Strut Shape Research Articles

Effects of Microstructures of Foam Core on the Thermal Shock Strength of Ceramic Foam Sandwich Structures

The effects of microstructures of foam core on the thermal shock strength of the sandwich slabs are investigated by macro-micro analysis. Each slab has two ceramic matrix composite (CMC) face sheets and an open-cell ceramic foam core. Asymmetric thermal shock fracture is evaluated at the macro continuum level. The position of the danger zone varying with the thickness of face sheets is determined. The finite element (FE) model based on 3D Voronoi tessellations consists of a local micro-scale region surrounding the crack tip. For a calculated stress intensity factor in macro model, the displacements along the boundary of the local model are calculated based on linear elastic fracture mechanics. The thermal shock strength of the sandwich structure is found to be dominated by the cell regularity, the cross-sectional shape of the cell struts and the relative density of foam core. The physical mechanisms responsible for those results are identified. The effective methods to improve the thermal shock resistance of the sandwich structures with ceramic foam core are proposed.

A particle-based model for endothelial cell migration under flow conditions

Endothelial cells (ECs) play a major role in the healing process following angioplasty to inhibit excessive neointima. This makes the process of EC healing after injury, in particular EC migration in a stented vessel, important for recovery of normal vessel function. In that context, we present a novel particle-based model of EC migration and validate it against in vitro experimental data. We have developed a particle-based model of EC migration under flow conditions in an in vitro vessel with obstacles. Cell movement in the model is a combination of random walks and directed movement along the local flow velocity vector. For model calibration, a set of experimental data for cell migration in a similarly shaped channel has been used. We have calibrated the model for a baseline case of a channel with no obstacles and then applied it to the case of a channel with ridges on the bottom surface, representative of stent strut geometry. We were able to closely reproduce the cell migration speed and angular distribution of their movement relative to the flow direction reported in vitro. The model also reproduces qualitative aspects of EC migration, such as entrapment of cells downstream from the flow-disturbing ridge. The model has the potential, after more extensive in vitro validation, to study the effect of variation in strut spacing and shape, through modification of the local flow, on EC migration. The results of this study support the hypothesis that EC migration is strongly affected by the direction and magnitude of local wall shear stress.

TCT-169 Early Scaffold Strut Coverage in Ultra-High Molecular Weight Amorphous PLLA Sirolimus-Eluting Bioresorbable Scaffolds: Impact of Strut Thickness Assessed in Normal Porcine Coronary Arteries

The speed and quality of stent/scaffold strut coverage by tissue are strongly impacted by strut thickness and shape, which influence shear stress and blood flow dynamics, affecting platelet accumulation and endothelial cell growth. In this study, we aimed to evaluate the short-term healing response

Layer geometry control for the fabrication of lattice structures by wire and arc additive manufacturing

Wire and arc additive manufacturing (WAAM) is an additive manufacturing (AM) technology that uses wire-form materials and arc discharges as the energy source. AM techniques can fabricate complicated shapes that cannot be obtained via conventional processing. Building lattice structures inside components enables weight reduction while maintaining high strength. Strut shapes must be constructed to form these lattice structures using WAAM. For fabricating strut shapes with high accuracy, the process parameters should be optimized. However, the relationship between layer geometry and process parameters is not clear. Therefore, in this study, struts were fabricated under various process conditions to investigate the influences of process parameters on the built object geometry. The results showed that fabrication of strut shapes depends on the heat input condition. Moreover, it was found that the arc discharge time had the highest influence on the layer height and diameter. The inclination angle of an overhanging shape had little influence on the dimensional accuracy of the built object. In addition, computer-aided manufacturing (CAM) system was developed for the fabrication of lattice structures, and the lattice structures were successfully built using WAAM. The build accuracy was measured using an x-ray computed tomography (CT) scanner; the deviation in the structures designed using the CAM system and the actual fabricated structures measured using the CT scanner was lower than approximately ±2.3 mm.

