Compression Mode Research Articles

Compressive anisotropy effects and crack evolution mechanisms of 3D5d braided composites induced by thermo-oxidative ageing

Ageing cracks and loading directions will affect mechanical behaviors of thermo-oxidative aged 3D braided composites. Here we report ageing crack propagations, compressive responses, in-situ strain evolutions, full-field damage distributions and crack evolution mechanisms of aged three-dimensional five-directional (3D5d) braided composites along out-of-plane (OP) and in-plane (IP) loading based on X-ray micro-computed tomography and digital image correlation technique. Ageing cracks are distributed at both axial ends that corresponding to the yarn arrangement and extend to the interior with the increase in ageing time. Compressive modulus, strength, retention rate and damage mode show ageing weakening behaviors and anisotropy effects. Strain distributions exhibit structure correlation. Ageing cracks affect the surface strain and damage distributions but not internal shear modes during OP loading. Yarn arrangement affects damage evolution and accumulation. Ageing cracks change the damage evolution path in both directions. The synergistic effect of ageing crack with IP loading is greater than that with OP loading. It is expected that this work can provide a clear guideline for the structural optimization and durability design of 3D braided composites.

Performance-Based Expressway Asphalt Pavement Structural Surface Layer Modulus Matching Mode Research

The current asphalt pavement design theory is a traditional line elastic theory. The voltage resistance recovery modulus is used as the material stiffness parameter in the design, which does not fully consider the significant differences between the road material pull-up modulus. Therefore, to improve the overall stress of the pavement structure, this article started from the perspective of the volume of the structure layer, analyzed the development characteristics of reflected cracks in terms of the modular volume of different surface layers, and studied the size of each layer. It is proposed to match the asphalt pavement layer modulus with the crack development level. Based on structural computing and simulation, the application of different modular matching modes was verified, and support was provided for the design of the pavement structure. The results showed that with the increase of the surface modulus, the stress intensity factor determined by a semi-analytical method showed a nonlinear decrease trend, and the change of stress intensity factor was not obvious when the modulus was greater than 10,000; the surface layer compression mode ratio increased, and the vertical deformation of the road surface and the top surface pressure of the road base slowly increased, especially when the volume of the pressure pull-up ratio was greater than 1.5. In addition, the impact of the constraint between the surface layer on the vertical deformation of the road table decreased with the decrease in the volume of the surface layer.

Analysis of stentering and heat-setting on the structure and compression behavior of 3D mesh fabric using X-ray micro-computed tomography and microscopic modeling

Three-dimensional (3D) mesh fabric is a key component in ventilated car seats for its excellent cushioning and ventilating performance. It is produced by stretching an as-knitted spacer fabric with closed surfaces via stentering and fixing the mesh form via heat-setting. This paper analyzes the effect of stentering and heat-setting processes on the structures and compression properties of a typical and commercial 3D mesh fabric by using X-ray micro-computed tomography (μCT) and finite element (FE) models. The monofilament architectures of the fabric after knitting, stentering, and heat-setting are reconstructed from μCT scanning, and the structural evolution is quantitatively investigated in terms of global dimension change, curvature, and torsion. The results from μCT reconstructions demonstrate that stentering shortens, widens, and thins the fabric due to yarn transfer to shorten monofilament loops and lengthen spacer monofilaments, which are then dispersed, bent, and twisted to increase the curvatures and torsions. The FE simulations indicate that the global and local monofilament architecture deformation shifts the fabric compression mode from constrained post-buckling to concurrent tilting and post-buckling of spacer monofilaments, expanding the plateau stage and decreasing plateau stress. Heat-setting shrinks monofilament and therefore deteriorates its mechanical performance. The fabric is further thinned during the heat-setting process, which also further bends and twists spacer monofilaments to slightly decrease the plateau stress. The structures and cushioning performance of 3D mesh fabrics can be engineered by employing a proper stentering ratio followed by a heat-setting process.

