Stress Transfer Process Research Articles

Experimental investigation of stress transfer and failure mechanism between existing concrete and CFRP grid-sprayed PCM

Abstract Carbon fiber reinforced polymer (CFRP) grid and sprayed polymer cement mortar (PCM) has been widely applied to retrofit and strengthen concrete structures. This paper focused on the stress transfer and failure mechanism between the concrete and CFRP grid with sprayed PCM. Pull-out tests of the single grid point in CFRP grid and plate-type specimens consisting of concrete, PCM and CFRP grid were carried out. Types of CFRP grids, grid interval and whether or not to use primer were selected as analysis parameters. The maximum tensile loads, failure modes and strains in the CFRP grid were obtained. The stress transfer process was investigated through analysis of the changing process for the resistant action of horizontal grid and the bond action of vertical grid. The stress transfer mode and its influence parameters were investigated based on the strain share ratios of grid points. In addition, the failure mechanisms of specimens were discussed through mechanical analysis. The research results indicated that the tensile strength of grid point was no more than 50% of the tensile strength of CFRP grid. The stress transfer mode was not constant, and it would gradually change with the increase of tensile load. The failure mechanism of the specimen could be evaluated based on the difference among the ultimate tensile action of CFRP grid, bond capacity between concrete and PCM with CFRP grid and the anchorage capacity of CFRP grid in the PCM. In order to make full use of the tensile strength of CFRP grid, the bond capacity between concrete and PCM with CFRP grid and the anchorage capacity of CFRP grid in the PCM should be larger than the ultimate tensile action of CFRP grid.

Study on Metal Plate Vibration Response Under Coal–Gangue Impact Based on 3D Simulation

The collision between the coal–gangue and the metal plate of the hydraulic support is a typical mechanical contact behavior in the surface coal mining, which will cause vibration in the tail beam and even in the whole hydraulic support. This paper presents a contact analysis of coal–gangue impact on the metal plate to discover how vibration signals are generated and propagated in the collision process. We build a dynamic model for rock impact on the metal plate based on the elastoplastic contact theory, the Drucker–Prager criterion and the Tavares and King particle impact damage model. We establish a finite element model of the impact system between the coal–gangue and the metal plate, conduct contact simulation using the software LS-DYNA and study the stress transfer process on the metal plate contact surface. We compare the parameters of coal and gangue impact on the metal plate and discuss how they are different by calculating the stress and displacement created in the metal plate, kinetic energy of the coal–gangue particles and the total energy of the metal plate. In addition, we study the effect of impact velocity on the vibration response of the metal plate. Results show that the stress wave diffuses outward from the center of the metal plate. The collision frequency, contact position of the rock sphere and re-collision time are random. As the impact velocity increases, the contact responses all increase. This study provides guidance for the analysis of vibration response of coal–gangue impact on tail beam .

Stress transfer at the nanoscale on graphene ribbons of regular geometry.

The knowledge of the mechanism of stress transfer from a polymer matrix to a 2-dimensional nano-inclusion such as a graphene flake is of paramount importance for the design and the production of effective nanocomposites. For efficient reinforcement the shape of the inclusion must be accurately controlled since the axial stress transfer from matrix to the inclusion is affected by the axial-shear coupling observed upon loading of a flake of irregular geometry. Herein, we study true axial phenomena on regular- exfoliated-graphene micro-ribbons which are perfectly aligned to the loading direction. We exploit the strain sensitivity of vibrational wave numbers in order to map point-by-point the strain built up along the length of graphene. By considering the balance of shear-to-axial forces, we identify the shear stress at the interface and develop a universal inverse-length parameter that governs the stress transfer process at the nanoscale. An important parameter that has come out of this approach is the prediction and measurement of the transfer length that is required for efficient stress in these systems.

Mechanoresponsive, Luminescent Polymer Blends Based on an Excimer-Forming Telechelic Macromolecule.

A well-known approach toward mechanochromic polymers relies on the incorporation of excimer-forming fluorophores into a matrix polymer and the disruption of aggregated chromophores when such materials undergo macroscopic mechanical deformation. However, the required aggregates and stress-transfer processes have so far only been realized with select dye/polymer combinations. As demonstrated here, the utility of this approach can be extended by tethering an excimer-forming cyano-substituted oligo(p-phenylene vinylene) fluorophore to the two ends of a telechelic poly(ethylene-co-butylene) and blending small amounts (0.1-2wt%) of the resulting aggregachromic macromolecule into polymer matrices such as poly(ε-caprolactone), poly(isoprene), or poly(styrene-b-butadiene-b-styrene). All blends display mechanofluorochromic responses, and the ratio between the monomer and excimer emission intensities can be used to correlate the luminescence signal to the extent of deformation and to follow subsequent relaxation processes. The developed approach significantly expands the scope of blend-based mechanoresponsive luminescent materials.

