Crack Growth Behavior Research Articles

An improved phase-field model for fatigue crack growth considering constraint effects

The phase field model is a promising method for simulating fatigue crack growth (FCG) behavior. However, the conventional phase field (PF) model may not adequately account for constraint effects, where fracture toughness is affected by geometries. Therefore, stress triaxiality is incorporated into the PF model by modifying the fracture energy release rates to consider constraint effects. The model successfully simulates the FCG behavior of different geometries, such as CT, SENB, and MT specimens, as well as the mixed-mode FCG behavior of CTS specimens and other complex geometries. All simulations agree well with experiments, proving that our model is capable to capture the constraint effects in FCG behavior. These findings indicate that stress triaxiality is important to capture the constraint effects.

Deformation, fracture, and fatigue crack growth behavior in TiB-reinforced near-α Ti matrix composites fabricated using powder metallurgy technique

Titanium matrix composites with TA15 alloy as the matrix and TiB (ranging from 0 to 4 vol%) as reinforcement were produced via powder metallurgy-based hot press sintering, employing AlB2 precursor. The addition of TiB to TA15 led to a substantial reduction in the colony and prior β grain sizes, and an increase in the lath thickness in the composites. Plastic deformation due to planar slip, which traverses across the α/β-ligament phase boundaries within each colony, may cause TiB particles located on colony boundaries to break, potentially more so than those within the colony. The composite containing 1 vol% TiB exhibited simultaneous improvements in strength, ductility, and strain hardening rate, attributed to colony refinement and possibly a sizable amount of intra-boundary TiB. Further increase in the TiB content led to significant drops in ductility, which was further exacerbated by the precipitation of α2-Ti3Al phase. A substantial decrease in the mode I fracture toughness, attributed to reduced colony size and diminished crack deflection, was noted in the composite with 1 vol% TiB. The colony size refinement also contributed to a reduction in the threshold stress intensity factor range, ΔK0, for the fatigue crack growth, owing to the decreased tortuosity of the fatigue crack front. This study elucidates the intricate relationship between TiB location, microstructure, and mechanical properties, providing insights for the design of TiB-reinforced titanium alloys with both α and β phases.

An analytical model for predicting fatigue crack arrest and growth behavior in metal plates strengthened with unbonded prestressed reinforcing strips

Fatigue crack arrest and life extension of metallic structures can be achieved by using prestressed strips for fatigue strengthening. However, the popularization of prestressed strengthening techniques has been limited by lack of versatile analytical methods. In this paper, an analytical method is developed for predicting the fatigue behavior of centrally cracked metal plates after being anchored with prestressed reinforcing strips, based on linear elastic fracture mechanics and the superposition method of stress intensity factors. The force transfer between reinforcing strips and metal cracked plates is modeled using the displacement compatibility principle. The finite-width correction factor for centrally cracked metal plates subjected to four-point concentrated forces is derived, allowing the model to analyze the effect of different anchorage locations on the effectiveness of strengthening. The analytical model is validated through experimental data and finite element analysis. The results show that the model is highly accurate and can be used to analyze how different anchorage locations affect fatigue reinforcement.

Study on hysteresis behavior of two kinds of reinforced CHS X-joints

Ultra-low cycle fatigue loading tests were conducted on two reinforced CHS X-joints (external stiffening rings and plates) and an unreinforced joint to analyze their post-yield failure modes, fracture mechanisms, and hysteretic properties. The findings indicate that these three joints exhibit varying failure modes post-yield: the unreinforced joints fail primarily due to chord fracture at the weld; the ring-reinforced joints initially see ring fracture, followed by chord crack at the weld; while the plate-reinforced joints initially fail from chord fracture and subsequently develop cracks at the plate-chord interface. The reinforced joints display plumper hysteretic loops and higher ultimate bearing capacities than the unreinforced joint. Compared to X-1, the tensile and compressive ultimate bearing capacities of RX-1 have increased significantly by 56.9 % and 74.1 %, respectively. In comparison, those of TX-1 have increased moderately by 2.2 % and 67.6 %, respectively, albeit with a slight compromise in ductility. The cumulative energy dissipation of RX-1 and TX-1 has risen by 22.62 % and 131.99 %, respectively, compared to the unreinforced joint. Notably, the dissipated energy before cracking accounts for 16.50 %, 18.16 %, and 22.29 % of the cumulative energy dissipationfor the three joints, respectively, highlighting the substantial contribution of crack propagation to energy dissipation under large deformations. The VUSDFLD subroutine based on Cyclic Void Growth Model (CVGM) is embedded into ABAQUS, this subroutine considers the initiation and propagation of joint cracks. Three finite element models with the same dimensions of the specimens were built to simulate the damage process of the joint. The results show that this method can accurately simulate such joints’ cracking and crack growth behavior under cyclic axial load. The simulated hysteresis curve is in good agreement with the test curve.

