[Design and Testing of a Pressure Measurement and Adjustment Device for Fracture Ends].

Objective: To design and test a device which is capable of accurately measuring and dynamically adjusting the axial pressure at the fracture end in real-time. Methods: Upon completion of the design, the pressure measurement and adjustment device was implemented in a canine tibial fracture external fixation model. A pressure sensor was mounted at the fracture end, and the displayed values of the pressure sensor were used as the standard for comparison. The relationship between the displayed values of the measurement and adjustment device and the pressure sensor under identical conditions was examined. Results: The device was utilized in external fixation models of tibial fractures in five beagles. A linear correlation was observed between the displayed values of the device and the pressure sensor at the fracture end. The measurement values from the device could be transformed into fracture end pressure through the application of coefficients, thereby facilitating accurate measurement and dynamic adjustment of the fracture end pressure. Conclusion: The pressure measurement and adjustment device at the fracture end is easy to operate, enabling precise measurement and dynamic regulation of the pressure at the fracture end. It is well-suited for animal experiments aimed at investigating the impact of axial compression on fracture healing, demonstrating promising potential for experimental applications.

The Potential of Multi-Task Learning in CFDST Design: Load-Bearing Capacity Design with Three MTL Models.

Concrete-filled double steel tubes (CFDSTs) are a load-bearing structure of composite materials. By combining concrete and steel pipes in a nested structure, the performance of the column will be greatly improved. The performance of CFDSTs is closely related to their design. However, existing codes for CFDST design often focus on how to verify the reliability of a design, but specific design parameters cannot be directly provided. As a machine learning technique that can simultaneously learn multiple related tasks, multi-task learning (MTL) has great potential in the structural design of CFDSTs. Based on 227 uniaxial compression cases of CFDSTs collected from the literature, this paper utilized three multi-task models (multi-task Lasso, VSTG, and MLS-SVR) separately to provide multiple parameters for CFDST design. To evaluate the accuracy of models, four statistical indicators were adopted (R2, RMSE, RRMSE, and ρ). The experimental results indicated that there was a non-linear relationship among the parameters of CFDSTs. Nevertheless, MLS-SVR was still able to provide an accurate set of design parameters. The coefficient matrices of two linear models, multi-task Lasso and VSTG, revealed the potential connection among CFDST parameters. The latent-task matrix V in VSTG divided the prediction tasks of inner tube diameter, thickness, strength, and concrete strength into three groups. In addition, the limitations of this study and future work are also summarized. This paper provides new ideas for the design of CFDSTs and the study of related codes.

Finite element analysis and bearing capacity calculation of cross-shaped CFST columns under compressive load.

The study investigated the load capacity of cross-shaped concrete-filled steel tubular (CFST) columns under axial and eccentric compression using finite element software ABAQUS. It analyzed six specimens with measured data and an additional 26 specimens with varied parameters, including eccentricity, slenderness ratio, section steel ratio and material properties such as concrete strength and steel yield strength.The objective was to understand how these parameters affect the load capacity of cross-shaped CFST columns. The research findings suggest that as eccentricity and slenderness ratio increase, the ultimate capacity decreases. Conversely, it increases with higher steel content, concrete strength and steel yield strength. Moreover, the bearing capacity deteriorates more rapidly with reduced eccentricity and concrete strength, while it demonstrates a nearly linear increase with greater steel content. Additionally, the study found that enhancing the resilience of the channel steel significantly boosts the load-bearing capacity of the column. Based on these findings, practical design equations were developed to determine the maximum bearing capacity of cross-shaped CFST columns under axial and eccentric compression. These equations are grounded in confined concrete theory and demonstrate robust applicability for practical design purposes.

Load-Carrying Capacity of Thin-Walled Composite Columns with Rectangular Cross-Section under Axial Compression.

The aim of the current study was to determine the load capacity of composite columns subjected to axial compressive load. The subjects of the study were two types of columns with a rectangular cross-section, with different edge lengths. The tested columns had a closed cross-section. Four different fiber arrangements were analyzed for both cross-sections studied. The research was realized using interdisciplinary methods to determine the mechanism of damage to the composite material, with particular emphasis on damage initiation and propagation. Experimental tests were realized on a testing machine, the analysis was carried out with an acoustic emission system, and image analysis using visual assessment system of deflections of the walls of the structure. In addition, a number of numerical analyses were realized based on advanced modeling techniques for fiber-reinforced composites. A comparative analysis of both quantitative and qualitative results is presented for both analyses. The innovation of the presented research lies in the development of a custom method for modeling structures made of composite material with special emphasis on the failure phase. This will allow to accurately reflect the modeling of thin-walled structures with closed cross-section subjected to loading in a complex stress state.

