Fast Crack Propagation Research Articles

Peridynamic modeling of crack propagation driven by hydrogen embrittlement

Hydrogen embrittlement is an important metal failure mechanism that affects the large-scale application of hydrogen energy. And developing an effective numerical model is vital for understanding and preventing hydrogen-assisted crack propagation. This paper proposes a peridynamic hydrogen embrittlement model based on the two-parameter model of bond-based peridynamics, where the limitation of Poisson’s ratio is relieved. The bond damage criterion that considers the hydrogen embrittlement mechanism is applied. Both the hydrogen diffusion and the hydrogen embrittlement phenomenon are simulated. The efficiency of the proposed peridynamic hydrogen embrittlement model is validated through four numerical examples, including the mode I crack propagations and complex crack paths simulations. The numerical results of the proposed peridynamic hydrogen embrittlement model are compared to the results of other numerical models and experiments. And the influences from hydrogen diffusion time steps and grain boundary diffusion coefficients on hydrogen-assisted crack propagation are investigated. The numerical results show that more diffusion time steps and larger grain boundary diffusion coefficients lead to faster hydrogen diffusion and faster crack propagation.

Corrosion Fatigue of Standard Duplex Stainless Steel X2CrNiMoN22-5-3 under Rotation Bending Load in Northern German Basin Environment

Standard duplex stainless steel X2CrNiMoN22-5-3 is resistant to both, corrosion and mechanical stress, but corrosion fatigue (CF) lowers the lifetime expectancy in a geothermal environment such as the Northern German Basin. Laboratory experiments used the in-situ electrolyte at 369 K in a specifically designed corrosion chamber applying rotation bending cyclic load to failure. CF behaviour was compared to pure push-pull load. Corrosion kinetics are independent of the load applied. Failure is initiated by pits resulting in mechanical degradation. Increased thickness of the passivating layer surrounding pits enhances degradation or delamination. Also, sharp notches located along the pit edge increase notch effects and stress concentration consequently leading to fast crack propagation and early failure.

The behavior of anisotropic fatigue short crack initiation and propagation for a directionally solidified superalloy CM247LC at room temperature

AbstractThe effects of mechanical and microstructural anisotropy on short fatigue crack initiation and propagation behaviors of a directionally solidified superalloy have been studied. An unusual result was found where the fatigue lives of specimens with grains longitudinally aligned along the loading direction fail at lower lifetimes than specimens with transversely loaded grains when the applied stress is close to the yield stress. This is mainly attributed to the lower Young's modulus of the longitudinal specimen, which induces more local plastic strain (at stress concentration features) leading to earlier crack initiation and faster crack propagation under the applied test stress.

Homogenization temperature dependent microstructural evolution and mechanical properties in a Nb-stabilized cast austenitic stainless steel

A Nb-stabilized cast austenitic stainless steel is processed by homogenization treatments at 1050 °C, 1150 °C, and 1250 °C respectively to investigate microstructure dependent deformation behaviors and mechanical properties. Homogenization treatment at 1050 °C could not change the size of primary NbC, while significant dissolution of primary NbC destroys the network connectivity after homogenization treatments at temperatures above 1150 °C. Meanwhile, dissolution of NbC will increase C and Nb content in austenite matrix, leading to an increase in SFE. Nucleation of microcracks within NbC at the initial stage of deformation is caused by plastic strain incompatibility at the NbC/γ interface. Fast crack propagation along the continuous network of primary NbC leads to premature fracture at a lower stress level, and resultant lower ultimate tensile strength and elongation in as-cast and 1050-H samples. A higher SFE reduces the susceptibility to localization of deformation and crack propagation along the discrete primary NbC is suppressed, resulting in an obvious improvement in the ultimate tensile strength and elongation in 1150-H and 1250-H samples. • The dissolution and precipitation of NbC during homogenization treatments by different temperatures were revealed. • A higher SFE is responsible for the lower rate of deformation-induced α'-martensitic transformation by suppressing the planar slip. • A higher SFE reduces the susceptibility to localization deformation and the crack propagation along the primary NbC is suppressed. • A noticeable improvement in the ultimate tensile strength and elongation after homogenization treatment at temperatures above 1150 °C.

