Predict Crack Initiation Research Articles

In-situ SEM observation and crystal plasticity finite element simulation of fretting fatigue crack formation in Ni-base single-crystal superalloys

Fretting fatigue behavior of Ni-base single crystal (NBSX) superalloys was investigated based on in-situ scanning electron microscope (SEM) observations. Evolutions of slip lines and initiations of cracks were observed. Slip lines were found on specimen free surface along three typical directions. Short cracks were observed to initiate along the slip line. Crystal plasticity constitutive model considering cyclic hardening effect was employed. Crystal plasticity finite element method (CPFEM) simulation showed the activations of crystallographic slip systems at contact region. The slip plane with the maximum plastic slip was determined as dominate slip plane. Equivalent plastic strain was proved effective to predict crack initiation. The simulation results including predicted slip lines, crack initiation locations and orientations were in good agreements with observations.

Cellular automata model for prediction of crack initiation and propagation in hot forging tools

The paper presents design and implementation of the cellular automata (CA) model, which predicts damage of forging tools due to fatigue. The transition rules for the model were developed on the basis of known information regarding crack initiation and propagation. The coefficients in the model were determined by the inverse analysis of the thermal fatigue tests performed on the Gleeble 3800 simulator and in the special device with a rotating disc. The CA model was coupled with the conventional abrasive wear model. The layers of cell in the CA space, which are in contact with the workpiece, were removed successively following the abrasive wear of the tool. The CA model was connected with the finite element (FE) programme, which simulates stresses in tools during forging. Since this multiscale approach appeared to be extremely demanding as far as computing times are considered, an efficient implementation of the model on heterogeneous hardware architectures was prepared. Results of simulations were compared with the industrial data and good predictive capabilities of the model were confirmed.

Modelling of fretting in the pressure armour layer of flexible marine risers

This paper presents a computational methodology for frictional contact mechanics of the pressure armour layer in flexible risers. This will allow, for the first time, quantification of key fretting variables, such as contact pressure, relative slip and sub-surface stresses in this complex geometry, under representative loading conditions. Fatigue lives are calculated using the 3-dimensional critical plane Smith–Watson–Topper multiaxial fatigue parameter. It is shown that COF has a significant effect on predicted trailing-edge tensile stresses in the pressure armour layer and, hence on fretting crack initiation in risers. It is also shown that operating pressure and bending-induced axial displacement significantly affect predicted crack initiation. These results will facilitate representative fretting wear and fretting fatigue testing of pressure armour layer material.

Fatigue crack initiation prediction of cope hole details in orthotropic steel deck using the theory of critical distances

AbstractOrthotropic steel decks are vulnerable to fatigue cracking in welded connections and complex geometrical details. A total of three fatigue tests were conducted on segments of orthotropic steel deck to evaluate the fatigue performance of trough‐to‐crossbeam connections with various cut‐out configurations. In the tests, the specimens were subjected to cyclic three‐point bending load and the fatigue cracks were more likely to initiate from the cope holes in the crossbeam web rather than the trough‐to‐crossbeam fillet welds. Three‐dimensional finite element models (FEM) of the specimens were built and validated by the measured deflections and stresses. Using the validated FEM, the characteristic stresses based on the theory of critical distances (TCD) were calculated for the stress concentrations along the cope holes. The fatigue crack initiation life, predicted by the TCD‐based stress combined with the plain material S–N curve, agreed reasonably with the fatigue test results. The TCD method could further form a basis of fatigue crack propagation analysis using the fracture mechanics approaches.

Modeling surface rolling contact fatigue crack initiation taking severe plastic shear deformation into account

A new model for the prediction of rolling contact fatigue crack initiation at the surface of railway rails and wheels has been developed, which takes contact-related plasticity effects into account. The model assesses the propensity towards fatigue crack initiation based on microstructural crack paths in the severely shear-deformed, anisotropic material near the surface. The key to differentiate between situations favoring crack initiation accompanied by wear and situations where only wear prevails, is the distribution of plastic shear strain in combination with stress in a crack initiation layer at the surface, rather than the maximum values of the plastic strain or stress. The model predicts the formation of head checks at the gauge corner of rails and the corresponding damage pattern on wheels. It is parameterized based on results from a full-scale test rig experiment. The model can be coupled to multi-body systems simulations of railway vehicles to account for the effect of plastic shear deformation on rolling contact fatigue crack initiation in such simulations. This allows systematic studies of contact conditions, material properties and railway vehicle dynamics behavior with regard to rolling contact fatigue crack initiation.

