Axial Stress State Research Articles

Mechanical analysis for deepwater drilling riser system with structural parameters uncertainty

Uncertain structural parameters increase the mechanical failure risk of the drilling riser system. The effect of uncertain structural parameters on the mechanical characteristics of the drilling riser system is unclear. Therefore, the mechanical characteristics of the drilling riser system with structural parameter uncertainty are analyzed. A mechanical model with uncertain structural parameters is established based on the basic analysis model and the local average theory. The basic analysis model takes into account the combined effects of platform motion, tensioner, riser string, and marine environment. The local average theory is suitable for situations where there is a lack of uncertainty information. A solution approach is proposed using the second-order central perturbation method and the Newmark-β method. The solution method reduces the load by simplifying degrees of freedom and random variables. The correctness of the solution method is verified using the Monte Carlo method. The analysis results indicate that uncertain structural parameters significantly affect the static response, while the dynamic response is less affected. The failure risk due to static axial stress and static shear stress is heightened by uncertain structural parameters. The failure risk arising from static shear stress at the bottom of the riser system experiences a significant increase.

Predicting Axial Stress State In Continuously Welded Rail Using Impulse-Generated Vibration Measurements

Continuously welded rails are connected without stress relief joints and, thus, thermally induced rail movement is constrained, which can result in the development of excessive axial stress and risk of rail failure. Nondestructive testing (NDT) methods that estimate in-place rail stress state or rail neutral temperature are desired. Some methods have been developed, but none satisfy the requirements for ideal monitoring in practice. We propose an NDT technique based on impulse-generated vibration, seeking high-frequency rail vibration resonances whose frequency maintains a consistent correlation with rail axial stress/strain across different temperatures, stress states, and rail support conditions. Rail temperature, axial strain, and vibration data were collected from an active Class 1 commercial rail line over a period of nearly two years. The frequencies of four consistent and clear resonance modes of the rail were monitored. One of the identified modes demonstrates a unique linear relation with axial strain across a range of temperatures and stress states at each of the two measurement locations. The developed linear relations were used to predict in-place strain and rail neutral temperature with acceptable accuracy across all the measurement data, although each test location exhibits a unique relation.

Static axial tension or compression stress effects on shear cracks initiating under cyclic torsion loads in multiaxial fatigue tests

This work proposes models that can better fit and explain apparently contradictory mean stress effects observed when superimposing axial tension or compression loads on cyclic torsion fatigue tests. It introduces new shear-based multiaxial fatigue damage models, modifying the classic Fatemi-Socie equation to consider the peak deviatoric stress normal to the initiating shear crack as the actual driving force for the peak or mean effects of the normal stresses. This deviatoric hypothesis can quantify the odd detrimental effects of static axial compression observed on axial shear cracks tested under cyclic torsion. Additionally, applying the peak deviatoric correction term only to the elastic component of the shear strain range allows an even better description of the increasing mean normal stress effects in high-cycle fatigue, eliminating the need for a variable normal stress sensitivity coefficient in the multiaxial fatigue damage parameter. To evaluate the proposed shear-based models, their life and crack initiation direction predictions are compared for cyclic torsion experiments both with and without superimposed static axial stresses for steels and aluminum alloys, including both literature and original data. The experiments show that axial compression can be detrimental to fatigue crack initiation lives, depending on the cracking mode and on the presence of bifurcations, which effects could be explained from the proposed deviatoric damage parameters.

Stress prediction of heated crude oil pipeline in permafrost region via fully coupled heat-moisture-stress numerical simulation and SVM algorithm

Large diameter crude oil transmission pipelines in unstable permafrost areas have been put into operation in China in recent dozen years. The perennial positive temperature operation of buried heating transmission pipelines leads to freeze–thaw of the soil around pipelines, and the axial stresses generated by thawing settlement of permafrost may induce fracture failure of weld crack. However, there are many shortcomings in existing frozen soil thawing settlement analysis and pipeline stress assessment methods, which makes it difficult to accurately analyze and predict the axial additional stress of pipelines induced by thawing settlement. Based on heat conduction and permafrost thawing consolidation settlement theory, numerical inversion analysis of centrifuge test was conducted to verify the accuracy of the finite element method. The key influencing factors such as wall thickness, insulation layer thickness, buried depth of pipe, and length of thawing settlement zone on the axial stress and strain state of buried pipeline under the condition of double pipe operation of Mo-da line were investigated. The results show that: Pipeline with an insulation layer, larger wall thickness and smaller buried depth is beneficial to improve the bearing capacity of buried pipeline under thawing settlement of permafrost. The axial stress varies nonlinearly with the increase of the thawing settlement zone length. Based on support vector regression (SVR) model, the axial stress prediction model of pipe crown and invert at different positions in the thaw settlement area was proposed and verified by field monitoring data of buried pipeline. The relative error between the prediction results and the field monitoring results is less than 15 %. This study can provide some theoretical support for the structural design of pipelines in thaw settlement area.

