Modulus Strain Research Articles

Comparison of the mechanical properties and constitutive models of carbon fiber-reinforced coral concrete cubes and prisms under uniaxial compression

Carbon fiber reinforced coral concrete (CFRCC) plays a crucial role in island construction. In engineering applications, CFRCC is often subjected to complex stress states. Multiaxial stress studies usually use cubes, and in order to maintain research consistency, uniaxial studies also often use cubes. Traditional uniaxial stress studies mainly use prisms. However, the differences in uniaxial compression mechanical properties between these two types of specimens have not been thoroughly studied. To fill this gap, this study conducted uniaxial compression tests on cubes and compared the results with previous tests on prisms.The results showed that the failure patterns of the two types of specimens were different. The main cracks of the cubes were vertically distributed, while the main cracks of the prisms mostly unfolded diagonally. The addition of carbon fibers (CFs) reduced the number of large cracks and increased the number of microcracks. The stress-strain curves, peak stress, peak strain, and initial elastic modulus of the two types of specimens show similar variations with CFs, and CFs significantly improve the mechanical properties of coral concrete. The constitutive models based on prisms cannot accurately describe the mechanical behavior of cubes, therefore, we proposed revised constitutive models. The revised constitutive models can accurately describe the mechanical behavior of cubes. This study laid the foundation for subsequent research on multiaxial stress.

Mechanical properties variation of samples fabricated by fused deposition additive manufacturing as a function of filler percentage and structure for different plastics.

One of the key factors in manufacturing products by fused deposition molding (FDM) or layer-by-layer printing technology is the material intensity of the product. The task of reducing the amount of material required to manufacture a product without significant loss of mechanical properties is one of the most practically important technological tasks. Material saving in FDM printing of products allows to reduce financial costs and increase the speed of manufacturing of the final product without reducing (or not significantly reducing) the quality properties of the product. In our work it is demonstrated that using Combs filling type and materials of poly lactic acid (PLA) and polyethylene terephthalate glycol (PETG) it is possible to achieve material savings of up to 23% at 50% filling for PLA and 17% at 75% filling for PETG without significant reduction of product strength in comparison with other filling types. Exceptions are PLA samples with 100% fill and Lateral fill. Application of Kruskal-Wallis criterion and Dunn's criterion with Bonferroni multiple comparison correction showed that there were no statistically significant differences within the strength limits of samples made by FDM printing technology from PLA and PETG plastics (p-value = 0.0514), as well as samples with Triangle and Grid filling type (p-value = 1). Based on this result, three groups of samples statistically significantly differing in ultimate strength were identified by methods of hierarchical cluster analysis; in each group (except for group 1, which included samples made of PLA plastic with Lateral filling type and 100% filling), correlation analysis was performed (Spearman correlation was used). The results of the correlation analysis showed a stable average correlation between the percentage of filling, modulus along the secant 0.05-0.2% strain, ultimate strength and strain corresponding to the yield stress. Analysis of the correlation graph showed that the main parameter correlating with all mechanical properties of the specimen is the 0.05-0.2% strain modulus. Based on this conclusion, robust regression equations predicting the 0.05-0.2% strain modulus as a function of the percentage of specimen filling were constructed for the two selected groups. Analysis of the equations showed that in the third group of specimens, the average modulus of 0.05-0.2% strain is more than twice the modulus of 0.05-0.2% strain in the second group. The detected statistical regularities can be explained by the mechanism of strain hardening, the actual value of which depends on the structure of the macrodefect (type of filling), properties and volume of the material (percentage of filling) used in the fabrication of samples using FDM printing technology.

Study on the compressive performance of hyperelastic seismic isolation materials under passive confinement of surrounding rock simulated by CFRP tubes

Tunnel engineering has played a significant role in the development of human society. However, due to geological constraints, tunnels are prone to losing stability and safety under strong seismic forces. High-elasticity, super-tough polymer-inorganic composite tunnel isolation materials (PTIM), based on polyurethane elastomers, can meet the requirements for tunnel isolation, but research on their passive confinement mechanical properties remains insufficient. This study designed an experimental method to simulate the passive confinement state of tunnel isolation materials. The method uses CFRP tube confinement and core loading to replicate the actual stress conditions within a confined tunnel space. Based on the experimental data, a constitutive model for PTIM under passive confinement was established. The results show that the initial elastic modulus (Ec0) of PTIM and the CFRP confinement stiffness (K) are significantly correlated with the material's stress-strain curve, axial peak stress and strain, circumferential strain, compressive performance, and elastic modulus. Based on these mechanical properties as characteristic values, a tri-linear constitutive model for PTIM with different mix ratios under various confinement stress conditions was established. The high degree of agreement between the predicted data and the experimental data validates the effectiveness of the model.

