Crack Nucleation Mechanism Research Articles

Early Stages of Crack Nucleation Mechanism in Fe39Mn20Co20Cr15Si5Al1 High-Entropy Alloy during Stress Corrosion Cracking Phenomenon: Pit Initiation and Growth

Early Stages of Crack Nucleation Mechanism in Fe39Mn20Co20Cr15Si5Al1 High-Entropy Alloy during Stress Corrosion Cracking Phenomenon: Pit Initiation and Growth

Analysis of Fatigue Crack Nucleation in Double-Network Hydrogels.

Hydrogel-based devices commonly have a high demand for material durability when subjected to prolonged or cyclic loads. To extend their service life, it is crucial to have a deep understanding of the fatigue mechanisms of hydrogels. It is well-known that double-network (DN) hydrogels are characterized by high strength and toughness and are thus recognized as a promising candidate under load-bearing conditions. However, the existing studies in the literature mainly focus on their resistant capability to fatigue crack growth, while the underlying mechanisms of fatigue crack nucleation are still inconclusive. This work aims to bridge this knowledge gap by formulating a fatigue life predictor for DN hydrogels within the framework of configurational mechanics to elucidate the underlying mechanisms governing fatigue crack nucleation. The fatigue life predictor for DN hydrogels is derived from the configurational stress by incorporating the corresponding constitutive models and the thermodynamic evolution laws for microdamage mechanisms and material viscoelasticity. With the developed fatigue predictor, the effect of the microdamage mechanism on fatigue is elucidated, i.e., the internal damage of the sacrificial network can improve the fatigue life of DN hydrogels. The fatigue life predictor is also adopted to evaluate the effects of some other factors on the fatigue crack nucleation, such as the loading rate, pre-stretching treatment, and water diffusion, identifying feasible loading profiles that could improve material durability. Overall, the theoretical framework and the modeling results in this work are expected to shed light on unveiling the fatigue mechanisms of DN hydrogels and advance the design of hydrogel-based devices.

Crack nucleation mechanism of Ti60 titanium alloy under ultrasonic fatigue loading at different temperatures

The crack nucleation mechanism of Ti60 titanium alloy under ultrasonic fatigue loading at different temperatures was investigated. The results show that there are two different crack nucleation mechanisms at RT, 300 °C, and 400 °C, i.e., the oversized facet and oxide shedding crack nucleation mechanisms. Almost all fatigue cracks initiate in the form of oversized facets at RT. At 300 °C, the crack initiates at the oversized facet under the low load test, while the crack initiates at the oxide shedding site under the high load test. At 400 °C, all the fatigue cracks initiate at the place where the oxide shedding. Higher temperature and applied stress will help the oxide shedding crack nucleation mechanism to become dominant.

Nonadiabatically Driven Subcritical Crack Nucleation in Solids

This paper discusses a subcritical crack nucleation mechanism in a brittle solid within a real range of applied stress. A medium deformed by uniaxial tension is considered as an open nonequilibrium system of nuclei and electrons. Structural relaxation of the medium begins with the excitation of dynamic displacements during nonadiabatic Landau–Zener transitions. Dynamic displacements induce the instability of the medium to the longitudinal displacement wave. The kinetics of structural relaxation is described by two nonlinear parabolic kinetic equations for dynamic order parameters. Conditions are derived for the existence of localized solutions (autosolitons). The excitation of autosolitons leads to local elongation and cross-sectional reduction of the specimen. The resulting neck is a subcritical crack.

Study on the crack nucleation mechanism of Ti-2Al-2.5Zr alloy in low cycle fatigue: Quasi in-situ experiments and crystal plasticity simulation

The fatigue crack nucleation mechanism of Ti-2Al-2.5Zr alloy was investigated using the coupling method of quasi in-situ tests with crystal plasticity (CP) simulation. The constitutive parameters in the CP model were validated by digital image correlation based on scanning electron microscope (SEM-DIC). It was found that the transgranular and intergranular cracks nucleated inside grains and at grain boundaries (GBs), respectively, due to slip intrusions/extrusions along prismatic planes with highest SFpris. values, while quasi transgranular cracks were induced by the adjacent microstructure or cracks. The angle Ω between the sample surface and the slip direction of slip planes where the transgranular cracks nucleated was within the range of 20–55°. Three factors for predicting intergranular cracks were identified, and the slip shearing mechanism of GBs was proposed. The CP results revealed that a significant plastic strain difference was generated at GBs where intergranular cracks nucleated. The band averaging method showed better prediction ability than the grain averaging method, and a unified fatigue indicator parameter (FIP) combining the accumulated plastic strain, the strain incompatibility parameter (SIP), and the abovementioned crack prediction factors, was proposed, which improved the unified prediction ability for both transgranular and intergranular cracks.

