Lower Fatigue Crack Growth Rate Research Articles

A crystal plasticity phase-field model for microstructure sensitive fatigue crack growth in a superalloy

In this work, a dislocation density-based crystal plasticity phase-field model (CP-PFM) is developed to simulate fatigue crack growth in nickel-based superalloys. Through normalization validation, the plastic dissipation work and crystallographic work are shown to be consistent with the fatigue indicator factors (FIPs), cumulative equivalent plastic strain and cumulative shear strain, and the two energies are computed as the main driving forces of the phase field. Both driving force models are able to obtain fatigue crack growth in close approximation to the experimental rate. However, the model with crystallographic work as the main driving force obtains crack growth paths that are in better agreement with electron backscattering pattern (EBSD) observations, which is attributed to its greater ability to characterize the microstructural susceptibility of fatigue crack growth. Specifically, the model is able to capture the tendency of cracks to crack along the close-packed planes and the hindering effect of grains with large misorientation angles on fatigue crack growth, which together contribute to the curved morphology of fatigue cracks. The combination of large grains or grains with small misorientation angles favors persistent slip band (PSB) formation and leads to softening of the crack tip, which results in lower fatigue crack growth rates.

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.

Influence of solid solution treatment on fatigue crack propagation behavior in the thickness direction of 2519A aluminum alloy thick plates

2519A aluminum alloy thick plates have excellent applications in the military industry due to their superior ballistic resistance and fatigue properties. However, the delamination cracking along the thickness direction of this alloy hinders its further utilization. To address this issue and enhance the fatigue properties of this alloy, the effect of single-stage solid solution treatment (SST) and two-stage solid solution treatment (DST) on the fatigue crack propagation (FCP) behavior in the thickness direction of the alloy was investigated. Compared to the optimum SST, the volume fraction of the secondary phase of the alloy was reduced by 28.6% after DST. The precipitated phases were finer with more uniform distribution, and the volume fraction and number density of the precipitation increased by 47% and 30%, respectively. The DST resulted in a lower fatigue crack growth rate (FCGR) and an increase of 2.92% in the critical stress intensity factor value (ΔKcr). The crack propagation mode of the alloy after SST was inter/transgranular crack propagation, while it was transgranular after DST. The aspect ratio of grain and the number of secondary phases in the alloy decrease after DST. Meanwhile, the grain boundary (GB) precipitation phase spacing, the precipitation-free zone (PFZ) width, and the spacing between the secondary phases increase. These changes result in decreased crack sources in the alloy and reduced the bridging effect of secondary phases on fatigue cracks, thus improving the fatigue performance of the alloy.

Fatigue crack growth rate and mechanical properties of one-step double-side friction stir welded AA6061-T6

Friction Stir Welding (FSW) is commonly used in transportation industries but faces challenges when joining thick plates. To address this, double-side FSW (DFSW) has been introduced, involving two steps: FSW on one side, flipping the plate, and then FSW on the opposite side. However, this process is time-consuming. To overcome this, a one-step DFSW technique was developed, where two tools simultaneously operate on both sides. This study aimed to investigate the impact of tool rotation speed on the fatigue crack growth rate of AA6061-T6 in one-step DFSW. One-step DFSW was performed at different upper and lower tool rotational speeds: 1500/900 rpm, 1500/1200 rpm, and 1500/1500 rpm. Tensile and fatigue crack growth tests were conducted on the joined specimens. The microstructure of the welded joint and the surface fractures resulting from the tests were analyzed using a scanning electron microscope (SEM). Results showed that the specimen welded at 1500/1500 rpm exhibited the lowest fatigue crack growth rate. The Paris constants (C and n) were determined as 4.348E-08 and 4.16, respectively.

