Preparation of porous PLLA/PCL blend by a combination of PEO phase and NaCl particulate leaching in PLLA/PCL/PEO/NaCl blend

In this study, the ternary blends containing microporosity based on poly(l-lactic acid) (PLLA), poly(ε-caprolactone) (PCL) and polyethylene oxide (PEO) were prepared using an internal mixer via a polymer leaching technique. The particulate leaching is the most widely used technique to create porosity. To introduce macroporosity besides micropores, NaCl particulates were incorporated into the ternary blends at 40–80 wt % and macropores were formed by particulate leaching. Samples porosity were evaluated by calculating the ratio of porous scaffold density (ρ*) to the non-porous material density (ρ _s). The results showed that with an increase in NaCl particulate content, the amount of porosity increased and the distribution of pore size was gradually transformed from monomodal into bimodal form. The porosity plays a key role in governing the compression properties. Mechanical properties are presented by Gibson–Ashby model. Compressive modulus decreased with an increase in NaCl particulate concentration due to the increase in porosity and thinning of pore wall that caused rupture at these weaker spots. Blending and forming of the bio-scaffold can be made using conventional polymer processing equipment. This process seems promising for a large-scale production of porous bio-scaffold of many sizes through an economic method.     

Anisotropy Effects in the Shape-Memory Performance of Polymer Foams

Isotropic and anisotropic shape-memory polymer foams are prepared by supercritical carbon dioxide foaming from a multiblock copolymer (PDLCL) consisting of poly(ω-pentadecalactone) and poly(ε-caprolactone) segments. Analysis by micro-computed tomography reveals for the anisotropic PDLCL foam cells a high shape anisotropy ratio of R = 1.72 ± 0.62 with a corresponding Young's compression moduli ratio between longitudinal and transversal direction of 4.3. The experimental compression data in the linear elastic range can be well described by the anisotropic open foam model of Gibson and Ashby. A micro-morphological analysis for single pores using scanning electron microscopy images permits the correlation between the macroscopic stress-compression behavior and microscale structural changes.     

Study on the mechanical properties of porous tin oxide

Despite the importance of tin oxide (SnO₂) in diverse functional applications, little information is available on the mechanical properties of bulk or porous SnO₂. In this study, porous SnO₂ was synthesized using an ice-templating method to produce a “dual” pore structure that comprises large wall pores (on the order of several micrometers) with small micropores (~2 µm) on their surfaces. The Vickers hardness decreased with increasing porosity and increased with increasing contiguity of struts. The compressive stress–strain curves of porous SnO₂ samples with porosity ranging from 48% to 73% were compared with both the Gibson–Ashby and the cellular-lattice-structure-in-square-orientation models, which generally represent the “lower” and “upper” bounds of yield strength for porous materials, respectively. As expected, the yield strength of the porous SnO₂ samples decreased with increasing porosity, and all the yield-strength values of porous SnO₂ fell between the two extreme prediction models. The sample with the lowest porosity of 48% exhibited sharply increasing elastic behavior followed by sudden rupture, as generally reported for bulk ceramics; however, the other samples with higher porosities ranging from 50% to 73% exhibited “porous-metal-like” behavior at strains of 15% or greater as a result of the fracturing of the solid walls between the pores.     

Compressive properties of pristine and SiC-Te-added MgB2 powders,green compacts and spark-plasma-sintered bulks

Pristine and (SiC+Te)-added MgB₂ powders, green and spark plasma sintered (SPS) compacts were investigated from the viewpoint of quasi-static and dynamic (Split-Hopkinson Pressure Bar, SHPB) compressive mechanical properties The amount of the additive (SiC+Te) was selected to be the optimum one for maximization of the superconducting functional parameters. Pristine and added MgB₂ show very similar compressive parameters (tan δ, fracture strength, Vickers hardness, others) and fragment size in the SHPB test. However, for the bulk SPSed samples the ratio of intergranular to transgranular fracturing changes, the first one being stronger in the added sample. This is reflected in the quasi-static K_IC that is higher for the added sample. Despite this result, sintered samples are brittle and have roughly similar fragmentation behavior as for brittle engineering ceramics. In the fragmentation process, the composite nature of our samples should be considered with a special focus on MgB₂ blocks (colonies) that show the major contribution to fracturing. The Glenn-Chudnovsky model of fracturing under dynamic load provides the closest values to our experimental fragment size data.     