Correlation between grain size variation and hydrogen embrittlement in a cost-effective Fe40Mn40Ni10Cr10 austenitic medium entropy alloy

The effect of grain size variation (11 μm, 34 μm) on the hydrogen-induced tensile properties degradation of a Co-free cost-effective Fe40Mn40Ni10Cr10 austenitic medium entropy alloy was investigated using a slow strain rate test. Despite improving both strength and ductility with decreasing the grain size in non-charged conditions, the fine-grain alloy showed a higher relative elongation loss after electro-chemical hydrogen charging. The larger ductility loss in the fine-grain alloy was ascribed to the fast propagation rate of major intergranular cracks and the drastic strain hardening rate drop of the alloy under hydrogen charging. The Schmid factor analysis showed that the enhanced dislocation activity in the fine-grain alloy compared with coarse-grain one was responsible for rapid hydrogen transfer to the grain boundaries, fast dislocation pile-up behind the grain boundaries, and, consequently, more severe hydrogen embrittlement. The significant stress concentration near the grain boundaries and fast intergranular crack propagation were recognized to be the main reason for premature fracture in fine-grain alloy.

Enhancing cavitation erosion resistance of sputtered NiCrAlTi coatings with coherent nanoprecipitates

Ceramic and composite coatings have been extensively studied as candidates for hydromechanical cavitation protection. However, fast crack propagation in hard coatings may lead to catastrophic destruction because of their poor fracture toughness. Here, NiCrAlTi nanocrystalline coating was prepared by magnetron sputtering. After introducing O2 in sputtering gas, moderate amount of oxygen was successfully doped in the coating. Subsequent annealing treatment drove oxygen to react with alloying elements to form in situ nanoprecipitates at the interface, these chemically bonded interfaces enhanced energy dissipation at the crack tip and deflected propagation of cracks in the coatings. Compared with the conventional NiCrAlTi coating, the NiCrAlTi(O) coating exhibited a high H3/E2 ratio of 0.037 GPa and a low cavitation erosion rate of 0.043 mg/h.

Abnormally fast crack propagation induced by short-range ordering in iron-cobalt alloys: A combined experiments and molecular dynamics simulations

We experimentally found that FeCo21.5Cr0.2Ni0.8 alloy quenched from 500 °C, slightly above the order-disorder transition temperature, surprisingly exhibited an extreme impact brittleness. Impact fracture morphology observed by scanning electron microscope (SEM) showed that the impact brittleness was caused by the fast crack growth and poor local plasticity. Short-range ordered (SRO) structures were found in the sample quenched from 500 °C by high-resolution transmission electron microscope (HRTEM) characterizations, and the average SRO size is ~1.58 nm. We contribute the impact brittleness to the formation of SROs. Molecular dynamics (MD) simulations were carried out to reveal the SRO-induced embrittlement mechanism caused by fast crack propagation under high-speed loading. MD simulations showed SROs significantly inhibited the deformation plasticity near crack tips by suppressing the dislocation proliferation, dislocation slip, and twinning. Generalized stacking fault energy (GSFE) curves were calculated to evaluate the effect of SROs on antiphase boundary (APB) energies, unstable stacking energy γusf, and intrinsic stacking energy γisf. SROs were found to prominently increase the APB energies and γisfγusf, fully supporting the SRO-induced poor dislocation and twinning plasticity near crack tips in MD simulations. These findings revealed the potential SRO-induced impact brittleness and its embrittlement mechanism, and provide a design consideration for the engineering application of materials under impact loading.

