Experimental investigation and numerical prediction of static strength and fracture behaviour of notched epoxy-based structural adhesives

The influence of stress concentrations on the tensile static strength and fracture behaviour of notched bulk specimens was investigated by comparing the response of two epoxy-based structural adhesives (a rubber-toughened and a polyurethane-toughened). Numerical predictions of failure stress were carried out using a 3D-FEA model with a hydrostatic dependent elastoplastic material behaviour and the equivalent plastic strain for failure assessment. The ductile adhesive, which plastically deforms more under high stresses, provided experimental evidence of a notch strengthening effect. Conversely, the less ductile adhesive has shown a reduction in tensile strength compared to un-notched samples. For both adhesives fracture surface analysis showed the presence of stress whitening and voids close to the notch regions. These regions could be correlated to higher values of stress triaxiality using numerical simulation. The more ductile adhesive underwent widespread stress whitening prior to failure, whereas the response of the less ductile adhesive was more localised. Numerically based predictions showed excellent agreement with experimental results with average error of 5.1% for different notch types in both adhesives.     

Compaction of aggregated ceramic powders: From contact laws to fracture and yield surfaces

This work describes a methodology based on Discrete Element Method (DEM) simulations to generate yield and fracture surfaces for aggregated ceramic powders. The DEM simulations, which consider the length scale of porous aggregates, are used as numerical triaxial experiments to obtain the behavior of a small volume element of powder under a given load. The experimental identification procedure, which relies on the Design Of Experiment method, is designed to limit the number of experiments and simulations needed to obtain the model material parameters. These material parameters, which model the interactions between aggregates in the DEM simulations are identified using two simple experiments on a Uranium diOxide powder: closed-die compaction and diametrical compression test. The yield and fracture surfaces obtained from the DEM simulations provide valuable information on the behavior of the powder for stress states that are difficult or impossible to attain in complex triaxial tests.     

Strength of porous oxide microspheres: The role of internal porosity and defects

Ceramic oxide green pellets for nuclear fuel or targets are manufactured by powder processes producing detrimental fines. Porous brittle microspheres offer an interesting alternative, providing that their fracture behavior is well controlled during pressing. Here we investigate, using experimental characterization and numerical simulations, the effect of porosity and internal defects on the strength of porous microspheres. We show that although the residual porosity is the main parameter affecting their strength, the shape and location of defects also play a role. Real defects characterized by X-ray tomography are reproduced with discrete element simulations, providing new insights on their fracture behavior. We also investigate the departure from monosized microspheres by simulating the fracture behavior of two microspheres of different sizes, and show that it is the minimum radius that allows for a consistent strength normalization. This study offers a method for anticipating agglomerate strength that can be generalized for any ceramic systems.     

Preparation and characterization of porous calcium-phosphate microspheres

Porous calcium-phosphate bioceramics are very important materials in bone tissue engineering. Recently, microsphere systems have been widely utilized in the treatment of defective tissues, including bone, cartilage and muscle. In this study, porous calcium-phosphate microspheres were prepared from calcium-deficient hydroxyapatite (d-HA) powders through a water-in-oil emulsion technique using camphene as the porogen and subsequently sintered at 700, 1100, 1200, or 1400 °C for 6 h. The microspheres produced in this study were characterized according to their morphology, properties, and biodegradation. The results indicated an interconnected porous structure with pore sizes ranging between several microns to as large as 250 μm. Approximately 35–50% of the pores were larger than 100 μm. In the microspheres sintered at 700 °C (Sample H), only the hydroxyapatite (HA) phase was present; when heated to 1100 °C (Sample BH), β-TCP was observed with HA; at 1200 °C (Sample ABH), the phase compositions included β-TCP and α-TCP, as well as a small quantity of HA; and at 1400 °C (Sample AH), the phases of samples included mainly α-TCP and HA. The degradation of the scaffolds was evaluated after immersion in distilled water for up to 28 days. Obvious dissolution and precipitation behavior was seen in the samples ABH and AH. The precipitates formed on the surface of ABH and AH could be carbonate-containing calcium-deficient HA (carbonated-CDHA) after immersion in distilled water for 28 days.     