Tensile and fatigue properties of the binder jet printed and hot isostatically pressed 316L austenitic stainless steel

The binder jet printed (BJP) metallic components are not yet considered for widespread structural applications that involve cyclic loading conditions due to the high levels of porosity typical to them. In the present study, the porosity in BJP 316L stainless steel (SS) was reduced from 3.5-5.6% to 1–1.2% by improving process parameters first and then employing the post-processing hot iso-static pressing (HIP) treatment. The reduced porosity in the as-sintered conditions resulted in ⁓15 and 5% improvement in tensile yield and ultimate tensile strengths, respectively. However, the high cycle fatigue strength (⁓250 MPa) did not improve even after a ⁓fivefold reduction in the porosity. HIP resulted in a further reduction in the porosity, which improved the elongation to failure by 30% while reduced the yield strength by ∼10% (due to grain coarsening that occurred during HIP). It, however, did not improve the unnotched fatigue strength. The microstructural coarsening during HIP also affected the short fatigue crack growth resistance of the BJP 316L SS specimens. Moreover, after reduction in overall porosity and the size of the pores, the void and fatigue crack nucleation mechanisms during tensile and fatigue loadings changed from pore assisted to inclusion assisted.

Dislocation Slip and Crack Nucleation Mechanism in Dual-Phase Microstructure of Titanium Alloys: A Review

Dislocation Slip and Crack Nucleation Mechanism in Dual-Phase Microstructure of Titanium Alloys: A Review

Experimental Study of VHCF Fractographic Features of Conventionally and Additively Manufactured Steels

Materials produced by additive manufacturing (AM) have been extremely related to literature. However, there is still unconsolidated knowledge about the fatigue life and respective mechanisms of initiation of cracks predominant in the VHCF regime for these materials. What has been observed in materials produced by conventional routes is that fatigue cracks tend to nucleate from intrinsic defects of the material located internally or in subsurface regions. The change in the evolution process of fatigue cracks leads to the formation of a characteristic morphology on the fracture surface, known as “fish-eye”. Another widespread aspect observed on the fracture surfaces is the formation of a fine granular area (FGA) nearby the initiation sites. This work aims to investigate the mechanisms of crack nucleation in VHCF of two distinct materials: conventional steel, DIN 34CrNiMo6 and AISI 316L stainless steel produced by L-DED. The ultrasonic tests were carried out at a frequency of 20±0,5 kHz and R= -1. The S-N curves were obtained and fracture surfaces were analyzed, fish-eye and FGA formation was verified. FGA sizes were compared to values estimated by empirical equations. FGA and fish-eye sizes were related to stress amplitude and maximum stress intensity factor (SIF).

Influence of V addition on the mechanical properties of FeCo alloys: a molecular dynamics study

Interatomic potentials for the Fe-Co and Fe-V binary systems have been developed based on an Fe interatomic potential with enhanced mechanical capabilities and merged to describe the Fe-Co-V ternary system based on the second nearest-neighbor modified embedded-atom method (2NN MEAM) formalism. Emphasis has been placed on the correct prediction of FeCo elastic properties and antiphase boundary (APB) energies to ensure proper description of the mechanical behaviors. The potentials are then used to analyze the crack nucleation and propagation mechanisms in polycrystalline FeCo and FeCo-2V alloys under different ordering degrees at grain boundaries (GB) and within the grains. The results show that disordering inside grains has a larger impact on the ductility of the binary FeCo. The mobility of the antiphase domains (APD) inside the grains delays the ultimate tensile strength (UTS) and fracture point. The observed fracture procedure shows that triple junctions of GBs serve as the focal points of stresses concentration. The poor mobility at the triple GBs promotes crack nucleation and crack propagation, leading to an intergranular fracture in ordered and disordered binary alloys. The effects of adding 2 at. % V to the FeCo alloy have also been investigated via molecular dynamics (MD) and hybrid grand canonical Monte Carlo-MD (GCMC-MD) simulations. It was shown that V prefers to migrate to (a) the GBs in an ordered state, and (b) GBs and APBs in a disordered state. The ordered FeCo-2V displays a decreased UTS and loss of ductility at room temperature. Introducing V exclusively to the APBs greatly increases the mobility of APD and migration of atoms to GBs boosting the ductility of the alloy.

