Tensile Regime Research Articles

Thermal vibration contribution to continuum stress in the elastic regime

Homogeneous deformation of an ordered crystalline solid at finite temperature can cause non-affine transformation of atomic trajectories. In such a case, continuum measures based on affine transformation of trajectories are insufficient to ensure energetic equivalence between the atomic and continuum scales. We use molecular dynamics simulation of fcc aluminum in NVT ensembles to demonstrate that the second moments of atomic positions about equilibrium show increasing deviation from affine behavior with strain and temperature. While the Cauchy-Born rule enforces affine deformation of the crystal in the static sense, second moments have been used to quantify non-affinity in thermal vibrations due to atomic trajectory transformation under macroscopic deformation. The evolution of second moments with applied strain becomes highly non-linear at high temperatures in the tensile regime as a result of the anharmonic potential energy surface of the material. The vibrational entropy is computed using the time-averaged second moments of position from the molecular dynamics simulation. Stress computed using the resulting free energy is significantly lower than the virial stress in the high tensile strain – high temperature regimes.

Local stress analysis of partial dislocation interactions with symmetrical-tilt grain boundaries containing E-structural units

ABSTRACTGrain boundaries containing porous E-structural units (SUs) are known to readily emit dislocations under tension. Using molecular dynamics simulations, we study the interactions between <112>{111} Shockley partial dislocations and <110> symmetrical-tilt Ni grain boundaries containing E-SUs. We show that the incoming Shockley partials can be accommodated by porous E-SUs along the grain boundary. However, the partial-absorption process disrupts the short-range interactions of incipient dislocations along the boundary, which generates high local tensile and compressive stress regimes emanating from the impingement sites. For the favoured grain boundary comprising only of E-SUs, incipient Shockley partials originating from E-SUs located within the tensile stress regime are subsequently re-emitted into the neighbouring grain. We demonstrate that the critical strength for re-emission of Shockley partials can be delineated into contributions from tensile stress generated by partial-absorption, intrinsic grain boundary tractions, as well as external loading. In the presence of other types of SUs, the incoming Shockley partials can also be transmitted through the boundary, or be stably absorbed by the boundary with no subsequent re-emission, depending on the impingement site.

A cone surface in three-dimensional analyses of slopes with tension cut-off

A three-dimensional limit analysis of slopes is presented in this paper, based on a right circular cone failure surface. The associated translational mechanism appears to yield better assessment of the safety in isotropic rock than the traditional wedge-type mechanism. The strength of a bonded geomaterial was described using a linear strength envelope truncated in the tensile regime. The tensile strength cut-off so introduced constitutes a non-linear portion of the strength envelope, and it allows constructing failure mechanisms that include rupture modes, in addition to the shear deformation mode. Consequently, the mechanisms in materials with tension cut-off are more critical than those in materials with a traditional linear strength envelope. It is also demonstrated that the failure mechanism based on the right circular cone failure surface in vertical slopes yields results as good as or better than the more complex rotational mechanisms, although the difference is not very significant.

Failure potential of infinite slopes in bonded soils with tensile strength cut-off

Infinite slope analyses are used in practice to assess the safety of slopes against shallow slides, with much of the soil mass moving parallel to a plane failure surface. Stability calculations typically involve the Mohr–Coulomb yield function, but the limitation on tensile strength is discussed and applied in stability considerations for bonded soils. Once the strength envelope is truncated in the tensile regime, the envelope becomes nonlinear and the normality flow rule admits deformation with large volumetric strains. For gentle slopes and in the absence of seepage, the difference in factors of safety calculated with and without tensile strength cut-off is small. However, in the presence of seepage and with increasing inclination of slopes, the difference becomes significant. As the deformation regime moves into the nonlinear portion of the yield envelope, application of the traditional definition of the factor of safety is not straightforward. If the strength envelope is truncated with a circular arc, then the envelope reduced by the factor of safety assumes an elliptical shape. The novelty in this paper is in presenting an analysis for bonded geomaterials with tensile strength cut-off, and indicating that tension cut-off can have a significant impact on the outcome of stability analysis for steep slopes subjected to seepage.

