Mechanical Isotropy Research Articles

Supercooling-Driven Homogenization and Strengthening of Hydrogel Networks.

By leveraging principles from metal grain refinement, we introduce a transformative technique for fabricating poly(vinyl alcohol) (PVA) hydrogels via supercooling-coupled wet annealing, significantly enhancing their mechanical robustness and isotropy while maintaining their exceptionally high water content. Our methodology involves the dissolving PVA in water at elevated temperatures, mirroring the homogeneity achieved with a molten metal, in order to ensure a uniform distribution of polymer chains. This uniformity facilitates a rapid cooling phase that generates ultrafine ice crystals, setting the stage for a crucial solvent exchange with ethylene glycol (EG). The EG-mediated supercooling technique ensures the polymer homogeneity and structure integrity and induces the PVA chains to aggregate and form high-density hydrogen bonds, leading to a uniformly distributed, interconnected PVA network with high crystallinity. The process is further strengthened by EG-enabled wet annealing, which promotes the formation of densely packed crystalline domains within the polymer network. This rigorous process yields PVA hydrogels with superior mechanical properties, including a tensile strength of 13.65 MPa and a fracture toughness of 35.39 MJ m-3, alongside remarkable water content nearing 80%. These advances not only surpass the capabilities of conventional hydrogels but also broaden their application potential, highlighting the innovative integration of supercooling principles in polymer science.

Investigation of the rheological behaviour for improved extrusion-based additive manufacturing of polymer composites based on dissociative dynamic covalent bonds

More complex 3D structures were manufactured out of reversible covalent polymer networks using multi-material extrusion-based additive manufacturing. The rheological behaviour of reversible polymer networks based on the thermoreversible Diels-Alder reaction was optimized through the addition of different types of nanoclays and carbon black. The extrudability and deposition of the composites improved up to 5 wt% nanoclay, as the viscosity behaviour increased by an order of magnitude in the nozzle and by two orders of magnitude at the print bed temperature, without affecting the reversible gel transition temperature and the healing performance. An electrically conductive composite with 20 wt% carbon black and 1 wt% nanoclay could be printed using nozzle sizes down to 0.3 mm, resulting in higher resolution and accuracy than the pristine polymer networks. The percolating filler network enabled the printing of overhangs and hollow structures without the need for support material with perfect mechanical and electrical isotropy. Multi-material printing combining the electrically conductive and non-conductive composites enabled manufacturing self-healing deformation and force sensors that could recover their sensing performance upon damage healing at 90 °C for one hour.

Powder-based 3D printed magnesium phosphate cement: Mechanical isotropy optimization using borax

Mechanical isotropy is essential for improving the integrity and application of 3D printed structures. In this paper, based on the high bonding, fast-hardening, and early strengthening characteristics of magnesium phosphate cement (MPC), the exothermic rate of MPC hydration reaction is optimized by controlling the content of borax (0.5%-3%) to improve the adhesive integrity for the interlayer interface of powder-based 3D printed MPC. In addition, the physical phase components, microscopic morphology, degree of hydration, and pore structure of borax-modified powder-based 3D printed MPC are analyzed by using various microscopic characterization methods of XRD, SEM, thermal analysis, and X-CT. Experimental findings demonstrate that borax, through its influence on the hydration environment and encapsulation of the hydration film, reduces the exothermic rate of the MPC reaction, thereby enhancing the structural integrity with improved mechanical isotropy. Printed MPC specimens with 2% borax content reach compressive strengths of 10.45–11.39 MPa at 28d in the X/Y/Z directions, with an optimum mechanical isotropy index of 3.6%. Furthermore, microscopic analysis reveals that the 2% borax-modified MPC displays a denser morphology, with the main hydration product struvite enhanced by 8.53% compared to the 0% specimen. The interlayer interfaces of the printed MPC demonstrate exceptional qualities, being small and homogeneous, with 4.20% less total porosity than the unmodified group.

