Crack Tip Deformation Research Articles

The role of hydrogen embrittlement in the near-neutral pH corrosion fatigue of pipeline steels

This study confirms that "Near-neutral pH Stress Corrosion Cracking" in steel pipelines propagates by hydrogen-enhanced corrosion fatigue. A critical cyclic loading frequency where maximum growth rate per cycle exists. The critical frequency at different temperatures is verified to correspond to hydrogen diffusion predictions. Growth is limited by crack-tip blunting below this frequency and hydrogen diffusion above this frequency. The critical frequency occurs when crack-tip deformation mechanisms that blunt the crack during the stress cycle overwhelm additional enhancement from diffusion. Growth occurs along cleavage planes and ductile slip planes, indicating hydrogen embrittlement, hydrogen-induced plasticity, and fatigue contribute to crack growth.

Fatigue crack growth in AA6082-T6/AA7050-T6 bi-materials: Effect of plastic zone ahead of crack tip

The main objective here is the understanding of the effect of plastic zone ahead of crack tip on fatigue crack growth (FCG). For that, a numerical simulation of FCG was made, based on cumulative plastic strain, considering a bi-material composed by the 6082-T6 and 7050-T6 aluminium alloys. The first one has a lower yield stress but a higher critical cumulative plastic strain. The values of da/dN are higher for the 6082-T6 which means that the effect of the easier plastic deformation dominates over the effect of the higher critical cumulative plastic strain. The bi-material 6082-7050 showed a pronounced decrease of da/dN near the interface, which starts when the monotonic plastic zone ahead of crack tip reaches the material transition. This indicates that the crack tip deformation is affected by the material inside the monotonic plastic zone. On the other hand, the bi-material 7050-6082 shows an accentuated increase of da/dN, which starts when plastic deformation is observed ahead of transition. The elimination of the contact of crack flanks reduces drastically the transient effects, which indicates that these are linked with plasticity induced crack closure effects. Crack tip blunting was proposed to be the mechanism of plasticity induced crack closure explaining the trends observed.

A material energy–momentum flux-driven phase field fracture mechanics model

This work introduces a novel phase-field solution for simulating and predicting fractures in elastic solids. In this phase-field fracture model, the crack growth is driven by the material energy–momentum flux, or the configurational force, inspired by a null divergence conservation law and its variational theory, as opposed to employing the commonly-used strain energy degradation-based crack-driving force. By doing so, the crack growth or material configuration change is solely determined by the physics-based Eshelby energy–momentum tensor density or the Rice J-integral flux, which aligns with the energy release criterion of the celebrated Griffith–Eshelby–Rice (GER) theory. The proposed method not only preserves the integrity of the balance of linear momentum principle at the crack tip region but also provides a correct stress asymptotic field in front of the crack tip, while accurately capturing the crack growth.The key idea of this approach is to use the monotonically increasing and irreversible phase field as the marker of the configurational change and form a phase field variational principle that couples the strain energy density with the configurational strain energy density. By doing so, we only damage the displacement field as a form of diffused crack, as opposed to damaging the stress field in the existing phase-field method. Moreover, from numerical simulations, we discovered two different forms of energy release rates: one drives crack growth by enforcing the crack-tip traction loading condition, and one drives crack growth by enforcing the crack-tip deformation condition, demonstrating the ability of the proposed method to naturally capture two distinct modes of fractures due to two different near-crack tip loading conditions. These advancements and contributions reveal previously unknown insights into fracture mechanics.

Multi-scale characterization of fatigue crack tip deformation and propagation in QP steel

A novel measurement method for multi-scale characterization of fatigue crack tip deformation and propagation behaviors in high strength and plasticity, multi-phase, fine-grain Quenching and Partitioning (QP) steel was proposed. The fatigue crack propagation (FCP) test system with the macro/micro images acquisition devices was built to measure the FCP rate and crack tip deformation simultaneously. First, at the mesoscale, the evolution characteristics of the crack tip deformation fields, plasticity induced crack closure (PICC) behaviors and plastic zone during the FCP process were analyzed using the sub-image stitching and matching digital image correlation (DIC) method. Then, the FCP models were established with special emphasis on the model considering the PICC effect based on the obtained crack opening load at the mesoscale. Moreover, the corresponding microscale surface crack and fracture morphologies, microstructure phase transformation and deformations were observed and analyzed by electron backscatter diffraction (EBSD) and scanning electron microscope (SEM) techniques.