Crystal crack dislocation model and micro-crack nucleation criterion in the hydrogen environment

Based on the theory of fracture mechanics, a hydrogen brittleness model of microcrystalline Al materials is proposed by using the distributed dislocation method. The distinguishing feature of this study is that the model can be used to clarify the dependence of hydrogen atoms at different positions and different numbers of hydrogen atoms on dislocation emission in the initial stage of corrosion fracture. At the same time, the mechanism of crack nucleation at the grain boundary in the hydrogen environment is studied. The results indicate that hydrogen can facilitate the emission, proliferation and motion of dislocations at the crack tip, and increase the accumulation energy of dislocations at the grain boundary, making it easier to generate micro-cracks. In the corrosive environment with hydrogen, a large number of hydrogen atom segregation at the grain boundary reduces the surface energy, and cracks are more likely to occur, and the length of microcracks is longer than that in the absence of hydrogen. These studies are helpful to deepen the understanding of the hydrogen embrittlement of materials.

Quantitative assessment of the microstructural factors controlling the fatigue crack initiation mechanisms in AZ31 Mg alloy

The deformation and fatigue crack nucleation mechanisms were studied by means of slip trace analysis and secondary electron microscopy in a textured AZ31B-O Mg alloy subjected to fully-reversed cyclic deformation at two different cyclic strain semi-amplitudes. Samples were deformed in two orientations leading to symmetric and non-symmetric cyclic stress-strain curves due to the activation of different deformation mechanisms. They were ascertained in longitudinal sections of the specimens, which included a large number of grains (from 1500 to 4500 for each specimen), to obtain statistically significant results. If the dominant deformation mechanisms were basal slip and tensile twinning/detwinning, the most damaging fatigue cracks were nucleated along twins in large grains, together with cracks parallel to basal slip bands associated with the localization of deformation in clusters of small grains suitably oriented for basal slip. If the main deformation mechanisms were tensile twinning/detwinning and pyramidal slip, the longest fatigue cracks were nucleated along pyramidal slip bands in large grains. Grain boundary cracks around small grains were found in all cases, but they were not critical from the viewpoint of fatigue failure. This information is relevant to assess the effect of the microstructural features on the fatigue life of Mg alloys and as input to simulate the fatigue behavior of Mg alloys using fatigue indicator parameters.

Face-core interfacial debonding characterization model of an all-composite sandwich beam with a hexagonal honeycomb core

In this paper, the mode I face-core interfacial debonding of an all-composite sandwich beam with carbon fiber reinforced polymer (CFRP) hexagonal honeycomb core is characterized through a theoretical model and double cantilever beam (DCB) tests. The theoretical model is established by combining Timoshenko’s beam theory for the face sheets and Extended High Order Sandwich Panel Theory (EHSAPT) for the CFRP hexagonal honeycomb core and the nonlinear exponential cohesive zone model (CZM) is used to model the face-core interfacial constitutive relation, which can couple the normal and the tangential tractions. Then three groups of all-composite sandwich beams with CFRP hexagonal honeycomb core are fabricated and DCB tests are conducted to characterize the mode I face-core interfacial debonding behaviors. The theoretical model is validated by comparing the results of fracture toughness obtained from theoretical calibration and the MBT method. The crack tip relative displacement and crack tip stress indicate the DCB test is mode I and mode II coupled debonding process and it is governed by mode I fracture. Moreover, the mechanism of crack nucleation and propagation between the face-core interface is revealed in detail. Investigation from this paper seeks and to explain the physical phenomenon of the debonding mechanism of all-composite sandwich beams. Finally, a parametric study is carried out to evaluate the effect of fracture toughness, characteristic length, face sheet thickness and fracture factors on the load–displacement response as well as the effect of fracture toughness, characteristic length, face sheet thickness and core thickness on the interfacial strength and damage zone length.

Elevated-temperature gigacycle fatigue properties of nickel based superalloy: Grain related cracking mechanism and life prediction modelling

Fatigue properties of a nickel-based superalloy at 25 °C and 750 °C were studied under stress ratio R = −1 & 0.1 to identify crack nucleation and growth mechanisms. Results show that interior grain-related cracking is induced by microstructure in Goss or Copper grain. Moreover, owing to competition between temperature and vacuum effects, threshold values for small and long cracks, and the transition crack size, are all lower at elevated temperature. Finally, based on modeling of crack nucleation and growth, a fatigue life model with good prediction results is proposed. The crack nucleation consumes more than 90% of fatigue life.