Role of graded microstructure and electrolyte distribution in electrochemical capacitance of compressible three-dimensional carbon nanotubes-polymer foam based supercapacitor

Three-dimensional microstructure of carbon materials has a prime importance in supercapacitor for energy storage. Carbon nanotubes (CNT) provide microstructurally porous structure via entangled nodes that dynamically changes with the uniaxial compression. A unique graded CNT-polymer based three-dimensional compressible cellular foam is used as an electrode material for the solid-state supercapacitor. A direct correlation was established between the applied strain and resulting electrochemical capacitance when the foam electrode was soaked and unsoaked in the electrolyte. In a novel finding, unsoaked electrodes revealed in an extraordinary enhancement of ∼1216% in gravimetric capacitance measured at a uniaxial strain of 80% and to no surprise, the electrochemical capacitance of electrolyte-soaked electrodes remained nearly invariant from its uncompressed state. The response of electrochemical capacitance under compression is also observed varying with the mode of compression i.e. quasi-static versus pre-compressed state to a targeted strain. The role of graded microstructure during uniaxial compression was verified by using as-grown CNT mat for the electrodes of a supercapacitor. The results demonstrated a dominating contribution of dynamic variation in the density of nodes in the compressed CNT mat. The impact of macroscopic variation on the response of electrochemical capacitance of a compressed cellular CNT-polymer foam was further understood using the separators of varying pore sizes. Our results pave the way to engineer the three-dimensional compressible cellular structures for extraordinarily large energy storage capacity.

Combined effects of freeze–thaw cycles and chemical corrosion on triaxial mechanical properties of sandstone

To study the combined effects of freeze–thaw cycles and chemical corrosion on deterioration mechanism of triaxial mechanical properties of sandstone in cold regions, triaxial compression tests were carried out on homologous sandstone specimens after corrosion action of acidic, alkaline or neutral solution and cyclic freeze–thaw action, by taking the pH value of the hydrochemical solution and the number of freeze–thaw cycles as control parameters. The failure mode of triaxial compression is shear-slip failure, and the deviatoric stress–strain curves can be divided into four stages of compaction, linear elastic, yield, and failure. The stress–strain curves and deterioration law of triaxial mechanical properties were analyzed. With the increase of freeze–thaw cycles, the peak stress, peak strain and modulus all decrease, the deterioration degree of the rock specimens increases, and the yield plateau and plastic characteristics in the yield stage become weaker. The strong acid solution shows the largest degree of damage, and the peak stress, peak strain and modulus decreased significantly. The precipitates produced by the reaction between the strong alkaline solution and the specimens adhere to the specimen surface, pores, and fissures, thereby inhibiting further damage and deterioration. Confining pressure is an essential factor affecting the rock mechanical properties. As confining pressure increases, the strength and deformation-resistant ability of specimens increase, and the yield stage on the stress–strain curve is more obvious. The sensitivity of acidic solution to the deterioration of mechanical properties of sandstone is obviously greater than that of alkaline solution. These results provide a theoretical basis for the construction of geotechnical engineering structures in cold regions.

Model for calculating local bearing capacity of concrete with spiral stirrups

This study evaluates the mechanical and failure modes of 30 concrete specimens under local compression. The experimental results indicate that the specimens undergo three types of local compressive failure modes. These modes primarily depend on the strengths of the tensile and compression regions. Based on the test results, a calculation model for the local bearing capacity of concrete with spiral stirrups Fl is formulated. In the model, the local compressive failure of concrete with spiral stirrups is classified into tension and compression. The yielding of spiral stirrups in the tensile region is regarded as tensile failure, and compressive failure is considered to have occurred when the strain value in the compression region reaches 0.002. To simplify the model calculation, the spacing s, diameter d, and yield strength fy of spiral stirrup as well as the local compression area ratio Ab/Al (ratio of the concrete bearing area Ab to the loading area Al) are comprehensively analyzed. Based on the analysis results, a formula for the bearing capacity of the reinforcement Ns is derived. The independent variables of the formula are yield force of spiral stirrup fyAssl, stirrup spacing s, and ratio of specimen side length to loading plate side length h/a. A simplified formula for calculating the tensile failure of the specimen is derived. To ensure simultaneous tensile and compressive failures, a critical reinforcement ratio of the spiral stirrup ρvu is proposed based on the calculation model. Test data from similar axial local compression tests are collected to verify the accuracy of the calculation model. The local bearing capacity of concrete is recalculated according to the calculation method proposed in this study. Results show that the calculation model is highly accurate. Accordingly, it may be used as reference for calculating the local bearing capacity.

