Crack Growth Behavior Research Articles

Fracture toughness and slow crack growth behaviour of metal-proton conducting ceramic composites

The fracture toughness and slow crack growth behaviour of two load-bearing metal ceramic components adopted in negatrode-supported electrochemical devices, namely Ni-BCZY27 and Ni-BCZY442, are investigated via double torsion technique.The investigation showed that the two metal-ceramic composites are considerably less sensitive to slow crack growth when compared to oxide ceramics such as stabilized zirconia and alumina, with KI0/KIC equal to approximately 0.8. On the other hand, they also showed lower fracture toughness than Ni-3YSZ, with Ni-BCZY442 and Ni-BCZY27 having a fracture toughness of respectively 1.74 and 3.04 MPa*m1/2 for porosities equal to 25.8% and 19.1% respectively, compared to Ni-3YSZ reported in previous work with similar Ni content and porosity of approximately 30% having a fracture toughness equal to 3.4 MPa*m1/2. Furthermore, the analysis of samples presenting extensive barium depletion indicated fast fracture propagation upon application of loads significantly lower than the critical fracture load.

Quasi-In Situ Observation of the Microstructural Response during Fatigue Crack Growth of Friction Stir Welded AA2024-T4 Joint.

The reliability of friction stir welded joints is a critical concern, particularly given their potential applications in the aerospace manufacturing industry. This study offers a quasi-in situ observation of the microstructural response during fatigue crack growth (FCG) of a friction stir welded AA2024-T4 joint, aiming to correlate fatigue crack growth behavior with mechanical properties investigated using electron backscatter diffraction (EBSD). Notched compact tension (CT) specimens corresponding to the morphology of the stir zone (SZ), advancing side (AS), and retreating side (RS) were meticulously designed. The findings indicate that the welding process enhances the joint's resistance to fatigue crack growth, with the base metal exhibiting a shorter fatigue life (i.e., ~105 cycles) compared to the welding zones (SZ ~ 3.5 × 105 cycles, AS ~ 2.5 × 105 cycles, and RS ~ 3.0 × 105 cycles). Crack propagation occurs within the stir zone, traversing refined grains, which primarily contribute to the highest fatigue life and lowest FCG rate. Additionally, cracks initiate in AS and RS, subsequently expanding into the base metal. Moreover, the study reveals a significant release of residual strain at the joint, particularly notable in the Structural-CT-RS (Str-CT-RS) sample compared to the Str-CT-AS sample during the FCG process. Consequently, the FCG rate of Str-CT-AS is higher than that of Str-CT-RS. These findings have significant implications for improving the reliability and performance of aerospace components.

Experimental and numerical insights into fatigue crack propagation of railway component with gradient residual stress

As a representative structure with an acute radial gradient in terms of residual stress (RS) and grains, the induction-hardened railway S38C axle exhibits excellent mechanical properties and fatigue resistance. To investigate the effect of gradient features on fatigue crack growth (FCG) behaviors, we designed three-point bending (TPB) tests with different gradient features, including the negative-gradient, positive-gradient, and matrix specimens. Finite element modeling was introduced to simulate the FCG of the TPB specimens in terms of the cyclic J-integral (ΔJ) and equivalent plastic strain at the growing crack tip. The results reveal that the FCG rates in both negative- and positive-gradient structures decrease and a more pronounced tendency of decline is achieved in the positive-gradient ones. The substantial compressive residual stress (CRS) within the negative-gradient structure directly influences the crack tip area, while the CRS in the positive-gradient structure indirectly mitigates the crack driving force by impeding the overall structural deformation. Coarser grains aid in enhancing energy absorption along the zigzag crack path, thereby diminishing the crack driving force. ΔJ and the equivalent plastic zone surrounding the crack tip indicate significant reductions in the presence of CRS.

