Stable Propagation Research Articles

Experimental study on effects of cyclic loading paths on cracking behavior and fracture characteristics of granite

To investigate the impact of cyclic loading paths on the Mode I fracture characteristics of granite, a series of fracture tests were conducted on semi-circular bend (SCB) specimens using acoustic emission (AE), digital image correlation (DIC) and 3D scanning. The tests included monotonic loading, variable amplitude (VA) cyclic loading, and tiered constant amplitude (TCA) cyclic loading. The results demonstrated that VA cyclic loading leads to an average enhancement of 5.6% in fracture toughness, whereas TCA cyclic loading results in an average reduction of 3.7% in fracture toughness. During cyclic loading and unloading, the TCA loading path exhibits a higher rate of cumulative plastic deformation growth and greater magnitude cumulative plastic deformation compared to the VA loading path. Additionally, detailed observations from 3D scanning revealed that under cyclic loading, the fracture trajectories and surface morphology exhibit more tortuosity and roughness compared to monotonic loading, particularly in the case of TCA cyclic loading. Furthermore, according to the AE and DIC analysis, the proportion of microcrack initiation and stable propagation stage, cumulative AE counts and fracture process zone (FPZ) size generated under VA cyclic loading are greater than those generated under TCA cyclic loading, displaying a consistent variation pattern with fracture toughness. The changes of fracture properties can be attributed to a competition between the compaction-induced hardening effect and the cracking-induced damage effect. A higher cyclic upper limit and incomplete unloading are more likely to accelerate the growth of the irreversible crack damage and rapid crack extension, resulting in weakened rock fracture properties. These findings provide important references for rock instability control in underground rock engineering.

Effects of diffusion barriers on reaction wave stability in Co/Al reactive multilayers

Bimetallic, reactive multilayers are uniformly structured materials composed of alternating sputter-deposited layers that may be ignited to produce self-propagating mixing and formation reactions. These nanolaminates are most commonly used as rapid-release heat sources. The specific chemical composition at each metal/metal interface determines the rate of mass transport in a mixing and formation reaction. The inclusion of engineered diffusion barriers at each interface will not only inhibit solid-state mixing but also may impede the self-propagating reactions by introducing instabilities to wavefront morphology. This work examines the effect of adding diffusion barriers on the propagation of reaction waves in Co/Al multilayers. The Co/Al system has been shown to exhibit a reaction propagation instability that is dependent on the bilayer thickness, which allows for the occurrence of unstable modes in otherwise stable designs from the inclusion of diffusion barriers. Based on the known stability criteria in the Co/Al multilayer system, the way in which the inclusion of diffusion barriers changes a multilayer's heat of reaction, thermal conductivity, and material mixing mechanisms can be determined. These factors, in aggregate, lead to changes in the wavefront velocity and stability.

In-situ monitoring of crack growth and fracture behavior in composite laminates using embedded sensors of rGO coated fabrics and GnP paper

Graphene-based nanomaterials have found significant interest in the development of embedded sensors to monitor the fracture behavior in composite structures. In this work, in-situ crack propagation and fracture behavior within a glass fiber reinforced polymer composite (GFRP) was monitored using embedded reduced graphene oxide (rGO) coated fabrics and highly conductive graphene nanoplatelet (GnP) paper. All laminates were fabricated using the resin infusion process. The piezoresistive performance of both types of laminates was evaluated using in-plane and out-of-plane mechanical tests. The effect of GnP paper thickness (50/150/240 µm) and loading rate was also evaluated using tensile and Mode-I fracture loadings. Piezoresistivity of the sensors was reduced by increasing loading rate during tensile tests. All laminates with GnP paper exhibited poor mechanical performance under tensile loading. The laminates with 50 µm GnP paper showed highest sensitivity under Mode-I loading. In comparison to pristine laminates, the interlaminar fracture toughness of laminates with 50 µm GnP paper was reduced by 70%. Furthermore, laminates with rGO coated fabrics demonstrated stable crack propagation under Mode-I loading as compared to GnP based laminates. The fractured surfaces were analyzed using scanning electron microscopy to investigate the underlying fracture mechanisms of the sensors in the composite laminates.

