Finite Element Crack Research Articles

Stochastic finite element based reliability analysis of steel fiber reinforced concrete (SFRC) corbels

In this study, reliability analyses of steel fiber reinforced concrete (SFRC) corbels based on stochastic finite element were performed for the first time in literature. Prior to stochastic finite element analysis, an experimental database of 84 sfrc corbels was gathered from literature. These sfrc corbels were modeled by a special finite element program. Results of experimental studies and finite element analysis were compared and found to be very close to each other. Furthermore experimental crack patterns of corbel were compared with finite element crack patterns and were observed to be quite similar. After verification of the finite element models, stochastic finite element analyses were implemented by a specialized finite element module. As a result of stochastic finite element analysis, appropriate probability distribution functions (PDF's) were proposed. Finally, coefficient of variation, bias and strength reduction (resistance) factors were proposed for sfrc corbels as a consequence of stochastic based reliability analysis.

Stochastic finite element based reliability analysis of steel fiber reinforced concrete (SFRC) corbels

In this study, reliability analyses of steel fiber reinforced concrete (SFRC) corbels based on stochastic finite element were performed for the first time in literature. Prior to stochastic finite element analysis, an experimental database of 84 sfrc corbels was gathered from literature. These sfrc corbels were modeled by a special finite element program. Results of experimental studies and finite element analysis were compared and found to be very close to each other. Furthermore experimental crack patterns of corbel were compared with finite element crack patterns and were observed to be quite similar. After verification of the finite element models, stochastic finite element analyses were implemented by a specialized finite element module. As a result of stochastic finite element analysis, appropriate probability distribution functions (PDF\'s) were proposed. Finally, coefficient of variation, bias and strength reduction (resistance) factors were proposed for sfrc corbels as a consequence of stochastic based reliability analysis.

A new two-dimensional cracked finite element for fracture mechanics

A novel finite element with a central crack is proposed for fracture applications. The introduced element is applicable in 2D problems. The crack is modeled by an additional flexibility matrix based on fracture mechanics. Among the topics discussed in this paper, the deficiencies of previous works related to cracked elements are modified. Furthermore, a method for calculating stress intensity factor (SIF) by applying cracked elements is suggested. To show the capability of the suggested element and proposed procedure, several numerical samples are performed.

Finite element crack width computations with a thermo-hygro-mechanical-hydration model for concrete structures

The paper presents an overview of a finite element approach for the analysis of the thermo-hygro-mechanical-hydration behaviour of concrete structures. The thermo-hygro component considers the mass balance equation of moisture as well as the enthalpy balance equation, and uses two primary variables, namely the capillary pressure and temperature. Heat of hydration is simulated using the approach of Schindler and Folliard. The basic mechanical model simulates directional cracking, rough crack closure and crushing using a plastic-damage-contact approach. Hydration dependency is introduced into the mechanical constitutive model. The material data from the Concrack benchmark are considered with the model. This includes data on adiabatic temperature changes during curing, changing elastic properties during curing, shrinkage and creep. The model, as implemented in the finite element program LUSAS, is used to analyse the Concrack benchmark beam RL1. Particular attention is paid to crack openings and the difference between predicted crack openings from analyses with and without time-dependent effects. It is concluded that ignoring time-dependent effects can result in a significant underestimate of crack openings in the working load range.

Finite element crack propagation analysis of shear loaded buckling panels

Purpose – In former work, test results of cracks in aluminium panels under cyclic shear buckling showed that cracks in the tensile stress field of a buckle propagate. The main influencing factor for the crack growth rate is the maximum principle stress. A simplified approach for crack propagation analyses based on this finding showed limitations for application on larger cracks because it disregarded the increasing out-of-plane deformation for larger cracks as well as stress redistributions. The purpose of this paper is to improve the results of the simplified approach with the help of finite element method (FEM). Design/methodology/approach – An approach for crack propagation based on FEM is presented taking into account the mutual interaction of cracks and buckling. The finite element (FE) model, which is described in detail, respects the boundary conditions of the test-set-up. Different initial crack positions, loads and panel thicknesses are analyzed. Results of the stress intensity factors KI calculated by the ABAQUS® FE model provide a function which is used to run a crack propagation analysis based on Forman law. Findings – The results of the FE-based crack propagation solution are in good agreement with test results and improve the prediction of the simplified approach. It is not restricted in terms of panel thickness, crack position or applied shear load. Research limitations/implications – Limitations of the FE-based crack propagation solution compared to the experimental results are discussed. These are, the sensitivity of crack propagation analyses to initial crack length and deviations of the experimental settings from the ideal FE model. Originality/value – The interaction of cracks and buckling in aluminium shells is mainly disregarded both in research and industrial work, but can be of interest considering, accidental damages in fuselage side shells. Cracks propagate under shear load as it was shown in former work. The FE modeling of the tests presented in this paper proves the mutual interactions of crack propagation and buckling deformation.