Interfaces in polyethylene oxide modified cellulose nanocrystal - polyethylene matrix composites

The interface between cellulose nanocrystals (CNCs) in thermoplastic matrices is one of the most important issues prohibiting the development of CNC-based polymeric composites prepared via melt processing. In the present work, polyethylene oxide (PEO) was used as a compatibilizer to enhance the interface with a polyethylene matrix. It was found that the composites produced using a PEO compatibilizer possess better overall mechanical properties and a higher degree of crystallinity of the polyethylene than the unmodified samples. An increase in both the tensile strength and modulus of the composites was observed for up to 1.5 wt.% of CNCs; beyond this point no significant increases were observed. When CNCs were added (up to 1.5 wt.%) to the matrix, the crystallization peak of the composites in the DSC thermograms shifted to higher temperatures. The stress-transfer process in the composites was monitored using Raman spectroscopy. Higher Raman band shift rates with respect to tensile strain of a peak corresponding to main chain molecular deformation were observed for the composites produced using the PEO compatibilizer. This demonstrates that stress is transferred from the matrix to the fillers more effectively with the presence of PEO. The simple PEO-modification approach adopted in this study avoids the classical solvent method of production of CNC-based nanocomposites which is not really applicable at an industrial scale.

Three-dimensional axisymmetric responses of exponentially graded transversely isotropic tri-materials under interfacial loading

In this paper, an analytical formulation is presented to study an exponentially graded transversely isotropic tri-material under applied axisymmetric point-load and patchload with the aid of Hankel transform and use of a potential function. The given formulation is shown to be reducible to the special cases of (1) an inhomogeneous nite layer on a rigid base; (2) exponentially graded bi-material or half-space under applied buried loads; (3) homogeneous tri-material or bi-material solid. Several numerical solutions are presentedto explain inhomogeneity e ect on the stress transfer process in the inhomogeneous threelayered medium by means of a fast and accurate numerical method.

Coexistence of interfacial stress and charge transfer in graphene oxide-based magnetic nanocomposites

In this paper, we establish the existence of both compressive stress and charge transfer process in hydrothermally synthesized cobalt ferrite-graphene oxide (CoFe2O4/GO) nanocomposites. Transmission electron microscopy (TEM) results reveal the decoration of CoFe2O4 nanoparticles on GO sheets. Magnetic response of nanocomposites was confirme from superconducting quantum interference device (SQUID) magnetometer measurement. Optical properties of these nanocomposites were investigated by Raman spectroscopy. Interfacial compressive stress involved in this system is evaluated from observed blue shift of characteristic G peak of graphene oxide. Increase in full width half-maximum ( FWHM) as well as up shift in D and G peaks are clear indicator of involvement of charge transfer process between GO sheets and dispersed magnetic nanoparticles. The effect of charge transfer process is quantified in terms of shifting of Fermi level of these nanocomposites. This is evaluated from variation in contact surface potential difference (CPD) using Scanning Kelvin probe microscopy (SKPM). XRD spectra of CoFe2O4/GO confirm the polycrystalline nature of CoFe2O4 nanoparticles. Lattice strain estimated from XRD peaks are correlated to the observed Raman shift.

The effect of the shape of the cohesive material law on the stress transfer at the FRP-masonry interface

Debonding of the fiber-reinforced polymer (FRP) from the substrate that occurs before its tensile strength is reached typically controls the effectiveness of the stress transfer mechanism between FRP composites and a quasi-brittle substrate, such as masonry. The FRP-substrate interface is usually modelled as a zero-thickness interface whose fracture Mode-II cohesive material law (CML) is defined in terms of shear stress and slip at the interface. In this paper, the effect of the shape of the CMLs of the FRP-brick and FRP-mortar interfaces on the stress transfer process will be presented and discussed. Several multi-linear CMLs, obtained from experimental data for both mortar and brick interfaces, will be adopted to obtain the load response of the FRP-masonry interface. It will be shown that the shape of the CMLs adopted does not strongly affect the load response if the fracture parameters are the same or similar among the CMLs. Finally, certain geometric relationships of brick and mortar joints imply more pronounced differences in terms of load response between the CMLs adopted.

