Hollow Strut Research Articles

Ti–6Al–4V hybrid-strut lattice metamaterials: A design strategy for improved performance

Metal hollow-strut lattices manipulate the strut and node sections to achieve higher strength. Although these metamaterials have recently surpassed the structural efficiency of conventional solid-strut lattices of comparable density and topology, they often observe localised yielding and fragmentation through the nodes that limit their structural efficiency. To quantify the effect of this localised failure and to develop mitigating strategies, we have integrated solid and hollow strut and node sections in singular Ti–6Al–4V simple cubic hybrid-strut lattices for a deployable relative density range of 15–30%. Fabricated through laser powder bed fusion these hybrid-strut lattices are stronger (32%), stiffer (15%), and tougher (65%) than solid-strut lattices of equivalent density. Interestingly, the metamaterials with hollow beams can transition from a bending-dominated to a stretch-dominated deformation response with increasing density, while the lattices with solid beams only observed a uniformly stretch-dominated deformation response. The combination of solid and hollow strut and node sections in the hybrid-strut lattice establishes a simple yet effective strategy to enhance structural efficiency and control of failure response without increasing density or manipulating unit cell topology.

Strength Increase of Alumina Foams by a Sodium Aluminosilicate Glass Coating with Subsequent Na+ ↔ K+ Ion Exchange (Gorilla Glass Coating)

Reticulated alumina ceramic foams manufactured by the Schwartzwalder technique are coated with a sodium aluminosilicate glass using an aqueous precursor containing alumina particles and sodium silicate. After a heat treatment, a well‐adhering glass layer with a thickness between 10 and 120 μm is obtained. Besides the strut surface, the glass phase also penetrates the hollow strut cavities as well as strut cracks. The total porosity of the glass‐coated foams is between 88% and 90%, and the open cellular structure is not negatively affected by the glass coating. The compressive strength increases by a factor of two under consideration of the decrease in total porosity; a compressive strength in the order of magnitude of 2 MPa is feasible. An ion exchange of Na+ by K+ into the glass coating layer (chemical strengthening) is successfully demonstrated, but does not result in an additional improvement of the compressive strength.

Tridimensional characterization of open cells and hollow strut cavities from SiC and ZrO2 foams: A study accomplished with open-source software tools

The characterization of ceramic foams via X-ray microtomography imaging method is often restricted to a general overview of samples, either to perform qualitative or quantitative analyses. To assess the foam with a focus on some specific components of its structure, such as hollow struts’ cavities and open cells, the generalized characterizations must be overcome. This work presents image analysis methodologies, based on open-source software, tools, and plugins, to achieve the aforementioned characterizations. SiC and ZrO2 foams samples were analyzed and some of the results were compared with those accomplished with pieces of commercial software. The results show good agreement between open-source and commercial software applications, indicating that the presented methodologies can be freely applied by any researcher to analyze their foams.

Additive manufacturing and performance of bioceramic scaffolds with different hollow strut geometries

Additively manufactured hollow-strut bioceramic scaffolds present a promising strategy towards enhanced performance in patient-tailored bone tissue engineering. The channels in such scaffolds offer pathways for nutrient and cell transport and facilitate effective osseointegration and vascularization. In this study, we report an approach for the slurry based additive manufacturing of modified diopside bioceramics that enables the production of hollow-strut scaffolds with diverse cross-sectional forms, distinguished by different configurations of channel and strut geometries. The prepared scaffolds exhibit levels of porosity and mechanical strength that are well suited for osteoporotic bone repair. Mechanical characterization in orthogonal orientations revealed that a square outer cross-section for hollow struts in woodpile scaffolds gives rise to levels of compressive strength that are higher than those of conventional solid cylindrical strut scaffolds despite a significantly lower density. Finite element analysis confirms that this improved strength arises from lower stress concentration in such geometries. It was shown that hollow struts in bioceramic scaffolds dramatically increase cell attachment and proliferation, potentially promoting new bone tissue formation within the scaffold channel. This work provides an easily controlled method for the extrusion-based 3D printing of hollow strut scaffolds. We show here how the production of hollow struts with controllable geometry can serve to enhance both the functional and mechanical performance of porous structures, with particular relevance for bone tissue engineering scaffolds.

