Loading Orientation Research Articles

Influence of load orientations with respect to twin boundaries on the deformation behaviors of high-entropy alloy nanocrystals

Influence of load orientations with respect to twin boundaries on the deformation behaviors of high-entropy alloy nanocrystals

A crystal plasticity investigation of grain size-texture interaction in magnesium alloys

This work investigates the microstructure-property linkages in magnesium (Mg) with an emphasis on understanding interaction effects between the grain size, texture, and loading orientation. A single crystal plasticity framework endowed with experimentally informed micro Hall-Petch type relations for the activation thresholds for slip and twinning is adopted to resolve polycrystalline microstructures over a broad texture-grain size space. The macroscopic trends from the simulations corroborate with experiments. The synergistic effects of microstructural engineering on the micromechanical characteristics are mapped, which reveal their role in the emergent macroscopic behaviors. The simulations predict reduced extension twinning with grain size refinement even though the micro Hall-Petch coefficient for twinning is smaller than that for the non-basal slip modes. While grain refinement and textural weakening generally reduce the net plastic anisotropy and tension-compression asymmetry, the degree to which these macroscopic behaviors are tempered depends on the loading orientation. The results offer preliminary insight into the roles that texture and grain size may play in the damage behavior of engineered Mg microstructures.

Riveted joints in composites, a practical tool to estimate stresses around the rivet hole

Riveted joints have different failure mechanisms, some associated to the failure of the rivet itself, and others associated to the materials to be joined. The failures associated to the material are mainly due to the stress concentrations at the rivet holes. In joints with metallic (ductile) materials, these stress concentrations might not play a relevant role in the failure initiation, but with unidirectional composite (brittle) materials, they do control the failure. Both the orthotropic behavior of joined materials and the non-linear contact conditions between the rivet and the hole, make that the problem does not have a general solution. In the present work, several finite element analyses have been carried out for basic load cases, different materials, and orientations of the fiber. Furthermore, a two-steps least squares adjustment has been performed to obtain a polynomial expression of the stress distributions for all cases. As a result, a tool is provided, allowing the users to easily estimate the stress distribution (σrr, σθθ) along the hole contour, and σxy along the tangential lines to the hole circumference. Such stress ditribution can be used for failure predictions. All input and output data are available to readers, by means of a spreadsheet, to estimate mentioned stress distributions.

Numerical life prediction of unidirectional fiber composites under block loading conditions using a progressive fatigue damage model

Practical applied components made of Fiber-Reinforced Plastics (FRPs) are usually subjected to cyclic loading with variable load amplitudes and arbitrary load orientation in the course of their service life. For a cost-effective design process of composite structures, it is essential to use computationally efficient and detailed fatigue damage design tools. This contribution focuses on the extension and application of a progressive Finite-Element-based Fatigue Damage Model (FDM) for unidirectional FRPs for lifetime prediction under different block loading conditions. The employed FDM relies on a layer- and energy-based approach, and it is capable of including basic fatigue phenomena such as load sequence effects and stress redistributions. First, the damage evolution laws on which the FDM is based are extended to block loading conditions with arbitrary load sequences. The effects of passive damage, which occur in this context under alternating cyclical tensile and compressive loading, are also taken into account. Secondly, appropriate block loading patterns are defined for the numerical prediction of the lifetime. Finally, the calculation results are presented and critically evaluated based on experimental findings from literature. The comparison of simulations with experiments demonstrates the high predictive capacity of the presented FDM.

Controlling and testing anisotropy in additively manufactured stochastic structures

Additive manufacturing (AM) enables fine control over the architecture and mechanical properties of porous lattice structures. Typical periodic unit cells used in porous structures are inherently anisotropic, may not be suitable for multi-axial applications and cannot reliably create features or struts with low build angle. This study used laser powder bed fusion (PBF) to create isotropic stochastic porous structures in stainless steel (SS316L) and titanium alloy (Ti6Al4V), with modifications that aimed to overcome PBF manufacturing limitations of build angles. The structures were tested in uniaxial compression (n = 5) in 10 load orientations relative to the structure, including the three orthogonal axes. The testing verified that no hidden peaks in elastic modulus existed in the stochastic structure. Modification to the structure reduced the standard deviation of the 10 elastic modulus values from 249 MPa to 101 MPa when made in SS316L and from 95.9 MPa to 52.5 MPa for Ti6Al4V, indicating the structures were more isotropic. These modified stochastic structures have improved stiffness isotropy and could be used for lightweighting and biomaterial applications, reducing the dependence of performance on build orientation, and allowing more flexibility for component orientation on the build platform.