Numerical simulation of laminar-turbulent transition in a supersonic boundary layer under the action of acoustic disturbances

In low disturbance environment, laminar-turbulent transition (LTT) is associated with excitation of unstable normal modes of small initial amplitudes (receptivity problem). These modes grow exponentially to a critical amplitude in accord with the linear stability theory (LST) and trigger the nonlinear breakdown. In the current study numerical simulation of laminar-turbulent transition (LTT) in the boundary layer on the upper surface of a flat plate in a supersonic free stream of Mach number M∞=3 and Reynolds number Re∞=2 × 107 is carried out. The flow is perturbed by fast or slow acoustic waves of low intensity, which excite unstable waves of the first mode. The latter have frequencies corresponding to the integral amplification N ≈ 9.16 typical for flight conditions. Two cases are considered: the angle of attack AoA=0° at which acoustic waves pass through a weak shock induced by viscous-inviscid interaction; AoA=5° at which acoustic waves pass through the expansion fan emanating from the plate leading edge. A holistic modeling of all stages of the transition from receptivity to the birth of turbulent spots is performed using direct numerical simulations. Linear stability theory is used to interpret the results. Feasibility of practical implementation of the amplitude method for predictions of the LTT onset in the considered and similar cases is discussed. It has been shown that to realize the amplitude method in the considered and similar cases, it is sufficient: to calculate the initial amplitudes of instability in a small vicinity of the leading edge and the propagation of instability from the leading edge to the station where the instability reaches the critical amplitude u′max≈4%. Analysis of known numerical and experimental data showed that this criterion weakly depends on the local Mach number in the range 0<Me<7, and it is acceptable for practical predictions of the transition onset points.

Relativistic-guided stable mode of few-cycle 20µm level infrared radiation.

The generation of intense infrared radiation with a wavelength greater than 10 µm is limited by the optical materials in traditional methods or the laser-plasma parameters of plasma-bubble methods. In this study, we propose a new method for generating an intense longitudinal radiation field of tens of GV/m. By utilizing the oscillations of the electron film on the inner surface of the micro-tube, excited by the relativistic electron beam propagating within it, it is possible to obtain tunable long-wavelength few-cycle infrared radiation, ranging from 20 to 30 µm and even longer. The radiation source is guided entirely by a relativistic electron beam and formed a stable TM propagation mode in the micro-tube. This opens up new opportunities for applications of the relativistic intensity infrared radiation to high-field physics, shorter attosecond pulses generation and charged particle acceleration.

In situ experiments in microgravity and phase-field simulations of the lamellar-to-rod transition during eutectic growth

In recent experiments on the solidification of the binary eutectic alloy succinonitrile-(D)camphor carried out on board of the International Space Station (ISS), a transition from rod to lamellar patterns was observed for low growth velocities. The transition was interpreted in terms of a competition between a propagative instability of lamellae and a drift induced by a transverse temperature gradient. Phase-field simulations of a symmetric model alloy support this scenario: for a fixed transverse temperature gradient, the transition from rods to lamellae occurs for a critical composition at fixed velocity, and for a critical velocity at fixed composition. Since the alloy and control parameters used in experiments and simulations are different, our results strongly suggest that this morphological transition is generic for eutectic alloys.

Effects of Injector Configuration on the Detonation Characteristics and Propulsion Performance of Rotating Detonation Engine (RDE)

Fuel injection and mixing affect the characteristics of detonation initiation and propagation, as well as the propulsion performance of rotating detonation engine (RDE). A study on the injector is carried out in the present investigation. A rectangular-shaped hole-type fuel injector (RHFI) and slit-type fuel injector (SFI) were designed and compared experimentally at equivalent conditions. The investigation of the detonation propagation modes and the analysis of propulsion performance were carried out using fast Fourier transform (FFT), short-time Fourier transform (STFT), and unwrapped image post-processing. Under 50, 75, and 100 g/s flow rate conditions at an equivalence ratio of 1.0 ± 0.05, the RHFI has relatively stable detonation propagation characteristics, higher thrust, and specific impulse performance. Additionally, the results of the experiment indicate that the number of detonation waves affects performance.

Mode I fracture of thick adhesively bonded GFRP composite joints for wind turbine rotor blades

This article investigates the effects of voids, joint geometry, and test conditions on the quasi-static Mode I fracture performance of thick adhesive Double Cantilever Beam (DCB) joints such as those prevailing in wind industry and shipbuilding. The specimens were made by glass fiber reinforced epoxy adherend and SikaPower®-830 epoxy adhesive in the cm thickness range. Side-grooved shape guided the crack propagation direction and assisted stable propagation, while lower cross-head displacement rates reduced the occurrence of unstable crack propagation and prevented crack deflection. Porosities, which are inevitable due to the high viscosity of the adhesive, led to unstable propagation and promoted crack path deviations. They could also decrease the apparent fracture energy release rate (SERR) since the crack surface is reduced. In conclusion, grooved DCB joints with low void content tested at low displacement rates showed stable crack propagation without significant crack path deviation. This method enables comparison with future adhesive formulations and the refinement of full-scale blade models.