Tensile and shear failure behaviour of compacted clay – hybrid failure mode

Tensile and shear behaviour of unsaturated soil have been investigated separately in the past. However, there is no comprehensive study on the transition from tensile to shear failure in compacted clayey soils. A series of uniaxial tensile, direct shear and triaxial compression tests were conducted on compacted Regina clay specimens to investigate the strength mobilisation and failure mode under different loading paths. It was observed that the matric suction increases the apparent cohesion and tensile strength of the material. At a given constant matric suction, the compacted clay displays a transition from tensile to shear failure with increasing confining stress which can be approximated by a bilinear failure envelope encompassing the transitional behaviour. The friction angles in tensile regime are greater than those in compressive regime, which can be explained by the transitional change in failure surface morphology from tensile ruptured to shear smoothened one.

Three-dimensional stability analysis of slopes in hard soil/soft rock with tensile strength cut-off

Many collapse analyses of slopes in soils or soft rocks use the classical Mohr-Coulomb yield function to define the strength of geomaterials. In the presence of bonded particles and grains, this function predicts uniaxial tensile strength and even greater isotropic tensile strength. Testing for material properties, however, is typically carried out in the compressive regime; the tensile strength is then burdened by uncertainties, as it is a result of extrapolation of test results into the tensile regime. A three-dimensional limit analysis of slopes is presented with the geomaterial described by a yield surface with tensile strength cut-off. The multiplicity of admissible collapse mechanisms is enriched, as the tension cut-off allows construction of mechanisms that include rupture modes. Stability factors for slopes with tensile strength cut-off are reduced compared to those based on the classical Mohr-Coulomb strength envelope, with the largest drop for steep slopes subjected to seepage. The stability factor for a 70-degree slope subjected to seepage can be reduced by as much as 69% when tensile strength cut-off is considered.

Observations on the desiccation and cracking of clay layers

Waterways and lakes in low-lying delta areas require regular dredging for maintenance. Often these sediments are placed on land, where they are allowed to ripen through a combination of drainage, consolidation and evaporation. When cracks develop during desiccation, the physical response of the soil is affected by changes in the overall strength, stiffness and permeability of the material. To better identify how cracks form and propagate, a series of tests was carried out in a controlled laboratory environment on samples of drying clay slurries under different initial and boundary conditions. The outcomes of this study indicate that the results from laboratory small scale models must be carefully analyzed, as they depend on the area and the thickness of the sample. However, common features from the different tests can be identified, which are mostly related to the intrinsic behavior of the material. For instance, the water content at which cracks initiate depends mostly on the drying rate and not only on the initial water content. Typically for the clayey soil investigated, the cracking water content is well above the shrinkage limit and in some instances even above the liquid limit. Cracks can form anywhere a defect is encountered, but it was observed that they propagate in horizontal directions below the soil surface. On the soil surface they tend to intersect with each other perpendicularly, suggesting that they are dominated by a tensile stress regime. Shear stresses also influence the response, but mainly near the boundaries of the samples, due to the interface friction.

Structural biomechanics of the craniomaxillofacial skeleton under maximal masticatory loading: Inferences and critical analysis based on a validated computational model

The trend towards optimizing stabilization of the craniomaxillofacial skeleton (CMFS) with the minimum amount of fixation required to achieve union, and away from maximizing rigidity, requires a quantitative understanding of craniomaxillofacial biomechanics. This study uses computational modeling to quantify the structural biomechanics of the CMFS under maximal physiologic masticatory loading. Using an experimentally validated subject-specific finite element (FE) model of the CMFS, the patterns of stress and strain distribution as a result of physiological masticatory loading were calculated. The trajectories of the stresses were plotted to delineate compressive and tensile regimes over the entire CMFS volume. The lateral maxilla was found to be the primary vertical buttress under maximal bite force loading, with much smaller involvement of the naso-maxillary buttress. There was no evidence that the pterygo-maxillary region is a buttressing structure, counter to classical buttress theory. The stresses at the zygomatic sutures suggest that two-point fixation of zygomatic complex fractures may be sufficient for fixation under bite force loading. The current experimentally validated biomechanical FE model of the CMFS is a practical tool for in silico optimization of current practice techniques and may be used as a foundation for the development of design criteria for future technologies for the treatment of CMFS injury and disease.

