Fatigue Crack Growth Rate Research Articles

Microstructure and fatigue crack growth behavior of heat-treated electron beam melted Ti-6Al-4V alloy

The effect of post-deposition heat treatment on microstructure and fatigue crack growth has been analyzed for electron beam melted Ti-6Al-4V plates. Samples have been heat-treated at temperatures of 950 °C and 1050 °C, and subsequently cooled at different cooling rates in the furnace and the water. The as-built sample possesses columnar prior β grains filled with exceptionally fine α+β Widmanstätten patterns and epitaxially grows in the vertical direction. Heat treatment with a slow cooling rate increases the α lath thickness, whereas fast cooling results in multiple needle-shaped α/α′ phases inside the prior β grains. The yield strength and ultimate tensile strength in as-built conditions are greater than the extruded mill annealed sample. The as-built samples show better crack growth resistance during the fatigue crack growth rate test than the heat-treated and mill-annealed samples. The lower plastic deformation of the as-built sample than the mill-annealed sample is attributed to the existence of fine α laths that restrict the motion of dislocation.

Insights into fatigue crack propagation behaviour of WAAM produced SS316 stainless steel: Molecular dynamics simulation and experimental analysis

The current work focuses on the crack growth mechanism / behavior of wire arc additively manufactured SS316L. Beside the experimental investigation, a molecular dynamics based polycrystalline model is developed to simulate the crack propagation under cyclic loading. The purpose of this work is to understand the crack growth behavior of wire arc additively manufactured SS316L, particularly the directional dependence of crack propagation. The experiments and molecular dynamics analysis are performed for the crack propagation along the build and traverse direction for the SS316L deposit. The experimental procedure involves fatigue crack growth rate tests, while the molecular dynamics simulations model the crack propagation in polycrystalline structures of different grain sizes. The ultimate tensile strength in build and traverse direction were 535±2 MPa and 495 ± 2 MPa, respectively. The experimental results reveal faster crack propagation along build direction in comparison to traverse direction. The values for Paris law material constants i.e. C and m are 7x10-15 and 6.78 along build direction while 8x10-14 and 6.02 along traverse direction, respectively. The crack growth along the build direction is primarily intergranular, which leads to high crack growth rate. The molecular dynamics simulations reveal the initiation and closure of secondary cracks for the traverse direction, which leads to slower crack growth rate along the traverse direction. The fractography of the fractured surfaces from fatigue crack growth rate test also indicate the formation of secondary cracks for the case of traverse direction. The molecular dynamics simulations performed with three grain sizes and thickness variation indicate the insensitivity of predicted crack growth mechanisms with respect to grain sizes used for simulations.

Experimental Research on the Low-Cycle Fatigue Crack Growth Rate for a Stiffened Plate of EH36 Steel for Use in Ship Structures

This paper presents a straightforward approach for determining the low-cycle fatigue (LCF) crack propagation rate in stiffened plate structures containing cracks. The method relies on both the crack tip opening displacement (CTOD) and the accumulative plastic strain, offering valuable insights for ship structure design and assessing LCF strength. Meanwhile, the LCF crack growth tests for the EH36 steel were conducted on stiffened plates with single-side cracks and central cracks under different loading conditions. The effects of stress amplitude, stress ratio, and stiffener position on the crack growth behavior were examined. Fitting and verifying analyses of the test data were employed to investigate the relationship between CTOD and the crack growth rate of EH36 steel under LCF conditions. The results showed that the proposed CTOD-based prediction method can accurately characterize the LCF crack growth behavior for stiffened plate of EH36 steel for use in ship structures.