Numerical Fracture Analysis of a Reactor Pressure Vessel based on Abaqus-FRANC3D Co-simulation Method

The reactor pressure vessel (RPV) is subjected to many cyclic loadings critical of which is the integrity risk posed by pressurized thermal shock (PTS) loadings induced by one of the most frequent anticipated operational occurrences - inadvertent operation of the safety injection system (SIS). In this paper, the fracture mechanics analysis for an ageing pressurized water reactor subjected to PTS induced by inadvertent actuation of the SIS is performed using a proposed simplified Abaqus-FRANC3D co-simulation method. A three-dimensional (3-D) finite element half-symmetric model of a typical pressured water reactor RPV is used to perform thermal-mechanical stress coupling analysis. 3-D fracture mechanic submodel with an assumed surface crack is created for the computation of the transient integrity parameter-stress intensity factor (SIF), using M-integral approach coupled in the proposed co-simulation method. Subsequently ASME method is used to evaluate the vessel’s material fracture toughness. Finally, the SIFs obtained with the simplified co-simulation method is compared with the conventional virtual crack-closure technique (VCCT), and the result show good agreement. This work serves as a useful reference for fast crack propagation and life prediction analysis of ageing PWR RPVs.

Mathematical models of supersonic and intersonic crack propagation in linear elastodynamics

This paper presents mathematical models of supersonic and intersonic crack propagation exhibiting Mach type of shock wave patterns that closely resemble the growing body of experimental and computational evidence reported in recent years. The models are developed in the form of weak discontinuous solutions of the equations of motion for isotropic linear elasticity in two dimensions. Instead of the classical second order elastodynamics equations in terms of the displacement field, equivalent first order equations in terms of the evolution of velocity and displacement gradient fields are used together with their associated jump conditions across solution discontinuities. The paper postulates supersonic and intersonic steady-state crack propagation solutions consisting of regions of constant deformation and velocity separated by pressure and shear shock waves converging at the crack tip and obtains the necessary requirements for their existence. It shows that such mathematical solutions exist for significant ranges of material properties both in plane stress and plane strain. Both mode I and mode II fracture configurations are considered. In line with the linear elasticity theory used, the solutions obtained satisfy exact energy conservation, which implies that strain energy in the unfractured material is converted in its entirety into kinetic energy as the crack propagates. This neglects dissipation phenomena both in the material and in the creation of the new crack surface. This leads to the conclusion that fast crack propagation beyond the classical limit of the Rayleigh wave speed is a phenomenon dominated by the transfer of strain energy into kinetic energy rather than by the transfer into surface energy, which is the basis of Griffiths theory.

Anisotropic stress-softening effect on fast dynamic crack in filler-reinforced elastomers

Deformation of filler-reinforced elastomers is accompanied by a complex internal damage due to stress softening (Mullins effect). The crack growth behavior of filler-reinforced elastomers is made significantly complex by the occurrence of a simultaneous directional stress softening. The influences of the directional stress softening on fast crack propagation normal to the pre-stretching direction are revealed by the comparison using three types of specimens: (I) a low-filled elastomer with almost no stress softening, (II) a pristine highly filled elastomer with a pronounced stress-softening feature, and (III) an elastomer obtained by imposing before uniaxial loading–unloading cycles with a large maximum stretch of specimen (II). The previous directional softening causes a reduction in the threshold tearing energy for velocity transition, and an increase in local strain in the softening direction. In contrast, the previous directional softening has no appreciable effect on the tearing energy dependence of velocity, crack-tip shape, strain field behind the crack-tip, and the strain singularity near the crack-tip.

