High Axial Strain Research Articles

DEM Investigation of Particle-Scale Mechanical Properties of Frozen Soil Based on the Nonlinear Microcontact Model Incorporating Rolling Resistance

Although frozen soil is in nature the discrete material, it is generally treated as the continuum material. The mechanical properties of frozen soil are so complex to describe adequately by conventional continuum mechanics method. In this study, the nonlinear microcontact model incorporating rolling resistance is proposed to investigate the particle-scale mechanical properties of frozen soil. The failure mechanism of frozen soil is explicated based on the evolution of contact force chains and propagation of microcracks. In addition, the effects of contact stiffness ratio and friction coefficient on stress-strain curve and energy evolution are evaluated. The results show that the nonlinear microcontact model incorporating rolling resistance can better describe the experimental data. At a higher axial strain, the contact force chains near shear band which can give rise to the soil arch effect rotate away from the shear band inclination but not so much as to become perpendicular to it. The propagation of microcracks can be divided into two phases. The stress-strain curve is strongly influenced by contact stiffness ratio. In addition, friction coefficient does not significantly affect the initial tangential modulus. Compared with frictional coefficient, the effect of contact stiffness ratio on stress-strain curve and energy evolution is greater.

The Influence of Dispersedly Distributed Cracks on Critical Current of the Nb3Sn Strand

The Nb3Sn strand is the presently most widely used high field superconductor. However, one important fact is that the transport properties of an Nb3Sn strand will degrade obviously when it is subjected to high mechanical loads. Based on the dispersed distribution of cracks in the bronze strand due to a high axial tensile strain, we propose an analytical strand model to describe the influence of cracks on the critical current of the Nb3Sn strand. The dependence of the critical current I c of the strand on crack density is investigated theoretically. It is shown that the calculation results by this model agree with the experimental data. The influence of filament-to-matrix resistance r c on transport degradation is also discussed since r c is a key parameter for the total voltage calculation and its realistic value is of great importance for accurate results. We also compared the influence of dispersed and collective cracks on transport properties with specific conditions.

High sensitivity axial strain and temperature sensor based on dual-frequency optoelectronic oscillator using PMFBG Fabry-Perot filter.

A dual-frequency optoelectronic oscillator (OEO) incorporating a polarization-maintaining fiber Bragg grating (PMFBG) Fabry-Perot filter for high-sensitivity and high-speed axial strain and temperature sensing is proposed and experimentally demonstrated. In the OEO loop, two oscillation frequencies are determined by a PMFBG Fabry-Perot filter with two ultra-narrow notches and two laser sources which operate as a dual-passband microwave photonic filter. The fiber birefringence affected by axial strain is far less than the temperature. Through monitoring the variations of two oscillating frequencies and beat frequency, the simultaneous measurement for the axial strain and temperature is realized. The sensitivities of the proposed OEO sensor for axial strain and temperature are experimentally measured to be as high as 100.6 or 100.5 MHz/με and -41 MHz/°C, respectively.

Inelastic compaction and permeability evolution in volcanic rock

Abstract. Active volcanoes are mechanically dynamic environments, and edifice-forming material may often be subjected to significant amounts of stress and strain. It is understood that porous volcanic rock can compact inelastically under a wide range of in situ conditions. In this contribution, we explore the evolution of porosity and permeability – critical properties influencing the style and magnitude of volcanic activity – as a function of inelastic compaction of porous andesite under triaxial conditions. Progressive axial strain accumulation is associated with progressive porosity loss. The efficiency of compaction was found to be related to the effective confining pressure under which deformation occurred: at higher effective pressure, more porosity was lost for any given amount of axial strain. Permeability evolution is more complex, with small amounts of stress-induced compaction ( < 0.05, i.e. less than 5 % reduction in sample length) yielding an increase in permeability under all effective pressures tested, occasionally by almost 1 order of magnitude. This phenomenon is considered here to be the result of improved connectivity of formerly isolated porosity during triaxial loading. This effect is then overshadowed by a decrease in permeability with further inelastic strain accumulation, especially notable at high axial strains ( > 0.20) where samples may undergo a reduction in permeability by 2 orders of magnitude relative to their initial values. A physical limit to compaction is discussed, which we suggest is echoed in a limit to the potential for permeability reduction in compacting volcanic rock. Compiled literature data illustrate that at high axial strain (both in the brittle and ductile regimes), porosity ϕ and permeability k tend to converge towards intermediate values (i.e. 0.10 ≤ ϕ ≤ 0.20; 10−14 ≤ k ≤ 10−13 m2). These results are discussed in light of their potential ramifications for impacting edifice outgassing – and in turn, eruptive activity – in active volcanoes.