Controlled mechanical and mass-transport properties of porous scaffolds through hollow strut

The rational design of bone implants is relatively complex because they should meet a unique combination of mechanical, mass-transport, and biological properties for favorable performance. However, in conventional topological structures of scaffolds, some important properties are coupled, which hampers the achievement of optimal performance. For example, it is difficult to enhance the stiffness and mass-transport properties of a scaffold simultaneously. This study aims to obtain controlled mechanical and mass-transport properties of scaffolds through adding an inner pore within the strut. The experimental results suggest that permeability and mechanical property were decoupled to a good extent, e.g., the elastic modulus of the four fabricated samples ranges from 1428.6 MPa to 3924.4 MPa, while the counterpart permeability distribution ranges from 11.08 × 10−9 m2 to 12.35 × 10−9 m2 (only 10% difference). Then, combination of the stiffness and permeability was controlled through porosity and inner pore. The simulated results show that permeability is sensitive to both the added inner pore and porosity while elastic modulus depends largely on the porosity, which provides a robust and straightforward approach to tailor the mechanical and mass-transport properties. The hollow-strut structure allows for greater design freedom in the combination of multi-physical properties, which provides a promising basis to the design of high-performance bone implants.

Using ductile cores for enhancing the mechanical performance of hollow strut β-TCP scaffolds fabricated by digital light processing

Hybrid scaffolds consisting of a three-dimensional network of hollow struts of β-tricalcium phosphate (β-TCP) with cores filled with polycaprolactone (PCL) were fabricated. For the development of the ceramic structures the additive manufacturing technique known as Digital Light Processing (DLP) was employed. After sintering, cores were infiltrated with molten PCL by suction. The enhancement of the scaffolds’ compressive strength and toughness upon infiltration was evaluated through uniaxial compression tests. The effect of the core diameter on the mechanical performance was also analyzed. Moreover, a comprehensive finite element analysis was carried out to broaden the study on the effect of core/shell respective diameters and to analyze the role of the modulus of the core material on the strength of the hybrid structure.

Preparation and enhancement of mullite reticulated porous ceramics for porous media combustion

The insufficient strength and thermal shock resistance of mullite reticulated porous ceramics (RPC) limited their application in porous media combustion. To strengthen mullite RPC, the struts with three layers were prepared using the combined approaches of polymeric replica and vacuum infiltration. The effects of residual stress within the struts on thermal shock resistance of mullite RPC were investigated by finite element analysis (FEA). The results showed that the strut was composed of the coating of reaction-bonded mullite, middle layer of mullite skeleton and the triangular filling layer. The triangular voids and surface cracks were eliminated in the formed struts, resulting in a higher strength of mullite RPC than that of hollow strut. In addition, FEA results indicated that the residual compressive stress formed in the coating of strut after mullite RPC was sintered at 1450 °C and 1500 °C, while the residual tensile stress formed at 1550 °C. The larger residual compressive stress within the strut coating was beneficial to enhance the thermal shock resistance of mullite RPC.

Correlative digital image correlation and infrared thermography measurements for the investigation of the mesoscopic deformation behaviour of foams

Open-cell metal/polymer hybrid foams are biomimetic open-porous composites, consisting of a polymeric foam electrochemically coated with a metallic layer. Due to their specific pore structure, they can be used in lightweight applications or as crash absorbers. The nanocrystalline coating leads to a strengthening effect and largely improved energy absorption capacity. Understanding the mesoscopic deformation behaviour of foams is essential for the design of intelligent and safe crash absorbers for the future. The present contribution focuses on the full-field thermomechanical analysis of the mesoscopic deformation of metal/polyurethane hybrid foams and hollow strut foams. Local strain fields from digital image correlation were directly coupled with local temperature fields from infrared thermography. A new software tool allows for the direct correlation of thermal and mechanical fields and for the corresponding experimental stress–strain data. Furthermore, the evolution of the energy absorption capacity as function of the strain rate was determined using infrared thermography.

