Initial Shear Stress Research Articles

Regulatory interaction between myogenic and shear-sensitive arterial segments: conditions for stable steady states.

Myogenic and shear stress-sensitive mechanisms control the caliber of a small blood vessel in this modeling study. This blood vessel in our model was composed of a pressure-sensitive (myogenic) component and a series-connected shear-sensitive component. The response of this model to imposed pressure and the conditions that result in a stable steady-state vessel diameter were investigated. The requirement that the model parameters need to satisfy for a stable steady state to exist are expressed by the numerical solution of simultaneous nonlinear equations. Also, if a vessel is put into an initial state that is not an equilibrium state, then the system must occupy a range of initial conditions to arrive at a stable equilibrium. These are described graphically for three cases. In general, the initial shear stress should be higher than the equilibrium value of shear stress, and/or the initial transmural pressure should be low, compared with the imposed feed pressure. Increasing the imposed pressure on the vessel can lead to elimination of the equilibrium state and vasospasm, according to this model. When a stable steady state is not reached, the model predicts elimination of the vessel or vasospasm. The model is in qualitative agreement with experimental observations that, during angiogenesis, vessels with low flow are often eliminated.

The state of stress on some faults of the San Andreas System as inferred from near‐field strong motion data

We present a simple method to calculate the stress produced on an earthquake fault during rupture. This method allows the complete evaluation of the stress spatio‐temporal history over the fault. We apply this approach to study the changes in shear stress produced during four of the largest earthquakes which occurred along the San Andreas fault system over the last 20 years: the Imperial Valley earthquake of 1979, the Morgan Hill earthquake of 1984, the Loma Prieta earthquake of 1989, and the Northridge earthquake of 1994. We use for this study the tomographic models of the fault rupture obtained from the inversion of the near‐field seismic data recorded during these earthquakes. The results obtained show that the static and the dynamic stress drops vary greatly over the fault. The peak values obtained for the four earthquakes studied range from about 20 to 100 MPa. These high values imply that the initial shear stress level on the fault at the onset of the earthquake was high on at least a significant portion of the fault. The regions of the fault which break with a high stress drop are also the regions where slip is large. This suggests that most of the slip produced in a large earthquake takes place over the “strong” areas of the fault. Low slip regions tend to break with low stress drops. After the earthquake, the shear stress is increased over a significant portion of the fault, which corresponds to low slip regions. Aftershock activity tends to be concentrated in these areas of stress increase. The apparent strength of the fault before the earthquake (that is the local shear stress increase which is required for rupture) is also extremely heterogeneous. The rupture velocity seems to be inversely correlated with this apparent fault strength, the rupture accelerating over the “weak” areas of the fault and slowing down over the high strength areas.

High-Temperature Creep Behavior in Ni<SUB>3</SUB>(Al, Ta) Single Crystals with Different Orientations

High temperature creep behavior of a nickel-rich Ni 3 (Al, Ta) with the LI 2 structure is investigated in order to clarify the influence of crystallographic orientation. The single crystals with four different orientations are deformed in compressive creep at the temperatures ranging from 1123 to 1273 K under a constant load with initial shear stresses being from 35 to 120 MPa for the (111)[101] slip system. The results show a distinct orientation dependence of creep strength, although shape of creep curves, stress exponent and the activation energy seem to be independent on orientation. It is shown, however, that the internal stress measured by strain transient dip tests is found to be orientation-dependent and that the creep behavior is independent on orientation if these are analyzed with the effective stress instead of the applied shear stress.

?Fracture? phenomena in shearing flow of viscous liquids

In start-up of steady shearing flow of two viscous unentangled liquids, namely low-molecular-weight polystyrene and α-D-glucose, the shear stress catastrophically collapses if the shear rate is raised above a value corresponding to a critical initial shear stress of around 0.1–0.3 MPa. The time dependence of the shear stress during this process is similar for the two liquids, but visualization of samples in situ and after quenching reveals significant differences. For α-D-glucose, the stress collapse evidently results from debonding of the sample from the rheometer tool, while in polystyrene, bubbles open up within the sample, as occurs in cavitation. Some similarities are pointed out between these phenomena and that of “lubrication failure” reported in the tribology literature.