Processing aramid nanofiber/modified graphene oxide hydrogel into ultrastrong nanocomposite film

Aramid nanofibers (ANFs) are promising polymeric nanoscale building block as matrix for high-strength composites, but remain to be explored fully. In this work, we demonstrate a hydrogel processing route to fabricate ultrastrong ANF-based nanocomposite film. A uniform mixed dispersion containing ANF and branched poly(ethylenimine)-modified graphene oxide (BGO) is converted into hydrogel, followed by chemical crosslinking and pre-stretching treatment. This unique hydrogel processing route allows step-by-step optimization of BGO loading, interfacial crosslinking and ANF orientation, and thus leads to well-organized ANF-based nanocomposite film. The nanocomposite film shows a tensile strength of ~616 MPa and a Young’s modulus of ~33 GPa, which are 4 and 15 times higher than those of pure ANF film and superior to all reported ANF-based nanocomposites. Experimental characterizations suggest that the superb mechanical property is benefited from uniform dispersion of BGO, multiple hydrogen bonding between BGO and ANF, alignment of ANF matrix and covalent crosslinking between BGO nanosheets. Our study provides innovative insights into the fabrication of ultrastrong ANF-based nanocomposites and is potentially extendable to their nanocomposites reinforced with other nanoscale fillers.

The relationship between the climate of organizations and the type of communication in a public institution

The organizational climate is a multi-dimensional construction that comprises a variety of work environment assessments. These assessments may refer to the dimensions of the environment, such as roles within the institution, load orientation and most often communication. The overall perceptions of the organizational climate develop as individuals attribute meaning to their organizational context based on the significance of the balance of work environment and individual values. The organizational climate fully in line with the type of effective communication dominates the public institutions in Romania, contributing to administrative efficiency.

Insight of molecular simulation to better assess deformation and failure of clay-rich rocks in compression and extension

Clay minerals constituent an essential component of clayey rocks. Macroscopic mechanical behaviors of those rocks can be significantly driven by local deformation and failure processes of clay minerals. In this study, the mechanical behavior of Montmorillonite (MMT) crystal at the atomistic scale is investigated by using molecular dynamics (MD) simulations. In accordance with macroscopic laboratory tests, both triaxial compression and extension with constant mean stress are considered. A series of MD simulations are performed for these two loading paths with different values of mean stress and respectively applied in the parallel and perpendicular directions to crystal layers. It is found that the mechanical behavior of MMT crystal exhibits a clear dissymmetry between compression and tension, a strong anisotropy along two loading orientations and an important dependency on mean stress. The inter-layer space collapse and atomic sheet bending are two failure mechanisms of the crystal structure. The results issued from MD simulations provide a microscopic insight into macroscopic deformation and failure of clay-rich rocks.

Inter-granular fracture behaviour in bicrystalline boron nitride nanosheets using atomistic and continuum mechanics-based approaches

Hexagonal boron nitride nanosheets (BNNS) are among the emerging nanomaterials with potential application in water purification, nanocomposites and ion separation. In this article, atomistic followed by sequential multiscale models in the framework of finite element was developed to investigate the inter-granular fracture properties of bicrystalline BNNS. Atomistic simulations were performed in the environment of classical mechanics-based molecular dynamics to capture the crack tip behaviour, and for developing traction separation law for bicrystalline BNNS. Moreover, the fracture toughness of bicrystalline BNNS is significantly dependent on the misorientation angle, GB configuration in terms of homoelemental bonds (B–B or N–N) and orientation of loading. The average crack propagation velocity was calculated at atomistic scale and was made to compare with the value predicted using linear elastodynamic theory. The average crack tip stresses predicted from atomistic simulations were found to be in good agreement with values calculated from the continuum model developed using the principle of sequential multiscale model. This work provides a systematic study of inter-granular fracture in bicrystalline BNNS, providing valuable information to develop future applications of 2D nanosheets.

Stress Analysis of Hole Orientation and Laminate Geometry Impacting on Boron/Epoxy Composites Laminates

Boron/epoxy laminates are used in aircraft and space vehicles for their high strength. Evaluation of stresses and residual strength of the laminate with square cutout are not analyzed in the literature. The present work is focused on studying the effect of hole orientation and laminate geometry on Boron/Epoxy composites laminates under in-plane loading. The analytical solution for stresses around holes in laminates is derived using Savins’s complex variables method to consider a multilayered plate with different hole shapes and orientations of loading. The basic equations of failure criteria available for plain laminates are derived to calculate the residual strength of the laminates with hole using the stresses obtained from the analytical solution. The derived analytical solution is validated by reproducing exactly the same results of earlier researchers even by other formulations and also by the results of finite element analysis using ANSYS. The [0/0]s laminate is not preferred due to highest stress concentrations at the corners that range between 12 to 12.45. Similarly, [45/-45]s laminate is also not preferred due to its higher values of stress concentrations which range from 9.5 to 28. The normalized stress for [0/90]s underx-axis loading is 9.6 and fory-axis loading it is 9.5 which is almost the same. Even for equi-biaxial loading, it is 8.5 and for shear loading, it is 12.45. Except for shear loading, [0/90]s laminate seems to be a better choice for a reasonable value of stress concentration for any general case loading. The analytical solution derived in the present work is the most general and unique as it can yield the stresses around any shape of hole and laminate geometry and all types of in plane loading. This solution will be able to reproduce the results of all other solutions available in the literature by different formulations.