Influence of platelet content on the fabrication of colloidal gels for robocasting: Experimental analysis and numerical simulation

The preparation of colloidal gels containing different amounts of platelet and sphere-like powders is presented for the production of textured structures using robocasting and templated grain growth methods. The influence of platelet amount on paste fabrication and extrusion is investigated experimentally and numerically. Rheology of the pastes showed the need to vary coagulant and total filling fraction to obtain a paste suitable for robocasting. Pastes containing up to 0.15 platelet fractions were successfully extruded through 0.5 mm and 1.5 mm nozzle diameters. Pastes with platelet content higher than 0.18 and different filling fractions could only be extruded through the 1.5 mm nozzle. Discrete Element Method simulation predicts that the extrusion of pastes containing a high amount of platelets through a thinner nozzle was not possible due to powder morphology and size. Thus, the simulation supports the assumption that a high amount of platelets might be unprintable using thinner nozzles.     

Numerical simulations of diametrical compression tests on agglomerates

The paper reports discrete element simulations of the diametrical compression test applied to two spherical agglomerates: one a dense agglomerate and the other a loosely packed agglomerate. The results obtained for the dense agglomerate show that the agglomerate fractures along a slightly inclined, approximately diametrical plane. Outwardly, the agglomerate shows all the characteristics of brittle fracture but half of the final number of broken bonds was progressively broken during loading. In the simulation on the loose agglomerate, significant flattening occurred at the platens and the agglomerate failed by crushing.     

Experimental and mathematical modelling of corrosion behaviour of CMAS coated oxide/oxide CMCs

CMAS corrosion of turbine blades is a crucial failure in turbine engines and their components. In this study, oxide/oxide CMCs (AS-N610), which are candidates for gas turbine (GT) applications, are investigated for its corrosion behaviour at different temperatures and time in presence of CMAS. The corrosion studies using CMAS coating of the CMCs reveal that CMCs had a weight gain of ～6% owing to formation of α-Al₂O₃ at 1000 °C. The SE image indicated the penetration of CMAS into the porous CMC. At 1000 °C, CMAS degraded to form a black glassy substance (Calcium alumino silicate) with traces of Mg which led to corrosion of the matrix. Indentation fracture toughness of the oxide/oxide CMCs was 7.78 ± 0.5 MPa m^0.5 which degraded by ～12% at 1000 °C after 10 h in the presence of CMAS. A mathematical model derived through diffusion equation indicated weight gain of ～0.3 g which was closer to experimental data.     

Experimental and numerical investigation of structure and hydrodynamics in packed beds of spherical particles

In chemical industry, flows often occur in nontransparent equipment, for example in steel pipelines and vessels. Magnetic resonance imaging is a suitable approach to visualize the flow, which cannot be performed with classical optical techniques, and obtain quantitative data in such cases. It is therefore a unique tool to noninvasively study whole‐field porosity and velocity distributions in opaque single‐phase porous media flow. In this article, experimental results obtained with this technique, applied to the study of structure and hydrodynamics in packed beds of spherical particles, are shown and compared with detailed computational fluid dynamics simulations performed with an in‐house numerical code based on an immersed boundary method‐direct numerical simulation approach. Pressure drop and the radial profiles of porosity and axial velocity of the fluid for three packed beds of spheres with different sizes were evaluated, both experimentally and numerically, in order to compare the two approaches. © 2018 The Authors AIChE Journal published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers AIChE J, 64: 1896–1907, 2018     