Edge-crack diagnosis using improved two-dimensional cracked finite element and micro genetic algorithm

In this paper, a crack diagnosis method based on an improved two-dimensional (2-D) finite element (FE) with an embedded edge crack, and micro genetic algorithm (μ-GA) is proposed. The crack is not physically modelled within the element, but instead, its influence on the local flexibility of the structure is accounted for by the reduction of the element stiffness as a function of the crack length. The components of the stiffness matrix for the cracked element are determined from the Castigliano’s first principle. The element was implemented in the commercial FE code ABAQUS as a user element subroutine. The accuracy of the proposed improved cracked element is verified by comparing the predicted first natural frequency with the available experimental data. Subsequently, a methodology to detect the crack location and size in conjunction with the proposed improved cracked element is formulated as an optimisation problem, and μ-GA is used to find the optimal location and depth by minimising the cost function ba...

TBC bond coat–top coat interface roughness: Influence on fatigue life and modelling aspects

Thermal barrier coatings (TBCs), when used in gas turbines, may fail through thermal fatigue, causing the ceramic top coat to spall off the metallic bond coat. The life prediction of TBCs often involves finite element modelling of the stress field close to the bond coat/top coat interface and thus relies on accurate modelling of the interface. The present research studies the influence of bond coat/top coat interface roughness on the thermal fatigue life of plasma sprayed TBCs. By using different spraying parameters, specimens with varying interface roughnesses were obtained. During thermal cycling it was found that higher interface roughness promoted longer thermal fatigue life. The interfaces were characterised by roughness parameters, such as Ra, Rq and R∆q, as well as by autocorrelation, material ratio curves and slope distribution. The variation of spray parameters was found to affect amplitude parameters, such as Ra, but not spacing parameters, such as RSm. Three different interface geometries were tried for finite element crack growth simulation: cosine, ellipse and triangular shapes. The cosine model was found to be an appropriate interface model and a procedure for obtaining the necessary parameters, amplitude and wavelength, was suggested. The positive effect of high roughness on life was suggested to be due to a shift from predominantly interface failure, for low roughness, to predominantly top coat failure, for high roughness.

Influence of substrate material on the life of atmospheric plasma sprayed thermal barrier coatings

Thermal barrier coatings (TBCs) are used in gas turbines to prolong the life of the underlying substrates and to increase the efficiency of the turbines by enabling higher combustion temperatures. TBCs may fail during service due to thermal fatigue or through the formation of non-protective thermally grown oxides (TGOs). This study compares two atmospheric plasma sprayed (APS) TBC systems comprising of two identical TBCs deposited on two different substrates (Haynes 230 and Hastelloy X). The thermal fatigue life was found to differ between the two TBC systems. The interdiffusion of substrate elements into the coating was more pronounced in the TBC system with shorter life, however, very few of the substrate elements (only Mn and to some extent Fe) formed oxides in the bond coat/top coat interface. Fractography revealed no differences in the fracture behaviour of the TBCs; the fracture occurred, in both cases, to about 60% in the top coat close to the interface and the remainder in the interface. Nanoindentation revealed only small differences in mechanical properties between the TBC systems and a finite element crack growth analysis showed that such small differences did not cause any significant change in the crack driving force. The oxidation kinetics was found to be similar for both TBC systems for the formation of Al2O3 but differed for the kinetics of non-Al2O3 TGOs where the TBC system with shortest life had a faster formation of non-Al2O3 TGOs caused by a faster Al depletion. The difference in non-Al2O3 TGO growth kinetics was considered to be the main reason for the difference in life.

IDENTIFICATION OF MULTI-CRACKS IN THE GATE ROTOR SHAFT BASED ON THE WAVELET FINITE ELEMENT METHOD

In order to have fault identification for the gate rotor shaft with multiple cracks effectively, the quantity identification method of the gate rotor shaft with multiple cracks based on wavelet finite element method is established and the frequency test of the gate rotor shaft based on blind source separation algorithm is analyzed. Firstly the wavelet finite element was established by combining the Daubechies wavelet theory and the traditional finite element theory and then the vibration equations of intact and cracked wavelet finite element were obtained, which were applied to compute the changing rate of the natural frequencies for the cracked gate rotor shaft. Then natural frequencies fitting curve surface of the cracked gate rotor shaft with were acquired based on the wavelet finite element method and the identification procedure of the cracks was confirmed and the gate rotor shaft with two different cracks was analyzed and the results showed that the wavelet finite element could predict the multi-cracks correctly.

