Accumulation Of Plastic Deformation Research Articles

Experimental and numerical investigations of accumulated plastic deformation in cement sheath during multistage fracturing in shale gas wells

Sustained Casing Pressure (SCP) has become a serious problem in the development of shale gas fields, which is believed to relate to the multistage fracturing. However, few researches were on this, especially the study of micro-annulus at the interface between the cement sheath and casing, which was one of the mechanisms of the cement sheath integrity failure, and always caused by accumulated plastic deformation. This paper presented experimental and numerical investigations of the emergence and development of plastic deformation occurred at the interface between the cement sheath and casing during multistage fracturing. Mechanical tests and physical simulation experiments were carried out so as to analyze the plastic deformation variation characteristic of the cement sheath under cyclical loading and unloading. 3-D numerical models were established to analyze the emergence and development of the accumulated plastic deformation based on the test results, considering the actual engineering and geological conditions during multistage fracturing in shale gas wells, which presented a new method to calculate the width of the micro-annulus between the cement sheath and casing. The research showed that the numerical results were in accordance with the experimental results. With the increase of cycle times, the accumulated plastic deformation increased, which was the greatest for the first time, and then increased linearly. Decreasing elastic modulus and increasing Poisson's ratio, cohesive force and internal friction angle could facilitate the protection of the cement sheath integrity. Finally, a new cement slurry system was proposed based on the results of study and applied in 5 wells with good effects acquired, none of them showed SCP after fracturing.

“Ʌ”-shaped trend of stress relaxation stability of Inconel718 superalloy with initial stress increasing

Stress relaxation tests were carried out to identify stress relaxation behavior of Inconel718 superalloy under different initial stresses. Experimental results indicate that: increasing initial stress from 260 MPa to 1160 MPa, stress relaxation stability of Inconel718 superalloy rises firstly and then falls. Above all, thermodynamic models of stress relaxation under different initial stresses are constructed, and the release of elastic strain energy density (▵W) and release ratio of elastic strain energy density (η) are identified to quantify thermodynamic characteristics of stress relaxation. The trends of ▵W and η demonstrate that plastic deformation decreases firstly and then increases with initial stress increasing. Besides, constitutive equation and relevant parameters are used to describe kinetic characteristics of stress relaxation behavior. Plastic strain rate increases continuously, but the tendency of stress exponent suggests that work hardening strengthens in the beginning and then weakens remarkably. Due to the coupling effect between the two factors, the accumulation of plastic deformation drops firstly and then rises significantly. Finally, based on microstructure evolution, it can be found that, at low initial stress, stress relaxation consists of two stages, and dislocation movement and grain boundary migration are dominant mechanisms of stress relaxation. While at high initial stress, stress relaxation can be divided into three stages. Twinning is the main mechanism in the first stage, leading to obvious plastic deformation. When it comes to the second stage, dislocation movement and grain boundary migration are dominant reasons. Eventually, stress relaxation achieves the third stage, referred to as the steady state.

New Asphalt Concrete Rutting Resistance Evaluation Method Based on Repeated-Load Test

AbstractRutting of asphalt pavement is the accumulation of plastic deformations and is one of its major distresses and concerns. To date, rutting of asphalt pavement cannot be accurately predicted ...

Comparative study on durability of different composite modified asphalt mixtures

The durability of asphalt mixture is one of the main indicators of the mechanical performance of asphalt pavement. Three kinds of composite modified asphalts (4%SBS/3%SBR, 4%SBS/15% rubber powder and 4%SBR/15% rubber powder) were chosen as binders. Three kinds of aggregate gradation (AC-13, AC-16, AC-20) were used to design composite modified asphalt mixture. The durability (ageing, accumulation and fatigue) of composite modified asphalt mixture was compared and analysed. The study includes comparative study on mechanical performance of composite modified asphalt with different ageing time (0, 5 and 20 h), comparative study on plastic deformation accumulation of composite modified asphalt mixture with different loading times and comparative study on fatigue characteristics of different composite modified asphalt mixtures based on strain. The findings have some theoretical and practical values for increasing the durability of asphalt pavement.