Characterizing fatigue damage evolution in asphalt mixtures using acoustic emission and Gaussian mixture model analysis

The identification and investigation of fatigue crack evolution and damage modes in asphalt mixtures are crucial for understanding the corresponding mechanisms and characteristics of fatigue failure. In this study, the four-point bending fatigue tests were performed on the asphalt mixture specimens with precast joints using the acoustic emission (AE) technique to analyze the damage evolution. The failure modes of the asphalt mixtures were identified using the Gaussian mixture model (GMM). The results revealed that the fatigue damage of the asphalt mixtures can be categorized into four stages: Stage I (void compaction), Stage II (microcrack initiation and stable propagation), Stage III (crack aggregation and unstable propagation), and Stage IV (complete fracture). This division was defined based on the inflection points of the axial displacement curves, the cumulative number of AE events, and the cumulative ringing counts. The evolution of the AE b value was found to be correlated with the crack propagation. The rapid decline in the b value in Stage III was considered a harbinger of the eventual complete fracture in the asphalt mixtures. The GMM clustering analysis indicated that the fatigue damage of the asphalt mixtures primarily involved tensile damage (90%), and shear damage made a small contribution of 10%. Existing AE studies have focused on the quantitative evaluation of AE parameters, while research on clustering identification algorithms for these parameters is scarce. The analytical methods and findings of this study enable an effective identification of fatigue damage in asphalt mixtures at the laboratory scale and offer insights into the fatigue damage mechanisms of asphalt mixtures.

Effect of atmospheric turbulence on modulational instability in laser-pulse propagation

Multiple filamentation is a major problem for laser pulse propagation in the atmosphere. In this article, we study the influence of a turbulent atmosphere on the growth of the modulational instability which is the cause of multiple filamentation. It is shown that the growth rate of this instability decreases when it is considered that the index of refraction has a stochastic behavior. A good qualitative agreement between the analytical and numerical results is obtained.

Study on the influence of crack depth on the safety of tunnel lining structure

In order to study the mechanism of the influence of crack depth on the bearing capacity and stability of tunnel lining structure, the calculation model of internal force of lining with cracks is established based on the theory of fracture mechanics, and the variation of stability coefficient of lining structure with Ⅰ-Ⅱ composite cracks is analyzed. With the increase of the crack depth of the vault, the bending moment of the vault gradually decreases, the stress intensity factor increases, and the stability factor f gradually decreases. The critical depth of tensile and compressive crack instability propagation is 54.38% and 74.01% of the lining thickness, respectively. The relationship between load and displacement, internal force and contact pressure, crack propagation depth and time, and strain variation of the secondary lining vault during the loading process of cracks at different depths are analyzed. It is found that cracks have a significant impact on the bearing capacity of the secondary lining, and the deeper the cracks, the lower the bearing capacity and stability. When the crack depth is large, the tensile stress peaks in a short time, and applying smaller loads can also cause the crack to expand. When the crack depth is 0.9d (d is the thickness of the secondary lining), the bearing capacity of the structure decreases by 72.57%. The research results can provide certain theoretical guidance for the prediction and prevention of lining crack propagation during tunnel operation.

Impact of interlamellar spacing and non-pearlitic features on mechanical properties and cyclic damage initiation in near-eutectoid pearlitic steels