Investigation of fatigue crack growth behavior and crack tip plastic zone characteristics in welded structures based on local displacement fields

The fatigue crack growth behavior in welds is difficult to evaluate due to complex internal stresses and microstructural differences. This paper uses the CJP model based on displacement field variations to describe the fatigue crack growth behavior and the size of the crack tip plastic zone in welds. First, fatigue crack growth rate tests were conducted on base metal and as-welded specimens. The digital image correlation technique was used to capture the speckle pattern variations on the specimen surface during crack growth, obtaining the crack tip displacement field required by the CJP model. Based on this, by calculating the stress intensity factor ranges ΔK and ΔKCJP, two da/dN–ΔK(ΔKCJP) curves defined by the traditional and CJP models were obtained, proving the model’s applicability for describing fatigue crack growth behavior in welded joints. Finally, the CJP model was used to calculate and compare the crack tip plastic zones of base metal and as-welded specimens. It was found that the maximum error for the as-welded specimen was 37.84%, occurring at the initial stage of crack propagation and gradually decreasing with the fatigue process.

Microstructure evolution and fatigue crack propagation of AlCoCrFeNi2.1 high entropy alloy after electron beam surface modification

AlCoCrFeNi2.1 eutectic high-entropy alloy has good fatigue properties. However, its application in engineering is hindered by as-cast defects. To solve this problem, vacuum electron-beam surface modification is used to strengthen the surface of as-cast high-entropy alloy. It has been discovered that the remelting zone is still composed of body-centered cubic and face-centered cubic phases and that its phase size is refined. A surface-modified structure consisting of the compact RZ and a heat affect zone is obtained, by applying the same fatigue load to the samples before and after electron-beam, respectively. The fatigue crack growth rate of the electron-beam surface-modified material is slower and its life to fracture is longer. Additionally, the fan-shaped fracture and tear edges are typical fracture characteristics of the specimens before and after modification. The results show that with a significant reduction in phase size, the phase boundary density increases. The enhancement of the high-angle grain boundary content and changes in the stress state are also critical factors for the inhibition of fatigue crack growth rate.

Investigating the effect of stress ratio on fatigue crack growth rate behaviour in friction stir welded AA7075-T651 aluminium alloy plates

This study examines the characteristics of Fatigue Crack Growth Rate (FCGR) in AA 7075-T651 Friction Stir Welded (FSW) joints with a thickness of 16 mm. Compact tension (CT) specimens from both the base and the FSW joints were utilized to assess FCGR under various stress ratios (0.2 to 0.5). Additionally, the study involved analyzing the transverse tensile characteristics of both the based and the welded joints. The microstructures of the welds were inspected using optical and transmission electron microscopes. Findings indicate that the FSWjoint critical stress intensity factor range (ΔKcr)is lower than the unwelded base, that can be related to precipitates dissolving in the weld zone when the friction stir welding process. Consequently, in contrast to the unwelded base, the fatigue life of AA 7075-T651 aluminium alloy FSW joints is significantly reduced. A scanning electron microscope carried out microstructural analyses of the fracture surfaces of the FSW and base under various stress ratioconditions. The results showed that, in contrast to the parent material, the FSW joint has ultra-fine grains resulting in intergranular cracks in the propagation zone.

High-temperature fracture behavior of an α/β Titanium alloy manufactured using laser powder bed fusion

The room and high temperature (up to 600 °C) mechanical properties, with emphasis on the fracture toughness and fatigue resistance, of an α/β titanium alloy that was additively manufactured using the laser powder bed fusion (LPBF) technique were evaluated in the as-printed (α’ lath martensite with negligible β phase content) and heat-treated (α/β structure with a significant volume fraction of the β phase) states for critically examining the β phase's role in the fracture and fatigue behavior (given the significantly higher diffusion rates of various species in β compared to α). Experimental results reveal that the heat-treated sample exhibits superior ductility and fracture initiation toughness at elevated temperatures, compared to the as-printed sample, due to the presence of β ligaments in the former. They also prevent strain localization along the prior β grain boundaries, and their absence in the as-printed state leads to cracking along those boundaries. While the fatigue crack growth (FCG) rates in both the as-printed and heat-treated samples deteriorated with increasing temperature, the threshold for FCG generally increased, which is attributed to creep-induced crack tip blunting. Moreover, a double Paris exponent phenomenon is observed for the heat-treated sample at 300 °C, attributed to the hydrogen-assisted crack growth, with β phase providing an accelerated diffusion pathway. Overall, the experimental results and their analyses illustrate the intricate relationship between creep, hydrogen diffusion, and the β phase in dictating the fracture behavior of LPBF α/β titanium at elevated temperatures.