Investigation on the size and distribution effects of O phase on fracture properties of Ti2AlNb superalloy by using image-based crystal plasticity modeling

Ti2AlNb superalloy consisting of O-phase and B-phase often exhibits deformation heterogeneity and incompatibility due to the mechanical characteristics of phases. To study the size and distribution effects of the O-phase on the fracture properties of Ti2AlNb superalloy, three types of material with different O-phase sizes and distributions were prepared through heat treatments, and the tensile properties of the heat-treatment samples were investigated. An image-based crystal plasticity model was built based on the backscattered electron images of the superalloy to probe the effects of O-phase size and distribution on stress, strain partitioning, and fracture. The strain hardening and fracture behaviors were discussed in terms of the strain partitioning between O-phase and B-phase, and the formation and propagation of micro-cracks, respectively, by combining the simulation and experimental results. Simulation results show the stress and strain partitioning between O/B phase interfaces increases with the increase of O-phase size, and micro-cracks are formed when the local stress concentration exceeds the critical value. The presence of micro-cracks imposes high shear stress on the neighboring B-phase matrix and promotes fast crack propagation through the B-phase matrix, resulting in cleavage fracture. Inversely, micro-cracks tips can be alleviated when the stress concentration at O/B phase interfaces can be relaxed by the plastic deformation of the B-phase matrix. The variation of stress and strain partitioning with deformation during plastic deformation shows that strain partitioning isn't related to O-phase size, but stress partitioning depends on it. Thus, the effects of O-phase size on fracture mainly depend on stress partitioning. This paper presents a meaningful method to model the phase distribution, and the effects of O-phase size and distribution on stress, strain partitioning, and fracture behavior are studied.

Nano-silica reinforced epoxy resin/nano-rubber composite material with a balance of stiffness and toughness

In the electronics and aerospace industries, epoxy resins are generally regarded as economical and efficient adhesives and have a high status. However, epoxy resins are highly crosslinked polymers and are very brittle adhesives where they are prone to fast crack propagation under dynamic loads. Therefore, it is very necessary to enhance the toughness of epoxy resin adhesives. Nano-rubber has been proved to be an important toughening agent for epoxy resin, which can significantly improve the fracture toughness of epoxy resin. However, increasing the toughness of epoxy resin by adding nanomaterials is often accompanied by decreasing the strength and stiffness of resin. Therefore, in this work, rigid nano-silica particles were added to improve the rigidity and tensile strength reduction caused by the addition of rubber particles. And further increase the toughness of the epoxy resin to obtain an epoxy adhesive with balanced stiffness-toughness. As a result, it can be found that the addition of silica particles can significantly improve the decrease in stiffness caused by the addition of rubber particles. For example, Young’s modulus and tensile strength are increased by 28%, and 23%, respectively, with 4% silica is added based on rubber particles. Through the single lap shear experiment, it is found that the shear strength of the epoxy/RnP/silica composite adhesive has increased, which further proves that the addition of nano-silica particles can increase the stiffness of the epoxy composite. The dynamic mechanical analysis experiment found that after adding nano-silica particles, the storage modulus of epoxy composites increased, which also shows that adding nano-silica particles can improve the stiffness of epoxy composites. Scanning electron microscopy analysis was performed to study the reinforcement mechanism of epoxy/RnP/silica composite materials. The thermal stability of epoxy composites was characterized by Dynamic mechanical analysis and thermogravimetric analysis.

Residual stress effects on fatigue crack propagation in Butt–Welded joints for 304 stainless steel sheets

Welded joints are sensitive to fatigue failure due to cyclic loading, as well as fatigue crack propagation influenced by the distribution of welding residual stress. In this study, the fatigue crack propagation rates in butt-welded joints for 304 stainless steel sheets were evaluated in the presence of welding residual stresses. The analysis consisted of two separate models: first, a 3D-finite element (FE) model was used to predict the residual stresses due to welding; second, a numerical study was undertaken to predict fatigue crack propagation in the presence and absence of residual stress using the extended finite element method (XFEM). The crack growth model (NASGRO) and available experimental data were applied to verify the simulation results. The XFEM without residual stress effects shows good agreement with the experimental data and the NASGRO model. However, in the presence of residual stress, the simulation results show less agreement with the NASGRO model. The level and the nature of residual stress have significant effects on crack growth. A faster crack propagation rate is recognized due to the effect of tensile residual stress at the crack tip, while a higher resistance to crack growth is developed due to a compressive residual stress field.