The relationship between microstructural characteristics and mechanical properties was investigated for C72 and C82 pearlitic steel rods patented at temperatures between 500 °C and 600 °C using a lead bath. Results revealed a refinement of interlamellar spacing upon reduction of patenting temperature. At the same time, the fraction of grain-boundary ferrite and Widmanstätten ferrite as common non-pearlitic regions was slightly increased, reaching a maximum of nearly 1.3 vol.-% in the case of C72 steel patented at 500 °C. Reduction of patenting temperature was associated with strengthening, as confirmed by tensile, fatigue and hardness tests. The strengthening was explained in terms of the Hall-Petch effect for ferrite due to increase in the boundary area. The Hall-Petch coefficient, obtained by taking the apparent interlamellar spacing as the mean free path of dislocations, was in the range 70–140 MPa.μm12. To some extent, these low values could be related to the low solute interstitial content of pearlitic ferrite. More importantly, the coefficients are underestimated due to the plate-shaped morphology of pearlitic ferrite, which causes the average mean free path of dislocations be longer than the apparent interlamellar spacing. Scanning electron microscopy examinations after interrupted fatigue tests indicated the preferred occurrence of intrusions and extrusions, as precursors to fatigue crack, in the non-pearlitic ferrite. The enhancement of fatigue limit at lower patenting temperatures—in spite of slight increase in the fraction of ferrite—is justified by the stress shielding effect of pearlite and necessity of crack propagation into pearlitic regions for stable crack propagation. In effect, as long as the fraction of non-pearlitic features remains low, the interlamellar spacing of pearlite serves as the most efficient strength-controlling microstructural parameter. Furthermore, due to the preferred growth of fatigue cracks parallel to lamellae and deflection at colony boundaries, the refinement of colonies at reduced patenting temperatures is an additional contribution to the fatigue strength.

Influence maximization algorithm based on group trust and local topology structure

Influence maximization is one of the important contents of social network analysis. Many classical influence propagation models assume that there is a stable information propagation phenomenon between adjacent users, and do not consider the influence of internal structure information of the network on the actual information propagation. Therefore, an influence maximization algorithm based on group trust and local topology structure is proposed. In order to make full use of the important role of group in information propagation, the concepts of intra-group connectivity, inter-group diffusion and group trust are defined based on the characteristics such as group tightness. Then, an influence propagation algorithm based on the local topological structure of the group is proposed to extract the local structure information of different topological positions in the group, and calculate the propagation probability between users. Finally, the seed nodes were selected according to the credibility ranking of the group for influence propagation. Experiments on multiple data sets show that compared with other algorithms, the algorithm can achieve higher propagation efficiency and wider influence effect, which verifies the rationality and effectiveness of the method.

Comparative analysis of thermal and non-thermal discharge modes on ultra-lean mixtures in an optically accessible engine equipped with a corona ignition system

Radio-Frequency corona-based ignition systems are currently being investigated by the engine research community for their capability to ensure robust combustion at challenging operating conditions such as lean mixture and/or high exhaust gas recirculation (EGR) dilution. The low-temperature plasma (LTP) produced by the corona discharge accelerates the early flame development thanks to the production of combustion precursors like radical and excited species. Plasma benefits can be maintained with extensive management of the control parameters, preventing the transition from the non-thermal discharge mode to a thermal one.However, under ultra-lean conditions, where the ignition probability significantly decreases, the LTP discharge may not be suitable for promoting the combustion process because of insufficient thermal energy transfer and scarcity of available fuel, which is essential for radical production. Therefore, it may be necessary to increase the volume in which the plasma interacts to ensure robust ignitions and stable flame propagations. For this reason, the present work aims to assess and quantify the differences, in terms of combustion performance, between a corona igniter operating at LTP mode and with electric arcs occurring, at ultra-lean operating conditions. In arc-mode, the igniter produces volumetric discharges, facilitating extensive mixing of the mixture and resulting in advantageous effects on kernel formation and propagation. Tests were carried out in a single-cylinder optical access engine under low load and low speed, using gasoline as fuel. Close to the leanest stable limit found, the LTP mode shows a lower cycle-to-cycle variability. Above such a threshold, the thermal discharge can guarantee stable combustion events while the LTP discharge leads to unstable working conditions. Indicating, imaging, and pollutant emission analysis are carried out to evaluate the effects of thermal and non-thermal plasma modes on combustion characteristics and their environmental impact.

A novel meso-damage constitutive model of rock under true triaxial stress with three-dimensional cracking strength, threshold and closure effect