The Effect of the Deposition Strategy and Heat Treatment on Cold Spray Additive Manufactured 316L Stainless Steel

Herein, the effect of heat treatment on the characteristics and properties of cold spray additive manufactured 316L stainless steel employing traditional and a new metal knitting strategy is investigated. 316L feedstock powder characteristics, the geometry of the bulk, microstructure, porosity, microhardness, mechanical isotropy, and residual stress are analyzed in both strategies in as‐sprayed and heat‐treated conditions. Results show that the traditional deposition strategy produced higher mechanical resistance, whereas metal knitting presents a better part geometry accuracy. The heat treatment significantly improves the material strength and the quality of the parts by recovery and recrystallization phenomena. The same microhardness and planar isotropy are achieved after heat treatment of samples produced by both strategies. A discussion about the mechanisms, microstructural, and residual stress evolution is presented.

3D Printed Architectured silicone composites containing a UV-curable rheological modifier with tailorable structural collapse

3D printed silicones, combining the unique physiochemical performances of silicones with the attractive design freedom of additive manufacturing, have aroused intensive interest from academia and industry owing to their attractive applications. Direct writing (DW) was the most used method to enable the 3D printing of silicones, but it was limited by its conflicting rheological requirements. Herein, we present novel design of an ultraviolet (UV)-thermal dual-cure ink for UV assisted DW of silicone composites. A UV curable rheological modifier was key to the ink design, which would not only increase the ink viscosity and yield stress substantially at low contents, but also further enhance its shape-retaining capability during printing by UV-assisted curing. As a result, challenging structures with good mechanical performances and isotropy were printable. Interestingly, the degree of structural collapse for the product could be controlled by tailoring the printing parameters, which proved to be a new design dimension for property manipulation of 3D-printed cellular architectures. Rheology of the inks and boundary conditions for structural collapse were discussed in detail. This work will open new opportunities for the development of 3D-printed silicones with customized architectures and tailored performances.

On the mechanical properties of dual-scale microlattice of starfish ossicles: A computational study

While engineering cellular solids, either stochastic foams or architected lattices, are usually made of polycrystalline or amorphous materials, echinoderms (e.g., sea urchins, starfish, and brittle stars) build their microporous skeletal elements with single-crystalline calcite. This class of biomineralized cellular solid, known as stereom, also exhibits a vast diversity in pore morphology, ranging from random open-cell foams to fully periodic lattices. In particular, the skeletal elements (also known as ossicles) of some starfish species are composed of a diamond-triply periodic minimal surface (diamond-TPMS) microlattice. In addition, the crystallographic symmetries at both atomic (calcite) and lattice (diamond-TPMS) levels are precisely aligned. Here we investigate the mechanical performance of this unique dual-scale microlattice and discuss the synergistic effects of atomic- and lattice-scale crystallographic coalignment. Our computational and theoretical analysis suggests that the mechanical isotropy of ossicles is enhanced due to the property compensation between the atomic-level and lattice-level architectures. Moreover, the observed 50 vol% relative density in the diamond-TPMS microlattice of ossicles may be a result of achieving overall mechanical isotropy, minimal surface curvature, and maximized surface area. We believe the methodology introduced here will be useful for understanding the mechanical behavior of natural crystalline cellular solids such as echinoderms’ stereom and designing dual-scale lattice materials.

Selective Laser Sintering versus Multi Jet Fusion: A Comprehensive Comparison Study Based on the Properties of Glass Beads‐Reinforced Polyamide 12

Selective laser sintering (SLS) and multi jet fusion (MJF) are the most widespread powder bed fusion additive manufacturing techniques for fabricating polymeric parts since they offer great design flexibility, productivity, and geometrical accuracy. However, these technologies differ in the thermal energy source used to melt the powders as well as the innovative use of printing agents featured in the latter one to promote material consolidation and to avoid thermal bleeding at the part contours. The use of a single powder made of glass beads‐reinforced polyamide 12 (PA12/GB) for the fabrication of MJF and SLS samples makes possible a systematic comparison of the printed parts properties. A thoughtful analysis of the microstructure and mechanical properties of the samples reveals differences and peculiarities between the two technologies. SLS exhibits lower porosity and higher mechanical performances when the parts are printed along the build plane thanks to the powerful heating ensured by the laser. In contrast, MJF samples show higher mechanical isotropy with great flexural and tensile behavior for vertically oriented parts. The role of glass beads in the material behavior is defined by their mechanical properties, meaning higher rigidity and lower strength compared to neat PA12, and fracture mechanism.