Effect of microstructure on oxygen rich layer evolution and its impact on fatigue life during high-temperature application of α/β titanium

The near alpha titanium alloy, Ti-6424S, is utilized in many critical high-temperature aerospace components due to its unique properties. However, oxygen ingress during elevated-temperature exposure induces formation of a subsurface brittle oxygen-rich layer (ORL), resulting in a deterioration of mechanical performance. This paper, for the first time, establishes the effect of the underlying microstructure on the formation and evolution of the ORL in α/β titanium alloys. In addition, models were developed to predict (i) the evolution of ORL as a function of the material microstructure, (ii) the effect of ORL on the critical strain for in-service crack initiation, and (iii) estimates of fatigue life of components made from a specific microstructure during in-service high temperature exposure and formation of ORL. In particular, five different microstructures were produced by tailored heat-treatments and thermally exposed at 650 °C up to 420 h. The base metal and the ORL were quantified using microhardness indentations, optical microscopy, and scanning electron microscopy (electron backscatter diffraction (EBSD), backscattered electron (BSE), and secondary electron (SE) imaging). The effective diffusion coefficients (Deff) for each microstructure were calculated and then integrated into a critical strain model to predict crack initiation strain as a function of exposure time. The predicted ORL thickness was used to estimate fatigue life using experimentally measured crack growth data. The largest Deff coefficient was observed in a colony microstructure, while a basketweave microstructure showed the smallest Deff. For several bimodal microstructures, Deff was noted to increase with increasing area fraction of secondary alpha colonies.

Microscopic Damage Evolution During Very High Cycle Fatigue (VHCF) of Tempered Martensitic Steel

Dimensioning of high-strength steels relies on the knowledge of Wöhler-type S/N data and the assumption of a fatigue limit for applications where the number of load cycles exceeds 107. Very high cycle fatigue (VHCF) experiments applied to a 0.5C-1.25Cr-Mo tempered steel (German designation: 50CrMo4) revealed surface crack initiation at prior austenite grain boundaries in medium strength condition (37HRC) and internal crack initiation at non-metallic inclusions at high strength condition (48HRC). Despite the formation of small cracks during cycling up to 109 cycles, it seems that the medium strength condition exhibits a real fatigue limit. Application of automated electron back-scattered diffraction (EBSD) within the shallow-notched area of electro-polished fatigue specimens had shown that prior austenite grain boundaries act as effective obstacles to crack propagation. High resolution thermography during cycling of the specimens allowed the identification of local plasticity, which led to crack initiation at a later stage of the fatigue life. It was found that Cr segregation rows play a decisive role in the crack initiation process. By means of high-resolution electron microscopy in combination with focused ion beam milling (FIB), evolution of cyclic plasticity and crack initiation was correlated with the material's microstructure. The results are discussed in terms of the completely different crack initiation mechanisms of medium and high strength variants of the same steel. EBSD and crack propagation data are used to adapt numerical modeling tools to predict crack initiation and short crack propagation.

Accuracy of 2D FE models for prediction of crack initiation in nested textile composites with inhomogeneous intra-yarn fiber volume fractions

The finite element simulation of nesting phenomena in textile composites is challenging, because of the difficulties that come with creating a 3D finite element model of a stack of nested plies. This paper proposes a 2D modeling approach for prediction of crack initiation locations in nested textile composites. The model is built up based on high resolution microscopic images of the polished edge of a tensile specimen and routines have been developed to take the inhomogeneous intra-yarn fiber volume fractions into account. The locations of critical stresses are compared with high-resolution microscopic images during tensile loading of the nested textile composite, and correlated with the locations of crack initiation. The methodology is validated against tensile experiments on a steel fabric reinforced epoxy laminate. The steel fabric has a high tendency for layer nesting and a high inhomogeneity in intra-yarn volume fractions, and it shows very good contrast in micro-tomography and microscopic images.