Designing graphene based nanofoams with nonlinear auxetic and anisotropic mechanical properties under tension or compression

In this study, the analysis of the mechanical response of realistic fullerene-nanotube nanotruss networks with face-centered cubic geometry is performed by using molecular dynamics with reactive potentials. In particular, the mechanical properties of these novel architectures are investigated in both compressive and tensile regimes, a number of truss geometries by straining along different directions. Our atomistic simulations reveal a similar behavior under tensile stress for all the samples. Conversely, under compressive regimes the emergence of a response that depends on the orientation of load is observed together with a peculiar local instability. Due to this instability, some of these nanotruss networks present a negative Poisson ratio in compression, like re-entrant foams. Finally, the performance of these nanotruss networks is analyzed with regards to their use as impact energy absorbers, displaying properties that outperform materials traditionally used in these applications.

Effects of Intercalation on the Electronic Properties of Multilayer Tmdc Materials

Over the past few years, Transition Metal Dichalgogenide (TMDC) materials have shown great potentials for next generation nanoelectronic applications. Both monolayers and van der Waals (vdW) heterostructures drawn from them have shown really interesting electronic and optoelectronic properties [1][2]. The electronic structure and properties of these materials can be significantly influenced by the number of material layers involved and van der Waals interaction between the layers of TMDC. Recent advancement in processing technology has allowed the controlled growth of vdW heterostructures by stacking and their successful integration in electronic device applications for low power electronics. Effects of intercalation of a 2D TMDC sheet in between two monolayers of MoS2 have been shown in recent literature [1]. In this work, we have provided a detailed simulation study of the effects of biaxial strain on electronic properties like bandgap and carrier effective mass in the conduction band of vdW intercalated trilayers (MoS2/MX2/MoS2 where, M=Mo/W and X=Se/S) from first principle simulation. We provide simulation results of three different stacking configurations: AAA, ABA and ACA as shown in fig. 1. The simulation was carried out using PWSCF package of Quantum Espresso [3] simulation tool. After geometry optimization and relaxation of different stacking configurations, self-consistent field (SCF) calculation was done to determine electronic configuration for the relaxed atomic structure followed by bandstructure calculation using the same package. We used normconserving pseudopotenial functions in our simulation framework and 24x24x1 Monkhorst-Pack k-point grids for sampling in the brillouin zone. After extracted of geometrically relaxed structure, biaxial strain was applied to the lattice and the effects on band structure and carrier effective mass were observed. The carrier effective mass at the brillouin zone symmetry points was calculated by simple parabolic fitting to the extracted ab-initio band structure. At first, we performed structural geometry optimization for ABA and ACA stacking configurations. We extracted the optimized values of lattice constant and interlayer separation. Our extracted results showed good agreement with the values reported in [1]. Fig 2. Shows the effect of variation of electron effective mass at K point in the brillouin zone for the simulated lattice structure under strain. The figure reveals lowering of effective mass at K points in the brillouin zone as we move from compressive to tensile strain regime. It is also seen that, for stacking configuration AAA in all three lattice structures, the simulation reveals a slightly lower carrier effective mass at K point for high compressive strain. However, as we move towards tensile strain regime, the stacking configurations reveal almost similar effective masses at K point. The gradual lowering of carrier effective mass at K point in the Brillouin zone is consistent with the results reported in literature[4]. Fig.3 shows the effect of strain on bandgap of different stacking configurations of multilayer TMDC materials as extracted from simulation. In all three material structures used in this study, we see a gradual lowering of bandgap with strain and therefore a gradual shift from semiconducting to metallic properties which is consistent with the results reported in [5] for bilayer TMDC heterostructures. Of the three structures used in this study, MoS2/WSe2/MoS2 shows the lowest bandgap and therefore the quickest transition to metallic state under tesile strain as seen from fig. 3(c). As observed from our simulation results, at unstrained condition, MoS2/WSe2/MoS2 trilayer showed a direct bandgap for all three stacking configurations. On the other hand, unstrained MoS2/MoSe2/MoS2 and MoS2/WS2/MoS2 trilayers showed indirect bandgap for all three stackings. The AAA stacking configuration is observed to be showing the lowest bandgap when compressive strain is being applied to the lattice. Some results extracted from our simulation for unstrained lattice configuration is listed below: LatticeStackingLattice Constant(A0) Bandgap, Eg(eV)Effective mass,mK(m0)AAA3.2390.7070.5035MoS2/MoSe2/MoS2 ABA3.240.7290.5129ACA3.2380.7520.5142AAA3.1861.2930.5753MoS2/WS2/MoS2 ABA3.1831.0740.5886ACA3.1861.1470.5877AAA3.2410.50350.5035MoS2/WSe2/MoS2 ABA3.2370.5420.5084ACA3.2430.5620.5097 In this work, we have provided a detailed simulation results on the effects of intercalation on electronic properties of TMDC materials. The findings of this study would be helpful in modeilng electronic devices using these material structures.