Microstructure, fatigue crack growth rate and low-cycle fatigue behavior of an Al[sbnd]Li alloy sheet with different aging

The microstructures, fatigue crack growth (FCG) rates and low-cycle fatigue (LCF) behaviors of 1445 Al-Li alloy sheet with T6 aging at 170 °C, single-step T8 (S-T8) aging at 150 °C and double-step T8 (D-T8) aging at 170 °C following aging at 150 °C for 4 h were investigated. The main precipitates in T6 and S-T8 samples are δ′ phases, but strain strengthening causes 40 MPa enhancement in the S-T8 sample yield strength (YS). Due to coherent shearable δ′ precipitates, the FCG rate curves of S-T8 samples are highly coincident. With the 2nd-step aging time extension, δ′ precipitates in D-T8 sample gradually coarsen, semi-coherent S′ phases form and their amount increases. The interaction between δ′ precipitates with dislocations transites from shearing to by-passing, leading to the YS enhancement and the FCG resistance decrease. Meanwhile, cyclic-hardening decreases and transfers to cyclic-softening during LCF at starin amplitude of 0.6% ∼ 0.9%. At 0.9% strain amplitude, due to greater cyclic-softening and cleavage fracture of S′ precipitates, the D-T8 sample with 2nd-step aging time of 120 h has a shorter LCF life. The D-T8 sample with 2nd-step aging time of 24 h possesses a comprehensive property of higher strength, lower FCG rate and better LCF life.

Fatigue Life and Crack Growth Rate Prediction of Additively Manufactured 17-4 PH Stainless Steel using Machine Learning

The present work is focused on machine learning-assisted predictions of the low cycle fatigue behaviour and fatigue crack growth rate (FCGR) of 17- 4 PH SS processed through L-PBF and post-processing. Various machine learning techniques reported in the literature provided a flexible approach for explaining the complex mathematical interrelationship among processing-structure-property of the materials. In the present work, four machine learning (ML) algorithms, such as K- Nearest Neighbor (KNN), Decision Trees (DT), Random Forests (RF), and Extreme Gradient Boosting (XGB) algorithms, are implemented to analyze the Fatigue Crack growth rate (FCGR) of 17-4 PH SS alloy. After optimizing the hyper parameters for these algorithms, the trained models were found to estimate the unseen data as equally well as the trained data. The four tested ML models are compared among each other over the training as well as the testing phase based on their mean squared error and R2 scores. Extreme Gradient Boosting model has performed better for the FCGR predictions providing the least mean squared errors and higher R2 scores compared to other models.

Fatigue crack growth behavior of selective laser melted martensitic stainless steel

This study investigated the fatigue crack growth (FCG) behavior of selectively laser-melted AISI 420 martensitic stainless steel built in two directions. The effect of tempering treatment at 400 °C was considered. In both as-built and tempered conditions, the parallel orientation exhibited lower fatigue crack growth rates (FCGRs) than the perpendicular orientation, indicating an anisotropic FCG behavior. Tempering treatment moderately reduced the anisotropy of FCG behavior, as the difference in FCGR between the two build orientations decreased in the tempered state. The anisotropic FCG behavior was attributed to the interaction between the crack path and the elongated cellular structure. Tempered martensite had a greater influence on the FCGR of the perpendicular group over the parallel group in tempered state.

State‐of‐the‐art review on developing lightweight fiber‐metal laminates based on synthetic/natural fibers

AbstractThe development of hybrid materials consisting of alternating layers of metal alloys and polymeric composites has revolutionized the engineering field. These metal‐composite laminates combine the merits of each individual component, forming advanced and promising structures which could be employed for structural applications. The low fatigue crack growth rate and high damage tolerance of these materials are particularly fascinating. When developing metal‐composite laminates with superb functional properties, the combination of several metal surface treatment techniques is a common practice to ensure optimum metal‐composite adhesion. Today, a new cutting‐edge manufacturing technique utilizing nano‐porous network film has been established to get rid of the massive infrastructure of conventional autoclave techniques and produce first‐rate materials. Even though various techniques can be adopted to manufacture metal‐composite laminates, the selection of the techniques is highly dependent on the nature of the polymer matrix. After the manufacture of the metal‐composite laminates, post‐stretching is considered a critical step to alleviate the unfavorable residual stress to ensure the optimum performance of these materials for long‐term service. When dealing with natural fibers, it is worth mentioning that the processing temperature should be limited to below 200°C regardless of manufacturing technique to avoid fiber degradation. Metal‐composite laminates consisting of synthetic/natural fibers are considered a viable option to offset the demerits of each kind of fibers. By incorporating fatigue‐resilient natural fibers in the laminates, it is expected that the overall fatigue resistance of the laminates can be apparently enhanced, which is in line with the initial goal of developing metal‐composite laminates.Highlights Fiber bridging offers fiber‐metal laminates with superior fatigue properties. Metal surface treatment is vital to ensure an optimum fiber‐bridging mechanism. A novel out‐of‐autoclave method can produce first‐grade fiber‐metal laminates. Post‐stretching is needed to lessen residual stress in fiber‐metal laminates. Hybrid fiber‐metal laminates are feasible for structural applications.