Comparative study of marine steel and welding joint in artificial seawater based on stress corrosion cracking and crack growth

In this work, the environmental fracture behavior of marine steel and its welding joint were compared in artificial seawater. The electrochemical measurement, slow strain rate tensile test, and low-frequency cyclic loading test were conducted. The results showed that the corrosion current density and stress corrosion cracking susceptibility of welding joints were higher than those of base metal, caused by high dislocation density and residual stress in the heat-affected zone. The acidification and ion concentration occurred at the crack tip of the welding joint and basic metal. pH value and Cl− concentration at the crack tip reached 4.1 and 3.8 mol/L, respectively. The crack growth rate of the welding joint was higher than that of base metal. The crack growth behavior was controlled by the mechano-electrochemical effect at the crack tip, and hydrogen embrittlement was the main factor.

Investigating the impact of manufacturing conditions on crack growth rate in nitrile butadiene rubber for enhanced service life – Part 02: Crosslink density via multi-stage swelling analysis

The importance of the state of cure of elastomer components with regard to fatigue behavior and thus also durability was already discussed in more detail in Part 01 of the paper. It was shown that crosslinking time and temperature have an enormous influence on the crack growth behavior in nitrile butadiene rubber (NBR). In the course of this, a fourfold deceleration in crack growth was observed with a 20 K increase in manufacturing temperature. To confirm this trend, test specimens were produced at 180 °C. Crack growth was found to be 40 times slower, especially in high tearing energy ranges, compared to 150 °C. To analyze the observed behavior in more detail, cylindrical samples were taken from the plain strain test specimens (used for the crack growth measurements), and multi-stage swelling was performed with them. This allowed a detailed analysis of the degree of cure and the sulfur chain composition. It was found that the crosslink density decreases with increasing manufacturing temperature due to decomposition and the proportion of polysulfidic crosslinks decreases with increasing heating time due to desulfurization. Furthermore, a correlation between the results found by means of the rubber process analyzer (RPA) and the swelling results could be established looking at the maximum transmitted torque during the isothermal crosslinking phase.

Experimental and crystal plasticity study on hydrogen-assisted fatigue crack growth behavior of IN625 superalloy

The fatigue crack growth (FCG) behavior of IN625 alloy was studied by experimental methods and crystal plasticity simulations. Based on the experimental FCG rate curves, it is evident that hydrogen significantly accelerates FCG, and calculations show that this acceleration factor reaches a maximum of 2.41 times at 1 Hz. Hydrogen results in smaller plastic deformation zones compared to hydrogen-free samples. The interaction between hydrogen and dislocations leads to the nucleation of micro-voids along the slip planes, promoting the hydrogen-assisted cracking process. Lower loading frequencies results in finer fatigue striations and more pronounced hydrogen embrittlement features on the fracture surface.

