Dynamic Normal Stress Research Articles

Direct shear behavior of dredged soil under dynamic normal load conditions

Dredged soil is commonly encountered in estuarine and coastal engineering constructions; however, its dynamic frictional behavior is still unclear. In this research, DJZ-500 shear apparatus was utilized for conducting direct shear test on the dredged soil. The periodic normal stress was employed in the vertical direction of soil sample, and simultaneously, shear stress was applied in the horizontal direction to supply a constant speed. The influences of different shear rates, dynamic normal stress frequencies, and dynamic normal stress amplitudes on the shear properties of the dredged soil under dynamic normal stress conditions were explored. It was found that shear stress periodically varied with the periodic normal stress, characterizing a phase lag with the peak shear stress. The dynamic normal stress manifested an enhancement effect and also a weakening effect on the shear strength. Generally, a small dynamic normal stress frequency, a high shear speed, and an appropriate dynamic normal stress amplitude induced frictional strengthening, whereas a small shear speed, a large dynamic normal stress frequency, and a very large or small dynamic normal stress amplitude caused frictional weakening. Therefore, the dynamic frictional behavior of the dredged soil depended on shear velocity and the frequency and amplitude of dynamic normal stress. The results of this paper provide technical support to improve the resistance of coastal constructions to dynamic disturbances, such as earth tides, ocean waves, earthquakes, typhoons, and traffic loads.

Frictional sliding of infilled planar granite fracture under oscillating normal stress

This paper investigates the frictional behavior of the infilled rock fracture under dynamic normal stress. A series of direct shear tests were conducted on saw-cut granite fractures infilled with quartz using a self-developed dynamic shear apparatus, and the effects of normal load oscillation amplitude, normal load oscillation period and sliding velocity were studied. The test results reveal that the shear response can be divided into three stages over a whole loading-unloading process, characterized by different time spans and stress variations. Generally, a smaller oscillation amplitude, a longer oscillation period and a fast shear velocity promote the stability of the friction system, which is also confirmed by the Coulomb failure criterion calculated based on the observed periodic apparent friction coefficient. The dynamic strengthening/weakening phenomenon is dependent on the oscillation amplitude and product of sliding velocity and oscillation period (vT). Also, the rate and state friction law incorporating the parameter α that characterizes the normal stress variation is employed to describe the dynamic friction coefficient but exhibits an incompetent performance when handling intensive variation in normal stress. Finally, the potential seismicity induced by oscillating normal stress based on the observed stress drop is analyzed. This work helps us understand the sliding process and stability evolution of natural faults, and its benefits for relative hazard mitigation.

Analysis of magnetorheological grease normal force characteristics in static and dynamic shear modes

In this paper, the normal stress behavior of magnetorheological grease (MRG) in a quiescent state and its dynamic shear modes were systematically studied. Three types of magnetorheological greases were prepared, MRG-30, MRG-50, and MRG-70 with 30%, 50%, and 70% carbonyl iron(CI) powder mass, respectively. Furthermore, static and dynamic shear tests were carried out using a rheometer, the effects of time, magnetic field strength, shear rate and temperature on normal stress were studied and analyzed under both static and dynamic conditions. In the static mode, the research results show that the normal force is related to the magnetic field strength and the content of magnetic particles in the MR grease, but has nothing to do with historical time. the static normal stress generates the evident fluctuation with changing the temperature when magnetic field strengths of 96 kA m−1 and 194 kA m−1. The fluctuation is very small when the magnetic field strengths are 0kA m−1, 391 kA m−1 and 740kA m−1. Similar to the static normal stress, the dynamic normal stress at the degaussing is greater than the normal stress during magnetic field loading. The dynamic normal stress are almost invariant with prolonging the time below 100 s−1 shear rate under the medium magnetic field strength of 194 kA m−1 whereas the stress is increased at the shear rate of 100 s−1. Besides, the dynamic normal stress is basically independent from the time above the shear rate 0f 1 s-1 at the high magnetic field of 740 kA m−1. Under other conditions, the normal force basically presents a stable value. At the absence of the magnetic field, there are quietly weak effects of temperature on the static and dynamic normal stress. Finally, when a magnetic field strength higher than 96 kA m−1 is applied to the material, the value of dynamic normal stress increases with temperature.

Transverse Impact on Rectangular Metal and Reinforced Concrete Beams Taking into Account Bimodularity of the Material.

