Predict Crack Initiation Research Articles

Verification and Validation of a 2D energy based peridynamic state‐based failure criterion

AbstractThe paper presents a validation approach for an enhanced energy‐based failure model for state‐based peridynamics. Model robustness and validity of predicted crack initiation and propagation is demonstrated for a double cantilever beam. Tensile tests using the same material and energy release rate from the prior simulations are used to predict specimen failure. The model is implemented in the software Peridigm and will be published open‐source.

Design of an interpretable Convolutional Neural Network for stress concentration prediction in rough surfaces

We present the application of a Convolutional Neural Network (CNN) to relate stress concentrations to surface roughness. Stress concentrations at the low points of rough surfaces are one of the primary causes of fatigue crack initiation but there is no generally accepted method for analyzing rough surfaces to predict crack initiation. Synthetically generated rough surfaces, instantiated in a mechanical model allow for the simulation of stress concentrations, creating a database of surface images and corresponding mechanical data. In this work, the CNN is designed and trained to interpret a height map of a surface and, from that data, to predict the stress concentrations created by the surface. Using a simple architecture, the CNN achieved R2 = 0.75 in prediction for test images, i.e., those not used in training. This CNN can be adapted for experimental surfaces thus creating a new and straightforward tool for prediction of crack initiation. Considerable care was taken to minimize the complexity of the CNN architecture and to make it interpretable via viewports.

Analysis of mechanisms inducing corrosion cracking of irradiated austenitic steels and development of a model for prediction of crack initiation

The austenitic steels used for RPV internals of light water reactors (LWR) are considered. Mechanisms having a potential effect on irradiation assisted stress corrosion cracking (IASCC) of austenitic steels in the LWR environment have been analyzed. Based on the analysis and generalization of reference and original data on IASCC, an IASCC initiation criterion has been formulated. The nature of low-temperature creep of irradiated austenitic steels has been considered, constitutive equations have been derived. Relying on the formulated criterion of grain-boundary microcrack nucleation and the derived creep equations, an IASCC initiation model has been developed. The model allows one to predict the dependence of the threshold stress σthIASCC on neutron dose and also to calculate the IASCC initiation time with stresses exceeding σthIASCC.

Crack initiation characteristics of ring chain of heavy-duty scraper conveyor under time-varying loads

Crack initiation characteristics of ring chain of heavy-duty scraper conveyor under time-varying loads were investigated in this study. The dynamic tension of ring chain of the heavy-duty scraper conveyor was obtained using the time-varying dynamic analysis. Finite element analyses of three-dimensional contacts between adjacent chain rings at straight and bending segments were carried out to explore three-dimensional stress distributions of chain rings. The crack initiation life of chain ring was predicted employing the multiaxial fatigue theory. The results show that the ring chain is subjected to time-varying dynamic tensions during operation. During tension-tension contact fatigue, as compared to the straight segment of ring chain, the bending segment engaging in the sprocket presents an overall lower (or higher) equivalent stress distribution in the case of flat (or vertical) chain ring, respectively. Maximum equivalent stresses at the contact regions of adjacent chain rings both present time-varying dynamic characteristics similar to evolutions of dynamic tension. During tension-torsion contact fatigue, an increase in torsion angle level causes unobvious difference between equivalent stress distributions at contact regions of adjacent rings. Predicted crack initiation lives of chain rings during tension-tension contact fatigue indicates more severe fatigue damages of the vertical chain ring at the straight segment and of the flat chain ring at the right bending segment.

Extended finite element method and anisotropic damage plasticity for modelling crack propagation in concrete

In this article, a new algorithm is developed for predicting crack initiation and growth direction in 3D solid concrete. The algorithm combines the anisotropic damage-plasticity model used for simulation of concrete material degradation (crack) and irreversible plastic deformation and the eXtended Finite Element Method (X-FEM). The Voyiadjis local anisotropic damage-plasticity model is expressed in a non-local form to prevent strain and damage localization in damaged points. Also, a new crack propagation direction criterion based on second order damage tensor of anisotropic damage, containing different material degradation components in different directions is proposed for 3D problems. Computational algorithms with an elastic predictor and plastic and damage corrector returning map are also developed. Finally, three numerical examples are solved and results of crack propagation paths predicted by the proposed algorithm are compared with available experimental data and those of other numerical simulations. It is shown that the results from the proposed method are in better agreement with experimental data compared to other reported numerical methods.

Development of new critical plane model for assessment of fatigue life under multi-axial loading conditions

The present work is aimed at development of new critical plane model based on envelope of strain energy density as fatigue damage parameter. This model eliminates the use of hypotheses for evaluation of resultant shear. The fatigue life has been predicted and compared with test life for a large set of multi-axial test data available on SA 333 Gr. 6, 16MnR, S460N and 7075-T651 Al materials. Predicted fatigue life is in good agreement with test life. The predicted crack initiation orientations are also comparable with the measured angles.The shortcomings of existing models have been brought out.