A comparative study of creep crack growth behaviour amongst the dissimilar P91-IN182-SS316LN and similar P91 weld joints

ABSTRACT Towards ensuring the structural integrity of the high-temperature components, it is essential to evaluate the creep crack growth (CCG) behaviour, especially for the weaker creep regions of the components. In this study, the CCG behaviour was investigated in the modified 9Cr-1Mo (P91) and Inconel (IN182) buttered layer interface regions of dissimilar metal weld (DMW) joints of P91 and type 316LN SS at 798 K and 898 K. For a quantitative assessment, the CCG rates were compared with the heat-affected zone (HAZ) of similar metal P91 welds (P91HAZ). The CCG rate was characterised using the creep fracture C* parameter as per ASTM E1457–17. The CCG rate in the P91/IN182 interface region was found to be higher than the P91HAZ, viz. 3.5 times and 4 times at 798 K and 898 K, respectively. This could be attributed to the propensity for creep cavitation in the interface region of the DMW joint.

Study on Corrosion Fatigue Behavior of 304L Austenite Stainless Steel in 325 °C High-Temperature Water Environment

The fatigue crack growth behavior of 304L austenitic stainless steel (SS) in a 325 °C high-temperature and high-pressure water environment were investigated by a corrosion fatigue test system, by electron back scatter diffraction (EBSD), and by a transmission electron microscope (TEM). The experimental results indicated that the crack growth rate (CGR) of 304L SS increases with increasing the stress intensity factor, stress level, and fatigue frequency (f). Compared to dissolved hydrogen (DH) in a high-temperature water environment, dissolved oxygen (DO) significantly enhances the CGR by about an order of magnitude higher. The crack tip of 304L SS after the corrosion fatigue test under higher stress levels is sharper, with more secondary cracks on the fracture surface, while the crack tip under lower stress levels is blunter with relatively fewer secondary cracks. The oxidation behavior at the crack tip was analyzed under different loading and water chemistry conditions, and a related effect on the crack tip and CGR was clarified.

Strain-Assisted Phase Transformation in Two-Dimensional Transition-Metal Dichalcogenides.

Two-dimensional polymorphic transition-metal dichalcogenides have drawn attention for their diverse applications. This work explores the complex interplay between strain-induced phase transformation and crack growth behavior in annealed nanocrystalline MoS2. Employing molecular dynamics (MD) simulations, this research focuses on the effect of grain size, misorientation, and annealing on phase evolution and their effects on the mechanical behavior of MoS2. First, examining phase transformation in monocrystalline MoS2 under various stress states reveals distinct behaviors depending on the initial phase (1T or 2H) and crystallographic orientation with respect to loading directions. Notably, transformation from a layered hexagonal to a body-centered tetragonal structure is more noticeable when strain in a zigzag direction is applied to the 1T sample. As such, single crystalline MoS2 with a 1T phase exhibits a 16% lower fracture stress in the armchair direction compared to that with a 2H phase. On the other hand, the 1T phase shows a 5% higher phonon lifetime compared to the 2H phase with similar phonon group velocities. Next, the influence of thermal energy and mechanical stress on the phase transformation of nanocrystalline MoS2 is investigated through annealing and quenching cycles, uncovering 60 and 44% irreversibility of phase transformation for an average grain size of 3 and 11 nm, respectively. Besides, the evolution of nanocrystalline samples with different initial phases and grain sizes is studied under uniaxial and biaxial stress. This study shows an inverse pseudo-Hall-Petch effect with exponents of 0.11 and 0.09 for 2H and 1T, respectively. The study reveals that phase transformation can occur concurrently with crack initiation and propagation with the 1T phase exhibiting a 19% lower grain size sensitivity of fracture stress compared to the 2H phase.

