Initial Compressive Modulus Research Articles

A cavity expansion theory-based hyperbolic model for the pull-out force of a compaction-grouted soil nail

Compared with conventional soil nails, the soil nail with grout bulbs (compaction-grouted soil nail) exhibits better performance in soil. In this study, a hyperbolic relationship was used to describe the relationship between pull-out stress (pull-out force divided by the cross-sectional area of a grout bulb) and displacement, in which the initial compressive modulus and ultimate compressive strength are the key factors that need to be determined in advance. In this case, the stress-strain behaviour of the soil around the grout bulb was investigated based on cavity expansion theory during compaction grouting and pull-out processes, and the initial compressive modulus with a compaction effect and the ultimate compressive strength were then quantified. Therefore, a cavity expansion theory-based hyperbolic model for the pull-out force of a compaction-grouted soil nail was proposed, in which the contribution of compaction grouting to the pull-out force was theoretically quantified. The proposed pull-out force model that combines the hyperbolic relationship and cavity expansion theory is simple and has high accuracy and will be useful for the design of the new soil nail with grout bulbs.

Study on dynamic deformation-damage-ignition mechanism of GAP/RDX/TEGDN propellant

ABSTRACT Investigations on high-energy and low-vulnerability propellants can provide a better understanding for improving the operational effectiveness and survivability of strategic and tactical missiles. In the present study, the dynamic compressive mechanical properties and damage-ignition response mechanism of the GAP/RDX/TEGDN (GRT) propellant under high strain rates have been studied using a split Hopkinson pressure bar apparatus and a high-performance high-speed camera. The experimental results suggested that the mechanical properties of the GRT propellant, in terms of yield stress, initial compressive modulus, and ultimate stress, were greatly affected by the loaded strain rate. The true stress-strain curves exhibited an initial linear elasticity region followed by yielding a subsequent strain hardening and strain softening region stage. It has been found that the yield stress and ultimate stress increased with the strain rates. Moreover, at higher strain rates, the impulse increases significantly, leading to earlier and more intense ignition. The critical strain rate and the critical impulse of the GRT propellant are 5000 s−1 and 13.28 N · s, respectively. A judgment basis for GRT propellant ignition is established. Finally, according to the high-speed images and the posttest SEM images, it has been concluded that the dominant ignition mechanism of GRT propellant is flow at the macroscopic level and internal frictional heating at the mesoscopic level.

An experimental study on dynamic deformation and ignition response of a novel propellant under impact loading

Investigations on high-energy and low-vulnerability propellants can provide a guarantee for improving the power and safety of solid rocket motors. In the present study, mechanical-thermal coupled responses were recorded by a high-speed infrared thermal imaging systems. The dynamic compressive properties and damage-ignition response mechanism under high strain rates of a novel type of propellant were studied, using a split Hopkinson pressure bar apparatus. The experimental results suggested that the mechanical properties of the novel propellant, in terms of yield stress, initial compressive modulus, and ultimate stress, were greatly affected by the loaded strain rate. The stress-strain responses exhibited an initial linear elasticity region, followed by yielding, and then a subsequent strain hardening or softening stage (plateau region), which was strongly dependent on the strain rate. It was found that the yield stress and ultimate stress increased with the strain rates. Moreover, the ignition response was accentuated at higher strain rates, resulting in an earlier and more intense ignition. According to the high-speed images and the post-test SEM images, the critical strain rate for ignition is 4500 s−1. Finally, by calculating the bulk and local temperature rise (considering the friction mechanism), it is clear that the dominant ignition mechanism of the novel propellant is the friction which is macroscopically represented as viscous shear flow.

Temperature Effects on the Compressive Behaviors of Closed-Cell Copper Foams Prepared by Powder Metallurgy.