Ultraviolet-assisted material extrusion of silicones with largely enhanced mechanical properties and isotropy

3D printed silicone elastomers, combining the favorable physiochemical performances of silicones with the unparalleled advantages of additive manufacturing, have gained significant attention from academia and industry owing to their attractive applications. However, there were several limitations in their existing printing techniques, such as the poor mechanical robustness of the products by vat polymerization and the conflicting rheological requirements for material extrusion. Herein, we report 3D printing of an ultraviolet (UV)-induced, platinum-catalyzed hydrosilylation silicone composite via UV-assisted material extrusion. It was found that UV assistance could enhance the shape-retaining capability during printing, and the achieved materials presented remarkably enhanced mechanical properties by almost an order of magnitude compared to other UV-cured silicones for 3D printing. The material was UV-curable and heat-curable, which allowed for manipulating the tensile, tearing, and relaxation properties by adjusting the curing technique. Moreover, the UV-cured silicone composites exhibited much enhanced mechanical isotropy compared to their heat-cured counterparts, indicating the potential to minimize the intrinsic hurdle of anisotropy for 3D printed products. Structural origins for the excellent properties were discussed in detail. This work will open new opportunities for developing high-performance 3D-printed silicones for engineering applications.

Multi-objective generative design of three-dimensional material structures

Generative design for materials has recently gained significant attention due to the rapid evolution of generative deep learning models. There have been a few successful generative design demonstrations of molecular-level structures with the help of graph neural networks. However, in the realm of macroscale material structures, most of the works are targeting two-dimensional, ungoverned structure generations. Hindered by the complexity of 3D structures, it is hard to extract customized structures with multiple desired properties from a large, unexplored design space. Here we report a novel framework, a multi-objective driven Wasserstein generative adversarial network (WGAN), to implement inverse designs of 3D structures according to given geometrical, structural, and mechanical requirements. Our framework consists of a WGAN-based network that generates 3D structures possessing geometrical and structural features learned from the target dataset. Besides, multiple objectives are introduced to our framework for the control of mechanical property and isotropy of the structures. An accurate surrogate model is incorporated into the framework to perform efficient prediction on the properties of generated structures in training iterations. With multiple objectives combined by their weight and the 3D WGAN acting as a soft constraint to regulate features that are hard to define by the traditional method, our framework has proven to be capable of tuning the properties of the generated structures in multiple aspects while keeping the selected structural features. The feasibility of a small dataset and the scalability of the objectives of other properties make our work an effective approach to provide fast and automated structure designs for various functional materials.

Improving the strength of β-TCP scaffolds produced by Digital Light Processing using two-step sintering

Digital Light Processing is combined with two-step sintering to obtain bioactive scaffolds with improved strength and mechanical isotropy. Highly loaded photosensitive suspensions were prepared from β-TCP powder to create scaffolds consisting of interpenetrating struts with two different designs. Two sintering methods were used: conventional sintering (CS) and two-step sintering (2SS). The latter resulted in a microstructure with uniformly shaped grains and reduced porosity. Their compressive strength was determined by uniaxial testing under two different load configurations, with the force applied parallel or perpendicular to the building plane of the scaffolds. Design optimisation and fine-tuning of the sintering process helped in reducing the presence of interlayer defects and minimise the shear-dominated fractures. Isotropic fracture behaviour was achieved, with similar central values of the Weibull distribution (49 ± 1 MPa vs. 51 ± 1 MPa) along both testing directions, showing a great potential for their use in load-bearing bone tissue engineering applications.