Low density biomorphic silicon carbide: microstructure and mechanical properties

Low density biomorphic SiC ceramics have been fabricated by melt infiltration of pine carbon templates. The resulting porous microstructure is anisotropic and presents a high level of connectivity. Some templates have been partially infiltrated to document the details of the infiltration process. The material has been tested at a constant compression with a strain rate of 2×10⁻⁵ s⁻¹ at high temperatures between 1150 and 1350 °C. The materials showed high compressive strengths relative to the density, with values up to 241 MPa in the axial direction. The compressive strengths relative to those of fully dense SiC are a good extrapolation of previous studies with denser samples, and present qualitative agreement with the model by Gibson and Ashby for plastic collapse of cellular solids.     

Dynamic compressive response of zirconium diboride-silicon carbide composites at high-strain rates

The quasi-static, dynamic compression experiments and micromechanical model were employed to declare the dynamic compressive response of ZrB₂–20%SiC composite at high-strain rates. The quasi-static compressive strengths were measured to determine the range of initial microcrack length in ZrB₂–20%SiC composite. The effects of the strain rate on dynamic compressive strength, critical stain, as well as fracture mechanisms were discussed based on experimental results. Dynamic mechanical properties of ZrB₂-based composites display obvious strain rate dependence. The dynamic increase factor in the compressive strength shows a rapid increase above a transition strain rate of 1228 s⁻¹. Moreover, a micromechanical model considering initial microcrack lengths is used to predict dynamic compressive strengths, which agree with the experimental results. Additionally, the critical strain has a linear increase tendency with the increase of strain rate. The dynamic compressive fracture mechanism of ZrB₂–20%SiC composite is relative to the combination effect between strain rate and microstructure. The size of flaw distribution is critical below the transition strain rate resulting in bigger fragments, whereas the flaw density is primary with more and smaller fragments above the transition strain rate.     

Mechanical modeling of silk fibroin/TiO2 and silk fibroin/fluoridated TiO2 nanocomposite scaffolds for bone tissue engineering

Biocompatible and biodegradable three-dimensional scaffolds are commonly porous which serve to provide suitable microenvironments for mechanical supporting and optimal cell growth. Silk fibroin (SF) is a natural and biomedical polymer with appropriate and improvable mechanical properties. Making a composite with a bioceramicas reinforcement is a general strategy to prepare a scaffold for hard tissue engineering applications. In the present study, SF was separately combined with titanium dioxide (TiO₂) and fluoridated titanium dioxide nanoparticles (TiO₂-F) as bioceramic reinforcements for bone tissue engineering purposes. At the first step, SF was extracted from Bombyx mori cocoons. Then, TiO₂ nanoparticles were fluoridated by hydrofluoric acid. Afterward, SF/TiO₂ and SF/TiO₂-F nanocomposite scaffolds were prepared by freeze-drying method to obtain a porous microstructure. Both SF/TiO₂ and SF/TiO₂-F scaffolds contained 0, 5, 10, 15 and 20 wt% nanoparticles. To evaluate the efficacy of nanoparticles addition on the mechanical properties of the prepared scaffolds, their compressive properties were assayed. Likewise, the pores morphology and microstructure of the scaffolds were investigated using scanning electron microscopy. In addition, the porosity and density of the scaffolds were measured according to the Archimedes’ principle. Afterward, compressive modulus and microstructure of the prepared scaffolds were evaluated and modeled by Gibson–Ashby’s mechanical models. The results revealed that the compressive modulus predicted by the mechanical model exactly corresponds to the experimental one. The modeling approved the honeycomb structure of the prepared scaffolds which possess interconnected pores.

     

Influence of carbon nanotubes as secondary phase addition on the mechanical properties and amorphization of boron carbide

The mechanical properties and amorphization response of a carbon nanotube (5 wt.%) boron carbide (CNT-B₄C) composite with 1 μm grain size are investigated, and compared to those of coarse-grained (10 μm grain size) and ultrafine-grained (0.3 μm grain size) monolithic boron carbides. The quasi-static and dynamic uniaxial compressive strengths for CNT-B₄C were statistically the same as those of the ultrafine-grained ceramic and higher than the coarse-grained material, contradicting the expected grain size hierarchy (Hall-Petch-type relationship). Addition of CNTs to B₄C resulted in decreased quasi-static hardness compared to the large grain size material; however, dynamic hardness was substantially improved compared to quasi-static values. CNT pullout and crack bridging were observed to be possible toughening mechanisms. Finally, Raman spectroscopy was used to quantify amorphization, and it was concluded that addition of CNTs to boron carbide does not alter the propensity for amorphization, but does improve mechanical properties by enhanced toughening.     