Stress-strain response analysis and protective device design for buried pipeline impacted by perilous rock

Collapse of perilous rocks is one of the most severe geological disasters for pipeline security. Stress-strain response of a buried pressure pipeline impacted by a perilous rock was simulated. Effects of impact velocity, rock’s radius, pipeline’s wall thickness, surrounding soil’s elastic modulus and Poisson’s ratio on stress, strain and deformation of the buried pipeline were investigated. The results show that the buried pipeline’s upper part is prone to instability under the rock impact. Plastic area of the buried pipeline becomes from oval to bat type with the impact load increases. Strain of the impact dent center is a compressive strain, while it is a tensile strain on the two sides of the dent. High stress area, axial strain and plastic strain of the buried pipeline increase with the increasing of impact velocity and rock’s radius, but they decrease with the increasing of wall thickness and soil’s elasticity modulus. Surrounding soil’s Poisson’s ratio has a small effect on the stress and strain of the pipeline. Impact dent’s size increases with the increasing of impact velocity and rock’s radius. Dent depth decreases with the surrounding soil’s elasticity modulus increases. With the wall thickness increases, impact dent depth first increases and then decreases. Finally, a protective device of buried pipeline is designed for preventing perilous rock impact. It can reduce the failure probability and improve the service life of buried pipeline for its simple structure and convenient installation.

Unconventional Gas: Experimental Study of the Influence of Subcritical Carbon Dioxide on the Mechanical Properties of Black Shale

An experimental study was performed to investigate the effect of subcritical carbon dioxide (CO2) adsorption on mechanical properties of shales with different coring directions. Uniaxial compressive strength (UCS) tests were conducted on shale samples with different CO2 adsorption time at a pressure of 7 MPa and a temperature of 40 °C. The crack propagation and the failure mechanism of shale samples were recorded by using acoustic emission (AE) sensors together with ARAMIS technology. According to the results, samples with parallel and normal bedding angles present reductions of 26.7% and 3.0% in UCS, 30.7% and 36.7% in Young’s modulus after 10 days’ adsorption of CO2, and 30.3% and 18.4% in UCS, 13.8% and 22.6% in Young’s modulus after 20 days’ adsorption of CO2. Samples with a normal bedding angle presented higher brittleness index than that with a parallel bedding angle. The strain distributions show that longer CO2 adsorption will cause higher axial strains and lateral strains. The AE results show that samples with a parallel angle have higher AE energy release than the samples with a normal angle. Finally, samples with longer CO2 adsorption times present higher cumulative AE energy release.

Behavior of hollow FRP–concrete–steel columns under static cyclic axial compressive loading

This paper presents the behavior of hollow fiber reinforced polymer–concrete–steel (HC-FCS) columns under axial compressive loading. The typical HC-FCS column consists of a concrete shell sandwiched between an outer fiber reinforced polymer (FRP) tube and an inner steel tube. The inner steel and outer FRP tubes provide continuous confinement for the concrete shell; hence, the concrete shell achieves a significantly higher strain, strength, and ductility compared to the unconfined concrete in conventional columns. The HC-FCS column represents a compact engineering system; the steel and FRP tubes act together as stay-in-place formworks. The effect of the fiber orientation and the steel tube diameter-to-thickness ratio (Di/ts) on the compressive behavior of HC-FCS columns was investigated. Ten HC-FCS cylinders with different steel tube Di/ts ratios and three concrete-filled fiber tubes (CFFTs) were manufactured and tested under static cyclic axial compressive loading in addition to three empty steel tubes. The behavior of the HC-FCS columns was complicated and related mainly to the stiffness of the FRP and steel tubes, which controlled the direction of the concrete dilation under axial load. HC-FCS columns with FRP tubes made with fibers oriented at ±45° showed low axial compressive strengths and high ultimate strains. HC-FCS columns with wet lay-up FRP tubes that had ±45° and 0° (hybrid FRP) exhibited high axial strengths and strains. The failure of the HC-FCS columns with hybrid FRP tubes consisted of two stages. The first stage was the rupture of the unidirectional FRP tube (outer tube), and the second stage was the reorientation of the oriented FRP tube exhibiting high axial strains.