Axial Compression Behavior of Circular Seawater and Sea Sand Concrete Columns Reinforced with Hybrid GFRP-Stainless Steel Bars.

The ductility of FRP-reinforced concrete structures is reduced by the brittleness of FRP bars. To address this issue, this study employs the hybrid reinforcement of stainless steel (SS) and GFRP bars to enhance the ductility of concrete columns. A total of 21 axially compressed seawater and sea sand concrete (SWSSC) circular columns are fabricated, including 15 hybrid GFRP and SS bar-reinforced SWSSC (GFRP-SS-SWSSC) columns, 3 GFRP bar-reinforced SWSSC (GFRP-SWSSC) columns, and 3 SS bar-reinforced SWSSC (SS-SWSSC) columns. The test results are analyzed in terms of failure mode, load-axial displacement curve, bearing capacity, and ductility. Results show that GFRP-SWSSC columns suffer brittle failure, while GFRP-SS-SWSSC columns and SS-SWSSC columns demonstrate ductile failure characteristics. Furthermore, the hybrid reinforcement contributes to an improvement in the bearing capacity of the columns. A calculation equation for the bearing capacity of axially compressed columns was established, providing reasonable predictions of bearing capacities, with a design compressive strain of 2000 µÎµ for GFRP bars. It was found that hybrid reinforcement enhanced the ductility of GFRP-SWSSC columns. In addition, when the percentage of the SS reinforcement ratio reaches 50%, the ductility indexes of the GFRP-SS-SWSSC columns closely approach those of the SS-SWSSC columns.

Effects of Freeze-Thaw Cycles on Axial Compression Behaviors of UHPC-RC Composite Columns.

Ultra-high performance concrete (UHPC) with excellent durability has broad application prospects in improving the durability of reinforced concrete (RC) structures. To clarify the influence of freeze-thaw cycles on the axial compression performance of UHPC-RC composite columns, axial compression tests were carried out on composite columns with different cycles (0, 100, 200, 300 cycles) and stirrup spacing (35, 70, 105 mm). The results showed that the UHPC shell did not fall off when the composite column was destroyed, even in the freeze-thaw environment. Under the action of freeze-thaw cycles, the peak load Nu,t and initial elastic modulus E of the composite column decreased, but the ductility coefficient µ increased. Increasing the stirrup spacing could significantly improve the ductility of the composite column. After 100 freeze-thaw cycles, the ductility coefficient µ of the 35 mm stirrup spacing specimen was 112.6% higher than that of the 105 mm specimen. A prediction model for the bearing capacity of UHPC-RC composite columns under freeze-thaw cycles was established, and the predicted results were in good agreement with the experimental results. This study lays a theoretical and experimental foundation for the application and design of UHPC-RC composite columns in the freeze-thaw environment.

Depth-Dependent Strain Model (1D) for Anisotropic Fibrils in Articular Cartilage.

The mechanical response of articular cartilage (AC) under compression is anisotropic and depth-dependent. AC is osmotically active, and its intrinsic osmotic swelling pressure is balanced by its collagen fibril network. This mechanism requires the collagen fibers to be under a state of tensile pre-strain. A simple mathematical model is used to explain the depth-dependent strain calculations observed in articular cartilage under 1D axial compression (perpendicular to the articular surface). The collagen fibers are under pre-strain, influenced by proteoglycan concentration (fixed charged density, FCD) and collagen stiffness against swelling stress. The stiffness is introduced in our model as an anisotropic modulus that varies with fibril orientation through tissue depth. The collagen fibers are stiffer to stretching parallel to their length than perpendicular to it; when combined with depth-varying FCD, the model successfully predicts how tissue strains decrease with depth during compression. In summary, this model highlights that the mechanical properties of cartilage depend not only on proteoglycan concentration but also on the intrinsic properties of the pre-strained collagen network. These properties are essential for the proper functioning of articular cartilage.