Phosphate overload via the type III Na-dependent Pi transporter represses aortic wall elastic fiber formation.

Phosphate (Pi) induces differentiation of arterial smooth muscle cells to the osteoblastic phenotype by inducing the type III Na-dependent Pi transporter Pit-1/solute carrier family member 1. This induction can contribute to arterial calcification, but precisely how Pi stress acts on the vascular wall remains unclear. We investigated the role of extracellular Pi in inducing microstructural changes in the arterial wall. Aortae of Pit-1-overexpressing transgenic (TG) rats and their wild-type (WT) littermates were obtained at 8 weeks after birth. The thoracic descending aorta from WT and TG rats was used for the measurement of wall thickness and uniaxial tensile testing. Structural and ultrastructural analyses were performed using light microscopy and transmission electron microscopy. Gene expression of connective tissue components in the aorta was quantified by quantitative real-time polymerase chain reaction. Aortic wall thickness in TG rats was the same as that in WT rats. Uniaxial tensile testing showed that the circumferential breaking stress in TG rats was significantly lower than that in WT rats (p<0.05), although the longitudinal breaking stress, breaking strain, and elastic moduli in both directions in TG rats were unchanged. Transmission electron microscopy analysis of the aorta from TG rats showed damaged formation of elastic fibers in the aortic wall. Fibrillin-1 gene expression levels in the aorta were significantly lower in TG rats than in WT rats (p<0.05). Pi overload acting via the arterial wall Pit-1 transporter weakens circumferential strength by causing elastic fiber malformation, probably via decreased fibrillin-1 expression.

Multi-functional Gleditsia sinensis galactomannan-based hydrogel with highly stretchable, adhesive, and antibacterial properties as wound dressing for accelerating wound healing

Design and development of a multifunctional wound dressing with self-healing, adhesive, and antibacterial properties to attain optimal wound closure efficiency are highly desirable in clinical applications. Nevertheless, conventional hydrogels face significant barriers in their mechanical strength, adhesive performance, and antibacterial properties. Herein, a tough hydrogel based on aldehyde-grafted galactomannan was synthesized through radical copolymerization and Schiff base reaction, incorporating hyaluronic acid, acrylamide, and the zwitterionic monomer to create a multi-crosslinked structure. The multiple crosslink structure pattern consisting of multiple hydrogen bonding, ionic interactions, reversible Schiff bases bonds, and molecular chain entanglement endowed this hydrogel with multiple functionalities, including high tensile strength (25 kPa), tensile strain (2200 %), toughness (391.59 kJ/m3), and Young's modulus (9.77 kPa). The presence of catechol groups and zwitterionic groups endow hydrogels with outstanding adhesion strength (42.21 kPa), which satisfied the adhesive demand for the ample motion of specific areas. The zwitterionic monomer provided long-lasting antibacterial properties and promoted migration and growth of negatively charged cells, capable of establishing efficient antibacterial barriers and serving as wound dressing. The in vivo and vitro experiments manifested that the optimized hydrogel demonstrated an inconspicuous inflammatory response, facilitating rapid healing of full-thickness skin wound in rat models. Therefore, this work provides a promising strategy and an ideal candidate for wound healing dressings in treating infected skin wounds.

Size effect and its atomistic origin on the mechanical properties of open-cell nanoporous amorphous alloy

Nanoporous amorphous alloys exhibit outstanding mechanical properties, including enhanced ductility, high strength-to-density ratio, and exceptional toughness. In this paper, atomistic models of nanoporous CuZr amorphous alloys (NP–CuZr AAs) with self-similar microstructures but varying ligament sizes are constructed. Molecular dynamics simulations are employed to examine the effects of ligament size on their mechanical properties. The yield strength, yield strain, and Young's modulus are found to be higher under tension than under compression. This tension-compression asymmetry stems from the surface effect, and it becomes more pronounced with decreasing ligament size. As the ligament size increases, the Young's modulus and compressive yield strength increase, while the tensile yield strength and ultimate tensile strength decrease. The tensile behavior comprises linear elastic deformation, strain-hardening, and ligament decay stages. During the ligament decay deformation stage, ligament necking and fracture are more severe with larger ligament sizes, resulting in relatively lower resistance stress.

