Stationary Crack Research Articles

Machine-Aided Bridge Deck Crack Condition State Assessment Using Artificial Intelligence.

The Federal Highway Administration (FHWA) mandates biannual bridge inspections to assess the condition of all bridges in the United States. These inspections are recorded in the National Bridge Inventory (NBI) and the respective state's databases to manage, study, and analyze the data. As FHWA specifications become more complex, inspections require more training and field time. Recently, element-level inspections were added, assigning a condition state to each minor element in the bridge. To address this new requirement, a machine-aided bridge inspection method was developed using artificial intelligence (AI) to assist inspectors. The proposed method focuses on the condition state assessment of cracking in reinforced concrete bridge deck elements. The deep learning-based workflow integrated with image classification and semantic segmentation methods is utilized to extract information from images and evaluate the condition state of cracks according to FHWA specifications. The new workflow uses a deep neural network to extract information required by the bridge inspection manual, enabling the determination of the condition state of cracks in the deck. The results of experimentation demonstrate the effectiveness of this workflow for this application. The method also balances the costs and risks associated with increasing levels of AI involvement, enabling inspectors to better manage their resources. This AI-based method can be implemented by asset owners, such as Departments of Transportation, to better serve communities.

In-situ SEM-EBSD investigation of the low-cycle fatigue deformation behavior of Inconel 718 at grain-scale

The low-cycle fatigue (LCF) deformation behavior of the Inconel 718 (IN718) specimen with micro-notch was investigated by an in-situ fatigue test inside a scanning electron microscope (SEM) combined with electron backscattered diffraction (EBSD). The initiation condition and propagation path of cracks were observed by SEM. The essential reasons for crack initiation, propagation and fatigue fracture are elaborately characterized by crystallographic information obtained from EBSD. Meanwhile, the strain evolution during fatigue deformation simulated by the crystal plastic finite element (CPFE) method is consistent with the experimental content, which further verifies and supplements the experimental results. Two LCF deformation mechanisms (location-related) of the IN718 specimen are proposed at grain-scale based on the direct observation of crack characteristics and in-depth analysis. Near the edge micro-notch region, the deformation mechanism is mainly related to crack initiation and propagation: the plastic strain localization along the most easily activated slip band leads to the initiation of fatigue microcracks inside the grain. The crack propagation is carried out by microcrack coalescence in different grains. Away from the edge micro-notch region, the deformation mechanism is mainly related to the instantaneous fatigue fracture: fatigue fracture comes from the intergranular plastic strain accumulation and heterogenous deformation at grain-scale. The result also deeply explains the fatigue crack propagation in metals (two consecutive modes).

ASR劣化を生じたRCはり部材の耐荷特性と内部ひび割れ状況

In order to establish a safety evaluation technique for ASR-deteriorated structures, crack observation, observation of ASR expansion such as reinforcing bar strain and concrete strain, and loading tests were conducted on RC beam specimens with ASR deterioration to evaluate the effects of ASR deterioration on the RC beam members. In addition, X-ray computed tomography (CT) images of core concrete taken from these RC beam specimens were taken to evaluate the cracking conditions in the ASR concrete. The test results showed that the ASR cracks in the specimens without shear reinforcement opened further under loading and cracks were generated, leading to shear failure. In addition, many fine cracks parallel to the loaded cracks were observed in the cores taken from the ASR specimens.