Crack Tip Deformation during Dwell Fatigue and Its Correlation with Crack/Fracture Surface Morphologies in a Bi-Modal Ti-6Al-4V Alloy

Crack Tip Deformation during Dwell Fatigue and Its Correlation with Crack/Fracture Surface Morphologies in a Bi-Modal Ti-6Al-4V Alloy

Investigation of frequency-dependent fatigue crack growth behavior in perfluorosulfonic-acid membrane: In-situ experiment, constitutive modeling and crack growth prediction

Fatigue crack growth behavior of perfluorosulfonic-acid membrane is investigated under distinct loading frequencies. The evolution of cyclic plastic zone (CPZ) is traced based on in-situ optical microscopy testing and digital image correlation (DIC) technique. The crack tip deformation process is directly observed by in-situ scanning electron microscope (SEM). The experimental results show that fatigue cracks experiencing a higher loading frequency display a comparatively reduced propagation rate. The CPZ can be solely used to interpret the fatigue crack growth rate variation under different loading frequencies. The crack-front microspores are easily formed under lower loading frequency due to the enhancement of near-tip plasticity deformation. Then, a cyclic elastic-viscoplastic constitutive model is developed, which successfully capture the mechanical behavior of the membrane. Finally, based on the plastically dissipated energy, a numerical method incorporated with the developed material’s constitutive relationship is established, which gives reasonable prediction of fatigue crack growth under various loading frequencies.

Study on fatigue crack propagation behavior of 2050 Al[sbnd]Li alloy during non-isothermal aging

The fatigue crack propagation behavior of 2050AlLi alloy at different stages during non-isothermal aging (NIA) was tested. It was found that when the aging temperature was reduced to 150 °C, the alloy exhibited higher fatigue crack growth resistance and lower fatigue crack growth rate and the fatigue crack growth path was mainly transgranular propagation in different stages of NIA. In addition, in the first stage of fatigue crack growth, the size and quantity of precipitates in the grain are the main factors affecting the crack growth. Because the tiny T1 phase can be sheared by dislocation without damage accumulation, it can effectively prevent crack propagation. In the second stage of fatigue crack growth, the strength difference between grain boundary and intragranular will directly affect the fatigue crack growth resistance of the alloy. The smaller the strength difference is, the more uniform the deformation of crack tip in the process of crack growth, and it is not easy to produce stress concentration and damage accumulation at the grain boundary. In this case, the alloy exhibits the better the fatigue crack growth resistance and the slower the crack growth rate.

Mixed‐mode I/II stress intensity factors of a biaxially loaded plate with a central slant frictional crack: An analytical comprehensive solution

AbstractThis work presents a comprehensive analytical solution for the mixed‐mode I/II stress intensity factor of a biaxially loaded plate that has a centrally straight inclined crack with frictional surfaces. The solution addresses every possible combination of coefficient of friction, biaxial ratio, crack length, and angle. The present results agree with the available corresponding finite element solutions. For the tension‐compression and compression‐tension biaxial loading, crack angles are recognized to achieve pure mode I, pure mode II, and mixed‐mode crack tip deformation in terms of stress biaxiality and friction coefficient. In the compression‐compression biaxial loading, only mode II crack tip deformation is operative. The ranges of crack angles necessary to produce dormant cracks are determined for the biaxial loading patterns, where a compressive stress is involved, as a function of the biaxial ratio and friction coefficient. It is possible to apply the present solution to cracked plates with two‐dimensional stresses that include a shear stress.