New Eco-Friendly Thermal Insulation and Sound Absorption Composite Materials Derived from Waste Black Tea Bags and Date Palm Tree Surface Fibers.

A tremendous amount of waste black tea bags (BTBs) and date palm surface fibers (DPSFs), at the end of their life cycle, end up in landfills, leading to increased pollution and an increase in the negative impact on the environment. Therefore, this study aims to utilize these normally wasted materials efficiently by developing new composite materials for thermal insulation and sound absorption. Five insulation composite boards were developed, two were bound (BTB or DPSF with polyvinyl Acetate resin (PVA)) and three were hybrids (BTB, DPSF, and resin). In addition, the loose raw waste materials (BTB and DPSF) were tested separately with no binder. Thermal conductivity and sound absorption coefficients were determined for all boards. Thermal stability analysis was reported for the components of the tea bag (string, label, and bag) and one of the composite hybrid boards. Mechanical properties of the boards such as flexural strain, flexural stress, and flexural elastic modulus were determined for the bound and hybrid composites. The results showed that the thermal conductivity coefficients for all the hybrid composite sample boards are less than 0.07 at the ambient temperature of 24 °C and they were enhanced as the BTB ratio was reduced in the hybrid composite boards. The noise reduction coefficient for bound and all hybrid composite samples is greater than 0.37. The composite samples are thermally stable up to 291 °C. Most composite samples have a high flexure modulus between 4.3 MPa and 10.5 MPa. The tea bag raw materials and the composite samples have a low moisture content below 2.25%. These output results seem promising and encouraging using such developed sample boards as eco-friendly thermal insulation and sound absorption and competing with the synthetic ones developed from petrochemicals in building insulation. Moreover, returning these waste materials to circulation and producing new eco-friendly composites can reduce the number of landfills, the level of environmental pollution, and the use of synthetic materials made from fossil resources.

Large Rupture Strain Cotton Ropes Hybridized with Affordable Fiberglass Chopped Strand Mat Sheets for Enhanced Compressive Behavior of Reinforced Concrete Columns

The hybrid confinement system combines various fiber types within a single matrix, allowing for the adjustment of volumetric ratios to optimize confinement performance. Synthetic FRPs are more expensive and have a higher carbon footprint due to significant CO2 emissions during production. In response, this study presents an innovative hybrid confinement approach using two natural materials: cotton ropes and FSMS (CFS) to improve concrete strength and ductility. Specimens, standardized at 300 mm height and 150 mm diameter with longitudinal steel bars and stirrups, were divided into two groups based on CFS configurations. The stress-strain response of CFS-confined concrete displayed distinctive behavior: an initial parabolic phase leading to peak compressive stress (ultimate strength), followed by a linearly degrading phase. Across all subgroups, CFS confinement significantly enhanced ultimate strength and corresponding compressive strains, with Subgroup 2A achieving the highest improvements of 246% in ultimate strength and 1477% in strain. Moreover, the ductility gain was reported as high as 20 for CFS-confined concrete. A non-proportional enhancement in the compressive behavior was observed with the increase in confinement ratio. Predictive models were developed for the idealized two-branch response of CFS-confined concrete, encompassing expressions based on nonlinear regression for ultimate strength, corresponding strain, ultimate strain, and elastic modulus. Two existing models were modified to trach each branch of the response. Integrating these two adjusted models closely replicated the experimental compressive curves.