Characterization of fatigue crack growth in directed energy deposited Ti-6Al-4V by marker load method

Fatigue crack growth properties of materials are crucial for evaluating damage tolerance in additive manufacturing (AM) metallic structures. However, the unique microstructures and defects of AM materials result in highly complex fatigue crack growth behaviors. Currently, there is a lack of systematic fatigue crack growth rate measurement methods specifically targeting this characteristic. Therefore, taking directed energy deposited Ti-6Al-4V titanium alloy as the object of the study, fatigue crack growth tests were conducted in three orthogonal build orientations of the material using marker load method. Additionally, the visual measurement and compliance methods were also employed to measure fatigue crack growth rates, and the anisotropy of fatigue crack growth property was analyzed. Subsequently, anisotropic fatigue crack growth behaviors were characterized by optical microscope, scanning electron microscope, confocal microscope, and electron backscatter diffraction, suggesting that the microstructure is the primary factor affecting overall fatigue crack growth. Furthermore, nanoindentation tests were conducted to obtain the micromechanical properties within and among columnar grains in different build orientations, clarifying the homogeneity and anisotropy of mechanical properties. Finally, a fatigue crack growth rate measurement method based on marker load method was established, and the advantages of this method in AM materials and structures were summarized by comparing the results with those obtained using these two mature methods.

A Simplified Failure Assessment to Identify Crack Growth Behavior in the Gas Pipeline by Post-Hydrostatic Pressure

Hydrostatic pressure tests on pipelines have been found to be an inadequate means of critical integrity management due to the frequency of false negatives, which result from the inability of these tests to detect crack growth. It can be argued that focusing on pipe leakage does not guarantee future operability. A study presents a failure assessment methodology based on the failure assessment diagram (FAD), which aims to predict crack growth during hydrotesting. The calibrated pipe spool is validated by the application of data from hydrostatic tests and analytical techniques to ascertain the potential growth in circumferential surface cracks. A variety of factors, including pressure, material grades, flaw dimensions, and elliptical flaw angles, were examined in an effort to assess cracks. The results demonstrate that there is no pipeline leakage and minimal trapped air. Despite its location within the plastic zone, with a normalized pressure index of ≤1, the pipeline is deemed to be within acceptable limits according to the criteria established by the FAD. The assessment point was found to be predominantly influenced by the toughness ratio of the material grade. The crack propagated in the opposite direction with a maximum length a/c= 0.125 and a crack depth a/t= 0.2, which limited the toughness ratio. The load ratio indicates uniformity in elliptical angle flaw results. In this simplified failure assessment, the parameter describing the flaw size, which exhibits a strong correlation with the toughness ratio, plays a pivotal role. Further research and recommendations are also proposed.

Investigating dwell fatigue crack growth in welded titanium alloy structures

ABSTRACT In this paper, dwell-fatigue crack growth tests under complex loading spectra were conducted for a titanium alloy used in marine structures. The influence of upper and lower peak dwell time on the dwell-fatigue crack growth behavior of the welded structure were comprehensively investigated. The results show that the longer the upper and lower peak dwell times were, the faster the crack growth rate was. Still, the impact of the upper and lower dwell times on the crack growth rate of titanium alloy had a specific saturation value. Based on the above findings, a new model was developed that combined the fracture mechanics theory, considering the effects of welding and load dwell on crack growth. By predicting the dwell-fatigue behavior of titanium alloy welded structures using the model, it was confirmed that the model has a strong prediction ability for the dwell-fatigue behavior of welded structures under complex loads..

Fracture toughness and fatigue crack growth in DMLS Co-Cr-Mo alloy: Unraveling the role of scanning strategies

Co-Cr-Mo alloy is crucial for biomedical implants and aerospace components. These parts often exhibit a high level of geometric intricacy. Direct metal laser sintering (DMLS) is ideal for these complex parts. In DMLS, choosing the right scanning strategies is vital, as it significantly affects the fatigue fracture behavior of the printed components. Thus, the present study investigates the effect of different scanning strategies (stripe, meander, and chessboard) on the fracture toughness and fatigue crack growth behavior of DMLS printed Co-Cr-Mo alloy. For each scanning strategy, fatigue crack growth tests have been performed to evaluate the threshold stress intensity factor and Paris law constants. To corroborate the obtained experimental results, microstructure analyses have been performed using electron backscattered diffraction. Further, failure mechanisms have been identified from fractographs obtained using field emission scanning electron microscopy. It is evident from the obtained test results that scanning strategies caused significant variation in fracture toughness and fatigue crack growth behavior. The stripe scanning strategy has exhibited higher resistance to fracture and fatigue crack growth. However, delayed crack initiation has been observed in the case of the chessboard scanning strategy. The present study provide the background for better selection of scanning strategies to mitigate fatigue fracture in DMLS-printed Co-Cr-Mo alloy designed for specific applications.