Damage evolution in delaminated woven fabric E-glass/epoxy composite plates under transverse static patch loading

Delaminations present in a laminated composite structure can grow under different loading conditions, thus further weakening the structure. Along with this, initiation and propagation of damages within the layers can take place because of in-plane stresses. Studies have been presented on damage evolution in delaminated woven fabric composite plates with initial delamination under pressure loading, normal to the plane of the plate, distributed over a small area. Delamination propagation has been studied using crack closure integral technique and finite element analysis. Initiation of failure of layers because of in-plane stresses has been studied using maximum stress failure criterion. Size and shape of the damage has been evaluated using the method presented and compared with the experimental results available in the literature.

Modelling of corrosion-induced cover cracking in reinforced concrete by an embedded cohesive crack finite element

Corrosion of a reinforcement bar leads to expansive pressure on the surrounding concrete that provokes internal cracking and, eventually, spalling and delamination. Here, an embedded cohesive crack 2D finite element is applied for simulating the cracking process. In addition, four simplified analytical models are introduced for comparative purposes.Under some assumptions about rust properties, corrosion rate, and particularly, the accommodation of oxide products within the open cracks generated in the process, the proposed FE model is able to estimate time to surface cracking quite accurately. Moreover, emerging cracking patterns are in reasonably good agreement with expectations.As a practical case, a prototype application of the model to an actual bridge deck is reported.

1913 原子炉機器溶接部の自動き裂進展解析システムの開発(OS19-3.破壊力学とき裂の解析・き裂進展シミュレーション(3),OS・一般セッション講演)

1913 原子炉機器溶接部の自動き裂進展解析システムの開発(OS19-3.破壊力学とき裂の解析・き裂進展シミュレーション(3),OS・一般セッション講演)

A three-dimensional edge-crack finite element for fracture mechanics applications

A three-dimensional extension of a previously published two-dimensional cracked finite element [Potirniche, G.P., Hearndon, J., Daniewicz, S.R., Parker, D., Cuevas, P., Wang, P.T., Horstemeyer, M.F., 2008. A two-dimensional damaged finite element for fracture applications. Engineering Fracture Mechanics 17(13), 3895–3908] is presented in this paper. The new element has an embedded edge crack, and was developed to model damage in three-dimensional structures using the finite element method. The element simulates the presence of a crack without physically inserting it in the three-dimensional finite element mesh. The method involves the derivation of a modified stiffness matrix that accounts for the change in the element flexibility due to the crack presence. The cracked element was analytically formulated and implemented in the finite element code ABAQUS Standard as a User-defined Element (UEL) subroutine. Tests of various cracked beam configurations were used to estimate the accuracy of the element by comparing two models: one with a UEL and another with an embedded edge crack. Beam deflections and natural frequencies were analyzed and compared for the two models. The results indicate that the new element has a good potential in modeling cracks in three-dimensional parts. Moreover, the method using this UEL computes the global response of damaged structures, in which cracks can be placed at various locations and in an unlimited number.

三次元き裂進展自動解析システムの構築 : 第2報,き裂進展解析システムとき裂進用有限要素法モデル生成

三次元き裂進展自動解析システムの構築 : 第2報,き裂進展解析システムとき裂進用有限要素法モデル生成

Probabilistic Upscaling of Material Failure Using Random Field Models – A Preliminary Investigation

Complexity of failure is reflected from sensitivity of strength to small defects and wide scatter of macroscopic behaviors. In engineering practices, spatial information of materials at fine scales can only be partially measurable. Random field (RF) models are important to address the uncertainty in spatial distribution. To transform a RF of micro-cracks into failure probability at full structural-scale crossing a number of length scales, the operator representing physics laws need be implemented in a multiscale framework, and to be realized in a stochastic setting. Multiscale stochastic modeling of materials is emerging as a new methodology at this research frontier, which provides a new multiscale thinking by upscaling fine-scale RFs. In this study, a preliminary framework of probabilistic upscaling is presented for bottom-up hierarchical modeling of failure propagation across micro-meso-macro scales. In the micro-to-meso process, the strength of stochastic representative volume element (SRVE) is probabilistically assessed by using a lattice model. A mixed Weibull-Gaussian distribution is proposed to characterize the statistical strength of SRVE, which can be used as input for the subsequent meso-to-macro upscaling process using smeared crack finite element analysis.