Fundamental mechanical behaviors of dental restorative materials resin composites due to compressive and flexural loading and internal damage evaluation

Development of internal damage within resin composites was evaluated under compressive loading in order to predict crack initiation and fracture. Moreover, three-point bending tests were also carried out in order to clarify mechanical behavior and fracture under tensile stress state in comparison with those under compressive stress state. Both of them were conducted for the purpose of obtaining data to formulate constitutive equations for resin composites and to implement precise numerical simulation. Columnar specimens for compression tests and square pole specimens for three-point bending tests were prepared by using clinical resin composites. In compression tests, loading–unloading (or –reloading) was given to columnar specimens and Young’s modulus was evaluated by the gradient of stress–strain curves under unloading. Internal damage was evaluated from the variation of Young’s modulus as a scalar damage variable based on the continuum damage mechanics. The variation of apparent density and residual strain at vanished stress were also discussed in association with the development of internal damage. Accumulation of internal damage was found on the stress–strain curve under loading–unloading–reloading in comparison with the curve under monotonic loading. On the other hand, in three-point bending tests, dependence of stress–strain curves on light curing time and strain rate was clarified. Since compression tests have been carried out under similar experimental conditions by authors so far, mechanical behaviors of resin composites under tensile stress state were discussed in comparison with those under compressive stress state. Brittleness under tensile stress state was indicated in comparison with compressive stress state.

Very High Cycle Fatigue Failure Analysis and Life Prediction of Cr-Ni-W Gear Steel Based on Crack Initiation and Growth Behaviors.

The unexpected failures of structural materials in very high cycle fatigue (VHCF) regime have been a critical issue in modern engineering design. In this study, the VHCF property of a Cr-Ni-W gear steel was experimentally investigated under axial loading with the stress ratio of R = −1, and a life prediction model associated with crack initiation and growth behaviors was proposed. Results show that the Cr-Ni-W gear steel exhibits the constantly decreasing S-N property without traditional fatigue limit, and the fatigue strength corresponding to 109 cycles is around 485 MPa. The inclusion-fine granular area (FGA)-fisheye induced failure becomes the main failure mechanism in the VHCF regime, and the local stress around the inclusion play a key role. By using the finite element analysis of representative volume element, the local stress tends to increase with the increase of elastic modulus difference between inclusion and matrix. The predicted crack initiation life occupies the majority of total fatigue life, while the predicted crack growth life is only accounts for a tiny fraction. In view of the good agreement between the predicted and experimental results, the proposed VHCF life prediction model involving crack initiation and growth can be acceptable for inclusion-FGA-fisheye induced failure.

Experimental and numerical study on crack initiation under fretting fatigue loading

Press-fitted railway axles and wheels are subjected to fretting fatigue loading with a potential hazard of crack initiation in press fits. Typically, the resistance against crack initiation and propagation in press fits is investigated in full-scale tests, which procedure is both costly and time consuming. In this context, combined experimental and numerical approaches are of increasing practical importance, as these may reduce the experimental effort and, moreover, provide a basis for the transferability of experimental results to different axle geometries and materials. This study aims at evaluating stress–strain conditions under which fretting fatigue crack initiation is likely to occur. Experiments on small-scale specimens under varying fretting fatigue load parameters and their finite-element modelling to characterize the resulting stress–strain fields are performed. Subsequently, different multiaxial fatigue parameters are applied to predict crack initiation under fretting fatigue conditions.

Fracture Mechanics-Based and Continuum Damage Modeling Approach for Prediction of Crack Initiation and Propagation in Integral Abutment Bridges

AbstractPiles in integral abutment bridges (IABs) are prone to fatigue damage crack attributable to cyclic load caused by daily and seasonal temperature variations. Previous research has shown that the fatigue life of these piles can be as low as three decades. It is essential to be able to predict the fatigue-crack initiation site and propagation path and have an accurate estimate of the total time to failure. This research presents a multiscale modeling approach used to accurately determine the local deformation and stresses in the critical pile. A continuum-based damage modeling approach is used to determine the location of the crack and its path as the number of daily cycles increase. The model is then verified using a fracture-based modeling approach, Paris Law, which assumes local plastic deformation at the tip of the crack. Paris Law constants for this case are generated and compared with literature values presented for the steel material used in the piles and show a very good correlation.