Phase-field finite element modelling of creep crack growth in martensitic steels

Creep fracture presents a major concern for structural materials and critical components operating at elevated temperatures, thus requiring effective computational models. This study presents a phase-field framework for modelling creep crack growth and fracture behaviour of modern high-temperature materials such as Creep Strength Enhanced Ferritic (CSEF) martensitic steels. The model was formulated using the thermodynamic principles of the variational phase field theory of fracture and considering some physical aspects of creep fracture. Within the modelling framework, a dissipation potential dependent on creep damage is introduced to capture the effect of creep cavitation on the fracture energy and the creep crack growth resistance of the solid in a phenomenological manner. An elastoplastic power-law creep model is coupled to the phase-field formulations to account for the non-linear deformation processes ahead of the crack tip due to inelastic deformations, as well as their contribution to fracture at high temperatures. The capability of the proposed model is assessed against experimental data from compact tension (CT) creep tests conducted on P91 and P92 steels. Good agreement was obtained between the FE-predicted creep crack growth behaviour and the experimental measurements, showcasing the model’s feasibility. Further, numerical experiments were conducted using the proposed model to elucidate some key aspects influencing the fracture behaviour of martensitic steels. The proposed computational framework not only demonstrated good capability but was also able to offer improved mechanistic insights into the influence of material tendency to develop creep cavities on crack growth behaviour. This work contributes valuable insights into understanding the fracture process of CSEF steels at elevated temperatures and further demystifies the role of creep ductility.

The impact of welding residual stresses on fatigue crack growth behaviour in aluminium alloy plates: a numerical and experimental investigation

ABSTRACT This paper presents the findings of an investigation into fatigue crack growth in Compact Tension (CT) samples made of an aluminium alloy that contains welding residual stresses. The samples were welded using the TIG welding process, and the fatigue crack growth was measured through experimental means. To model the welding process and resulting residual stresses, as well as their impact on the rate of fatigue propagation, 3-D thermal-mechanical finite element (FE) analyses were conducted. The simulations revealed that the residual stresses induced by welding relaxed during fatigue loading. Both the experimental and FE results demonstrated similar relaxation behaviour of the specimens under fatigue loading. This study highlights the significant influence of residual stresses on the propagation of fatigue cracks in aluminium alloy specimens. By combining experiments and numerical simulations, a comprehensive understanding of how welding residual stresses evolve during fatigue loading and affect crack growth rates is achieved. The agreement between the experimental and FE models confirms the effectiveness of the finite element method in investigating the influence of residual stresses on the behaviour of fatigue crack propagation in structures fabricated through welding processes.

Influence of oxygen content on fatigue crack growth behavior of FGH96 superalloys

The fatigue crack growth (FCG) mechanism of FGH96 superalloys influenced by oxygen (O) was investigated in terms of grain structure and phase composition. The results reveal that the thinner powder oxide film process facilitates easier dissolution, and thus the fewer undissolved oxides as well as the movement of high-angle grain boundaries promote the generation of a uniform distributed small grains (SGs) within both the prior particle boundaries (PPB) interface and powder interior in the alloy with O content of 140 ppm. In contrast, the large number of undissolved oxide particles provides more recrystallization sites and thus the SGs clusters aggregate at the PPB interface in the alloy with O content of 340 ppm during sintering. A homogeneous distribution of SGs induces the uniform deformation of the alloy and thus stimulates the transgranular fracture of the coarse grains in alloy with O content of 140 ppm, in favor of a higher fatigue life. On the contrary, in the alloy with O content of 340 ppm, the SGs clusters forming on PPB interface results to the strain concentration at PPB, leading to the PPB fractured morphology and a serious reduction of the fatigue life. This study reveals that the generation of SGs is responsible for the FCG behavior and failure mechanism of the alloys, and suggests that regulating the uniform distribution of SGs or reducing the number of SGs during sintering plays an important role in improving the fatigue properties of powder metallurgy superalloys.

