Crack Nucleation Research Articles

Fatigue assessment of welded joints and crack growth considering residual stress

There are different approaches to investigate the fatigue life of welded joints, however, less has been done considering residual stresses. The heat and residual stress distribution of a butt weld joint are determined using ABAQUS software and the results are validated against experimental and numerical studies in the literature. The software FE-SAFE is used to study the fatigue life of a GMAW welded butt joint under high cycle loading considering the residual stress. The corresponding S-N curve is determined, and the results show that the longer the length of welds, the higher residual stress and lower fatigue life. The crack nucleation and propagation in the butt weld simulated using XEFM software, and it is found that crack starts from the position of highest restrain.

Effect of pad rotation angle and coefficient of friction on fretting fatigue crack nucleation behavior

Fretting fatigue may occur on surfaces that are in contact, subjected to a normal load, and where there is a cyclic small relative movement between them. It could occur in press-fitted shafts, bolted and dovetail joints, and many other engineering components. It is recognized that the coefficient of friction of the contacting bodies is a primary variable in the fretting process. Also, a recent study indicates that its behavior might be greatly affected by the possible rotation of the pad. Consequently, the aim of this investigation is to evaluate the impact of diverse angles of rotation of the pad and different coefficients of friction on crack initiation angle, location, and life. For this purpose, a numerical model based on a finite element model of a cylinder-plane test configuration is constructed. The Fatemi-Socie damage parameter for multiaxial fatigue is used to evaluate initiation behavior. Firstly, different angles of rotation are applied to the pad, in order to model a lack of stiffness of the test rig, while the coefficient of friction is kept constant and equal to 0.75. Secondly, several coefficients of friction between the pad and the specimen are utilized, while considering a cyclic rotation of the pad between 0.19° and – 0.19°. For the case studied, the results indicate that an increase of the coefficient of friction (μ) may cause a substantial reduction of crack initiation life. However, a variation of μ in the range 0.75 ≤ μ ≤ 0.85 does not produce a significant change in life estimation. This suggests that it cannot be stated that an increase in the coefficient of friction always leads to a reduction in nucleation life. Regarding the angle of initiation of the crack, the results show that an increase of μ causes an approximately linear reduction of the angle. In the case of an increase of the pad angle of rotation, this causes a longer nucleation life and a smaller crack nucleation angle. Even the rotation angle of 0.23° produces an increase of initiation life from 16.6 to 154 thousand cycles. It is concluded that there is a major impact on initiation behavior of the coefficient of friction and the rotation of the pad, which may be due to the lack of rigidity of the test setup. Therefore, controlling the stiffness of the test rig and measuring accurately the coefficient of friction are important factors to consider in fretting fatigue research.

Adaptive phase-field modeling of fracture propagation in bi-layered materials

We study fracture propagation in bi-layered materials using an adaptive phase-field method. The method combines a phase-field representation of fracture with spatial adaptivity to provide mesh refinement in the smeared zone of damage, where higher accuracy is needed to resolve the solution. We place a particular emphasis on studying the competition between crack arrest, deflection, and penetration at the interface of two dissimilar materials. To explore this competition, we perform tension tests on various models with different combinations of material and geometric properties. We begin by validating the adaptive phase-field model through a uniaxial tension test. The material mismatch and locations of the initial damage for this example are chosen such that the fracture propagation behavior is identical to that in a homogeneous material. Next, we independently vary the mismatches in elastic stiffness and critical energy release rate to study their influence on crack growth. Our observations show that both mismatches contribute to an effective toughening of the model, but in different ways. Critical energy release rate mismatch causes crack deflection along the interfaces, while elastic stiffness mismatch results in retardation of crack growth. We also note that for a specific combination of material mismatch and location of initial damage, crack nucleation in the adjacent material is preferred over propagation from the existing notch. Finally, through models with inclined interfaces, we demonstrate the effect of the relative orientation between the approaching crack and the material interface on crack penetration, deflection, and nucleation. The numerical results from this study provide valuable insight into the various failure mechanisms in bi-layered materials.