Effect of compression reinforcement on modulus of elasticity of RC beams under flexure

In structural analysis, precise estimation of modulus of elasticity, cross-sectional area and moment of interia of the section play an important role. It is a key factor while carrying out structural analysis for indeterminate structures. Most of the international codes quantify material behaviour for concrete as well as steel in elastic zone and is equally correct. Indian codal provisions express modulus of elasticity of concrete in terms of characteristic strength of concrete only similar to all other international codes. Structural analysis of a high rise building demands cross-sectional area of unreinforced concrete. However, area of reinforcement and stirrups provided are overlooked in the analysis. Contemporary design practice necessitates additional cycle of analysis along with design. First cycle of analysis shall yield the conventional design of sections and are of reinforcement. Based on calculated area of steel in first cycle, it is possible to arrive at effective area of concrete and respective moment of inertia by considering reinforcement in tension and compression zone. The analysis of sections can further be carried out in pre and post cracking conditions. At this stage, material as well geometrical properties play an important role. Present aim of study is to determine modulus of elasticity of reinforced concrete beams with percentage of compression reinforcement ranging from 0.29 to 0.43%. The outcomes are used to propose an empirical equation for evaluation of modified modulus of elasticity of reinforced concrete material which is a dependent on concrete grade along with percentage of tension and compression reinforcement.

Structural performance assessment of RC flexural members through crack-based fibre beam-column model updating

For reinforced concrete (RC) structures in service, original finite element models (FEMs) based on design information usually deviate from actual structures due to structural damage such as cracks, displacement and material degradation. Although conventional FEMs can be updated to give results that match the response data used during the updating process, they can hardly predict the subsequent structural performance. In this study, an innovative fibre beam-column model updating method based on static deflection and crack width is proposed. First, the self-developed fibre beam-column finite element program COMPONA-MARC is modified to efficiently simulate the nonlinear cracking behaviour of RC flexural members. Second, sensitivity studies using the discrete central difference method based on the Lagrange interpolation function are conducted to select updating parameters. Third, with the combination of modified COMPONA-MARC and an automation program written in Python, plenty of sample data are obtained, and artificial neural networks (ANNs) are trained by Bayesian Regularization Backpropagation (BRB). Finally, the updated parameters are acquired by inputting the measured static deflections and crack widths. Typical RC beams and RC slabs from the literature are utilized to validate the proposed methodology and explore potential implementation scenarios. The results show that updated FEMs can satisfactorily predict mechanical behaviour and assess structural performance. The proposed method offers enlightening insights about combining structural health monitoring and inspection (SHMI) results with model updating, which can promote the applications of digital twins in performance assessments of RC structures.

A coupling approach of the isogeometric–meshfree method and peridynamics for static and dynamic crack propagation

A coupling approach of the isogeometric–meshfree method and the peridynamic method is developed for static and dynamic crack propagation. The coupling approach exhibits advantages in the flexibility of modeling cracks and the exactness of geometry representation. The isogeometric–meshfree method, which adopts the moving least-squares approximations to establish the equivalence between meshfree shape functions and isogeometric basis functions, is capable of obtaining the exact geometry. The isogeometric–meshfree nodes located on the geometry can be directly coupled with peridynamic points by the balanced force principle, which is straightforward and effective. With this approach, the boundary conditions can be enforced directly using the isogeometric–meshfree method, and the surface effects of peridynamics are effectively eliminated. Moreover, the present approach is flexible in switching the isogeometric–meshfree nodes to peridynamic points and achieving adaptive coupling with the same convergence rate as the one for the isogeometric–meshfree method, which is higher than the one for the original peridynamic method. The dual-horizon peridynamic method is adopted for non-uniform discretization. Based on the exact geometry representation, the coupling approach is further extended to crack problems with contact loading. Several representative examples are presented to validate the effectiveness of the present approach in solving static/dynamic crack problems and studying fractures caused by contact.

Dealkylation in Fluid Catalytic Cracking Condition for Efficient Conversion of Heavy Aromatics to Benzene-Toluene-Xylene.