Transduction of Single Nanomechanical Pillar Resonators by Scattering of Surface Acoustic Waves.

One of the challenges of nanoelectromechanical systems (NEMS) is the effective transduction of the tiny resonators. Vertical structures, such as nanomechanical pillar resonators, which are exploited in optomechanics, acoustic metamaterials, and nanomechanical sensing, are particularly challenging to transduce. Existing electromechanical transduction methods are ill-suited as they put constraints on the pillars' material and do not enable a transduction of freestanding pillars. Here, we present an electromechanical transduction method for single nanomechanical pillar resonators based on surface acoustic waves (SAWs). We demonstrate the transduction of freestanding nanomechanical platinum-carbon pillars in the first-order bending and compression mode. Since the principle of the transduction method is based on resonant scattering of a SAW by a nanomechanical resonator, our transduction method is independent of the pillar's material and not limited to pillar-shaped geometries. It represents a general method to transduce vertical mechanical resonators with nanoscale lateral dimensions.

Numerical model of cumulative damage evolution of offshore wind turbine grouted connection under cyclic axial load

GCs in OWTs are exposed to fatigue loads that can lead to damage of the grout before ultimate limit state is reached. A uniaxial and fatigue compression test of high strength grout material were carried out to obtain the compressive strength, failure mode, fatigue life and load-displacement relationship of test cubes. A uniaxial and fatigue tests were carried out on GCs specimens to study the damage evolution of the GCs. The fatigue test on GC was done by applying six incremental amplitude cyclic load ranges of 30%–80% of ultimate load on the GC. The scalar damage degradation parameter (SDEG) from the result of the numerical simulation was used to estimate a damage index which measured the cumulative damage evolution. The cumulative damage evolutions were influenced by the cyclic load amplitude. The larger the load amplitude, the higher the damage index accumulated. The numerical simulation results compared with the DNV code method showed that the DNV code predicted lower damage values for fatigue loads ratio of up to 0.725 and predicted higher damage values beyond the 0.725 loads ratio for the GC. The proposed numerical model predicted the laboratory test results of the grout cube and the grouted connections.

Abstract #1408105: Treatment of Bisphosphonate Associated Atypical Femur Fracture with a Combination of Teriparatide and a Novel Surgical Technique

Atypical femur fractures (AFF) caused by long term Bisphosphonate use are associated with high rates of delayed healing and non-union, posing a significant challenge in the management of these patients. We describe the case of a 64-year-old female with osteopenia on Bisphosphonate therapy for 15 years who sustained a displaced left AFF following a fall from standing height. She reported a 6-month history of antecedent left groin pain. Radiographs of the left femur showed an acute displaced transverse diaphyseal fracture with lateral cortical thickening and beaking. The right femur radiograph showed features of abnormal cortical remodeling with thickened, irregular cortices. DEXA showed T-scores of -2.1 at the left femoral neck, -1.4 at the left total hip, and -1.4 at the lumbar spine. She underwent an open reduction and internal fixation with insertion of a cephalomedullary nail placed in compression mode. A novel technique of intraoperative removal of the endosteal hypertrophied cortical bone at the fracture with a Midas rex cutting burr was used and there was excellent cortical contact after reduction. She had immediate relief of her pain and was permitted to fully weight-bear postoperatively. Her bisphosphonate was stopped immediately and Teriparatide was initiated at 7 weeks post-operatively. Radiographs 8 weeks after initiation of Teriparatide showed evidence of normal fracture union with mature callus formation. AFF caused by prolonged Bisphosphonate use have a high rate of delayed healing and non-union due to abnormal bone remodeling. Use of Teriparatide post-operatively has been shown to reduce healing time in recent small studies in surgically treated patients. Our case demonstrates expedited healing time when Teriparatide was combined with a novel surgical technique involving removal of a portion of the abnormal fracture end and placement of an intramedullary nail in compression mode. Furthermore, the patient had radiographic evidence of cortical thickening on her right femur, increasing her risk for contralateral AFF. Prophylactic surgical stabilization was recommended but the patient has deferred this. She remains fracture-free and pain-free on the right side, suggesting a possible benefit of Teriparatide use in patients at risk for impending AFF. Our case demonstrates an expedited healing time (3.5 months) relative to previously reported average healing times for AFF (10.7 months), supporting the use of the combination of Teriparatide and a novel surgical technique.