Experimental and numerical investigation of the hydroerosive grinding

This paper presents an Euler-Euler approach for the numerical simulation of the hydroerosive grinding (HE) process. It describes a two-phase slurry flow consisting of a liquid and a dispersed solid phase which causes wear at walls of devices. The continuous fluid phase is solved using a finite volume scheme in which the Large Eddy Simulation (LES) [1] model is applied to resolve large-scale turbulent structures. The solid phase is dispersed and treated as a second continuum in which drag and lift forces as well as added mass, pressure and history force are taken into account. Considering particle-particle interactions, the granular model from Gidaspow [2] is used for particle volume concentrations over 1%. Investigations of erosion processes proofed that non-spherically shaped particles as well as harder particles increase the wear on devices significantly. Consequently, non-spherical particles are utilised for the hydroerosive grinding. Their steady drag, unsteady drag and lift coefficients, depending on the particle Reynolds number, are determined by a direct numerical simulation via an in-house LES Lattice-Boltzmann solver. This Lattice-Boltzmann method was presented for laminar flows by Hölzer and Sommerfeld [3]. In this work, interpolating functions of these coefficients are implemented in the Euler-Euler approach which enables the simulation of non-spherical particle transport. Hydroerosive grinding experiments in 3D throttles and 3D planar geometries are carried out to determine an erosion model depending on particle impact velocity, particle size, particle concentration and wall hardness. Implementation of a mesh-morphing algorithm combined with the Euler-Euler scheme of the commercial solver ANSYS CFX11 [4] enables an online simulation of the hydroerosive grinding process. Additionally, the online simulation is used to validate the applied numerical methods. Very good agreements are achieved and will be presented in this paper.     

Experimental and numerical investigation on the strength of polymer-metal hybrid with laser assisted metal surface treatment

The aim of this study was to evaluate the effect of laser assisted treating metal surface on the strength of polymer-metal hybrid. The oxide film on the metal surface was removed by caustic soda and nitric acid solution. After that, the metal surface was treated by fiber laser, and the hot pressing connection between polymer and metal was completed by plate vulcanizing machine. And then, the tensile strength was obtained by using universal testing machine. The effect of different laser power, different scanning line width and different scanning speed on the bonding strength of polymer-metal hybrid was investigated. The correlation between the characteristics of metal surface and the bonding strength of polymer-metal hybrid was analyzed based on the micro structure morphology and scanning electron microscope (SEM). The results show that the bonding strength of polymer-metal hybrid increases first and then decreases with the increase of laser power. With the increase of scanning line width, the strength of polymer-metal hybrid increases. When the scanning speed is 500?mm/s, the strength of polymer-metal hybrid is the lowest. Based on the experiment, a simplified model is established and analyzed. Through using ABAQUS to conduct the numerical simulations, the results are consistent with the theoretical analysis and experimental data.     

An investigation of the comparative behaviour of alternative contact force models during elastic collisions

In this paper the rebound kinematics obtained using different contact force models are compared for the simple problem of an elastic sphere impacting obliquely with a target wall. It is shown that, for an appropriately calibrated normal spring stiffness and a realistic ratio of the tangential to normal spring stiffnesses, excellent results can be obtained by using a simple linear spring model. The paper also demonstrates that for non-linear contact models, integral equations for the tangential force-displacement cannot be used as the spring stiffness varies during the collision. Finally some comments are provided regarding the limitations of the linear spring model and alternatives are discussed.     

Incorporating Darcy's law for pure solvent flow through porous tubes: Asymptotic solution and numerical simulations

A generalized solution for pressure‐driven, incompressible, Newtonian flow in a porous tubular membrane is challenging due to the coupling between the transmembrane pressure and velocity. To date, all analytical solutions require simplifications such as neglecting the coupling between the transmembrane pressure and velocity, assuming the form of the velocity fields, or expanding in powers of parameters involving the tube length. Moreover, previous solutions have not been validated with comparison to direct numerical simulation (DNS). We comprehensively revisit the problem to present a robust analytical solution incorporating Darcy's law on the membrane. We make no assumptions about the tube length or form of the velocity fields. The analytic solution is validated with detailed comparison to DNSs, including cases of axial flow exhaustion and cross flow reversal. We explore the validity of typical assumptions used in modeling porous tube flow and present a solution for porous channels in Supporting Information. © 2012 American Institute of Chemical Engineers AIChE J, 2012     

