Coalescence Of Multiple Cracks Research Articles

Simulation of thermal barrier coating spallation induced by the initiation/growth/coalescence of internal crack and interfacial crack based on a real image model

In the furnace cycle test, the growth of oxide film leads to the propagation and coalescence of multiple cracks near the interface, which should be responsible for the spallation of thermal barrier coatings (TBCs). A TBC model with real interface morphology is created, and the near-interface large pore is retained. The purpose of this work is to clarify the mechanism of TBC spallation caused by successive initiation, propagation, and linkage of cracks near the interface during thermal cycle. The dynamic growth of thermally grown oxide (TGO) is carried out by applying a stress-free strain. The crack nucleation and arbitrary path propagation in YSZ and TGO are simulated by the extended finite element method (XFEM). The debonding along the YSZ/TGO/BC interface is evaluated using a surface-based cohesive behavior. The large-scale pore in YSZ near the interface can initiate a new crack. The ceramic crack can propagate to the YSZ/TGO interface, which will accelerate the interfacial damage and debonding. For the TGO/BC interface, the normal compressive stress and small shear stress at the valley hinder the further crack propagation. The growth of YSZ crack and the formation of through-TGO crack are the main causes of TBC delamination. The accelerated BC oxidation increases the lateral growth strain of TGO, which will promote crack propagation and coalescence. The optimization design proposed in this work can provide another option for developing TBC with high durability.

Modeling the entire progressive failure process of rock slopes using a strength-based criterion

To simulate the entire process of the progressive failure of rock slopes, a series of techniques are incorporated into the original NMM (numerical manifold method). To reflect the stress concentration near the crack tip, the Williams' displacement series is adopted to enrich the global displacement function of the NMM. Furthermore, the most recently proposed strength-based LT criterion, which can account for tensile cracks, tensile-shear cracks and compressive-shear cracks, is adopted to determine the crack propagation direction and length. Three typical numerical examples, including a Mode-I crack problem, a Mode-II crack problem and a Brazilian disc problem are adopted to verify the numerical model. The numerical results indicate that the numerical model is capable of accurately simulating the Mode-I crack propagation problem, the mode-II crack propagation problem and the failure process of Brazilian disc. Furthermore, the numerical model is adopted to investigate the entire progressive failure process of two rock slopes. The corresponding results indicate that the numerical model can not only simulate the propagation and coalescence of multiple cracks in rock masses but also the opening/sliding of rock blocks along discontinuities. The proposed numerical model warrants further investigation.

Modeling of structural failure of Zircaloy claddings induced by multiple hydride cracks

Zirconium alloys have been serving as primary structural materials for nuclear fuel claddings. Structural failure analysis under extreme conditions is critical to the assessment of the performance and safety of nuclear fuel claddings. This work focuses on simulating structural failure of Zircaloy tubes with multiple hydride defects through modeling explicit crack propagation in ductile media. First, we developed an integrated cladding failure model by taking into account both crack initiation induced by hydride/matrix interface separation and ligament tearing-off between activated hydride cracks. Second, to accommodate the initiation, propagation, and coalescence of multiple cracks in finite plastic media we incorporated this structural failure model into a coupled continuous/discontinuous Galerkin (DG) based finite element code, a traditionally preferred implicit numerical framework. Third, to improve the adaptive placement of DG interface elements for crack propagation and to identify potential coalescence of cracks due to the interaction between adjacent hydride cracks, we defined a special failure index for the assessment of potential failure zones using both true plastic strain developed and predicted failure strain based on the Johnson–Cook material failure criterion. Finally, by calibrating the proposed material failure model using a cluster of Zircaloy material experimental tests, we successfully simulated a complete failure process of a fuel cladding tube with multiple hydride cracks.