Meso-mechanics-based microstructural modelling approach to predict low cycle fatigue properties in additively manufactured alloys

Grain, grain boundaries, voids, alloying elements and other anomalies have an important impact on the stress distribution at the sub-grain meso level which consequently affect the strain controlled low cycle fatigue (LCF) in additively manufactured (AM) alloys. In alloys AM materials yielding with large stress–strain gradients tends to form around voids and other defects, thus inducing preferential crack nucleation sites and accelerating fatigue failure of the substrate. In order to investigate the effects of these microscopic features on the LCF performance in AM alloys, this paper proposes an idealized grain/grain boundary model using Voronoi tessellation technique and to quantify the effects of different features such as randomly distributed internal pores and hard interstitial on the LCF performance. The proposed method is an important meso-scale continuum damage modelling approach that can be developed to study and analyze the LCF properties with the consideration of microscopic features.

Effect of phase content on mechanical properties and deformation mechanism in TC21 titanium alloy: An in-situ tensile investigation

This work fabricated a series of microstructures consisting of different equiaxed α (αeq) and β phase contents to investigated the effect of phase content on mechanical properties and deformation mechanism, including slip initiation, slip transfer, stress-induced martensite (SIM) and crack nucleation of TC21 titanium alloy. The results show that, with the decrease of αeq phase content, the YS of the alloy firstly decreases and then increases, the UTS gradually increases, and the EL gradually decreases. The significant variation of mechanical properties is closely related to the deformation mechanism of microstructures with different αeq phase contents. In sample with high αeq phase content, αeq phase is deformed by slip prior to β phase, and with the increase of strain, β phase can coordinate deformation by slip initiation and exhibits a better ability to coordinate plastic deformation. In samples with medium αeq phase content, the stability of β phase decreases, slip in αeq grain and SIM in β grain can be initiated simultaneously. However, the formation of SIM would harden β phase, and with the increase of strain, it would restrain the ability of β phase to further coordinate plastic deformation. In samples with low αeq phase content, the stability of β phase decreases sharply, which could induce the precipitation of fine-lamellar secondary α (αs) and acicular martensite α'' in β grain during annealing. These precipitated αs and α'' can not only inhibit the formation of SIM, but also dramatically reduce the ability of β phase to coordinate plastic deformation. Meanwhile, the evolution of deformation mechanism in different samples can significantly influence the crack nucleation location (slip band or αeq/β interface) during deformation. This paper analyzes in detail the effect of αeq phase content on mechanical properties of TC21 titanium alloy based on deformation mechanism, it would contribute to the establishment of a relationship among phase content, deformation mechanism and mechanical properties.

Crack Nucleation in Shells with Through-the-Thickness Microstructure

Crack Nucleation in Shells with Through-the-Thickness Microstructure

Evaluating fracture properties and interfacial strengths of silicon oxycarbide thin-films: XFEM simulations and experimental validation

Silicon oxycarbide (SiCO) thin films with unique nano-domain structure show various functional properties and high-temperature stability, and the mechanical property at the interface between SiCO and substrate is one of the critical issues for enhancing reliabilities of microelectronic thin-film devices. In this work, the fracture properties and interfacial strengths of SiCO thin films are investigated by experiments and extended finite element (XFEM) method. The elastic and plastic properties of the SiCO/substrate interfaces are obtained by fitting the XFEM calculated P-h curves with those from nano-indentation experiments, fracture toughnesses and energy release rates are obtained to improve the description of interfacial crack nucleation and propagation. Then the properties are used as the inputting parameters of XFEM model for scratch simulations, and the critical loads for evaluating interfacial strength are achieved. The calculated critical loads are comparable to the results of experimental scratch, and the influences of layer thickness on interfacial strength of SiCO thin-film are nicely predicted.

The origin of the energy split in phase‐field fracture and eigenfracture

AbstractThe energy splits, which are used in phase‐field fracture and eigenfracture, are frequently motivated by the statement that crack nucleation and crack growth have to be prevented at compression loads. The intention of the article at hand is to show that the energy decomposition is an essential part of such fracture models, and to investigate the underlying problem and its physical interpretation. Finally, a general framework for the derivation of physically motivated energy splits is presented, which is not limited to certain bulk material models.