CO enhancement by magnetohydrodynamic waves

Context. The formation of molecular gas in interstellar clouds is a slow process, but can be enhanced by gas compression. Magneto-hydrodynamic (MHD) waves can create compressed quasi-periodic linear structures, referred to as striations. Striations are observed at the column densities at which the transition from atomic to molecular gas takes place. Aims. We explore the role of MHD waves in the CO chemistry in regions with striations within molecular clouds. Methods. We targeted a region with striations in the Polaris Flare cloud. We conducted a CO J = 2−1 survey in order to probe the molecular gas properties. We used archival starlight polarization data and dust emission maps in order to probe the magnetic field properties and compare against the CO morphological and kinematic properties. We assessed the interaction of compressible MHD wave modes with CO chemistry by comparing their characteristic timescales. Results. The estimated magnetic field is 38–76 µG. In the CO integrated intensity map, we observe a dominant quasiperiodic intensity structure that tends to be parallel to the magnetic field orientation and has a wavelength of approximately one parsec. The periodicity axis is ~17° off from the mean magnetic field orientation and is also observed in the dust intensity map. The contrast in the CO integrated intensity map is ~2.4 times higher than the contrast of the column density map, indicating that CO formation is enhanced locally. We suggest that a dominant slow magnetosonic mode with an estimated period of 2.1–3.4 Myr and a propagation speed of 0.30–0.45 km s−1 is likely to have enhanced the formation of CO, hence created the observed periodic pattern. We also suggest that within uncertainties, a fast magnetosonic mode with a period of 0.48 Myr and a velocity of 2.0 km s−1 could have played some role in increasing the CO abundance. Conclusions. Quasiperiodic CO structures observed in striation regions may be the imprint of MHD wave modes. The Alfvénic speed sets the dynamical timescales of the compressible MHD modes and determines which wave modes are involved in the CO chemistry.

Highly Stretchable Printed Poly(vinylidene fluoride) Sensors through the Formation of a Continuous Elastomer Phase

Stretchable piezoelectric stress/strain sensing materials have attracted substantial research interest in the fields of wearable health monitoring, motion capturing, and soft robotics. These sensors require operation under dynamic loading conditions with high strain range, changing strain/loading rates, and varying pre-stretch states, which are challenging conditions for existing sensors to produce reliable measurements. To overcome these challenges, an intrinsically stretchable poly(vinylidene fluoride) (PVDF) sensor is developed through the polymer blending of PVDF and acrylonitrile butadiene rubber (NBR). Through precipitation printing and vulcanization, the resulting PVDF/NBR blends exhibit strong β phase PVDF and a blend morphology with submicron-level phase separation, but also strains up to 544%. Both the blend morphology and the mechanical properties indicate that this PVDF/NBR blend can be considered as a continuous elastomer phase above micron scale. After electric poling and adding electrodes, the PVDF/NBR blends have excellent piezoelectric properties to be used as both stretching mode strain sensors and compression mode stress/force sensors. The stretching mode sensors can measure strain up to 70% without strain rate and pre-stretch dependence, while the compression mode sensors have a loading-rate-independent linear voltage-stress relationship up to 4.8 MPa stress and a negligible pre-stretch dependence. Therefore, the PVDF/NBR sensors can provide accurate and reliable stress/strain measurements when attached to soft structures, which paves the way for sensing and calibration of soft robots under dynamic loading conditions.