A numerical study of the size effect in fracture mechanical bending tests with the cohesive zone method

A finite element crack growth model is presented which is based on the cohesive zone method and which handles crack initiation and crack growth with a prescribed traction–separation law. In this two parameter approach, the fracture behaviour is determined by the cohesive strength and the separation energy. The model is applied to simulate three-point bending tests of unirradiated reduced-activation ferritic–martensitic steels. The specimen size is varied from nearly standard to miniaturized dimensions to investigate its influence on the crack resistance curve. Additionally, the effect of side grooving on crack initiation and the early stage of crack growth is studied with the presented numerical method.

Numerical simulation of cracking in reinforced concrete members by an embedded model

PurposeThe purpose of this paper is to describe the development of an embedded crack finite element (FE) model for reinforced concrete (RC) structures, including a bond‐slip methodology to take into consideration the steel contribution in the rupture process, capable of capturing the global behavior of the structure as well as details of cracking phenomenon.Design/methodology/approachThe reinforcement contribution is added in the equilibrium at element level in an embedded crack FE model, based on displacement localization lines inside the elements.FindingsThe model is able to determine the steel stress in the crack besides the volumetric average steel stress. It is shown that the steel stress in the crack can be considerable greater than the average value. Other important aspect detected is the contribution of the concrete softening in the steel stress in the crack and in the overall behavior. The number, the distribution and the opening of cracks can be estimated too.Practical implicationsThe yield of the steel in the cracking process can be detected more precisely by this methodology, allowing a better design and understanding of RC structures. In addition, the knowledge of crack openings is an important information to predict corrosion and other degradation phenomena of the reinforcement bars.Originality/valueThe bond‐slip procedure is linked with the embedded crack model in an original way: sliding gives the crack width. Moreover, the inclusion of steel forces in the crack equilibrium balance was not a usual procedure and permits an understanding of reinforcement effect in both levels (macro and micro) studied in this work.

Finite element crack propagation of adhesively bonded repaired panels in general mixed-mode conditions

In order to simplify the analyses, in the previously developed mixed-mode fracture analyses of the single-side repaired panels with composite patches, the effect of mode-III was neglected. In those analyses, it was also assumed that the crack-front remains perpendicular to the panel's surfaces during the crack propagation which is in defeat with the experimental observations. In this paper, finite element analyses are performed to investigate the crack propagation of single-side repaired panels using composite patches containing an initial central inclined crack. In these analyses the general mixed-mode fracture conditions of mode I, II and III and the real three-dimensional crack-front are considered in the finite element crack growth modeling. For this purpose, Modified Virtual Crack Closure Technique (MVCCT) is used to calculate the three strain energy release rate components corresponding to various fracture modes at the crack-front. The obtained crack-front shapes at various crack growth steps, crack trajectories at both patched and un-patched sides and life of the repaired panels with various patch lay-ups are discussed.

Effect of thickness on fracture criterion in general yielding fracture mechanics

In this research work, the effect of thickness on fracture criterion is studied for extra deep drawn (EDD) steel sheets. Experimental results are generated on fracture toughness of EDD steel sheets using compact tension specimens and a ‘maximum load’ as a fracture criterion. Critical crack tip opening displacement (CTOD) is found with the help of three methods: plastic hinge model (PHM), crack flank opening angle (CFOA) and finite element model (FEM). The fracture toughness is found to increase with increase in thickness of specimens. The fracture behaviour exhibited characteristics of general yielding fracture mechanics.

Probabilistic Fracture Mechanics Analysis for Optimization of High-Pressure Vessel Inspection

The use of High-pressure Vessel in eco-friendly Natural Gas Vehicles (NGV) is technologically feasible nowadays. Common applications of High-pressure Vessel are to carry Compressed Natural Gas (CNG), hydrogen for fuel-cell vehicle, and high-compression air in the new air-car technology. High-pressure Vessel is subjected to extreme compression-decompression cycles that could cause fatigue failure. Therefore, vessel shall be inspected regularly to detect if there is crack inside. The objective of this paper is to optimize the inspection interval of CNG Highpressure Vessel by means of Probabilistic Fracture Mechanics Analysis. Vessel is made of highalloy steel and assumed to have distributed elliptical cracks. Three length-to-depth crack ratios (a/c), i.e. 3, 8, and 15, are simulated. Crack is assumed to propagate in fixed ratio. Stress Intensity Factors at each crack tip are calculated by Finite Element Analysis and Crack Closure Technique. Fatigue crack growth is simulated by Cycle-by-Cycle Integration Technique. The Fracture Mechanics Analysis is then expanded to probabilistic analysis by considering stochastic nature of analysis parameters. Probability of failure is computed by Guided Direct Simulation Method using software which is specially written for this project [1]. Based on simulation result, High-pressure Vessel is recommended to be inspected every 3 years.