A fabrication technology of closed-cell copper foams (CCCFs) based on powder metallurgy is proposed, by using the expanded polystyrene foams (EPS) spheres with the prescribed diameter as the space holder before sintering. The material characterization and the quasi-static compressive behaviors of both uniform and graded CCCFs at different temperatures were experimentally studied. A high temperature weakens the initial compressive modulus, plateau stress, and effective energy absorption for both uniform and graded CCCFs; meanwhile, the onset strain of densification and the maximum energy absorption efficiency are less sensitive to temperature, especially for the graded CCCFs. Compared with the uniform CCCF, the graded CCCF with even a small relative density exhibits superiority in terms of the effective energy absorption and the maximum energy absorption efficiency, attributed to the much larger onset strain of densification for the gradient pore arrangement. Finite element simulations based on the ideal sphere foam model can basically mimic the compressive performance of the CCCF samples. It is also found that both the decrease of pore diameter and the increase of cell wall thickness could improve the compressive performance of the CCCFs.

Dynamic Out-of-Plane Compressive Failure Mechanism of Carbon/Carbon Composite: Strain Rate Effect on the Defect Propagation and Microstructure Failure

Abstract Out-of-plane compression experiments with the strain rate from 0.0001/s to 1000/s are performed on a three-dimensional (3D) fine weave-pierced Carbon/Carbon (C/C) composite using a universal testing machine, a high-speed testing machine, and a split Hopkinson pressure bar (SHPB). The compressive failure mechanism of the composite is analyzed by a multi-scale analysis method, which ranges from micro-scale defect propagation, through meso-scale microstructure failure, to macro-scale material failure. In order to predict the out-of-plane compressive properties of 3D fine weave-pierced C/C composite at different strain rates, a strain-rate-dependent compressive constitutive model is proposed. The results show that the out-of-plane compressive behavior of the 3D fine weave-pierced C/C composite is sensitive to strain rate. With increasing the strain rate, the initial compressive modulus, the maximum stress, and the strain at the maximum stress increase. The difference in mechanical behavior between quasi-static and high strain rate compression is owing to the strain rate effect on the defect propagation of the 3D fine weave-pierced C/C composite. The proposed constitutive model matches well with the experimental data.

A tough polysaccharide-based cell-laden double-network hydrogel promotes articular cartilage tissue regeneration in rabbits

Hydrogels used for cartilage tissue engineering purposes must be designed to mimic the biological and physical features of the native extracellular matrix (ECM). Here, we developed three different types of physical and chemical chondro-inductive hydrogels with the controlled drug release ability based on sulfated alginate (SAlg) and chitosan (CS). Bioconjugation of catechol to SAlg (SAlg-Cat) resulted in formation of SAlg-Cat/CS hydrogels with higher mechanical stability than physical SAlg/CS hydrogel. In the chemical double-network hydrogel (DN/SAlg-Cat), the presence of catechol and amine groups within the network resulted in dynamic and reversible bonding with energy dissipation. Our DN/SAlg-Cat hydrogel with initial compressive modulus of 34.86 kPa exhibited adequate compressive strength (421.45 kPa), toughness (89.69 kJ/m3), ability to self-heal and recoverability, which could be exceptional for utilizing in a load-bearing tissue like cartilage. Among hydrogels, the DN/SAlg-Cat hydrogel displayed less degradation rate (6.81%) and higher mechanical stability (81% retention of stiffness) in the presence of lysozyme (15 μg/mL) during 21 days. Incorporation of the sulfate and catechol groups within these hydrogels mediated the sustained release of TGF-β1 and kartogenin (KGN) chondrogenic agents, and resulted in proper chondrogenesis of the encapsulated mesenchymal stem cell (MSC) and chondrocyte (CH) co-culture. The catechol-enriched hydrogels (SAlg-Cat/CS and DN/SAlg-Cat) showed better regeneration of full-thickness articular defects in rabbits after 12 weeks due to the high affinity of the catechol group for cell adhesion, tissue integration, and immobilization of the biological factors. Our results demonstrated a role for a biomimetic microenvironment in efficient chondrogenesis.