This article is devoted to the stress–strain state (SSS) study of metal and reinforced fiber-reinforced concrete beam under static and shock loading, depending on the bimodularity of the material, the mass of the beam, and the location of the reinforcing bars in zones under tension and compression. It is known that many materials have different tensile and compression properties, but in most cases, this is not taken into account. The calculations were carried out by using load-bearing metal beams made of silumin and steel and reinforced concrete beams under the action of a concentrated force applied in the middle of the span. The impact load is considered as the plastic action of an absolutely rigid body on the elastic system, taking into account the hypothesis of proportionality of the dynamic and static characteristics of the stress–strain state of the body. The dependences of the maximum dynamic normal stresses on the number of locations of reinforcing bars in zones under tension and compression, the bimodularity of the material, and the reduced mass of the beam are obtained. A numerical study of SSS for metal and concrete beams has shown that bimodularity allows the prediction of beam deflections and normal stresses more accurately.

Embedded piezoelectric lead-zirconate-titanate-based dynamic internal normal stress sensor for concrete under impact

The direct stress measurement is critical for the constitutive law investigation of concrete materials and dynamic behavior study of concrete structures under strong dynamic loadings where the strain-rate effect and the plastic behavior should be considered. In this article, an embedded dynamic internal normal stress sensor for concrete by employing piezoelectric lead-zirconate-titanate (PZT) patch as a sensing element is developed. After being calibrated under sinusoidal loadings with different frequencies, periodical square loadings, and random loadings, the developed dynamic internal normal stress sensor is further calibrated with a unique self-developed free-falling drop hammer with different impact velocities. The sensitivity of the developed normal stress sensor is determined accordingly. Finally, the developed sensors are employed to monitor the dynamic normal stress time history at different locations of concrete cylinder specimens under impacts with different velocities using a large-scale drop hammer test facility. The dynamic increase factors of concrete strength under impact are determined and compared with the values determined by the Comité Euro-International du Béton code. The experimental results indicate that the PZT-based embedded dynamic internal normal stress sensor provides a novel way for the direct dynamic normal stress measurement of concrete specimens under impact loadings.

Pressure-sensitive nutrient consumption via dynamic normal stress in rotational bioreactors

Pressure-sensitive biological response is simulated in “rotating-cup” bioreactors with unidirectional modulations in compressive stress at the cylindrical wall that stimulate bone-tissue growth. Anchorage-dependent mammalian cells (i) adhere to a protein coating, (ii) receive nutrients and oxygen from an aqueous medium via radial diffusion toward the active surface, and (iii) respond to physiological modulations in centrifual-force-induced fluid pressure at the cell/aqueous-medium interface. This process is modeled by the classic diffusion equation (i.e., Fick's second law), with a time-dependent reaction/diffusion boundary condition at the wall. Non-reversing angular velocity modulations resemble pulsations at physiological frequencies. Computer simulations of nutrient consumption profiles suggest that rotational bioreactor designs should consider the effects of normal stress when the pressure-sensitive Damköhler number (i.e., ratio of the pressure-dependent zeroth-order rate of nutrient consumption relative to the rate of nutrient diffusion toward active cells adhered to the cylindrical wall), evaluated under steady rotation, is greater than ≈ 10–20% of the stress-free Damköhler number (i.e., β 0,1st- order = 0.025) for simple 1st-order stress-free kinetics, and ≈ 1% of the stress-free Damköhler number (i.e., β 0,2nd- order = 0.40) for complex 2nd-order stress-free nutrient consumption. When the peak-to-peak amplitude of angular velocity modulations of the cylindrical wall is the same as or larger than the angular velocity for steady rotation, the effect of non-reversing centrifugal-force-induced dynamic normal stress in rotational bioreactors, superimposed on steady rotation, can be significant when one is below the critical value of the pressure-sensitive Damköhler number that has been identified under steady rotation.

Stability of frictional slipping at an anisotropic/isotropic interface

The stability of steady, quasi-static slip at a planar interface between an anisotropic elastic solid and an isotropic elastic solid is studied. The paper begins with an analysis of anti-plane sliding at an orthotropic/isotropic interface. Friction at the interface is assumed to follow a rate- and state-dependent law. The stability to spatial perturbations of the form exp(i kx 1), where k is the wavenumber and x 1 is the coordinate along which the interface is studied. An expression is derived for the critical wavenumber ∣ k∣ cr above which there is stability. In-plane sliding at an anisotropic/isotropic interface is subsequently studied. In this case, slip couples with normal stress changes and a constitutive law for dynamic normal stress changes is adopted. Again a formula for ∣ k∣ cr is derived and the results are specialized to the case of an orthotropic/isotropic interface. Numerical plots of the dependence of ∣ k∣ cr on the orientation of the orthotropic solid, as well as on the material parameters are provided.