Mode II delamination failure mechanisms of polymer matrix composites

The failure process of mode II delamination fracture is studied on the basis of the microscopic matrix failure modes (microcracks and hackles) as well as fracture mechanics principles. The crack tip matrix stresses leading to delamination is analysed by examining an adhesive bond with a crack analogous to a delamination crack in the resin layer of a composite. Such crack tip stresses induce matrix microcracks involving two major events: (a) single microcrack initiation and (b) development of multiple microcracks with regular spacing. The microcrack initiation shear stress τ* is found by the use of fracture mechanics to be related to certain resin properties (shear modulus G and mode I fracture toughness GIC) and microcrack length of the order of the resin layer thickness t (related to resin content).

Effects of hoop stress, initial shear stress, and shear viscosity ratio on the steady two-phase fiber spinning

Effects of hoop stress, initial shear stress, and shear viscosity ratio on the steady two-phase fiber spinning

Cyclic strength and deformation of crushable carbonate sand

Cyclic triaxial tests have been carried out on a skeletal carbonate sand from the west coast of Eire. Results are presented for undrained cyclic shear tests on samples with 80% of relative density consolidated under both isotropic and anisotropic conditions. Failure was defined as a 5% double amplitude cyclic strain and a 5% peak axial strain for stress-reversal and non-reversal stress conditions, respectively. Using this definition the cyclic strength for isotropically consolidated samples was affected by the confining pressure although the angular platey nature of the sand resulted in higher cyclic strengths than for a comparable silica sand. For anisotropically consolidated samples the cyclic strength increased with increasing initial static shear stress while on the other hand the cyclic strength normalised in the usual way with respect to the initial confining pressure decreased as this pressure increased.

Buckling behaviour of laminated composite structures using a discrete higher-order displacement model

A higher-order theory is used to develop a discrete finite element model for the linear buckling analysis of multilaminated composite plate-shell structures. This model is based on an eight node isoparametric element with 10 degrees of freedom per node. The geometric stiffness matrix is developed by taking into consideration the effects of the higher order terms on the initial in-plane and transverse shear stresses. The model is applied to study several cases that take into consideration different number of layers, lamination angles, length-to-thickness ratio, as well as symmetry and non-symmetry on the laminates. The accuracy of the present formulation is evaluated by comparing the present results with alternative solutions.

Strain Rate and Preshear Effects in Cyclic Resistance of Soft Clay

Cyclic constant-volume direct simple shear tests were performed on intact specimens of a sensitive clay reconsolidated to the in-situ stress state resulting in an overconsolidation ratio (OCR) of 2.2. The tests were conducted for different values of initial static undrained shear stress. The results confirmed that the high strain rate associated with cyclic loading has a significant effect on the cyclic resistance of soft clays. For up to 12 cycles, the strain-rate effect fully compensates the degradation of shear strength associated with cycling. The presence of an initial static shear stress decreases the cyclic resistance but increases the total undrained shear resistance, due to the partial or total disappearance of stress and strain reversal. The results indicate that for such a clay, the static undrained shear strength provides a conservative estimate in a pseudostatic analysis.

Orientation Dependence of High Temperature Creep Strength and Internal Stress in Ni3Al Alloy Single Crystals

ABSTRACTHigh temperature creep behavior of a nickel-rich Ni3(Al,Ta) with the L12 structure is investigated in order to clarify the influence of crystallographic orientation with respect to the stress axis. The single crystals with four different orientations are deformed in compressive creep at temperatures ranging from ‘M 23 to 1273 K under a constant load, initial shear stress being 35 to 120 MPa for (111)[101] slip system. The results show a distinct orientation dependence of creep strength, although shape of creep curves, stress exponent and the activation energy seem to be independent of the orientation. It is shown, however, the internal stress, being measured by strain transient dip tests, is found to be orientation dependent and the creep behavior is independent on orientation if it is interpreted using the effective stress instead of the applied shear stress.