Refitting of Zirconia Toughening into Open-Cellular Alumina Foams by Infiltration with Zirconyl Nitrate.

The present work describes the combination of the well-established dispersion infiltration of the hollow struts in reticulated porous ceramics (RPCs) and the salt solution infiltration of the remaining strut porosity. This approach is applied on alumina foams, which are loaded subsequently with a dispersion of sub-micrometer alumina particles and a ZrO(NO3)2 solution. The zirconyl nitrate is converted into a ZrO2 transformation toughening phase during the final sintering step. As a consequence of the complex microstructure evolution during the consecutive infiltration cycles, the reinforcement phase concentrates selectively at the weak spots of RPC structures—namely, the hollow strut cavities and longitudinal cracks along the struts. As a consequence, a severe improvement of the compressive strength is observed: The average compressive strength, normalized to a porosity of 91.6 vol.%, is 1.47 MPa for the Al2O3/ZrO2 infiltrated foams, which is an improvement by 40% with respect to alumina-only loaded foams (1.05 MPa) or by 206% compared to uninfiltrated alumina RPCs (0.48 MPa). The compressive strength results are correlated to infiltration parameters and the properties of the infiltration fluids, for example the rheological behavior and the size of the Zr solute species in the respective ZrO(NO3)2 solution.

Selective Laser Melting Strategy for Fabrication of Thin Struts Usable in Lattice Structures

This paper deals with the selective laser melting (SLM) processing strategy for strut-lattice structure production which uses only contour lines and allows the porosity and roughness level to be managed based on combination of the input and linear energy parameters. To evaluate the influence of a laser scanning strategy on material properties and surface roughness a set of experiments was performed. The single welds test was used to find the appropriate processing parameters to achieve continuous welds with known width. Strut samples were used to find a suitable value of weld overlapping and to clarify the influence of input and linear laser energy on the strut porosity and surface roughness. The samples of inclined hollow struts were used to compare the wall thickness with single welds width; the results showed about 25% wider welds in the case of a hollow strut. Using the proposed SLM strategy it is possible to reach a significantly lower porosity and surface roughness of the struts. The best results for struts with an inclination of 35.26° were achieved with 25% track overlapping, input energy in the range from 9 J to 10.5 J and linear energy Elin from 0.25 to 0.4 J/mm; in particular, the relative density of 99.83% and the surface roughness on the side of the strut of Ra 14.6 μm in an as-built state was achieved.

Enhancement of mechanical properties of hollow-strut foams: Analysis

Hollow-strut foams are predicted to have superior mechanical properties compared with solid-strut foams of the same relative density. Due to the enlarged bending stiffness of the hollow strut, the enhancements of stiffness, buckling strength, plastic collapse strength, and brittle failure strength and fracture toughness were substantial (even an order of magnitude) according to the analysis and comparisons. The hollow-strut foam is much more damage tolerant than the solid-strut foam. The enhancement of the robustness to waviness influence is dramatic. The stiffness enhancement of the wavy hollow-strut foam will exceed an order of magnitude. With hollow struts, foams have the comparable (even stiffer and stronger) mechanical properties compared with stretching dominated lattice truss materials.

Failure modes in open-cell foams with hollow struts

Failure modes in open-cell foams with hollow struts are analyzed. As the walls of the struts become thin, the failure mode of the foam can change from plastic collapse of struts loaded in bending to plastic collapse of struts loaded in direct compression. For very thin strut walls an elastic strut wrinkling failure mode is possible. Expressions for the failure modes are derived, and a failure mode map is presented to clarify when the various modes are expected. The analysis suggests that a hollow strut foam can be substantially stronger than a solid strut foam with the same relative density.

Struts of Minimum Weight

IT is intended to show in this article the magnitude of the weight saving which can be achieved on simple struts. It is necessary for any comparison to derive the weight and shape of the solid or hollow strut theoretically most efficient. This can be done most conveniently using the Calculus of Variations and it will be assumed in all cases that the strut is of such a length that bending stresses predominate at the instant of failure and that the ends of the strut are rotationally free.