A strain energy density criterion for the initiation of edge debonding

Material or geometrical discontinuities within a structure induce a localized stress concentration which may initiate a crack leading to catastrophic failure in brittle materials. To assess crack initiation in the vicinity of a V-notch, a criterion based on the strain energy density (SED) averaged over a volume surrounding the notch tip is an attractive approach. It is demonstrated that this criterion can also be used to predict crack initiation along an interface. The precise definition of the control volume is obtained with the help of a coupled criterion which relies on finite fracture mechanics.

Interior crack initiation and growth behaviors and life prediction of a carburized gear steel under high cycle fatigue and very high cycle fatigue

Abstract

Thematic issue: Advanced modeling and innovative processes

The thematic issue on BAdvanced Modeling and Innovative Processes^ is a result of a kind invitation from the Editorial Board of the International Journal of Material Forming. Therefore, the Invited Editors would like to acknowledge the Editorial Board, in the person of the Editor-in-Chief Prof. Francisco Chinesta, the ESAFORMAssociation, in the person of its President Prof. Dorel Banabic and, last but not least, Springer Editions for this great opportunity. The idea of this thematic issue is to give a summary (but comprehensive) overview of the most recent and diversified topics in the field of advanced modeling techniques, when directly applied to innovative processes. One of the main challenges of Engineering, in general aspects, is to provide a link between fundamental mathematical/physical concepts and industrial applications, leading to economic development as well as scientific one. This thematic issue is devoted to this landmark. The contributions chosen to be included in the present thematic issue try to provide, as mentioned, a wide variety of examples where this link can be achieved. For instance, polymer crystallization phenomena are dealt with and simulated by means of numerical techniques including multi-scale effects. In other publication, also dealing with constitutive modeling, nonquadratic anisotropic criteria are considered for the correct simulation of metallic alloys. In order to achieve a higher level of efficiency in the numerical algorithms to be used in industrial applications, procedures for obtaining equivalent parameters from different yield criteria and hardening laws are studied and analyzed in another contribution. Directly related to industrial applications and constantly seeking for efficient, real-time computations, another contribution proposes innovative ways to choose optimum tool paths for incremental forming processes, based on sequencing and multi-step mesh morphing procedures. From the point of view of reliability and robustness of numerical simulation results, the thematic issue presents a paper dealing with an inevitable nonlinearity in forming processes: contact modeling, and its influence in results as coming from early stage decisions on the way to properly describe tools’ surfaces. Also related to metal forming tools, another contribution deals with and presents a set of experimental setups able to properly investigate the wear of coatings, with results that can provide valuable insights into process simulation techniques, afterwards. Another analysis of coatings’ wear, but this time applied to turning operations, is given in another contribution, widening the range of applications included in this thematic issue. Modeling and numerical analysis should also rely, when possible, on inverse techniques and optimization and a contribution is included in this issue dealing with an integrated approach for the SHPB test and ranging from numerical to experimental analysis. Another relevant field of study in the processes’ modeling is the prediction of crack initiation and, subsequently, the monitoring of its growth. This topic is seen in a contribution dealing with FSW joining/processing techniques and the analysis of process parameters in the appearance of fracture zones and crack propagation. Finally, and also with the goal to widen the range of * Robertt Valente robertt@ua.pt

Energy release rate cannot predict crack initiation orientation in domains with a sharp V-notch under mode III loading

The energy release rate (ERR) proposed by Irwin based on a theory by Griffith (1920) and Irwin (1957) has been extensively used as a fracture criterion in 2D for brittle domains. Under in-plane mixed mode loading (modes I+II), the direction of crack initiation from cracks and sharp V-notches was determined by the orientation at which the ERR attains its maximum.Using the newly developed asymptotic expansion presented in Mittelman (2014) verified by direct results from finite element analyses we demonstrate that the ERR under mode III cannot predict the fracture initiation direction correctly. The ERR maximum value is always obtained along the V-notch bisector, contrary to experimental observations. This forbids the ERR to be applied as a failure initiation criterion in cases where mode III is dominant.