Contact Mechanics of Ultrahigh-Molecular-Weight Polyethylene for Artificial Knee Joint

To investigate the wear mechanism of an ultrahigh-molecular-weight polyethylene (UHMWPE) for an artificial knee joint, the pin-on-disk-type wear test and the numerical analysis of contact mechanics of the UHMWPE were performed. In this analysis, to calculate the cyclic deformation of the UHMWPE, a constitutive equation for cyclic plasticity was used. In the initial stage of deformation, the UHMWPE was deformed only plastically ; beyond this stage, however, wear of the UHMWPE progressed rapidly with weight loss. In this initial stage, shear stress and equivalent plastic strain were accumulated by cyclic loading on the contact surface of the UHMWPE and it was expected that the microcracks that create wear particles would be generated on the surface.

Rotation of the principal stress directions due to earthquake faulting and its seismological implications

Abstract Earthquake faulting results in stress drop over the rupture area. Because the stress drop is only in the shear stress and there is no or little stress drop in the normal stress on the fault, the principal stress directions must rotate to adapt such a change of the state of stress. Using two constraints, i.e., the normal stress on the fault and the vertical stress (the overburden pressure), which do not change before and after the earthquake, we derive simple expressions for the rotation angle in the σ1 axis. For a dip-slip earthquake, the rotation angle is only a function of the stress-drop ratio (defined as the ratio of the stress drop to the initial shear stress) and the angle between the σ1 axis and the fault plane, but for a strike-slip earthquake the rotation angle is also a function of the stress ratio. Depending on the faulting regimes, the σ1 axis can either rotate toward the direction of fault normal or rotate away from the direction of fault normal. The rotation of the stress field has several important seismological implications. It may play a significant role in the generation of heterogeneous stresses and in the occurrence and distribution of aftershocks. The rotation angle can be used to estimate the stress-drop ratio, which has been a long-lasting topic of debate in seismology.

In search of potential earthquake source regions in the Chinese mainland in the light of ambient shear stress field

Earth media are incomplete media. There exist many cracks in it. The achievements of fracture mechanics show that the strength of the incomplete materials will be much lower than that of the complete materials. We consider that earthquake occurrence is the result of unstable propagation of a crack in crust media in proper condition and the earthquake rupture is the phenomenon of a failure by fast fracture under applied low shear stress. It has already been explained by fracture mechanics. The occurrence of failure by fast fracture is necessarily associated with the presence of high level concentration of local stress and strain. The elastic/plastic stress analysis in cracked pieces by Dugdale indicates that the state of stress at the tip of a crack takes a very important role to crack propagation. A plastic zone has necessarily formed in the tip of a crack due to stress concentration. Therefore, the dislocations at the tip of a crack are naturally a plastic displacement, rather than elastic one. The plastic displacementδ=πaτ 0 2 /2µτ y , whereτ 0 is applied shear stress which is equivalent to initial or tectonic shear stress when the quake occurs,a is the half length of a crack,μ is the rigidity,τ y is the yield stresses in shear. The main seismic dislocations take place exactly at the ends of the crack where the plastic zone had been formed. So, a critical assumption is adopted, i. e. we assume the dislocationD(ξ 1,t) as formula (5) in text. The maximum earthquake dislocationD max=πLτ 0 2 /4πτ y , whereL is the fault length. Ifμ is taken the value in the upper crust,μ=33 GPa; andτ y is taken the average value given from laboratories,τ y =30 MPa. Thus, according to observation values ofD max andL, using the formula, one can estimate the initial shear stresses for large earthquakes. Computations show that the initial shear stresses for large earthquakes all over the world are about 5–20 MPa which have some differences between regions. We further research the characteristics of source spectra and have derived the dependent relation of body wave magnitudem b on the shear stressτ 0 and seismic momentM 0 as formula (11) in text. Thus, the formula provides a possibility of computation of large amount of tectonic shear stress values from seismic data. We consider that the tectonic shear stress field is a main factor which controls the earthquake occurrence. The regions with high tectonic shear stress values are considered to be prone to occur great earthquakes (M s⩾6) and called earthquake hazard regions. Based on this criterion,τ 0 values for all earthquakes withm b⩾3.8 all over China since 1987 have been computed, and the great earthquake hazard regions with magnitude ranges have been zoned in the Chinese mainland. During April 1992–January 31, 1994, there were 9M s⩾6 earthquakes which occurred in the Chinese mainland, 8 earthquakes of the 9 had fallen into the regions delineated by us prior to the earthquake occurrence, with only one failure. This new approach as a method for medium-term prediction of strong earthquakes has been proved by practice to be an efficient one. It has good physical bases and bright prospect and worth further research.