Prediction of fatigue life in aluminium alloy (AA7050-T7451) structures in the presence of multiple artificial short cracks

Although methods for predicting the growth and coalescence of cracks have been introduced, uncertainties persist in structures with multiple short cracks. Quantitative fractographic methods were used to study the way in which multiple short cracks in AA7050-T7451 of varying sizes on the same and on different planes grew under low amplitude cyclic loading. The accuracy of existing theoretical methods for predicting the growth and coalescence of multiple cracks was evaluated. Experimental results corroborate the findings that growth of the ‘largest’ crack dominated the fatigue life of a specimen with very little influence from relatively shorter cracks.

A continuous-discontinuous cellular automaton method for cracks growth and coalescence in brittle material

A method of continuous-discontinuous cellular automaton for modeling the growth and coalescence of multiple cracks in brittle material is presented. The method uses the level set to track arbitrary discontinuities, and calculation grids are independent of the discontinuities and no remeshing are required with the crack growing. Based on Griffith fracture theory and Mohr-Coulumb criterion, a mixed fracture criterion for multiple cracks growth in brittle material is proposed. The method treats the junction and coalescence of multiple cracks, and junction criterion and coalescence criterion for brittle material are given, too. Besides, in order to overcome the tracking error in the level set approximation for crack junction and coalescence, a dichotomy searching algorithm is proposed. Introduced the above theories into continuous-discontinuous cellular automaton, the present method can be applied to solving multiple crack growth in brittle material, and only cell stiffness is needed and no assembled global stiffness is needed. Some numerical examples are given to shown that the present method is efficient and accurate for crack junction, coalescence and percolation problems. Open image in new window

Multiple Cracks Interaction Model under Creep Conditions

Multiple flaws are often found in many high-temperature structural components. The interaction and coalescence of multiple cracks may significantly influence the service lives of these high-temperature components. In this paper, the interaction effect on multiple cracks is investigated under creep conditions. At the same time, interaction is considered during crack growth calculation by including the appropriate magnification factors in the calculations of creep fracture parameters. Based on these analyses, the assessment method of high-temperature structure containing multiple flaws is developed.

Experimental and Numerical Research on 3D Crack Growth in Rocklike Material Subjected to Uniaxial Tension

AbstractUniaxial tension experiments on precracked rocklike samples and the related numerical simulations using FRANC3D were performed to investigate the growth mechanism of three-dimensional (3D) cracks in rocks under tension as well as the strength and failure behavior of samples. Experimental results indicate that the geometric characteristics of preexisting cracks, including crack dip angle, crack spacing, and crack intensity, have significant effects on the strength and failure modes of the samples. Failure of samples resulted from the propagation of a single crack and the coalescence of multiple cracks if the spacing between adjacent cracks is less than the length of the cracks. Distribution of mixed-mode stress-intensity factors (SIFs) and energy-release rates (ERRs) along the fronts of cracks with different geometric characteristics was investigated numerically, and the results can provide an interpretation for the experimental results. Based on numerical simulations of the propagation processes o...

Multiscale numerical modeling of propagation and coalescence of multiple cracks in rock masses

A new multiscale numerical model in which the coordination between the internal boundary (such as cracks and holes) and meshes is not necessary for simulating the damage evolution of crack-weakened rock masses is proposed based on the extended finite element method and global–local analysis. The connection between different sizes of meshes behaves smoothly through combining the displacement projecting method and displacement loading method. The contact constraint on crack surfaces is embedded within the total stiffness matrix using the penalty method. The path of crack propagation and stress fields are determined through iterative computations. Because additional discontinuous functions and enriched tip elements are added into the displacement field, the geometry of cracks is independent of the finite element mesh. As a result, re-meshing is not necessary to model crack propagation using the present new model. As the frictional contact of crack surfaces is taken into account, the present method is suitable for modeling the growth and coalescence of multiple cracks in geomaterials under compressive loads as well as tensile loads. Finally, the present method is employed to simulate multiple cracks growth when frictional contact exists on the crack surfaces. Comparison between the numerical and experimental results shows that the present numerical results are in good agreement with the experimental ones.