Fabrication of high-resolution strain sensors based on wafer-level vacuum packaged MEMS resonators

The paper reports on the fabrication and characterization of high-resolution strain sensors for structural materials based on Silicon On Insulator flexural resonators manufactured by polysilicon Low-Pressure Chemical Vapour Deposition vacuum packaging. The sensors present sensitivity of 164Hz/με and strain resolution limit of 150pε on steel for a measurement time of 315ms, in both tensile and compressive strain regimes. The readout of the sensor is implemented with a transimpedance oscillator circuit implemented on Printed Circuit Board, in which a microcontroller-based reciprocal frequency counter is integrated. The performance of the sensors on steel are investigated for measurement bandwidths from 1.5 to 500Hz and a comparison with conventional metal strain gauges is proposed.

Insights for the melt migration, the volcanic activity and the ultrafast lithosphere delamination related to the Yellowstone plume (Western USA)

The Yellowstone–East Snake River Plain hotspot track has been intensely studied since several decades and is widely considered to result from the interaction of a mantle plume with the North American plate. An integrated conclusive geodynamic interpretation of this extensive data set is however presently still lacking, and our knowledge of the dynamical processes beneath Yellowstone is patchy. It has been argued that the Yellowstone plume has delaminated the lower part of the thick Wyoming cratonic lithosphere. We derive an original dynamic model to quantify delamination processes related to mantle plume–lithosphere interactions. We show that fast (∼300 ka) lithospheric delamination is consistent with the observed timing of formation of successive volcanic centres along the Yellowstone hotspot track and requires (i) a tensile stress regime within the whole lithosphere exceeding its failure threshold, (ii) a purely plastic rheology in the lithosphere when stresses reach this yield limit, (iii) a dense lower part of the 200 km thick Wyoming lithosphere and (iv) a decoupling melt horizon inside the median part of the lithosphere. We demonstrate that all these conditions are verified and that ∼150 km large and ∼100 km thick lithospheric blocks delaminate within 300 ka when the Yellowstone plume ponded below the 200 km thick Wyoming cratonic lithosphere. Furthermore, we take advantage of the available extensive regional geophysical and geological observation data sets to design a numerical 3-D upper-mantle convective model. We propose a map of the ascending convective sheets contouring the Yellowstone plume. The model further evidences the development of a counter-flow within the lower part of the lithosphere centred just above the Yellowstone mantle plume axis. This counter-flow controls the local lithospheric stress field, and as a result the trajectories of feeder dykes linking the partial melting source within the core of the mantle plume with the crust by crosscutting the lithospheric mantle. This counter-flow further explains the 50 km NE shift observed between the mantle plume axis and the present-day Yellowstone Caldera as well as the peculiar shaped crustal magma chambers.