Elastic plastic deformation of TC6 titanium alloy analyzed by in-situ synchrotron based X-ray diffraction and microstructure based finite element modeling

The phase properties and distribution of multi-phase materials not only produce heterogeneous stress distributions between phases, but also stress gradients inside grains. However, due to limitations of stress/strain partitioning at the submicron scale, mechanical property characterizations of such alloys are typically based on evaluating averaged responses, e.g. macroscopic stress-strain curves. Here, the elastic plastic deformation of α/β dual phase TC6 titanium alloy is investigated in depth on the microstructural level by a two dimensional microstructure based finite element method (FEM). In-situ synchrotron based X-ray diffraction was also used to characterize the lattice strain evolutions during deformation. The microscopic stress and strain evolutions were successfully tracked by FEM during the elastic plastic transition process. The α phase bears a relatively high stress compared to the β phase during the elastic deformation stage. In the elastic to plastic deformation stage, a stress inversion phenomenon was observed in the microstructure by path tracing to elucidate the stress transfer process in detail. In the plastic deformation stage, strong plastic strain concentration occurs in the α phase near phase boundaries. The current local strains and stresses of α and β phases are not synchronous with the applied macroscopic true strains and the corresponding constitutive stresses. Moreover, different current local strain rates produce obvious stress gradients inside the α grains.

Stone column effectiveness in soils with creep: a numerical study

ABSTRACTWhile it is well established that vibro stone columns reduce primary settlement and improve bearing capacity, their effect on creep compression has largely been overlooked to date. However, with increasing pressure to develop marginal sites underlain by soft organic soils, the effect of ground treatment on creep is an important emerging issue in geotechnical engineering. In this paper, a series of axisymmetric unit cell analyses have been carried out using the PLAXIS 2D finite element program in conjunction with the Soft Soil Creep (SSC) model. Examination of the evolution of settlement improvement factor with time has indicated that the presence of creep leads to a lower ‘total’ improvement factor than would be obtained for primary consolidation settlement alone. Separate ‘primary’ and ‘creep’ improvement factors have also been derived; the latter are much lower than the former, but are nevertheless greater than unity. Creep results in a stress transfer process; as the soil creeps, vertical stress is transferred from the soil to the stone column. The additional load carried by the column induces additional yielding and shear-plane formation in closely-spaced columns. The additional increment of stress transferred to the already yielded column reduces its efficacy.

Stress-directed compositional patterning of SiGe substrates for lateral quantum barrier manipulation

While vertical stacking of quantum well and dot structures is well established in heteroepitaxial semiconductor materials, manipulation of quantum barriers in the lateral directions poses a significant engineering challenge. Here, we demonstrate lateral quantum barrier manipulation in a crystalline SiGe alloy using structured mechanical fields to drive compositional redistribution. To apply stress, we make use of a nano-indenter array that is pressed against a Si0.8Ge0.2 wafer in a custom-made mechanical press. The entire assembly is then annealed at high temperatures, during which the larger Ge atoms are selectively driven away from areas of compressive stress. Compositional analysis of the SiGe substrates reveals that this approach leads to a transfer of the indenter array pattern to the near-surface elemental composition, resulting in near 100% Si regions underneath each indenter that are separated from each other by the surrounding Si0.8Ge0.2 bulk. The “stress transfer” process is studied in detail using multiscale computer simulations that demonstrate its robustness across a wide range of applied stresses and annealing temperatures. While the “Si nanodot” structures formed here are not intrinsically useful as quantum structures, it is anticipated that the stress transfer process may be modified by judicious control of the SiGe film thickness and indenter array pattern to form more technologically useful structures.

Dynamic Numerical Analysis of the “Foot – Training Shoe“Model

This paper focus on the three - dimensional dynamic research of the”foot - shoe“system. Based on the analysis on the physiology of human foot, the dynamic explicit Finite Element model of the foot and two kinds of training shoes’ sole are established by ANSA, ANSYS, DYNA and other software. Firstly the stress transfer process of falling foot is numerically simulated. Secondly the effective stresses of the foot by the new-fashioned and old-fashioned shoe are compared. Then the load characteristics of the”foot - shoe“model is studied through the acceleration, the energy, etc. Finally the research conclusion and the suggestion are summarized, which prove that the sole with the multilayer structure, anti-torsion bridge design and optimum material selection is very important for absorbing the shock and protecting the human arch, the ankle, etc., which can reduce the stress peak value of about 80%.