An anisotropic conductive hydrogel for strain sensing and breath detection

Conductive hydrogels, with their excellent conductivity and flexibility, are highly suitable for use as strain sensors. However, they often exhibit weak mechanical properties and isotropy, which can limit their application potential. Moreover, the incorporation of self-powering capabilities within hydrogels would provide a significant advantage for sensor applications. In this study, we utilized hydrophilic delignified wood as a scaffold and impregnated it with FeCl3 solution to fabricate conductive wood. Subsequently, a precursor solution of polyvinyl alcohol (PVA), chitosan (CS), and 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) was prepared by heating and mixing. The conductive wood was then fully infiltrated with the precursor solution and transformed into a conductive wood-based hydrogel (CWH) using a freeze-thaw method. The aligned fibers in the conductive wood scaffold imparted anisotropic mechanical and electrical properties to the hydrogel, with a fracture stress anisotropy ratio of 47.5 (ratio of values parallel to the fiber direction to values perpendicular to the fiber direction) and an electrical conductivity anisotropy ratio of 1.55. The hydrogel remained soft after bonding with the wood scaffold, preserving its potential as a sensor. Experimental results demonstrated that the CWH sensor exhibited stable strain sensitivity (gauge factor of 0.2805 with R²=0.998 in the range of 0∼90°), enabling the monitoring of human joint bending and capturing facial smiles. The CWH also exhibited excellent resistance to expansion while maintaining high water content (79.4%), making it more suitable for use in humid environments compared to traditional hydrogels. Furthermore, we assembled the CWH into a humidity-driven generator and conducted power generation tests, revealing its promising power generation potential. The voltage significantly increased when exposed to humid air (from 6.2 mV to 66.1 mV). Lastly, we integrated the humidity-driven generator with a mask to create a breath detector, which was connected to an electrochemical workstation for testing. The test curve demonstrated the successful capture of orderly breathing activities from volunteers. The combination of these advantages positions CWH for a wide range of applications, including wearable flexible strain sensors, breath detection, bioelectrodes, and green energy generation devices.

Improving mechanical properties and isotropy of laser DED 17-4 PH stainless steel by combining ultrasonic vibration

Nanoscale Cu-rich precipitates (CRPs) are important to achieve high strength for the extensively applicated 17-4 precipitation hardening (PH) stainless steel (SS). Traditionally, the precipitation of CRPs requires high-temperature and long-time heat treatments. In this work, ultrasonic vibration (UV) was introduced into the laser direct energy deposition (L-DED) process to fabricate 17-4 PH deposits, a large density of nanoscale CRPs was directly and uniformly precipitated during depositing, the microstructure was also improved. Consequently, the mechanical properties of the deposit, both at the vertical and horizontal directions, were enhanced remarkably compared with those of the deposit manufactured by normal DED. Especially, the elongation was improved significantly, reached to around 20%. Moreover, the isotropy of mechanical property was improved, all the counterpart mechanical properties of the UV-DED sample were nearly close. This investigation supplied a potential method to mitigate the post heat treatments during fabricating 17-4 PH with L-DED.

Multifunctional mechanical metamaterials with tunable double-negative isotropic properties

This research was focused on innovative design of lattice metamaterials that can exhibit tunable double-negative mechanical properties and elastic isotropy simultaneously. A discrete topology optimization method using a multi-material ground structure was developed to create microlattices exhibiting both negative thermal expansion coefficient and negative Poisson’s ratio in a single integrated design, while maintaining elastic isotropy. First, the numerical homogenization method with beam elements was used to estimate the effective thermal and elastic properties of a microlattice. Second, the topological design, subject to required geometric constraints, was formulated as a mixed integer programming problem to discover a series of multi-material microlattices that present customized isotropic values of negative thermal expansion coefficient and negative Poisson’s ratio. Finally, several three-dimensional multi-material microstructures were produced by altering either the cross-sections or constituent materials of struts to demonstrate their tunable mechanical properties.

Overcoming the strength-ductility trade-off and anisotropy of mechanical properties of Ti6Al4V with electron beam powder bed fusion