Crack tip field analysis for thermo-mechanical fatigue loading

This study aimed to establish a computational strategy for the thermo-mechanical fatigue (TMF) simulations of crack tip fields considering both the in-phase (IP) and out-of-phase (OOP) loading histories. To this end, an algorithm for multi-physics numerical calculations was developed and implemented by incorporating Maxwell 3D, Fluent, and the transient structural modules of ANSYS 2021R1, to understand the mechanics of crack tip deformation under thermo-mechanical loading conditions. These computations are based on the coupled heat loss from magnetic field eddy currents and the forced convective air-cooling accounted for by turbulence model response, which provides the gradients of mechanical elastic–plastic deformations. The proposed algorithm was applied to replicate and analyse the crack growth test conditions of polycrystalline XH73M nickel-based alloys subjected to a triangular waveform of IP and OOP cycles and a temperature range of 400–650 °C. Further, multi-physics finite element (FE) modelling of the stress, strain, and displacement fields in uncracked and fractured single-edge-notch tension specimens was performed. The novelty of the results lies in the crack tip fields, which strongly influenced by the phase relation between mechanical loading and temperature, provided the electromagnetic and fluid dynamics characteristics and the elastic–plastic properties of the material are dependent on the current temperature and time across a particular deformation cycle. Notably, the complete three-dimensional multi-physics FE analysis presented herein is expected to contribute toward a better understanding of the different mechanisms driving TMF crack growth and also help address the related outstanding questions.

Effects of the exposure to moist air on crack-tip deformation and striation marking in a 7175 T7351 aluminum alloy

This paper investigates the effects of the exposure to moist air on the mechanisms of fatigue crack growth in 7175 T7351 aluminum alloy. A special attention is paid to the crack-tip deformation mechanisms, controlling the fatigue crack growth rate as well as the fatigue striation markings, firstly under moist air and at low loading frequency, corresponding to a high exposure rate, and then as compared to a low exposure rate condition obtained in an inert atmosphere. It is shown that a high exposure rate induces the development at the crack-tip of a 800 nm-thick surface layer, divided into subdomains characterized with a pronounced and highly localized lattice rotation. In contrast, under very low exposure rate, a significantly lower lattice rotation associated with a smoother gradient is observed. This suggests that the formation of subdomains at high exposure rate is actually induced by some environmental effect, presumably in relation with an assistance of hydrogen highly accumulated in a thin subsurface layer in the intense localization of crack-tip deformation resulting into lattice rotation. Finally, it is clearly established that fatigue striation spacing in moist air exactly coincides with the distance separating two successive subdomains exhibiting the highest lattice disorientation.

Cohesive-zone based fracture mechanics model of an edge delamination in bimaterial beam under mixed-mode bending test

The problem of an edge-split bilayer beam consisted of bonded dissimilar materials under mixed-mode bending (MMB) is investigated by using a structural mechanics procedure. In this analytical procedure, the bilayer beam is modeled as two Timoshenko beams joined by using interface tensile and shear springs, and the fracture mechanics parameters including the strain energy release rate and phase angle are estimated from the spring responses. Numerical examples of bilayer beams consisted of isotropic or orthotropic bimaterials and with symmetric or asymmetric dimensions are presented and compared to continuum based finite-element solutions with either cohesive-zone or perfectly-bonded interface models. It is shown that the strain energy release rates obtained from these models are in good agreement. The phase angles obtained from the analytical model and the cohesive-zone-interface based continuum model are also consistent with each other, and agree to the crack-tip deformation. On the other hand, the phase angles obtained from the oscillating crack-tip fields related to the perfectly-bonded interface do not match to the crack-tip deformation as well as the cohesive-zone interface models. Consequently, it is recommended to characterize the mode-mixity dependent interfacial adhesion and fatigue responses by using the fracture mechanics parameters obtained by using the analytical solution for the MMB fracture problem.

Phase-field-crystal simulation of nano-single crystal microcrack propagation under different orientation angles

The propagation mechanism of microcracks in nanocrystalline single crystal systems under uniaxial dynamic and static tension is investigated using the phase-field-crystal method. Both dynamic and static stretching results show that different orientation angles can induce the crack propagation mode, microscopic morphology, the free energy, crack area change, and causing fracture failure. Crack propagation mode depends on the dislocation activity near the crack tip. Brittle propagation of the crack occurs due to dislocation always at crack tip. Dislocation is emitted at the front end of the crack tip and plastic deformation occurs, which belongs to ductile propagation. The orientation angles of 9° and 14° are brittle--ductile mixed propagation, while the orientation angles of 19° and 30° are brittle propagation and no dislocation is formed under dynamic tension. The vacancy and vacancy connectivity phenomenon would appear when the orientation angle is 14° under static tension, and the crack would be ductile propagation. While the orientation angle is 19° and 30°, the crack propagates in a certain direction, which is a kind of brittle propagation. This work has some practical significance in preventing material fracture failure and improving material performance.

Exceptional fracture toughness of CrCoNi-based medium- and high-entropy alloys at 20 kelvin.