Experimental and Theoretical Investigation on Bistable Symmetric Shells Built by Locally Nanostructuring Isotropic Rectangular Plates

A new bistable shell with two symmetric configurations is proposed by using the nanotechnology, surface mechanical attrition treatment (SMAT), to locally treat a rectangular region. The impacts from randomly fast moving balls during the process induce nanotwins and mesh material grains into nanoscale on originally flat plates, which largely increase the material’s yield strength and elastic deformation capacity. Also, the plastic deformations accumulated from thousands of impacts may stretch the plate under the constraint from the untreated region, while inducing internal compressive forces in the processed region. The experiments show that, when the accumulated plastic deformations are large enough, the locally nanostructured plate may buckle transversely by the internal forces to hold two different stable configurations, resulting in the bistable feature. An analytical model is developed to predict the stable configurations, which is numerically verified and experimentally validated. The parameters, including the SMAT region, plate dimensions, and SMAT process, to design the stable configurations of the bistable shells are systematically studied experimentally, analytically, and numerically.

Effect of infill walls on cumulative plastic deformation energy of RC frame structures

PurposeA large number of earthquake damages showed that infill walls have obvious influence on the seismic damage performance of RC frame structures. The purpose of this paper is to study the effect of infill walls on the cumulative plastic deformation energy of RC frame structures, for which four RC frame structures are build and the time-history response analysis under unidirectional seismic action is presented.Design/methodology/approachThe time-history response analysis under unidirectional seismic action is presented. Then the effect of periodic reduction coefficient on the cumulative plastic deformation energy of the structures, the beams and the columns is investigated.FindingsFinally, the quantitative calculation formulas are provided. The results show that the periodic reduction coefficient has an obvious effect on the distribution of the accumulated plastic deformation energy, and the influence rules are presented here.Originality/valueThe effect of infill walls on the cumulative plastic deformation energy of RC frame structures is quantitatively analyzed here.

Atomic-scale viscoplasticity mechanisms revealed in high ductility metallic glass films

The fundamental plasticity mechanisms in thin freestanding Zr65Ni35 metallic glass films are investigated in order to unravel the origin of an outstanding strength/ductility balance. The deformation process is homogenous until fracture with no evidence of catastrophic shear banding. The creep/relaxation behaviour of the films was characterized by on-chip tensile testing, revealing an activation volume in the range 100–200 Å3. Advanced high-resolution transmission electron microscopy imaging and spectroscopy exhibit a very fine glassy nanostructure with well-defined dense Ni-rich clusters embedded in Zr-rich clusters of lower atomic density and a ~2–3 nm characteristic length scale. Nanobeam electron diffraction analysis reveals that the accumulation of plastic deformation at room-temperature correlates with monotonously increasing disruption of the local atomic order. These results provide experimental evidences of the dynamics of shear transformation zones activation in metallic glasses. The impact of the nanoscale structural heterogeneities on the mechanical properties including the rate dependent behaviour is discussed, shedding new light on the governing plasticity mechanisms in metallic glasses with initially heterogeneous atomic arrangement.

Numerical investigation of the deformation properties of rock materials subjected to cyclic compression by the finite element method

Rock materials, which are involved in a myriad of engineering projects, are often subjected to cyclic loading, such as earthquakes. Understanding the dynamic deformation properties of rock materials under cyclic loading is necessary for evaluating the stability of rock engineering structures. This study numerically investigated the influence of cyclic loading conditions (e.g., waveform, frequency, maximum loading stress and amplitude) on the deformation properties of rock samples using the finite element method (FEM). On the basis of the Drucker–Prager (D-P) yield criterion and sub-loading surface theory, a dynamic constitutive model for reproducing the hysteresis loops and the accumulative plastic deformation of rock materials subjected to cyclic loading was established, numerically implemented using FEM, and validated through comparison with experimental results on basalt and granite model materials. Then, cyclic uniaxial loading tests were simulated with a range of loading parameters, and the deformation properties of rock samples were analyzed. The results indicated that the hysteresis loops and accumulated plastic deformation of rock materials during the cyclic process can be numerically reproduced in an effective manner. The loading parameters significantly affected dynamic deformation properties such as the maximum strain, irreversible strain and hysteresis loop. The simulation results were almost consistent with the experimental results in the published literature. Therefore, through the proposed dynamic constitutive model, FEM can be used to numerically simulate the influence of cyclic loading conditions on the deformation properties of rock materials.