Achieving a combination of excellent ductility and mechanical property isotropy is highly desirable for conventional biomedical materials, although this has been a long-standing problem. Here, we present a novel approach, in situ alloying of Cu (3.5 wt%) into Ti6Al4V through electron beam powder bed fusion (EB-PBF), through which the aforementioned issues can be addressed. The ultimate tensile strength (UTS) and elongation (EL) to failure of the Ti6Al4V-3.5Cu (TAVC) alloy fabricated by EB-PBF were found to reach values of 1053 MPa and 15.3%, respectively, which are markedly superior to those of the PBF-EB/Ti6Al4V (UTS: 902 MPa; EL: 13.5%). However, an alloy with identical composition made by spark plasma sintering exhibited a significant brittleness characteristic (UTS: 1051 MPa; EL). Moreover, the generation of multiple micro/nanosized Ti2Cu phases (1–5 μm, 50–100 nm, and 5–10 nm) in the TAVC alloy samples is thought to contribute a significant strengthening (46.24%), which effectively prevents plastic deterioration caused by brittle lamellar (Ti2Cu + α-Ti) structures. Further investigation revealed that the ductile enhancement of the TVAC alloy can be attributed to the microstructural evolution that encompasses the generation of twin structures and spheroidization of α-Ti and results from the addition of Cu coupled with the intrinsic heat treatment during the EB-PBF manufacturing process. Our findings not only provide an in-depth understanding of the formation mechanism of multi-scale Ti2Cu precipitates but also offer critical guidance for improving the overall mechanical properties of Cu-containing biomedical titanium alloys.

Novel isotropic mechanical properties of laser powder-bed fusion Sc/Zr modified Al alloy

To explore the mechanical anisotropy of high-strength Al alloys for laser powder bed fusion (LPBF) additive manufacturing, the Al–Mn–Mg-Sc-Zr alloys were built along X, Y, Z axial and 45° directions by LPBF subjecting to a series of different heat treatments. All the printed and heat treated components displayed a columnar/equiaxed bimodal microstructure, where the secondary Al6Mn and nano-size Al3Sc precipitates ubiquitously dispersed in the columnar and equiaxed grain regions, respectively. The Al–Mn–Mg-Sc-Zr alloy exhibited a continuous strength increase from 453 MPa to 577 MPa after the heat treatment through the synergistic effect of the solid solution strengthening, grain boundary strengthening and Orowan dislocations strengthening from Al6Mn as well as Al3Sc precipitates. But its ductility suffered from a decrease from 14.1% to 2.3%. Especially, the optimal collaboration between hard equiaxed grain and soft columnar grain contributed to the mechanical properties isotropy of the printed and heat-treated alloy components along different tensile directions with strength difference only 1.3–2%. Thus, the synergetic effects of hierarchical bimodal microstructure and secondary precipitations could provide the effective avenues to tune the mechanical property and anisotropy of LPBF Al alloys.

Composite alkali-activated materials with waste tire rubber designed for additive manufacturing: an eco-sustainable and energy saving approach

There is an increasing trend in research projects and case studies to demonstrate the potential of Additive Manufacturing (AM) with concrete, better known as 3D concrete printing. Like ordinary construction, the latest upgrades on this topic are strongly focused towards improving eco-sustainability in terms of low-carbon materials. Low-carbon binders’ alternative to Portland cement and the utilisation of selected waste materials in place to virgin aggregates has high potential in fulfilling the sustainable development goals. In this paper, an experimental study was performed by incorporating ground waste tire rubber aggregates of different size gradation (0–1 mm and 1–3 mm) and replacement levels (50 v/v% and 100 v/v%) in a “greener” alkali-activated mix designed for 3D printing applications. First, the experimental program involved the optimization of mix design rheology and printing parameters to successfully integrate rubber aggregates into the printable alkali-activated mixtures. Then, a comprehensive characterization, including static mechanical testing, dynamic thermo-mechanical analysis, thermal conductivity testing, and acoustic insulation measurements was conducted. Comparison with identical Portland-based rubberized formulations designed for AM revealed better mechanical isotropy, flexural strength, thermo-mechanical behaviour, heat insulation, and high-frequency acoustic insulation for alkali-activated composites. The influence of rubber aggregate size on the fresh and hardened state behaviour of the mixes was also studied and discussed. Keeping the losses in mechanical strength restrained, the rubberized composites designed in this study have demonstrated significant thermal and acoustic insulation properties that are desired for energy-saving applications in buildings. The research verified the practicability of using waste aggregates in low-carbon binders for sustainable lightweight and thermo-acoustically effective applications, establishing an attractive starting point to address future research on material optimization for practical purposes.