Strain localization and fatigue crack formation at [formula omitted] twist boundaries in titanium alloys

The process of crack initiation has been investigated in three widely used titanium alloys with different microstructures and loading conditions. Using low-cycle fatigue tests, a unique crack nucleation mechanism involving strain localization at (0001) twist boundaries has been identified. In order to constitute a potential crack initiation site, the twist boundary must experience a high resolved shear stress and a high normal stress. Crack initiation at these boundaries is most frequently associated with twist angles spanning the 10° - 20° range. Deformation prior to crack initiation at these rare microstructural configurations has been characterized using transmission electron microscopy and high-resolution digital image correlation across large fields of view. The (0001) twist boundaries are preferential locations for early and intense strain localization. Prior to crack nucleation, deformation proceeds via shear along such boundaries where no β layer at the interface was evidenced. The presently discussed crack formation mechanism is believed to be of broad relevance as it is not significantly influenced by microstructural parameters such as the α grain size, the degree of microtexture, the β phase fraction or the surrounding microstructure as well as α and β compositions.

Ultrahigh cycle fatigue deformation of polycrystalline NiTi micropillars

Bulk polycrystalline superelastic NiTi shape memory alloy usually has limited fatigue lives. Here we show that polycrystalline superelastic NiTi micropillars under compressive stresses of 1 GPa and 1.5 GPa can endure over 108 phase transition cycles, two orders of magnitude larger than the reported fatigue life of 1.12×106 cycles under 1.2 GPa of bulk polycrystalline NiTi pillars with a similar grain size. The dominant crack nucleation and propagation mechanisms in bulk samples are suppressed in the micropillars due to the elimination of internal micro-voids and surface scratches in the small physical dimensions. Transformation-induced dislocations and the associated residual martensites are constrained at the nanoscale and do not lead to crack nucleation. The research reveals the importance of sample dimensions in bringing changes in the fatigue performance and mechanisms of the material. The observed sample size effects can be exploited to obtain ultrahigh fatigue life at the microscale.

Experimental and analytical investigation on service life of film cooling structure for single crystal turbine blade

In this study, regarding the film cooling structure of single crystal (SC) turbine blade, the life tests simulating the service condition are conducted, and the life prediction modeling as well as damage mechanism analysis are carried out. In life test, the SC specimen with multiple film cooling holes is designed to simulate the geometrical characteristic of film cooling structure. The creep-fatigue loading spectrum characterizing a typical flight process is employed. The loading system is developed to simulate the cyclic service load in laboratory environment. Meanwhile, the strain distribution of film cooling structure is measured using digital image correlation (DIC) method. After that, the life tests are conducted, and eight groups of test lives are obtained. Taking the strain distribution measured by DIC as input, the model based on theory of critical distance (TCD) is improved to predict the service lives of film cooling structure. Furthermore, the crack nucleation mechanism of film cooling structure is discussed. Based on the microscopic observation, it can be found that the microcrack nucleation around hole edge is induced by persistent slip bands (PSBs). The matrix channel that significantly influences the dislocation accumulation rate and PSBs formation is chosen as the concerned microstructure. Under the effect of stress distribution of film cooling structure, the uneven spatial distributions of mean values and standard derivations for matrix channel width are quantificationally revealed. And the distribution of dislocation density on film cooling structure is measured using electron back-scatter diffraction (EBSD). It can be found that the concentration of dislocation density exists around the hole edge, and the distribution of dislocation density is quite unsmooth due to the scatter of matrix channel width. These findings can be employed in the life prediction and fault diagnosis for film cooling structure under service condition.

Cyclic Deformation Induced Residual Stress Evolution and 3D Short Fatigue Crack Growth Investigated by Advanced Synchrotron Tomography Techniques.

Diffraction and phase contrast tomography techniques were successfully applied to an austenitic–ferritic duplex stainless steel representing exemplarily a metallic material containing two phases with different crystal structures. The reconstructed volumes of both phases were discretized by finite elements. A crystal plasticity finite-element analysis was executed in order to simulate the development of the experimentally determined first and second order residual stresses, which built up due to the manufacturing process of the material. Cyclic deformation simulations showed the single-grain-resolved evolution of initial residual stresses in both phases and were found to be in good agreement with the experimental results. Solely in ferritic grains, residual stresses built up due to cyclic deformation, which promoted crack nucleation in this phase. Furthermore, phase contrast tomography was applied in order to analyze the mechanisms of fatigue crack nucleation and short fatigue crack propagation three-dimensionally and nondestructively. The results clearly showed the significance of microstructural barriers for short fatigue crack growth at the surface, as well as into the material. The investigation presented aims for a better understanding of the three-dimensional mechanisms governing short fatigue crack propagation and, in particular, the effect of residual stresses on these mechanisms. The final goal was to generate tailored microstructures for improved fatigue resistance and enhanced fatigue life.

