Extended Finite Element Model Research Articles

Crack Detection of Asphalt Concrete Using Combined Fracture Mechanics and Digital Image Correlation

Cracking is a common failure mode in asphalt concrete (AC) pavements. Many tests have been developed to characterize the fracture behavior of AC. Accurate crack detection during testing is crucial to describe AC fracture behavior. This paper proposes a framework to detect surface cracks in AC specimens using two-dimensional digital image correlation (DIC). Two significant drawbacks in previous research in this field were addressed. First, a multiseed incremental reliability-guided DIC was proposed to solve the decorrelation issue due to large deformation and discontinuities. The method was validated using synthetic deformed images. A correctly implemented analysis could accurately measure strains up to 450%, even with significant discontinuities (cracks) present in the deformed image. Second, a robust method was developed to detect cracks based on displacement fields. The proposed method uses critical crack tip opening displacement (δc) to define the onset of cleavage fracture. The proposed method relies on well-developed fracture mechanics theory. The proposed threshold δc has a physical meaning and can be easily determined from DIC measurement. The method was validated using an extended finite-element model. The framework was implemented to measure the crack-propagation rate while conducting the Illinois-flexibility index test on two AC mixes. The calculated rates could distinguish mixes based on their cracking potential. The proposed framework could be applied to characterize AC cracking phenomenon, evaluate its fracture properties, assess asphalt mixture testing protocols, and develop theoretical models.

A new extended finite element model for gas flow in shale gas reservoirs with complex fracture networks

A new extended finite element model for gas flow in shale gas reservoirs with complex fracture networks

Analysis of the hydraulic fracturing mechanism and fracture propagation law with a new extended finite element model for the silty hydrate reservoir in the South China Sea

The existing hydrate exploitation technology is characterized by low gas production in single wells and a small exploitation range, owing to the low porosity and low permeability of submarine hydrate reservoirs; therefore, commercial exploitation is difficult to achieve. There is an urgent need to improve exploitation efficiency by means of reservoir reconstruction. Hydraulic fracturing technology may be useful for low-permeability reservoirs such as silty hydrate reservoirs in the South China Sea (SCS). In this study, we established a new hydraulic fracturing model of a horizontal single well for gas-hydrate-bearing-sediments (GHBS) using the cohesive zone method (CZM) based on the extended finite element method (XFEM). To this end, we utilized field measured data of GHBS at site GMGS3-W19. The effect of fluid-solid coupling was considered, and the fracture propagation criterion was set as the maximum principal stress criterion to analyze the hydraulic fracture propagation law of hydrate reservoirs in the SCS. First, we illustrated the effectiveness of the CZM based on XFEM in simulating the fracture morphology of GHBS hydraulic fracturing. A Parametric study showed that the hydrate saturation, horizontal geo-stress coefficient, perforation angle, and injection rate play crucial roles in the fracture propagation path and characteristic parameters of GHBS hydraulic fractures. The fracture development pattern and propagation law of a silty hydrate reservoir within the SCS were theoretically obtained. The results of the present study partly provide theoretical support for hydrate exploitation engineering in the SCS.

Numerical Simulation of Cracking in Asphalt Concrete Through Continuum and Discrete Damage Model

Abstract: This study describes the development of Continuum and Discrete Damage Models in commercial finite element code Abaqus/Standard. The Concrete Damage Plasticity Model has been simulated, analysed, and compared the result with the experimental data. For verification, the Cohesive Zone Model has been simulated and analysed. Furthermore, the Extended Finite Element Model and concrete damage model are discussed and compared. The continuum damage model tends to simulate the complex fracture behaviour like crack initiation and propagation along with the invariance of the result, while the cohesive zone model can simulate and propagate the crack as well as the good agreement of the result. Further work in the proposed numerical models can better simulate the fracture behaviour of asphalt concrete in near future. Keywords: Model, Concrete, Cohesive Zone, Finite element, Abaqus.