Microstructure evolution during the co-sintering of Ni/BaTiO3 multilayer ceramic capacitors modeled by discrete element simulations

Electrode discontinuities are a critical issue for manufacturing ultrathin multilayer ceramic capacitors (MLCCs). The Discrete Element Method is used to simulate, at the particle length scale, the microstructure evolution during the co-sintering of state-of-the-art nickel based-MLCCs. Electrode discontinuities are considered to originate from the heterogeneities in the initial powder packing and to grow because of the constraint imposed by adjacent dielectric layers. A parametric study demonstrates that: (i) fast heating rate leads to lower electrode discontinuity during heating, (ii) green density and thickness of the electrode should be optimized to improve the electrode connectivity, (iii) rearrangement of the nickel particles plays a significant role in electrode discontinuity, and (iv) the addition of non-sintering inclusions can improve the electrode connectivity. These findings can be generalized to other multilayer components.     

Experimental and numerical investigation on performance of a porous medium burner with reciprocating flow

Experimental and two-dimensional numerical investigations on the performance of an inert porous media burner with reciprocating flow are presented. Attention was focused on the combustion temperature and pressure loss in the burner, which was, respectively, packed with 4PPC (Pores Per Centimeter) ceramic foams or alumina pellets with various sizes. Results show that material and structures of porous media have significant influence on the burner performance, and that ceramic foam with high porosity is suitable for using in the combustion region whereas alumina pellets should be placed in the heat exchange zone. In addition, the highly two-dimensional characteristics of the porous media burner are validated by the numerical model, which include temperature distributions, species profile and flame structure. Numerical results were validated against experiment data.     

Experimental and numerical investigation on three-point bending behaviors of unidirectional warp-knitted composites

This study develops a novel specimen model, which considers the progressive damage of the layers, the knitting yarns, and the interlaminar cohesive zone, to investigate the flexural properties and the interlaminar shear properties of the unidirectional warp-knitted composites. Three-point bending tests are conducted as verification of the numerical model. Improved strain-based Hashin criteria are proposed to analyze the shear nonlinearity. Results show that specimens with small span-to-thickness ratios exhibit obvious nonlinear behaviors and the corresponding simulation results are sensitive to the value of the shear nonlinear factor. Failure mechanism and stress distributions are analyzed based on numerical simulations. The effect of the specimen size on bending behaviors is discussed. The influence of the width is found to be negligible but that of the span-to-thickness ratio is significant. The ranges of the span-to-thickness ratio corresponding to different failure modes are given. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48132.     

Large-scale numerical investigation of solids mixing in a V-blender using the discrete element method

Over the past few years, the discrete element method (DEM) has been used in models for the simulation of granular flows in various mixing applications. If these models have shown rather efficient, they have so far been applied to predict the behavior of small numbers of particles over limited spans of time. The objective of this work is to show that DEM-based models can be used to predict the flow behavior of large numbers of particles over large spans of time and, more particularly, mixing phenomena that take time to manifest in such systems. To this end, several large-scale DEM-based numerical investigations of the flow of monodisperse and bidisperse blends of up to 225 000 particles over a span of 120 s in a V-blender will be discussed using entities such as the particle velocity and granular temperature, the torque of the mixing system, RSD curves and mixing times.     

Experimental characterization and computational simulations of the low‐velocity impact behaviour of polypropylene

The objective of this work is to validate predictive models for the simulation of the mechanical response of polypropylene undergoing impact situations. The transferability of material parameters deduced from a particular loading scenario (uniaxial loading) to a different loading situation (multiaxial loading) was studied. The material was modelled with a modified viscoplastic phenomenological model based on the G'Sell–Jonas equation. To perform the numerical simulations, a user‐material subroutine (VUMAT) was implemented in the ABAQUS/explicit finite element code. Constitutive parameters for the model were determined from isostrain rate uniaxial tensile impact test data using an inverse calibration technique. In addition, falling‐weight low‐energy impact tests were performed on disc‐shaped specimens at velocities in the range 0.7 to 3.13 m s^?1. The model predictions were evaluated by comparison of the experimental and finite element response of the falling‐weight impact tests. The G'Sell–Jonas model showed much better predictability than classical elastoplasticity models. It also showed excellent agreement with experimental curves, provided stress‐whitening damage observed experimentally was accounted for in the model using an element failure criterion. © 2013 Society of Chemical Industry     