Buckling Analysis of Thin-Walled Composite Structures with Rectangular Cross-Sections under Compressive Load.

The purpose of this research was the analysis of the stability of compressed thin-walled composite columns with closed rectangular cross-sections, subjected to axial load. The test specimens (made of carbon-epoxy composite) were characterized by different lay-ups of the composite material. Experimental tests were carried out using a universal testing machine and other interdisciplinary testing techniques, such as an optical strain measurement system. Simultaneously with the experimental studies, numerical simulations were carried out using the finite element method. In the case of FEA simulations, original numerical models were derived. In the case of both experimental research and FEM simulations, an in-depth investigation of buckling states was carried out. The measurable effect of the research was to determine both the influence of the cross-sectional shape and the lay-up of the composite layers on the stability of the structure. The novelty of the present paper is the use of interdisciplinary research techniques in order to determine the critical state of compressed thin-walled composite structures with closed sections. An additional novelty is the object of study itself-that is, thin-walled composite columns with closed sections.

Feasibility of 3D-Printed Locking Compression Plates with Polyether Ether Ketone (PEEK) in Tibial Comminuted Diaphyseal Fractures.

The applicability of a polyether ether ketone locking compression plate (PEEK LCP) fabricated using FDM (fused deposition modeling)-based 3D printing to treat actual patients was studied. Three different tests-bending, axial compression, and axial torsion-were conducted on tibial non-osteoporotic comminuted diaphyseal fracture samples fixed with the commercial titanium alloy LCP and 3D-printed PEEK LCP. Comparing the outcomes of these tests revealed that the commercial titanium alloy LCP underwent plastic deformation in the bending and axial torsion tests, though the LCP did not fail even when an external force greater than the maximum allowable load of the tibia fixture of the LCP was applied. Elastic deformation occurred in the 3D-printed PEEK LCP in the bending and axial torsion tests. However, deformation occurred even under a small external force, and its stiffness was 10% compared to commercial titanium alloy LCP. Thus, 3D-printed PEEK LCP can be applied to the fracture conditions in non-weight-bearing regions. The experimental results reveal detailed insights into the treatment of actual patients by considering the stiffness and high toughness of 3D-printed PEEK LCP.

Axial Compressive Behavior of Cross-Shaped CFST Stub Columns with Steel Bar Truss Stiffening.

Concrete-filled steel tube (CFST) columns have been widely used in residential buildings due to their high bearing capacity, good ductility, and reliable seismic performance. However, conventional circular, square, or rectangular CFST columns may protrude from the adjacent walls, resulting in inconvenience in terms of the arrangement of furniture in a room. In order to solve the problem, special-shaped CFST columns, such as cross-shaped, L-shaped, and T-shaped columns, have been suggested and adopted in engineering practice. These special-shaped CFST columns have limbs with the same width as the adjacent walls. However, compared with conventional CFST columns, the special-shaped steel tube provides weaker confinement to the infilled concrete under axial compressive load, especially at concave corners. The parting at concave corners is the key factor affecting the bearing capacity and ductility of the members. Therefore, a cross-shaped CFST column with steel bar truss stiffening is suggested. In this paper, 12 cross-shaped CFST stub columns were designed and tested under axial compression loading. The effects of steel bar truss node spacing and column-steel ratio on the failure mode, bearing capacity, and ductility were discussed in detail. The results indicate that the columns with steel bar truss stiffening can change the final deformation mode of the steel plate from single-wave buckling to multiple-wave buckling, and the failure modes of columns also subsequently change from single-section concrete crushing failure to multiple-section concrete crushing failure. The steel bar truss stiffening shows no obvious effect on the axial bearing capacity of the member but significantly improves the ductility. The columns with a steel bar truss node spacing of 140 mm can only increase the bearing capacity by 6.8% while nearly doubling the ductility coefficient from 2.31 to 4.40. The experimental results are compared with those of six design codes worldwide. The results show that the Eurocode 4 (2004) and the Chinese code CECS159-2018 can be safely used to predict the axial bearing capacity of cross-shaped CFST stub columns with steel bar truss stiffening.

Study on Quasi-Static Axial Compression Performance and Energy Absorption of Aluminum Foam-Filled Steel Tubes.