Interaction of compression failure mechanisms in composites with ply waviness defects

This article presents results from experimental and computational investigations of composite laminates with embedded ply waviness defects of different geometries under compression. The effect of the ply waviness defect on the initiation, interaction, and evolution of failure mechanisms of surrounding plies are studied. Using high-resolution imaging and digital correlation analysis, the strain localization phenomenon is monitored, and the sequence of failure mechanisms is established. The experimental observations are supported with computational finite element progressive failure analyses of the same composites with ply waviness defects using an energy based continuum damage mechanics approach. For the case of laminates with ply waviness defects of high angles, a delamination crack between the wavy plies and adjacent transverse plies is the dominant mode of failure, which interacts strongly with the fiber compression and matrix tension modes. For the case of laminates with ply waviness defects of low angles, failure does not initiate at the waviness defect but at locations where the laminate strength was locally reduced due to other microstructural defects. The combined experimental and computational analyses presented here advanced the understanding of the interaction of compression failure mechanisms in composites with ply waviness defects, which is highly challenging to experimentally measure.

Effect of Silicone Modifier on the Physical Properties of Flexible Silica Aerogels.

Research on the development of flexible silica aerogels (FSAs) has been ongoing due to their excellent thermal insulation, low density, and high elasticity. However, the physical properties of FSAs, such as density, thermal conductivity, mechanical strength, and surface wettability, are highly dependent on the preparation conditions. To achieve the desired properties of FSAs for various applications, it is necessary to develop a method to fine-tune their physical properties. In this paper, two modifiers of methyltrimethoxysilane (MTMS)/trimethylethoxysilane (TMES) were employed to fine-tune the bulk density of a series of flexible silica aerogels (FSAs), reflecting a series of FSAs with fine-tunable physical properties. First, the precursor was synthesized by a click reaction between vinyltrimethoxysilane (VTMS) and 2,2' (ethylenedioxy) diethanethiol (EDDET). The VTMS, EDDET, and the as-prepared precursor were characterized by FT-IR and NMR spectroscopy. Subsequently, the precursor was converted into a series of FSAs (denoted by FSA, FSA-M, and FSA-T) through conventional sol-gel reactions with/without MTMS/TMES. Chemical structures of synthesized FSAs were confirmed by 13C and 29Si solid-state NMR spectroscopy. The porous structure of FSAs was identified by BET and SEM, respectively. Physical properties, such as thermal conductivity, mechanical strength, and surface wettability of FSAs were determined by a Hot Disk, durometer/DMA in compression mode, and contact angle measurements, respectively. This study found FSAs containing none, 1 wt%, 5 wt%, and 10 wt% of MTMS increase the density of FSAs from 0.419 g/cm3 (FSA), 0.423 g/cm3 (FSA-M1), 0.448 g/cm3 (FSA-M5), and 0.456 g/cm3 (FSA-M10). It should be noted that the thermal conductivity, surface hardness, bulk mechanical strength, and hydrophobicity of FSA-Ms of increasing MTMS loading were all found to show a rising trend, while FSA-Ts exhibited lower density. FSA-T10 exhibited lower thermal conductivity, surface hardness, and bulk mechanical strength as compared to FSA. However, it was found to show higher hydrophobicity as compared to that of FSA.

Application of Miller yield criterion to develop a unified fatigue law of asphalt mixture for different loading modes

AbstractFatigue performance of asphalt mixture has been tested through different types of loading modes, including the uniaxial tension (UT), uniaxial compression (UC), and indirect tension (IDT) modes. However, fatigue test results from different modes are found to be dissimilar. This research aimed to develop a unique fatigue law suitable for different loading modes, using the Miller yield criterion. First, the strength and fatigue behaviors of asphalt mixture under UT, UC, and IDT modes were compared and discussed. Then, the strength yield surfaces and stress paths were obtained by the Miller yield criterion. Finally, a new stress ratio indicator was proposed to generate a unified fatigue law of asphalt mixtures that are suitable for different loading modes. The findings in this research are expected to provide references for deriving the fundamental fatigue damage law for the asphalt mixture based on the strength yield criterion.