Fatigue crack growth of WC–Co cemented carbides: a comparative study using small indentation flaws and long through-thickness cracks

The fatigue crack growth behavior of a submicron-grained WC–Co hardmetal is investigated by artificially introducing small flaws by means of sharp indentation. Similar fatigue testing is also conducted on notched specimens with long through-thickness cracks for comparison purposes. The use of controlled small indentations flaws is shown to be a valid and successful approach for studying and describing crack growth behavior under cyclic loading for the material under consideration. This statement is based on the similitude found in fatigue mechanics and mechanisms between both crack types. Regarding the former, accounting of the indentation-induced residual stresses is key to rationalize the experimental findings. Concerning the latter, inspection of crack-microstructure interaction as well as fracture surfaces permit to discern similar features and scenarios, at both meso- and micrometric length scales. Results from this research yield an immediate practical implication, as indentation techniques may then be proposed as an alternative testing route for investigating fatigue crack growth behavior of hardmetal grades where sharp indentation is capable to induce well-developed radial crack systems.

A finite crack growth energy release rate for interlaminar fracture analysis of composite material at different temperatures

The interlaminar fracture toughness of unidirectional fiber reinforced polymer composite material is temperature and crack growth history dependence. The non-constant interlaminar fracture toughness challenges the wide application of the linear elastic fracture mechanics-based approach for analyzing the interlaminar crack growth in composite material and structures. In addition, a quantitative relationship between the ductility of the matrix and interlaminar fracture toughness of the fiber reinforced composite is still highly desirable. In this paper, both pure resin and composite double cantilever beam (DCB) are tested at different temperatures. A finite crack growth energy release rate is used to analyze the interlaminar crack growth behavior of composite material at different temperatures. The plasticity of the composite DCB is modeled by Hill’s anisotropic plasticity, with the properties determined from multiscale analyses. It is found that a single constant finite growth energy release rate can well predict the interlaminar mode I crack growth of composite material at different temperatures. The roles of the ductility of matrix, thickness of the DCB, and crack growth history on the interlaminar mode I fracture toughness of composite material are quantitatively determined by using the present method.

Fatigue Crack Propagation of 51CrV4 Steels for Leaf Spring Suspensions of Railway Freight Wagons.

Leaf springs are critical components for the railway vehicle safety in which they are installed. Although these components are produced in high-strength alloyed steel and designed to operate under cyclic loading conditions in the high-cyclic fatigue region, their failure is still possible, which can lead to economic and human catastrophes. The aim of this document was to precisely characterise the mechanical crack growth behaviour of the chromium-vanadium alloyed steel representative of leaf springs under cyclic conditions, that is, the crack propagation in mode I. The common fatigue crack growth prediction models (Paris and Walker) considering the effect of stress ratio and parameters such as propagation threshold, critical stress intensity factor and crack closure ratio were also determined using statistical methods, which resulted in good approximations with respect to the experimental results. Lastly, the fracture surfaces under the different test conditions were analysed using SEM, with no significant differences to declare. As a result of this research work, it is expected that the developed properties and fatigue crack growth prediction models can assist design and maintenance engineers in understanding fatigue behaviour in the initiation and propagation phase of cracks in leaf springs for railway freight wagons.

Effects of build direction and microstructure on fatigue crack growth behavior of Ti6Al4V fabricated by laser powder bed fusion

AbstractThis study compares the fatigue crack growth (FCG) behavior of Ti6Al4V parts fabricated by laser powder bed melting (LPBF) with four build directions (0°, 30°, 60°, and 90°) and two heat treatments and analyzed the effect of build direction and microstructure on FCG. The results show that the build direction has a significant effect on the FCG rate. Particularly, specimens with 90° build direction have the best FCG resistance while the 0° specimens have the worst. Furthermore, the fragile deposition layer boundary is the main cause of the crack deflection. The microstructure shows that the columnar β grains have less effect on FCG. The increase in resistance to FCG after heat treatment is attributed to the lath‐like α grains consuming the energy for FCG. As compared to needle‐like α′ grains, the lath‐like α grains are more likely to initiate intergranular fracture and secondary cracking, leading to rugged crack tip paths.