On the Elastic Properties and Fracture Patterns of MoX2 (X = S, Se, Te) Membranes: A Reactive Molecular Dynamics Study

We carried out fully-atomistic reactive molecular dynamics simulations to study the elastic properties and fracture patterns of transition metal dichalcogenide (TMD) MoX2 (X = S, Se, Te) membranes, in their 2H and 1T phases, within the framework of the Stillinger–Weber potential. Results showed that the fracture mechanism of these membranes occurs through a fast crack propagation followed by their abrupt rupture into moieties. As a general trend, the translated arrangement of the chalcogen atoms in the 1T phase contributes to diminishing their structural stability when contrasted with the 2H one. Among the TMDs studied here, 2H-MoSe2 has a higher tensile strength (25.98 GPa).

A New General Paradigm for Understanding and Preventing Li Metal Penetration through Solid Electrolytes

Summary The use of lithium (Li) or sodium (Na) metal anodes together with highly ion-conductive solid electrolytes (SEs) could provide batteries with a step improvement in volumetric and gravimetric energy densities. Unfortunately, these SEs face significant technical challenges, in large part because Li and Na dendrites can penetrate through SEs, leading to short circuits. The ability of such a soft material (Li or Na metal) to penetrate through ceramic is surprising from the point of view of models widely used in the Li-battery field. We introduce a concept, new to the battery field, for preventing penetration of lithium dendrites through SEs by putting the SE surfaces into a state of residual compressive stress. For a sufficiently high compressive stress, cracks have difficulty forming, and cracks that do form are forced to close, inhibiting dendrite penetration. This approach is widely used to solve commercially important stress corrosion cracking problems in metals and static fatigue problems in ceramics and glasses (e.g., Gorilla Glass). However, the technique will not be useful for SEs if the Li-ion transport rate through a SE is substantially reduced when the SE is under compression. Our molecular dynamics calculations for Li-ion transport through a common SE demonstrate that the introduction of even very high residual compressive stresses (∼10 GPa) has only a modest effect on Li-ion transport kinetics, suggesting that this approach is viable and capable of providing a new paradigm for developing high-performance and mechanically stable SEs.

Temperature Effects on the Fracture Dynamics and Elastic Properties of Popgraphene Membranes.

Popgraphene (PopG) is a new 2D planar carbon allotrope which is composed of 5-8-5 carbon rings. PopG is intrinsically metallic and possesses excellent thermal and mechanical stability. In this work, we report a detailed study of the thermal effects on the mechanical properties of PopG membranes using fully-atomistic reactive (ReaxFF) molecular dynamics simulations. Our results showed that PopG presents very distinct fracture mechanisms depending on the temperature and direction of the applied stretching. The main fracture dynamics trends are temperature independent and exhibit an abrupt rupture followed by fast crack propagation. The reason for this anisotropy is due to the fact that y-direction stretching leads to a deformation in the shape of the rings that cause the breaking of bonds in the pentagon-octagon and pentagon-pentagon ring connections, which is not observed for the x-direction. PopG is less stiff than graphene membranes, but the Young's modulus value is only 15 % smaller.

Fretting fatigue crack initiation and propagation in Ti6Al4V sheets under tribocorrosive conditions of artificial seawater and physiological solutions

The interaction of mechanical components experiencing relative movements and cyclic loads in a corrosive environment is known as fretting corrosion or tribocorrosion. In the current work, the mechanism of crack initiation and propagation in dovetail slots of Ti6Al4V samples (in contact with carbide rods) under fretting corrosion conditions was investigated. A newly developed test rig installed on a universal testing machine was used to conduct tests at 20 Hz frequency under 5 and 7.5 kN fretting loads. Tests were conducted at room temperature in 3.5% NaCl and phosphate-buffered saline solutions. Crack propagation in all samples was examined by a metallurgical microscope, and the detailed analysis of fractured samples was carried out by a scanning electron microscope. In comparison to dry conditions, early crack initiation and faster crack propagation were observed in salt and physiological solution environments. Colored spots and large amounts of chlorine, sodium, and oxygen were found around cracks, and plastically deformed regions in the 3.5% NaCl environment provided the evidence of a corrosive attack. Large amounts of oxygen, phosphorous, chlorine, potassium, and sodium were detected in the phosphate-buffered saline environment.