Modeling mixed mode cracking in concrete through a regularized extended finite element formulation considering aggregate interlock

Mixed mode cracking in concrete keeps being a challenging issue in computational fracture mechanics. While uncracked concrete develops cracks perpendicular to the direction of the maximum principal tensile stress, the crack edges will expectedly open, mutually slide, and transmit both normal and shearing stresses. In the present formulation, we replace the damage band with a regularized strain band by enriching the space of the approximating functions in compliance with the regularized extended finite element model developed by the authors. Our clue is that a discriminant factor to choose the appropriate expression of the regularized strain relies upon the concept of stress compatibility modified to consider aggregate interlock. In particular, we resort to a simplified phenomenological saw-tooth model of the crack surface asperities. We show the capability of the methodology to capture both structural behavior and crack paths in a set of bending tests carried out on concrete specimens. Though phenomenological, the resulting approach is novel, conceptually simple and reliable for it directly reproduces the expected kinematics and structural results.

Automated finite element modeling and analysis of cracked reinforced concrete beams from three dimensional point cloud

AbstractModeling and assessing the load‐carrying capacity of reinforced concrete (RC) structures that are cracked is important because the condition of an RC structure changes due to natural aging and accumulated damage. The current approach to determining the residual capacity of a cracked RC structure is time‐consuming and error‐prone due to a limited capacity to obtain and process structural information. In this study, an automated approach was proposed to determining residual capacity by performing finite element (FE) analysis of an RC structure using point cloud three dimensional modeling. Two simply supported RC beams were subjected to bending tests to provide validation data. The initial activity was to extract dimensional data from laser‐scanned point cloud data of a beam. The information obtained was used to automatically generate nonlinear FE models of the beam that was used for nonlinear FE analyses in Abaqus. An initial crack was added to the beam to model a cracked beam in real conditions. Three different models were used to simulate initial cracks, a weakened element model, a cohesive zone model, and an extended FE model. The results indicate that the proposed method of automating the combination of point cloud data with nonlinear FE analysis provides a novel practical approach to modeling and assessing cracked RC beams.

The Prediction of Road Cracks through Acoustic Signature: Extended Finite Element Modeling and Experiments

Abstract Traffic produces vibrations and noise that affect the livability and structural integrity of the built environment. Despite the fact that many studies focused on traffic-induced vibrations and noise, there is a lack of studies linking the vibrations propagating into the road pavement and the related acoustic response (or acoustic signature) as a means to assess the structural health status. Indeed, monitoring this response can lead to an estimation of the road layer structural condition and an identification of cracks that occurred because of the traffic. Consequently, the objectives of this study are to (i) model the involved phenomena through a Finite Element Method (FEM) analysis; (ii) compare data and simulations; and (iii) set up an Extended Finite Element Model (EXFEM) that is able to forecast the change of the road acoustic signature over time because of the presence of occurred cracks. Loads and sound-related phenomena (generation, transmission, interaction with cracks) were simulated through an EXFEM software. In addition, in order to estimate the effectiveness of the study, the aforementioned simulations were compared with real data gathered from a Dense Graded Friction Course road pavement in different and controlled structural conditions through a specially designed, microphone-based electronic system. Even if further studies are needed to better fix the measurement chain and better carry out the FEM analyses, preliminary results show that the EXFEM model is able to reproduce, with good approximation, the measured signals and that this model can be used to forecast the effects of different types of cracks on the propagation of vibration into road pavements.

A cohesive XFEM model for simulating fatigue crack growth under various load conditions

This study presents calibration and validation of a cohesive extended finite element model for fatigue crack propagation in ductile materials. The approach relies on a separation between plasticity around the crack tip and fatigue crack growth at the crack tip such that the influence of plasticity on fatigue driving forces is predicted. This implies that characterization of crack growth requires effective Paris parameters. It is shown that the calibrated model can capture fatigue crack growth behaviour in ductile materials for in-phase and out-of-phase biaxial fatigue loading as well as in-phase biaxial loading with an overload.