Micro-deformation behavior of laser melting deposited multi-gradient TC4/TC11 alloy under tension–tension cyclic loading

The microstructure of additive manufacturing titanium alloy is closely related to its mechanical properties. In this study, a TC4/TC11 titanium alloy with 20 layers was prepared by laser melting deposition (LMD) technology. Microstructure analysis revealed a transformation of the columnar grains into equiaxed grains along the build direction (BD), accompanied by the presence of heat affected zone (HAZ) bands and pore defects. The defect families of specimens in different directions were characterized by X-ray micro-computed tomography (micro-CT). The porosity of BD specimens was 0.0021%, and the porosity of the transverse direction (TD) specimens was 0.0001%, indicating that the material was dense. Specimens with different directions were subjected to tension–tension cyclic loading, and their deformation was analyzed by digital image correlation (DIC). The results show that the presence of noticeable pore defects leads to crack initiation around these regions due to local strain concentration. Moreover, the dispersion and anisotropy of mechanical properties are attributed to the influence of porosity and microstructure deformation, the latter being the dominant factor. In addition, the deformation mechanism and failure mode of the microstructure in LMD TC4/TC11 are proposed in this paper. The deformation of the α laths of TD specimen is hindered by the columnar grain boundary, resulting in improved resistance against crack nucleation and propagation compared to the BD specimen.

Experimental observations of nucleation and crack growth paths of hydrogen-induced cracking in pipeline steel

The microstructural features that favor hydrogen-induced cracking (HIC) in internal, middle, and external layers of pipeline steels were investigated. The electrochemical cathodic charging was used to induce HIC, and straight beam ultrasonic inspection was identified in which steel layers HIC nucleate and grow. The cracked plates were examined by optical microscope and scanning electron microscope (SEM). The hydrogen microprint technique (HMT) was used to visualize the site of high hydrogen concentrations. Electron backscatter diffraction (EBSD) was used to analyze the role of crystallographic texture in HIC pathways. The results revealed that HIC nucleation and growth are influenced by the segregation degree and inclusion contents, which are higher in the internal surface layer of our studied steels but not consistently so in the middle thickness layer. Further, HIC nucleated in cuboidal and spinal inclusions and propagated along ferrite/pearlite bands with high misorientations. The HMT results showed that hydrogen is present in pearlite colonies, grain boundaries, and non-metallic inclusions. The results indicated that HIC is not solely controlled by the crack geometry.

From stress concentrations between inclusions to probability of breakage: A two-dimensional peridynamic study of particle-embedded materials.

Particle-embedded materials consist of a dispersed phase of particles in a sticking matrix. We used a bond-based peridynamics method to investigate their elastic properties, rupture, and probability of failure. We performed an extensive two-dimensional parametric study where particles are disk-shaped inclusions diluted into a pore-filling matrix. Both particle and matrix are considered to be brittle elastic with a perfectly bonded interface. The inclusion volume fraction φ and the inclusion matrix toughness ratio β (β≥1) were varied from 0.254-0.754 (jamming point) and 1.5-100.0, respectively. A total of 5000 uniaxial tensile tests up to failure were performed. We showed that the Halpin-Tsai model fits well all Young elastic moduli even for nearly in-contact particles. The stress distribution strongly depends on φ and β. As the highest stresses (at the origin of crack nucleation) occur between neighboring particles, we analyzed the average stress in gaps. We found that, regardless of the particle volume fraction, the yield stress is a power law of a grain-scaled stress concentration factor. We also investigated the probability of failure of the samples. We found that whatever φ and β, this probability follows a classical Weibull law. Finally, we showed that Weibull modulus, normalized by its value for infinitely rigid particles, is inversely proportional to a function of the stress concentration factor.

Failure mechanisms of equipment hatch area in prestressed concrete containment vessel

The failure mechanisms of the equipment hatch area under two load cases (LCs), i.e. in-plane shear/tension and tension/tension to consider the combined action of ground motion and internal pressure and the action of internal pressure only, respectively, were experimentally studied with three specimens for each LC.The results show that the failure of the LC1 specimens is due to the eccentric tension in the vertical direction. The shear force in the lateral direction changes the stress distribution in the specimensand thus influence the crack nucleation and propagation. The failure modes of the LC2 specimens depend on the magnitude of the lateral tension. With the increase of the maximum lateral tension, the failure modes vary in the sequence of the horizontal cracking of concrete at the bottom of the specimen, the critical diagonal cracks on the top of the specimen and the failure of the narrowest part of the specimen.