CrCoNi-based medium- and high-entropy alloys display outstanding damage tolerance, especially at cryogenic temperatures. In this study, we examined the fracture toughness values of the equiatomic CrCoNi and CrMnFeCoNi alloys at 20 kelvin (K). We found exceptionally high crack-initiation fracture toughnesses of 262 and 459 megapascal-meters½ (MPa·m½) for CrMnFeCoNi and CrCoNi, respectively; CrCoNi displayed a crack-growth toughness exceeding 540 MPa·m½ after 2.25 millimeters of stable cracking. Crack-tip deformation structures at 20 K are quite distinct from those at higher temperatures. They involve nucleation and restricted growth of stacking faults, fine nanotwins, and transformed epsilon martensite, with coherent interfaces that can promote both arrest and transmission of dislocations to generate strength and ductility. We believe that these alloys develop fracture resistance through a progressive synergy of deformation mechanisms, dislocation glide, stacking-fault formation, nanotwinning, and phase transformation, which act in concert to prolong strain hardening that simultaneously elevates strength and ductility, leading to exceptional toughness.

Crack Tip Deformation during Dwell Fatigue and Its Correlation with Crack/Fracture Surface Morphologies in a Bi-Modal Ti–6Al–4V Alloy

Crack Tip Deformation during Dwell Fatigue and Its Correlation with Crack/Fracture Surface Morphologies in a Bi-Modal Ti–6Al–4V Alloy

Fatigue crack growth mechanism in ferroelectric ceramics under alternating electric field: An investigation by digital image correlation technique

The deformation of crack tip in PZT ceramics under cyclic electric field loading was characterized by Digital Image Correlation (DIC) technique. Two other specimens without speckles were used to observe the physical processes during the crack propagation in-situ. The deformation of the region of interest is elucidated. Near the U-shaped notch, the DIC results directly reveal the existence of incompatible deformation. However, in the front of the newly formed crack, no incompatible deformation was observed. Additionally, in-situ observation reveals that, the crack growth occurs at the stage of crack closure when the crack propagates steadily. The observed pop-in behavior of the crack during the first cycle of electric field loading is rationalized by the interaction between the incompatible domain switching zones with and without U-shaped notch, which is consistent with the explanation of Westram et al. The subsequent crack propagation after the pop-in cannot be rationalized by the model exploited by Zhu and Yang, Westram et al., and Liu and Fang, which is thought to be related to three different factors – arcing, wedging and grain-to-grain interaction. The mechanism for the appearance and the disappearance of the incompatible domain switching zone near the U-shaped notch and around the tip of the newly formed crack is discussed based on the electrical boundary condition (EBC) of the crack surfaces we proposed recently.

High-cycle-fatigue properties of selective-laser-melted AlSi10Mg with multiple building directions

Axial loading fatigue tests with stress ratio of 0 were conducted in investigating the influence of building directions (0°, 45° and 90°) on high cycle fatigue property of a selective laser melted AlSi10Mg in combination with two-dimensional microscope observation and three-dimensional ultra-depth imaging. Results show that fatigue strength decreases with the increase of building angle due to larger defects and sparser melting pool boundaries. Two failure modes, i.e. surface and subsurface failures, are mainly induced by irregular lack of fusion whose size arises as building angle increases. Also, the roughness of crack nucleation region is mainly attributed to the plastic deformation of crack tip under tensile stress. Based on the stress intensity factor evaluation at crack tip, both long crack growth threshold value and transition size from small to long crack growth all drop as building angle increases. Based on these, the microstructure-based crack nucleation and propagation behavior is clarified, and an energy based fatigue life prediction model considering the stress concentration effect of defect is proposed. The agreement between experimental and predicted data is satisfied.

In-situ experimental study on the fatigue crack propagation behavior of 7075-T6 and 2024-T3 aluminium alloys under variable amplitude loading and retardation model modification

Fatigue crack propagation behavior of Al7075-T6 and Al2024-T3 under four typical variable amplitude loading (VAL) patterns are investigated by using in-situ testing. The near-tip strain fields and crack opening level are measured combining in-situ optical microscopy testing and digital image correlation (DIC) technique. The crack tip deformation process is directly observed by in-situ scanning electron microscope (SEM). The results show that both the crack propagation behaviors and the crack-tip strain distributions are greatly influenced by the VAL patterns. The shape effect of the overload-induced plastic zone may prolong the crack growth retardation. The crack-front plasticity plays a dominant role in controlling fatigue crack growth. The crack-front plasticity supplies a link between crack closure and crack growth. The crack tip resharpening caused by the underload applied immediately after the overload may reduce the retardation extent. Finally, a modified retardation model is proposed and gives reasonable prediction results of the crack growth rate under VAL.