Fatigue and fracture in dual-material specimens of nickel-based alloys fabricated by hybrid additive manufacturing

The integration of additive manufacturing with traditional processes, termed hybrid additive manufacturing, has expanded its application domain, particularly in the repair of gas turbine blade tips. However, process-related defects in additively manufactured materials, interface formation, and material property mismatches in dual-material structures can significantly impact the fatigue performance of components. This investigation examines the low cycle fatigue and fatigue crack growth behaviors in dual-material specimens of nickel-based alloys, specifically the additively manufactured STAL15 and the cast alloy 247DS, at elevated temperatures. Low cycle fatigue experiments were conducted at temperatures of 950 °C and 1000 °C under a range of strain levels (0.3%–0.8%) and fatigue crack growth tests were conducted at 950 °C with stress ratios of 0.1 and −1. Fractographic and microscopic analyses were performed to comprehend fatigue crack initiation and crack growth mechanisms in the dual-material structure. The results consistently indicated crack initiation and fatigue fracture in the additively manufactured STAL15 material. Notably, fatigue crack growth retardation was observed near the interface when the crack extended from the additively manufactured STAL15 material to the perpendicularly positioned interface. This study highlights the importance of considering yield strength mismatch, as well as the potential effects of residual stresses and grain structure differences, in the interpretation of fatigue crack growth behavior at the interface.

Fracture evolution characteristics of asphalt concrete using digital image correlation and acoustic emission techniques

The cracking damage of asphalt concrete significantly affects the service life of pavement. The objective of this research is to investigate fracture mechanisms of asphalt concrete by employing a synergistic approach combining digital image correlation (DIC) and acoustic emission (AE) techniques under three-point bending tests. Firstly, the crack tip opening displacement (CTOD) and the fracture process zone (FPZ) derived from DIC were utilized to classify fracture stages and describe the entire crack propagation process. Meanwhile, AE-related parameters were applied to detect the internal damage and fracture characteristics. Subsequently, the results from DIC and AE were utilized for comparative analysis to evaluate the cracking resistance of different materials. Finally, the fractal dimension and AE b-value were used to distinguish the characteristics of the critical damage conditions. The results indicated that both techniques are effective in classifying the fracture stage of asphalt concrete. DIC can visualize and analyze crack propagation processes and paths on concrete surfaces, especially at the stage of macrocrack initiation and propagation. AE allows for continuous monitoring and highly sensitive detection of early-stage damage inside concrete. The distinct advantages of DIC and AE result in different fracture stage identification results at the early stage, but consistent after the appearance of macrocracks. Based on the relationship between CTOD and AE cumulative count, a simple approach is proposed to directly evaluate the cracking resistance of different asphalt concretes. The formation of macrocracks in asphalt concrete can be reflected by a shift where the correlation dimension drops steeply to the minimum value, and a minimum inflection point in the AE b-value curve acts as a precursor of complete fracture. The research offers an in-depth insight of both surface deformation and internal damage, thus enhancing the understanding of crack growth behavior and fracture process in asphalt concrete.

A Model to Account for the Effects of Load Ratio and Hydrogen Pressure on the Fatigue Crack Growth Behavior of Pressure Vessel Steels.

A phenomenological model for estimating the effects of load ratio R and hydrogen pressure PH2 on the hydrogen-assisted fatigue crack growth rate (HA-FCGR) behavior in the transient and steady-state regimes of pressure vessel steels is described. The "transient regime" is identified with crack growth within a severely embrittled zone of intense plasticity at the crack tip. The "steady-state" behavior is associated with the crack growing into a region of comparatively lower hydrogen concentration located further away from the crack tip. The model treats the effects of R and PH2 as being functionally separable. In the transient regime, the effects of the hydrogen pressure on the HA-FCGR behavior were negligible but were significant in the steady-state regime. The hydrogen concentration in the steady-state region is modeled as being dependent on the kinetics of lattice diffusion, which is sensitive to pressure. Experimental HA-FCGR data from the literature were used to validate the model. The new model was shown to be valid over a wide range of conditions that ranged between -1≤R≤0.8 and 0.02≤PH2≤103 MPa for pressure vessel steels.