A study of loading rate effect fracture behavior of concrete based on digital image correlation and finite-element method

To study the rate effect on the fracture properties of concrete, 700 mm × 150 mm × 100 mm specimens with a 60-mm notch were used for three-point bending test at the loading rate of 0.0005, 0.005, 0.05, and 0.5 mm/s, respectively. In the test, digital image correlation was used for monitoring the fracture process. The result shows that the fracture stress, unstable fracture toughness, and fracture energy have rate sensitivity. The numerical simulation was performed by extended finite-element model. The model calibration had a very good agreement with the experimental result under different loading rate. Meanwhile, fracture process zone length calculated by digital image correlation is similar to the result obtained by the extended finite-element model under different loading rate.

An orthotropic multi-surface damage-plasticity FE-formulation for wood: Part II – Numerical applications

The present study investigates the modelling capabilities offered by the multi-surface damage-plasticity FE-formulation illustrated in Part I [1] when integrated with a regularized extended finite element model. The aim is to tackle discontinuous displacements and interfaces. In particular, we show how to embed a regularized discontinuity within an element, carry out the transition from the continuous to the regularized discontinuous regime and obtain physically consistent results. We present a wide set of applications ranging from brittle to ductile failure, displaying the effectiveness of the proposed FE-approach versus the experimental results.

Level Set-Based Extended Finite Element Modeling of the Response of Fibrous Networks Under Hygroscopic Swelling

Abstract Materials like paper, consisting of a network of natural fibers, exposed to variations in moisture, undergo changes in geometrical and mechanical properties. This behavior is particularly important for understanding the hygro-mechanical response of sheets of paper in applications like digital printing. A two-dimensional microstructural model of a fibrous network is therefore developed to upscale the hygro-expansion of individual fibers, through their interaction, to the resulting overall expansion of the network. The fibers are modeled with rectangular shapes and are assumed to be perfectly bonded where they overlap. For realistic networks, the number of bonds is large, and the network is geometrically so complex that discretizing it by conventional, geometry-conforming, finite elements is cumbersome. The combination of a level-set and XFEM formalism enables the use of regular, structured grids in order to model the complex microstructural geometry. In this approach, the fibers are described implicitly by a level-set function. In order to represent the fiber boundaries in the fibrous network, an XFEM discretization is used together with a Heaviside enrichment function. Numerical results demonstrate that the proposed approach successfully captures the hygro-expansive properties of the network with fewer degrees-of-freedom compared to classical FEM, preserving desired accuracy.

Recycled Stone/ABS particulate composite: Micromechanical finite element fracture analysis

New stone composite samples, fabricated using nano- to micro-sized recycled granite particles with irregular shapes and random distribution within a ABS matrix, demonstrate a highly nonlinear and complex fracture behavior. To model this behavior, a mixed mode cohesive zone finite element model is identified using the single-leg bending (SLB), in order to represent the granite particles-ABS interfacial debonding. An inverse methodology is then proposed to determine the parameters of the cohesive zone (CZM). A direct method based on J-integral approach is employed to determine the effective parameters of the traction-separation law. A two dimensional micromechanical extended finite element model (XFEM) of the composite is generated using X-ray micro-computed tomography (XMT) to mimic the actual shape of the particles. Finally, the identified cohesive model parameters have been employed to simulate the crack growth within the granite particulates/ABS composite. The comparison of the numerical and experimental results of the SLB test demonstrated the effectiveness of the proposed simulation framework.