Compressive stress-strain response of freshly compressed rubberized concrete: An experimental and theoretical study

Although using crumb tire rubber concrete (CTRC) in constructing structural elements is accompanied by various environmental and economic advantages, it always comes with challenges due to a significant loss in some mechanical properties of this concrete type. To tackle this challenge, a novel technique of compressing fresh concrete was used, and its impacts on improving the mechanical features of CTRC were investigated. For this purpose, concrete specimens containing crumb tire rubber (CTR), as the fine aggregate substitute, were constructed considering the variables of the water-to-cement ratio (w/c), volume percentage of CTR, and the fresh concrete pressure level as a fraction of steel tube (mold) yielding stress. After the application of a specified short-term pressure to the fresh mix inside the steel molds and the time required for the concrete setting, the steel tubes were removed after curing, and the concrete core alone was exposed to an axial compressive load. According to the test results, the mechanical properties, weight loss resulting from compressing the fresh concrete, energy absorption, failure pattern, the concrete microstructure, water absorption, and porosity were evaluated. By applying an initial axial pressure of 25–80 MPa on the mixes containing 0 %, 15 %, and 30 % CTR replacing the fine aggregate, the compressive strength, the ultimate strain, the toughness, and the initial modulus of elasticity experienced 100–140 % increase, 130–420 % increase, 120–1140 % increase, and 66–70 % decrease, respectively, compared with the uncompressed specimen. Finally, a regression analysis was employed to propose prediction models for the compressive capacity, modulus of elasticity, peak strain, ultimate strain, and relative toughness of freshly compressed concrete containing CTR particles; these models show proper agreement with the experimental results.

Bicuspid Valve Aortopathy: Is It Reasonable to Define a Different Surgical Cutoff Based on Different Aortic Wall Mechanical Properties Compared to Those of the Tricuspid Valve?

In this study, we examined and compared ex vivo mechanical properties of aortic walls in patients with bicuspid (BAV) and tricuspid (TAV) aortic valve aortopathy to investigate if the anatomical peculiarities in the BAV group are related to an increased frailty of the aortic wall and, therefore, if a different surgical cutoff point for ascending aortic replacement could be reasonable in such patients. Ultimate stress tests were performed on fresh aortic wall specimens harvested during elective aortic surgery in BAV (n. 33) and TAV (n. 77) patients. Three mechanical parameters were evaluated at the failure point, under both longitudinal and circumferential forces: the peak strain (Pstr), peak stress (PS), and maximum elastic modulus (EM). The relationships between the three mechanical parameters and preoperative characteristics were evaluated, with a special focus on evaluating potential risk factors for severely impaired mechanical properties, cumulatively and comparatively (BAV vs. TAV groups). The patient populations were inhomogeneous, as BAV patients reached surgical indication, according to the maximum aortic dilatation, at a younger age (58 ± 15 vs. 64 ± 13; p = 0.0294). The extent of the maximum aortic dilatation was, conversely, similar in the two groups (52 ± 4 vs. 54 ± 7; p = 0.2331), as well as the incidences of different phenotypes of aortic dilatation (with the ascending aorta phenotype being the most frequent in 81% and 66% of the BAV and TAV patients, respectively (p = 0.1134). Cumulatively, the mechanical properties of the aortic wall were influenced mainly by the orientation of the force applied, as both PS and EM were impaired under longitudinal stress. An age of >66 and a maximum dilatation of >52 mm were shown to predict severe Pstr reduction in the overall population. Comparative analysis revealed a trend of increased mechanical properties in the BAV group, regardless of the position, the force orientation, and the phenotype of the aortic dilatation. BAV aortopathy is not correlated with impaired mechanical properties of the aortic wall as such. Different surgical cutoff points for BAV aortopathy, therefore, seem to be unjustified. An age of >66 and a maximum aortic dilatation of >52 mm, however, seem to significantly influence the mechanical properties of the aortic wall in both groups. These findings, therefore, could suggest the need for more accurate monitoring and evaluation in such conditions.