In-situ re-melting and re-solidification treatment of selective laser sintered polycaprolactone lattice scaffolds for improved filament quality and mechanical properties

Selective laser sintering (SLS) is a promising additive manufacturing technique that produces biodegradable tissue-engineered scaffolds with highly porous architectures without additional supporting. However, SLS process inherently results in partially melted microstructures which significantly impair the mechanical properties of the resultant scaffolds for potential applications in tissue engineering and regenerative medicine. Here, a novel post-treatment strategy was developed to endow the SLS-fabricated polycaprolactone (PCL) scaffolds with dense morphology and enhanced mechanical properties by embedding them in dense NaCl microparticles for in-situ re-melting and re-solidification. The effects of re-melting temperature and dwelling time on the microstructures of the SLS-fabricated filaments were studied. The results demonstrated that the minimum requirements of re-melting temperature and dwelling time for sufficient treatment were 65 °C and 5 min respectively and the size of the SLS-fabricated filaments was reduced from 683.3 ± 28.0 μm to 601.6 ± 17.4 μm. This method was also highly effective in treating three-dimensional (3D) PCL lattice scaffolds, which showed improved filament quality and mechanical properties after post-treatment. The treated PCL scaffolds with an initial compressive modulus and strength of 3027.8 ± 204.2 kPa and 208.8 ± 14.5 kPa can maintain their original shapes after implantation in vivo for 24 weeks. Extensive newly-grown tissues were found to gradually penetrate into the porous regions along the PCL filaments. Although degradation occurred, the mechanical properties of the implanted constructs stably maintained. The presented method provides an innovative, green and general post-treatment strategy to improve both the filament quality and mechanical properties of SLS-fabricated PCL scaffolds for various tissue engineering applications.

In situ synthesis of bilayered gradient poly(vinyl alcohol)/hydroxyapatite composite hydrogel by directional freezing-thawing and electrophoresis method

Bilayered poly(vinyl alcohol) (PVA)/hydroxyapatite (HA) composite hydrogels with anisotropic and gradient mechanical properties were prepared by the combination of directional freezing-thawing (DFT) and electrophoresis method. Firstly, PVA hydrogels with aligned channel structure were prepared by the DFT method. Then, HA nanoparticles were in situ synthesized within the PVA hydrogels via electrophoresis. By controlling the time of the electrophoresis process, a bilayered gradient hydrogel containing HA particles in only half of the gel region was obtained. The PVA/HA composite hydrogel exhibited gradient mechanical strength depending on the distance to the cathode. The gradient initial tensile modulus ranging from 0.18MPa to 0.27MPa and the gradient initial compressive modulus from 0.33MPa to 0.51MPa were achieved. The binding strength of the two regions was relatively high and no apparent internal stress or defect was observed at the boundary. The two regions of the bilayered hydrogel also showed different osteoblast cell adhesion properties.

Strain rate and temperature dependence of the compressive behavior of a composite modified double-base propellant

The quasi-static and dynamic compressive properties of a composite modified double-base (CMDB) propellant were studied over a wide range of strain rates from 10−4s−1 to 103s−1 at low temperatures of −40°C to 0°C, using a conventional universal testing machine and a split Hopkinson pressure bar apparatus. The experimental results show that the mechanical properties of the CMDB propellant, in terms of yield stress, initial compressive modulus, and ultimate strain, were greatly affected by the applied strain rate and the temperature, and the stress–strain responses exhibited an initial viscoelastic phase, followed by yielding, and then a subsequent strain hardening or softening stage, which was strongly dependent on the strain rate and temperature. It was found that the yield stress increased with both increasing strain rate and decreasing temperature. According to the experimental results, the strain rate effect, but not the temperature effect, was accentuated at higher rates. Moreover, the Eyring cooperative model was employed to predict the yielding behavior, and a master curve of reduced yield stress versus strain rate was built. The results show that the Eyring cooperative model provides reasonable prediction over a wide range of strain rates under low temperatures.

Tailoring properties of microsphere‐based poly(lactic‐co‐glycolic acid) scaffolds

Biodegradable polymer scaffolds are being extensively investigated for uses in tissue engineering because of their versatility in fabrication methods and range of achievable chemical and mechanical properties. In this study, poly(lactic-co-glycolic acid) (PLGA) was used to make various types of microspheres that were processed into porous scaffolds that possessed a wide range of properties. A heat sintering step was used to fuse microspheres together around porogen particles that were subsequently leached out, allowing for a 10-fold increase in mechanical properties over other PLGA scaffolds. The sintering temperature was based on the glass transition temperature that ranged from 43 to 49°C, which was low enough to enable drug loading. Degradation times were observed to be between 30 and 120 days, with an initial compressive modulus ranging from 10 to 100 MPa, and after 5 days of degradation up to 10 MPa was retained. These scaffolds were designed to allow for cell ingrowth, enable drug loading, and have an adjustable compressive modulus to be applicable for soft or hard tissue implants. This study combined well-established methods, such as double emulsion microspheres, polymer sintering, and salt leaching, to fabricate polymer scaffolds useful for different tissue engineering applications.