Continuum damage mechanics based computational framework for prediction of the lifetime and degradation of wind turbine coatings with defects

The repeated raindrop impact significantly decreases the life of wind turbine blades (WTB) coating. Several computational frameworks have been developed in the literature to understand the damage behaviour and to predict the safe life of WTB's coating under such conditions. Experimental findings show that coatings may contain voids which adversely affect the coating's life. However, the computational models of coating often ignore this effect. This paper develops a new framework based on Continuum Damage Mechanics (CDM) to predict the life of WTB’s coating with voids due to rain. To determine the fatigue life of WTB’s coating, (i) a stochastic rain field simulation model has been employed, which provides data on the raindrop size distribution and impact locations. (ii) The stresses due to the impact of these raindrops are computed using smoothed particle hydrodynamics. (iii) Thereafter, CDM is implemented to predict fatigue damage life. Since damage can be visualized as nucleation and propagation of pits, voids, and cracks, hence, the complete life of the wind turbine blade's coating can be predicted by CDM. The computational result of the proposed CDM framework has been verified with fatigue damage results available in the literature. This study establishes that the proposed CDM-based computational framework is a reliable tool to predict the rain-induced fatigue life of the blade's coating in the presence of flaws.

Microstructural characterisation of brittle fracture initiation sites in reactor pressure vessel steels

Neutron embrittlement of reactor pressure vessel (RPV) steels potentially limits the safe operational time of nuclear reactors. In tested RPV steel samples microcracks are known to originate from cleaved precipitates, inclusions or grain boundaries, which eventually lead to brittle fracture. In most cases, these precipitates are reported as carbides. Latest research indicates the critical fracture event to be the crossing of grain boundary instead of the particle–matrix interface by the microcrack. Very few works have performed detailed fractography, which revealed the size and type of the feature found in the crack nucleation centre, together with metallographic bulk investigations. Additionally, the effect of subsequent neutron irradiation on microcrack nucleation is unknown. In the present work, detailed fractography and metallography is performed on a RPV steel in both its unirradiated as well as neutron irradiated state. A particle in the centre of the initiation site has been identified in a majority of the cases, while no specific type of particles were involved in more than 1/3 of all samples. Those particles are either Mo-rich carbides or Al-rich inclusions. In comparison, the metallographic analysis of the bulk steel reveals Mo-rich and Mn-rich carbides. Mn-rich carbides constitute the majority of all carbides in the steel but are never found to be responsible for brittle fracture initiation. Only a small fraction of observable carbides is Mo-rich which are responsible for fracture. Thus, the present work demonstrates a discrepancy between particles found in brittle fracture initiation sites and particles that are directly observable in bulk steel. Some of the initiating particles are extremely rare in the bulk. Furthermore, it is revealed that the local fracture stress does not depend on the type or size of particle or grain involved at the initiation site. In conclusion, the fracture stress strongly depends on the type of the grain boundary, rather than on a stress based upon a Griffith criterion, which considers particle or grain size alone. The brittle fracture mechanism was found to be unaffected by neutron irradiation. These findings could help refine modelling of the critical brittle fracture stress and fracture toughness using microstructural parameters as input.

A dimensional analysis based multi-scale thermal damage framework of brittle materials at elevated temperatures

Research on macro-scale damage prediction and micro-scale damage mechanisms of brittle materials at elevated temperatures is extremelyimportantto the design and application of practical engineering structures. In this study, a multi-scale thermal damage framework is proposed for both describing macro-scale damage evolution and collective micro-scale cracks behavior of brittle materials atelevatedtemperatures. In the framework, the evolution equation of ideal micro-crack system is deduced to establish the relationship between the macro-scale damage and the micro-scale crack nucleation and growth based on dimensional analysis method. To support the multi-scale thermal damage framework, the multi-scale damage of the benchmark fracture test of the Brazilian disk at elevated temperatures is predicted and compared with the experimental results. The results support the effectiveness and ability of the multi-scale thermal damage framework, which can achieve thermal damage prediction of brittle materials in macro-scale as well as physical mechanisms description in micro-scale.

A robust interface finite element formulation for modeling brittle material failure problems

AbstractFailure of many brittle materials and structures can be modeled using interface‐oriented finite elements combined with intrinsic cohesive zone models. The discontinuous Galerkin (DG) finite element method provides an innovative framework for modeling brittle crack propagation with zero‐thickness interface elements, which can accommodate extrinsic cohesive laws to avoid the artificial compliance required in intrinsic cohesive models. However, robust formulations and implementations of DG methods are critical in alleviating the well‐known convergence issues for both crack nucleation and propagation with reduced instability. This paper presents a robust interface element formulation by modifying the incomplete interior penalty Galerkin (IIPG) method, which successfully avoids the initial element interface penetration across elements that occurs prior to crack nucleation, and thereby greatly reduces the instability issue as cracks open. We further verified and validated our implementation by using a bar tension test and a beam fracturing benchmark. The robustness of our proposed interface element method was demonstrated by a micromechanics fiber/matrix debonding problem with 64 fibers embedded in a bulk matrix.