Development of high performance fiber reinforced cement composites (HPFRCC) for application as a transition layer of reinforced beams

This study presents the development and behavior analysis of high performance fiber reinforced cement composites (HPFRCC). The describedmaterials were specifically developed for application as a transition layer: a repair layer that constitutes the stressed chord of reinforcedconcrete beams strengthened in flexure with carbon fiber reinforced polymers (CFRP). Nineteen different composites were produced by thehybridization process, varying the conventional short steel fiber and steel microfiber (manufactured exclusively for this research) contentsto modify the microstructure of the material, thus enhancing the stress transfer process from the cement matrix to the fibers. To analyze theresponse to flexural loading, the composites underwent three point bending tests in notched prism specimens. The response of the materialwas obtained considering strength and tenacity parameters (flexural and fracture). There was evidence of high performance by the composites with a pseudo-hardening behavior.

Stress transfer analysis of unidirectional composites with randomly distributed fibers using finite element method

A three-dimensional representative volume element (RVE) of unidirectional composites with both randomly distributed fibers and periodic geometry was generated using DIGIMAT-FE software. Finite element analysis of the stress transfer mechanisms around a fiber break in the RVE was performed via ABAQUS/Standard. The influences of distance to the broken fiber, fiber/matrix stiffness ratio and fiber volume fraction on the stress transfer process of intact fibers were discussed for the case of perfect fiber/matrix adhesion. The study shows that the nearest fibers and the second nearest fibers share the stress released from the broken fiber. The stress transfer coefficient and the ineffective stress transfer length of the nearest fibers was found to increase with the increasing distance to the broken fiber and the stiffness ratio, while decrease with the increasing fiber volume fraction. However, the trends in the two stress transfer parameters of the second nearest fibers are slightly different from those of the nearest fibers due to the random distribution of other intact fibers.

Micro-deformation mechanism of Zr-based metallic glass/porous tungsten composite by in-situ high-energy X-ray diffraction and finite element modeling

Micromechanical behaviors of Zr-based metallic glass/porous tungsten composite under quasi-static uniaxial compression at room temperature were investigated by in-situ high-energy X-ray diffraction (HEXRD) technique based on the synchrotron source and finite element modeling (FEM). During the process of compression, the main load phase was tungsten phase until it yielded at an applied stress of 1175MPa. Subsequently, metallic glass phase became the main load phase. The plastic misfit strain of the two phases resulted in stress concentration near interfaces between them, which accelerated the process of stress transfer from the tungsten phase to the metallic glass phase. The metallic glass phase started to yield at 1500MPa; at that time defects weakened the mutual restriction between the metallic glass phase and the tungsten phase, leading the tungsten phase to fail or the interfaces between two phases to separate by the increasing load.

Investigating Ductile Lithosphere Deformation in Southern California

A workshop to investigate properties and processes of ductile lithosphere deformation was held in Menlo Park, Calif. This workshop provided overviews of the newest results from lab, field, and geodetic modeling disciplines relevant to better understanding earthquake processes in Southern California. A principal goal of the workshop was to bring together niche experts in tectonic geodesy, laboratory rock mechanics, and field observations of ductile rocks to gain better interdisciplinary understanding of the stress transfer process between brittle‐elastic upper crust and ductile lithosphere in Southern California.

Orientation and deformation of wet-stretched all-cellulose nanocomposites

Cellulose nanowhiskers (CNWs) are used to reinforce an all-cellulose composite. This composite comprises a matrix formed by dissolution of plant cellulose using a lithium chloride/N,N-dimethylacetamide solvent system into which high stiffness CNWs are dispersed. It is shown that the Young’s modulus and strength of the composites decrease dramatically when the material is wetted. Raman spectroscopy is used to show how the two bands located at 1095 and 895 cm−1 can be used to follow both the molecular deformation and orientation of the CNWs and the matrix phases, respectively, both in the wet and dry states. The disruption of the stress transfer process is observable via the lack of shift in the position of both Raman peaks upon deformation in the wet state. This shift is restored when the samples are dried, and some recovery is noted in the materials. The orientation of both the matrix and CNWs phases is monitored during stretching, both in wet and dry states. Little orientation of the CNWs is observed at low strains. Significant orientation of the CNWs occurs at high strain, for samples deformed both in the wet and dry states. The most significant orientation is observed for cellulose molecular chains present in the matrix phase. It is thought that this component of the orientation contributes significantly to the mechanical properties of the nanocomposites. The use of these approaches opens up opportunities to use wet processing to induce orientation of cellulose nanofibers, and to monitor the development of mechanical properties using Raman spectroscopy in a wet environment.

