Shear resistance improvement of oil-contaminated ballast layer with rubber shred inclusions

Railway ballast, which form an integral part of rail tracks, is highly susceptible to subsistence due to both vibration transmitted by the passing trains, as well as the breakage of ballasts with repeated impact. The resulting subsistence necessitates regular monitoring and maintenance, involving cost- and time- consuming remedial actions, such as stone-blowing and ballast renewal. Measures to minimize the wear and tear effect are therefore desirable to prolong the lifespan of the ballast layer. It is even more critical when the ballast is contaminated with oil and grease from braking wheels and leakages. This paper describes the inclusion of rubber shreds (≤10 mm in length, 1.5 mm thick) derived from the inner tubes of motorcycle tyres in oil-contaminated ballast layer for shear resistance improvement. The tests are mainly carried out in a standard direct shear test setup, i.e. shear box measuring 60 mm x 60 mm. Granitic stones of suitable sizes were sieved and used as representative samples of typical ballast. The samples were soaked in lubricant oil for 14 days to simulate the contamination. The direct shear test results indicated rubber shreds inclusion could effectively improve the shear resistance of ballast and expedient in deformation control with increased ductility of the composites. This could potentially improve absorption of impact, hence reduction of breakages of the ballasts. Clearly both mechanisms contribute to the overall reduced subsistence, accompanied by an increase in the shear resistance. However, further investigations in a dynamic test setup are necessary for verifications prior to field implementation

Development of experimental concepts for investigating the strength behavior of fine-grained cohesive soil in the Spacelab/space shuttle zero-g environment

Three different sets of tests are proposed for the NASA Spacelab experimental program. The first of tests, designed to measure the true cohesion of several different soils, would be carried out in space through use of a specially prepared direct shear apparatus. As part of this first series of tests, it is recommended that a set of drained unconfined compression tests be performed terrestrially on the same soils as tested in space. A form of the direct tension test is planned to measure the true tensile strength of the same types of soils used in the first series of tests. The direct tension tests could be performed terrestrially. The combined results of the direct shear tests, direct tension tests, and unconfined compression tests can be used to construct approximate failure envelopes for the soils tested in the region of the stress origin. Relationships between true cohesion and true tensile strength can also be investigated. In addition, the role of physio-chemical variables should be studied. The third set of tests involves using a multiaxial cubical or true triaxial test apparatus to investigate the influence of gravity induced fabric anisotropy and stress nonhomogeneities on the stress strain behavior of cohesive soils at low effective stress levels. These tests would involve both in space and terrestrial laboratory testing

Evaluation of shear mounted elastomeric damper

Viton-70 elastomeric shear mounted damper was built and tested on a T-55 power turbine spool in the rotor's high speed balancing rig. This application of a shear mounted elastomeric damper demonstrated for the first time, the feasibility of using elastomers as the primary rotor damping source in production turbine engine hardware. The shear damper design was selected because it was compatible with actual gas turbine engine radial space constraints, could accommodate both the radial and axial thrust loads present in gas turbine engines, and was capable of controlled axial preload. The shear damper was interchangeable with the production T-55 power turbine roller bearing support so that a direct comparison between the shear damper and the production support structure could be made. Test results show that the Viton-70 elastomer damper operated successfully and provided excellent control of both synchronous and nonsynchronous vibrations through all phases of testing up to the maximum rotor speed of 16,000 rpm. Excellent correlation between the predicted and experienced critical speeds, mode shapes and log decrements for the power turbine rotor and elastomer damper assembly was also achieved

Influence of time-dependence on failure of echelon rock joints through a novel DEM model

This is an Accepted Manuscript of an article published by Taylor & Francis Group in [European Journal of Environmental and Civil Engineering] on [September 2015], available online at: http://www.tandfonline.com/10.1080/19648189.2015.1064624This article investigates the time-dependent influence on the shear failure behaviour of parallel rock joints in the echelon arrangement due to chemical weathering, which can be treated as a generalised time-dependency of the rock material. A time-dependent parameter alpha, identifying the accumulated relative mass removal of bonding material, has been implemented into a novel distinct element method bond contact model. This model is based on a series of mechanical test on bonded aluminium rods with different bond geometries. The numerical direct shear test results of echelon rock joints characterised by different values of alpha show that increasing time-dependent parameter alpha can lead to a lower crack initiation and peak stresses. This is accompanied by a growing ratio of the microscopic compressive-shear-torsional (CST) bond failure number of bond failures to the total number of failures, except for the case without weathering influence. High values of alpha render the material bridge a weaker part to be cut through, generating a large number of CST bond breakages along the central shear axis.Peer ReviewedPostprint (author's final draft

