A plastic damage model for concrete structure cracks with two damage variables

Based on the concepts of continuum damage theory,a new plastic damage model for concrete crack failure is developed through studying the basic damage mechanics.Two damage variables,tensile damage variable for tensile damage and shear damage variable for compressive damage,are adopted to represent the influence of microscopic damage on material macromechanics properties under tensile and compressive loadings.The yield criteria and flow rule determining the plasticity of concrete are established in the effective stress space,which is convenient to decouple the damage process from the plastic process and calibrate material parameters with experimental results.Meanwhile,the plastic part of the proposed model can be implemented by back-Euler implicit algorithm,and the damage part is explicit.Consequently,there exist robust algorithms for integrating the constitutive relations using finite element method.Comparison with several experimental results shows that the model is capable of simulating the nonlinear performance of concrete under multiaxial stress state and can be applied to practical concrete structures.

Evaluation of Blast-Resistant Performance Predicted by Damaged Plasticity Model for Concrete

in order to evaluate the capacity of reinforced concrete （RC） structures subjected to blast Ioadings, the damaged plasticity model for concrete was used in the analysis of the dynamic responses of blast-loaded RC structures, and all three failure modes were numerically simulated by the finite element software ABAQUS. Simulation results agree with the experimental observations. It is demonstrated that the damaged plasticity model for concrete in the finite element software ABAQUS can predict dynamic responses and typical flexure, flexure-shear and direct shear failure modes of the blast-loaded RC structures.

Damage Detection in Reinforced Concrete Berthing Jetty Using a Plasticity Model Approach

A conventional method of damage modeling by a reduction in stiffness is insufficient to model the complex non-linear damage characteristics of concrete material accurately.In this research,the concrete damage plasticity constitutive model is used to develop the numerical model of a deck beam on a berthing jetty in the Abaqus finite element package.The model constitutes a solid section of 3D hexahedral brick elements for concrete material embedded with 2D quadrilateral surface elements as reinforcements.The model was validated against experimental results of a beam of comparable dimensions in a cited literature.The validated beam model is then used in a three-point load test configuration to demonstrate its applicability for preliminary numerical evaluation of damage detection strategy in marine concrete structural health monitoring.The natural frequency was identified to detect the presence of damage and mode shape curvature was found sensitive to the location of damage.

Caution to Apply Magnetic Barkhausen Noise Method to Nondestructive Evaluation of Plastic Deformation in Some Ferromagnetic Materials

Magnetic Barkhausen Noise(MBN) method is known as an effective nondestructive evaluation(NDE) method for evaluation of residual stress in ferromagnetic materials. Some studies on the feasibility of the MBN method for NDE of residual strains were also conducted and found applicable. However, these studies are mainly focused on the state of residual strains which were introduced through a one-cycle-loading process. In practice, however, structures may suffer from an unpredictable and complicated loading history, i.e., the final state of plastic strain may be induced by several times of large loads. Whether the loading history has influences on MBN signals or not is of great importance for the practical application of the MBN method. In this paper, several ferromagnetic specimens with the same final state of residual strain but of different loading history were fabricated and inspected by using a MBN testing system. The experimental results reveal that the loading history has a significant influence on the detected MBN signals especially for a residual strain in range less than 1%, which doubts the feasibility to apply the MBN method simply in the practical environment. In addition, micro-observations on the magnetic domain structures of the plastic damaged specimens were also carried out to clarify the influence mechanism of loading history on the MBN signals.

THE DAMAGE STATES AND TENSILE FAILURE BEHAVIOR OF TORSIONAL PRESTRAINED STEELS

In this paper,the damage state of a torsional prestrained steel is examined by means of the concepts of continuum damage mechanics and then the tensile properties and fracture ductility of two kinds of steels under various torsional prestrained conditions are investigated from both macroscopic and microscopic points of very slight as contrasted with tensile damage;(2)after torsional prestraining,both yielding strength and ultimate tensile strength become higher for 20 steel and lower for 40Cr steel;(3)when the torsional prestrain exceeds a critical value,that is about 70% of pure torsional shear fracture strain,the ductile-brittle transition of tensile fracture behavior may initiates.Moreover,the advantages and applicable conditions of torsional prestrain strengthening technique are also discussed.