Synergistic Stimuli by Hydrodynamic Pressure and Hydrophilic Coating on PLGA Scaffolds for Extracellular Matrix Synthesis of Engineered Cartilage

Scaffold surface properties and mechanical stimuli have been known to have a critical influence on cell proliferation and extracellular matrix synthesis in cultured cells for tissue engineering. Hydrophilic surface and hydrodynamic pressure (HP) stimulation have been shown to have a beneficial effect on chondrocyte activity. The aim of this study was, thus, to assess the synergic influences of HP and hydrophilic coating on cell activity using primary porcine chondrocytes inoculated in hydrophilic-coated poly(lactide-co-glycolide) (PLGA) sponge scaffolds. The natural materials hyaluronic acid (HA), chitosan and HA/chitosan were cross-linked on porous PLGA as a hydrophilic surface modification. HP was applied to scaffolds at an amplitude of 2.24 MPa and a frequency of 0.1 Hz for 30 min per day, twice a week, over a period of 28 days. Cell activities were determined by the MTS assay and the dimethylmethylene blue assay for glycosaminoglycan (GAG) quantification. Our results displayed that PLGA coated with both HA and chitosan had the best hydrophilicity (contact angle 49.46°) and initial compressive modulus (1.10 ± 0.13 MPa) among the tested scaffold groups. Additionally, HP stimulation enhanced cell proliferation as well as GAG production (up to 3-fold in culture medium and 15-fold in scaffolds at 28 days compared to static culture of PLGA alone in the scaffold group) in the hydrophilic-coated scaffold groups. The synergistic benefit from hydrophilic coating and HP stimulation may be imperative in regenerating engineered cartilage in the long-term.

Effect of Hydroxyapatite on Biodegradable Scaffolds Fabricated by SLS

Selective laser sintering (SLS) has the potential to fabricate bioresorbable polymer / ceramic composite scaffolds with pre-designed external and internal architecture that can be used for bone tissue engineering applications. Scaffolds were fabricated using poly-ε-caprolactone as the base material. The effect of 15 and 30 wt% of hydroxyapatite (HA) addition was investigated in terms of compressive properties, accuracy, surface topology, and wettability. Fabricated dimensions of PCL microstructures showed great deviations from their nominal values. Average surface roughness was found to be Ra=25±4 µm. Increased HA content had no statistically significant effect on accuracy and surface roughness. However the addition of HA had a significant influence on compressive properties, hydrophobicity and wettability of the samples. Addition of 30 wt% HA improved initial compressive modulus of pure PCL scaffolds from 1.31±0.08 MPa to 1.58±0.18 MPa. Yield strength values increased from 0.14±0.07 MPa to 0.17±0.01 MPa by adding 15 wt% of HA, but decreased with further HA addition. Yield strain for all compositions was over ε=0.06. Increased HA content decreased hydrophobicity and increased wettability of scaffold surfaces. The study demonstrated the ability of SLS to fabricate tissue engineering scaffolds, and the positive effect of HA particle reinforcement in terms of compressive mechanical properties and surface characteristics

Controlled drug release from a novel injectable biodegradable microsphere/scaffold composite based on poly(propylene fumarate)

