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Dynamics Near the Subcritical Transition of the 3D Couette Flow I: Below Threshold Case
The authors study small disturbances to the periodic, plane Couette flow in the 3D incompressible Navier-Stokes equations at high Reynolds number Re. They prove that for sufficiently regular initial data of size $\\epsilon \\leq c_0\\mathbf {Re}^-1$ for some universal $c_0 > 0$, the solution is global, remains within $O(c_0)$ of the Couette flow in $L^2$, and returns to the Couette flow as $t \\rightarrow \\infty $. For times $t \\gtrsim \\mathbf {Re}^1/3$, the streamwise dependence is damped by a mixing-enhanced dissipation effect and the solution is rapidly attracted to the class of \"2.5 dimensional\" streamwise-independent solutions referred to as streaks.
eBook
Experimental study on shear failure mechanism of loose spoil soil considering the adverse factor of water
Loose spoil soil (LSS) is a kind of granular material with unstable structure, for which the deformation and even failure should be induced by changing of water conditions. This research was conducted based on an engineering spoil dump, and considered the relative density (Dr) and existed stress environment of LSS, the authors designed detail experimental plans and improved experimental equipment, a comprehensive study focused on adverse factor of water in shear failure mechanism of LSS has been achieved, conclusions were obtained as follows: 1. Under the three selected Dr0 (0.35/0.40/0.45), LSS showed strain hardening during whole shear process, shear deformation mechanism could be summarized as the big friction deformation of unstable granular materials; 2. the adverse effect of water on LSS was significant, as the increase of main shear strain εs0, volume strain εv under wet and saturated states showed continuously shrinkage in whole process, while under dry state, εv showed an evolution characteristics of contractancy–stable–dilatancy, and based on specific εs0–εv curves, the concept of critical equilibrium normal stress (CENS) was proposed; 3. The shear strength of LSS could be considered to fully follow a linear relationship, and it is greatly affected by dry/wet states, for three determined Dr0 (0.35/0.40/0.45), their internal friction angles φ were 33.1°–38.0°, 22.9°–27.7° and 20.9°–27.3°, respectively, under dry, wet and saturated states, and it showed that strength deterioration of LSS had been basically released in wetting process; 4. LSS showed characteristics of relatively uniform friction deformation under dry state, while under wet and saturated states, thickness of shear zone narrowed significantly, and when shear stress level exceeded 0.70, local strain increment of shear zone increased sharply, localized strain should be the main reason in strength attenuation; 5. 3D particle flow method was considered to make a simple shear numerical test of LSS with a rapid shear process, it was found that under lower normal stress condition, LSS exhibited semi-fluid characteristics, which could help us to reveal the phase transition of LSS from a mechanistic perspective.
Journal Article
Damage analysis of shear mechanical behavior of pile–structural soil interface considering shear rate effect
It is of great significance to investigate the shear mechanical behavior of pile–structural soil interface (PSSI) for guiding the design and construction of pile foundations in structural soft soil areas. The available studies rarely consider the effect of shear rate on the shear behavior of the PSSI under large deformation, and a relevant constitutive model is still lacking. In this paper, the effect of shear rate on the shear behavior of the PSSI under large deformation is investigated using a series of ring shear tests with different shear rates. Based on the test results, damage theory is introduced to analyze the mechanism of shear damage of the PSSI, and an interface damage constitutive model is proposed to describe the shear mechanical behavior of the PSSI under large deformation. Finally, a method for calculating the brittleness index of the PSSI is proposed based on the interface constitutive model. The evolution of the shear stress of the PSSI is analyzed, and the shear mechanical behavior of the PSSI under any normal stress is evaluated. The results show that the PSSI subjected to large deformation exhibits strain softening and has stable residual strength. The shear strength of the interface first decreases and then increases with increasing shear rate. The interface constitutive model can accurately reflect the shear behavior of the PSSI under large deformation. The shear rate has a positive effect on the brittleness index of the PSSI, and the normal stress amplifies this effect. The interface constitutive model can also predict the effect of normal stress on the brittleness index.
Journal Article
The effect of enzymatic GAG degradation on transverse shear properties of porcine cornea
The structural integrity of cornea depends on properties of its extracellular matrix, mainly a mixture of collagen fibers and soluble proteoglycans (PGs). PGs are macromolecules of negatively charged sulphated glycosaminoglycans (GAGs) covalently attached to a protein core. GAGs appear as bridges between adjacent collagen fibers and could facilitate force transfer between them. Furthermore, GAGs are responsible for corneal hydration by attracting and maintaining water molecules into the extracellular matrix. Based on these observations, GAGs are expected to be essential for biomechanical properties of cornea. The primary objective of the present study was to determine the effects of GAGs on shear properties of cornea. For this purpose, GAGs were enzymatically removed from porcine corneal disks by keratanase II enzyme. After confirming the successful removal of GAGs by histochemical methods, torsional rheometry was performed to characterize the shear stiffness of GAG-depleted samples as a function of axial strain. It was found that the shear modulus of all samples was a function of applied shear strain and compressive strain. Beyond the range of linear viscoelastic response, the average complex shear modulus decreased with increasing the shear strain. Increasing the axial strain from 0% to 40% significantly increased the average complex shear modulus of corneal disks in all groups. Finally, the enzyme treatment with keratanase II enzyme significantly decreased the shear stiffness. The experimental measurements were discussed in terms of microstructural and compositional properties of corneal extracellular matrix and it was concluded that GAGs play a significant role in defining shear properties of cornea.