Damage modeling of ballistic penetration and impact behavior of concrete panel under low and high velocities

This work presents a numerical simulation of ballistic penetration and high velocity impact behavior of plain and reinforced concrete panels.This paper is divided into two parts.The first part consists of numerical modeling of reinforced concrete panel penetrated with a spherical projectile using concrete damage plasticity(CDP)model,while the second part focuses on the comparison of CDP model and Johnson-Holmquist-2(JH-2)damage model and their ability to describe the behavior of concrete panel under impact loads.The first and second concrete panels have dimensions of 1500 mm1500 mm150 mm and 675 mm675 mm200 mm,respectively,and are meshed using 8-node hexahedron solid elements.The impact object used in the first part is a spherical projectile of 150 mm diameter,while in the second part steel projectile of a length of 152 mm is modeled as rigid element.Failure and scabbing characteristics are studied in the first part.In the second part,the comparison results are presented as damage contours,kinetic energy of projectile and internal energy of the concrete.The results revealed a severe fracture of the panel and high kinetic energy of the projectile using CDP model comparing to the JH-2 model.In addition,the internal energy of concrete using CDP model was found to be less comparing to the JH-2 model.

Atomic-Scale Insights into Damage Mechanisms of GGr15 Bearing Steel Under Cyclic Shear Fatigue

Alternating shear stress is a critical factor in the accumulation of damage during rolling contact fatigue,severely limiting the service life of bearings.However,the specific mechanisms responsible for the cyclic shear fatigue damage in bearing steel have not been fully understood.Here the mechanical response and microstructural evolution of a model GGr15 bearing steel under cyclic shear loading are investigated through the implementation of molecular dynamics simulations.The samples undergo 30 cycles under three different loading conditions with strains of 6.2%,9.2%,and 12.2%,respectively.The findings indicate that severe cyclic shear deformation results in early cyclic softening and significant accumulation of plastic damage in the bearing steel.Besides,samples subjected to higher strain-controlled loading exhibit higher plastic strain energy and shorter fatigue life.Additionally,strain localization is identified as the predominant damage mechanism in cyclic shear fatigue of the bearing steel,which accumulates and ultimately results in fatigue failure.Furthermore,simulation results also revealed the microstructural reasons for the strain localization(e.g.,BCC phase transformation into FCC and HCP phase),which well explained the formation of white etching areas.This study provides fresh atomic-scale insights into the mechanisms of cyclic shear fatigue damage in bearing steels.

Stresses and Shear Fracture Zone of Jinshazhou Tunnel Surrounding Rock in Rich Water Region

Field evidence has shown that large-scale and unstable discontinuous planes in the rock mass surrounding tunnels in rich water region are probably generated after excavation. The tunnel surrounding rock was divided into three zones, including elastic zone, plastic damage zone and shear fracture zone for assessing the stability of the tunnel surrounding rock. By local hydrogeology, the stresses of surrounding rock of Jinshazhou circular tunnel was analyzed and the stress solutions on the elastic and plastic damage zones were obtained by applying the theories of fluid-solid coupling and elasto-plastic damage mechanics. The shear fracture zone generated by joints was studied and its range was determined by using Molar-Coulomb strength criterion. Finally, the correctness of the theoretical results was validated by comparing the scopes of shear fracture zones calculated in this paper with those from literature.

A new method to predict fatigue crack growth rate of materials based on average cyclic plasticity strain damage accumulation

By introducing a fatigue blunting factor, the cyclic elasto-plastic Hutchinson-Rice-Rosengren （HRR） field near the crack tip under the cyclic loading is modified. And, an average damage per loading-cycle in the cyclic plastic deformation region is defined due to Manson-Coffin law. Then, according to the linear damage accumulation theory-Miner law, a new model for predicting the fatigue crack growth （FCG） of the opening mode crack based on the low cycle fatigue （LCF） damage is set up. The step length of crack propagation is assumed to be the size of cyclic plastic zone. It is clear that every parameter of the new model has clearly physical meaning which does not need any human debugging. Based on the LCF test data, the FCG predictions given by the new model are consistent with the FCG test results of Cr2Ni2MoV and X12CrMoWVNbN 10-1-1. What＇s more, referring to the relative researches, the good predictability of the new model is also proved on six kinds of materials.