Virtual estimation of the Griffith’s modulus and cohesive strength of ultra-high performance concrete

This investigation presents a computational methodology to estimate the unit fracture energy (Griffith’s modulus) and the cohesive strength of ultra-high performance concrete (UHPC). The hydration of plain UHPC mixtures is simulated using the National Institute of Standards and Technology (NIST) Virtual Cement and Concrete Testing Laboratory (VCCTL) hydration simulator. The Young’s modulus of elasticity of the hardened concrete is obtained via finite-element analysis of a sample cube of its micro-structure. Applying the Extended Finite Element modeling capabilities of ABAQUS to previously grooved simple structural elements under gradually-increasing load, the results of computational simulations of the start of crack growth are linked to the corresponding predictions by regressions on experimental observations in classical linear elastic fracture mechanics physical tests. A procedure is developed to estimate both the Griffith’s modulus and the cohesive strength of the UHPC, which are difficult to assess experimentally. As an application, the effect on the horizontal thrust at the abutments of a two-hinged symmetric arch with a crown crack and uniformly loaded is analyzed using the data provided by the pure-bending test on a pre-grooved simply supported beam.

On the undrained and drained hydraulic fracture splits

SummaryThe simulation of hydraulic fracturing (HF) involves the solution of a hydro‐mechanically coupled system. This article presents a new iterative sequential coupling algorithm, the undrained HF split, that improves the simulation of HFs in impermeable media. A poromechanics analogy is used to derive a stable split for the hydro‐mechanically coupled system in which the mechanical subproblem is solved first. The proposed undrained HF split is applied to the simulation of cohesive HFs in an impermeable elastic medium. The cubic law is used as the constitutive model for simulating fluid flow in fractures. A minimum hydraulic aperture is assumed in the cohesive tip zone, where the mechanical aperture smoothly vanishes. While general in its nature, the undrained HF splitting scheme is employed within the context of a two‐dimensional eXtended finite element model for the fractured solid, and a regular finite element model for fluid in the fracture. The undrained HF split is successfully used to simulate self‐similar plane strain HFs as well as the propagation of HFs from a wellbore under anisotropic stress conditions. Fracture trajectories and local alteration of stress field are investigated. The solution of the undrained HF split converges to the same solution as the fully coupled model, whereas the commonly used P→W sequential algorithm, referred to in this article as the drained HF split, generates spurious oscillations and fails to converge in many problems. The undrained HF split is shown to be stable and robust in applications where the drained HF split is unstable.

FEM of Cracking during Nanoindentation and Scratch Testing in the Hard W-C Coating/Steel Substrate System

Finite element modelling (FEM) and eXtended FEM (XFEM) combined with the experimental nanoindentation and scratch tests have been used to simulate the process of cohesive cracking in W-C coating on softer and more ductile steel substrate during nanoindentation and scratch testing. The formation of single and multiple circular “frame” cohesive cracks in the sink-in zone during nanoindentation were explained by the development of high local tensile stresses in the coatings controlled by the plastic deformation of the substrate. Analogous mechanisms were successfully applied to the simulation of multiple Chevron type cracking during scratch testing. Thus, the ability of XFEM to predict the formation of different types of cohesive cracks was confirmed. It was also demonstrated that both nanoindentation and scratch tests in combination with XFEM can be used as the methods to determine the strength and fracture toughness of thin coatings.

Use of extended finite element method and statistical analysis for modelling the corrosion-induced cracking in reinforced concrete containing metakaolin

This study presents an improved technique to predict the time for corrosion-induced cracking in concrete containing metakaolin (MK) based on combining extended finite element model (XFEM) and statistical analysis. The prediction model was developed based on the percentage of MK in the mixture, binder content, water-to-binder (W/B) ratio, and concrete cover thickness. The developed model was also validated experimentally using an accelerated corrosion test. Moreover, design charts were developed in this study using statistical analysis to facilitate and simplify the use of the prediction model. The results indicated that the corrosion pressure required to crack the concrete cover increased with higher percentages of MK, higher binder content, and (or) lower W/B ratio. The most significant factors affecting the time for corrosion-induced cracking was found to be the concrete cover, W/B ratio, MK replacement, and binder content, respectively, in order of significance. The results also indicated that the time required for corrosion-induced cracking obtained from the developed prediction model showed a good agreement with the experimental results of the accelerated corrosion samples. Also, the cracks predicted by the XFEM showed a similar trend of variation with that found in the accelerated corrosion samples.

