This paper presents the formulation of a constitutive model for amorphous thermoplastics using a thermodynamic approach with physically motivated internal state variables. The formulation follows ...current internal state variable methodologies used for metals and departs from the spring-dashpot representation generally used to characterize the mechanical behavior of polymers like those used by Ames et al. in Int J Plast, 25, 1495–1539 (2009) and Anand and Gurtin in Int J Solids Struct, 40, 1465–1487 (2003), Anand and Ames in Int J Plast, 22, 1123–1170 (2006), Anand et al. in Int J Plast, 25, 1474–1494 (2009). The selection of internal state variables was guided by a hierarchical multiscale modeling approach that bridged deformation mechanisms from the molecular dynamics scale (coarse grain model) to the continuum level. The model equations were developed within a large deformation kinematics and thermodynamics framework where the hardening behavior at large strains was captured using a kinematic-type hardening variable with two possible evolution laws: a current method based on hyperelasticity theory and an alternate method whereby kinematic hardening depends on chain stretching and material plastic flow. The three-dimensional equations were then reduced to the one-dimensional case to quantify the material parameters from monotonic compression test data at different applied strain rates. To illustrate the generalized nature of the constitutive model, material parameters were determined for four different amorphous polymers: polycarbonate, poly(methylmethacrylate), polystyrene, and poly(2,6-dimethyl-1,4-phenylene oxide). This model captures the complex character of the stress–strain behavior of these amorphous polymers for a range of strain rates.
A physically motivated and thermodynamically consistent formulation of small strain higher-order gradient plasticity theory is presented. Based on dislocation mechanics interpretations, gradients of ...variables associated with kinematic and isotropic hardenings are introduced. This framework is a two non-local parameter framework that takes into consideration large variations in the plastic strain tensor and large variations in the plasticity history variable; the equivalent (effective) plastic strain. The presence of plastic strain gradients is motivated by the evolution of dislocation density tensor that results from non-vanishing net Burgers vector and, hence, incorporating additional kinematic hardening (anisotropy) effects through lattice incompatibility. The presence of gradients in the effective (scalar) plastic strain is motivated by the accumulation of geometrically necessary dislocations and, hence, incorporating additional isotropic hardening effects (i.e. strengthening). It is demonstrated that the non-local yield condition, flow rule, and non-zero microscopic boundary conditions can be derived directly from the principle of virtual power. It is also shown that the local Clausius–Duhem inequality does not hold for gradient-dependent material and, therefore, a non-local form should be adopted. The non-local Clausius–Duhem inequality has an additional term that results from microstructural long-range energy interchanges between the material points within the body. A detailed discussion on the physics and the application of proper microscopic boundary conditions, either on free surfaces, clamped surfaces, or intermediate constrained surfaces, is presented. It is shown that there is a close connection between interface/surface energy of an interface or free surface and the microscopic boundary conditions in terms of microtraction stresses. Some generalities and utility of this theory are discussed and comparisons with other gradient theories are given. Applications of the proposed theory for size effects in thin films are presented.
► Failure loads in 6061-T6 aluminum resistance spot welded joints were investigated. ► Force, displacement, and time were captured prior complete failure. ► Nugget and microstructure characteristics ...were quantified. ► We used laser beam profilometry and electron back scatter diffraction techniques. ► Process sensitivity was captured and optimal welding conditions were established.
This study offers a novel research approach to compare weld quality for different welding conditions in order to achieve optimal end-product results. Using electron back scatter diffraction (EBSD) scanning, tensile testing, and laser beam profilometry (LBP) measurements along with optical microscopy (OM) images, failure loads and deformation of 6061-T6 aluminum alloy, resistance spot welded (RSW) joints were experimentally investigated. Three welding conditions, nugget and microstructure characteristics were quantified according to predefined process parameters. Quasi-static tensile tests were used to characterize the failure loads in specimens based upon these same process parameters. Profilometer results showed that the larger the applied welding current, the deeper the weld imprints. In addition, good correlation was obtained between the EBSD scans and the welding conditions. A strong dependency was found between the grain size and orientation and the welding parameters.
► Residual stresses in 6061-T6 aluminum resistance spot welded joints were investigated. ► Neutron diffraction technique was used. ► We used electron back scatter diffraction techniques to analyze ...weld microstructure. ► Bulk (“in-depth”) residual stresses were measured. ► Entire 3-D residual stress field was quantified.
Residual strains of resistance spot welded joints of 6061-T6 aluminum alloy sheets were measured in three different directions denoted as in-plane longitudinal (σ11), in-plane transversal (σ22), and normal (σ33). The welding process parameters were established to meet or exceed MIL-W-6858D specifications (i.e., approximately 5.7mm weld nugget and minimum shearing force of 3.8kN per weld confirmed via quasi-static tensile testing). Electron backscatter diffraction (EBSD) and optical microscopy (OM) were performed to determine grain size and orientation. The residual stress measurements were taken at a series of points along the weld centerline at depths corresponding to the weld mid-plane and at both 1mm below the top surface of the plate and 1mm above bottom surface. The residual stresses were captured on the fusion zone (FZ), heat affected zone (HAZ) and base metal (BM) of the resistance spot welded joint. Neutron diffraction results show residual stresses in the weld are approximately 40% lower than yield strength of the parent material. The maximum variation in residual stresses occurs, as expected, in the vertical position of the specimen because of the orientation of electrode clamping forces that produce a non-uniform solidification pattern. Despite the high anisotropy of the welding nugget and surrounding area, a significant result is that σ33 measured stress values are negligible in both the horizontal and vertical directions of the specimen. Consequently, microstructure–property relationships characterized here can indeed inform continuum material models for application in multiscale models.