Fatigue properties of continuous wave and pulsed wave laser cold-wire welding of thick section AA6005-T6 aluminum alloys

The effect of laser wave modes on the fatigue behavior of laser cold-wire welding made of 4.8 mm thick AA6005-T6 aluminum alloy was investigated using continuous and pulsed wave lasers. Due to the inherent differences in these two wave laser modes, different welding parameters were used, while keeping the interaction time constant. The mechanical properties of welded joints were measured using tensile tests, while their fatigue performances were quantified using a constant amplitude force-controlled technique to obtain S-N curves. The pulsed wave laser mode produced higher fatigue resistance as compared to the continuous wave mode. The fatigue strength corresponding to the run-out condition (i.e., 107 cycles, in this study) was about 28% higher for the pulsed wave mode than for the continuous wave laser mode. At a high stress amplitude (30 MPa), the lifespan of the pulsed wave joints was about twice as high as that with continuous wave joints. These fatigue results were cross-referenced with a 3D topographic map, a 2D microhardness map, a metallographic study, and a fractographic analysis to better understand the crack nucleation and crack propagation mechanisms. A microhardness analysis performed along the cross-section of the joints did not reveal any significant difference between pulsed wave and continuous wave modes. A fractographic analysis confirmed crack propagation within the fusion zone and that 83 to 90% of cracks nucleated from the root undercuts. Topographic maps of the joints before fracture revealed that the continuous wave laser mode produces deeper and narrower (i.e., more acute) undercut defects than does the pulsed wave mode. A Weibull approach based on the biggest defects found on the fracture surfaces also confirmed that the continuous wave process produces larger defects at the root. Top defects are significantly small (55%), but they have a larger size dispersion than do root defects. Since the root undercuts act as the main stress concentrators, they are mainly responsible for the lower fatigue performance of the joints, and their sizes and shapes should be minimized during further process development of the welding process.

Misorientation effect of twist grain boundaries on crack nucleation from molecular dynamics

Grain boundaries (GBs) have profound effects on the dislocation motion in materials so as to cause crack nucleation. However, the experimental observation on how the misorientation of twist GBs affects the crack nucleation is scarce. In this paper, we proposed a theoretical model to quantify the dislocation energy per unit length. With it, the critical resolved shear stress at GBs was calculated to assess the crack nucleation. It was found that crack nucleation is significantly influenced by the surface energy, grain boundary energy, system potential energy, boundary stress and misorientation. With the increase of the twist angle, high-angle GBs are more favorable for crack nucleation as general. Interestingly, some specific low-angle GBs possess comparable small critical resolved shear stress as the high-angle GBs and they are favorable for crack nucleation as well. Through the analysis of the resolved shear stress distribution of GBs, it is revealed that the crack nucleation mechanism of these specific low-angle GBs is similar to that of general high-angle GBs. It is inferred that the special low-angle GB systems could generate a crack under a relatively low resolved shear stress.

Effect of corrosion on the fatigue behaviour of butt welds of G20Mn5QT cast steel and Q345D hot rolled steel in 3.5‐wt% NaCl solution

AbstractCorrosion fatigue experiments are carried out on butt welds of G20Mn5QT cast steel and Q345D hot rolled steel in simulated seawater. The cyclic stress versus number of cycles to failure (S‐N) curve was fitted. The mechanism of crack nucleation at different fracture locations and at different stress levels is analysed. The results show that compared with the S‐N curve of laboratory air, for the S‐N curve of simulated seawater, the number of fatigue failure cycles is significantly reduced when the maximum cyclic stress is less than 190 MPa. At higher stress levels, stress concentrations caused by internal defects near the surface of the weld zone and the heat‐affected zone of G20Mn5QT cast steel cause crack nucleation. At lower stress levels, the main reason for crack nucleation is the stress concentration caused by corrosion pitting. However, there are no obvious internal defects in the heat‐affected zone of the Q345D steel, and corrosion pitting is the main cause of crack nucleation. The stress concentrations caused by pitting lead to crack nucleation.