Based on the characteristics of typical C9 + aromatics in naphtha fractions, the effects of key process parameters and heavy aromatic composition on product distribution of fluid catalytic cracking (FCC) of heavy aromatics (HAs) were investigated. The results show that catalysts with large pore size and strong acid sites are favorable for the conversion of HAs to benzene-toluene-xylene (BTX) at higher reaction temperatures and moderate catalyst-oil ratios (C/O). With a Y zeolite-based catalyst which was hydrothermally pretreated for 4 h, the conversion of Feed 1 at 600 °C and C/O of 10 may reach 64.93%. Meanwhile, the yield and selectivity of BTX are 34.80 and 53.61%, respectively. The proportion of BTX can be adjusted within a certain range. The HAs from different sources show high conversion and good BTX selectivity, which provides strong support for the technological development of HAs to light aromatics in FCC operation.

Deformation State Analysis and Design Method of Bottom-Confined Gravity Retaining Walls

The deformation failure modes of gravity retaining walls include overturning, toppling, deep sliding, and eccentric failure. The complex eccentric failure mode, which involves the multi-stage deformation of retaining walls, suffers from a lack of mechanical models and design theories. Based on the basic theory of reinforced concrete construction, this study established a deformation calculation model of bottom-confined reinforced concrete gravity retaining walls (RC-GRWs), which are composed of tensile, compressive, and rigid zones. Then, this study defined RC-GRWs’ normal-operation, cracking, yield, and ultimate limit states, as well as the displacement of RC-GRWs in these states, according to the concrete cracking and steel reinforcement yield conditions. Accordingly, this study proposed a design method and process to determine the deformation of bottom-confined RC-GRWs. A project case shows that the calculation method for RC-GRW deformation proposed in this study can effectively assess the deformation state of in-service retaining walls and that optimized cracked retaining walls can meet the deformation-based design requirements.

Finite element study of V-shaped crack-tip fields in a three-dimensional nonlinear strain-limiting elastic body

We study the crack-tip fields in a three-dimensional (3D) plate with a V-shaped crack employing Rajagopal’s theory of elasticity with the strain-limiting effect. The classical linear elastic fracture mechanics (LEFM) theory bears a well-documented inconsistency, i.e., for a static crack problem under the traction-free boundary condition, the LEFM theory predicts singular crack-tip strains. Such results are inconsistent with the constitutive relation derived using the assumption of uniform bound for the strain values. Instead, Rajagopal’s recent theory of elasticity provides implicit constitutive relations between the linearized strain and Cauchy stress in which one can impose an a-priori upper bound for strains. We formulate the strain-limiting theory comprehensively in this article. The linearized strain is expressed as a nonlinear function of the Cauchy stress. The equation for the equilibrium of the linear momentum under a special type of nonlinear constitutive relation reduces to a second-order quasilinear elliptic boundary value problem (BVP). Using a Picard-type linearization and a continuous Galerkin-type finite element technique, we solve several BVPs with tensile and shear displacement loading. The numerical results for the strains, the stresses, the stress intensity factor (SIF), and strain energy density (SED) are obtained for different Poisson values, strain-limiting parametric values, and the V-shaped angles. The growth of the near-tip strains is far less than that of stresses, and the results for both the SIF and SED are also consistent with the results from LEFM. Our work demonstrates that the framework of Rajagopal’s theory of elasticity can provide a basis for developing physically meaningful and mathematically well-posed BVPs to study evolution of cracks, damage, nucleation, or failure in elastic bodies.

Study on the electric resistance method in crack depth measurements

Obtaining geometric parameters, especially depth, and describing the morphological characteristics of cracks are of great significance to control engineering disasters and accidents caused by cracks. The electric resistance method is based on the principle of differences in electrical properties between cracks and soil, which could be used to measure the single crack depth at project sites. There exists an Rmin value corresponding to a specific electrode distance d value at each Rf-d value obtained by laboratory experiments. Furthermore, a two-dimensional finite element model of soil with a single crack is established to carry out numerical simulation analysis considering the crack width W, crack depth D and complex crack conditions. The results reveal dynamic variation rules of soil resistance after crack development, and for each Rf-d value, the electrode distance d value corresponding to the Rmin value is approximately equal to the crack depth D. In the range of the electric field, the offset and rotation of the crack have little effect, while the measurement results have a strong dependence on relocation movement. The regulation gives guidance to the inversion analysis of crack depth D at project sites and has been applied in crack depth measurements of an expansive soil slope. The electric resistance method as a proposed integrated approach is of great significance and brings new perspectives into the study of crack depth measurements for field applications.