Investigation of the resin infusion process and mechanical performance of carbon fiber reinforced plastic composites from recycled carbon fiber

The disposal of Fiber Reinforced Composites (FRCs) waste is becoming a very critical aspect, from both a technical and an economic point of view, due to the continuous increase of the production of such a class of materials. Thus, the possibility to produce new composites by employing recycled materials is a crucial aspect both to guarantee the circularity of the manufacturing processes and the reduction of costs. In this work, the manufacturing process, together with the performance of a carbon fiber-reinforced plastic (CFRP) laminate obtained by employing into sandwich structures a novel commercial recycled non-woven carbon fabric with an epoxy resin, have been investigated. The fabrication process based on the Resin Infusion under Flexible Tooling (RIFT) has been selected and performed in different conditions. The mechanical behavior of the resulting laminates has been investigated by quasi-static tensile, and flexural characterization, to evaluate both the optimal conditions for the process, and the possible anisotropy of the product. Moreover, thermogravimetric analysis (TGA) was applied to complete the post-processing characterization of the resulting laminate by determining the actual carbon fibers (CFs) content, thus, to validate the reproducibility of the manufacturing process. The laminates obtained by the RIFT method exhibited good mechanical properties and isotropy if cross-ply stratification is adopted. The use of recycled CFs allows high sustainability and reduced cost for the composites, making the RIFT method investigated a scalable process and the so-manufactured products a valid candidate for numerous applications.

Elimination of in-plane mechanical anisotropy and enhancement of strength-ductility of rolled Mg sheets via electric-associated twinning and recrystallization

A novel method referred as electric-associated corrugated wide limit alignment (E-CWLA) was proposed to address the contradiction between strength and plasticity of hot-rolled Mg sheets. The counterpart without electricity was known as CWLA. A huge number of tensile twins (TTs) was induced in CWLAed sample R300, and dynamic recrystallization (DRX) occurs. Strong transverse direction (TD)-titled texture accounts for the huge difference in yielding strength between different loading directions, which are closely related to Schmid factors of prismatic <a> and pyramidal <c+a> slips, resulting in different ultimate tensile strength (UTS) and ductility of R300 along different directions. E-CWLAed sample E300 presents good in-plane mechanical isotropy with the maximum UTS of 272.1 MPa, yielding strength of 213.6 MPa and elongation of 22.7%, increasing by 32%, 53% and 83% compared with the as-received sheets, respectively. Regardless loading direction, basal and non-basal slips possess similar SFs and greatly facilitates uniform plastic deformation and mechanical isotropy of E300. The special microstructure including lots of fine grains, some large grains and TTs results in a bimodal basal texture with very weak TD-titled component promotes its good deformation compatibility. Electricity contributes little to TTs formation of E300, and its thermal effect promoting dislocation mobility and DRX and the athermal effect increasing DRX nucleation rate are responsible for final grain size distribution.

Achieving synergy of mechanical isotropy and tensile properties by constructing equiaxed microstructure in as-printed Ti alloys

Achieving synergy of mechanical isotropy and tensile properties by constructing equiaxed microstructure in as-printed Ti alloys

Geometry effect on mechanical properties and elastic isotropy optimization of bamboo-inspired lattice structures

Inspired by the geometry of bamboo, this study proposes a novel bamboo-inspired body-centered cubic (B-BCC) lattice structure consisting of tapered and hollow struts. Using representative volume elements applied with periodic boundary conditions, the mechanical properties and deformation behaviors of the B-BCC lattice structures are thoroughly evaluated by considering a large number of combinations of geometric parameters and volume fractions. Results reveal that the geometric parameters highly influence the deformation behavior of the B-BCC lattice structures under uniaxial compression (e.g, from bending- to stretching-dominated) but little under shear load. For this reason, tunable elastic modulus across a broad range can be realized via adjusting the geometric parameters and elastic isotropy can be obtained across all volume fractions. On this basis, a combination of artificial neural network and elastic isotropy optimization is proposed to obtain the isotropic B-BCC lattice structures with superior elastic modulus. The optimization results show that the elastic modulus of the isotropic B-BCC lattice structures increased by 271.24–1335 % and 17.72–43.63 %, as compared to the original BCC and isotropic hollow BCC lattice structures, respectively. Finally, the multi-layer simulation and compression experiments are applied to validate the optimization results. Good agreements are observed comparing the numerical and experimental results, demonstrating the effectiveness of the proposed bamboo-inspired design and optimization method for lightweight applications with desired properties.