April 2012 intra-oceanic seismicity off Sumatra boosted by the Banda-Aceh megathrust

Large earthquakes nucleate at tectonic plate boundaries, and their occurrence within a plate's interior remains rare and poorly documented, especially offshore. The two large earthquakes that struck the northeastern Indian Ocean on 11 April 2012 are an exception: they are the largest strike-slip events reported in historical times and triggered large aftershocks worldwide. Yet they occurred within an intra-oceanic setting along the fossil fabric of the extinct Wharton basin, rather than on a discrete plate boundary. Here we show that the 11 April 2012 twin earthquakes are part of a continuing boost of the intraplate deformation between India and Australia that followed the Aceh 2004 and Nias 2005 megathrust earthquakes, subsequent to a stress transfer process recognized at other subduction zones. Using Coulomb stress change calculations, we show that the coseismic slips of the Aceh and Nias earthquakes can promote oceanic left-lateral strike-slip earthquakes on pre-existing meridian-aligned fault planes. We further show that persistent viscous relaxation in the asthenospheric mantle several years after the Aceh megathrust explains the time lag between the 2004 megathrust and the 2012 intraplate events. On a short timescale, the 2012 events provide new evidence for the interplay between megathrusts at the subduction interface and intraplate deformation offshore. On a longer geological timescale, the Australian plate, driven by slab-pull forces at the Sunda trench, is detaching from the Indian plate, which is subjected to resisting forces at the Himalayan front.

Micromechanics of TEMPO-Oxidized Fibrillated Cellulose Composites

Composites of poly(lactic) acid (PLA) reinforced with TEMPO-oxidized fibrillated cellulose (TOFC) were prepared to 15, 20, 25, and 30% fiber weight fractions. To aid dispersion and to improve stress transfer, we acetylated the TOFC prior to the fabrication of TOFC-PLA composite films. Raman spectroscopy was employed to study the deformation micromechanics in these systems. Microtensile specimens were prepared from the films and deformed in tension with Raman spectra being collected simultaneously during deformation. A shift in a Raman peak initially located at ~1095 cm(-1), assigned to C-O-C stretching of the cellulose backbone, was observed upon deformation, indicating stress transfer from the matrix to the TOFC reinforcement. The highest band shift rate, with respect to strain, was observed in composites having a 30% weight fraction of TOFC. These composites also displayed a significantly higher strain to failure compared to pure acetylated TOFC film, and to the composites having lower weight fractions of TOFC. The stress-transfer processes that occur in microfibrillated cellulose composites are discussed with reference to the micromechanical data presented. It is shown that these TOFC-based composite materials are progressively dominated by the mechanics of the networks, and a shear-lag type stress transfer between fibers.

The June 2010 Nicobar Earthquake: Fault Reactivation on the Subducting Oceanic Plate

The similar to 1300-km-long rupture zone of the 2004 Andaman-Sumatra megathrust earthquake continues to generate a mix of thrust, normal, and strike-slip faulting events. The 12 June 2010 M(w) 7.5 event on the subducting plate is the most recent large earthquake on the Nicobar segment. The left-lateral faulting mechanism of this event is unusual for the outer-rise region, considering the stress transfer processes that follow great underthrusting earthquakes. Another earthquake (M(w) 7.2) with a similar mechanism occurred very close to this event on 24 July 2005. These earthquakes and most of their aftershocks on the subducting plate were generated by left-lateral strike-slip faulting on north-northeast-south-southwest oriented near-vertical faults, in response to north-northwest-south-southeast directed compression. Pre-2004 earthquake faulting mechanisms on the subducting oceanic plate are consistent with this pattern. Post-2004, left-lateral faulting on the subducting oceanic plate clusters between 5 degrees N and 9 degrees N, where the 90 degrees E ridge impinges the trench axis. Our study observes that the subducting plate off the Sumatra and Nicobar segments behaves similarly to a chip of the India-Australia plate, deforming in response to a generally northwest-southeast oriented compression, an aspect that must be factored into the plate deformation models.