Structural behaviour of hybrid FRP pultruded columns. Part 1: Experimental study

The design of glass fibre reinforced polymer (GFRP) pultruded members is often governed by deformability and buckling phenomena, preventing the full exploitation of the material potential. Hybridization – the partial replacement of the glass reinforcement with (stiffer) carbon fibres – is a possible approach to improve the performance of GFRP thin-walled profiles. This paper presents an experimental study on the structural behaviour of I-section hybrid fibre reinforced polymer (FRP) pultruded columns made of glass and carbon fibres (GF and CF) embedded in a polyester resin. A bare GFRP reference profile and four series of hybrid C-GFRP profiles, with different types and architectures of CF reinforcement, were designed, manufactured and tested under compression in three different lengths – short, intermediate and long. Particular attention was given to the buckling behaviour of the columns and to the delamination at the interface between GFRP and CFRP layers. In terms of serviceability performance, results obtained confirm the hybridization’s effectiveness in increasing the axial stiffness of GFRP compressive members. In terms of ultimate limit states behaviour, hybridization increased the load carrying capacity of the long columns, which exhibited global buckling. In opposition, for the short and intermediate columns, which failed respectively due to local buckling and a combination of global and local buckling, the load carrying capacity of the hybrid columns was lower than that of the reference profile; such worse performance seems to have been caused by the delamination of the CF layers, owing to the relatively high axial strains that developed in those columns. In a companion paper (Part 2), the experimental data presented and discussed herein is compared with predictions from numerical models and analytical formulae, which also provide further information about the delamination and progressive failure of the hybrid columns.

Offshore In-Service Resistance of Double Joints Under Strain-Based Conditions

Tenaris, Centro Sviluppo Materiali, and Saipem launched a cooperative program aimed at evaluating the in-service flaw tolerability of girth welded seamless pipes for offshore applications. Full-scale testing was conducted considering severe biaxial loading scenarios replicating actual offshore in-service conditions. Tearing by an occasional high axial strain was taken into account in order to consider suitability for strain-based conditions. Different notch sizes and sampling positions were also taken into account, thus improving the experimental database, which may assist the designer to evaluate the pipeline residual resistance when it is subjected to specific installation and in-service loading conditions.

Mechanical behaviour analysis of buried pressure pipeline crossing ground settlement zone

Numerical simulation model of buried pipeline crossing ground settlement zone was established considering pipeline–soil interaction. Mechanical behaviour of the buried pipeline was investigated, and effects of ground settlement, pipeline parameters and surrounding soil parameters on mechanical behaviour of the buried pipeline were discussed. These results show that there are two high stress areas on both sides of the dividing plane. High stress areas are oval on the top and bottom of the pipeline. Z-shape bending deformation appears under the action of ground settlement. In ground settlement zone, axial strain on the top of the pipeline is compression strain, and axial strain on the bottom of the pipeline is tension strain. On the contrary, they are tension strain and compression strain respectively in no settlement zone. Bending deformation, axial strain and plastic strain of the buried pipeline increase with the increase in ground settlement. Von Mises stress, high stress area, axial strain and plastic strain of the buried pipeline increase with the increasing diameter-thick ratio and internal pressure, but they decrease with the increase in buried depth. Diameter-thick ratio and internal pressure have a small effect on the bending deformation of the buried pipeline. Bending deformation decreases with the increase in buried depth in ground settlement zone. Von Mises stress and high stress area increase with the increasing surrounding soil’s elasticity modulus and cohesion, but they increase first and then decrease with the increase in Poisson’s ratio. Bending deformation of the pipeline in no settlement zone increases with the increase in elasticity modulus and Poisson’s ratio, but it is affected little by the cohesion. Axial strain and plastic strain have a bigger relationship with the elasticity modulus and Poisson’s ratio. Axial strain and plastic strain of the buried pipeline increase with the increase in cohesion, and the change rates increase with the increase in ground settlement.

Stress relaxation behavior in Virginia Beach sand

Effects of strain rate on the stress–strain and subsequent stress relaxation behaviors have been studied by performing triaxial compression tests on dense Virginia Beach sand specimens at three different strain rates (ratio of 256 between the slowest and the fastest) under low and high confining pressures. For the tests performed under low confining pressure, the specimens that were initially sheared at a faster rate showed a slightly higher amount of stress relaxation, but almost identical stress–strain behaviors were achieved. For tests performed under high confining pressure, the same amount of strength was achieved at high axial strains (10% to 20%), but specimens sheared at higher strain rates showed a slightly stiffer stress–strain response at low axial strains (up to 10%). Similar to the tests performed under low confining pressure, higher strain rates produced higher amounts of stress relaxation to some extent. Effects of correction of axial strain due to load cell expansion and drainage condition during stress relaxation have also been studied and the results indicated that correction of axial strain and undrained condition will both increase the observed amount of stress relaxation. Moreover, a 1 day stress relaxation curve was obtained by connecting the ending stress–strain points of six stress relaxation tests initiated at different deviator stress levels, and this curve was found to be different from the 1 day creep curve obtained from a previous study. A long-term stress relaxation test was also performed, and it showed linear reduction of deviator stress with the logarithm of time during stress relaxation. Observations made are all aligned with the phenomenon of static fatigue and the proposed mechanism for time effects in granular materials.

