Dynamic Shear Stiffness Research Articles

Experimental study of dynamic shear stiffness decay characteristics of interbedded soil: a case study in Yangtze River floodplain

Experimental study of dynamic shear stiffness decay characteristics of interbedded soil: a case study in Yangtze River floodplain

Dynamic properties of high-density polyethylene–sand by large-scale direct shear experiments

Recycling and reuse of plastic waste in civil projects is one way to reduce its negative impacts on the environment. High-density polyethylene (HDPE) is a highly abundant plastic waste found widely and in high volumes worldwide. Although sand has a relatively high frictional resistance, if its dynamic parameters are improved, it can be used in geotechnical projects that require higher dynamic parameters than those of pure sand. This study aimed to investigate the dynamic properties of sand mixed with crushed HDPE waste and its application to mitigate its negative environmental impacts. Crushed HDPE was added to sand at 0, 4 and 8% by weight. Under three vertical stresses and two shear amplitudes, the dynamic parameters of the mixture, including damping and shear stiffness, were investigated by conducting a large-scale cyclic direct shear test under ten cycles and are discussed. The results showed that with increasing HDPE percentage, the damping ratio rose and the shear stiffness decreased compared with those of pure sand due to the elasticity, deformation, tensile properties and high potential of HDPE for energy absorption. Furthermore, the rise in shear amplitude decreased the dynamic shear stiffness and raised the damping ratio of the mixtures.

Discrete element method analysis of non-linear stiffness for granular media

This contribution considers the nonlinearity of stiffness in granular materials using 3D DEM simulations, with a primary focus being placed on three issues. First, it assesses stress-dependent stiffness and density-dependent stiffness based on DEM simulations of monotonic triaxial tests. The modified formulation of hyperbolic relation based on the DEM data is generated to capture stiffness degradation, which can take the various types of an S-shaped curve due to the initial mean effective stress and the sample density. The use of various void ratio correction functions is made to remove the effect of density on the stiffness curve. DEM results reveal that inclusion of the coordination number in both forms of the void ratio function serve a better role in unifying the stiffness dataset from samples with different densities. Second, the framework of kinematic modified yielding points proposed by Jardine (1992) that identifies three main zones (i.e. linear elastic behaviour, non-linear elastic behaviour and elasto-plastic behaviour) provides a benchmark for the gap between the dynamic stiffness and the static stiffness to be studied in an effective manner. A key observation is that the dynamic shear stiffness tends to increase to a peak value before experiencing of a decrease in magnitude. Third, the investigation into the nature of energy dissipation during triaxial shearing tests is made. Several DEM simulations were performed to assess effects of the coefficient of uniformity (Cu) on the non-linear behaviour of granular materials, arriving at some key observations that: (i) a higher amount of work should be input for samples with coarse particles to attain a particular strain level; and (ii) more energy dissipation is observed for samples with a lower Cu value.

Experimental Investigation of Dynamic Characteristics of Subsea Sand‐Silt Mixtures

In this paper, extensive resonant column tests were conducted to investigate dynamic responses of subsea sand‐silt mixtures. The effects of confining pressure, mixture ratio, curing age, and cement content were evaluated. For the test condition considered in this study, the measured damping ratio is the smallest when the ratio of subsea sand to silt is in a range of 1.5 to 2.0. Moreover, unsolidified subsea sand‐silt mixed at a ratio of 1.5 has almost the same maximum shear stiffness as the pure sand. For solidified subsea sand‐silt mixture, cement can significantly increase the dynamic shear stiffness when the curing age is less than 14 days. However, the increase of the maximum dynamic shear stiffness is negligible when the curing age is longer than 14 days. When the cement content is 2%, the damping ratio of the solidified mixtures is very close to that of the unsolidified mixture. When the cement content is higher than 4%, the damping ratio of the solidified mixtures reduces significantly. This is mainly due to hydration reactions occurring in the solidified mixtures.

Dynamic shear stiffness and damping ratio of marine calcareous and siliceous sands

Shear stiffness and damping ratio of two marine calcareous and siliceous sands were evaluated through an experimental program. Resonant column and cyclic triaxial experiments were conducted to measure the dynamic properties of the sands in small and large shear strain amplitudes. The tests were conducted under various initial stress-density conditions. The influence of effective confining pressure on the dynamic properties of the sands was assessed and compared in a preceding paper. It was shown that the calcareous sand has higher shear stiffness and lower damping ratio in comparison to the siliceous sand. In this note, the results are presented in more details and the dynamic behavior curves of the studied sands are compared with some available models, mostly developed based on the laboratory data of siliceous sands. This comparative study reveals that the previous models predict the dynamic properties of the calcareous sand in less precision than those of the siliceous sand.