Influences of grinding with Toric CBN grinding tools on surface and subsurface of 1.3344 PM steel

Residual stresses have an important role concerning service life of forming tools. The grinding of the tool is one of the major manufacturing steps inducing residual stresses into the subsurface of the tool. Tool and process parameters for grinding with toric grinding pins have been investigated for their influences on residual stresses and surface finish. Residual stresses were measured using the sin2ψ method, while the surface finish was analyzed using white light confocal microscopy. Results show that the grinding strategy has a major influence on residual stress generation for both principle directions of the process. In addition cutting grain size has the major impact on residual stresses transverse to cutting direction, while feed rate has the main influence on residual stresses in cutting direction. A bigger grain size results in more compressive residual stresses, while a higher feed rate shifts stresses towards the tensile regime. A good surface finish is achieved with small cutting grain size, low feed rates and frontal grinding strategy.

Comparison of continuous and discontinuous constitutive models to simulate concrete behaviour under mixed‐mode failure conditions

SummaryThe paper presents detailed FE simulation results of concrete elements under mixed‐mode failure conditions according to the so‐called shear‐tension test by Nooru‐Mohamed, characterized by curved cracks. A continuous and discontinuous numerical two‐dimensional approach was used. In order to describe the concrete's behaviour within continuum mechanics, two different constitutive models were used. First, an elasto‐plastic model with isotropic hardening and softening was assumed. In a compression regime, a Drucker–Prager criterion with a non‐associated flow rule was used. In turn, in a tensile regime, a Rankine criterion with an associated flow rule was adopted. Second, an isotropic damage constitutive model was applied with a single scalar damage parameter and different definitions of the equivalent strain. Both constitutive laws were enriched by a characteristic length of micro‐structure to capture properly strain localization. As an alternative approach, the extended finite element method was used. Our results were compared with the experimental ones and with results of other FE simulations reported in the literature. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Pore orientation mediated control of mechanical behavior of scaffolds and its application in cartilage-mimetic scaffold design

Scaffolds with aligned pores are being explored in musculoskeletal tissue engineering due to their inherent structural anisotropy. However, influence of their structure on mechanical behavior remains poorly understood. In this work, we elucidate this dependence using chitosan–gelatin based random and aligned scaffolds. For this, scaffolds with horizontally or vertically aligned pores were fabricated using unidirectional freezing technique. Random, horizontal and vertical scaffolds were characterized for their mechanical behavior under compressive, tensile and shear loading regimes. The results revealed conserved trends in compressive, tensile and shear moduli, with horizontal scaffolds showing the least moduli, vertical showing the highest and random showing intermediate. Further, these scaffolds demonstrated a highly viscoelastic behavior under cyclic compressive loading, with a pore orientation dependent relative energy dissipation. These results established that mechanical behavior of porous scaffolds can be modulated by varying pore orientation alone. This finding paved the way to recreate the structural and consequent mechanical anisotropy of articular cartilage tissue using zonally varied pore orientation in scaffolds. To this end, monolithic multizonal scaffolds were fabricated using a novel sequential unidirectional freezing technique. The superficial zone of this scaffold had horizontally aligned pores while the deep zone consisted of vertically aligned pores, with a transition zone between the two having randomly oriented pores. This depth-dependent pore architecture closely mimicked the collagen alignment of native articular cartilage which translated into similar depth-dependent mechanical anisotropy as well. A facile fabrication technique, biomimetic pore architecture and associated mechanical anisotropy make this multizonal scaffold a promising candidate for cartilage tissue engineering.

Polyamorphism in tetrahedral substances: Similarities between silicon and ice.