Investigation of microstructural evolution and mechanical properties for in-service nickel-based superalloy

In this study, the microstructural evolutions are systematically investigated in the IN718 alloy that has been in-service for eight years. Correspondingly, the effect of in-service process on mechanical properties are explored in terms of tensile properties, fatigue performance and crack growth behavior. Results show that the grain sizes at different sampling locations vary from 9.8 to 15.5 μm, exhibiting the inhomogeneous microstructures for the in-service IN718 alloy. The volume fraction of δ phase and kernel average misorientation (KAM) at outermost layer are the maximum ones, followed by the middle and innermost layers. In addition, the high magnitude of δ phase is caused by the transformation from strengthening phase γ′′. From the perspective of mechanical properties, the in-service IN718 alloy can maintain excellent yield strength and elongation at the high temperature of 650 °C, while the tensile performance is degenerated at room temperature (RT) compared with those of new IN718 alloy. The fatigue life at 650 °C is similar to that at RT under the stress levels lower than 600 MPa. As the stress level increases to 800 MPa, an obvious decrease in fatigue life at 650 °C compared with that at RT. This special fatigue performance is revealed based on the fractography examinations under different loading conditions. Moreover, the investigation of fatigue crack growth for the in-service IN718 alloy is carried out under different stress ratio and temperatures.

Fatigue growth behavior of mode II crack in headed stud steel used in steel–concrete composite structures

The fracture mechanics method has been employed for fatigue performance evaluation of stud used in steel–concrete composite structures while the fatigue crack growth behavior in headed stud steel has not been systematically studied. The present work is proposed to investigate the fatigue growth behavior of mode II crack in headed stud steel and its welded joint. Compact tensile shear (CTS) specimens extracted from the base material (BM) and weld metal (WM) of stud are tested under mode II loading to investigate the fatigue crack growth (FCG) trajectories and rates with different stress ratio. The crack closure level and FCG threshold are determined and discussed. Results show that the crack deflection in BM meets both the MTS criterion and S-criterion, while crack deflection in WM does not meet the two criterions. The 2% compliance offset criterion in the origin ASTM method is suitable to determine the crack opening force both for BM and WM of stud. The low crack closure level in WM indicates the tensile residual stress in welded joint, resulting a more quickly fatigue crack growth rate (FCGR) in WM than that in BM. The calculation formulation of equivalent SIF range proposed by Borrego et al. is verified with the best normalization effects and is recommended to fit the FCGR model both for headed stud steel and its welded joint under mode II loading.

Investigation of the fracture resistance of high strength ferritic steel welds in gaseous hydrogen environment

Hydrogen is expected to play a major role in the decarbonization of the energy grid. As a fuel, it possesses an elevated energy density per unit mass, about twice as much as natural gas but also a very low mass density; for this reason, it is preferably stored and transported at elevated pressures in the gas phase to achieve a comparable volumetric energy density. High strength quenched and tempered ferritic steels are widely used for gaseous hydrogen storage despite the fact that hydrogen affects material properties and result in higher fatigue crack growth rate and lower fracture toughness.This work presents the results of a comprehensive fracture toughness test program in gaseous hydrogen for the characterization of base metal, heat affected zone and weld metal on a high strength, quenched and tempered steel, suitable for the construction of hydrogen pressure equipment.Full penetration, butt welded joints were made using partly mechanized gas metal arc welding technique on a weldable, high strength steel, namely EN 101216-3 grade P690 QL2. Two different welding procedures were evaluated, and fracture toughness tests were conducted according to ASTM E1820 requirements, in high purity (99.9995%) hydrogen gas at a pressure of 200 bar on specimens extracted from base metal, heat affected zone and weld metal. While base metal and weld metal exhibited stable crack growth behavior, heat affected zone showed variable behavior as a function of the post-weld heat treatment conditions. It was found that local micro-hardness spots are a concern and should be controlled through dedicated post weld heat treatment operation to limit hardness values for use of welds in H2 gas conditions.