The simulation of dynamic crack propagation using the cohesive segments method

The cohesive segments method is a finite element framework that allows for the simulation of the nucleation, growth and coalescence of multiple cracks in solids. In this framework, cracks are introduced as jumps in the displacement field by employing the partition of unity property of finite element shape functions. The magnitude of these jumps are governed by cohesive constitutive relations. In this paper, the cohesive segments method is extended for the simulation of fast crack propagation in brittle solids. The performance of the method is demonstrated in several examples involving crack growth in linear elastic solids under plane stress conditions: tensile loading of a block; shear loading of a block and crack growth along and near a bi-material interface.

Fatigue Crack Propagation Life of Welded Joints Related to the Statistical Characteristics of Multiple Surface Cracks

Multiple collinear surface cracks distributed that are randomly along a weld toe have a strong influence on the fatigue crack propagation life of welded joints. This issue is investigated using statistical approaches based on series of systematic experiments, in which initial crack numbers, their locations and crack sizes, i.e. depth and length, are taken into account. The number of initial cracks follow a normal distribution, and the probability of initial crack depths and lengths can be accurately described by the Weibull distribution. These characteristics are used to calculate the fatigue crack propagation life, in which the mechanisms of the mutual interaction and the coalescence of multiple cracks are considered as well as the Mk-factors. The automatic calculation of fatigue crack propagation life is achieved by the application of NESUSS, where parameters such as the number, location and size of cracks are all treated as random variables. The random variables are dealt through a Monte-Carlo simulation with sampling random numbers of 2,000. The results of the simulation provide the statistical characteristics of the fatigue crack propagation life for welded joints as a function of the number of initial cracks. The sum of the simulation results and the fatigue crack initiation life referred from a previous paper is in good agreement with the experimental results.

Crack initiation model for sensitized 304 stainless steel in high temperature water

To investigate the initiation behavior of stress corrosion cracking (SCC) for sensitized Type 304 stainless steel in high temperature water, a constant load SCC test method combined with in situ crack observation technique was employed. The in situ crack observation system allowed us to detect small cracks of at least 100 μm. As a result, a fracture time decreased with an increase in an applied stress. The first cracks were observed at most 3 h before the specimen was fractured under all the stress conditions. After that, many cracks were initiated in a short time to fracture. The fracture was caused by coalescence of multiple cracks rather than by growth of some primary cracks. The simulation model for surface crack initiation was newly developed using a Monte Carlo method, which was based on damage mechanics and stress analysis around the existing cracks. The simulation could represent the empirical results of changes in the crack distribution and the cumulative number of cracks during the SCC tests. It was concluded, therefore, that the crack initiation process should be considered in simulating the life prediction of the material in this SCC system.

Effects of particle size on fatigue crack initiation and small crack growth in SiC particulate-reinforced aluminium alloy composites

Fatigue crack initiation and small crack growth were studied under axial loading using powder metallurgy 2024 aluminum-matrix composites reinforced with SiC particles of three different sizes of 5, 20 and 60 μm. The 5 and 20 μm SiC p/Al composites exhibited nearly the same fatigue strength as the unreinforced alloy, while the 60 μm SiC p/Al composite showed a significantly lower fatigue strength due to its inferior crack initiation resistance that could be attributed to interface debonding between particles and the matrix. Small crack growth behaviour was different depending on stress level. At a low applied stress, the addition of SiC particles enhanced the growth resistance, particularly in the composites reinforced with coarser particles, while at a high applied stress, the 60 μm SiC p/Al composite showed a considerably low growth resistance, which could be attributed to interaction and coalescence of multiple cracks. In the 5 μm SiC p/Al composite, small cracks grew avoiding particles and thus few particles appearing on the fracture surfaces were seen, particularly in small crack size region. In the 20 and 60 μm SiC p/Al composites, they grew along interfaces between particles and the matrix and the number of particles appearing on the fracture surfaces increased with increasing crack size or maximum stress intensity factor.