Dynamic behavior of fractured gabbro treated by high temperatures

Rock masses in deep engineering are often subjected to high geothermal effect, and blasting is a key technique for deep rock excavation. It is crucial to study the dynamic characteristics of fractured rock masses under high geothermal conditions and blasting excavation. This paper focuses on fractured gabbro, examining the effects of high temperatures on its surface chromaticity and mass. Dynamic splitting tests were conducted using a Split Hopkinson Pressure Bar (SHPB) apparatus, and the microstructure was analyzed with scanning electron microscopy (SEM). The results indicate that: (1) High temperatures significantly affect the surface chromaticity and mass of gabbro samples. (2) Elevated temperatures lead to the progression from a microstructure with no noticeable microcracks to the development of extensive microcracks and mineral particle fragmentation. (3) The impacts of high temperatures on the splitting strength, peak strain, and dynamic elastic modulus of gabbro samples reach a critical point around 500 °C. At this temperature, the strengthening effect of high temperature is evident, whereas at 800 °C, these properties significantly decrease due to extensive mineral crystal fragmentation and microcrack expansion, weakening the sample's strength, stiffness, and deformation resistance. (4) From 25 °C to 600 °C, the fissure angle significantly influences the splitting strength, peak strain, and dynamic elastic modulus of gabbro. Although the fissure angle still impacts these properties at 800 °C, the overall effect of high temperature is more pronounced. (5) The failure of fractured gabbro samples is primarily characterized by tensile and shear failures, with the crack initiation angle closely related to the fissure angle.

Assessing design-induced elasticity of 3D printed auxetic scaffolds for tissue engineering applications

Auxetic scaffolds fabricated via additive manufacturing can enable cyclic mechanical stimulation to promote the biomechanical functionalization of engineered tissues. Typical designs of additively manufactured scaffolds used in tissue engineering literature (e.g., 0/90˚ strand laydown) are not amenable to cyclic loading due to their rigidity, which is in part due to the high stiffness of biopolymers such as polycaprolactone (PCL). Auxetic scaffolds can help overcome this due to their design-induced elasticity while recapitulating negative Poisson’s ratios seen in various natural tissues. In this study, we investigated the effects of auxetic design patterns and unit cell sizes on the mechanical properties of 3D bioplotted PCL scaffolds. First, we assessed the monotonic tensile properties of two auxetic patterns – re-entrant honeycomb and missing rib (unit cell = 3 × 3 mm2 for both) – in comparison to a uniaxial control (0/0˚ strand laydown) using finite element analysis (FEA) and experimental design (n = 3/group). The results showed that the scaffold design significantly impacted scaffold elasticity (p < 0.05), with the missing rib auxetic design demonstrating significantly higher yield strain (48.2 %) compared to the re-entrant honeycomb design (11.0 %) and the control (4.8 %). The missing rib design also possessed significantly lower elastic modulus and tensile strength (11.5 MPa/g and 10 MPa/g, respectively) compared to the re-entrant honeycomb (58 MPa/g and 35.7 MPa/g, respectively) (p < 0.05). For the missing rib design, we further investigated the effect of unit cell size (2 × 2, 3 × 2, 3 × 3 mm2) on the mechanical properties. Both 3 × 2 and 3 × 3 mm2 unit cell scaffolds (n = 3/group) possessed similar mechanical properties whereas the 2 × 2 mm2 unit cell scaffolds possessed significantly lower yield strain and higher elastic modulus and tensile strength (p < 0.05). The missing rib auxetic scaffolds were also tested under tensile cyclic loading for up to 6000 cycles at 10 % of maximum strain at 0.5 Hz. The 2 × 2 mm2 unit cell scaffolds degraded significantly faster than the other two groups. Overall, the 3 × 2 mm2 unit cell scaffolds performed better under cyclic loading in terms of maintaining their tensile strength. Finally, biocompatibility testing of the missing rib 3 × 2 mm2 unit cell scaffolds demonstrated their ability to support the adhesion and viability of fibroblast cells. In future, this knowledge will be leveraged to engineer scaffolds for connective tissues such as tendons and cardiac muscle.

Damage constitutive model of RAC under triaxial compression based on weibull distribution function