Study on fatigue crack propagation behavior of 2050 Al[sbnd]Li alloy during non-isothermal aging

The fatigue crack propagation behavior of 2050AlLi alloy at different stages during non-isothermal aging (NIA) was tested. It was found that when the aging temperature was reduced to 150 °C, the alloy exhibited higher fatigue crack growth resistance and lower fatigue crack growth rate and the fatigue crack growth path was mainly transgranular propagation in different stages of NIA. In addition, in the first stage of fatigue crack growth, the size and quantity of precipitates in the grain are the main factors affecting the crack growth. Because the tiny T1 phase can be sheared by dislocation without damage accumulation, it can effectively prevent crack propagation. In the second stage of fatigue crack growth, the strength difference between grain boundary and intragranular will directly affect the fatigue crack growth resistance of the alloy. The smaller the strength difference is, the more uniform the deformation of crack tip in the process of crack growth, and it is not easy to produce stress concentration and damage accumulation at the grain boundary. In this case, the alloy exhibits the better the fatigue crack growth resistance and the slower the crack growth rate.

Effect of grain structure on fatigue crack propagation behavior of Al-Cu-Li alloys

Recrystallization behavior during optimized heat treatments provides a potential to obtain desirable grain structure, which significantly improves the mechanical properties of aluminum alloys. The influence of grain structures on fatigue crack propagation (FCP) behaviors of Al-Cu-Li alloy with hot-rolled (HR) and cold-rolled (CR) was investigated. Subgrain boundaries have a significant impact on small crack growth rates, which is reflected in the pronounced fluctuation of fatigue crack growth of HR specimens after solution treatment. Moreover, the specific cellular structure within grains can improve the deformation capacity of alloys due to their accommodation of plastic deformation, which contributes to the lower fatigue crack growth rates and higher threshold values in HR specimens. The intragranular deflection also decelerates the FCP rate and occurs in these regions of large grain without subgrain boundaries. Recrystallization occurs in the CR specimens, resulting in small anisotropy on the fatigue resistance for the different orientations in the Paris stage due to the recrystallization texture. Fatigue cracks can be deflected and tend to propagate along the grain boundaries when it goes into the grain with a relatively low Schmidt factor value.

Effect of microstructure on fatigue crack growth behavior of surface‐ and middle‐layer materials of thick high‐strength bridge steel plates

AbstractThe effects of microstructure on the fatigue crack growth behavior of surface‐ and middle‐layer materials of high‐strength bridge steel (including steel Q370qE, Q420qE, and Q500qE) plates were investigated. The microstructures are captured via an optical microscope, and fracture morphologies are examined by scanning electron microscopy. The results indicate that for materials with different strength grades of steel plates at the same sampling locations, the higher the strength grade, the lower the fatigue crack growth rate (FCGR). In addition, materials with smaller equivalent grain sizes or higher volume fractions of the second hard phase exhibit lower FCGR for the surface‐ and middle‐layer materials of the identical strength‐grade steel plate. The fatigue fracture morphological examination results show that typical fatigue striations along with secondary cracks are observed in the stable crack growth region of three high‐strength bridge steel plates. Moreover, fatigue steps are observed in steel Q500qE, which has the lowest FCGR, indicating that the fatigue steps contribute significantly to resisting the fatigue crack propagation.

Fatigue performance and acoustic emission behavior of remanufactured low-carbon steel made by wire and arc additive manufacturing

The fatigue behavior of low-carbon steel made by wire and arc additive manufacturing (WAAM) is studied. The role of the microstructures underpins the difference in fatigue performance between WAAM and hot-rolled materials is investigated using scanning electron microscopy (SEM), optical metallography, and in-situ acoustic emission. Results show the WAAM low-carbon steel has a lower fatigue crack growth rate (FCGR) comparing with the hot-rolled steel. The mechanisms enhancing the fatigue performance are a lower volume fraction of pearlite and the interlocking basket weave microstructure in WAAM specimens. The anisotropy in the FCGR of WAAM low-carbon steel is very small.