To study the axial compression performance of aluminum foam-filled steel tube and empty steel tube as objects, such tubes are studied in this paper, which explores the carrying capacity and deformation behavior of aluminum foam-filled steel tube with different lengths under a quasi-static axial load through experimental research. The carrying capacity, deformation behavior, stress distribution, and energy absorption characteristics of empty steel tubes and foam-filled steel tubes are compared through finite element numerical simulation. The results indicate that, compared with the empty steel tube, the aluminum foam-filled steel tube still presents a large residual carrying capacity after the axial force exceeds the ultimate load, and the whole compression process reflects steady-state compression. In addition, the axial and lateral deformation amplitudes of the foam-filled steel tube decrease significantly during the whole compression process. After filling the foam metal, the large stress area decreases and the energy absorption capacity improves.

A lumbar disc degeneration model established through an external compressive device for the study of microcirculation changes in bony endplates.

Objective: To establish a new animal model of intervertebral disc degeneration (IDD) by axial compression on lumbar spine of rabbits and to investigate the changes of microcirculation in bony endplates during the progress of IDD. Methods: 32 New Zealand white rabbits were equally divided into 4 groups as follows: Control group with no operation and compression, Sham operation group with apparatus placement only, 2-week compression group and 4-week compression group with the devices installed and compressed for their preset duration. All groups of rabbits underwent MRI, histological evaluation, disc height index measurement and Microfil contrast agent perfusions to examine the ratio of endplate microvascular channels. Results: The new animal model of IDD was successfully established after axial compression for 4 weeks. The MRI grades for the 4-week compression group was 4.63 ± 0.52 and significantly different to the sham operation group (P < 0.05). Histologically, decrease of normal NP cells and extracellular matrix and disorganization of the architecture of the annulus fibrosus apparently occurred in 4-week compression group, which was different to the sham operation group (P < 0.05). There was no statistically difference between the 2-week compression and sham operation group no matter in the histology and MRI assessment. The disc height index slowly decreased as the compression duration rose. The ratio of microvascular channel volume within the bony endplate in 2-week and 4-week compression group were both reduced whereas the 4-week compression group obtained significantly less vascularization volume (6.34 ± 1.52 vs. 19.52 ± 4.63, P < 0.05). Conclusion: A new model of lumbar IDD was successfully established by axial compression and the volume of microvascular channels in the bony endplate gradually decreased as the grade of IDD increased. This model provides a new choice for etiological studies on IDD and investigation of nutrient supply disturbance.

Numerical Analysis on Dynamic Response of CFRP-Wrapped RC Columns under Lateral Impact Loading.

This paper presents a numerical study examining the dynamic response and resistance mechanism of reinforced concrete (RC) columns strengthened with or without carbon-fiber-reinforced polymer (CFRP) wraps under lateral impact loading by using the software LS-DYNA. First, the information of eight column models was briefly introduced as part of the laboratory experimental program from the literature. Secondly, finite element (FE) models were established in terms of the geometries of impact tests. Then, a detailed comparison between numerical results and experimental results was made, and FE models showed a relatively high simulation accuracy. Subsequently, a series of parametric analyses were carried out with a focus on the effects of axial compression ratio, the boundary condition at the column top, the layer number of CFRP wraps, and the impact velocity and impact height on the dynamic responses of plain and strengthened columns. The results demonstrated that the CFRP retrofit mechanism was not activated during the initial Stage-I when the impact force rapidly increased to the first peak and then decreased to zero. CFRP strengthening came into play in the second stage, Stage-II, and affected the response of the shear force and moment along the column height, as well as had a great influence on the control of shear damage. The dynamic response of RC columns was more sensitive to the impact velocity than to other parameters, regardless of whether CFRP wrapping was applied. The axial compression ratio would have a different influence on the column failure mode if the impact velocity was varied. The variation in impact height and boundary condition at the column top had little influence on the damage mode of strengthened columns.

On a three-dimensional model for the description of the passive characteristics of skeletal muscle tissue.