Undrained Cyclic Shear Behaviour of Normally Consolidated Clay Subjected to Initial Static Shear Stress

A series of undrained cyclic triaxial compression tests has been performed on a high plasticity marine clay. Testing was performed using various combinations of initial static and subsequent cyclic shear stress on isotropically and anisotropically consolidated specimens. Additionally, monotonic triaxial compression and extension tests were also performed on the same clay with the same initial condition. The cyclic shear strength was considered first, and then, the accumulated shear strain was investigated related to the effective stress ratio at the peak cyclic stress. Based on the experimental results, a semi-empirical model is proposed for evaluating the development of pore pressure and residual shear strain during cyclic loading. The model successfully explains the behaviour of clay subjected to various magnitudes of initial static and subsequent cyclic shear stresses.

Evaluation of liquefaction potential of foundation soils at Duncan Dam

A comprehensive program of field, laboratory, and analytical investigations was carried out to evaluate the potential of liquefaction for the foundation soils at Duncan Dam. Duncan Dam was completed in 1967 under the Columbia River Treaty in southeastern British Columbia. The 39 m high zoned embankment dam is founded on a sequence of sands, silts, and gravels. Some of the foundation soils may liquefy during earthquake loading and this would affect the stability and performance of the dam. The liquefaction studies were carried out in two phases to characterize the engineering properties of the foundation soils and to assess its potential for triggering liquefaction using the total stress approach. This paper describes methods of assessment of liquefaction potential using soil parameters based on field penetration data (Seed's method) and laboratory testing of undisturbed soil samples obtained in situ after freezing the ground (Lab method) and presents the results of triggering analysis. Influence of confining stress (Kσ) and initial static shear stress (Kα) on liquefaction were investigated and site-specific Kσ and Kα curves were developed.For the design earthquake (M 6.5, PGA = 0.12g) both the Lab method and Seed's method predict a significant extent of liquefaction of the foundation soils under the downstream slope in the right half of the dam. Key words : sand, liquefaction, confining stress, density, cyclic resistance ratio.

Phase Stability of Chemically Derived Enstatite (MgSiO3) Powders

A modified Pechini chemical preparation technique was used to produce enstatite (MgSiO3) powder. By this method, low‐temperature stable orthoenstatite (OE) was obtained at 850°C after 2 h of calcination. The effects of annealing temperature/time, initial particle size, chemical dopants, and shear stress on the conversion of protoenstatite (PE) to clinoenstatite (CE) on the powder have been studied. The results indicated that the transformation of orthorhombic PE to monoclinic CE was sensitive to the initial powder particle size as well as to the type and amount of chemical dopant used. Sodium ions (Na+), which were found to cause the formation of a glassy phase around the PE grains, destabilized the PE phase physically. In comparison, manganese ions (Mn2+), were found to preferentially substitute for the smaller Mg2+ ions in solid solution and stabilized the PE phase chemically. The powders with different chemical dopants were examined by TEM and EDS.

Vein adaptation to the hemodynamic environment of infrainguinal grafts.