Study on fatigue crack growth behavior of the TA15 titanium alloy repaired by laser deposition

Laser deposition repair was employed to repair the body and surface of the forged TA15 titanium alloy. Subsequently, the fatigue crack growth behavior of the repaired components after annealing was investigated. The results showed that the fatigue crack growth rate (FCGR) of the body-repaired specimen decreased from the initial cracks in the heat-affected zone (HAZ), interface, and repaired zone (RZ). The increase in the repair layer thickness drastically reduced the FCGR in the surface-repaired specimen. The main crack growth mode was crossing the primary α and transformed β in the substrate zone (SZ), primarily crossing the lamellae α and along lamellae α in the HAZ, and crossing the lamellae α in the RZ. The RZ developed a tighter bond with the substrate interface, making crack growth more difficult. Crossing the lamellae α in the basket-weave structure required higher amount of energy and exhibited significant increase in tortuosity. Hence, the FCGR of the initial crack within the RZ exhibited the lowest rate. An increase in the thickness of the repair layer increased the proportion of the basket-weave, improving the resistance against crack growth, and increased the tortuosity associated with the crack path, reducing the FCGR. It was confirmed that the proposed body and surface repair methodology improved the FCGR of investigated specimens.

The effect of pipeline root weld microstructure on crack growth behaviour

The girth weld is the weak link of high-pressure pipeline, which is easy to crack under the combined action of internal and external loads, defect and stress concentration. At present, the safety hazards of high-strength pipeline girth welds are prominent, and it is of great significance to study the failure law of girth welds to ensure the safe operation of pipelines. In this paper, the failure analysis of 6 welded joints with crack defects excavated on site in a gas transmission company was carried out. Through metallographic analysis, scanning electron microscope (SEM) observation and electron backscatter diffraction (EBSD) analysis, the relationship between root welding, weld microstructure and crack generation was analyzed in detail. The influence of grain boundary distribution behavior of weld microstructure on crack propagation path was studied, and the mechanism of welding inclusions on crack propagation and aggregation nucleation was revealed. The results show that the cracks in root welding, hot welding, filling welding and heat affected zone are all transgranular propagation types, and the deflection of crack propagation path may be related to the complex stress state at the welded joint. Local strain concentration often exists in the near crack surface, secondary crack and crack deflection position on the crack propagation path, which indicates that the formation and propagation of root cracks are likely to be related to the deterioration of weld metal properties caused by local strain concentration at the root. The research conclusions have certain reference value for the intrinsic safety design, construction, and integrity operation and maintenance of pipeline girth welds.

Characterization of fatigue crack growth behavior in welded tubular T-joint

This paper focuses on investigating the fatigue crack growth (FCG) characteristics and residual fatigue life of tubular T-joints which are prone to suffer fatigue damage at the brace and chord intersections under multi-axial stress. Static and fatigue loading tests are performed to investigate the FCG behaviors of tubular T-joints, combining beach marking technique. The evolution of FCG characteristics is studied during the crack growth, convergence and wall penetration through an efficient co-simulation system that established using the multi-scale modeling technique. Fracture morphology analysis is conducted to gain insight into the FCG behaviors combining the scanning electron microscope (SEM) observation. Results indicate that multiple cracks initiate at the crown regions and subsequently evolve circumferentially around the weldments in a doubly-curved shape towards wall-thickness. Interaction effects between adjacent cracks extend the fatigue life along the outer surface, attributed to the premature exposure of converged crack front and the induced variation of stress intensify factor (SIF) distribution along the crack front. The dominant failure mode of tubular T-joints is characterized by an opening mode crack, with the contribution of anti-plane shear mode crack gradually increasing as structural symmetry diminishes. The established co-simulation system shows advantage in capturing the FCG behavior, predicting the fatigue life and characterizing the FCG characteristics with a good balancing of simulation efficiency and calculation accuracy.