Hydraulic and volume change behaviors of compacted highly expansive soil under cyclic wetting and drying

The wide engineered application of compacted expansive soils necessitates understanding their behavior under field conditions. The results of this study demonstrate how seasonal climatic variation and stress and boundary conditions individually or collectively influence the hydraulic and volume change behavior of compacted highly expansive soils. The cyclic wetting and drying (CWD) process was applied for two boundary conditions, i.e. constant stress (CS) and constant volume (CV), and for a wide range of axial stress states. The adopted CWD process affected the hydraulic and volume change behaviors of expansive soils, with the first cycle of wetting and drying being the most effective. The CWD process under CS conditions resulted in shrinkage accumulation and reduction in saturated hydraulic conductivity ( k sat ). On the other hand, CWD under CV conditions caused a reduction of swell pressure while has almost no impact on k sat . An elastic response to CWD was achieved after the third cycle for saturated hydraulic conductivity ( k sat ), the third to fourth cycle for the volume change potential under the CV conditions, and the fourth to fifth cycle for the volume change potential under the CS conditions. Finally, both swell pressure ( σ s ) and saturated hydraulic conductivity ( k sat ) are not fundamental parameters of the expansive soil but rather depend on stress, boundary and wetting conditions.

Monolithically integrated triaxial high-performance micro accelerometers with position-independent pure axial stressed piezoresistive beams

With the increasing demand for multidirectional vibration measurements, traditional triaxial accelerometers cannot achieve vibration measurements with high sensitivity, high natural frequency, and low cross-sensitivity simultaneously. Moreover, for piezoresistive accelerometers, achieving pure axial deformation of the piezoresistive beam can greatly improve performance, but it requires the piezoresistive beam to be located in a specific position, which inevitably makes the design more complex and limits the performance improvement. Here, a monolithically integrated triaxial high-performance accelerometer with pure axial stress piezoresistive beams was designed, fabricated, and tested. By controlling synchronous displacements at both piezoresistive beam ends, the pure axial stress states of the piezoresistive beams could be easily achieved with position independence without tedious calculations. The measurement unit for the z-axis acceleration was innovatively designed as an interlocking proof mass structure to ensure a full Wheatstone bridge for sensitivity improvement. The pure axial stress state of the piezoresistive beams and low cross-sensitivity of all three units were verified by the finite element method (FEM). The triaxial accelerometer was fabricated and tested. Results showing extremely high sensitivities (x axis: 2.43 mV/g/5 V; y axis: 2.44 mv/g/5 V; z axis: 2.41 mV/g/5 V (without amplification by signal conditioning circuit)) and high natural frequencies (x/y axes: 11.4 kHz; z-axis: 13.2 kHz) were obtained. The approach of this paper makes it simple to design and obtain high-performance piezoresistive accelerometers.

Influence of the punch shape on the core and shell structure of press-coated tablets

Press-coated tablets are a key technology to achieve delayed releases in chronotherapeutics. The drug release properties of this kind of tablets are linked to its unique core–shell structure. It is thus important to understand the influence of the process parameters on this structure. As different shapes can be used in the industry, we focused, in this study, on understanding the influence of punch shape on the final structure of a press-coated tablet. Experiments were performed using flat, bevel-edged and concave punches for the coating-compression to study the effect of the punch shape on the final properties of the core but also on the density distribution in the shell. The experiments were supported by numerical simulation to understand the mechanical effects in the powder compression process. It was found that the radial and axial stress state in the shell and in the core during compression is very dependent on the punch shape. The use of concave punches results in a more hydrostatic stress state compared to flat punches. The consequences on the structure are a more homogenous shell and less deformation of the core, which confirms that the tooling shape is a critical parameter to consider for the production of press-coated tablets.