Crushing behavior of alumina inclusions with different porosity during hot compression

Alumina inclusions in commercial as-cast 2.25Cr1Mo0.25V aluminum deoxidized steel exhibited a feature of porous structure. In order to investigate the crushing characteristics of alumina inclusion during hot working, a series of alumina blocks with different porosity whose properties are similar to the alumina inclusions in ingots were prepared using spark plasma sintering. The crushing behavior of alumina blocks during hot compression with quasi-static load was studied. A prediction model of compressive strength of alumina inclusions considering apparent porosity was established on basis of hyperbolic sine Arrhenius equation. A novel crushing mode diagram for alumina inclusions characterized by Z parameter was proposed. The crushing mechanism of alumina inclusions under different deformation parameters was clarified by fracture characteristics. The results showed that the hot compression process of alumina presented a typical brittle fracture, the compressive strength was more sensitive to deformation conditions at lower apparent porosity as compared with the conditions of higher apparent porosity. With the increase of Z, the crushing mode of alumina inclusions gradually changed from intergranular fracture to transgranular fracture.     

Compressive properties and fracture behavior of ceramic fiber-reinforced carbon aerogel under quasi-static and dynamic loading

The quasi-static and dynamic compressive properties of a ceramic fiber-reinforced carbon (CFRC) aerogel were investigated using a universal test machine and a split Hopkinson pressure bar. The fracture surface of the CFRC aerogel was studied by scanning electron microscopy. Results show that the compressive behavior of CFRC aerogel exhibits a significant strain rate strengthening effect. The quasi-static failure strain is higher than the dynamic failure strain. Under quasi-static compressive loading, the carbon aerogel matrix breaks into small pieces at a strain of 0.75 and fibers separate from the matrix. The deformation of the fibers is not obvious, indicating that fibers suffer little stress. Under dynamic compressive loading, the aerogel matrix shatters into fragments at a strain of 0.62 and shows a “bursting” phenomenon. The high speed compression of gas in the aerogel results in an increase of the internal stress. Fibers bend, break and separate from the matrix, indicating that fibers carry partially the applied loading. The carbon nanoparticles are squeezed closer with nearly no voids remaining after both quasi-static and dynamic compression. The increase of the internal stress and the fracture of fibers lead to strain rate strengthening and earlier fracture of the CFRC aerogel at high strain rates.     

Preparation and characterization of magnesium aluminate (MgAl2O4) spinel ceramic foams via direct foam-gelcasting

Lightweight MgAl₂O₄ spinel ceramic foams with high mechanical strength and good dielectric properties were prepared with a direct foam-gelcasting method using MgAl₂O₄ and TiO₂ (rutile phase, as sintering aid) powders. The effects of calcination temperature and foam volume on bulk density, apparent porosity, and on the mechanical and dielectric properties of the ceramic foams were investigated. Tailored porosity (75.14–82.46%), pore size (10–200 μm), dielectric constant (1.66–2.05), and compressive strength (4.0–14.3 MPa), were obtained based on the change of the foam volume in the foamed slurries, and the calcination temperature of porous ceramics. The compressive strength and dielectric constant of the as-manufactured spinel foam with a porosity of ~75.14% was as high as 14.3 MPa and 2.05, respectively. The spinel ceramic foam which had a porosity of 81.84% was prepared with a foam volume of 350 mL and a sintering temperature of 1500 °C, and exhibited heterogeneous pore structures, whereby large and open spherical cells involved in small circular windows on the internal walls with a mean pore size of ~66.26 μm and a grain size of ~8 μm. The experimental dielectric constant matches well with that calculated by the modified Bruggeman model. The dependence of the mechanical strength on the relative density can be represented by the Gibson and Ashby model. The fitted index values of the power relationship were 3.504 and 3.533, compared to the theoretical value of 1.5. The ceramic foam can potentially become a new type of electromagnetic wave-transmitting radome material due to its low dielectric constant (1.66–2.05) and dielectric loss (0.0026–0.006) values.     

Shear thickening behavior of dilute poly(diallyl dimethyl ammonium chloride) aqueous solutions