Numerical modelling of the quasi-brittle behaviour of refractory ceramics by considering microcracks effect

In the steelmaking industry, the inner lining of ladles is made of refractory ceramics, which are constantly subjected to thermal shocks during their service. Experimentally, it is observed that pre-existing microcracks could significantly increase the thermal shock resistance of these ceramics. The presence of such microcracks network within the refractory microstructure could lead to a non-linear quasi-brittle mechanical behaviour.To model this quasi-brittle behaviour, a suitable numerical approach is the Discrete Element Method (DEM), which can circumvent the limitations of more conventional continuum approaches in capturing microstructural effects required to simulate multi-fracture propagation.Here, it is aimed to simulate such quasi-brittle behaviour by initial well-distributed damages, with a strength dispersion following a Weibull distribution. In this way, the microcracks effect on the quasi-brittle behaviour of a numerical sample under uniaxial and cyclic tensile tests is investigated. Ultimately, a quantitative DEM model to simulate such a complex behaviour is proposed.     

Hydrodynamic modelling of dense gas-fluidised beds: Comparison of the kinetic theory of granular flow with 3D hard-sphere discrete particle simulations

A novel technique to sample particle velocity distributions and collision characteristics from dynamic discrete particle simulations of intrinsically unsteady, non-homogeneous systems, such as those encountered in dense gas-fluidised beds, is presented. The results are compared to the isotropic Maxwellian particle velocity distribution and the impact velocity distribution that constitute the zeroth-order Enskog approximation for the kinetic theory of granular flow. Excellent agreement with the kinetic theory is obtained for elastic particles. The individual particle velocity distribution function is isotropic and Maxwellian. A good fit of the collision velocity distribution and frequency is obtained, using the radial distribution function proposed by Carnahan and Starling (J. Chem. Phys. 51 (1969) 635). However, for inelastic and rough particles an anisotropic Maxwellian velocity distribution is obtained. It is concluded that the formation of dense particle clusters disturbs spatial homogeneity and results in collisional anisotropy. Analysis of the impact velocity shows that, in dense gas-fluidised beds, not all impact angles are of equal likelihood. The observed anisotropy becomes more pronounced with increasing degree of inelasticity of the particles.     

Experimental investigation and numerical modelling of the mechanical response of a semi-structural polyurethane adhesive

The use of adhesives in load-carrying structures and components has increased recently, especially in the automotive industry. There has been many studies on structural adhesives, but when it comes to semi-structural adhesives, there is a lack of literature. In this article, a semi-structural two component polyurethane adhesive has been studied experimentally and modelled numerically. It was performed uniaxial tension tests at rates ranging from 10^-3s^-1 to 10^-1s^-1. The tests were monitored by two perpendicular digital cameras and a thermal camera. Similarly, uniaxial compression tests were performed at rates ranging from 10^-3s^-1 to 350s^-1, where a split Hopkinson pressure bar (SHPB) was used for the highest rates. The low-rate tests were recorded with high-resolution digital cameras, while a high-speed camera and a thermal camera were used for the SHPB tests. In addition, it was performed notched tensile tests at a low rate to study failure. These tests also served as a validation case for the numerical simulations. A high-resolution camera was used, such that the local strains in the notch could be captured using digital image correlation. The experiments indicated that the adhesive behaved similar as rubbers. Therefore, the Bergström-Boyce constitutive model was applied in the numerical simulations. The overall prediction of the test results was seen to be satisfactory, but the initial stiffness was too high compared to the response measured from the experiments. An investigation of the numerical results indicated that this mismatch was likely linked to the formulation of the inelastic shear rate.