Post-fatigue properties of high-strength concrete subjected to coupled 3D fatigue-static loading

Concrete structures suffer some damage yet not failed under fatigue load, and then continue to bear 3D redistributed stress. For this, the 3D fatigue loading considering various fatigue factors (e.g. confining stress, axial static stress (ASS) and force amplitude (FA), frequency, and cycle number) is first carried out, and then the 3D static loading is performed. The post-fatigue characteristics of high-strength concrete (e.g. P-wave velocity, S-wave velocity, porosity, gas permeability, triaxial compression strength, and elastic modulus) are gained. The results indicate that 3D fatigue loading weakens mechanical properties, delays wave propagation, and increases seepage paths. An obvious stress threshold is exhibited with increasing axial static stress and force amplitude, that is 80% triaxial compressive strength, where the physico-mechanical characteristics are rapidly weakened due to the energy dissipation caused by crack growth rises increasingly. Compared with 1D fatigue loading, the frequency turning point of weakening effect from decreasing to increasing is advanced under 3D fatigue loading due to the application of 3D stress exacerbates the heat accumulation and creep damage generation. Interestingly, the fatigue damage is likely to be more sensitive to axial load compared to confining stress during 3D fatigue loading. In other words, the promotion effect of axial fatigue load on damage is larger than the restriction of confining stress. Furthermore, the fatigue damage models considering various fatigue factors are proposed. Then, the empirical prediction models of this damage variable to mechanical parameters (strength and elastic modulus) and permeability are established to predict the capacity loss caused by fatigue loading in the design of concrete construction. The testing results in this context could facilitate our understanding of post-fatigue characteristics of high-strength concrete subjected to 3D fatigue loading and guide the safe design of concrete construction.

Dog-bone Samples may not Provide Direct Access to the Longitudinal Tensile Strength of Clear-wood

Background: Testing standards prescribe dog-bone samples for the determination of clear-wood longitudinal tensile strength. However, the literature reports a high number of invalid tests due to the unexpected failure of the sample outside the gauge length. Motivation: The paper aims at understanding the reason for the premature failure of dog-bone samples and suggesting possible strategies for improving testing protocols. Methods: The paper starts with a comparative review of standards for different orthotropic materials. Thereafter, it analyzes the stress distribution in a clear-wood dog-bone sample using a recently proposed stress-recovery procedure and Finite Elements. Finally, the sample failure is considered applying Tsai-Wu and SIA criteria. Results: Comparative review highlights the controversy on the choice of the sample geometry. Both analytical and numerical results confirm the presence of shear and transversal stresses in necking regions, overlapping with axial stress greater (up to 2%) than the one in the gauge region. As a consequence, clear-wood dog-bone samples fail not due to a pure axial stress state in the gauge region (as expected), but due to complex stress state in necking region, where failure index is 4 ~ 5% greater than the one in gauge region. Conclusion: Assuming that dog-bone samples fail in the gauge region due to pure axial stress is simplistic, as demonstrated by analytical and numerical evidence. As a consequence, interpretations of experimental results based on this belief are misleading and testing protocols should be refined. Indeed, the presence of spurious stresses interfering with expected pure axial stress seems unavoidable. Therefore, clear-wood testing standards should allow to use prismatic samples or, alternatively, to consider as valid also tests on samples breaking outside the gauge region. Both the proposed solutions apparently reduce the accuracy of the experiments, while in contrast, they provide the best achievable results, speeding up the testing procedure and reducing the testing costs.

Creep behavior of steel bonded reinforced concrete members under small eccentric compression

In this paper, the creep behavior of steel bonded reinforced concrete members under small eccentric compression is studied. Firstly, based on the existing mechanical characteristics of concrete under multi axial stress state, the stress-strain relationship of steel bonded small eccentric concrete is theoretically deduced and analyzed by considering the reduction coefficient of hoop force. Secondly, according to the creep characteristics of confined core concrete, combined with the existing concrete creep model and the analysis of its influencing factors, the influence of sticking steel on the creep of small eccentric reinforced concrete compression members is discussed. The corresponding creep calculation formula is established. Finally, the existing experimental data are used to verify the proposed formula, and its applicability is analyzed.