Using dynamic light scattering (DLS) and capillary dynamic viscoelasticity (DVE) analyzer, we investigated dilute (0.5 mg/ml) poly(diallyl dimethyl ammonium chloride) (PDADMAC) aqueous solution properties for three different molecular weights of PDADMACs mixed with various concentrations of NaCl. The dependence of PDADMAC molecular chain conformations in aqueous solutions on polymer molecular weight and NaCl concentration were studied. By analyzing dynamic shear viscosity η′(ω), viscoelastic relaxation times t_r, and shear rate at tube wall ?_a(ω) of PDADMAC aqueous solutions in oscillatory flows, we proposed that polymer chain conformations varied with increasing shear frequency ω via the following steps: intra-polymer associations, dissociation of intra-polymer associations, stretching of polymer chains, inter-polymer aggregations, and dissociations of inter-polymer aggregations. The intra-polymer associations lowered the n′ exponent of storage modulus G′(ω) (G′(ω) ∼ ω^n′) with n′ < 2, and the polymer chain stretching and inter-polymer aggregations caused shear thickening (i.e. upturn of η′(ω)) of PDADMAC aqueous solutions. The behaviors of the lowering of n′ exponent with n′ < 2 and the shear thickening were favored by increasing ionic strength of solutions. By comparing η′(ω) data with DLS hydrodynamic radii (R_h) data, we also confirmed the possibility of inter-polymer aggregations in dilute solutions when polymer chains were stretched in oscillatory flows.     

Compression strength of tungsten carbide–cobalt hard metals

Eight tungsten carbide (WC) materials containing different cobalt (Co) contents (3‒12 wt.%) and with different WC grain sizes (.4‒1.85 μm) were subjected to compressive loading under quasi-static and dynamic conditions using a dumbbell-shaped specimen geometry. The materials exhibited varying degrees of inelastic strain prior to final fracture under both loading conditions. Inelastic strain was consistent under dynamic loading but varied with Co content and WC grain size under quasi-static loading. The only material to exhibit a strain-rate-dependent compression strength was the material containing the highest level of Co (12%) and the largest WC grain size (1.85 μm) indicating a potential threshold level of Co content and/or WC grain size where the compressive strength is insensitive to the strain rate.     

应变率对乙烯基酯树脂压缩力学行为影响的研究

为研究应变率对乙烯基酯树脂压缩性能的影响,在准静态与动态试验条件下,比较分析了不同加载速率时乙烯基酯树脂的压缩力学行为.研究发现,乙烯基酯树脂的压缩性能受加载速率影响显著,具有明显的应变率效应.动态试验条件下的屈服应力比准静态下提高57.0%～85.2%,而屈服应变略有降低,其屈服形貌和破坏特征也明显不同.     

Experimental research on the dynamic compressive properties of lightweight slag based geopolymer

The present study aims at preparing lightweight slag based geopolymer (LW-SG) and studying its mechanical properties under dynamic and quasi-static loads. Firstly, three LW-SG with different densities were prepared by replacing the slag with expanded perlite (EP). Secondly, the density, wave velocity and pore structure of LW-SG with different EP contents were tested. Thirdly, the mechanical properties under quasi-static and dynamic loads were compared. Finally, the effects of the strain rate and EP content on the mechanical properties and failure modes of LW-SG were discussed. The results showed that with the EP contents increase, the dry density and longitudinal wave velocity gradually decreased, while the porosity increased. In addition, the quasi-static compressive strength and elastic modulus of LW-SG increased with curing ages, but decreased with EP contents increased. The dynamic compressive strength, dynamic increase factor, strain energy density and damaged degree of LW-SG all showed an increasing tendency with the strain rates increase, which exhibits an obvious strain rate dependence. Under the same strain rate, the dynamic compressive strength and strain energy density decreased with the EP contents increase, while the damaged degree increased with the EP contents increase.     

Effects of strain rate and temperature on compressive strength and fragment size of ZrB2–SiC–graphite composites

The quasi-static (strain rate of 10⁻⁴ s⁻¹) and dynamic compression experiments (strain rate of 200–1500 s⁻¹) of ZrB₂–SiC–graphite composites are conducted at 293 K and 1073 K. The initial compressive strength and Weibull modulus are calculated to handle the discrete quasi-static experimental data. Considering effects of strain rate and temperature, the compressions of ZrB₂–SiC–graphite composites are investigated. The results show that both compressive strength and fragment size are higher at 1073 K than those at room temperature. The compressive strengths increase with increasing strain rate at room temperature and 1073 K, whereas fragment sizes decrease. Moreover, a micromechanical model is utilized to characterize the effect of strain rate on the compressive strength. The predictions of this micromechanical model are good agreement with the experimental results. Meanwhile, the fragment sizes of dynamic compressive specimens are analyzed through analytical approaches.     