Relevance of factor of safety based on number of cycles in the prediction of fatigue crack initiation as per A16 sigma-d approach

The focus of this paper is to bring out the conservatism present in the fast reactor design standard RCC MRx A16 in predicting fatigue crack initiation for a component that has a crack-like defect. Full-scale pipe bend test results were used for comparing the fatigue life predicted based on the RCC MRx A16 approach. A representative pipe bend of size 570 mm outer diameter and 15 mm thickness has been used for the purpose. The selected pipe bend is made up of austenitic stainless steel (SS 316 LN) and is provided with a surface notch 22.5 mm long and 2.1 mm in depth to simulate the potential flaw, which could be present in the piping system. Cyclic testing has been carried out to determine fatigue life. These tests were conducted in two stages. During the first stage, the finite-sized notch was sharpened into a crack-like defect allowing it to develop into a natural crack (85,150 cycles). In the second stage, the testing was continued to estimate the number of cycles required to advance the crack by > 50 μm and thereby compare prediction capability of the sigma-d approach given in the RCC MRx A16. The number of cycles required for crack incubation and advancement was obtained by periodic measurement of the measured readings obtained from the crack depth gauge. A comparison of the number of cycles was predicted by the RCC MRx A16 method, and the experimental results show that the A16 prediction was very conservative. Useful suggestions were proposed in this paper for improving the fatigue crack initiation prediction capability.

Numerical Analysis of Crack Initiation Direction in Quasi-brittle Materials: Effect of T-Stress

A two-dimensional finite element analysis was adopted to assess the effect of T-stress on predicting crack initiation angle in a quasi-brittle material. Asymmetric semicircular PMMA specimen containing a vertical edge crack subjected to three-point bending was employed. The specimen was assumed as an isotropic and homogeneous material. Relative crack length ratios of 0.3, 0.4, 0.5, 0.6 and 0.7 were examined. Several relative bottom span ratios were included to develop a wide range of mixed-mode I/II loading conditions. The conventional maximum tangential stress (MTS) criterion could not precisely predict the crack initiation angle through the total range of pure mode I to pure mode II. The generalized maximum tangential stress (GMTS) criterion showed a significant effect of T-stress on the numerical prediction of the crack initiation angles in PMMA specimens. In the present study, neglecting the T-stress in the MTS criterion overestimates the crack initiation angle. The numerical predictions using the GMTS criterion showed a good agreement with the relevant experimental data found in the literature. The ability of GMTS in predicting the crack initiation angle is improved by considering the T-stress.

Fatigue crack initiation prediction using phantom nodes-based extended finite element method for S355 and S690 steel grades

The assessment of fatigue crack initiation behavior of steel structures is essential and important especially to improve the application of high strength steel in construction. For a complete understanding of fatigue endurance, it is necessary to combine the phenomenological damage model with finite element numerical approach. In this paper, phantom nodes-based extended finite element method is used to predict the fatigue crack initiation of steel material, considering a prediction by XFEM of coupon tests made of steel grades S355 and S690. A user-defined fatigue damage initiation subroutine based on Smith, Watson, and Topper (SWT) damage model combined with non-linear isotropic/kinematic cyclic hardening model is implemented to predict fatigue crack initiation. The proposed method is successfully validated based on fatigue coupon test results of both steel grades, S355 and S690.

Numerical Simulation of Crack Propagation in Flexible Asphalt Pavements Based on Cohesive Zone Model Developed from Asphalt Mixtures.

To give engineers involved in planning and designing of asphalt pavements a more accurate prediction of crack initiation and propagation, theory-based models need to be developed to connect the loading conditions and fracture mechanisms present in laboratory tests and under traffic loading. The aim of this study is to develop a technical basis for the simulation of fracture behavior of asphalt pavements. The cohesive zone model (CZM) approach was applied in the commercial FE software ABAQUS to analyze crack propagation in asphalt layers. The CZM developed from the asphalt mixtures in this study can be used to simulate the fracture behavior of pavements and further optimize both the structure and the materials. The investigations demonstrated that the remaining service life of asphalt pavements under cyclic load after the initial onset of macro-cracks can be predicted. The developed CZM can, therefore, usefully supplement conventional design methods by improving the accuracy of the predicted stress states and by increasing the quality, efficiency, and safety of mechanical design methods by using this more realistic modeling approach.