Deep underground engineering is in a true three-dimensional stress state, and the adjustment of the three-dimensional stress state caused by engineering excavation will induce the fracture or even instability of the surrounding rock. However, three-dimensional mechanical model research suitable for the stability analysis of deep surrounding rock is very scarce. Therefore, a series of tests under different true triaxial stresses on two rocks (rhyodacite and marble) were conducted, and the characteristic strength (crack stable propagation initiation stress, crack unstable propagation initiation stress and peak strength) and deformation characteristics were further analyzed. After that, using the Lemaitre strain equivalence hypothesis and rock statistical damage theory, a new statistical damage constitutive model at true triaxial stress states was proposed, which introduced the three-dimensional strength criterion Modified Wiebols Cook to characterize the three-dimensional strength of the rock microelement. Therefore, the intermediate principal stress can be reasonably considered. The damage threshold, initial compaction effect and residual strength of the rock microelement at different true triaxial stress conditions were also considered. Then the relationships between the proposed model parameters and σ2 and σ3 were analyzed. Furthermore, sensitivity analysis of the influence of parameters m and F0 in proposed model on the shape of rock stress–strain curve and peak strength was also investigated. The comparison between the results predicted by proposed model and the experimental data shows that the new model established in this study can well simulate the prepeak and postpeak deformation characteristics of rock and the intermediate principal stress effect under true triaxial stress conditions.

A linear parameters study of ion cyclotron emission using drift ring beam distribution

Ion Cyclotron Emission (ICE) holds great potential as a diagnostic tool for fast ions in fusion devices. The theory of Magnetoacoustic Cyclotron Instability (MCI), as an emission mechanism for ICE, states that MCI is driven by a velocity distribution of fast ions that approximates to a drift ring beam. In this study, the influence of key parameters (velocity spread of the fast ions, number density ratio, and instability propagation angle) on the linear MCI is systematically investigated using the linear kinetic dispersion relation solver BO (Xie 2019 Comput. Phys. Commun. 244 343). The computational spectra region considered extends up to 40 times the ion cyclotron frequency. By examining the influence of these key parameters on MCI, several novel results have been obtained. In the case of MCI excited by super-Alfvénic fast ions (where the unique perpendicular speed of fast ion is greater than the perpendicular phase velocity of the fast Alfvén waves), the parallel velocity spread significantly affects the bandwidth of harmonics and the continuous spectrum, while the perpendicular velocity spread has a decisive effect on the MCI growth rate. As the velocity spread increases, the linear relationship between the MCI growth rate and the square root of the number density ratio transitions to a linear relationship between the MCI growth rate and the number density ratio. This finding provides a linear perspective explanation for the observed linear relation between fast ion number density and ICE intensity in JET. Furthermore, high harmonics are more sensitive to changes in propagation angle than low harmonics because a decrease in the propagation angle alters the dispersion relation of the fast Alfvén wave. In the case of MCI excited by sub-Alfvénic fast ions (where the unique perpendicular speed of fast ion is less than the perpendicular phase velocity of the fast Alfvén waves), a significant growth rate increase occurs at high harmonics due to the transition of sub-Alfvénic fast ions to super-Alfvénic fast ions. Similarly, for MCI excited by greatly sub-Alfvénic fast ions (where the unique perpendicular speed of fast ion is far less than the perpendicular phase velocity of the fast Alfvén waves), the growth rate at high harmonics also experiences a drastic increase compared to the low harmonic, thereby expanding the parameter range of the velocity spread.

Fatigue damage propagation of FRP under coupling effect of cyclic load and corrosion

AbstractInterlaminar fracture or delamination is one of the major failure modes for fiber reinforced polymer (FRP) laminate. Especially, the synergistic effect of environmental exposure and fatigue loads accelerates interface degradation. The main objective in this work was to investigate the fatigue damage propagation of basalt FRP (BFRP) laminate under coupling of acidic environment and fatigue load. The fatigue cycling of 40,000 and the maximum stress of 30% fu (fu = ultimate tensile strength) were adopted. The fatigue specimens under air condition (air fatigue), immersing in water (water fatigue) and immersing in acid (acid fatigue) were introduced. The double cantilever beam (DCB) test was used to evaluate the interlaminar damage of BFRP laminate after fatigue cycling. The fatigue damage propagation mechanism was explored by means of failure modes, load displacement curves, mode I fracture toughness and crack propagation resistance model. The results show that the fatigue weakened the interlaminar properties and resulted in reduced fiber bridging. Especially for the acid fatigue, the laminate was damaged by the coupling of fatigue and acid corrosion. The linear loading stage, nonlinear slow growing stage, stable crack propagation stage and linear unloading stage for DCB after fatigue were observed in the load–displacement curve. The fatigue cycling has caused fracture toughness decrease of 15.0%, 13.8% and 24.1% on air fatigue, water fatigue and acid fatigue respectively. Compared to air fatigue, additional 9.1% interlaminar toughness decrease was evidenced for acidic corrosion fatigue due to the coupling effect of chemical corrosion and physical swelling. The fluctuating decline of crack resistance was caused by discontinuous interface damage, and the interlayer damage was randomly distributed. Thus the interface damage model was proposed to characterize the damage distribution and the damage size.Highlights Degradation law of BFRP under the coupling effect of acid immersion and fatigue load. Interface damage model for characterization of the damage distribution and damage size. Fatigue damage propagation of BFRP under coupling effect of cyclic load and corrosion.