Performance optimization of Si/SiC ceramic triply periodic minimal surface structures via laser powder bed fusion

SiC ceramic lattice structures (CLSs) via additive manufacturing (AM) have been recognized as potential candidates in engineering fields owing to their various merits. Compared with traditional SiC CLSs, SiC triply periodic minimal surface (TPMS) CLSs could possess more outstanding properties, making them more promising for wider applications. Since SiC CLSs are hard to be fabricated through stereolithography techniques because of inferior light performance, the laser powder bed fusion (LPBF) process via selective sintering is an effective method to prepare near-net-shaped SiC TPMS lattices. As the mechanical performances of lattice structures are the foundation for future practical applications, it is of great significance to optimize the preparation process, thus improving the mechanical properties of SiC TPMS structures. In this work, the optimal printing parameters of the LPBF and liquid silicon infiltration process for SiC ceramic TPMS CLSs with three different volume fractions were systematically illustrated and analyzed. The effects of the printing parameters and carbon densities on the fabrication accuracy, microstructure, and mechanical performance of SiC TPMS CLSs were defined. The mechanism of the reactive sintering process for the SiC TPMS lattice structure was revealed. The results reveal that Si/SiC TPMS CLSs with optimum preparation have superior manufacturing accuracy (most less than 6%), relatively high bulk densities (about 2.75 g/cm³), low residual Si content (6.01%), and excellent mechanical properties (5.67, 15.4, and 44.0 MPa for Si/SiC TPMS CLSs with 25%, 40%, and 55% volume fractions, respectively).     

Study on compression–expansion behaviour of PBXs substitutive materials

ABSTRACT

In this work, polymer-bonded sugars were used as a substitutive material for polymer-bonded explosives (PBXs). Their production process and damage mechanism under compressive process were analysed. We also investigated the initial Poisson’s ratio of PBXs as well as their mechanical properties under quasi-static compression and variation of Poisson’s ratio during the one-dimensional compressive process. Compressive expansion behaviour due to compressive loading was investigated comprehensively. It was found that due to the damage caused by the loading process, volumetric strain and Poisson’s ratio showed three stages characteristic. Experimental results proved that compression–expansion had occurred during the one-dimensional compressive process. Simulation results verified that PBXs crystal underwent mainly transgranular fracture under quasi-static compression.     

Solvents effect on the physical properties of semi-dilute poly(N-isopropyl acryl amide) solutions

The physical properties of 5 wt% poly(NIPAM) (M_v=3.22×10⁵) semi-dilute solutions in H₂O, D₂O, and THF (tetrahydrofuran) solvents were studied using dynamic light scattering (DLS) and dynamic shear viscosity (DSV) measurements. The DLS data showed that there were poly(NIPAM) slow mode inter-polymer chains associations in H₂O and D₂O solvents. However, no DLS slow mode was observed in poly(NIPAM)/THF solutions. The DSV data showed that there are shear thickening behavior in these three poly(NIPAM) solutions, resulting in a maximum shear viscosity η_peak in the viscosity η′(ω) versus shear frequency ω curve. The slow mode hydrodynamic radius 〈R_hs〉 of DLS measurements and the zero shear rate viscosity η₀ and maximum viscosity η_peak data of DSV measurements from poly(NIPAM)/H₂O and poly(NIPAM)/D₂O solutions show two critical transition temperatures with T_cr1=30-32 °C and T_cr2=32-34 °C. Poly(NIPASM)/D₂O has higher T_cr1 and T_cr2 than poly(NIPASM)/H₂O. However, no transition temperatures of poly(NIPAM)/THF solution were observed. The different temperature dependencies of these three solutions were attributed to the ‘solubility’ and ‘hydrogen bonding’ effects between poly(NIPAM) with H₂O, D₂O, and THF solvents. Without considering the polymer-solvent hydrogen bonding, the solubility of poly(NIPAM) in solvents decreases in the following sequence: THF>H₂O>D₂O and the degree of polymer-solvent hydrogen bonding increases in the following sequence: THF<H₂O<D₂O. The effects of the degree of ‘hydrogen bonding’ and the ‘solubility’ of polymer in solvents on the physical properties of poly(NIPAM) solutions are discussed.     

Compressive behavior and deformation kinetics of ultrafine grained aluminum processed by equal channel angular pressing

ABSTRACT

In the present work, the uniaxial compressive mechanical proper\ties of ultrafine grained (UFG) aluminum produced by equal channel angular pressing method were investigated experimentally over a wide temperatures ranging from 77 to 473 K under both quasi-static and dynamic loading conditions. Based on the experimental results, the strain rate, temperature sensitivity, and the apparent activation volume were estimated. The coupling effects of both experimental temperature and applied strain rate on thermal-activated plastic deformation behavior were also investigated. Based on the thermal activation theory, the rate-controlling mechanisms for the UFG aluminum under both quasi-static and dynamic loading conditions were discussed.