isolates recovered from patients and environment in two intensive care units in a Moroccan hospital

Background Carbapenem-resistant Acinetobacter baumannii has recently been defined by the World Health Organization as a critical pathogen. The aim of this study was to compare clonal diversity and carbapenemase-encoding genes of A. baumannii isolates collected from colonized or infected patients and hospital environment in two intensive care units (ICUs) in Morocco. Methods The patient and environmental sampling was carried out in the medical and surgical ICUs of Mohammed V Military teaching hospital from March to August 2015. All A. baumannii isolates recovered from clinical and environmental samples, were identified using routine microbiological techniques and Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry. Antimicrobial susceptibility testing was performed using disc diffusion method. The carbapenemase-encoding genes were screened for by PCR. Clonal relatedness was analyzed by digestion of the DNA with low frequency restriction enzymes and pulsed field gel electrophoresis (PFGE) and the multi locus sequence typing (MLST) was performed on two selected isolates from two major pulsotypes. Results A total of 83 multidrug-resistant A. baumannii isolates were collected: 47 clinical isolates and 36 environmental isolates. All isolates were positive for the bla OXA51-like and bla OXA23-like genes. The coexistence of bla NDM-1 /bla OXA-23-like and bla OXA 24-like /bla OXA-23-like were detected in 27 (32.5%) and 2 (2.4%) of A. baumannii isolates, respectively. The environmental samples and the fecally-colonized patients were significantly identified (p < 0.05) as the most common sites of isolation of NDM-1-harboring isolates. PFGE grouped all isolates into 9 distinct clusters with two major groups (0007 and 0008) containing up to 59% of the isolates. The pulsotype 0008 corresponds to sequence type (ST) 195 while pulsotype 0007 corresponds to ST 1089.The genetic similarity between the clinical and environmental isolates was observed in 80/83 = 96.4% of all isolates, belonging to 7 pulsotypes. Conclusion This study shows that the clonal spread of environmental A. baumannii isolates is related to that of clinical isolates recovered from colonized or infected patients, being both associated with a high prevalence of the bla OXA23-like and bla NDM-1genes. These findings emphasize the need for prioritizing the bio-cleaning of the hospital environment to control and prevent the dissemination of A. baumannii clonal lineages