On the Importance of Displacement History in Soft-Body Contact Models

Two approaches are commonly used for handling frictional contact within the framework of the discrete element method (DEM). One relies on the complementarity method (CM) to enforce a nonpenetration condition and the Coulomb dry-friction model at the interface between two bodies in mutual contact. The second approach, called the penalty method (PM), invokes an elasticity argument to produce a frictional contact force that factors in the local deformation and relative motion of the bodies in contact. We give a brief presentation of a DEM-PM contact model that includes multi-time-step tangential contact displacement history. We show that its implementation in an open-source simulation capability called Chrono is capable of accurately reproducing results from physical tests typical of the field of geomechanics, i.e., direct shear tests on a monodisperse material. Keeping track of the tangential contact displacement history emerges as a key element of the model. We show that identical simulations using contact models that include either no tangential contact displacement history or only single-time-step tangential contact displacement history are unable to accurately model the direct shear test

Comparison of residual shear strength determined by different methods

The shear stress of stiff or dense soils increases with the displacement and reaches itsmaximum value, and then shear stress decreases and remains a constant value. The minimumand constant shear stress of soils reached at large shear displacements is called as residualshear strength. Residual shear strength generally has a great importance in design ofengineering structures constructed on fissured overconsolidated clays and long-term slopestability analysis in geotechnical engineering. In laboratory testing, modeling the residualconditions of a soil requires large shear displacements attained in drained conditions.Reversal direct shear test (RDS), consolidated-drained triaxial test (CD) and torsional ringshear test (RS) are the widely used testing methods to determine residual shear strengthparameters. These methods have some advantages or limitations when compared with eachother. In this study, residual shear strength parameters of soil samples having different clayfractions were determined by the three different drained tests, the results were compared, andeffect of the testing methods on residual shear strength was investigated. The variation ofresidual shear strength angle versus liquid limit and plasticity index were studied. The resultswere compared with previous studies. As a result, it is found that the residual shear strengthangle determined by the ring shear test is lower than the others, and the residual shear strengthangle decreases with the increasing liquid limit and plasticity index

Shear properties of the strain hardening cementitious composite material

Recently, Strain Hardening Cementitious Composite (SHCC) material has been used for the shear strengthening and the structural rehabilitation of reinforced concrete structures. However, the shear behavior of this material has not been yet fully understood due to lack of an appropriate and accurate direct shear test method. This paper aims to investigate the shear properties of the SHCC material. For this purpose, Iosipescu shear test was selected, where loads are applied in antisymmetric four points bending, assuring a pure shear section at the center of the specimen. A special geometry for the specimen was adopted in order to assure a uniform shear stress distribution in the pure shear section. This experimental test can characterize the shear behavior of SHCC material. The experimental test was simulated by the FEM-based computer program, FEMIX. To predict the average shear stress-sliding response, the shear crack softening diagram, available in the multi-directional fixed smeared crack model, was used. After demonstration the good predictive performance of the numerical model, a parametric study was carried out to evaluate the influence of shear retention factor, fracture energy of mode II, and crack shear strength on the average shear stress-sliding response of the SHCC. The advantage of SHCC instead of conventional mortar was also studied.Fundação para a Ciência e a Tecnologia (FCT) - PTDC/ECM/114511/200

Towards comprehensive characterisation and modelling of the forming and wrinkling mechanics of engineering fabrics

Through a combination of direct measurement and inverse modelling, a route to characterising the main mechanical forming properties of engineering fabric is demonstrated. The process involves just two experimental tests, a cantilever bending test and a modified version of the uniaxial bias extension test. The mechanical forming properties of a twill weave carbon fabric have been determined, including estimates of the in-plane bending stiffness and the torsional stiffness of a sheared fabric. As a result of measuring and incorporating all the main mechanical properties of the fabric in forming simulations (tensile, shear, out-of-plane bending, in-plane bending & torsion), the specimen size-dependent shear kinematics and wrinkling response measured in experiments, is faithfully reproduced in simulations of the uniaxial bias extension (UBE) test

A three-dimensional turbulent separated flow and related mesurements

The applicability of and the limits on the applicability of 11 near wall similarity laws characterizing three-dimensional turbulent boundary layer flows were determined. A direct force sensing local wall shear stress meter was used in both pressure-driven and shear-driven three-dimensional turbulent boundary layers, together with extensive mean velocity field and wall pressure field data. This resulted in a relatively large number of graphical comparisons of the predictive ability of 10 of these 11 similarity models relative to measured data over a wide range of flow conditions. Documentation of a complex, separated three-dimensional turbulent flow as a standard test case for evaluating the predictive ability of numerical codes solving such flows is presented