Mechanical properties of polymeric microfiltration membranes

The influence of the pore topology and polymer properties on mechanical characteristics of asymmetric polyethersulfone (PES) and symmetric polyvinylidene fluoride (PVDF) microfiltration membranes was investigated by conducting elongation, creep, stress relaxation, small-amplitude oscillatory and bubble point pressure tests. The main aspects of the membrane stress-strain curves were found to be similar despite significant differences in the pore topology and polymer properties. While the Kelvin-Voigt model for solid polymers described the membrane viscoelastic response below the transition to ductile yielding, the stress-strain curves of membranes and solid polymers above the yield point appeared to be drastically different. All tested membranes demonstrated weak strain hardening, low sensitivity to strain rate, significant elastic recovery, stress relaxation and reduction of the bubble point pressure with accumulation of plastic deformation. Therefore, tensile stresses exerted on a membrane under assembling and process conditions should be smaller than the yield stress to assure that they will not impair filter performance. The novelty of our approach is the use of models for perforated plates to evaluate membrane mechanical properties as ductile yielding for both proceeds via localized plastic deformation around pores. Presented results provide a reliable framework for development of membranes with properties tailored to applications.

Modelling the cyclic elasto-viscoplastic behaviour of polymers

The mechanical behaviour of polymeric materials presents some peculiarities, especially regarding the evolution of their hardening under stresses above the proportional limit. For many types of polymers (Epoxy, Polyamides, Polyvinylidene, etc.) the elastic properties are not rate-dependent, and the behaviour is elasto-viscoplastic. Also, in the case of load-unload tensile tests, preliminary studies show that the kinematic hardening of this kind of material can be much more significant than the isotropic hardening, and a negative plastic strain rate may occur even with a positive stress. Thereby, nonlinear phenomena may occur during tractive load-unload cyclic tests executed above the proportionality limit, leading to dissipation of mechanical energy. The present paper aims at the development of a general elasto-viscoplastic model for polymers, able to describe the progressive accumulation of plastic deformation for an arbitrary loading history. This model considers dissipative phenomena by the coupled effect of the internal variables with nonlinear hardening equations. Since a volume variation can be observed during plastic deformation, a modified von Mises yield criterion that accounts for the influence of the hydrostatic pressure is proposed. All parameters arisen from theory can be identified experimentally from tensile tests with different strain rates and load-unload tensile tests. The model adequacy was tested for particular types of Polyvinylidene fluoride (PVDF) and of Polyamide (PA 12), which are materials commonly used as an internal pressure sheath in offshore flexible pipes in the Brazilian pre-salt basin. One-dimensional numerical simulations were performed aiming to compare the numerical results with the experimental ones, showing a very good agreement.

Heterogeneous strain responses of as-sintered cemented carbide

The heterogeneous interactions between the pre-existing thermal residual stress and external compression in the WC-Co cemented carbide are investigated, where the microstructural characteristics are taken into account simultaneously. A new method for creating three-dimensional finite element model based on real microstructure of the cemented carbides was proposed. The deformation behavior of the as-sintered cemented carbide was quantified and demonstrated in detail. The results indicate that among the heterogeneous distribution of strain responses in the dual-phase microstructure during compressive loading, the layer-like metal binder distributing in the transverse cross-section with respect to the direction of compression has the earliest strain response. The microcracks may preferentially nucleate at these regions due to the largest accumulation of plastic deformation. It is suggested that the activation sequence of dislocation slip systems in the metal binder can be changed by the anisotropic distribution of stress.

Shear stress relaxation behavior of rock discontinuities with different joint roughness coefficient and stress histories

Stress relaxation is an important time-dependent behavior of rock mass. To investigate the stress relaxation behavior and the influence of stress history and surface morphology on the stress relaxation of rock discontinuity, iso-stress cyclic relaxation tests and shear stress relaxation tests with loading–unloading stress histories were conducted on artificial rock discontinuity samples and natural rock discontinuity samples. The characteristics of the stress relaxation curve with different stress histories were investigated in detail, and the relationship between the accumulated plastic deformation and relaxation stress was established. Results revealed that the stress history caused the weakening of the stress relaxation ability. The greater the joint roughness coefficient (JRC), the more noticeable is the stress relaxation. Moreover, the stress history presents a greater effect on stress relaxation as the JRC increases. The stress relaxation exhibits a linear relationship with the plastic deformation that occurred before the stress relaxation. It demonstrates that the relaxation stress and plastic deformation are homogenous and that the elastic energy is the power of stress relaxation. The roughness or asperities of rock discontinuity provides the “space” for the stress relaxation.