Effect of combustion chamber diameter size on scramjet rotating detonation under high Mach number flow conditions

This study conducted 3D numerical simulations to investigate the impact of combustion chamber diameter on the combustion characteristics of liquid kerosene scramjet rotating detonation under high Mach number flow conditions (Ma6/28 km). After ignition, a local hotspot near the contact surface between the combustion product and fresh reactant facilitates the generation of new detonation wave heads, resulting in a co-directional multi-wave mode in the detonation combustion flow field. The higher total temperature of the incoming flow restricts the accumulation of a substantial fuel gas layer in the axial direction of the combustion chamber, resulting in a smaller fuel distribution area and a lower wave head. Increasing the inner diameter of the combustion chamber leads to an increase in the number of wave heads but a decrease in overall height. Specifically, when using diameters of 125 and 150 mm, we observed significant periodic low-frequency oscillations in the peak pressure of the detonation wave during stable propagation. The specific impulse of the fuel does not vary significantly across different combustion chamber diameters. However, when the inner diameter is 75 mm, periodic oscillations occur, which reduce thrust stability. These findings provide valuable insights into optimizing combustion chamber design and improving the efficiency and stability of liquid kerosene scramjet rotating detonation propulsion systems under high Mach number flow conditions.

Numerical simulation and experimental verification of fatigue crack propagation in high-strength bolts based on fracture mechanics.

To investigate the fatigue crack propagation behavior of high-strength bolts for high-speed train brake discs, the fatigue crack propagation of high-strength bolts with initial defects under various load ratios was numerically simulated and experimentally verified based on fracture mechanics in this paper. Firstly, the fracture mechanics model of a three-dimensional hexahedral mesh with initial root defects was established using ABAQUS-FRANC3D interactive technology. Then the stress intensity factor (SIF) of the crack front was calculated by the stress superposition of the crack surface to simulate the coupling effect of preload and axial cyclic load. Based on it, fatigue crack propagation was simulated. Finally, the corresponding fatigue experiments on prefabricated crack bolts were carried out. The results show that mode I cracks dominate in the process of crack propagation. The stable crack propagation zones of the fractured high-strength bolts all show a semi-elliptical cross-section. The SIF of the crack front decreases with the increase of the load ratio, thus making the crack propagation life increase with the increase of the load ratio. The experimental outcomes are in great agreement with the simulation results, which verify that the numerical simulation method can effectively and accurately evaluate the fatigue life of high-strength bolts with initial defects and provides an effective means for predicting the fatigue crack propagation life of the same type high-strength bolts in engineering applications.

Effect of microstructure heterogeneity on the cryogenic fracture toughness of the dissimilar joint between SA645 and 304L made by laser welding

The cryogenic fracture toughness of the SA645/304L dissimilar weld, produced using continuous wave IPG fiber laser welding with a 0.3 mm beam offset towards the 304L side, and the microstructural effects on the fracture behavior were systematically investigated. The weld metal consists of ∼89% columnar dendritic martensite and ∼11% retained austenite (RA), where the ununiform distribution of constituent phases as well as the distinctions between martensite and RA in terms of morphology and mechanical properties lead to the microstructure heterogeneity of weld metal. Pop-in phenomenon appears on the Force-Displacement (F-V) curve during the crack tip opening displacement (CTOD) test for the weld. The CTOD value for the weld is ∼0.057 mm, about 8 times less than that when pop-in effect is ignored (∼0.563 mm). Rapid propagation of the pre-fatigue crack tip along the center of the weld leads to the pop-in phenomenon. Fracture surface in pre-fatigue crack tip (PCT) region shows quasi-cleavage features, while fracture surface in stable crack propagation (SCP) region presents a ductile-fractured surface with dimples. The crack propagation path in SCP region is frequently deflected. Inhibition effect of grain boundaries on crack propagation and TRIP effect of retained austenite (RA) result in the improvement in fracture toughness of the SCP region.