In this work, a three-dimensional model was developed to describe the passive mechanical behaviour of anisotropic skeletal muscle tissue. To validate the model, orientation-dependent axial ([Formula: see text], [Formula: see text], [Formula: see text]) and semi-confined compression experiments (mode I, II, III) were performed on soleus muscle tissue from rabbits. In the latter experiments, specimen deformation is prescribed in the loading direction and prevented in an additional spatial direction, fibre compression at [Formula: see text] (mode I), fibre elongation at [Formula: see text] (mode II) and a neutral state of the fibres at [Formula: see text] where their length is kept constant (mode III). Overall, the model can adequately describe the mechanical behaviour with a relatively small number of model parameters. The stiffest tissue response during orientation-dependent axial compression ([Formula: see text] kPa) occurs when the fibres are oriented perpendicular to the loading direction ([Formula: see text]) and are thus stretched during loading. Semi-confined compression experiments yielded the stiffest tissue ([Formula: see text] kPa) in mode II when the muscle fibres are stretched. The extensive data set collected in this study allows to study the different error measures depending on the deformation state or the combination of deformation states.

Mesoscale Analysis of the Effect of Interfacial Transition Zone on the Compressive Damage of Concrete Based on Discrete Element Method.

The coarse aggregate-mortar interface transition zone (ITZ) has a great influence on the mechanical properties of concrete, which cannot be easily studied using laboratory tests in the mesoscale. In this paper, a series of axial compression tests were conducted using the discrete element method (DEM) on concrete specimens for four phases: coarse aggregates, mortars, aggregate-mortar interface transition zones, and voids. The effects of ITZ strength on macroscopic stress and microscopic cracks under different strength reduction factors were investigated through axial compression testing. With the increase in interface transition strength, the compressive strength of the concrete becomes stronger; moreover, the number of cracks decreases, and the anisotropy of contact orientation becomes weaker. Meanwhile, the direction of crack development and the damage mode of compressed concrete specimens were also dependent on the coarse aggregate-mortar interface strength coefficient.

Theoretical Analysis and Experimental Verification of the Stress and Strain of Axially Compressed Steel-Reinforced Concrete Columns under Long-Term Loads.

The objective of this study is to provide a theoretical method to accurately calculate the stress and strain of steel-reinforced concrete (SRC) columns under long-term axial compression. First, considering the cross-sectional stress redistribution and the influence of each stress increment in the process, the theoretical formula of stress and strain under long-term loading was deduced. Then, the stress and strain calculation program of SRC columns under long-term axial compression was programmed by using object-oriented Visual C++ language. Finally, an experimental study on the long-term deformation performance of SRC axial compression columns was performed to validate the accuracy of the proposed theoretical method. By comparing the calculated results with the experimental results, the influence of steel bars on the long-term stress and strain of SRC columns under axial compression was analyzed and the corresponding long-term stress-strain variation law was studied. Results show that the changing trend of the long-term strain of plain concrete (PC) and SRC with loading time is basically the same, increasing rapidly in the first 270 days and gradually tending to be stable beyond 270 days. After 750 days, the maximum difference in the total strain between the PC columns and SRC columns reaches 26.60%, and the steel bars have a strong influence on the long-term strain of the concrete columns. The errors between the measured values of the two SRC columns, and the calculated results are 2.96% and 5.78%, respectively. Therefore, the derived stress-strain calculation formula and calculation program of SRC columns under long-term loads are accurate and reliable. When the loading time is 750 days, the tensile stress increment of 1.92 MPa and a compressive stress increment of 168.26 MPa are produced in concrete and steel bars. The long-term stress of concrete columns is markedly influenced by steel bars. In the first three years, the stress and strain of the concrete and steel bars develop rapidly and then gradually slow down.

Axial Compression Property Test of GFRP Tube-Confined Coal Gangue Steel Fiber Short Concrete Column.

In order to study the axial compression property of a GFRP (glass fiber-reinforced polymer) tube-confined coal gangue steel fiber short concrete column, a test was carried out. The whole process of deformation and failure of the specimen under axial compression load was observed, and the whole process of the stress-strain curve of the specimen was obtained. The results show that the thickness of the GFRP tube has the most significant effect on the mechanical properties. The thickness of the 7 mm tube is 4.3 times the axial ultimate stress and 21.5 times the ultimate strain of the unconstrained short column. Under a certain volume fraction, the ultimate axial strain of the wave fiber is 10.1% higher than that of the hook fiber short column, and the ductility coefficient is 9.6% higher. The fiber volume fraction significantly increases the strain of the short column, and the 3% fiber content is 50.1% higher than that of the non-fiber short column. Finally, three classical strength models of confined concrete were selected for comparative calculation, and a new stress correction model was proposed.