Strengthening and embrittlement are controlled by the interactions between dislocations and hydrogen (H)–induced defect structures that can inversely affect the deformation mechanisms in materials. ...Here we present a simulation framework to understand fundamental issues associated with H-assisted dislocation nucleation and mobility using Monte Carlo (MC) and molecular dynamics (MD). In order to study the interaction between H and dislocations and its effect on material failure, we extensively examined mode I loading of an edge crack using MD simulations. The MD calculations of the total structural energy in the nickel (Ni)–H system shows that H atoms prefer to occupy octahedral interstitial sites in the fcc Ni lattice. As H concentration is increased, the Young’s modulus and the energy required to create free surface decreased, resulting in H-enhanced localized plasticity. The MD simulations also indicate that H not only facilitates dislocation emission from the crack tip but also enhances dislocation mobility, leading to softening of the material ahead of the crack tip. While the decrease in surface energy suggests H embrittlement, the increase in local plasticity induces crack blunting and prohibits crack propagation. The mechanisms responsible for transitioning from a ductile to brittle crack behavior clearly depend on the H concentration and its proximity to the crack tip. Enhanced plasticity will occur within a general field of H atoms that results in lower stacking fault and surface energies, yet H interstitials on preferential slip planes can inhibit dislocation nucleation.
Food-based dietary guidelines (FBDG) aim to address the nutritional requirements at population level in order to prevent diseases and promote a healthy lifestyle. Diet quality indices can be used to ...assess the compliance with these FBDG. The present study aimed to investigate whether the newly developed Diet Quality Index for Adolescents (DQI-A) is a good surrogate measure for adherence to FBDG, and whether adherence to these FBDG effectively leads to better nutrient intakes and nutritional biomarkers in adolescents. Participants included 1804 European adolescents who were recruited in the Healthy Lifestyle in Europe by Nutrition in Adolescence (HELENA) Study. Dietary intake was assessed by two, non-consecutive 24 h recalls. A DQI-A score, considering the components' dietary quality, diversity and equilibrium, was calculated. Associations between the DQI-A and food and nutrient intakes and blood concentration biomarkers were investigated using multilevel regression analysis corrected for centre, age and sex. DQI-A scores were associated with food intake in the expected direction: positive associations with nutrient-dense food items, such as fruits and vegetables, and inverse associations with energy-dense and low-nutritious foods. On the nutrient level, the DQI-A was positively related to the intake of water, fibre and most minerals and vitamins. No association was found between the DQI-A and total fat intake. Furthermore, a positive association was observed with 25-hydroxyvitamin D, holo-transcobalamin and n-3 fatty acid serum levels. The present study has shown good validity of the DQI-A by confirming the expected associations with food and nutrient intakes and some biomarkers in blood.
We treat mathematical and computational issues related to the incorporation of strain gradient terms in a phenomenological plasticity model. The strain gradients are associated with incompatibilities ...due to geometrically necessary dislocations, and are quantified by the Nye dislocation density tensor. When incorporated within a flow rule for the plastic strain rate, this tensor fundamentally alters the mathematical structure of the theory. Several computational complexities also arise as a result. These problems are posed in the setting of a variationally-based multiscale method. It allows the circumvention of some of the mathematical and computational difficulties associated with this model. The phenomenological plasticity model, its enhancement by strain gradients, formulation within a multiscale context and two numerical examples are presented.
In this paper, we present numerical simulations with local and nonlocal models under dynamic loading conditions. We show that for finite element (FE) computations of high-velocity, impact problems ...with softening material models will result in spurious post-bifurcation mesh dependency solutions. To alleviate numerical instability associated within the post-bifurcation regime, a characteristic length scale was added to the constitutive relations based on calibration of the series of different notch specimen tests. This work aims to assess the practical relevance of the modified model to yield mesh independent results in the numerical simulations of high-velocity impact problems. To this end, we consider the problem of a rigid projectile moving at a range of velocities between 89-107 m/s, colliding against a 6061-T6 Aluminum disk. A material model embedded with a characteristic length scale in the manner proposed by Pijaudier-Cabot and Bazant (1987), but in the context of concrete damage, was utilized to describe the damage response of the disk. The numerical result shows that the addition of a characteristic length scale to the constitutive model does eliminate the pathological mesh dependency and shows excellent agreements between numerical and experimental results. Furthermore, the application of a nonlocal model for higher strain rate behavior shows the ability of the model to address intense localized deformations, irreversible flow, softening, and final failure.
In April 2020, the QUality Assessment and REProducibility for Instruments and Images in Light Microscopy (QUAREP-LiMi) initiative was formed. This initiative comprises imaging scientists from ...academia and industry who share a common interest in achieving a better understanding of the performance and limitations of microscopes and improved quality control (QC) in light microscopy. The ultimate goal of the QUAREP-LiMi initiative is to establish a set of common QC standards, guidelines, metadata models, and tools, including detailed protocols, with the ultimate aim of improving reproducible advances in scientific research. This White Paper 1) summarizes the major obstacles identified in the field that motivated the launch of the QUAREP-LiMi initiative; 2) identifies the urgent need to address these obstacles in a grassroots manner, through a community of stakeholders including, researchers, imaging scientists, bioimage analysts, bioimage informatics developers, corporate partners, funding agencies, standards organizations, scientific publishers, and observers of such; 3) outlines the current actions of the QUAREP-LiMi initiative, and 4) proposes future steps that can be taken to improve the dissemination and acceptance of the proposed guidelines to manage QC. To summarize, the principal goal of the QUAREP-LiMi initiative is to improve the overall quality and reproducibility of light microscope image data by introducing broadly accepted standard practices and accurately captured image data metrics.
PDF