The Effects of Moisture Content and Loading Orientation on Some Physical and Mechanical Properties of Tiger Nut

The physical and mechanical properties of the newly “Zhongyousha No.1” and the traditional “Yousha” Chinese tiger nut varieties are studied. Several physical and mechanical properties of the two tiger nut varieties were determined as a function of moisture content (at five levels of 40.0, 32.0, 24.0, 16.0 and 8.0% wet basis). The physical and mechanical properties including length, width, thickness, geometric mean diameter, surface area, sphericity, compression rupture force, firmness, shear rupture force and shear strength. All tiger nut mechanical properties were significantly affected by moisture content (P<0.01). The results shows that the physical properties decreased linearly by decreasing moisture content. The compression rupture force increased linearly from the average of 175.56-208.77 N as moisture content increased from 8 to 40%. While for the shear rupture force and shear strength decreased linearly from the average of 109.73-91.72 N and 1.03-0.63 MPa, respectively. There is a quadratic relationship between firmness and moisture content from the average of 51.80-65.20 N/m. The compression rupture force in the vertical loading orientation is significantly greater than that of the horizontal, while the shear rupture force is higher in the vertical than horizontal. “Yousha” tiger nut possessed higher mechanical strength than “Zhongyousha No.1”. The results can be used as design parameters for suitable harvesting machinery and food processing equipment.

Strength and energy consumption of inherently anisotropic rocks at failure

Using a discrete-element approach and a bonding interaction law, we model and test crushable inherently anisotropic structures reminiscent of the layering found in sedimentary and metamorphic rocks. By systematically modifying the level of inherent anisotropy, we characterize the evolution of the failure strength of circular rock samples discretized using a modified Voronoi tesselation under diametral point loading at different orientations relative to the sample’s layers. We characterize the failure strength, which can dramatically increase as the loading becomes orthogonal to the rock layers. We also describe the evolution of the macroscopic failure modes as a function of the loading orientation and the energy consumption at fissuring. Our simulation strategy let us conclude that the length of bonds between Voronoi cells controls the energy being consumed in fissuring the rock sample, although failure modes and strength are considerably changing. We end up this work showing that the microstructure is largely affected by the level of inherent anisotropy and loading orientation.

Influence of topology and porosity on size effects in stripes of cellular material with honeycomb structure under shear, tension and bending

Cellular solids are known to exhibit size effects, i. e.differences in the apparent effective elastic moduli, when the specimen size becomes comparable to the cell size. The present contribution employs direct numerical simulations (DNS) of the mesostructure to investigate the influences of porosity, shape of pores, and thus material distribution along the struts, and orientation of loading on the size effects and effective moduli of regular honeycomb structures. Beam models are compared to continuum models for simple shear, uniaxial loading and pure bending of strips of finite width. It is found that the honeycomb structure exhibits a considerable anisotropy of the size effects and that honeycomb structures with circular pores exhibit considerably stronger size effects than those with hexagonal pores (and thus straight struts). Positive (stiffening) size effects are observed under simple shear and negative (softening) size effects under bending and uniaxial loading. The negative size effects are interpreted in terms of the stress-gradient theory

Practical constraints in the container loading problem: Comprehensive formulations and exact algorithm

This paper addresses the Single Container Loading Problem. We present an exact approach that considers the resolution of integer linear programming and constraint programming models iteratively. A linear relaxation of the problem based on packing in planes is proposed. Moreover, a comprehensive set of mathematical formulations for twelve practical constraints that arise in this problem are discussed. These constraints include complete shipment, conflicting items, priorities, weight limit, cargo stability, load-bearing, multi-drop, load-balancing, manual loading, grouping, separation, and multiple orientations. Extensive computational experiments are carried out on instances from the literature to show the performance of the proposed approach and state how each practical constraint affects the container’s occupancy, the approach runtime, and the number of packing patterns evaluated. In general, the approach could optimally solve instances with around ten items types and a total of 110 items, besides obtaining the optimal solution for more than 70% of all instances.