Evaluation of Mechanical Properties of Different Casing Drilling Steels

An investigation into the mechanical properties of K55, N80, and P110 steels commonly used for casing drilling was carried out together with microstructural and fractographic analysis. The results show that P110 steel consisted of almost fully tempered martensite and exhibited a synergy combination of static tensile, dynamic impact, and fatigue crack propagation properties among the three steels, possessing a higher fatigue limit and deeper crack tolerance before failure occurred. The K55 steel consisted of the pearlite and network structure of ferrite and possessed a high strain hardening exponent and low impact property, which led to the more suitable application under incidental large overload and temperature change, but it was unsuitable under the condition of higher impact force. The properties of N80 steel were moderate, and its fatigue property was higher than that of K55 and lower than that of P110; its incidental overload resistance was also higher than that of P110. The casing drilling steel can be selected according to the environment.

Conversion of light cycle oil to benzene and alkylated monoaromatics over monometallic and bimetallic CoMo catalysts in the presence of hydrogen donor

The conversion of diaromatics and triaromatics present in light cycle oil (LCO) to monoaromatics such as benzene, toluene, and xylene (BTX) is studied under fluid catalytic cracking conditions over zeolite-based monometallic (Co, Mo) and bimetallic (CoMo) bifunctional catalysts. Instead of H2, n-hexadecane (n-HD) is used as the hydrogen donor source at atmospheric pressure, which results in a high yield of the desired products and low coke formation. Additionally, n-HD undergoes cyclization, dehydroaromatization, and cracking as well. The total monoaromatics and BTX yield for various feed compositions follows the order: CoMo/Beta > CoMo/Y > Mo/Beta > Co/Beta > Mo/Y > Co/Y > Beta > Y, with CoMo/Beta resulting in 1-Methylnaphthalene conversion and monoaromatics yield of 84.3 % and 43.9 %, respectively. The monoaromatics and BTX yield over various catalysts is correlated to the coke formation, which is the least over bimetallic catalysts. The presence of rod-type CoMoO4 particles is detected on the bimetallic catalysts, and is supported by XRD, H2-TPR, and Raman analysis. For various feeds and catalysts, the cracking of triaromatics is fast, and the yield of monoaromatics is primarily dependent on the catalytic activity towards the cracking of diaromatics. An inverse correlation between the selectivity for 2-ring monoaromatics and BTX shows the formation of 2-ring monoaromatics as intermediates. Various diaromatic compounds with similar reactivity are identified, which could be used for the development of a lumped kinetic model. Based on a detailed product characterization, reaction pathways for the conversion of 1-Methylnapthalene and anthracene are proposed.

Electromechanical phase-field fracture modelling of piezoresistive CNT-based composites

We present a novel computational framework to simulate the electromechanical response of self-sensing carbon nanotube (CNT)-based composites experiencing fracture. The computational framework combines electrical-deformation-fracture finite element modelling with a mixed micromechanics formulation. The latter is used to estimate the constitutive properties of CNT-based composites, including the elastic tensor, fracture energy, electrical conductivity, and linear piezoresistive coefficients. These properties are inputted into a coupled electro-structural finite element model, which simulates the evolution of cracks based upon phase-field fracture. The coupled physical problem is solved in a monolithic manner, exploiting the robustness and efficiency of a quasi-Newton algorithm. 2D and 3D boundary value problems are simulated to illustrate the potential of the modelling framework in assessing the influence of defects on the electromechanical response of meso- and macro-scale smart structures. Case studies aim at shedding light into the interplay between fracture and the electromechanical material response and include parametric analyses, validation against experiments and the simulation of complex cracking conditions (multiple defects, crack merging). The presented numerical results showcase the efficiency and robustness of the computational framework, as well as its ability to model a large variety of structural configurations and damage patterns. The deformation-electrical-fracture finite element code developed is made freely available to download.