Size effect on dynamic compressive failure mode and mechanical properties of recycled concrete

AbstractIn order to study the size effect behavior of recycled concrete under dynamic compression load, a meso‐scale model of recycled concrete was established. The particle size of coarse aggregate, replacement ratio of recycled coarse aggregate, and section size were used as parameters to explore the mechanical properties of recycled concrete under uniaxial dynamic compression load, such as failure mode, compressive strength, peak strain, and residual strength. The effects of strain rate, particle size of recycled coarse aggregate, and replacement rate of recycled coarse aggregate on the size effect of dynamic compression of recycled concrete were further analyzed. The results showed that under uniaxial dynamic compression load, the damage degree of recycled concrete deep and along with the change of the strain rate change, recycled concrete dynamic compression strength and strain rate were positively correlated. There is the critical strain rate is 1/s, which weaken the influence of the size effect, when the strain rate reaches 10/s, the dynamic compressive strength of recycled concrete increases with the increase of section size. At the same time, when the strain rate is high, the particle size of recycled coarse aggregate and the replacement rate of recycled coarse aggregate have little effect on the size effect behavior of recycled concrete.

Evaluation of Hybrid Fiber Multiscale Polymer Composites for Structural Confinement under Cyclic Axial Compressive Loading

Fiber reinforced polymer (FRP) confinement is recognized as the most promising technique for the strengthening and retrofitting of concrete structures. In order to enhance the performance of conventional epoxy-based FRP composites, nano filler modification of the epoxy matrix was implemented in the current study. In particular, the cyclic loading response of standard concrete specimens externally confined by epoxy-based natural and hybrid fiber reinforced polymer systems was investigated. The confinements were realized with sisal fiber reinforced polymer (SFRP) and hybrid sisal basalt fiber reinforced polymer (HSBFRP). Moreover, the effects of multiwalled carbon nanotubes (MWCNT) were also investigated. Three different specimen sets were considered for study: (i) unconfined specimens, (ii) epoxy-based FRP confined specimens and (iii) MWCNT incorporated epoxy-based FRP confined specimens. The specimens were tested in repeated compressive mode in loading-unloading cycles at increasing displacement levels. The test results revealed that FRP wrapping could enhance the mechanical behavior of unconfined columns in terms of strength and ductility. Moreover, it was evident that the mechanical properties of the epoxy matrix were enhanced by MWCNT incorporation. The developed epoxy-based FRP confinement containing MWCNT ensures improvement in axial strength by 71% when compared with unconfined specimens. The epoxy-based FRP confinement, with and without MWCNT, exhibited a high strain redistribution behavior around the concrete core. In comparison to the unconfined specimens, the confinement could increase the sustained axial strain from 0.6 to 1.4% using epoxy-based FRP confinement and to 1.6% with MWCNT incorporated epoxy-based FRP confinement. Further, an empirical model was developed to predict the ultimate axial stress of concrete columns confined externally with FRP jackets. The ultimate compressive strength obtained from the experimental study was compared with the proposed model, and the observed deviation was lower than 1%.

Reuse of waste rubber in pervious concrete: Experiment and DEM simulation

The reuse of waste rubber has a profound impact on environmental protection and resource recycling. To evaluate the feasibility of adding waste rubber into pervious concrete, the effects of rubber content and particle size on the mechanical properties of pervious concrete are investigated through experiments and discrete element method (DEM) simulation. The compressive failure mode of pervious concrete gradually transits from tensile failure to shear failure with the increase of rubber content, showing a decrease in compressive strength, and the larger the particle size is, the stronger the attenuation effect on the strength is. Rubber particles play a role in isolating coarse aggregates from the cement matrix and form a weak bond with the matrix, showing a weak bridging effect in stress transmission at the left and right ends of the crack, so that the splitting tensile strength of pervious concrete significantly decreases with the increase of rubber content, and the weakening effect of coarse rubber on the splitting tensile strength is stronger than that of fine rubber. Due to the low strength of rubber itself and weak bonding with cement matrix, pervious concrete with high rubber content breaks in advance at the bottom of the midspan, resulting in the decline of flexural capacity, and it decreases more obviously with the increase of rubber size. Although the direct tensile strength of pervious concrete gradually decreases with the increase of rubber content, the addition of rubber improves the plasticity of pervious concrete. Moreover, the plastic reinforcement effect of pervious concrete is the strongest when the rubber content reaches approximately 10%, and this toughening effect increases with the decrease of rubber size. On the basis of experimental research, a universal DEM model considering rubber content and particle size is established, which can well predict the mechanical performance of rubberized pervious concrete in terms of strength, failure mode, elastic modulus and ultimate strain.