Role of Initial Crack Tip Shape, Plastic Compressibility and Strain Softening on Near-Tip Stress-Strain State in Fatigue Cracks during Simulation of a Finite Deformation based Elastic-Viscoplastic Constitutive Model

This paper deals with the effect of initial crack tip shape, plastic compressibility, and strain softening on near-tip stress-strain fields for a mode I crack subjected to fatigue loading under plane strain and small scale yielding. A finite strain-based elastic-viscoplastic constitutive equation with bilinear hardening and hardening-softening-hardening hardness functions is taken up for simulation. It is observed that plastic compressibility and strain softening have a significant impact on crack tip opening displacement (CTOD) and tip propagation. Furthermore, it has been viewed that the initial shape of a crack tip can significantly influence both the CTOD and the crack tip extension for the bilinear hardening material; however, with identical conditions for the hardening-softening-hardening material, the initial crack tip shape affects the fatigue crack growth much lesser though the CTOD is influenced considerably. In comparison to the crack growth in the plastically incompressible hardening-softening-hardening solids, the variation of the crack growth (with respect to the tip curvature radius) is more and peculiar in the corresponding plastically compressible solid. To explain and to get a better insight of the crack tip deformation, the near-tip plastic strain and hydrostatic stress have been illustrated.

Effect of interatomic potential on modelling fracture behavior in hcp titanium: a molecular dynamics study

Molecular dynamics (MD) simulations have unique advantages in capturing the details of the crack tip deformation at the atomic scale. As the accuracy of simulation results is heavily dependent on the selected interatomic potential, a comprehensive study of the anisotropic crack tip behavior of hcp titanium under plane strain mode-I loading with the commonly utilized potentials is presented in this paper. The typical crystallographic planes in hcp materials including {0001}, {101¯0}, {112¯0}, {101¯1} and {112¯2} are considered as the pre-existing crack planes. The intrinsic brittleness and ductility of different crack systems are discussed from the perspective of linear elastic fracture criteria. Depending on the selected potential, the dominant crack tip deformation mechanism may be basal a slip, prismatic a, pyramidal c+a slip or twinning modes in different crack systems. Finally, in comparison with experimental results, the different potentials are evaluated in the light of their capability to reproduce the experimentally observed crack tip deformation. Overall, the recent developed Mendelev-II potential is recommended for MD simulation of fracture behavior under mode-I loading, as this potential gives a more realistic crack tip response than other studied potentials.

Effect of crack-tip deformation on fatigue crack growth: A comparative study under overload/underload conditions

With the aim to provide the structural design stage with data on the crack retardation/acceleration effects caused by overload/underload and to explore the corresponding mechanisms, we conducted a series of comparative fatigue crack growth rate (FCGR) tests on S355J2W+N steel. The various amplitudes of overload/underload were investigated comprehensively under different stress-ratio baseline loads for single and periodic overloading/underloading processes. Moreover, the crack-tip deformation behaviors (i.e., the strain/stress fields and crack closure responses following the overload/underload) were observed using a digital image correlation (DIC) technique and calculated through the finite element analysis. The relevant fracture morphologies and crack-tip dislocation structures were characterized and analyzed. The results showed that the crack propagation cycles could be influenced to different degrees by a single overload (OL), a single underload (UL), and periodic underloads (PULs). Compared with the transient crack acceleration induced by a UL, a lasting crack growth retardation occurred following an OL. The OL&UL sequences had various effects on the changes in FCGR. Furthermore, a higher stress-ratio baseline loading and larger-amplitude OL increased the retardation effect; a lower stress-ratio baseline loading and larger-amplitude UL enhanced the acceleration effect. The degrees of crack retardation or acceleration following the OL/UL were determined by the changes in the crack closure and residual stress state near crack tip, which were closely related to the baseline loading stress ratio and local residual strain characteristics. Correspondingly, the OL and PULs also changed the dislocation density and structures in the microscopic view.