A Validated Computational Study of Lubricants under White Etching Crack Conditions Exposed to Electrical Fields

The problem of White Etching Cracks (WEC) leading to failures in the life cycle of industrial and automotive drive trains has existed for more than 30 years. Many parameters leading to WEC, such as the presence of electricity, unfavorable lubricants, and other factors, have been identified by the use of test rigs. However, since (a) within lifecycle operation these impacts appear incidentally, and (b) the identified impacts stimulate each other as stated in WEC research, the appearance of WEC in a drive train is impossible to predict for any given application. This difficulty causes WEC to be a serious problem. Obtaining reliable failure prediction rates using construction guidelines is still impossible, resulting in warranty claims. The aim of this paper is, first, to establish a routine to determine if and how lubricants could be described numerically with respect to their susceptibility toward electrical fields and to see how this factor could be added in a later stage to construction guidelines. Second, this paper aims to give advice to the applicant in terms of countermeasures. Currently, as predictors are missing, extensive and time-consuming testing is required. Contradictory test results in the field of application, indicate the need for fundamental parameters to determine in which circumstances the application is exposed to WEC risk and to suggest countermeasures. The current study presents a simulation method based solely on the chemical structure of lubricant components, investigating the appearance of WEC and their response to increasing electrical fields. The results show a clear pattern in WEC criticality with respect to lubricants: if two or more components present in the lubricant create clusters through their dipolar interaction, an apparent WEC risk could be accurately predicted apart from test rig results. These clusters are charged like one big particle in an early stage of electrical field exposure. As a result, the surface area increases, facilitating a higher uptake of charge. The incidental breakdown of this charge is assumed. As the charge of a cluster is higher than that of single molecules, WEC critical lubricants are subjected to this pattern. The study validates these results by comparing them using lubricants known to be critical with respect to WEC, suggesting further tests in the near future. Despite the fact that some lubricants seem more critical than others related to WEC, each lubricant clearly might lead to WEC under given conditions. Thus, constructing guidelines for a prediction routine is essential.

Finite element modeling of dynamic fracture in concrete through the initial fracture toughness-based criterion and field variable transference technique

Objective of the present study is to model the mode I dynamic crack propagation of concrete three-point bending (TPB) beams by using an initial fracture toughness-based criterion, and to evaluate the contribution of the rate and the inertia effects to the dynamic responses of concrete beams during crack growth. Firstly, the theory and implementation of the initial fracture toughness-based criterion are described in detail. Specifically, a field variable transference (FVT) technique is introduced to realistically reflect the coupling effect of the moving cracks and stress waves. Then, analysis procedures for static and dynamic crack growth are established. The comparison between the simulated and experimental impact and reaction forces indicates that, the initial fracture toughness-based criterion can accurately predict the mode I dynamic crack extension in concrete TPB beams. Moreover, the effects of the inertia and rate effects on dynamic responses (i.e., load, deformation, crack propagation velocity, and energy evolution and partition) are evaluated by performing numerical experiments on the mode I crack growth of TPB beams under a wide range of loading rates from 0.0002 mm/s to 2000 mm/s. It is concluded that, for the adopted concrete TPB beams, the inertia effect can be neglected and only the rate effect needs to be taken into account if the loading rate does not exceed 2 mm/s. Otherwise, if the loading rate exceeds 500 mm/s, only the inertia effect needs to be considered. In addition, the effect of damping on dynamic responses is investigated, and it is found that the damping effect can be ignored at high loading rates.