Viscoplastic constitutive modelling of the ratchetting behavior of 35CrMo steel under cyclic uniaxial tensile loading with a wide range of stress amplitude

The rate-dependent ratchetting characteristics of 35CrMo alloy under repeated uniaxial tension considering different stress amplitudes were studied at 500 °C. It is found that the applied stress rate and stress amplitude influence the accumulated plastic deformation greatly. The existing visco-plastic ratchetting constitutive model can well predict those features considering the effect of stress rate, but not so well when the stress amplitude effect is involved, especially for the cases of relatively low stress amplitude. Accordingly, a modified rate-dependent ratchetting constitutive model has been developed, which can well predict the ratchetting deformations in a wide range of stress amplitude and stress rate. Furthermore, the modified rate-dependent ratchetting constitutive model can be used to assess the ratchet boundary of the alloy with relatively high accuracy.

Transitions of nanostructure-induced bistable disks actuated by line forces

The transition processes of nanostructure-induced bistable disks triggered by line forces are analytically and experimentally investigated. An analytical model based on the Ritz method is formulated to optimise the placement of axisymmetric line forces for converting the bistable disks between the two stable configurations. Different transition paths are captured for the bistable disks upon applying the line forces at different locations. The transition features, including the snapping forces and transition types, of the bistable disks under different line forces are analysed with respect to the design parameters, such as the nanostructured region ratio, accumulated plastic deformation and disk slenderness. Strategies for placing the line forces to achieve the shape transitions using small line forces or small total forces are proposed. Transition tests of manufactured bistable disks reveal a local buckling phenomenon when the line forces are placed around the disk centre, which is also predicted by numerical modelling. The experimental investigation validates the analytical studies, which are also verified by numerical models. This study provides information for the design of mechanical actuations to achieve the shape reconfiguration of bistable disks.

Prediction of plastic deformation and wear in railway crossings – Comparing the performance of two rail steel grades

Railway crossings are subjected to a severe load environment leading to damage and degradation of rail profiles. The damage is the result of the high magnitudes of contact pressure and slip generated in the wheel–rail contacts during each wheel transition between wing rail and crossing nose. In this paper, numerical predictions of the long-term accumulation of plastic deformation and wear are presented and compared for two rail grades used in crossings. The comparison is performed using a framework that includes simulations of dynamic vehicle–track interaction, wheel–rail contact and rail damage. A load sequence generated by means of Latin hypercube sampling, taking into account variations in worn wheel profile, vehicle speed and wheel–rail friction coefficient, is considered. A Hertzian-based metamodel for wheel–rail normal contact accounting for inelastic material response is used for the simulation of wheel–rail contact. The methodology is demonstrated by calculating the plastic deformation for one cross-section of the crossing nose using an Ohno-Wang cyclic plasticity model, while wear is predicted by means of the Archard model for sliding wear. For the studied conditions and after the simulation of 41400 load cycles, representing an accumulated traffic load of 0.8 MGT, it is concluded that the fine-pearlitic rail grade R350HT experiences half of the ratchetting strain compared to the austenitic hot-rolled manganese steel Mn13. Based on a wear model calibrated for Mn13, the maximum wear of the studied cross-section is about 2% of the maximum plastic deformation.

Undrained dynamic behaviour of peaty organic soil under long-term cyclic loading, Part II: Constitutive model and simulation

Based on the experimental evidences presented in Part I, an empirical equation for the peaty organic soil is derived for calculating the accumulated plastic strain, in which the influencing factors such as the amplitude and number of cyclic loading, the cyclic static deviator stress and the static strength are considered. By regarding the accumulated plastic deformation under long-term cyclic loading as static creep, a constitutive model is presented. The modified Cam Clay model is incorporated in the proposed model to determine the initial stress condition which is necessary for subsequent calculation of accumulated plastic strain. Then, the proposed model is implemented in a finite-element formulation by using an implicit stress integration algorithm. Finally, this model is evaluated and validated through the applications to specific boundary value problems including element tests presented in Part I, a flexible shallow foundation subjected to long-term cyclic load and a road with a low embankment subjected to traffic load. The proposed methodology can be used to calibrate the design of dynamic machine foundation, road and others that involve long-term repetitive loading.