Fly ash (FA) and ground blast furnace slag (GGBS) can be used to densify the pore structure of recycled aggregate concrete (RAC). However, there remains a lack of deep exploration into the effects of confining pressure and mineral admixtures on RAC's crack formation and damage analysis. This study thus focused on investigating the combined impact of the replacement rate of recycled coarse aggregate (RCA), FA, and GGBS contents on the damage mechanisms of RAC under varying stress conditions by microscopic tests and triaxial compression tests. Results identified that increasing confining pressures enhance the peak stress, peak strain, and elastic modulus of RAC, with shear cracks predominating under triaxial compression. Whereas higher the replacement rate of RCA reduced the mechanical performance of RAC, incorporating 20 % FA and 20 % GGBS can significantly enhance its strength and modulus. Electron microscopy (ESEM) images proved that the incorporation of FA and GGBS lead to a denser internal structure compared to that of RAC without mineral admixtures. The constructed damage constitutive model agreed well with experimental data of this study and literatures, showing slower increases in damage variables (D) under triaxial stress compared to uniaxial compression. Model parameters indicated that increasing confining pressure decreased parameter m and increased F0, resulting in flatter post-peak stress-strain curves, and RAC incorporated with 20 % FA and 20 % GGBS had smaller m value under different stress states. These findings provide valuable insights for applying mineral-admixed RAC in diverse engineering scenarios, offering a robust reference for optimizing RAC formulations with FA and GGBS in practical applications.

Effects of palm oil fuel ash and crumb rubber on mechanical and thermal properties of sustainable engineered cementitious composites

The construction sector using natural sand causes rapid depletion and environmental harm, including erosion, biodiversity loss, and aquatic habitat destruction. To address these challenges and advance sustainability, this study systematically investigated the behaviour of engineered cementitious composites (ECC) incorporating 10–30 % ground palm oil fuel ash (GPOFA) and crumb rubber (CR) as a fine aggregate substitution. Two different sizes of GPOFA, P600 (in the range of 300 μm–600 μm) and P300 (less than 300 μm), were utilized to substitute the fine aggregate in the mix. Dry density, workability, thermal conductivity, modulus of elasticity, tensile strength and strain, compressive strength, flexural strength, and ultrasonic pulse velocity (UPV) were investigated. Furthermore, the microstructures and mineralogical compositions of the selected ECC samples were also examined. The results indicate that increasing GPOFA and CR content reduced ECC's flowability and density. Incorporating GPOFA reduced the compressive strength by 23 % for P600, 12 % for P300, and 74 % for CR, for 30 % substitution at 28 days. Both substitutions reduced the modulus of elasticity and UPV of ECC due to reduced packing efficiency and increased porosity. Increasing GPOFA improved the first cracking load slightly but decreased ultimate flexural strength, whereas CR significantly reduced both. In addition, tensile strain and strength dropped more significantly for ECC with CR compared to ECC with GPOFA. ECC with CR recorded the highest shrinkage and the lowest thermal conductivity (0.66W/mK). Overall, P600 outperformed P300, and CR showed the lowest performance in ECC across all parameters studied. This study highlights that partial replacement of silica sand with GPOFA could produce greener ECC with acceptable mechanical properties, while CR effectively reduces the thermal conductivity of ECC. Integrating GPOFA and CR adds significant value to these wastes while simultaneously providing a sustainable solution for waste disposal.

Understanding the effects of mineralization and structure on the mechanical properties of tendon-bone insertion using mesoscale computational modeling

Tendon-bone fibrocartilaginous insertion, or enthesis, is a specialized interfacial region that connects tendon and bone, effectively transferring forces while minimizing stress concentrations. Previous studies have shown that insertion features gradient mineralization and branching fiber structure, which are believed to play critical roles in its excellent function. However, the specific structure-function relationship, particularly the effects of mineralization and structure at the mesoscale fiber level on the properties and function of insertion, remains poorly understood. In this study, we develop mesoscale computational models of the distinct fiber organization at tendon-bone insertions, capturing the branching network from tendon to interface fibers and the different mineralization scales. We specifically analyze three key descriptors: the mineralization scale of interface fibers, the mean, and relative standard deviation of the local branching angles of interface fibers. Tensile test simulations on insertion models with varying mineralization scales of interface fibers and structures are performed to mimic the primary loading condition applied to the insertion. We measure and analyze five representative mechanical properties: Young's modulus, strength, toughness, resilience, and failure strain. Our results reveal that mechanical properties are significantly influenced by the three key descriptors, with tradeoffs observed between mutually exclusive properties. For instance, strength and resilience plateau beyond a certain mineralization scale, while failure strain and Young's modulus exhibit monotonic decreasing and increasing trends, respectively. Consequently, there exists an optimal mineralization scale for toughness due to these tradeoffs. By analyzing the mesoscale deformation and failure mechanisms from simulation trajectories, we identify three fracture regimes closely related to the trends in mechanical properties, supporting the observed tradeoffs. Additionally, we examine in detail the effects of the mean and relative standard deviation of local branching angles on mechanical properties and deformation mechanisms. Overall, our study enhances the fundamental understanding of the composition-structure-function relationships at the tendon-bone insertion, complementing recent experimental studies. The mechanical insights from our work have the potential to guide the future biomimetic design of fibrillar adhesives and interfaces for joining soft and hard materials.