Contribution of Stable Crack Extension to Variable Amplitude Fatigue Crack Growth

Abstract Variable-amplitude fatigue is invariably associated with periodic tensile overloads that can induce static crack extension component in addition to fatigue crack growth. A simple equation is proposed to account for the static crack growth component. The procedure is validated against fractographic evidence from a programmed load sequence. Accounting for static crack extension is likely to improve cycle-by-cycle crack growth modeling and may also explain scale effects.

Particle Discontinuous Deformation Analysis of Static and Dynamic Crack Propagation in Brittle Material

Particle Discontinuous Deformation Analysis of Static and Dynamic Crack Propagation in Brittle Material

Parallel parametric analysis approach based on an s-version FEM for fracture mechanics analysis in designing hole positions

This study proposes a parallel parametric analysis approach, in which parametric analysis for a local shape, such as a hole, is automated and parallelized. For the automation, the present approach is based on an s-version FEM, namely, the coupling-matrix-free iterative s-version FEM. In this method, the hole can be detached from the mesh of the global structure. This feature overcomes a difficulty in generating a mesh with a hole, particularly together with a crack for fracture mechanics analysis. Moreover, the parallelization is implemented by Message Passing Interface (MPI). In the present approach, a large number of analyses for different hole positions are performed in parallel. In each analysis, the hole position is changed using the s-version FEM. Then, this approach was applied to numerical examples of stationary crack problems, namely, a plate with an edge crack and a V-bending die with a circular surface crack. Both examples had a hole, the position of which was the parameter of the parametric analysis. In each numerical example, changes in stress intensity factor of the crack for different hole positions were investigated. It was numerically demonstrated that some hole positions decreased the stress intensity factor of the crack. Moreover, our approach achieved high speedup and parallel efficiency in a strong scaling test.

Numerical Modelling of Drying Induced Cracks in Wood Discs Using the Extended Finite Element Method

Drying crack is a common phenomenon occurring during moisture discharge from wood, reducing efficient wood utilization. Drying crack is primarily caused by drying stress, and the reasonable methods for determining drying stress are sparse. In this study, the initiation and propagation of cracks during wood discs drying were simulated using the extended finite element method (XFEM). The distribution of drying stress and displacement was analyzed at different crack conditions based on the simulation results. This study aimed to solve the problem of the limitation of drying stress testing methods and provide a new idea for the study of wood drying stress. The numerical simulation results are found in good agreement with the experimental results, thus corroborating the feasibility of XFEM in modeling drying crack of wood discs. The stress concentration was observed at the crack tip region, while a minor stress was presented in the region of crack passing through, indicating that the crack formation process was also a process of releasing drying stress. Further, more energy was required to form double cracks in comparison with the single crack mode.

Effect of Process Parameters and Nugget Growth Rate on Liquid Metal Embrittlement (LME) Cracking in the Resistance Spot Welding of Zinc-Coated Steels

Although the influence of weld process variables on LME cracking was known to be significant, limited studies have been conducted on the effect of process variables with systematic approaches in the equivalent nugget growth behavior and heat input. This study aimed to identify the effect of weld process variables on LME sensitivity with the equivalent nugget diameter and underlying mechanism with induced tensile stress for cracking. Among the welds with equivalent nugget diameters in the combination of different welding current and time, higher LME sensitivity was observed with the high welding current and short welding time combination than that with the low welding current and long welding time combination for the equivalent nugget diameter. Because a high current and short time combination resulted in faster weld nugget growth than the low welding current and long welding time combination, it rapidly increased the surface temperature along with the cooling from the electrode. These combined effects induced a higher thermal gradient and thermally induced tensile stress on the weld surface, satisfying the conditions of the LME cracking. The simulation results also confirmed that the critical weld cycle time of the LME cracking (<i>t<sub>c</sub></i>), which is the cross point between the nugget growth diameter and a contact diameter of the electrode, could be different with the combination of the weld process variables with the equivalent weld nugget size. Therefore, <i>tc</i> can be applied for the sensitivity index of LME cracking of the resistance spot weldment considering complex weld variables.