Crack Growth Behavior Research Articles

Fatigue Crack Growth Behavior of Additively Manufactured Ti Metal Matrix Composite with TiB Particles

Fatigue crack growth behavior of additively manufactured Ti metal matrix composite with TiB particles at room temperature was studied using a compact tension specimen and at the stress ratio of 0.1 (R = 0.1). The composite studied in this work was manufactured with a unique additive technique called plasma transferred arc solid free-form fabrication, which was designed to manufacture low-cost near-net-shaped components for aerospace and automotive industries. The fatigue crack growth rate experiments were carried perpendicular and parallel to the additive material build, aiming to find any fatigue anisotropies at room temperature. The findings reveal that additively manufactured Ti-TiB composite shows isotropic fatigue properties with respect to fatigue crack growth. Furthermore, the fatigue crack growth mechanisms in this additive composite material were identified as void nucleation/coalescence and the bypassing of particles and matrix, depending on the interparticle distance.

Multi-pass weld influence on metallurgical, tensile, impact toughness and fatigue crack growth behavior of gas tungsten arc welded Ti-6Al-4V joints

A comparative study on the influence of changing the number of weld passes on the metallurgical and mechanical properties of Ti-6Al-4 V alloy was carried out. Two welded joints involving 3-pass and 4-pass welding procedure were fabricated for 7 mm thick Ti-6Al-4 V plates using gas tungsten arc welding (GTAW) process. A compatible solid filler of Ti-6Al-4 V was used to weld these plates. A significant variation in the microstructure and the microhardness of the upper part of the fusion zones of these welds was observed. Equiaxed microstructure possessed by the base metal resulted into relatively better transverse tensile strength, ductility, and impact toughness than the 3-pass and the 4-pass weld. Widmanstätten and martensite-α′ microstructure associated with the fusion zone of the 3-pass weld resulted into a larger fraction of high angle grain boundaries (HAGBs) which accounted for higher resistance to fatigue crack propagation in this weld. Fusion zone of the 4-pass weld possessed lamellar and basketweave microstructure which had a lower fraction of HAGBs and, hence exhibited relatively lower resistance to fatigue crack propagation as compared to the 3-pass weld. However, equiaxed microstructure of the base metal could not offer much resistance to fatigue crack propagation, which resulted into relatively poor fatigue performance of the base metal. These fatigue studies indicate that both the weld joints exhibited better fatigue crack resistance than the base metal. This study established that 7 mm thick Ti-6Al-4 V joint welded using 3-pass welding procedure resulted into superior mechanical properties than those obtained by using 4-pass welding procedure.

A deformation-diffusion interactive model to study crack-tip behaviour and predict crack growth rate under fatigue and hydrogen-embrittlement conditions

Hydrogen is accepted as a cleaner and more sustainable energy source, since it carries abundant energy and does not produce any effluent gas from burning when compared to fossil fuels. However, long-term degradation in mechanical properties, attributed to hydrogen embrittlement, is reported for metallic materials exposed to gaseous hydrogen, leading to accelerated crack growth behaviour under fatigue. In this paper, a visco-plastic deformation-diffusion interactive model is developed to study fatigue crack growth behaviour in a nickel-based superalloy under hydrogen-embrittlement condition. In the model, cyclic deformation is described by visco-plasticity constitutive model, while hydrogen diffusion is simulated by a modified form of Fick’s first law of diffusion which can address hydrogen diffusion driven by the hydrostatic stress gradient. In particular, trapped hydrogen, expressed as dependent on both diffusible hydrogen concentration and inelastic strain, is considered in the modelling of hydrogen diffusion. The model also describes the effects of hydrogen concentration on mechanical deformation by considering its influence on the evolution of isotropic hardening as well as on the strain state. Interaction between cyclic deformation and hydrogen diffusion is therefore studied for a crack tip in a compact tension specimen subjected to fatigue and hydrogen attack, showing a strong dependency on both loading frequency and stress intensity factor range. Subsequently, a two-parameter criterion is proposed for fatigue crack growth prediction, which accounts for the contributions of both cyclic deformation and hydrogen diffusion. The predictions show a good agreement with experimental data in terms of crack growth rate under fatigue and hydrogen-embrittlement conditions.

Role of nitrogen on fatigue crack growth behaviour of low carbon nitrogen alloyed stainless steel at different load ratios and temperatures

Role of nitrogen on fatigue crack growth behaviour of low carbon nitrogen alloyed stainless steel at different load ratios and temperatures

Residual Fatigue Life Estimation of Damaged Welded Structures

This paper presents a finite element modeling procedure to determine of crack growth behaviour of butt welded joints under the subject of load for mode I. This paper presents a computation procedure to determine the ratio of fatigue crack growth in butt welded plates for mode I fracture mechanics loading conditions. The presence of residual stresses in welded structures can significantly affect the material’s resistance to fatigue under cyclic loading. The presence of tensile residual stresses adversely affects the fatigue crack growth rate increased it. Change of microstructure and hardening material as a result of the welding process also has a negative impact on the growth of the crack. Accurate prediction and reliable assessment of the residual stress are important for the structural integrity and residual life assessment of welded parts design. Although there are several techniques for the determination of residual stresses, the finite element method (FEM) is one of the most convenient and useful. This paper presents a finite element modeling procedure to determine of crack growth behaviour of butt welded joints under tensile load for mode I. Keywords: Welding, Residual stress, Residual life, Crack growth, FEM, Butt welded joint

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.

Fatigue crack growth behavior of Alloy 247DS brazed joints at high temperatures

Fatigue crack growth behavior of Alloy 247DS brazed joints at high temperatures

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.

Corrosion fatigue crack growth model for aluminum alloys in jet fuel

AbstractAluminum alloys are primary structural materials in aircraft fuel systems, where jet fuel can influence the fatigue crack growth (FCG) behavior of materials. This paper presents a model for calculating the FCG rate of aluminum alloys in jet fuel environment. The model is based on elastoplastic fracture mechanics and revises the interaction terms in the linear superposition model by accounting for the corrosive environment and the crack closure effect. The derivation process of the model is discussed in detail. To validate the efficacy of the model, FCG tests were conducted on three types of aviation aluminum alloys, namely, 2524‐T3, 7050‐T7451, and 7075‐T62 in the jet fuel. The experimental results were compared with FCG rates in the laboratory air environment. Findings indicate that the proposed model effectively captures the primary trends observed in the experimental data. In addition, the failure surfaces of the specimens were observed using a super‐depth‐of‐field optical microscopy system.

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.