Laboratory Determination of Rock Fracture Shear Stiffness Using Seismic Wave Propagation and Digital Image Correlation

Seismic wave propagation and digital image correlation were used during direct shear experiments on Indiana limestone specimens to investigate the stiffness of rock discontinuities (fractures) approaching shear failure. An instrumented direct shear apparatus was used to apply shear stress to the discontinuity. Compressional and shear wave pulses were transmitted through and reflected from the discontinuity, whereas digital images of the specimen surface were acquired during the test. To measure the dynamic shear stiffness of the rock discontinuities, the displacement discontinuity theory was used and the stiffness was calculated based on the ratio of transmitted to reflected wave amplitudes. The static shear stiffness was calculated based on the ratio of an increment in the applied shear stress to the corresponding increment of relative shear displacement (slip) along the discontinuity. The dynamic shear stiffness measured by seismic wave propagation showed roughly five to ten times greater magnitude than the static values measured by digital image correlation technique. This observation is found to be in agreement with available studies indicating that the frequency-dependent fracture stiffness arises from probabilistic and spatial distributions of stiffness and that dynamic moduli are typically greater than the static values.

Precursors of dynamic failure on a tectonic fault

A new method for controlling the dynamic shear stiffness of interblock contact during stick-slip is proposed. This method enables us to reveal changes in the mechanical properties of the contact long before the macroscopic slip will be recorded. In the experiments conducted, the time of precursor manifestation was about 1/3 of the duration of the “seismic cycle.”

Dynamic viscoelastic modeling of magnetorheological elastomers

A micromechanics-based viscoelastic constitutive model is proposed to estimate the zero-magnetic-field- and magnetic-field-dependent dynamic shear stiffness and damping behavior for magnetorheological elastomers (MREs). The effect of imperfect interfacial condition between the ferromagnetic particles and the elastomeric matrix on those properties is incorporated in the proposed model. A concept of effective volume fraction of particles is introduced to take into account the particle agglomeration in MREs. The magnetic dipole interaction is further employed to evaluate the magnetic-field-induced increase in shear stiffness of MREs. Numerical simulations are conducted and compared with experimental data to verify the proposed model.

Seismic Bearing Capacity of Piles in Liquefiable Soils

An investigation into the base capacity of piles in passing through loose, liquefiable sand and founded in underlying dense sand is presented based on the results of a series of dynamic centrifuge tests on instrumented model pile groups. Excess pore pressures equal in magnitude to the initial effective vertical stress were observed to be generated in the bearing layer of dense sand at both shallow (15 m) and deep (26 m) depths. This induced a dramatic reduction in base capacity and consequently, large settlements of the piles by as much as ~5 D 0 . A spherical cavity expansion solution for base capacity was validated against measured values showing good agreement, provided that excess pore pressure and dynamic shear stiffness in the bearing layer are known. A simple closed-form relationship, applicable to end-bearing piles, between the degree of liquefaction and the initial pile static safety factor was then developed against plunging failure at the pile base which can be used in design.

The generalized Cauchy relation: a probe for local structure in materials with isotropic symmetry

The elastic properties of the isotropic state of condensed matter are given by the elastic constants ell and c44. In the liquid state the static shear stiffness c44 vanishes whereas at sufficient high probe frequencies a dynamic shear stiffness may appear. In that latter case the question about the existence of a Cauchy relation appears. It will be shown that a pure Cauchy relation can appear only under special conditions which are rarely fulfilled. For all investigated materials, including ceramics, liquids and glasses, a linear relation between ell and c44 called generalized Cauchy relation is observed, which, surprisingly, follows a linear transformation

The Shear Strength and Dynamic Shear Stiffness of Some New Zealand Volcanic ASH Soils

ABSTRACT A significant proportion of the surface deposits of the central and upper half of the North Island of New Zealand consists of volcanic ash soils. Given that New Zealand is situated within an active seismic region, the dynamic properties of these soils are an important geotechnical earthquake engineering consideration. A systematic investigation of three volcanic ash sites was performed to help characterise the dynamic properties of these soils. In situ down-hole and cross-hole seismic tests produced small strain shear modulus (Gmax) values ranging between 17 and 300 MPa, with excellent agreement between the two test methods. Bender element and free vibration torsion testing of specimens trimmed from high quality samples produced Gmax values which were 28% to 75% lower than those measured in the field. This disparity was considered to result from negative pore pressures potentially present in the field and irreversible changes in the structure of the soil that occurred during removal of the soil from its in situ stress environment. Normalised reduction curves (G/Gmax versus shear strain) produced results which were comparable to those presented by other researchers for cohesive soils, though the influence of the plasticity index on the form of the reduction curves was observed to be negligible. Investigation of the Gmax/Su relationship from field investigations of volcanic soils produced average values that ranged between 560 and 890.