Development of a new connection for precast concrete walls subjected to cyclic loading

The Industrialized Building System （IBS） was recently introduced to minimize the time and cost of project construction. Accordingly, ensuring the integration of the connection of precast components in IBS structures is an important factor that ensures stability of buildings subjected to dynamic loads from earthquakes, vehicles, and machineries. However, structural engineers still lack knowledge on the proper connection and detailed joints o fiBS structure construction. Therefore, this study proposes a special precast concrete wall-to-wall connection system for dynamic loads that resists multidirectional imposed loads and reduces vibration effects （PI2014701723）. This system is designed to connect two adjacent precast wall panels by using two steel U-shaped channels （i.e., male and female joints）. During casting, each joint is adapted for incorporation into a respective wall panel after considering the following conditions： one side of the steel channel opens into the thickness face of the panel; a U-shaped rubber is implemented between the two channels to dissipate the vibration effect; and bolts and nuts are used to create an extension between the two U-shaped male and female steel channels. The developed finite element model of the precast wall is subjected to cyclic loads to evaluate the performance of the proposed connection during an imposed dynamic load. Connection performance is then compared with conventional connections based on the energy dissipation, stress, deformation, and concrete damage in the plastic range. The proposed precast connection is capable of exceeding the energy absorption of precast walls subjected to dynamic load, thereby improving its resistance behavior in all principal directions.

Dynamic Simulation on Collision Between Ship and Offshore Wind Turbine

By using ABAQUS/Explicit, the dynamic process of an offshore wind turbine(OWT) stricken by a ship of 5000DWT in the front direction is simulated. The OWT is located on a large-scale prestressing bucket foundation constructed by an integrated installation technique. According to the simulation results, under the ship collision, a certain range of plastic zone appears within a local area of arc transition structure of the bucket foundation, and the concrete plastic zone is seriously damaged. As the stress level of OWT tower is relatively low, the OWT tower is less affected. A great inertial force is generated at the top of the OWT tower as the mass of nacelle and blades is up to 400 t. The displacement of the tower is in the opposite direction of the ship collision at the end of 1 s under the action of inertial force. There is only a minor damage in the ship bow. Most of the kinetic energy is transformed into the plastic dissipation and absorbed by the arc transition structure of bucket foundation.

YIELD CRITERION IN PLASTIC-DAMAGE MODELS FOR CONCRETE

A class of plastic-damage models for concrete require an unambiguous definition of cohesion in the yield criteria. For this reason, the Lubliner yield criterion has been adopted by many investigators and the commercial FE program Abaqus. As is well known, this criterion has achieved great success especially in plane stress states. In this paper, we are trying to extend it to triaxial compression stress states. First, a major limitation of the Lubliner criterion is analyzed. Then, a revised version of the Lubliner criterion is proposed, which shows appropriate properties over a wide range of stress states often encountered in engineering structures, and the predicted failure envelopes fit well with experimental data. For the concrete damaged plasticity model in Abaqus, a calibration strategy is suggested for uniformly confined concrete.

Deformation coordination analysis of CC-RCC combined dam structures under dynamic loads