Investigation of Flow Behavior and Porous Medium Resistance Coefficients for Metallic-Cloth Fibers

The flow through porous metallic-cloth fibers influences the cloth seal leakage performance. Measuring the actual seal leakage proves difficult with challenging turbine operating conditions. A non-Darcian porous medium Computational Fluid Dynamics (CFD) model was employed for the flow within porous metallic-cloth fibers. CFD analyses need leakage data depending on the pressure load to calibrate flow resistance coefficients. A test rig was built to measure leakage with respect to the pressure load and weave orientation in four directions. The Sutherland-ideal gas approach was utilized to determine the flow resistance coefficients for Dutch twill metallic-cloth fibers as a function of pressure load. The results show that metallic-cloth fiber leakage is a linear function of pressure load. The best–worst order for leakage performance was the warp, diagonal, shute, and cross directions. For the best sealing performance, the flow direction in metallic-cloth fibers would be the warp direction. The flow resistance coefficients depend on the evaluation of the pressure level, which changes over the weave flow thickness. This is represented with the pressure constant (Cdown). The best match between the test and CFD leakages was obtained for the weave directions of warp (0.9), shute (0.9), diagonal (0.7), and cross (0.0). Calibrating the resistance coefficients with respect to the pressure and temperature enables performing CFD analyses in turbine conditions.

Effect of non-zero mean stress bending-torsion fatigue on fracture surface parameters of 34CrNiMo6 steel notched bars

Abstract Modern methods of testing materials require the use of the latest technologies and combining measurement and calculation methods. It is important to find a quantitative way of describing, among other things, the failures so that it can help to design with high accuracy. This paper studies loading orientations on crack shape and fracture surface changes. The advantage of the entire fracture surface method is simplicity and applicability in studies on other materials, shapes and loadings. A higher values of fracture surface parameters (Sx, Vx) was observed in failure specimens with lower σ/τ (B/T) ratios. It has been observed that largest crack lengths with a small number of cycles occur for loading combinations different then B=T. As well as analyzed surface parameters Sx, Vx, are higher for larger number of cycles to crack initiation (Ni) values.

Effects of crystallography on hot-spot formation in porous RDX single crystals

The thermomechanical behavior of porous RDX single crystals is studied under dynamic loading using an anisotropic dislocation-based crystal plasticity model that accounts for deformation-induced heating. A micromechanics-based framework is proposed to account for micro-inertially confined dynamic collapse of pores. A suite of finite element calculations are carried out to systematically elucidate the role of crystallographic orientation on hot-spot formation. Under particular loading orientations, the porous RDX crystals are found to be quite sensitive to deviatoric and volumetric strain localizations, even for loading situations that do not particularly promote strain localization in isotropic materials. Due to the anisotropy-induced strain localizations, fairly severe hot-spots are predicted even under moderate applied strains. The susceptibility of porous RDX to strain localize and form severe hot-spots under particular loading orientations has important implications for accidental detonation of these energetic materials.

Influence of load orientations with respect to twin boundaries on the deformation behaviors of high-entropy alloy nanocrystals

Abstract

Mechanistic analysis of the enhanced RNAi activity by 6-mCEPh-purine at the 5' end of the siRNA guide strand.

A key approach for improving siRNA efficacy is chemical modifications. Through an in silico screening of modifications at the 5′-end nucleobase of the guide strand, an adenine-derived compound called 6-(3-(2-carboxyethyl)phenyl)-purine (6-mCEPh-purine) was identified to improve the RNAi activity in cultured human cells and in vivo mouse models. Nevertheless, it remains unclear how this chemical modification enhances the siRNA potency. Here, we used a series of biochemical approaches to quantitatively evaluate the effect of the 6-mCEPh-purine modification at each step in the assembly of the RNAi effector complex called RISC. We found that the modification improves the formation of mature RISC at least in two different ways, by fixing the loading orientation of siRNA duplexes and increasing the stability of mature RISC after passenger strand ejection. Our data will provide a molecular platform for further development of chemically modified siRNA drugs.

Modelling the influence of the microstructure on the high cycle fatigue crack initiation in short fibre reinforced thermoplastics

In this work, a new criterion to predict fatigue crack initiation in plain Short Fibre Reinforced Thermoplastics is proposed. Several experimental observations in the literature showed that the initiation of a macro crack results from the accumulation of damage in the matrix at the micro-scale. Accordingly, in the proposed criterion the life to crack initiation is predicted through an effective stress acting in the most critical regions in the matrix (close to the fibre ends and lateral surfaces). More precisely, the effective stress is evaluated within a control volume defined as a certain percentage of the matrix volume subjected to the highest stresses. A multi-scale (macro-meso-micro) procedure is adopted for estimating the local stress fields in the polymer. The criterion was proved to successfully predict the influence of the material microstructure (fibre content and orientation distribution) and load orientation.