A multi-scale model for predicting the thermal shock resistance of porous ceramics with temperature-dependent material properties

This paper develops a novel multi-scale thermal/mechanical analysis model which not only can efficiently measure the thermal shock response but also highly reflects the effects of diversiform micro-structures of porous ceramics. Knowledge of the temperature distribution and time-varied thermal stress intensity factors (SIF) is derived by finite element/finite difference method and the weight function method in the macro continuum model. The finite element analysis employs a micro-mechanical model in conjunction with the macro model for the purpose of relating the SIF to the thermal stress in the struts of the porous ceramics. The micro model around the crack tip was established by using Voronoi lattices to accurately explore the micro-architectural features of porous ceramics. Hot shock induced center crack and cold shock induced edge crack are both considered. Effects of relative density and pore size on the thermal shock resistance are investigated and the results are well coincident with the experimental tests. The influence of cell regularity and cross section shape of the cell struts is discussed and the corresponding explanations are provided. The importance of incorporating temperature-dependent material properties on the thermal shock resistance prediction is quantitatively represented. These multi-faceted models and results provide a significant guide to the design and selection of porous ceramics against the thermal shock fracture failure for the future thermal protection system of space shuttle.

Validation of an experimental-numerical approach for the high temperature behaviour of open-cell ceramic foams

Open-cell ceramic foams are used as filters in casting processes of molten metal to reduce the amount of non-metallic inclusions. These filters are exposed to high temperatures and loadings, sometimes for a longer time. Therefore, the dimensional stability has to be considered, which suffers from inelastic deformations, in particular creep deformations.A recent work of the authors describes the creep deformation behaviour of such foam structures using an experimental-numerical approach based on bulk material creep tests. Now, the results of this prediction are validated by creep curves of real foam samples. This comparison shows a good accordance for the considered parameters (load, temperature, time, and creep phase).Moreover, the influences of different structural parameters on the elastic foam stiffness and foam creep resistance are investigated. The relative foam density has the greatest impact, followed by uniaxial pore stretching in one direction and strut shape, whereas polydispersity has only a very small effect.

A fundamental investigation of gas/solid mass transfer in open-cell foams using a combined experimental and CFD approach

In this work, we combine numerical (CFD) simulations and experimental measurements in a fundamental investigation of the fluid-solid mass transfer properties of open-cell foams, which are promising support for catalytic applications limited by external heat and mass transfer. CFD simulations are exploited to gain insight into the complex transport mechanisms and to enable a parametric analysis of the geometrical features by means of virtually-generated structures. Catalytic activity experiments under diffusion control are used to validate the CFD results and to extend the range of conditions and foam morphologies investigated. Analysis of the flow field by CFD simulations provides a rational basis for the choice of the average strut size as a physically sound characteristic length for mass transfer correlations. Results from both numerical simulations and experimental tests are interpreted according to a fully-theoretically based geometrical model for the prediction of the specific surface area, which accounts for the detailed node-strut geometry. The effects of cell size and strut shape are properly included in the functional dependence of the Sherwood number on the Reynolds number. The effect of porosity requires one additional dependence, wherein the Sherwood number is inversely proportional to the square of the void fraction. The resulting Sherwood–Reynolds correlation is in excellent agreement with experimental data and CFD simulations. It enables accurate (±15%) estimation of the external mass transfer coefficients for open-cell foams when coupled with the proposed geometrical model from two readily accessible pieces of geometrical information, i.e. the void fraction and either the cell size or the pore diameter of the foam. The derived correlation can be applied to the design of novel enhanced open-cell foam catalyst substrates and structured reactors.