A combined dam structure using different concrete materials offers many practical benefits.There are several real-world cases where largevolume heterogeneous concrete materials have been used together.From the engineering design standpoint,it is crucial to understand the deformation coordination characteristics and mechanical properties of large-volume heterogeneous concrete,which affect dam safety and stability.In this study,a large dam facility was selected for a case study,and various design schemes of the combined dam structure were developed by changing the configurations of material zoning and material types for a given dam shape.Elastoplastic analysis of the damfoundation-reservoir system for six schemes was carried out under dynamic conditions,in which the concrete damaged plasticity(CDP)model,the Lagrangian finite element formulation,and a surface-to-surface contact model were utilized.To evaluate the mechanical properties of zoning interfaces and coordination characteristics,the vertical distribution of the first principal stress at the longitudinal joint was used as the critical index of deformation coordination control,and the overall deformation and damage characteristics of the dam were also investigated.Through a comparative study of the design schemes,an optimal scheme of the combined dam structure was identified:large-volume roller-compacted concrete(RCC)is recommended for the dam body upstream of the longitudinal joint,and high-volume fly ash conventional concrete(CC)for the dam body downstream of the longitudinal joint.This study provides engineers with a reference basis for combined dam structure design.

An enhanced damage plasticity model for predicting the cyclic behavior of plain concrete under multiaxial loading conditions

Some of the current concrete damage plasticity models in the literature employ a single damage variable for both the tension and compression regimes,while a few more advanced models employ two damage variables.Models with a single variable have an inherent dificulty in accounting for the damage accrued due to tensile and compressive actions in appropriately different manners,and their mutual dependencies.In the current models that adopt two damage variables,the independence of these damage variables during cyclic loading results in the failure to capture the effects of tensile damage on the compressive behavior of concrete and vice-versa.This study presents a cyclic model established by extending an existing monotonic constitutive model.The model describes the cyclic behavior of concrete under multiaxial loading conditions and considers the influence of tensile/compressive damage on the compressive/tensile response.The proposed model,dubbed the enhanced concrete damage plasticity model(ECDPM),is an extension of an existing model that combines the theories of classical plasticity and continuum damage mechanics.Unlike most prior studies on models in the same category,the performance of the proposed ECDPM is evaluated using experimental data on concrete specimens at the material level obtained under cyclic multiaxial loading conditions including uniaxial tension and confined compression.The performance of the model is observed to be satisfactory.Furthermore,the superiority of ECDPM over three previously proposed constitutive models is demonstrated through comparisons with the results of a uniaxial tension-compression test and a virtual test.

Determination of the Cement Sheath Interface and the Causes of Failure in the Completion Stage of Gas Wells

The bonding quality of the cement sheath interface decreases during well completion because of the change in the casing pressure.To explore the root cause of such phenomena,experiments on the mechanical properties and interface bonding strength of a cement sheath have been carried out taking the LS25-1 high-temperature and high-pressure(HTHP)gas field as an example.Moreover,a constitutive model of the cement sheath has been defined and verified both by means of a full-scale HTHP cement sheath sealing integrity evaluation experiment and three-dimensional finite element simulations.The results show that the low initial cementing surface strength is the root cause of cement sheath interface bonding failure.When the pressure in the casing exceeds a certain limit,the stress caused by the change in the internal pressure in the casing is transmitted to the cement sheath,resulting in the degradation of the interface stiffness of the cement sheath.However,with an increase in the casing wall thickness,the stress transmission capacity decreases.Therefore,it is concluded that improving the interfacial cementing strength,appropriately increasing the casing wall thickness and increasing the initial stress of the cement sheath are the keys to ensuring the sealing integrity of the cement sheath in high-temperature and high-pressure gas wells.

Analysis on damage causes of built-in corridor in core rock-fill dam on thick overburden:A case study

The stress state of the built-in corridor in core rock-fill dam on thick overburden is extremely complex,which may produce cracking and damage.The purpose of this paper was to investigate the effect of thick overburden on the stress and deformation of the built-in corridor in a rock-fill dam,and ascertain the damage causes of the corridor.The rationality of the analysis method for corridor with similar structure is another focus.The approach is based on finiteelement method and the calculation result accuracy is verified by the field monitoring data.The improved analysis method for corridors with similar structure is proposed by comparing various corridor load calculation methods and concrete constitutive models.Results demonstrate that the damage causes of the corridor are the deformability difference between the overburden and concrete and the special structural form.And the calculation model considering dam construction process,contact between concrete and surrounding soil,and concrete damage plasticity can reasonably reflect the mechanical behavior of the corridor.The research conclusions may have a reference significance for the analysis of tunnels similar to built-in corridors.