Effects of single overloads on fatigue crack propagation behaviour of AZ31B magnesium alloy under different temperatures

An experimental study examined the influence of single overload ratio (OLR) on fatigue crack growth (FCG) behaviour in AZ31B magnesium alloy at different temperatures (300 K, 373 K, and 473 K). The results showed that FCG rates and stress intensity factor ranges (ΔK) vary with temperature and overload ratios. After applying a single overload (OL), the FCG rate initially decreased, then increased rapidly, and stabilized at a value consistent with constant amplitude loading (CAL). The fatigue life increased by 86 % to 143 % at 300 K, 75 % to 132 % at 373 K, and 75 % to 134 % at 473 K for OLRs ranging from 1.75 to 2.25 when compared to the fatigue life at CAL. A single overload accumulates energy at the crack tip, forming an overload region with tearing ridges, secondary cracks, dimples, and fatigue striations under varying temperatures.

A fatigue crack tip field model considering residual stress and plasticity-induced closure for welded structures

In order to more accurately understand and predict the fatigue crack growth behavior in welded structures, a fatigue crack tip field model considering residual stress and plasticity-induced closure (RSPC model) is presented in this paper. Building upon the existing CJP model, which adequately explains plasticity-induced crack closure, the RSPC model further considers the influence of tensile residual stress induced by welding. The model, through a detailed derivation and analysis of the residual stress function, establishes the theoretical stress and displacement fields at the crack tip of the butt-welded joint. Integrating these fields with the CJP model, the theoretical framework of the RSPC model is formulated. Subsequently, through fatigue crack growth experiments and digital image correlation analysis, a comparison and validation of the predictive performance of the CJP and RSPC models are conducted. It is observed that the latter aligns more closely with experimental results in calculating plastic zone area, with a maximum error of only 10%. In comparison to the CJP model, the average error decreases by 19.143%, indicating higher predictive accuracy. Furthermore, the study focuses on the contribution of tensile residual stress in butt welded joints to the growth of the plastic zone at the fatigue crack tip and the underlying reasons for its variations. Finally, the research investigates the constrained relationship between plastic zone size and fatigue crack growth rate. Corresponding relationship functions are fitted, providing more accurate guidance for crack control and material design.

Insights into fatigue crack propagation mechanism of T91 steel in liquid lead-bismuth eutectic at 150–450 °C

The fatigue crack growth (FCG) behaviors of T91 steel in oxygen-saturated liquid lead–bismuth eutectic (LBE) at 150–450 °C were investigated. The FCG rate increased gradually with the increase of temperature at 150–350 °C, is firstly low at 450 °C and then comparable with that at 350 °C. The macroscopic trans-granular cracks preferentially propagated along the deformation-induced low-angle grain boundaries (LAGBs) near the crack tip at the microscopic scale. Intergranular precipitation-enhanced wetting of matrix by liquid LBE results in the reduction of atomic bonding force and formation of micro-cracks along the LAGBs, which triggers brittle cracking.