Controlling hot cracking in Ni-based Inconel-718 superalloy cast sheets during tungsten inert gas welding

The pre-welding condition of the base metal, the welding heat input, and the state of the interlayer in multi-layer welding are examined to identify means of controlling the hot cracking in Inconel-718 cast sheets during tungsten inert gas (TIG) welding. A finite element method (FEM) incorporating the cast microstructure and alloy chemical composition was used to calculate the welding stresses and predict crack initiation. The crater crack, weld-seam center longitudinal solidification crack, and the weld toe compound crack mixed with the transversal solidification crack and the heat-affected zone (HAZ) liquation crack are discussed. A pre-welding homogenization heat treatment, lower welding heat input, and post-welding crater grinding of each individual welding seam can decrease its hot cracking susceptibility.

Microstructure-based thermo-mechanical modelling of thermal spray coatings

Abstract This paper demonstrates how microstructure-based finite element (FE) modelling can be used to interpret and predict the thermo-mechanical behaviour of thermal spray coatings. Validation is obtained by comparison to experimental and/or literature data. Finite element meshes are therefore constructed on SEM micrographs of high velocity oxygen-fuel (HVOF)-sprayed hardmetals (WC–CoCr, WC–FeCrAl) and plasma-sprayed Cr 2 O 3 , employed as case studies. Uniaxial tensile tests simulated on high-magnification micrographs return micro-scale elastic modulus values in good agreement with depth-sensing Berkovich micro-indentation measurements. At the macro-scale, simulated and experimental three-point bending tests are also in good agreement, capturing the typical size-dependency of the mechanical properties of these materials. The models also predict the progressive stiffening of porous plasma-sprayed Cr 2 O 3 due to crack closure under compressive loading, in agreement with literature reports. Refined models of hardmetal coatings, accounting for plastic behaviours and failure stresses, predict crack initiation locations as observed by indentation tests, highlighting the relevance of stress concentrations around microstructural defects (e.g. oxide inclusions). Sliding contact simulations between a hardmetal surface and a small spherical asperity reproduce the fundamental processes in tribological pairings. The experimentally observed “wavy” morphologies of actual wear surfaces are therefore explained by a mechanism of micro-scale plastic flow and matrix extrusion.

Characterization of plasticity and fracture of shell casing of lithium-ion cylindrical battery

Most of the literature on lithium-ion battery cells is concerned with modeling of jellyroll with little attention to properties of shell casing. However, shell casing provides substantial strength and fracture resistance under mechanical loading and therefore must be an important part of modeling of lithium-ion batteries. The paper reports on a comprehensive test program on commercially available empty shell casing of 18650 lithium-ion cylindrical cells. Part of the tests was used to determine plastic and fracture properties from sub-size specimens cut from lateral part of the cans. The other part served to validate plasticity and fracture models under various loading conditions. The associated flow rule was used to simulate plasticity behavior and Modified Mohr-Coulomb (MMC) fracture model was adopted to predict crack initiation and propagation of shell casing. Simulation results confirmed that present plasticity and fracture models could predict global plastic behavior of the cells under different loading conditions. The jellyroll model with volumetric hardening was introduced to compare the performance of empty shell casing, bare jellyroll and complete battery cell. It was shown that in many loading situations, for example, three point bending of the cylindrical cells, the metallic shell casing provides most of mechanical resistance.

Finite fracture mechanics analysis using the scaled boundary finite element method

The polygon-based scaled boundary finite element method is applied to two finite fracture mechanics based failure criteria to predict the crack initiation from stress concentrations, i.e. notches and holes. The stress and displacement fields are modelled by the scaled boundary finite element method through semi-analytical expressions that resemble asymptotic expansions around cracks and notches. Important fracture parameters, i.e. energy release rate and stress, are accurately and conveniently computed from the solutions of stresses and displacements via analytical integration. One distinguished advantage of applying the scaled boundary finite element method to finite fracture mechanics is that the required changes in the mesh are easily accommodated by shifting the crack tip within the cracked polygon without changing the global mesh structure. The developed framework is validated using four numerical examples. The crack initiation predictions obtained from the scaled boundary finite element method agree well with the reference finite element results.