Numerical modelling of reinforced concrete flexural members strengthened using textile reinforced mortars

Externally bonded(EB)and near-surface mounted(NSM)bonding are two widely adopted and researched strengthening methods for reinforced-concrete structures.EB composite substrates are easy to reach and repair using appropriate surface treatments,whereas NSM techniques can be easily applied to the soffit and concrete member sides.The EB bonded fiber-reinforced polymer(FRP)technique has a significant drawback:combustibility,which calls for external protective agents,and textile reinforced mortar(TRM),a class of EB composites that is noncombustible and provides a similar functionality to any EB FRP-strengthened substrate.This study employs a finite element analysis technique to investigate the failing failure of carbon textile reinforced mortar(CTRM)-strengthened reinforced concrete beams.The principal objective of this numerical study was to develop a finite element model and validate a set of experimental data in existing literature.A set of seven beams was modelled and calibrated to obtain concrete damage plasticity(CDP)parameters.The predicted results,which were in the form of load versus deflection,load versus rebar strain,tensile damage,and compressive damage patterns,were in good agreement with the experimental data.Moreover,a parametric study was conducted to verify the applicability of the numerical model and study various influencing factors such as the concrete strength,internal reinforcement,textile roving spacing,and externally-applied load span.The ultimate load and deflection of the predicted finite element results had a coefficient of variation(COV)of 6.02%and 5.7%,respectively.A strain-based numerical comparison with known methods was then conducted to investigate the debonding mechanism.The developed finite element model can be applied and tailored further to explore similar TRM-strengthened beams undergoing debonding,and the preventive measures can be sought to avoid premature debonding.

Capacity and failure mechanism of laterally loaded jet-grouting reinforced piles: Field and numerical investigation

This study presents the results of field and numerical investigations of lateral stiffness, capacity, and failure mechanisms for plain piles and reinforced concrete piles in soft clay. A plastic-damage model is used to simulate concrete piles and jet-grouting in the numerical analyses. The field study and numerical investigations show that by applying jet-grouting sur- rounding the upper 7.5D （D = pile diameter） of a pile, lateral stiffness and beating capacity of the pile are increased by about 110% and 100%, respectively. This is partially because the jet-grouting increases the apparent diameter of the pile, so as to en- large the extent of failure wedge and hence passive resistance in front of the reinforced pile. Moreover, the jet-grouting pro- vides a circumferential confinement to the concrete pile, which suppresses development of tensile stress in the pile. Corre- spondingly, tension-induced plastic damage in the concrete pile is reduced, causing less degradation of stiffness and strength of the pile than that of a plain pile. Effectiveness of the circumferential confinement provided by the jet-grouting, however, diminishes once the grouting cracks because of the significant vertical and circumferential tensile stress near its mid-depth. The lateral capacity of the jet-grouting reinforced pile is, therefore, governed by mobilized passive resistance of soil and plastic damage of jet-grouting.

Nonlinear numerical simulation of punching shear behavior of reinforced concrete flat slabs with shear-heads

This paper examines the structural response of reinforced concrete flat slabs,provided with flillyembedded shear-heads,through detailed three-dimensional nonlinear numerical simulations and parametric assessments using concrete damage plasticity models.Validations of the adopted nonlinear finite element procedures are carried out against experimental results from three test series.After gaining confidence in the ability of the numerical models to predict closely the full inelastic response and failure modes,numerical investigations are carried out in order to examine the influence of key material and geometric parameters.The results of these numerical assessments enable the identification of three modes of failure as a function of the interaction between the shear-head and surrounding concrete.Based on the findings,coupled with results from previous studies,analytical models are proposed for predicting the rotational response as well as the ultimate strength of such slab systems.Practical recommendations are also provided for the design of shear-heads in RC slabs,including the embedment length and section size.The analytical expressions proposed in this paper,based on a wide-ranging parametric assessment,are shown to offer a more reliable design approach in comparison with existing methods for all types of shear-heads,and are suitable for direct practical application.