Although arteries appear to remodel in response to changes in hemodynamic parameters such as shear stress, little is known about functioning human vein grafts. This study was designed to explore diameter changes in human saphenous vein grafts after infrainguinal bypass. We used duplex ultrasonography to measure hemodynamic variables that might affect the diameter of 48 in situ saphenous vein grafts during the first year after infrainguinal arterial bypass. Volumetric flow rate, average velocity, peak systolic velocity, and vein diameter in the proximal and distal thirds of these grafts were each measured at 1 week and at 3, 6, and 12 months after operation. Veins were divided into three groups based on initial size (1 week after bypass) in the below-knee segment: small, < 3.5 mm diameter; medium, 3.5 to 4 mm diameter; and large, > 4 mm diameter. Distal vein diameters at 1 week for small, medium, and large grafts were 2.9 +/- 0.1, 3.7 +/- 0.1, and 4.3 +/- 0.1 mm, respectively (p < 0.001), but by 12 months these diameters were 3.6 +/- 0.2, 3.8 +/- 0.2, and 3.9 +/- 0.2 mm, respectively (p = 0.54). Large veins decreased in diameter, whereas small veins increased in diameter, as confirmed by linear regression of percent change in diameter versus initial vein graft diameter (r = -0.62, p < 0.001). Volumetric flow rate, peak systolic velocity, and shear stress also tended to approach uniform values over time. Of the hemodynamic variables studied, the best predictor of diameter change was shear stress (linear regression of percent change in diameter vs shear stress, r = 0.67, p < 0.001). Veins with a diameter increase greater than 10% over time had significantly higher initial shear stress than veins with a diameter decrease greater than 10% over time (28.6 +/- 3.8 vs 13.1 +/- 1.8 dynes/cm2, p < 0.01), whereas initial volumetric flow rates in these two groups were similar (135 +/- 23 vs 130 +/- 15 ml/min). Infrainguinal in situ vein graft diameter, volume flow rate, peak systolic velocity, and shear stress all tend to stabilize at uniform values regardless of the initial vein graft diameter. Of the hemodynamic variables studied, shear stress is most strongly associated with the change in diameter over time. Thus human saphenous vein appears to be capable of adapting to its hemodynamic environment after arterial grafting by modulating diameter to normalize shear stress.

Critical shear stress and critical flow rates for initiation of rilling : Gilley, J E; Elliot, W J; Laflen, J M; Simanton, J R J HydrolV142, N1/4, Feb 1993, P251–271

Critical shear stress and critical flow rates for initiation of rilling : Gilley, J E; Elliot, W J; Laflen, J M; Simanton, J R J HydrolV142, N1/4, Feb 1993, P251–271

Critical shear stress and critical flow rates for initiation of rilling

This study was conducted to identify critical shear stress and critical flow rates required initiate rilling on selected sites. The data used in this investigation were collected from soils located throughout the USA where crop residues had been removed, and moldboard plowing and disking had occurred. Runoff and soil loss measurements were made on sites where simulated rainfall was applied to preformed rills. Multiple regression analyses were used to relate critical shear stress values and critical flow rates to selected soil properties. The soil-based regression equations were found to provide reliable estimates. Information identified in this study will improve our ability to understand and properly model upland runoff and erosion processes.

Cyclic and permanent deformation of saturated sand with initial static shear stress during cyclic loading : Hyodo, M; Murata, H; Yasufuku, N; Fujii, T Proc 10th European Conference on Soil Mechanics and Foundation Engineering, Florence, 26–30 May 1991V1, P111–114. Publ Rotterdam: A A Balkema, 1991

Cyclic and permanent deformation of saturated sand with initial static shear stress during cyclic loading : Hyodo, M; Murata, H; Yasufuku, N; Fujii, T Proc 10th European Conference on Soil Mechanics and Foundation Engineering, Florence, 26–30 May 1991V1, P111–114. Publ Rotterdam: A A Balkema, 1991