Surrogate models of the influence of the microstructure on the mechanical properties of closed- and open-cell foams

The mechanical behavior of closed- and open-cell foams was analyzed using a computational homogenization strategy based on the finite element simulation of a representative volume element of the foam microstructure. The representation of the foam took into account the main microstructural features, namely density, fraction of solid material in cell walls and struts, cell anisotropy, cell size and distribution and strut shape. A parametric study was carried out to ascertain the influence of these parameters on the elastic modulus and the plateau stress under compression. It was found that these properties mainly depended on the density, fraction of solid material in cell walls and struts and cell anisotropy. Building upon the results of the parametric study, surrogate models were developed to relate the influence of the microstructure on the mechanical properties of the foams. It is believed that these models can be used for the design of foams with optimized properties for particular applications.

Evolution of mass distribution in walls of rigid polyurethane foams

In this work, we describe a theoretical model for the simulation of reactive foaming of polyurethane (PU) consisting of three parts – reaction kinetics, foam expansion and wall evolution, which are coupled together. The advantage of this approach is that it provides comprehensive details about the development of PU foam – from the evolution of temperature and foam density to morphology features like bubble size, wall thickness and strut shape at the same time. The performance of the model is evaluated by analysing the model predictions for realistic process conditions and comparing to experimental data. It is demonstrated that the increased viscosity of the reaction mixture will lead to foams with thicker walls. The presented model can serve as a useful tool for the optimization of PU foaming process.

The control of stem cell morphology and differentiation using three-dimensional printed scaffold architecture.

In this work, we investigated the interactions of human mesenchymal stem cells (hMSCs) with three-dimensional (3D) printed scaffolds displaying different scaffold architectures. Pressure-assisted microsyringe system was used to fabricate scaffolds with square (SQR), hexagonal (HEX), and octagonal (OCT) architectures defined by various degrees of curvatures. OCT represents the highest degree of curvature followed by HEX, and SQR is composed of linear struts without curvature. Scaffolds were fabricated from poly(L-lactic acid) and poly(tyrosol carbonate). We found that hMSCs attached and spread by taking the shape of the individual struts, exhibiting high aspect ratios (ARs) and mean cell area when cultured on OCT scaffolds as compared with those cultured on HEX and SQR scaffolds. In contrast, cells appeared bulkier with low AR on SQR scaffolds. These significant changes in cell morphology directly correlate with the stem cell lineage commitment, such that 80 ± 1% of the hMSCs grown on OCT scaffolds differentiated into osteogenic lineage, compared with 70 ± 4% and 62 ± 2% of those grown on HEX and SQR scaffolds, respectively. Cells on OCT scaffolds also showed 2.5 times more alkaline phosphatase activity compared with cells on SQR scaffolds. This study demonstrates the importance of scaffold design to direct stem cell differentiation, and aids in the development of novel 3D scaffolds for bone regeneration.

Influence of Foam Morphology on Effective Properties Related to Metal Melt Filtration

In this article, a numerical study on the sensitivity, related to the performance of open‐cell foams used for the depth filtration of liquid metals, on two characteristic morphological properties is presented. Therefore, simulations of fluid flow and particle transport inside an artificial foam structure are carried out, whose porosity and strut shape is varied within a certain expected range. For comparison purposes, however, the simulations are also performed for three typical ceramic foam filters (CFF) with pore densities of 20 and 30 PPI, whose geometries are obtained from CT scans. In order to allow for a comparison between the different structures, a reference length is introduced that relies upon the actual ratio of pores per volume. The evaluation is mainly based on the comparison of the hydraulic tortuosity, the viscous and the inertial permeability coefficients as well as the initial filtration coefficient for alumina inclusions, with their size ranging from of 10 to 40 μm at process conditions typically encountered during the aluminum filtration. It is shown that the ratio of filtration coefficient and pressure drop increases with the porosity, while the material distribution between the struts and the joints is less influential. Finally, the article also provides information on the anisotropy of CFFs and on the transition behavior from steady to unsteady flow in open‐cell foams.