Microstructure and fracture toughness of laser wire deposited Ti-6Al-4V

In this paper, the microstructure, and mechanical properties of Ti-6Al-4V fabricated through Laser Directed Energy Deposition (L-DED) process are investigated and discussed. Deposited coupons were produced on Ti-6Al-4V wrought substrate using Ti-6Al-4V ELI grade wire with the laser wire deposition (LWD) process. Characterization efforts led to the evaluation of the microstructure, hardness, tensile behavior, and fatigue crack growth resistance of the build-up alloy. Microstructure of the deposit consists of columnar prior β grains and basket-weave α/β phase mixture. The size of the alpha laths is measured as 1.2 ± 0.3 µm and 0.8 ± 0.2 µm in the banded zone and band-free zone, respectively. Hardness of the deposited block is found to be uniform and ranging between 321 and 323 Hv. Tensile and fatigue crack growth properties of the deposited block were evaluated in various orientations. Tensile specimens loaded in the deposition direction exhibited 824 MPa and 930 MPa for yield and ultimate tensile strength, respectively. Tensile specimens loaded in the build direction exhibited 782 MPa and 907 MPa for yield and ultimate tensile strength, respectively. The analysis reveal that the tensile properties of the deposited material match the strength requirements for ASTM F1108 Cast Ti-6Al-4V. However, they fell just below the AMS 4911P Wrought Ti-6Al-4V standard. Fatigue Crack Growth Rate tests were conducted for three directions: parallel to the deposition direction, parallel to the build direction and at 45° of the deposition direction. No significant differences in crack growth properties were observed between the different orientations with average crack initiation near threshold (ΔKth) and the fracture toughness (Kc) value of 3.6 MPa√m and 69 MPa√m, respectively. The crack propagation properties are similar to cast & wrought materials.

Prediction of corrosion fatigue crack growth rate in aluminum alloys based on incremental learning strategy

The 2xxx series aerospace aluminum alloys, vital for aircraft structures due to their excellent mechanical strength and corrosion resistance, face challenges with corrosion fatigue failure, impacting material integrity and aircraft safety. Accurately predicting the corrosion fatigue crack growth rate is crucial for these materials’ reliability in aerospace applications, ensuring safety and operational efficiency. Various unique incremental learning strategies like structural optimization, regularization, and replay were used to enhance the predictive model’s adaptability to continually updated data without the need to retrain the entire model, thus overcoming “catastrophic forgetting.” Based on these strategies, a series of XGBoost models for the prediction of corrosion fatigue crack growth rate in aerospace aluminum alloys have been developed. The results demonstrate that the replay strategy excels in maintaining the accuracy of historical data, while structural optimization and regularization strategies stand out in adapting to new data and effectively preventing overfitting. Combining these three strategies, the models maintain good accuracy and a low forgetting rate across all steps, with an accuracy rate exceeding 0.95 and a forgetting rate below 0.05 in all incremental steps, showcasing excellent knowledge retention capabilities. The models achieve a balanced performance in learning new tasks and retaining old knowledge.

Development of corrosion-fatigue analysis method of cable hangers in tied-arch bridges considering train-bridge interaction

This study proposes an integrated corrosion-fatigue analysis method for bridge hangers by combining the dynamic bridge-train interaction analysis for random train arrivals with various speeds and the linear elastic fracture mechanics (LEFM)-based crack propagation analysis considering the corrosion-fatigue effects. Accordingly, a three-stage process, namely, zinc coating corrosion, pit-crack transition, and corrosion-fatigue crack propagation for wires in hangers, was proposed to simulate the total service life. A train-bridge-interaction analysis was conducted on a tied-arch bridge with multiple cable hangers using the vector form intrinsic finite element method, where the stresses in the hangers were generated under high-speed train loads. The pit-crack transition was analyzed based on the pit growth and fatigue growth; whose rates compete to dominate the transition phenomenon. The fatigue crack growth rate was estimated using a LEFM approach considering the equivalent stress ranges and number of stress cycles. The effects of reducing the cable cross-section and updating the internal stress of the multi-layered wires were investigated. The present method was compared with the continuum damage mechanics model for validation. A relatively short remaining corrosion-fatigue life was found in the cables under the train loads, after corrosion of the zinc coating and pit-crack transition in the steel wires were developed.

Failure analysis of fatigue crack propagation in specimens of AlSi10Mg aluminum alloy produced by L-PBF: Effect of different heat treatments

Fatigue crack growth behavior of AlSi10Mg aluminum alloy processed by laser powder bed fusion (L-PBF) was studied for three material conditions: as-built, stress relieved and hot isostatic pressed. An improvement in the fatigue crack growth resistance was found for the heat-treated conditions which was attributed to the residual stress relief induced by the heat treatments. The stress ratio did not affect the fatigue crack growth in regime II which was explained by the absence of crack closure. Fatigue crack growth rates were predicted using a model based on the cyclic plasticity of the tested aluminum alloy. The proposed model agreed well with the experimental results.