Effect of aging treatments on fatigue properties of 6005A aluminum alloy containing Sc

A new type of alloy, modified based on 6005A alloy, with addition of 0.06% Sc was prepared by ingot metallurgy. The alloy ingots were homogenized, extruded, quenched and solution-aged into 8 mm thick plates. The effect of different aging treatments on the fatigue properties of 6005A alloy was investigated. It is revealed that optimum resistance against fatigue crack growth (FCG) was obtained in the under aged (UA) sample. The lowest fatigue crack growth rate (FCGR) was evident in the UA sample compared with the peak aged (PA) and over aged (OA) samples.

Cohesive zone modelling of hydrogen assisted fatigue crack growth: The role of trapping

We investigate the influence of microstructural traps in hydrogen-assisted fatigue crack growth. To this end, a new formulation combining multi-trap stress-assisted diffusion, mechanism-based strain gradient plasticity and a hydrogen- and fatigue-dependent cohesive zone model is presented and numerically implemented. The results show that the ratio of loading frequency to effective diffusivity governs fatigue crack growth behaviour. Increasing the density of beneficial traps, not involved in the fracture process, results in lower fatigue crack growth rates. The combinations of loading frequency and carbide trap densities that minimise embrittlement susceptibility are identified, providing the foundation for a rational design of hydrogen-resistant alloys.

Estimation of Fatigue Crack Growth Rate in Heat-Resistant Steel by Processing of Digital Images of Fracture Surfaces

The micro- and macroscopic fatigue crack growth (FCG) rates of a wide class of structural materials were analyzed and it was concluded that both rates coincide either during high-temperature tests or at high stress intensity factor (SIF) values. Their coincidence requires a high level of cyclic deformation of the metal along the entire crack front as a necessary condition for the formation of fatigue striations (FS). Based on the analysis of digital fractographic images of the fatigue fracture surfaces, a method for the quantitative assessment of the spacing of FS has been developed. The method includes the detection of FS by binarization of the image based on the principle of local minima, rotation of the highlighted fragments of the image using the Hough transform, and the calculation of the distances between continuous lines. The method was tested on 34KhN3M steel in the initial state and after long-term operation (~3 × 105 h) in the rotor disk of a steam turbine at a thermal power plant (TPP). Good agreement was confirmed between FCG rates (both macro and microscopic, determined manually or using digital imaging techniques) at high SIF ranges and their noticeable discrepancy at low SIF ranges. Possible reasons for the discrepancy between the micro- and macroscopic FCG rates at low values of the SIF are analyzed. It has also been noted that FS is easier to detect on the fracture surface of degraded steel. Hydrogen embrittlement of steel during operation promotes secondary cracking along the FS, making them easier to detect and quantify. It is shown that the invariable value of the microscopic FCG rate at a low SIF range in the operated steel is lower than observable for the steel in the initial state. Secondary cracking of the operated steel may have contributed to the formation of a typical FS pattern along the entire crack front at a lower FCG rate than in unoperated steel.

Influence of types and shapes of 3D printed polymeric lattice on ductility performance of cementitious backfill composites

The compression properties of cementitious backfill composite (CBC) is strong, but their tension properties are weak and presents little ductility because of their low fatigue crack growth rate and resistance. Thus, proper reinforcement techniques are needed to improve CBC’s ductility without rupture. This paper deals a system for boosting CBC’s ductility by reinforcing it with several types (e.g., ordinary resin OR, transparent resin TR, and nylon NY) and shapes (e.g., hexagon, cube, and rhombus) of 3D printed polymeric lattice (3D PPL) by agreeing the digital image correlation method. Specimens were subjected to three-point bending and SEM experiments to study their flexural and microstructural behavior. Results shown that: the flexural strengths of OR and NY 3D PPL reinforced CBC specimens were larger than TR and non-3D-PPL (N-3D-PPL) reinforced ones. But TR 3D PPL reinforcement effect was not so vibrant. For an ideal flexural strength, the best type and shape of 3D PPL were OR and rhombus, respectively. The deflection of 3D-PPL reinforced CBC specimens was larger than those N-3D PPL reinforced ones. For an ideal deflection value, the optimal shape and 3D PPL types were cube and OR. 3D PPL reinforced CBC specimens exhibited great ductility and toughness while N-3D PPL reinforced ones exhibited obvious brittleness. The most abundant elements were oxygen, calcium, and silicon, which gathered in the large amounts around C-S-H gels’ location. Finally, the outcomes of this work state that 3D PPL has great potential for converting brittle CBC into a ductile material without dropping its hardening performance.