Effect of surrounding soil on stress–strain response of buried pipelines under ground loads

In order to determine the stress–strain response of buried pipelines under the ground load, the pipeline–soil coupling finite element model was established. The mechanical behaviours of buried pipelines in the soil stratum and the rock stratum, as well as the effects of surrounding soil's elasticity modulus, Poisson's ratio and cohesion on stress and strain of buried pipelines were investigated. The results show that the maximum von Mises stress, high stress area, axial strain and plastic strain increase with the increasing ground load. Buried pipeline in the soil stratum is more prone to failure than in the rock stratum under the same ground load. The maximum axial compressive strain appears at the bottom of buried pipelines when there is no buckling, and the maximum tensile strain appears on the two sides. High stress area, the axial strain, the plastic strain and the plastic area decrease with the increase of the soil's elasticity modulus and cohesion. But the surrounding soil's Poisson's ratio has a s...

The impact of particle form on the packing and shear behaviour of some granular materials: an experimental study

This paper reports the results of experiments carried out to explore the effects of particle form, sometimes referred to as sphericity, on the depositional density and subsequent shear behaviour of granular material. Previous experimental studies have generally neglected this important control, considering only rotund or bulky particles. However, the results of a few numerical studies have suggested its importance. Pluviated granular materials were tested in triaxial compression under free-end conditions, achieved using enlarged lubricated platens, which allowed specimens to be taken to high axial strain levels without strain localisation. Strains were measured locally. The results show that the limiting void ratios, \(e_{max}\) and \(e_{min}\), the void ratio range (\(e_{max}- e_{min})\) and the depositional anisotropy increase as particles become platy. Almost all aspects of behaviour during shear are significantly affected by particle form, including mobilised friction at the onset of dilation, peak state and ultimate (or critical) state, dilation and particle breakage. For extremely platy material it appears that dilatancy may be completely suppressed, even for dense packings of particles.

Optimized FRP Wrapping Schemes for Circular Concrete Columns under Axial Compression

This study investigates the behavior and failure modes of fiber-reinforced polymer (FRP) confined concrete wrapped with different FRP schemes, including fully wrapped, partially wrapped, and nonuniformly-wrapped concrete cylinders. By using the same amount of FRP, this study proposes a new wrapping scheme that provides a higher compressive strength and strain for FRP-confined concrete, in comparison with conventional fully wrapping schemes. A total of 33 specimens were cast and tested, with three of these specimens acting as reference specimens and the remaining specimens wrapped with different types of FRP (CFRP and GFRP) by different wrapping schemes. For specimens that belong to the descending branch type, the partially-wrapped specimens had a lower compressive strength but a higher axial strain as compared to the corresponding fully-wrapped specimens. In addition, the nonuniformly-wrapped specimens achieved both a higher compressive strength and axial strain in comparison with the fully-wrapped specimens. Furthermore, the partially-wrapping scheme changes the failure modes of the specimens and the angle of the failure surface. A new equation that can be used to predict the axial strain of concrete cylinders wrapped partially with FRP is proposed.

Experimental and numerical developing of reduced length buckling-restrained braces

Buckling-restrained braced frames (BRBFs) have been widely used as an efficient seismic load resisting system in recent years mostly due to their symmetric and stable hysteretic behavior and significant energy dissipation capacity. However, buckling-restrained braces (BRBs) are heavier and more expensive in comparison to other concentric bracing systems. In order to facilitate the use of BRBs, the idea of reducing the length of the core and the encasings which can result in lighter and more replaceable BRBs is proposed and experimentally investigated in this paper. Two relatively similar all-steel reduced length BRBs (RLBRBs) are designed, detailed and constructed using a special debonding and stopper mechanism. The design and construction procedure is accomplished by paying special attention to low-cycle fatigue (LCF). The specimens were tested under the quasi static loading protocol, and withstood high axial strains of 4–5% without any global or local failure. The hysteretic responses of the specimens were stable and symmetric. Moreover, numerical models were developed and nonlinear cyclic analyses were performed to provide better insight into the core and encasing performance as well as application of the RLBRB in the brace configuration.