Bonded and prestressed fatigue crack retarders based on Fe-SMA

A bonded and prestressed retarder concept is proposed in this work. The shape memory effect of an iron-based shape memory alloy (Fe-SMA) has been exploited to realize a desirable self-prestressing mechanism, resulting in compressive stresses in the parent structure. The synergistic prestressing and added fail-safe features of bonded and prestressed Fe-SMA retarders desirably restrain the fatigue crack growth. The fatigue crack growth behaviour in metallic plates reinforced with bonded and prestressed retarders is experimentally studied in this work. A test campaign has been implemented to investigate the effects of prestressing, as well as the influence of the relative location between the Fe-SMA retarders and the initial crack tip on the crack growth behaviour and fatigue crack life extension. The experimental results show that the prestressing of retarders desirably extends the fatigue crack growth life. More extension in fatigue life can be achieved by positioning the bonded prestressed retarders from 40 to 20 mm before the initial crack tip, the desirable retardation due to beneficial compressive prestressing and added load path on the small crack is significant. The findings in this work imply that it is beneficial to proactively reinforce the fatigue-prone locations in metallic structures with bonded and prestressed Fe-SMA retarders in order to extend the service life.

A model for modifying the S‐N curve considering the effect of boundary conditions on the fatigue crack growth of welded components

AbstractThe present study proposes a novel model to modify master S‐N curves of components according to their load redistribution capability reflected in different boundary conditions (BCs) based on the fracture mechanics analysis. To that end, a comprehensive numerical study was conducted on a Single Edge Notch Bend (SENB) specimen constrained with different kinematic BCs using discrete fatigue crack growth (FCG) simulation. It was observed that BCs indeed can have a significant effect on the crack growth behavior and consequently on the resulting fatigue life under the same nominal loading conditions. The proposed model was applied to the S‐N curve of a T‐welded joint, and the predicted fatigue life was validated against 3D FCG simulations. Finally, FCG tests were conducted on SENB specimens to experimentally corroborate the effect of BCs on the FCG rate.

A combined cyclic viscoplasticity and entropy generation approach for modelling fatigue crack growth behavior of a nickel-based superalloy at high temperature

This paper developed an entropy-based approach to estimate the fatigue crack growth (FCG) behavior of GH4169 at high temperature. Firstly, systematic FCG experiments of GH4169 at 650 °C were conducted to obtain the experimental data. Then, a finite element analysis combined with Chaboche viscoplasticity and node release technology was developed to obtain cyclic stress–strain responses and entropy generation fields. Based on thermodynamic analysis, numerous discrete material elements as sub-systems were divided along the crack growth direction in simplified representative 2D middle section. Fracture process zone (FPZ) was defined equal to the size of discrete material elements (ρ) which can be determined by distribution of normal stress perpendicular to the crack plane. Subsequently, an effective cyclic entropy generation related to ρ was defined as the crack driving force. Effective cumulative entropy generation was calculated when crack increment was ρ and the results indicated that effective cumulative entropy generation float within a certain range. The average of all effective cumulative entropy generation values was defined as crack growth entropy. Finally, an entropy-based FCG rate model was established to estimate the FCG behavior of GH4169, and a good correlation between experimental results and model predictions are achieved for all the high temperature FCG tests.

Bayesian analysis of acoustic emission data for prediction of fatigue crack growth in concrete

Acoustic emission (AE) is a valuable technique for non-destructive testing, enabling real-time monitoring and measurement of fatigue crack growth in engineering structures. The present study focuses on investigating the fatigue crack growth behaviour in plain concrete specimens through Bayesian analysis of AE data collected during the testing of beam specimens under three-point bending. A log-linear relationship is established between parameters extracted from AE waveforms and the fatigue crack growth in concrete. Additionally, AE energy is identified as the most suitable parameter for characterising the fatigue behaviour of concrete. Bayesian regression is employed for estimating model parameters and their posterior distributions. The proposed model is validated through post-processing and Bayesian analysis of experimental data from the literature. Furthermore, it is demonstrated that the estimated model parameters remain unaffected by the frequency of fatigue loading and the size of the specimen.