Influence of Alternating Current Frequency on Output Voltage at Electromagnetic Method of Metal Control

The equipment of oil refineries and other hazardous production facilities operate under high pressures and temperatures. Such operation conditions require continuous control and equipment remaining operation life period assessment. The existing methods of diagnostics are based on probabilistic remaining life assessment and use data regarding wall thickness variation during the operation process. The present article presents the method of accumulated damage assessment and its approximation to the limiting state, based on electromagnetic processes studying by means of eddy current control method. The main purpose of studies was determination of optimal value of input signal frequency, which could the most informative for determination of regularity of electric signal parameters change depending on the level of accumulated damages. Steel grade 09Г2С samples were used as the subject of studies. The samples were exposed to static tension under constant rate and during the process of samples deformation we measured the value of electric signal under three frequencies: 100 Hz, 10 kHz, and 1 MHz Based on the obtained results we prepared output signal voltage-relative elongation dependencies, which showed that accumulation of plastic deformations in metal leads to reduction of signal amplitude. Particularly interesting was dependence under 1 MHz frequency, under which electromagnetic processes occur in subsurface and surface layers. This dependence was of some regular nature, which was described by means of the sinusoidal function. Graph of the obtained function qualitatively describes the experimental dependence. On the basis of obtained results we can make a conclusion that optimal input signal frequency is within megahertz range, under which difference between the sinusoidal function graph and the empirical curve is minimum.

Elasto-plastic analysis of a multi-stage excavation in bonded geomaterials

The deformation characteristics of braced and unbraced excavations in a dry structured soil are investigated numerically. A constitutive model based on the theory of bounding surface plasticity is used to predict the plastic deformation of the soil. A plastic cementation index is introduced in the hardening modules to capture soil degradation. In the model, the progress of the degradation is associated with the stress magnitude and the accumulated plastic deformation. The constitutive model is implemented in a finite difference code to analyse the lateral deformation of braced and unbraced excavations. The implemented model captures the strength increase of soils due to cementation, and predicts the subsequent debonding and strength degradation due to excavation. It is shown that soil cementation significantly influences the compression and tensile strengths of soils, and alters the deformation profile of the excavated face.

Influence of hard and soft inclusions inside a ferritic matrix

In the case of an interior fracture mode in fatigue, material impurities play a fundamental role in the generation and the propagation of cracks. In general, the presence of hard or soft inclusions, voids, materials imperfections, etc., alter the local stress state in the matrix generating the accumulation of plastic deformations which lead to the crack formation. In terms of inclusions, high strength steels are often characterized by the presence of aluminum oxides Al2O3 or Manganese sulfide MnS. The experimental works reported by several authors pointed out that the critical location for the crack nucleation is, often, at the edge of the inclusion and it subsequently propagates on a plane orthogonal to the loading direction. In a previous work, the authors investigated the stress distribution around a spherical inclusion inside a ferritic matrix, pointing out the role of the material anisotropy and the different crystallographic orientation of the matrix. However, the investigations dealt with simple loading conditions and linear elasticity. The present paper aims to extend the field of investigations to the elasto-plastic domain, focusing the attention on the role of the crystallographic orientation and comparing the results with the previous study.

Achieving ultra-high strength and ductility in equiatomic CrCoNi with partially recrystallized microstructures

Despite having otherwise outstanding mechanical properties, many single-phase medium and high entropy alloys are limited by modest yield strengths. Although grain refinement offers one opportunity for additional strengthening, it requires significant and undesirable compromises to ductility. This work therefore explores an alternative, simple processing route to achieve strength by cold-rolling and annealing an equiatomic CrCoNi alloy to produce heterogeneous, partially recrystallized microstructures. Tensile tests reveal that our approach dramatically increases the yield strength (to ∼1100 MPa) while retaining good ductility (total elongation ∼23%) in the single-phase CrCoNi alloy. Scanning and transmission electron microscopy indicate that the strengthening is due to the non-recrystallized grains retaining their deformation-induced twins and very high dislocation densities. Load-unload-reload tests and grain-scale digital image correlation are also used to study the accumulation of plastic deformation in our highly heterogeneous microstructures.