Strain and damage sensing in additively manufactured CB/ABS polymer composites

In this study, an experimental investigation is performed to observe the electromechanical response of CB (carbon black)/Acrylonitrile butadiene styrene (ABS) additive manufactured composite under quasi-static (tensile, shear, and mode-I fracture) and dynamic (mode-I fracture) loading conditions for the potential damage sensing applications. Dog bone tensile, double v-notch shear, and single edge notch bending (SENB) specimen printed with three different configurations (0°/90°, +45°/-45°, and 0°) are considered for the quasi-static condition. A modified split Hopkinson pressure bar along with high-speed video camera is used for dynamic fracture experiments. Four-point probe technique coupled with a high-resolution data acquisition system is employed to obtain the real-time electrical response. In the case of tensile loading, +45°/-45° printed specimens show a nonlinear change of electrical resistance due to unique failure mode. Under the shear loading, electrical resistance remains unchanged during the elastic deformation. After the damage evolution, +45°/-45° printed specimens exhibit a higher rate of change in electrical resistance due to alignment of the filaments along the maximum principle shear stress direction. For both static and dynamic fracture loading, a minimal change of electrical resistance is observed before crack initiation. However, after the crack initiation, a sharp change of electrical resistance for 0°/90° printed specimens indicates a faster crack propagation as compared to the +45°/-45° printed specimens.

In-Situ SEM Fracture Analysis of W-Eurofer Brazed Joints Under Three-Point Bending Test Configuration

The fracture mechanism of W-Eurofer brazed joints was studied by in-situ SEM three-point bending tests using two different notched sample configurations. The objective of these tests was to analyze and understand the possible mechanical and fracture behavior of the joint during service life. The results showed that the mechanical behavior of this dissimilar brazed joint is very sensitive to defects on the tungsten side. When the notch is located in the tungsten base material, the stress concentration located in this material causes a fast crack propagation from the notch to W-braze interface, causing premature joint failure. However, when the notch is made in the Eurofer base material, the stronger Eurofer-braze interface and toughening behavior of the steel completely change the fracture mechanism, leading to improved mechanical behavior of the brazed joint.

Thickness debit effect in Ni-based single crystal superalloys at different stress levels

In order to investigate the mechanism of thickness debit effect at different stress levels, creep tests of thin-walled Ni-based superalloys were conducted under three test conditions: 220 MPa, 275 MPa and 330 MPa at 980 °C. Results showed that the creep life reduced with the decrease of specimen thickness at 220 MPa and 275 MPa. At 330 MPa, this variation tendency was not so evident. Metallographic observations revealed that the specimen thickness and the external stress level affected the depth of γ' depleted region and the distribution of voids and cracks of the specimen. Thus, a phase-field model and a crack growth model were used to consider the influence of γ' depleted region and crack growth on life, respectively. It was found from the phase-field model that the existence of γ' depleted region led to more considerable plastic strain and equivalent stress in the normal γ/γ' phase region than that in the γ' depleted region. This phenomenon was more evident in the thin specimen at low stress level. At high stress level, the redistribution of the stress field was not so distinct due to the premature plastic deformation in the normal γ/γ' phase region. The crack growth models showed a shorter critical value of the crack length and faster crack propagation in the thin specimen. However, the increased external stress levels would decrease the disparity of cracks propagation between the thin and thick samples. Comprehensive consideration of the phase-field method and the crack growth model revealed the thickness debit effect at different stress levels.