Micromechanical study on hard rock strainburst using the discrete element method

Rock strainburst is a geological hazard occurred under high triaxial stress states, posing a great threat to the safety of underground constructions. It is well-known that the in-situ axial stress state of hard rock has significant impacts on its strainburst strength and fracture pattern. However, the effects of axial stress on the strainburst process and micro-cracking behavior ofrock have not been well understood. This paper aims to investigate the strainburst mechanism from the micromechanics perspective using Discrete Element Method (DEM). The microscale parameters of rock are obtained through a calibration process with different laboratory tests of hard rock under both tension and compression loadings. Consequently, a series of true strainburst tests of rock under a wide range of axial stresses are simulated, and numerical results show good agreements with experimental data, which verify the capability of the calibrated model in reproducing the mechanical response of rock strainburst. Based on this, the effects of axial stress on cracking behavior are further analyzed. It is found that the crack density and strainburst strength of specimens tend to increase with growing axial stress, and this trend is shown to be independent of sample size and frictional properties. In addition, the strainburst crack mode exhibits a transition from tensile dominated to tensile-shear mixed when the axial stress increases.

Elasto-Damage Constitutive Modelling of Recycled Aggregate Concrete

Use of recycled aggregates in concrete has proved to be beneficial in attaining sustainable construction without compromising overall material and structural performance when compared with concrete containing natural aggregates. However, use of the recycled aggregates in concrete have resulted in reduction in compressive and tensile strengths with the recycled aggregates percentage increase in concrete. Furthermore, it is important to note that most of the finite element software used in the construction industry use concrete model derived from the test results of the conventional concrete, therefore, they may not always predict safe solution for recycled aggregates concrete (RAC). Therefore, in this investigation elasto-damage, proposed by Khan and Zahra, for natural aggregate concrete (NAC) was modified to incorporate the influence of recycled aggregates on the behaviour of concrete. Model use four parameters α, β, γ critical energy release rate (Rc) to predict the behaviour of recycled aggregate concrete for multi axial stress states. Parameters α, β and γ are used to predict the different behavior of concrete in tension and compression while Rc controls the damage growth rate. These parameters are defined as a function of concrete compressive strength (fc/) and its initial elastic modulus (Eo). Existing test results for uniaxial compressive state of stress were used to validate this model and it was found that it predicts better post cracking and post peak-behaviour of RAC as compared to the commercially available models for conventional concrete

Static and coupled dynamic testing of granite for geological disposal of high-level waste

The Tianhu area of Xinjiang Province has been pre-selected for the geological disposal of high-level radioactive waste. To engineer the design of the disposal site, one must understand the static and dynamic properties of the granite in the borehole. In the present study, rock was sampled at the same depth in the same borehole, and its tensile and uniaxial-compression strengths were evaluated. Both strengths were steady and representative. The dynamic tensile and dynamic compression strengths increased with loading rate, confirming the loading rate effect on the brittle material. A coupled static-and-dynamic tension test was then carried out. Under increasing axial static stress, the dynamic tensile strength first increased and then decreased in a stable manner. The dynamic tensile strength was maximized when the static axial pre-pressure reached approximately 50% of the static-tensile strength. Meanwhile, the coupled static-and-dynamic strength increased under increasing axial static pressure, reaching approximately three times the static tension strength, and 1.5 times the dynamic tension strength. The failure model in the dynamic test was mainly tension failure, consistent with the static tension test. Therefore, the rock in this area is mechanically stable and putatively suitable for the geo-disposal of high-level waste (HLW). For a complete assessment, the rock should be tested by other methods such as seepage and in situ stress testing. After a detailed analysis of the rock in this area, the testing results and theoretical knowledge will become available for deep geo-engineering blasting and excavation, and HLW geo-disposal.