High and low cycle fatigue properties of selective laser melted AISI 316L and AlSi10Mg

In the last years, additive manufacturing has widely adopted to enable lightweight design based on the topological optimization. In fact, this technology allows generation of lattice structures with complex geometries and small thicknesses. In this work, both the low-cycle-fatigue and high-cycle-fatigue behaviors of selective laser melted AISI 316L and AlSi10Mg were investigated. Fatigue samples were designed to characterize small parts and tested in the as-built condition since reticular structures are usually adopted without any finishing operation. Microstructural features were studied by light-optical microscopy and scanning-electron microscopy. Finally, fatigue failures were studied considering the fracture mechanics principles with the Kitagawa-Takahashi diagram. The analysis of fracture surfaces revealed that crack nucleation mainly occurs close to the surface because of both poor surface quality and presence of near-surface defects. As expected, because of the face-centered cubic lattice, the final rupture of all the investigated alloys was characterized by micro-dimples confirming the presence of a ductile behavior.

Micro-mechanisms underlying enhanced fatigue life of additively manufactured 316L stainless steel with a gradient heterogeneous microstructure

The fatigue life of additively manufactured 316L stainless steel (SS) in an aqueous corrosive medium is shown to improve markedly when the surfaces were processed by surface mechanical attrition treatment (SMAT). SMAT induced surface nano-crystallization owing to severe plastic deformation and yielded a gradient microstructure beneath the surface of additively manufactured SS 316L, which increases the resistance to fatigue crack initiation and propagation. During cyclic deformation, dislocations are generated and accumulated in the cell boundary, resulting in the evolution of persistent slip bands (PSBs) at the surface of the as-manufactured specimen. Continuous intrusions and extrusions of PSBs provide crack initiation sites due to strain localization at the surface of the as-manufactured specimen, whereas the crack initiation region moved from the surface to the sub-surface layer after SMAT. Mainly, the mutual crossing of shear bands or twin boundaries accommodates most of the plastic deformation and provides crack nucleation sites at subsurface regions in this case. To elucidate the fatigue failure mechanism associated with fatigue loading (interactions of stacking faults, slip bands, and deformation twins), electron back-scattered diffraction and transmission electron microscopy analysis of the fatigue-failed alloy after SMAT was performed at different distances adjacent to the fatigue-failed tip. The interfaces between dislocation forest and twin-twin boundaries hinder dislocation motion and disrupt stress concentration. In addition, amorphous bands were formed at the interfaces between twin and shear bands which co-deform with the matrix during dynamic loading and act as a natural sink for dislocations, delaying decohesion and commencement of fracture. The changes in the crack initiation site and fatigue micro-mechanism cause considerable changes in fatigue life. This work provides insight into the failure mechanisms of additively manufactured SS for designing surface modification techniques to enhance the fatigue life of the parts post-printing.

Crack‐like effectiveness of some discontinuities in AA2024

AbstractMaintaining aircraft airworthiness to ensure the fleet's safe operation and maintain its readiness is critically dependent on accurate modelling and reliable predictions of fatigue crack growth. In this process, a knowledge of the representative initial discontinuity sizes that cause fatigue crack nucleation and early growth in aircraft is essential. Here the effective pre‐crack size of aluminum alloy 2024, from samples of aircraft production material and tested under aircraft spectra, are considered.

Evolution of Fretting-Induced Cracks and Delamination Wear on Alloy 690 Steam Generator Tubes During Mixed Fretting Regime in Air at Room Temperature

The occurrence of fretting-induced cracks and delamination wear processes in a mixed fretting regime on Alloy 690 steam generator tubes against 304L stainless steel pads was analyzed. Fretting tests in 90° cross-cylinder configuration were performed in air at room temperature. Changes in the contact conditions were evaluated using the energy ratio and the sliding ratio criteria. Damage was characterized using light microscopy, scanning electron microscopy, and optical profilometry. Vickers microindentation was applied to characterize the initial plastic deformation process. It was found that during the initial 1 × 103 cycles, plastic deformation constitutes the main damage mechanism. Thereafter, between 1 × 103 and 1 × 104 cycles, cracks nucleation and propagation accompanied by negligible wear were detected. Beyond 1 × 104 cycles, the dominant main damage mechanism consists of delamination wear assisted by cracks coalescence, and a change from a U-shaped to a W-shaped scar profile was observed in correspondence with the appearance of the delamination wear process.