Predicting crack initiation site in polycrystalline nickel through surface topography changes

Fatigue loading generates plastic deformation and strain localization on the surface of polycrystalline metals, which are known to be the precursor for crack initiation. The plastic deformations manifest themselves in the form of surface roughening that can be characterized using surface profiling techniques. In this study, surface topology changes on the surface of a nickel specimen under cyclic loading are monitored and analyzed to investigate the correlation between surface topography changes and crack initiation. Defining the surface roughness parameters as the damage index, an algorithm was formulated to predict the crack initiation location well before the formation of a visible crack. In addition, the crack initiation time can also be determined based on the saturation of the surface roughness damage index as well as the rebound of the surface height near the crack initiation location.

Numerical analysis of the influence of micro-voids on fretting fatigue crack initiation lifetime

Abstract In this paper, the influence of the heterogeneity in the predicted crack initiation lifetime under fretting fatigue conditions is analysed for a regular and a random distribution of micro-voids. A critical plane analysis with two multiaxial damage criteria is performed to assess the crack initiation lifetime. The predicted initiation lifetime in the heterogeneous material is compared with the results obtained in the homogeneous case. The numerical results show that the heterogeneity has a noticeable influence on the predicted initiation lifetime. Furthermore, the numerical model suggests that a crack may firstly initiate at the upper edge of the micro-voids located close to the contact edge, leading to a mean reduction of the predicted crack initiation lifetime. However, in some cases, the introduction of micro-voids reduces the stress intensity at the contact edge and thus decreasing the predicted crack initiation lifetime.

3D finite element study of the fatigue damage of Ti–6Al–4V in presence of fretting wear

Fretting damage analysis is carried out for several 3D contact geometries (cylinder-on-plane and punch-on-plane) under different slip regimes. A robust numerical environment is implemented to simulate the consequences of fatigue-fretting, in terms of wear or fatigue crack initiation. The numerical model allows to characterize the abrasive wear phenomena observed experimentally, and introduces specific damage post-processing to predict crack initiation. The numerical process includes an automatic mesh generation for updating the geometry due to wear effect. The difference between 2D and 3D cases is highlighted. Cases where a 3D numerical approach appears to be mandatory to obtain a quantitative agreement between simulation and experiment are mentioned.

Elasticity and fracture of brick and mortar materials using discrete element simulations

Natural materials such as bone and the nacre of some seashells are made of relatively weak building blocks and yet often exhibit remarkable combinations of stiffness, strength, and toughness. Such performances are due in large part to their brick and mortar architectures. Many efforts are devoted to translate these design principles into synthetic materials. However, much of the progress is based on trial-and-error approaches, which are time consuming and do not guarantee that an optimum is achieved. Modeling is an appealing alternative to guide the design and processing routes of such materials. However, the current analytical approaches cannot describe the extrinsic toughening mechanisms that takes place during crack propagation and are responsible for the remarkable properties of such materials. Here we show that the Discrete Element Method (DEM) can be used to predict the elastic and fracture behavior of brick and mortar materials and capture non-continuous phenomena such as multi-cracking. In contrast with most analytical shear-lag models, which only predict crack initiation, the model proposed here can also tackle crack propagation. DEM simulations are compared to analytical results with special attention to the shear transfer at the interface. The case of nacre-like alumina–a ceramic/ceramic brick and mortar composite with a brittle interface–is investigated to illustrate the potential of the method. We demonstrate in particular the importance of controlling the interface strength for further optimization of the mechanical properties. This method could be extended to predict and investigate the behavior of brick and mortar composites with a ductile interface, such as polymer/ceramic or metal/ceramic composites.

Predicting Crack Initiation of Solder Joints with Varying Sizes Under Bending

The critical strain energy release rate for crack initiation, Jci, was measured under mode I loading for SAC305 solder joints between two copper substrates. Fracture tests were performed using double cantilever beam specimens at a strain rate of 0.03 s−1. Different bond-line widths (i.e., joint size in the out-of-plane dimension) and thicknesses, were examined. The fracture force per unit width and Jci (the average value of four J-integral contours encircling the crack tip) were relatively insensitive to the width of the joint ranging from 8 mm to 21 mm. Variations in bond-line thickness (i.e., 150 μm, 250 μm and 450 μm) also had an insignificant influence on the fracture energy of solder joints. This behavior was explained in terms of stress distribution, crack-tip plastic zone area and triaxiality factor produced in the solder layer. Crack initiation in the specimens was then predicted using Jci as a property. Finally, a cohesive zone model was developed using a single set of parameters that was successfully used to predict the fracture loads of the joints with different sizes.