Flow patterns and wall stresses in a moving granular filter bed with an asymmetric louvered-wall and obstacles

This study investigated the flow patterns and wall stresses in a two-dimensional asymmetric louvered-wall moving granular filter bed with an obstacle placed at each stage of the granular filter bed. The obstacles utilized were: (1) a single vertical plate and (2) a wedge-shaped obstacle comprising a left-vertical-plate and a right-inclined-plate. A digital camcorder was used to record the flow pattern histories of the granular solids in the moving granular filter bed. The instantaneous vertical velocity distribution at a certain height from the bottom of the third stage in a two-dimensional asymmetric louvered-wall granular filter bed was determined from two successive flow images. The static stresses and the variations in the dynamic wall stresses with time were obtained using a two-directional pressure gauge, which could simultaneously measure the normal and shear stresses of the granular solids. The preliminary theoretical study of the obstacle's placement in the asymmetric louvered-wall moving granular filter bed was proposed. The magnitude of the average dynamic normal stress on the convergent section of the louvered-wall greatly exceeds that on the vertical section of the louvered-wall. A wedge-shaped obstacle (or a single vertical plate) placed at a proper location, which causes two asymmetric flowing streams in the granular bed, may increase the velocity and refreshing rate of the filter granules in the quasi-stagnant zone near the louvered-wall and further the overall dust collection efficiency of the filter granules because the central flowing core above exit is not evident in the granular bed. Additionally, a wedge-shaped obstacle installed at an appropriate location, which consists of a left-vertical-plate and a right-inclined-plate, satisfies the following requirements: (1) it keeps the granules in the left part of the granular bed away from the obstacle, preventing interference therewith; (2) it shrinks stagnant zone and declines the normal stress on the convergent section of the louvered-wall in the right part of the granular bed; and (3) it solves the potential problem of a roof-type insert, on which the stagnant zone may sit.

Flow patterns and wall stresses in a moving granular filter bed with a curved symmetric louvered wall

Abstract This study investigated the flow patterns and wall stresses in a two-dimensional moving granular filter bed with a curved and symmetric louvered wall. A digital camcorder was used to record the flow pattern histories of the granular solids in the moving granular filter bed. The numerical predictions of the flow patterns were implemented using the discrete element method and compared with the experimental results. The variations in the dynamic wall stresses with time were obtained using a pressure gauge, which could simultaneously measure the normal and shear stresses of the granular solids. The effects of the curvature of the louvered wall and the exit width upon the flow patterns and the wall stresses were also elucidated. For a fixed value of radius of curvature, the ratio of the quasi-stagnant zone (Q-SZ) area in the third stage to the granular bed area in the third stage increases with decreasing width of the exit, but the average dynamic normal stress on the curved louvered wall increases with increasing width of the exit. For a fixed value of the exit width, both the ratio of the Q-SZ area in the third stage to the granular bed area in the third stage and the average dynamic normal stress on the curved louvered wall increase with increasing radius of curvature. Additionally, the curved louvered wall can not only shrink the area of the Q-SZ in the granular bed, but also reduce the dynamic normal stress on the louvered wall.

Vibration of horizontally curved box girder bridges due to vehicles

The purpose of this paper is to develop a procedure for obtaining the dynamic response of thin-walled curved box girder bridges due to truck loading and to get their basic impact characteristics. The box girder bridge is divided into a number of thin-walled beam elements. Both warping torsion and distortion are considered in the study. The analytical vehicle is the AASHTO HS20-44 truck simulated as a nonlinear vehicle model with 11 independent degrees of freedom. Four different classes of road-surface roughness generated from power spectral density function for very good, good, average, and poor roads are used in the analysis. In comparison with the other approaches developed by former investigators, the proposed procedure not only facilitates a physical understanding of the general structural response, but also greatly reduces the computing time required and output obtained. The analytical results show that most impact factors of torsion and distortion are much larger than those of vertical bending response. The impact factors of normal stress at different points in the same cross section are quite different. The larger the normal stress, the smaller the impact factor will be. Though the static normal stresses at different positions in the same cross section may be extremely different for a curved box girder bridge, their dynamic normal stresses tend to be uniform.

Theory of dynamic shear viscosity and normal stress coefficients of dense fluids

On the basis of the nonlinear evolution equation for the stress tensor derived from the kinetic equation for dense simple fluids, we examine the shear rate and frequency dependence of the shear viscosity and normal stress coefficients. An analytic expression for nonlinear shear viscosity has been obtained, which is in agreement with nonequilibrium molecular dynamic simulation results. Based on the material functions obtained in the analysis, a theory of rheological corresponding states is developed by showing that there exists a set of reduced material functions which depend on the reduced shear rate, reduced frequency, reduced static normal coefficients and limiting values of the reduced imaginary parts of dynamic normal stress coefficients. It is also shown that the dynamic normal stress coefficients can change its sign as the critical value of the frequency is crossed.