Effects of Structural Parameters on the Poisson’s Ratio and Compressive Modulus of 2D Pentamode Structures Fabricated by Selective Laser Melting

Metamaterials have been receiving an increasing amount of interest in recent years. As a type of metamaterial, pentamode materials (PMs) approximate the elastic properties of liquids. In this study, a finite-element analysis was conducted to predict the mechanical properties of PM structures by altering the thin wall thicknesses and layer numbers to obtain an outstanding load-bearing capacity. It was found that as the thin wall thickness increased from 0.15 to 0.45 mm, the compressive modulus of the PM structures increased and the Poisson’s ratio decreased. As the layer number increased, the Poisson’s ratio of the PM structures increased rapidly and reaches a stable value ranging from 0.50 to 0.55. Simulation results of the stress distribution in the PM structures confirmed that stress concentrations exist at the junctions of the thin walls and weights. For validation, Ti–6Al–4V specimens were fabricated by selective laser melting (SLM), and the mechanical properties of these specimens (i.e., Poisson’s ratio and elastic modulus) were experimentally studied. Good consistency was achieved between the numerical and experimental results. This work is beneficial for the design and development of PM structures with simultaneous load-bearing capacity and pentamodal properties.

Sand control effect of HDPE sandbreak nets with different porosity structure

Straw checkerboard sand barriers with a porous structure that consists of a pervious upper portion and a dense lower portion are widely used to achieve great sand control effect.Considering this,and resolving the serious earth surface undercutting problem after HDPE sandbreak net checkboard barriers setting,the authors used HDPE(high-density polyethylene)materials to prepare new sandbreak materials with a similar porous structure.Through wind tunnel simulations and field sand control monitoring,we compared the sand control effect of three HDPE sandbreak nets with different porosity structure.Compared to the sandbreak net with uniform porosity structure,the three types of HDPE sandbreak nets with different porosity structure had poorer effect on reducing sand transport rates,but had longer effective protection distance before sandbreak nets at low wind velocity conditions(<12 m/s),longer effective protection distance at high wind velocity(>14 m/s)and longer effective protection distance between sandbreak nets at all experimental wind velocity conditions.Wind and sand control effect characteristics of HDPE sandbreak nets with different porosity structure provide an ideal material on semiburied checkerboard sand barriers for sand stabilization.By contrast,uniform-type sandbreak nets are used as materials on high upright sand fences for sand blocking.These HDPE sandbreak nets can be used to replace traditional sandbreak materials and have a very high potential for widespread and popular application in aeolian sand disaster control.

Very high cycle fatigue for GCr15 steel with smooth and hole-defect specimens

Very high cycle fatigue（VHCF） properties of a low temperature tempering bearing steel GCr15 with smooth and hole-defect specimens are studied by employing a rotary bending test machine with frequency of 52.5 Hz.Both smooth and hole-defect specimens break in VHCF regime with some difference in fatigue crack initiation.For smooth specimens,a fine granular area（FGA） is observed near the grain boundary in the fracture surface of the specimens broken after 10~7 cycles. But no FGA is observed in the hole-defect specimens broken in VHCF regime,and the VHCF crack does not initiate from the small hole at the surface as it does at low or high cycle fatigue regime. Internal stress is employed to explain the VHCF behavior of these two types of specimens.At last,an advanced dislocation model based on Tanaka and Mura model is proposed to illustrate the internal stress process and to predict fatigue crack initiation life with FGA observed in the fracture region.

Airflow patterns upwind of obstacles and their significance for echo dune formation:A field measurement of the effects of the windward slope angle

The velocities in front of five topographic obstacles with windward slope angles between 45° and 80° were measured using ultrasonic anemometers in the field at the southeastern margin of the Tengger Desert.The characteristics of the horizontal and vertical velocities indicated that the airflow pattern was significantly influenced by the obstacle's windward slope angle.The horizontal flow decelerated but the vertical flow accelerated upwind of the obstacles when the windward slope was less than 60°,but a reversed flow formed in front of steeper obstacles.The size of the area occupied by the reversed horizontal component increased with increasing windward slope,but the region of reversed vertical velocity became narrower and taller.Airflow turbulence was strengthened as the wind approached the obstacle,and low-frequency high-energy velocity fluctuations were observed upwind of the obstacle,and could affect the entrainment and saltation processes of sand particles.Turbulence intensities for both the horizontal and vertical components of the airflow increased with increasing windward slope and with decreasing distance from the obstacle.The deceleration and downward movement of airflow upwind of the obstacle would result in supersaturation of the air-sand flow,leading to deposition of moving sand,and these deposited grains would become the material basis for the formation of echo dunes.The reversed airflow would both determine the initial position of the echo dune and provide the necessary motive power to subsequently shape the echo dune.