Permeability Experiment of Fractured Rock with Rough Surfaces under Different Stress Conditions

To investigate the permeability changes and the mechanisms of fractured rock under dynamic and static stresses produced by earthquakes, permeability experiments on fractured rock with rough surfaces under axial dynamic and static stresses were conducted on the MTS815 Rock Mechanics Testing System. Surface asperity was investigated by scanning the specimen surfaces before and after testing. The results show that the roughness of fracture surface has a great influence on the permeability when the axial displacement is not enough to cause the fracture rock to slip. Moreover, the rougher fracture surface leads to severer surface damage as indicated by the more gouge productions. The accumulation of gouge materials on larger roughness fracture surfaces causes a slow drop in permeability. The fracture surfaces experience larger degradations, but it has small weights of gouge materials on fracture surface after testing under axial dynamic stress. The reason is that the gouge material transport and mobilization tend to occur in process of dynamic loading. Therefore, the permeability drops of axial dynamic stress are larger than those of axial static stress.

Effects of axial static stress on stress wave propagation in rock considering porosity compaction and damage evolution

A wave equation of rock under axial static stress is established using the equivalent medium method by modifying the Kelvin-Voigt model. The analytical formulas of longitudinal velocity, space and time attenuation coefficients and response frequency are obtained by solving the equation using the harmonic method. A series of experiments on stress wave propagation through rock under different axial static stresses have been conducted. The proposed models of stress wave propagation are then verified by comparing experimental results with theoretical solutions. Based on the verified theoretical models, the influences of axial static stress on longitudinal velocity, space and time attenuation coefficients and response frequency are investigated by detailed parametric studies. The results show that the proposed theoretical models can be used to effectively investigate the effects of axial static stress on the stress wave propagation in rock. The axial static stress influences stress wave propagation characteristics of porous rock by varying the level of rock porosity and damage. Moreover, the initial porosity, initial elastic modulus of the rock voids and skeleton, viscous coefficient and vibration frequency have significant effects on the P-wave velocity, attenuation characteristics and response frequency of the stress wave in porous rock under axial static stress.

Physico-mechanical Behaviors of Granite Under Coupled Static and Dynamic Cyclic Loadings

The stress distribution around constructions in deep rock can be modified by dynamic disturbances, such as earthquakes. For this, we proposed a new loading method applied to granite sample under coupled static and dynamic cyclic loading (CSDCL) condition. The variations in P-wave velocity, gas permeability, and mechanical properties of granite before and after CSDCL are revealed. It shows that with increasing axial static stress, the dynamic cyclic loading amplitude and cycle number, P-wave velocity, uniaxial compressive strength (UCS) and elastic modulus decrease, whilst the permeability and Poisson’s ratio increase. It seems that variations in those parameters have a close relation with the exciting frequency. In this case, CSDCL is applied for crack development along the axial direction in the rock samples, and subsequently results in degradation of mechanical properties, delay of P-wave propagation, and increase of the transport paths. The results show that the permeability and Poisson’s ratio are likely to be more sensitive to the CSDCL, in particular under the axial static stress. The empirical relationships of the P-wave velocity with permeability, UCS, and elastic modulus are established for a pragmatic monitoring purpose. From design point of view, we have established the relationships of the disturbance factors to damage variable. Then the correlations of the damage variable to permeability, UCS, and elastic modulus are analyzed. The testing results in this context could facilitate our understanding of rock stability upon excavation subjected to dynamic disturbances.

Static analysis of graphene reinforced composite plate

In this paper static analysis of grapheme reinforced composite plate has been analyzed. A multiquadric radial basis function and meshless method has been analyzed toughly to determine the static axial stresses and in-plane stress over the domain of Graphene-reinforced (GRC) composite plate. Third order shear deformation theory (TSDT) is employed to solve such computational problem. Extended Halpin-Tsai homogenization techniques is used to evaluate the thermo mechanical properties of the composite media. An in-house mat lab code is generated to show the results by validating with some benchmark literatures.