Investigating the Interaction Behavior Between Two Arbitrarily Oriented Surface Cracks Using Multilevel Substructuring

The existence of arbitrarily oriented multiple cracks is a common problem in brittle materials. Some of these materials, such as ceramics, are used in mechanical and aerospace structures that suffer from aging. Because of that, such structures have shown some signs of sudden partial or total failure. The interaction and coalescence of multiple cracks may significantly affect the designed lives of aging structures. Knowledge of the growth behavior of interacting cracks is still limited. In this paper, a novel submodeling meshing algorithm is used to construct different cases of arbitrarily oriented identical surface cracks in a plate subjected to remote tension. These cases are solved using finite element analysis (FEA) and covered a wide range of crack geometries. The stress intensity factors (SIFs) and the energy release rates (G) for these cracks are calculated as a function of their relative orientation and the position along the interaction crack-front. In this paper, the studied ratio of crack depth to plate thickness, a/t, and to crack length, a/c, are kept at 0.2 and 0.3, respectively. Where possible, a comparison of the 3-D results with 2-D ones is also considered.

Investigating the Effect of Crack Shape on the Interaction Behavior of Noncoplanar Surface Cracks Using Finite Element Analysis

In the last two decades, multiple cracks are often found in aging aerospace and mechanical structures. The interaction and coalescence of multiple cracks may significantly affect the service lives of these aging structures. Knowledge of the behavior of interacting cracks is still limited. The calculation of the crack-tip stress intensity factor, SIF, along the interacting crack fronts is considered a major contribution for the application of any linear fracture mechanics model to investigate the growth life of these cracks. In this paper, a parametric study is presented for two parallel surface cracks in an infinite plate subjected to remote tension or to pure bending loads. This study focuses on constructing a finite element (FE) model that combines the submodeling technique with its ability to generate crack submodels of different lengths and depths, and a mesh generator that can build up a mesh grid based on the size, depth, and orientation of the interacting crack sub-models. The stress intensity factors for these cracks are calculated as a function of the crack front position, depth, shape, and plate thickness. In this paper, the values of the studied crack depth to length ratio, a/c, are 0.33, 0.5, 0.67, and 1.0. Where possible, a comparison of the 3-D with 2-D results is also considered.

Fatigue Life Prediction of Welded Joints for Offshore Structures Under Variable Amplitude Loading

This paper describes the development of a fracture mechanics model for the prediction of fatigue life of welded joints subjected to variable amplitude loading. The prediction program is based on the integration of the Paris equation. A novel feature of this program are relationships which account for the development, growth and coalescence of multiple cracks that occur at the weld toe. These relationships or forcing functions were developed from extensive monitoring of the initiation and crack shape development of multiple cracks during experimental tests. Data is also presented on a series of variable amplitude tests performed on welded T-plate joints loaded in 3-point bending using the common load sequence (COLOS) at a stress ratio of R=−1. These results are presented in terms of effective and root mean square stress ranges. The experimental data are used to validate the life prediction results from the growth model. A discussion is also included on the proportion of the compressive stress range to be included as an effective stress range in the prediction model calculations.

Fatigue and fracture behavior of straight pipe with flaws in inner surface

Fatigue and fracture tests of piping models with flaws in the inner surface were carried out to investigate the fatigue crack growth, coalescence of multiple cracks and fracture behavior. Two straight test pipes with and without weldment in the test section of AISI type 304L stainless steel were tested under almost the same test conditions by imposing moment loads. Three artificial defects were machined in the inner surface of the test section of the test pipes and the fatigue test was performed until the cracks coalesced and grew through the thickness. Subsequently, a static load was imposed on the test pipe which contained a large crack in the test section. The fatigue test results are compared with an analytical crack growth behavior predicted by the method described in the Section XI of ASME Code, and show slower crack growth than that of the prediction. From the fracture test results, it is found that the test pipes can endure considerably high load.