Universal gravity-driven isothermal turbulence cascade in disk galaxies

While interstellar gas is known to be supersonically turbulent, the injection processes of this turbulence are still unclear. Many studies suggest a dominant role of gravitational instabilities. However, their effect on galaxy morphology and large-scale dynamics varies across cosmic times, in particular, due to the evolution of the gas fraction of galaxies. In this paper, we propose numerical simulations to follow the isothermal turbulent cascade of purely gravitationally driven turbulence from its injection scale down to 0.095 pc for a gas-poor spiral disk and a gas-rich clumpy disk. For this purpose, and to lift the memory-footprint technical lock of sufficiently resolving the interstellar medium of a galaxy, we developed an encapsulated zoom method that allows us to self-consistently probe the self-generated turbulence cascade over three orders of magnitude on spatial scales. We followed this cascade for 10 Myr. We find that the turbulent cascade follows the same scaling laws in both setups. Namely, in both cases, the turbulence is close to equipartition between its compressive and solenoidal modes, the velocity power spectrum follows the Burgers scaling, and the density power spectrum is rather shallow, with a power-law slope of −0.7. Last, gravitationally bound substructures follow a mass distribution with a −1.8 slope, similar to that of CO clumps. These simulations thus suggest that gravity-driven isothermal turbulent cascades are universal in disk galaxies across cosmic time.

The compressive responses and failure behaviors of composite graded auxetic re-entrant honeycomb structure

This paper investigated the quasi-static compressive performances and failure behaviors of composite auxetic re-entrant honeycomb sandwich structure. Three types of auxetic re-entrant honeycomb structures with different gradient configurations were manufactured and tested. The carbon/epoxy prepreg was used to fabricated the auxetic re-entrant honeycomb sandwich structure. The reaction force and displacement of the test fixture were collected by the transducer of the universal testing machine. The compressive processes of specimens were recorded by high-resolution camera. Combined with the stress–strain curves and deformation processes of composite structures, the compressive responses and deformation mechanisms was analyzed. The Poisson’s ratio, energy efficiency and plateau stress of re-entrant honeycomb sandwich structures were defined and utilized to study the auxetic performance and energy absorption ability. Furthermore, a high-fidelity numerical model was established and finite element software ABAQUS was used to analyzed the compressive performances and failure behaviors of composite graded auxetic re-entrant honeycomb structure. The compressive failure modes of the numerical simulation results were compared with that of the experimental results, and the numerical simulation results agreed well with the experimental results. The results indicated that average structure exhibited best energy absorption performance and bidirectionally graded auxetic honeycomb structure had the best negative Poisson’s ratio performance.

A Numerical Study on the Influence of Cerebrospinal Fluid Pressure on Brain Folding

AbstractOver the past decades, the buckling instability of layered materials has been the subject of analytical, experimental, and numerical research. These systems have traditionally been considered with stress-free surfaces, and the influence of surface pressure is understudied. In this study, we developed a finite element model of a bilayer experiencing compression, and found that it behaves differently under surface pressure. We investigated the onset of buckling, the initial wavelength, and the post-buckling behavior of a bilayer system under two modes of compression (externally applied and internally generated by growth). Across a wide range of stiffness ratios, 1 < μf/μs < 100, we observed decreased stability in the presence of surface pressure, especially in the low-stiffness-contrast regime, μf/μs < 10. Our results suggest the importance of pressure boundary conditions for the stability analysis of bilayered systems, especially in soft and living matter physics, such as folding of the cerebral cortex under cerebrospinal fluid pressure, where pressure may affect morphogenesis and buckling patterns.