Enhancing the magneto-optical Kerr effect through the use of a plasmonic antenna.

We employ an extended finite-element model as a design tool capable of incorporating the interaction between plasmonic antennas and magneto-optical effects, specifically the magneto-optical Kerr effect (MOKE). We first test our model in the absence of an antenna and show that for a semi-infinite thin-film, good agreement is obtained between our finite-element model and analytical calculations. The addition of a plasmonic antenna is shown to yield a wavelength dependent enhancement of the MOKE. The antenna geometry and its separation from the magnetic material are found to impact the strength of the observed MOKE signal, as well as the antenna's resonance wavelength. Through optimization of these parameters we achieved a MOKE enhancement of more than 100 when compared to a magnetic film alone. These initial results show that our modeling methodology offers a tool to guide the future fabrication of hybrid plasmonic magneto-optical devices and plasmonic antennas for magneto-optical sensing.

Three dimensional crack propagation through mesh-based explicit representation for arbitrarily shaped cracks using the extended finite element method

In this study a three-dimensional extended finite element modeling work is developed for crack propagation analysis of arbitrarily shaped crack(s) under mixed mode loading conditions. An explicit method is proposed for crack geometry characterizations, slicing of the enriched elements, and crack propagations. The method can be used to model multiple cracks propagation and allows for crack geometry transition from internal to surface, and further to through-thickness modalities under arbitrary loading conditions. Benchmark cases are studied and satisfactory agreement between theoretical and numerical results is observed. In addition, comparison between fully explicit and conventional implicit methods is made at different finite element mesh densities. The results show that compared to the conventional implicit method, there is a significant improvement in the obtained stress intensity factors for the fully explicit method at coarse linear hexahedral and quadratic tetrahedral meshes.

Multiple cohesive cracking during nanoindentation in a hard W-C coating/steel substrate system by FEM

Stress evolution and subsequent cohesive cracking in the hard and stiff W-C coating on steel substrate during nanoindentation have been investigated using finite element modelling (FEM) and eXtended FEM (XFEM). The FEM simulations showed that the maximum principal stresses in the studied system were tensile and always located in the coating. They evolved in several stages. At indentation depths below 15% of the relative indentation depth, the maximum principal tensile stresses of ∼3GPa developed at the top surface of the coating along the indenter/coating interface. At relative depths range 15–60%, the maximum tensile stresses of ∼6–8GPa concentrated under the indenter tip in the coating along the interface with the substrate. At relative depths exceeding 60%, the maximum stresses gradually increased up to 10GPa and they were located in the sink-in zone outside the indent as well as below the indenter tip. The first and subsequent cohesive cracks developed when the maximum tensile stresses in the sink-in zone at the top surface of the coating (and at the coating/substrate interface under the indenter) repeatedly reached the ultimate tensile strength of the coating. The hardness profile as well as cohesive cracking is controlled by the deformation of the substrate defined by the ration of the yield stresses of the coating and substrate. Very good correlation between the experimentally obtained cracks and multiple cracks predicted by XFEM confirmed the ability of the applied modelling in the prediction of fracture behavior of the studied coating/substrate system.

Failure of FRP-strengthened SFRC beams through an effective mechanism-based regularized XFEM framework

Failure of beams made of Steel Fiber Reinforced Concrete (SFRC) and strengthened with plates of Fiber Reinforced Polymer (FRP) typically occurs after the detachment of the FRP plate, and depends on the content of steel fibers. Relevant finite element simulations are few, and, usually, neglect the modeling of the detachment process. On the contrary, the proposed regularized eXtended Finite Element Model (XFEM) focuses on the detachment process. In particular, the present approach takes into account the fiber-bridging effect exerted by the steel fibers at the detachment zone, while modeling the detachment through a mechanism-based procedure. The obtained results are fully consistent with the experimental data.