Tetrahedral substances, such as silicon, water, germanium, and silica, share various unusual phase behaviors. Among them, the so-called polyamorphism, i.e., the existence of more than one amorphous form, has been intensively investigated in the last three decades. In this work, we study the metastable relations between amorphous states of silicon in a wide range of pressures, using Monte Carlo simulations. Our results indicate that the two amorphous forms of silicon at high pressures, the high density amorphous (HDA) and the very high density amorphous (VHDA), can be decompressed from high pressure (∼20 GPa) down to the tensile regime, where both convert into the same low density amorphous. Such behavior is also observed in ice. While at high pressure (∼20 GPa), HDA is less stable than VHDA, at the pressure of 10 GPa both forms exhibit similar stability. On the other hand, at much lower pressure (∼5 GPa), HDA and VHDA are no longer the most stable forms, and, upon isobaric annealing, an even less dense form of amorphous silicon emerges, the expanded high density amorphous, again in close similarity to what occurs in ice.

Dynamic mechanical response of polyetheretherketone (PEEK) exposed to cyclic loads in the high stress tensile regime

Tensile fatigue tests of PEEK at high load levels were carried out and analyzed regarding the dynamic mechanic behavior of the material during the tests. The experiments were conducted in the high stress tensile regime of PEEK. After a short period of unsteady temperature increase distinct material changes were observed during the tests at medium and high load levels. The storage modulus increased continuously while the loss modulus, loss factor (tanδ) and the dissipation energy rate decreased continuously. It was concluded that the accumulation of residual stresses because of partially irreversible deformation causes this effect.

Fatigue behavior of rolled and forged tungsten at 25°, 280° and 480 °C

Pure tungsten has been chosen as the target material at the European Spallation Source facility in Lund. Calculations show that the target temperature can reach 500 °C momentarily during the spallation process, leading to thermal fatigue. Target life estimations require fatigue data at different temperatures and this work focuses on generating such data for pure, unirradiated, rolled and forged tungsten in the range 25°–480 °C.For specimens oriented in the rolling direction, tensile tests at room temperature indicated Young's modulus values in the range 320–390 GPa, low levels of plasticity (<0.23%) and UTS values in the range 397 MPa (unpolished) and 705 MPa (Polished). UTS for forged specimens were around 500 MPa.Stress-controlled fatigue tests were conducted in the tensile regime, with a runout limit of 2 × 106 cycles. At 25 °C, unpolished specimens had fatigue limits of 150 MPa (rolling and transverse direction), and 175 MPa (forged). For polished specimens in the rolling direction, fatigue limits were higher at 237.5 MPa (25 °C) and 252.5 MPa (280 °C). The forged specimens showed slightly better fatigue properties and marginal cyclic hardening at 480 °C.

On modeling of discrete propagation of localized damage in cohesive‐frictional materials

SummaryIn this paper, the problem of propagation of localized deformation associated with formation of macrocracks/shear bands is studied in both tensile and compressive regimes. The main focus here is on enhancement of the constitutive law with embedded discontinuity to provide a discrete representation of the localization phenomenon. This has been accomplished by revising the formulation and coupling it with the level‐set method for tracing the propagation path. Extensive numerical studies are conducted involving various fracture modes, ranging from brittle to frictional, and the results are compared with the experimental data as well as those obtained using XFEM methodology. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Capturing the strain hardening and softening responses of cementitious composites subjected to impact loading

Ultra-high performance fiber reinforced concrete (UHP-FRC) is a type of cementitious composite that has extended hardening and softening responses when subjected to tension. The length of the tensile loading regime complicates the development of test setups that can capture the full tensile response at high strain rates. To address this challenge, analytical and finite element modeling are used to propose modifications to an existing test set up to enable it to conduct accurate and practical testing of UHP-FRC specimens in direct tension, under high strain rate. The test device employs suddenly released strain energy to generate an impact pulse and a sufficiently long transmitter bar to channel the signal and measure it. Tests conducted on UHP-FRC specimens at strain rates of 90–1451/s show that, under increasing strain rates, the material maintains its strain capacity and has highly enhanced strain dissipation capacity, making it particularly well suited for blast and impact applications.