Multiscale crystal-plasticity phase field and extended finite element methods for fatigue crack initiation and propagation modeling

This paper presents a physics-based prediction of crack initiation at the microstructure level using the phase field (PF) model without finite element discretization, coupled with an efficient and accurate modeling of crack propagation at macro-scale based on extended finite element method (XFEM). Although the macro-scale model assumes linear elastic material behavior, at micro-scale the behavior of plastically deforming heterogeneous polycrystals is taken into account by coupling the PF model and a crystal plasticity model in the fast Fourier transform computational framework. A sequential coupling has been established for the multiscale modeling where the macro-scale finite element (FE) model determines the hot spots at each cyclic loading increment and passes the associated stress/strain values to the unit-cell phase-field model for accurate physics-based microstructure characterization and prediction of plasticity induced crack initiation. The PF model predicts the number of cycles for the crack initiation and the phenomenological crack growth models are employed to propagate the initiated crack by the appropriate length to be inserted in the FE mesh. Finally, the XFEM solution module is activated to perform mesh independent crack propagation from its initial crack size to the final size for the total life prediction. The effectiveness of the proposed multiscale method is demonstrated through numerical examples.

RA-based fretting fatigue life prediction method of Ni-based single crystal superalloys

Abstract Fretting fatigue damage can significantly reduce the service life of Ni-based single crystal (NBSX) superalloys turbine blades. In this work, we proposed a fretting fatigue life prediction method by considering cracking, wear and their mutual interaction. Specifically, a parameter, denoted as RA, is developed with combination of Resolved shear stress based damage factor and Accumulated dissipated energy based damage factor, by which the contribution from cracking and wear are described respectively. The proposed method is validated with experimental data in the literature. Finite element analyses are performed to study the distribution of stresses and strains. Results show that the predicted crack initiation site, failure plane and life cycles are consistent with the experimental results.

Predicting Fatigue Crack Initiation in 3D Structures with Adapcrack3D

The total service life of structures or components that have been under cyclic loading is the combination of load cycles required to initiate a crack or multiple cracks and the load cycles needed to propagate those cracks. Thus prediction of fatigue crack initiation as well as fatigue crack growth is very important in determining the lifetime of structures. Various simulation programs are available to foresee the fatigue crack growth and to calculate load cycles required for crack growth. Adapcrack3D is one such automatic crack growth simulation program which uses finite element method to simulate crack growth behavior in 3D structures. It also calculates the crack path, crack growth rate and the load cycles needed for the crack growth up until failure of the structure. Adapcrack3D generally uses mechanical and thermal loading conditions. Until now the software only calculates load cycles required for propagating an already initiated crack. Thus the user provided FE-Model contains the location and shape of the initial crack which Adapcrack3D uses for simulating the growth of the crack.This paper is an attempt to introduce a numerical procedure in Adapcrack3D for automatic crack initiation in 3D structures. 3D models are created with necessary loading conditions and without the initial technical crack. Simulations are performed on these models for finding the stress tensor and maximum principal stresses in the structure. The software then uses this information to calculate the position and the surface where a crack is most likely to occur. The load cycles required to initiate the crack are also calculated using the Smith–Watson–Topper damage parameter. The automatically initiated crack is then used for a crack growth simulation, thereby determining the total lifetime of structure.

Enhanced models of creep crack initiation prediction coupled the stress-regime creep properties and constraint effect

In this work, enhanced theoretical analysis and finite-element (FE) simulation were carried out to evaluate the creep crack initiation times by coupling the stress-regime creep strain rate and creep ductility and constraint effect. Firstly, the enhanced theoretical approaches were derivated based on the ductility exhaustion model, 2RN (2 regimes Norton) creep model and the Fermi's fit (between the creep ductility and creep strain rate), and several analytical models of CCI prediction were proposed under the different stress field condition. Then the constraint effect was coupled into the enhanced C*-Q* approaches. The comparison of CCI time predictions between the FE solutions and the enhanced C*-Q* approaches verified the accuracy of the latter. Reasonable and conservative predictions of CCI time could be obtained from the theoretical solutions when the constraint effects were considered in the K-RR and HRR-RR controlled models when compared with FE solutions. The K-RR and solutions were accurate when initial stress intensity factor K was at low and high range, respectively. Finally, the accuracy of the enhanced approaches and the suitability of finite element simulation were validated by the experimental data of 316H steel.

Mixed-mode I/II fracture criterion for crack initiation assessment of composite materials

This paper presents a mixed-mode I/II fracture criterion to investigate the crack initiation in orthotropic materials in which the crack is directed along fibers. The minimum strain energy density criterion is extended to investigate the cracked orthotropic materials. The reinforced isotropic solid model based on collinear crack propagation along fibers is proposed as an advantageous model to study the fracture behavior of composites. This model introduces fibers as reinforcements of the isotropic matrix in orthotropic materials in which their effects are qualified by defining reinforcement factors at tension and shear modes. The proposed criterion can predict the crack initiation phenomenon. Therefore, in this study a new concept of linear fracture toughness for orthotropic materials is proposed. Experimental data are used to validate the output of the proposed criterion. The coincidence of fracture limit curves and experimental data indicates the ability of the new criterion to predict crack initiation in orthotropic materials.