Journal Article
Experimental characterization of the mechanical properties of 3D-printed ABS and polycarbonate parts
Purpose
This paper aims to present the methodology and results of the experimental characterization of three-dimensional (3D) printed acrylonitrile butadiene styrene (ABS) and polycarbonate (PC) parts utilizing digital image correlation (DIC).
Design/methodology/approach
Tensile and shear characterizations of ABS and PC 3D-printed parts were performed to determine the extent of anisotropy present in 3D-printed materials. Specimens were printed with varying raster ([+45/−45], [+30/−60], [+15/−75] and [0/90]) and build orientations (flat, on-edge and up-right) to determine the directional properties of the materials. Tensile and Iosipescu shear specimens were printed and loaded in a universal testing machine utilizing two-dimensional (2D) DIC to measure strain. The Poisson’s ratio, Young’s modulus, offset yield strength, tensile strength at yield, elongation at break, tensile stress at break and strain energy density were gathered for each tensile orientation combination. Shear modulus, offset yield strength and shear strength at yield values were collected for each shear combination.
Findings
Results indicated that raster and build orientations had negligible effects on the Young’s modulus or Poisson’s ratio in ABS tensile specimens. Shear modulus and shear offset yield strength varied by up to 33 per cent in ABS specimens, signifying that tensile properties are not indicative of shear properties. Raster orientation in the flat build samples reveals anisotropic behavior in PC specimens as the moduli and strengths varied by up to 20 per cent. Similar variations were observed in shear for PC. Changing the build orientation of PC specimens appeared to reveal a similar magnitude of variation in material properties.
Originality/value
This article tests tensile and shear specimens utilizing DIC, which has not been employed previously with 3D-printed specimens. The extensive shear testing conducted in this paper has not been previously attempted, and the results indicate the need for shear testing to understand the 3D-printed material behavior fully.
Journal Article
A Comparative Study on the Shear Behavior of UHPC Beams with Macro Hooked-End Steel Fibers and PVA Fibers
Structural members made of ultra-high-performance concrete (UHPC) have been attractive to engineers and researchers due to their superior mechanical properties and durability. However, existing studies were focused on the behavior of UHPC members reinforced with micro straight steel fibers at a volume fraction between 1 and 3%. There is a lack of studies on the influence of different types and amounts of fibers on the shear behavior of UHPC structural members. The objective of the study was to experimentally investigate the shear behavior of UHPC beams with macro hooked-end steel (MHS) fibers and polyvinyl alcohol (PVA) fibers, which are two of the most used fibers for high-performance fiber-reinforced cementitious composites. The shear behavior of ten large-scale non-prestressed UHPC beams was studied. The experimental parameters included the shear span-to-effective depth ratio, the fiber volume fraction, and the type of fibers. It was found that both MHS fibers and PVA fibers were effective in enhancing the shear performance of the UHPC beams whether the shear transfer mechanism was governed by arch action or beam action. Moreover, the measurement results of the average crack spacing imply the distinct difference in the fiber bridging effects of the MHS fibers and PVA fibers in the UHPC beams.
Journal Article
Improvements to the Duplicate Shear Test (DST) Device for Measuring the Fundamental Shear Properties of Asphalt Concrete Mixes
This study aimed to simplify the testing procedure of the duplicate shear tester (DST) and modify it for testing different shapes of Hot Mix Asphalt (HMA) specimens. The DST steel plates were replaced with aluminum plates of smaller dimensions to reduce its weight. The device was also modified to accommodate asphalt concrete specimens of 101.6 mm and 152.4 mm diameter cylindrical specimens and rectangular specimens cut parallel and perpendicular to the direction of compaction. Three asphalt mixes of different aggregate sizes and the same binder grade were collected from an asphalt plant located in North Texas. The frequency sweep test at constant height (FSCH) and repeated shear test at constant height (RSCH) were conducted following AASHTO T-320 test protocols using the DST on a total of ninety-six samples. The device proved its ability to provide repeatable measurements with a coefficient of variation less than 11%, 10%, and 13% for the measured dynamic shear modulus, shear-phase angle, and permanent shear deformation, respectively. Statistical analysis showed that testing different shapes of HMA specimens would affect the mean values for the measured shear parameters. However, the classical method of testing 152.4 mm. diameter cylindrical samples with the DST is recommended to determine the fundamental shear properties of asphalt mixes with good precision.
Journal Article
Measurement of the Shear Properties of Extruded Polystyrene Foam by In-Plane Shear and Asymmetric Four-Point Bending Tests
The shear modulus and shear strength of extruded polystyrene foam were obtained by the in-plane shear and asymmetric four-point bending tests. In addition, the test data were numerically analysed, and the effectiveness of these tests was examined. The numerical and experimental results suggest that the shear modulus and shear strength obtained from the in-plane shear test are significantly smaller than those obtained from the asymmetric four-point bending test because the influence of the stress concentration was less significant. Although the in-plane shear test is standardised in ASTM C273/C273M-11, it is considerable to adopt the asymmetric four-point bending test as another candidate for obtaining the shear properties of extruded polystyrene foam.
Journal Article