Effect of cross sectional shape of struts on the mechanical properties of aluminum based pyramidal lattice structures

Three types of cross sectional shapes of struts are designed in a pyramidal lattice truss structure including circular, semi-circular and U-type, and fabricated using pure aluminum by 3-D printing based investment casting technology. The compressive mechanical behaviors of the lattice truss structures were examined and the related mechanisms were analyzed. It is shown that, when the inclination angle was 70°, the compression strengths of semi-circular and U-type lattice truss structures were twice that of circular strut structure due to the increased area moment of inertia of struts contributing to higher buckling resistance.

Predicting pressure drop in open-cell foams by adopting Forchheimer number

Interconnected struts arranged in 3-D foam structures pose a challenge in understanding fluid flow, which is significantly different from that in traditional porous media. Different flow regimes (Darcy, transition and weak inertia regimes) and thus, different flow laws in open-cell foams are used. The impact of characteristic lengths’ choices based on both, morphological and hydraulic parameters on flow law formulation has been studied. Ambiguities in definitions and measurements of several key parameters have been shown and limitations in the use of some parameters have been pointed out.An equivalent Reynolds number in the form of Forchheimer number (Fo) has been proposed to establish the friction factor relationship in order to avoid any morphological ambiguities. This number takes into account hydraulic characteristics of viscous and inertia regimes simultaneously. It has been observed that when Fo < 0.1, the flow through open-cell foams remains in the Darcy regime while the occurrence of weak inertia regime dominates when Fo > 1. Transition regime occurs in a narrow range of flow velocity when 0.1 < Fo < 1. The limits of transition for regime identification are found to be independent of foam morphologies. The form drag coefficient varies in relation with foam morphological parameters and is not a “universal” constant.Empirical correlations have been derived to predict hydraulic characteristics and friction factor data for different strut shapes and porosities. An excellent agreement has been obtained between predicted and numerical/experimental flow data.

Influence of Morphology on Flow Law Characteristics in Open-Cell Foams: An Overview of Usual Approaches and Correlations

Foam structures have been a subject of intensive research since the last decade. The pore space in open-cell foam is interconnected, forming perforated channels of varying cross-sectional areas where fluid can flow. Knowledge of pressure drop induced by these foam matrices is essential for successful design and operation of high-performance industrial systems. In this context, analytical correlations were derived for the determination of Darcian permeability (KD) and Forchheimer inertia coefficient (CFor) in open-cell foams of different strut shapes. It has been shown that the flow law characteristics are strongly dependent on strut shape, strut characteristic dimension, and length. The applicability of new correlations was examined by comparing and validating the numerical and experimental flow law characteristics data against the predicted ones. An excellent agreement has been observed for the foam structures of different materials and variable texture in a wide range of porosity and Reynolds number.

Strut protrusion and shape impact on endothelial shear stress: insights from pre-clinical study comparing Mirage and Absorb bioresorbable scaffolds

Protrusion of scaffold struts is related with local coronary flow dynamics that can promote scaffold restenosis and thrombosis. That fact has prompted us to investigate in vivo the protrusion status of different types of scaffolds and their relationship with endothelial shear stress (ESS) distributions. Six Absorb everolimus-eluting Bioresorbable Vascular Scaffolds (Absorb, Abbott Vascular) and 11 Mirage sirolimus-eluting Bioresorbable Microfiber Scaffolds (Mirage, Manli Cardiology) were implanted in coronaries of eight mini pigs. Optical coherence tomography (OCT) was performed post-scaffold implantation and obtained images were fused with angiographic data to reconstruct the three dimensional coronary anatomy. Blood flow simulation was performed and ESS distribution was estimated for each scaffold. Protrusion distance was estimated using a dedicated software. Correlation between OCT-derived protrusion and ESS distribution was assessed for both scaffold groups. A significant difference was observed in the protrusion distances (156 ± 137 µm for Absorb, 139 ± 153 µm for Mirage; p = 0.035), whereas difference remained after adjusting the protrusion distances according to the luminal areas. Strut protrusion of Absorb is inversely correlated with ESS (r = −0.369, p < 0.0001), whereas in Mirage protrusion was positively correlated with EES (r = 0.192, p < 0.0001). Protrusion distance was higher in Absorb than in Mirage. The protrusion of the thick quadratic struts of Absorb has a tendency to lower shear stress in the close vicinity of struts. However, circular shape of the less thick struts of Mirage didn’t show this trend in creating zone of recirculation around the struts. Strut geometry has different effect on the relationship between protrusion and shear stress in Absorb and Mirage scaffolds.