Dynamic Mechanical Characterization of a Mutable Collagenous Tissue: Response of Sea Cucumber Dermis to Cell Lysis and Dermal Extracts

The dermis of the holothurian Cucumaria frondosa is a mutable collagenous tissue (MCT). In this study, the inner and outer regions of the dermis were separated and used to make two different tissue extracts. These extracts were applied to intact pieces of dermis, one invoking a stiff mechanical state and the other invoking a compliant state. The extracts were effective on tissues incubated in artificial sea water (ASW) and in those incubated in Ca(2+)-chelated ASW. Furthermore, the extracts were effective on both fresh tissues and tissues in which the cells had been lysed by freeze-thawing, indicating that the sites of action are in the extracellular matrix. Dynamic oscillatory shear tests and analyses were used to measure both the dynamic shear stiffness (G*) and the relative damping (tan delta ) of the tissue. These two parameters proved to be inversely related to each other (i.e. when G* increased, tan delta decreased). A theoretical viscoelastic model is constructed to interpret the results of these tests. It is concluded that changes in the mechanical state of the tissue involve interactions between elastic elements within the tissue rather than an alteration of its viscous components.

Dynamic fracture characteristics of the osteochondral junction undergoing shear deformation

This paper presents a biomechanical study of the dynamic fracture response of the osteochondral regions of both immature and mature cartilage-bone laminates induced through shear loading. An instrumented impact machine, providing both mechanical and real-time macro-photographic fracture data, was used to load the osteochondral region in shear by means of a direct compressive force applied to the cartilage layer only, and in a direction parallel to the plane of the osteochondral junction. This force was applied at a known velocity of 2500 mm/s. Our results show that under these conditions of shear loading the dynamic mode II fracture toughness of the osteochondral region of the mature tissue is approximately 1.5 times that of its immature counterpart, and that the derived dynamic shear stiffness of the immature tissue is about 4 times that of the mature. The structural studies demonstrated that the mature tissue delaminates within a well-defined tidemark region whilst the immature fractures through the subchondral bone into which fingers of articular cartilage penetrate. We suggest that this pseudo-brittleness of the immature tissue could explain why there is an increased susceptibility to osteochondral failure in the younger human joint. Finally, we note that the strength-to-toughness relationship, in which stiffer engineering materials are known to exhibit lower fracture toughness, also applies to the cartilage-bone system.

Viscoelastic Stability Model for Elastomeric Isolation Bearings

A viscoelastic model accounting for the stability effect is proposed for high‐damping elastomeric bearings used in aseismic base isolation. This model is consistent with Haringx's theory, which takes into consideration the significant shear deformation of the elastomer. The method of mode superposition is used to obtain an alternative solution to the “Haringx column,” and this solution is readily generalized to include viscoelasticity. Each mode is an eigensolution of the corresponding stability problem. The rapid fourth‐order convergence permits the use of only the first mode to give a very good approximation. The dynamic shear stiffness, damping characteristics, and height reduction of bearings are obtained by the first‐mode consistent model. In particular, the phenomenon of increasing energy dissipation due to the axial load is explained by the model in a consistent way. The applicability of the model is then verified by experiments conducted on multilayer elastomeric isolation bearings with and withou...

Use of the complex dynamic shear stiffness of a visco‐elastic liquid to illustrate the origins of structural damping

The same complex stiffness notation which causes damping of solids and structures to appear to be independent of frequency and dependent on amplitude is shown to have descriptions of frequency‐dependent, velocity‐dependent, viscous damping hidden within it. That revelation is consistent with recent papers which report that only a viscous component of structural damping need be measured or modelled mathematically. Those papers also report that viscous damping accounts for all of the energy loss from built‐up structures (as elementary dimensional analysis shows it must), This paper draws upon measured values of the dissipative and elastic components of the complex dynamic viscosity and stiffness of a viscoelastic liquid to illustrate conceptual continuity from damping in liquids to damping in solids.