Strain distribution in the intervertebral disc under unconfined compression and tension load by the optimized digital image correlation technique

The unconfined compression and tension experiments of the intervertebral disc were conducted by applying an optimized digital image correlation technique, and the internal strain distribution was analysed for the disc. It was found that the axial strain values of different positions increased obviously with the increase in loads, while inner annulus fibrosus and posterior annulus fibrosus experienced higher axial strains than the outer annulus fibrosus and anterior annulus fibrosus. Deep annulus fibrosus exhibited higher compressive and tensile axial strains than superficial annulus fibrosus for the anterior region, while there was an opposite result for the posterior region. It was noted that all samples demonstrated a nonlinear stress-strain profile in the process of deforming, and an elastic region was shown once the sample was deformed beyond its toe region.

Can quantitative synthetic aperture vascular elastography predict the stress distribution within the fibrous cap non-invasively

An imaging system that can detect and predict the propensity of an atherosclerotic plaque to rupture would reduce stroke. Radial and circumferential strain elastograms can reveal vulnerable regions within the fibrous cap. Circumferential stress imaging could predict the propensity of rupture. However, circumferential stress imaging demands either accurate knowledge of the geometric location of the fibrous cap or high quality strain information. We corroborated this hypothesis by performing studies on simulated vessel phantoms. More precisely, we computed stress elastograms with (1) precise knowledge of the fibrous cap, (2) no knowledge of the fibrous cap, (3) imprecise knowledge of the fibrous cap. We computed stress elastograms with accuracy of 8%, 15%, and 23% from high precision axial and lateral strain elastograms (i.e., 25 dB SNR) when precise, imprecise, and no geometric information was included the stress recovery method. The stress recovery method produced erroneous elastograms at lower noise level (i.e., 15 dB SNR), when no geometric information was included. Similarly, it produced elastograms with accuracy of 13% and 30% when precise and imprecise geometric information was included. The stress imaging method described in this paper performs well enough to warrant further studies with phantoms and ex-vivo samples.

Fracture analysis of single-layer graphene sheets with edge crack under tension

In this study, the fracture of single-layered graphene sheets (SLGSs) with edge crack under simple tension is investigated using molecular dynamics simulations, and the variations in fracture strength of SLGSs with crack length, strain rate and temperature are analysed. It is found that the existing edge crack weakens mechanical properties of SLGSs. Fracture strength and strain decrease with the increase in crack length and temperature, but increase with the increase in strain rate. It is also shown that shorter initial cracks propagate faster than longer initial cracks, but shorter initial cracks begin propagating at higher axial strain at a certain temperature and strain rate. And cracks are found to propagate faster in higher strain rates.

Further Insight into the Depth-Dependent Microstructural Response of Cartilage to Compression Using a Channel Indentation Technique

Stress relaxation and structural analysis were used to investigate the zonally differentiated microstructural response to compression of the integrated cartilage-on-bone tissue system. Fifteen cartilage-on-bone samples were divided into three equal groups and their stress relaxation responses obtained at three different levels of axial compressive strain defined as low (~20%), medium (~40%) and high (~60%). All tests were performed using a channel indenter which included a central relief space designed to capture the response of the matrix adjacent to the directly loaded regions. On completion of each stress relaxation test and while maintaining the imposed axial strain, the samples were formalin fixed, decalcified, and then sectioned for microstructural analysis. Chondron aspect ratios were used to determine the extent of relative strain at different zonal depths. The stress relaxation response of cartilage to all three defined levels of axial strain displayed an initial highly viscous response followed by a significant elastic response. Chondron aspect ratio measurements showed that at the lowest level of compression, axial deformation was confined to the superficial cartilage layer, while in the medium and high axial strain samples the deformation extended into the midzone. The cells in the deep zone remained undeformed for all compression levels.

Molecular dynamics simulation study on graphene-nanoribbon-resonators tuned by adjusting axial strain

A tunable graphene-nanoribbon (GNR)-resonator was investigated via classical molecular dynamics simulations. Resonance frequencies increased with increasing externally applied gate-force and axial-strain, and could be tuned above several hundred GHz. Tunable resonance frequencies achieved from the gate force were higher than those achieved from the axial-strain. The operating frequencies of GNR-resonators without axial-strain or with small axial-strains were most widely tuned by the gate, and almost linearly increased with increasing mean deflection. As the axial strain increased, the tunable ranges of the GNR-resonators were exponentially decreased, although the operating frequencies increased. GNR-resonators without axial-strain could be applied to wide-range-tuners, whereas GNR-resonators with high axial-strain could be applied to high-frequency-fine-tuners.