Assessment of fracture criteria for cracking behavior of asphalt concrete using various SCB specimen sizes

AbstractIn this research, five fracture criteria (maximum tangential stress (MTS), generalized maximum tangential stress (GMTS), minimum strain energy density (SED), generalized minimum strain energy density (GSED), and modified maximum tangential stress (MMTS)) were evaluated for asphalt concrete using fracture data from tests conducted under various loading modes and conditions. Tests were performed on semi‐circular bend (SCB) specimens of various sizes. Based on the results, the MTS criterion tends to overestimate fractures, while the GMTS criterion exhibits more precise predictions. The SED and GSED criteria inaccurately predict fractures, especially under certain loading conditions. The MMTS criterion shows superior predictive capability for asphalt concrete fractures. Specimen size influences fracture resistance, with larger specimens exhibiting higher fracture values. Overall, the MMTS criterion closely mirrors the crack growth behavior of asphalt concrete, highlighting its precision in predicting fracture results. The errors between the predictions of the fracture criteria MTS, GMTS, SED, GSED, and MMTS, and the experimental results ranged from 0% to 42.8%, 0% to 22.5%, 0% to 52.7%, 0% to 42.4%, and 0% to 14.5%, respectively.

Ultrasonic vibration assisted directed energy deposition of titanium alloy: Microstructure control, strengthening mechanisms and fatigue crack behavior

Large-scale titanium alloy structural components fabricated using direct energy deposition (DED) technology have broad application prospects. However, the columnar grain structures and inhomogeneous microstructure due to the inherent characteristics of additive manufacturing seriously affects the mechanical properties of components. In this study, high-intensity ultrasound vibration was introduced into the wire-arc DED deposition of TC4-DT titanium alloy. The results showed that ultrasonic vibration assistance (UVA) significantly improves the grain structure (refine prior-β grain size and weaken α texture) of the as-deposited component, without introducing additional porosity. Compared to conventional wire-arc DED deposited components, the yield strength and tensile strength of UVA alloys increased by 22.23 % and 15.06 %, respectively. Furthermore, the difference in α grain structure caused by UVA results in different fatigue crack growth behaviors. The disorder orientation of the α laths enhanced the fatigue crack initiation resistance of the wire-arc DED component with UVA. This study shows the excellent mechanical properties of UVA DED fabricated TC4-DT alloy, paving the way for the design, fabrication, and application of large structural components of titanium alloys.

Mechanisms of near-wellbore fracture growth considering the presence of cement sheath microcracks and their implications on wellbore stability

Placement of intended perforations is usually extensive enough into the reservoir to minimize any near wellbore influence on the expected fracture growth. However, some unplanned fractures do not necessarily originate from the intended shot holes. A sufficient volume of published work exists on the processes of crack growth from the near wellbore regions such as from the cement sheath or from fractures observed on the formation wall surrounding the wellbore. A robust quantification of wellbore stability issues should not detach such fracture development, which can potentially result into issues such as cement sheath damage and unnecessary formation damage in the near-wellbore regions. Such fractures may become fluid injection points during stimulation related wellbore pressurization, leading to pronounced development. In such scenarios, fracture development may become a nuisance if it jeopardises the intended structural support and zonal isolation integrity of the cement sheath around the wellbore, leading to wellbore failure, and a potential abandonment. The pressure transmission across this structural seal affects the zonal isolation integrity and its intended structural support leading to wellbore instability. Understanding crack growth behaviour in such cement-rock structural framework is still illusional yet it poses the wellbore to a risky state of instability. Our study provides insight into mechanical and elastic behavior of a tightly bonded cement-rock framework with presence of initial cracks. It also provides insight into how the acting far-field stresses, applied wellbore pressures, crack length, and orientation angle influence the crack growth morphologies including planar fractures, curved and cyclic fractures. The study is important not only for a proper selection of the cement slurry but also for the definition of allowable pressures during the wellbore lifespan. Inappropriately selected parameters might lead to the loss of wellbore integrity to a point as early as the start of a well’s life.