CROSSLINK DENSITY AND ITS DISTRIBUTION IN HEAT AND OIL RESISTANT ELASTOMERS BY DOUBLE QUANTUM NUCLEAR MAGNETIC RESONANCE

ABSTRACT Double quantum (DQ) nuclear magnetic resonance (NMR) was used to characterize the crosslink density, crosslink density distribution, and defect level in a series of heat and oil resistant elastomers. A wide range of defect levels, crosslink densities, and crosslink density distributions was measured, and results depended on elastomer type and compound formulations, including the vulcanization system. The sol fraction defect level generally correlated with the concentration of added plasticizer in the formulation. The presence of polar side chains appeared to cause additional dynamic contributions to the dangling chain end fraction. The large differences in elastomer composition and rubber formulations prevented meaningful correlation of the measured crosslink densities with the low strain modulus. Fast Tikhonov regularization and log normalization fitting of the corrected DQ build-up curve was extremely useful to provide insight into the modality and widths of the crosslink density distributions. A high degree of heterogeneity of the crosslink network of heat and oil resistant elastomers was found. Crosslink density distributions were explained in terms of the polymer chain structure comprised of monomer sequencing coupled with the position of the crosslinking sites. The type of vulcanization system had a lesser effect of the nature of the crosslink density distribution. The primary polymer chain crosslinking sites may become segregated from the continuous phase due to polarity differences seen in the microstructure of oil and heat resistance elastomers. The development of such micromorphologies can favor curative partitioning. The sole use of DQ NMR can provide valuable insight into the nature of the polymer chain structure and crosslink network in rubber.

Dynamic characteristics of compacted landfill waste material from cyclic triaxial tests

This study conducted a series of cyclic and monotonic triaxial tests on reconstituted landfill waste material from a closed landfill site in Sydney, Australia, to assess its dynamic behaviour under various testing conditions. Specifically, the effects of cyclic deviatoric stress, loading frequency, and effective confining stress on the cumulative plastic axial strain, resilient modulus, and damping ratio under undrained cyclic loading conditions were investigated. Results indicated that the plastic deformation, resilient modulus, and material damping are significantly influenced by dynamic stress and confining stress, with a lesser impact from loading frequency. Notably, as the number of loading cycles increased, the cumulative plastic axial strain and resilient modulus exhibited an increase, whereas the damping ratio decreased. Furthermore, increasing cyclic deviatoric stress led to an increase in both cumulative plastic axial strain and damping ratio, while an increase in confining stress resulted in a decrease in these parameters. Conversely, the resilient modulus showed an increase with rising cyclic deviatoric stress and confining stress. The influence of loading frequency on cumulative plastic axial strain and resilient modulus was minor, and its effect on the damping ratio was rather negligible. The study observed that initial loading cycles caused rearrangement and reorientation of waste components and the mobilisation of fibres with tensile forces as loading progressed, suggesting that these landfill waste samples behaved comparably to fibrous soil with randomly distributed fibres. Through nonlinear regression analysis, an empirical relationship for cumulative plastic axial strain incorporating cyclic deviatoric stress, confining stress, number of cycles, and frequency was derived. This research contributes valuable insights into the behaviour of compacted landfills as railway subgrades, providing a foundation for informed decision-making in the design of transport infrastructure over closed landfill sites.