Comparative analysis of trabecular bone structure and orientation in South African hominin tali

Tali of several hominin taxa are preserved in the fossil record and studies of the external morphology of these often show a mosaic of human-like and ape-like features. This has contributed to a growing recognition of variability characterizing locomotor kinematics of Australopithecus. In contrast, locomotor kinematics of another Plio-Pleistocene hominin, Paranthropus, are substantially less well-documented, in part, because of the paucity of postcranial fossils securely attributed to the genus. Since the talus transmits locomotor-based loads through the ankle and its internal structure is hypothesized to reflect accommodation to such loads, it is a cornerstone structure for reconstructing locomotor kinematics. Here we quantify and characterize trabecular bone morphology within tali attributed to Australopithecus africanus (StW 102, StW 363, StW 486) and Paranthropus robustus (TM 1517), making quantitative comparisons to modern humans, extant non-human apes, baboons, and a hominin talus attributed to Paranthropus boisei (KNM-ER 1464). Using high-resolution images of fossil tali (25 μm voxels), nine trabecular bone subregions of interest beneath the articular surface of the talar trochlea were segmented to quantify localized patterns in distribution and primary strut orientation. It was found that trabecular strut orientation and shape, in some cases, can discriminate amongst species characterized by different locomotor foot kinematics. Discriminant function analyses using standard trabecular bone structural properties align TM 1517 with Pan and Gorilla, while other hominin tali structurally most resemble those of baboons. In primary strut orientation, Paranthropus tali (KNM-ER 1464 and TM 1517) resemble the human condition in the anterior-medial subregion, where strut orientation appears positioned to distribute compressive loads medially and distally toward the talar head. In A. africanus tali (particularly StW 486), primary strut orientation in this region resembles that of apes. These results suggest that Paranthropus may have had a human-like medial weight shift during the last half of stance phase but Australopithecus did not.

The effect of open-cell metal foams strut shape on convection heat transfer and pressure drop

Metal foams are promising materials for applications where enhancement of heat transfer is needed. Their performance depends on morphological parameters, such as the diameters of cells, pores, and struts, the porosity, the shape of struts.The effect of strut shape on convection heat transfer and pressure drop in open-cell metal foams has been investigated numerically in this paper by introducing the foam shape factor, a parameter that characterizes the shape of the strut. The analysis has been carried out both on real and ideal foams. The geometry of three 40 Pores Per Inch (PPI) real foams, with 0.87, 0.94 and 0.96 average measured porosities, was determined with X-ray Computed Microtomography (XCT) and a morphological analysis of the XCT data was carried out. The geometry of the ideal foams, based on the Lord Kelvin foam model, with the same PPI and porosities as those of the real foams, was generated with the free-to-use software Surface Evolver, building up foams with different strut shapes. Governing equations have been solved with a finite element scheme, assuming a uniform heat flux condition at the solid/fluid boundary of the foam.Results are presented in terms of convection heat transfer coefficients and pressure distributions as a function of strut shape parameters. The comparison between predictions for ideal and real foams, for different strut shapes, confirms that, in all cases, the closer the form of the ideal strut to that of the real strut the better the agreement among predictions. It is also pointed out that the convection heat transfer coefficient is maximized when, at equal porosity, the ligament shape is convex, while a concave strut shape maximizes the pressure drop.