Influence of load ratio on fatigue life assessment of AZ31B magnesium alloy under different temperatures

First ever fatigue crack growth experiments were conducted under various load ratios (R = 0.1 to 0.8) at 300 K, and (R = 0.1, 0.3, 0.5, and 0.7) at 327 K and 473 K, respectively, on AZ31B magnesium alloy. The results reveal that temperature-dependent variations in both fatigue crack growth rates and stress intensity factor ranges (ΔK) across different load ratios. It was found that at relatively higher load ratio (R = 0.8) leads to a lower ΔK = 8.0287 MPa√m and a lower load ratio (R = 0.1) result in a higher ΔK = 15.5981 MPa√m, indicating that the material undergoes fatigue crack propagation at a higher level of stress intensity factor range at room temperature. A slower crack propagation was seen at higher load ratio (R = 0.8), whereas lower load ratio (R = 0.1) exhibits faster crack growth at 300 K. Similarly, crack growth generally increases at higher temperatures (i.e., 373 K and 473 K). The fatigue life of the magnesium alloy AZ31B increased from 12,410 to 87,031 cycles at 300 K, 9,620 to 32,667 cycles at 373 K, and 9,140 to 30,143 cycles at 473 K as the load ratio increases. The average fatigue life decreases to 59 % and 63 % as the temperature is varied from 300 K through 373 K to 473 K. The fractographic and surface roughness carried out by field emission scanning electron microscopy and 3D-Profilometer respectively, illustrate that roughness of fracture surfaces correlates with an increase in the fatigue crack growth rate for a specific value of ΔK.

High cycle fatigue behavior of bulk 6061Al prepared by cold spray-friction stir processing composite additive manufacturing

In this work, the high cycle fatigue behavior of bulk 6061Al prepared by cold spray-friction stir processing composite additive manufacturing (CFAM) was studied for the first time. The microstructure and fatigue fracture morphology of CFAM sample were characterized by scanning electron microscopy equipped with electron backscattering diffraction. The stress versus number of cycles of failure (S-N) curve for CFAM sample was established, and the fatigue crack growth behavior of CFAM sample was analyzed. The results showed that the microstructure of CFAM sample was denser and more homogeneous compared with cold spray (CS) sample, and the average grain size was refined to 3.3 μm due to dynamic recrystallisation. The dense, homogeneous and fine microstructure delayed the initiation and growth of fatigue cracks, and thus resulted in a 178% increase in fatigue strength in the CFAM sample over CS sample. The fatigue surface of the CFAM sample could be divided into fatigue source zone, crack growth zone, and transient fracture zone, showing typical fatigue fracture characteristics. The fatigue crack growth rate in the CFAM sample was significantly lower than that of CS sample for the same stress intensity factor range (ΔK). This was because the fine grains uniformly distributed in CFAM sample increased the proportion of grain boundaries and hindered the fatigue cracks growth. Therefore, CFAM is a promising technology for preparing bulk 6061 Al with excellent fatigue properties.

Laser shock peening for enhanced fatigue resistance in steel structures: Insights from Q960 steel study

Fatigue damage of steel structures is often likened to the “cancer” of structural engineering. Fatigue failure, characterized by the absence of apparent signs before occurrence, usually leads to significant losses. To investigate the potential of laser shock peening (LSP) as an effective method for improving the fatigue performance of steel structures, square plates and compact tension (CT) specimens made of Q960 structural steel were designed. The surface integrity, fatigue crack growth (FCG) performance, microstructural grain size, and fracture morphology of specimens were compared at different laser shock parameters. The results show that LSP implanted a high level of beneficial residual compressive stress in a mild way, and no obvious surface defects were introduced. A noticeable retardation effect was found in the FCG of LSPed specimens, and this effect becomes more pronounced with the increase of laser pulse energy. Compressive residual stress is a more dominant factor in enhancing fatigue performance compared to grain refinement for the small-grained metals studied. LSP can effectively reduce the FCG rate of Q960 high-strength steel, showing promising application prospects in strengthening the fatigue hazard zone of steel structures.