Study on the influence mechanism of interfacial inclination angle on the mechanical behavior of coal and concrete specimens

In recent years, the proposed distributed underground reservoir technology has attracted a lot of attention from experts and scholars in the field of engineering. This technology not only avoids the waste of mine water resources, but also realizes the rational utilization of coal mine underground space, and promotes the green and sustainable development of coal mining. As a storage facility of the underground reservoir, the stability of the coal-concrete composite structure will directly affect the long-term safe operation of the underground reservoir. In light of this situation, this study conducted uniaxial compression and acoustic emission experiments on coal-concrete specimens with interface inclinations of 0°, 15°, 30°, 45°, 60°, 75°, and 90° to investigate the impact of interface inclination on coal-concrete specimen stability. The test results show that the stress-strain curve passes through three stages: compression, elastic deformation, and deformation failure. The peak stress, peak strain, and elastic modulus exhibit a “U”-shaped distribution with varying angles, reaching a minimum at 60°. Crack analysis using RA-AF reveals that tension cracks predominate at 0°-30°, while shear cracks predominate at 45°-90°. The application of the Mohr-Coulomb criterion to coal-concrete specimens is discussed, and a derivation of the formula for calculating the ultimate principal stress is provided. The research in this paper provides a theoretical reference for the structural design and parameter selection of underground reservoir dams in coal mines.

Analysis of the Impact of Cooling Lubricants on the Tensile Properties of FDM 3D Printed PLA and PLA+CF Materials.

This study investigates the impact of infill density on the mechanical properties of fused deposition modeling (FDM) 3D-printed polylactic acid (PLA) and PLA reinforced with carbon fiber (PLA+CF) specimens, which hold industrial significance due to their applications in industries where mechanical robustness and durability are critical. Exposure to cooling lubricants is particularly relevant for environments where these materials are frequently subjected to cooling fluids, such as manufacturing plants and machine shops. This research aims to explore insights into the mechanical robustness and durability of these materials under realistic operating conditions, including prolonged exposure to cooling lubricants. Tensile tests were performed on PLA and PLA+CF specimens printed with varying infill densities (40%, 60%, 80%, and 100%). The specimens underwent tensile testing before and after exposure to cooling lubricants for 7 and 30 days, respectively. Mechanical properties such as tensile strength, maximum force, strain, and Young's modulus were measured to evaluate the effects of infill density and lubricant exposure. Higher infill densities significantly increased tensile strength and maximum force for both PLA and PLA+CF specimens. PLA specimens showed an increase in tensile strength from 22.49 MPa at 40% infill density to 45.00 MPa at 100% infill density, representing a 100.09% enhancement. PLA+CF specimens exhibited an increase from 23.09 MPa to 42.54 MPa, marking an 84.27% improvement. After 30 days of lubricant exposure, the tensile strength of PLA specimens decreased by 15.56%, while PLA+CF specimens experienced an 18.60% reduction. Strain values exhibited minor fluctuations, indicating stable elasticity, and Young's modulus improved significantly with higher infill densities, suggesting enhanced material stiffness. Increasing the infill density of FDM 3D-printed PLA and PLA+CF specimens significantly enhance their mechanical properties, even under prolonged exposure to cooling lubricants. These findings have significant implications for industrial applications, indicating that optimizing infill density can enhance the durability and performance of 3D-printed components. This study offers a robust foundation for further research and practical applications, highlighting the critical role of infill density in enhancing structural integrity and load-bearing capacity.

Effects of specimen size and loading rate on the mechanical property of Bentonil-WRK bentonite for engineered barrier system

This study aims to investigate the effects of specimen size and loading rate on the mechanical property of Bentonil-WRK bentonite buffer material for engineered barrier system. Highly instrumented unconfined compressive strength tests were performed on cylindrical specimens prepared using cold isostatic pressing (CIP) technique at varying testing conditions: (a) CIP pressures of 29.43 MPa, 39.24 MPa, and 58.86 MPa, (b) specimen sizes of 30 × 60mm, 40 × 80mm, and 50 × 100mm, and (c) loading rates of 0.5 %/min, 1.0 %/min, and 1.5 %/min. As CIP pressure increases, the unconfined compressive strength (qu), failure strain (εf), and elastic modulus (E) of Bentonil-WRK increase similar to Gyeongju compacted bentonite (KJ-II). Increase in diameter results in a decrease in qu and εf, and increase in E. Minimal effect of loading rate was observed on qu and εf. However, increasing loading rate tends to increase E. In addition, decrease in diameter increases the elastic threshold axial strain (εe,th). Increase in diameter and loading rate slightly increases Poisson's ratio. Prediction models are provided for assessing the effects of specimen diameter on qu and εf. Furthermore, correlation models of qu to ρd and E are proposed for Bentonil-WRK bentonite buffer material.