An Experimental Investigation of the Performance Impact of Swirl on a Turbine Exhaust Diffuser/Collector for a Series of Diffuser Strut Geometries

A comprehensive experimental investigation was initiated to evaluate the aerodynamic performance of a gas turbine exhaust diffuser/collector for various strut geometries over a range of inlet angle. The test was conducted on a 1/12th scale rig developed for rapid and accurate evaluation of multiple test configurations. The facility was designed to run continuously at an inlet Mach number of 0.40 and an inlet hydraulic diameter-based Reynolds number of 3.4 × 105. Multihole pneumatic pressure probes and surface oil flow visualization were deployed to ascertain the effects of inlet flow angle and strut geometry. Initial baseline diffuser-only tests with struts omitted showed a weakly increasing trend in pressure recovery with increasing swirl, peaking at 14 deg before rapidly dropping. Tests on profiled struts showed a similar trend with reduced recovery across the range of swirl and increased recovery drop beyond the peak. Subsequent tests for a full diffuser/collector configuration with profiled struts revealed a rising trend at lower swirl when compared to diffuser-only results, albeit with a reduction in recovery. When tested without struts, the addition of the collector to the diffuser not only reduced the pressure recovery at all angles but also resulted in a shift of the overall characteristic to a peak recovery at a lower value of swirl. The increased operation range associated with the implementation of struts in the full configuration is attributed to the deswirling effects of the profiled struts. In this case, the decreased swirl reduces the flow asymmetry responsible for the reduction in pressure recovery attributed to the formation of a localized reverse-flow vortex near the bottom of the collector. This research indicates that strut setting angle and, to a lesser extent, strut shape can be optimized to provide peak engine performance over a wide range of operation.

Basic Research on Lattice Structures Focused on the Strut Shape and Welding Beads

This survey is about the requirements of lattice structures which are made by Selective Laser Melting. The process is based on Additive Manufacturing. It allows the generation of almost every shape. Even complex lattice structures can be manufactured using this technology. Moreover it is possible to integrate high performance lightweight structures into applications. Lattice structures are distinguished into periodic structures and stochastic cellular structures, each type with its individual properties. In a great field of applications periodic lattices are integrated. Because of the great demand for high performance applications, the industry has a big interest in lattice structures. A survey on this topic is indispensable. The main aim is to extract the most influencing process parameters that effect the properties of the structures and to create a construction guideline using the extracted results. Due to the fact that periodic lattices consist of struts, the basic research is focused on those.

Development of a new pressure drop correlation for open-cell foams based completely on theoretical grounds: Taking into account strut shape and geometric tortuosity

Owing to their outstanding geometry and material related properties, open-cell foams as structured packing in reactors, columns or heat exchangers offer clear advantages over their traditional counterparts. For the reliable and efficient design of such systems, an accurate knowledge or prediction of the pressure drop is of fundamental importance, as it plays an important economic role in situations involving high space velocities (high Reynolds numbers). The state-of-the-art pressure drop correlations for open-cell foams are not predictive as they tend to show large deviations from measured data. These deviations are observed mainly due to empirical coefficients in the correlations that have been fitted to a particular data set, but without theoretical meaning these coefficients cannot be extrapolated to other foam structures. In addition, the incorporation of inappropriate definitions (originating from misinterpretation or negligence of important aspects) of the structural and geometrical parameters of foams can also induce a considerable error. In this paper, using the notion of geometric tortuosity and starting from the basic Hagen–Poiseuille equation, a generalized pressure drop correlation for open-cell foams is developed which is based completely on theoretical grounds. The correlation is validated for a wide range of open porosities, pore sizes and materials and its applicability for different working fluids is demonstrated. Furthermore, the definitions of structural and geometrical parameters (as mentioned, these are crucial for pressure drop correlations but are sometimes misinterpreted or neglected) of open-cell foams are reviewed. A consensus on these definitions and their incorporation into the pressure drop correlations is proposed. Consequently, with the proposed correlation, pressure drop in open-cell foams can be predicted by knowing only two structural parameters: the open porosity and the window diameter.