A micro-macro confined compressive fatigue creep failure model in brittle solids

Fatigue properties under cyclic static compressive loading and unloading (i.e., compressive fatigue/cyclic creep properties) have an important meaning for evaluation of lifetime in brittle solids containing numerous microcracks. The total deformation during cyclic static compression consists of time-independent elastic, time-dependent viscoelastic, time-dependent viscoplastic and time-independent plastic deformations. In this study, the viscoplastic and plastic deformations are collectively called as plastic deformation below. The plastic deformation dominates the lifetime of brittle solids. The rebound of elastic and viscoelastic deformations at static unloading is widely studied. However, the rebound of plastic deformation at static unloading is rarely studied under constant lateral confining pressure, and the total time-dependent deformation during cyclic static compressive failure also is rarely studied in theory. Furthermore, the rebound of plastic deformation has a great significance for cyclic static compressive failure, and microcrack variable influences seriously plastic mechanical properties of brittle solids. However, the theoretical relationship between plastic deformation rebound and microcrack variable in brittle solids during a cyclic static compressive failure is rarely established. In this study, an analytical solution is proposed by coupling the confined cyclic static loading and unloading path, the Hooke-Kelvin viscoelastic model and the formulated micro-macro model, which explains the total time-dependent visco-elastic-plastic deformation caused by microcracks variable during cyclic static compressive failure. Effects of residual static axial stress after unloading and confining pressure on cyclic creep failure are studied. Irreversible elastic and viscoelastic deformations caused by residual static stress after unloading are illustrated. An interesting rebound phenomenon of plastic deformation induced by microcrack recovery is found. A critical value of axial stress causing a rebound of plastic deformation is also found. Effects of parameters Δσ1a, Δσ1b, and Δt in cyclic static loading and unloading path on the mechanical properties of cyclic static compressive failure are discussed. The proposed theoretical model provides a certain help for evaluation and prediction of fatigue lifetime in brittle solid engineering.

The role of three-dimensionality and alveolar pressure in the distribution and amplification of alveolar stresses

Alveolar stresses are fundamental to enable the respiration process in mammalians and have recently gained increasing attention due to their mechanobiological role in the pathogenesis and development of respiratory diseases. Despite the fundamental physiological role of stresses in the alveolar wall, the determination of alveolar stresses remains challenging, and our current knowledge is largely drawn from 2D studies that idealize the alveolar septal wall as a spring or a planar continuum. Here we study the 3D stress distribution in alveolar walls of normal lungs by combining ex-vivo micro-computed tomography and 3D finite-element analysis. Our results show that alveolar walls are subject to a fully 3D state of stresses rather than to a pure axial stress state. To understand the contributions of the different components and deformation modes, we decompose the stress tensor field into hydrostatic and deviatoric components, which are associated with isotropic and distortional stresses, respectively. Stress concentrations arise in localized regions of the alveolar microstructure, with magnitudes that can be up to 27 times the applied alveolar pressure. Interestingly, we show that the stress amplification factor strongly depends on the level of alveolar pressure, i.e, stresses do not scale proportional to the applied alveolar pressure. In addition, we show that 2D techniques to assess alveolar stresses consistently overestimate the stress magnitude in alveolar walls, particularly for lungs under high transpulmonary pressure. These findings take particular relevance in the study of stress-induced remodeling of the emphysematous lung and in ventilator-induced lung injury, where the relation between transpulmonary pressure and alveolar wall stress is key to understand mechanotransduction processes in pneumocytes.

Fatigue and cyclic plasticity of 304L stainless steel under axial-torsional loading at room temperature

This work investigates the axial-torsional fatigue and cyclic deformation behaviour of 304L stainless steel at room temperature. The testing programme included four fully reversed strain-controlled loading paths (axial, torsional, proportional axial-torsional, and 90° out-of-phase axial-torsional) and a fully reversed shear strain-controlled with static axial stress. The experiments were carried out at equivalent strain amplitudes ranging from 0.20% to 1.00%, resulting in fatigue lives from 102 to 106 cycles. For axial, torsional, and torsional with axial static stress loading, secondary hardening related to martensitic transformation was observed, while it did not occur for nonproportional loading. For proportional loading, secondary hardening occurred only for an equivalent strain amplitude equal to or greater than 0.80%. Neither the accumulated plastic strain nor the equivalent plastic strain amplitude could predict whether secondary hardening would occur for the investigated loading conditions. Almost no variation in the cyclic Young’s and shear moduli with the number of loading cycles was observed even for the tests that exhibited considerable secondary hardening. Fatigue life estimates calculated using the stress-strain hysteresis loops at maximum softening and at half-life were similar for the Smith–Watson–Topper, Fatemi–Socie, and Jiang models. It was observed that macroscopic surface crack orientation depends on the equivalent strain amplitude for torsional, proportional, nonproportional and torsional with static stress loading. The capability of the models to estimate the observed crack orientations is discussed.