Effect of Aggregate Size on the Axial Compressive Behavior of FRP-Confined Coral Aggregate Concrete.

Using locally available raw materials for preparing concrete, such as coral reefs, seawater, and sea sand, is conducive to compensating for the shortage of construction materials used on remote islands. Jacketing fiber-reinforced polymer (FRP), as passive confinement, is a practical approach to enhance the strength, ductility, and durability of such coral aggregate concrete (CAC). Rational and economical CAC structural design requires understanding the interactions between the CAC fracture process and FRP confinement. The coral aggregate size is the critical parameter of their interaction since it affects the crack propagation of CAC and FRP confinement efficiency. This study conducted axial compression tests on FRP-confined CAC cylinders with varying coral aggregate sizes and FRP confinement levels. The test results indicate that the coral aggregate sizes affected the unconfined CAC strength. In addition, the dilation behavior of FRP-confined CAC varied with aggregate sizes, showing that CAC with smaller coral aggregate featured a more uniform hoop strain distribution and larger FRP rupture strain. These coupling effects are epitomized by the variation in the transition stress on the stress-strain curve, which makes the existing stress-strain models not applicable for FRP-confined CAC. A modified stress-strain model is subsequently proposed. Finally, the practical and environmental implications of the present study are discussed.

Diameter-Change-Induced Transition in Buckling Modes of Defective Zigzag Carbon Nanotubes.

In general, the insertion of Stone-Wales (SW) defects into single-walled carbon nanotubes (SWNTs) reduces the buckling resistance of SWNTs under axial compression. The magnitude of reduction is more noticeable in zigzag-type SWNTs than armchair- or chiral-type SWNTs; however, the relation between the magnitude of reduction and aspect ratio of the zigzag SWNTs remains unclear. This study conducted molecular dynamics (MD) simulation to unveil the buckling performance of zigzag SWNTs exhibiting SW defects with various tube diameter. The dependencies of energetically favorable buckling modes and the SW-defect induced reduction in the critical buckling point on the tube diameter were investigated in a systematic manner. In particular, an approximate expression for the critical buckling force as a function of the tube diameter was formulated based on the MD simulation data.

Correlating the microstructural architecture and macrostructural behaviour of the brain.

The computational simulation of pathological conditions and surgical procedures, for example the removal of cancerous tissue, can contribute crucially to the future of medicine. Especially for brain surgery, these methods can be important, as the ultra-soft tissue controls vital functions of the body. However, the microstructural interactions and their effects on macroscopic material properties remain incompletely understood. Therefore, we investigated the mechanical behaviour of brain tissue under three different deformation modes, axial tension, compression, and semi-confined compression, in different anatomical regions, and for varying axon orientation. In addition, we characterised the underlying microstructure in terms of myelin, cells, glial cells and neuron area fraction, and density. The correlation of these quantities with the material parameters of the anisotropic Ogden model reveals a decrease in shear modulus with increasing myelin area fraction. Strikingly, the tensile shear modulus correlates positively with cell and neuronal area fraction (Spearman's correlation coefficient of rs=0.40 and rs=0.33), whereas the compressive shear modulus decreases with increasing glial cell area (rs=-0.33). Our study finds that tissue non-linearity significantly depends on the myelin area fraction (rs=0.47), cell density (rs=0.41) and glial cell area (rs=0.49). Our results provide an important step towards understanding the micromechanical load transfer that leads to the non-linear macromechanical behaviour of the brain. STATEMENT OF SIGNIFICANCE: Within this article, we investigate the mechanical behaviour of brain tissue under three different deformation modes, in different anatomical regions, and for varying axon orientation. Further, we characterise the underlying microstructure in terms of various constituents. The correlation of these quantities with the material parameters of the anisotropic Ogden model reveals a decrease in shear modulus with increasing myelin area fraction. Strikingly, the tensile shear modulus correlates positively with cell and neuronal area fraction, whereas the compressive shear modulus decreases with increasing glial cell area. Our study finds that tissue non-linearity significantly depends on the myelin area fraction, cell density, and glial cell area. Our results provide an important step towards understanding the micromechanical load transfer that leads to the non-linear macromechanical behaviour of the brain.