The ideal biomaterial for the repair of bone defects is expected to have good mechanical properties, be fabricated easily into a desired shape, support cell attachment, allow controlled release of bioactive factors to induce bone formation, and biodegrade into nontoxic products to permit natural bone formation and remodeling. The synthetic polymer poly(propylene fumarate) (PPF) holds great promise as such a biomaterial. In previous work we developed poly(DL-lactic-co-glycolic acid) (PLGA) and PPF microspheres for the controlled delivery of bioactive molecules. This study presents an approach to incorporate these microspheres into an injectable, porous PPF scaffold. Model drug Texas red dextran (TRD) was encapsulated into biodegradable PLGA and PPF microspheres at 2 microg/mg microsphere. Five porous composite formulations were fabricated via a gas foaming technique by combining the injectable PPF paste with the PLGA or PPF microspheres at 100 or 250 mg microsphere per composite formulation, or a control aqueous TRD solution (200 microg per composite). All scaffolds had an interconnected pore network with an average porosity of 64.8 +/- 3.6%. The presence of microspheres in the composite scaffolds was confirmed by scanning electron microscopy and confocal microscopy. The composite scaffolds exhibited a sustained release of the model drug for at least 28 days and had minimal burst release during the initial phase of release, as compared to drug release from microspheres alone. The compressive moduli of the scaffolds were between 2.4 and 26.2 MPa after fabrication, and between 14.9 and 62.8 MPa after 28 days in PBS. The scaffolds containing PPF microspheres exhibited a significantly higher initial compressive modulus than those containing PLGA microspheres. Increasing the amount of microspheres in the composites was found to significantly decrease the initial compressive modulus. The novel injectable PPF-based microsphere/scaffold composites developed in this study are promising to serve as vehicles for controlled drug delivery for bone tissue engineering.

Encapsulating chondrocytes in degrading PEG hydrogels with high modulus: engineering gel structural changes to facilitate cartilaginous tissue production.

A major challenge when designing cell scaffolds for chondrocyte delivery in vivo is creating scaffolds with sufficient mechanical properties to restore initial function while simultaneously controlling temporal changes in the gel structure to facilitate tissue formation. To address this design challenge, degradable photocrosslinked hydrogels based on poly(ethylene glycol) were investigated. To alter the gel's initial mechanical properties, hydrogels were fabricated by varying the initial macromer concentration from 10% to 15% to 20%. A twofold increase in macromer concentration resulted in an eightfold increase in the initial compressive modulus from 60 to 500 kPa. Gel degradation was tailored by incorporating fast-degrading crosslinks that enable maximal extracellular matrix (ECM) diffusion with time and a minimal number of nondegrading (or slowly degrading) crosslinks to maintain scaffold integrity and prevent complete gel erosion during tissue formation. Chondrocytes encapsulated in these gels produced cartilaginous tissue rich in glycosaminoglycans and collagen as seen biochemically and histologically. Interestingly, mass loss appeared to more closely match tissue secretion in gels fabricated from a 15% macromer concentration. However, the spatial ECM distribution was grossly similar in all three gels. By tailoring gel degradation and controlling network evolution during degradation, gels with optimal properties can be fabricated to support initially physiologic compressive loads while simultaneously supporting the formation of a neotissue.

Rheology of Boiled Rice Grain and Starch Granules in the Gelatinization Process

The taste of boiled rice depends primarily on various mechanical properties of boiled rice grains and the internal structure is responsible for these mechanical properties.(1) The time dependency of strain in compressed boiled rice, a Japanese delicacy known as“oshi-zushi”, during its formation process was studied by applying a given compressive stress. Most of the properties of compressed boiled rice grains could be explained by a mechanical nine-element model, and the components of the strain in each element were determined by the thickness of boiled rice grains before compression, the compressive stress, and compression time.(2) Twenty-four types of domestic rice were examined for their mechanical properties. There was a strong correlation between the taste category and the experimental results, including the initial compressive modulus, adhesive energy, total strain, and relaxation time.(3) Deterioration of the taste of boiled rice with the retrogradation time and its physical factors were studied. Generally, the change in the degree of crystallinity obtained from an X-ray diffraction intensity diagram correlated well with the change in the compressive energy obtained from compression experiments.(4) Changes of structure and mechanical properties were studied in the gelatinization process of suspensions of starch granules from white potatoes, non-glutinous rice, and non-glutinous rice pretreated by NaOH, and that of gel of starch granules from non-glutinous rice. Before the heat treatment, four-leaf clover images were seen in the light scattering photographs. As the gelatinization proceeded, a maximum appeared in the viscosity-time relation. Streaks and speckles were observed in the succeeding light scattering measurements.(5) Furthermore changes in the structure and mechanical properties of starch granules from white potatoes during the gelatinization process were studied. The light scattering photographs and polarization microscopy during the heating process demonstrated that there was a relationship between the disintegration characteristics of granules and change in viscosity. Also, there was a definite difference in the thermal resistance between the surface layers and the inner part of the same starch granule.