Isotropy of fatigue crack growth in hybrid additive manufactured Ti6Al4V ELI titanium alloy

In the present paper, the effects of macrostructure and microstructure on fatigue crack growth (FCG) behavior of wire and arc additive manufacturing (WAAM) and hybrid additive manufacturing with in-situ rolling (HAMR) were investigated in three sampling directions for Ti6Al4V ELI. The results demonstrate that WAAMed samples exhibit coarse columnar grains with zigzag boundaries, interlayer heat affected band (HAB), and basketweave microstructures. The fatigue crack growth rate (FCGR) is lowest in the Y-X direction while being similar between the X–Y and X-Z directions, indicating significant anisotropy. This difference can be attributed to changes in plastic region size and internal slip characteristics near the crack tip caused by variations in macrostructure and microstructure. After applying HAMR with quasi-β heat treatment, interlayer HABs are eliminated, resulting in uniform equiaxed grains and colony microstructures. The FCGR curves in three directions significantly overlap without any observed anisotropy, which are lower than those obtained for traditionally forged materials. Therefore, the isotropic FCGR of plastic materials can be reduced by achieving a nearly flawless metallurgical quality and ensuring uniform equiaxed grain matching with the corresponding critical colony size and fine thickness lamellae. The findings suggest that the application of quasi-β heat treatment in conjunction with HAMR offers a promising high-efficiency manufacturing technique for producing isotropic titanium alloy forgings characterized by exceptional damage tolerance.

Fatigue crack growth study on rolled and cryogenically treated AZ31B Mg alloy plate

This paper investigates the fatigue crack growth rate (FCGR) of AZ31B Mg alloy after cryogenic treatments. Shallow and deep cryogenic treatments were carried out separately at −80οC and −196οC for 12 and 72 hrs, respectively. The microstructural characterization of the cryogenically treated AZ31B specimens was carried out by optical microscope (OM) and X-Ray diffraction (XRD). The small crystalline size structure generated in AZ31B alloy due to the cryogenic treatment further promoted the nucleation of Mg17Al12 phase. Accumulation of dislocation density was also observed in the cryogenically treated specimens. CT specimens were prepared for all three conditions to estimate the FCGR. The as-rolled specimen (R) had higher fatigue resistance than the cryogenically treated specimens. The R specimen’s threshold intensity factor (ΔKth) was higher than the cryogenically treated specimens. The exponent (m) value of the R specimen was lower than the shallow and deep cryogenic treated specimens. Hence, the crack growth resistance was marginally higher in the R specimen compared to the cryogenically treated specimens. However, the critical stress intensity factor (ΔKcr) of cryogenically treated specimens was slightly higher than the R specimen. The post-fracture characterization of the specimens was carried out at three different regions by SEM. The cleavage faces, tear ridges, and voids were noticed in all specimens. The fatigue failure of the R specimen was due to ductile and brittle mode, whereas the cryogenically treated specimen was dominated entirely by brittle fracture.

Viability of WAAM for fabrication/repair of SS316: Fatigue crack growth behavior for varied notch locations in the vicinity of WAAM-substrate interface

The tensile and fatigue crack growth rate (FCGR) behavior of SS316L wire deposited on SS316 substrate using wire arc additive manufacturing (WAAM) was studied to assess its potential for fabrication and repair applications. Five initial notch locations near the WAAM-substrate interface were considered for FCGR. Cracks in the deposition direction propagated towards the WAAM region. FCGR of cracks in the build direction accelerated in the WAAM region due to its favorable columnar grain orientation. Overall, FCGR behavior near the WAAM-substrate interface is comparable with wrought alloy, suggesting WAAM’s viability for SS316 fabrication and repair, with satisfactory structural integrity observed in tensile and FCGR behavior.