3D Printed Ultra-thin Acoustic Metamaterials with Adaptable Low-frequency Absorption Performance

The inherent absorption frequency of traditional sound absorbers makes it difficult to solve the problem of acoustic wave removal in a changeable acoustic environment.In this study,acoustic absorption metamaterials(AAMs)with adaptable sound absorption performance were innovatively designed using the structural combination concept and fabricated via 3D printing.Accordingly,two coiled-up channels were combined in a single cell,which could effectively broaden the absorption bandwidth in a limited space.The longitudinal movement of the coiled-up channels endowed the tunable entire depth and internal cavity of the AAMs;thus,the sound absorption performance could be tailored accordingly.Through computational analysis and experimental verification,it was demonstrated that the depth of the AAM could be adjusted from 10 mm to 20 mm,and the corresponding absorption frequencies of the two channels ranged from 206 Hz to 179 Hz and 379 Hz to 298 Hz,respectively.In addition,the finite element results also indicate that the sound absorption bandwidth of AAMs could be further improved by the periodic arrangement of the units.This work opens a promising structural design approach for presenting a route toward acoustic devices with adaptable absorption performances.

A Century in Retrospect: Several Theoretical Problems of Russian Formalism From a Paradigm Perspective

The study of Russian Formalism from a paradigm perspective has an integrative methodological value compared with traditional approaches. From the three different levels(philosophy, knowledge and operation) inherent to the paradigm perspective, this paper reexamines several theoretical problems that were previously thought to be settled: the theoretical construction of Russian Formalism, its development and its theoretical foundation. It shows that the Russian Formalistic paradigm is not only a literary phenomenon, but also relates to the fields of linguistics and philosophy, and at the knowledge level consists of the three schools of Formalistic Literature, Formalistic Linguistics and Formalistic Functionalism. The paper also points out that the lifespan of the paradigm from beginning to decline was longer than the ten years generally believed by scholars. Finally, the paradigm's theoretical foundation does not originate from Husserl's phenomenology or the literary schools related to it, as previously assumed, but from the linguistic methodologies of historical comparativism and structuralism, as well as the common aesthetic theory within philosophy.

Status and trends in the stability of the three largest ice shelves in Antarctica

The Ross,Filchner-Ronne,and Amery ice shelves are the three largest ice shelves in Antarctica,playing a crucial role in supporting the Antarctic ice sheet.However,current studies on the stability of the three largest ice shelves primarily focus on singular or limited factors,lacking a comprehensive assessment of multiple parameters.To systematically and in-depth study the stability and trend of the three largest ice shelves,we comprehensively collected and analyzed key parameters,including elevation changes,basal melting,surface meltwater,major rifts propagation rate,suture zones,ice front area change rate,grounding lines,ice velocity,and mass balance.Additionally,we selected the collapsed Larsen B Ice Shelf(LBIS),the rapidly changing and structurally weakened Pine Island Ice Shelf(PIIS),and the accelerating Totten Ice Shelf(TIS)as reference ice shelves.By comparing and analyzing the key parameters between these reference ice shelves and the three largest ice shelves,we find the status and trends in the stability of the latter.Our findings reveal that most key parameters of the three largest ice shelves present relatively minor variations compared to those of the reference ice shelves.Specifically,50%of the parameters are smaller than those of the accelerating TIS,88%are smaller than those of the rapidly changing PIIS,and all parameters are smaller than those of the collapsed LBIS.Furthermore,after analyzing parameters that are not smaller than those of the TIS,it is observed that they remain in a stable state.Hence,the three largest ice shelves are currently undergoing natural changes that do not threaten their stability in the short term.Nevertheless,the evolution of the ice shelves under global climate change remains uncertain,making long-term observation and monitoring essential to assess their impact on sea level rise.