Effect of microstructure on fatigue crack growth of wire arc additive manufactured Ti–6Al–4V

In the present paper, the effect of microstructure on fatigue crack growth (FCG) behavior of wire arc additive manufactured (WAAMed) Ti–6Al–4V was investigated. The fatigue crack growth rate (FCGR) in WAAMed sample with heat-treated was lower than that in the forged sample. The 1050-AA (1050 °C/2h/air cooling (AA)+550 °C/4h/air cooling (AC)) sample showed the lowest FCGR because of its highest ΔKT (18.01 MPa m1/2), while the forged sample exhibited the highest FCGR because of its lowest ΔKT (13.3 MPa m1/2). The larger the plastic zone (rp) at the crack tip, the higher values of the stress intensity factor transition factor (ΔKT), the lower FCGR in the sample. In addition, the sample with continuous grain boundary α (αGB) or discontinuous αGB showed different FCG behavior and its mechanisms were discussed. For the sample with continuous grain boundary α (αGB), the crack in the horizontal sample (H-sample) passed through the continuous αGB and grew along with basket-weave microstructure and colony microstructure, while the crack in the vertical sample (V-sample) grew along with basket-weave microstructure. Therefore, the FCGR in these samples showed anisotropy. For the sample with discontinuous αGB, the crack in the H-sample and the V-sample passed through the discontinuous αGB and grew along with colony microstructure or basket-weave microstructure. The two samples showed the same FCG behavior, which indicated that there was no anisotropy.

Warm deformation enhances strength and inhibits hydrogen induced fatigue crack growth in metastable 304 and 316 austenitic stainless steels

The effect of deforming temperature on fatigue crack growth rate (FCGR) was investigated for cold/warm deformed 304 and 316 stainless steels under 5.0 MPa hydrogen and argon gas atmospheres. The slope of the FCGR vs ΔK curve in both steels declined with increasing deforming temperature, especially in type 304 steel. Strain-induced α′ martensite was prone to flat-facetted, step-like and quasi-cleavage fracture. Warm deformation (400 °C) can inhibit the formation of α′ martensite around crack tip and cause cellular dislocation structures, which led to multi-directional crack pathway and lower FCGR. The microstructure was adjusted by warm deformation, and then hydrogen embrittlement resistance was improved on the basis of high-strength in metastable austenitic stainless steels.

A new numerical approach to estimate stress intensity factors under very high cycle fatigue testing

Ultrasonic very high cycle fatigue testing finds many advantages in fatigue studies especially in very low fatigue crack growth rate investigations. In this application, the determination of the stress intensity factor has a key role and presenting a general approach to calculate stress intensity factor in reasonable time and ultimate accuracy is necessary. The present research proposes a new numerical approach (hybrid method) based on reduce order modeling (dynamic substructuring), multi-harmonic balance method, and M-integral to evaluate stress intensity factors under very high cycle fatigue testing in the most accurate and fast way possible. The verification of the hybrid method was done through a benchmark and implicit solver implemented in ABAQUS commercial software as the reference solution. Investigations proved that the presented hybrid method here could determine stress intensity factor in ultrasonic regime with the ultimate accuracy and save computational time at least 50%.

Experimental Investigation of Fatigue Crack Growth Behavior in Banded Structure of Pipeline Steel

The effect of a ferrite/mixed phase banded structure on the fatigue crack growth behavior of pipeline steel has been investigated in this work. Constant stress-controlled fatigue tests were carried out on unilateral notch specimens in the longitudinal direction (LD) and the transverse direction (TD). The strain accumulation was measured by digital image correlation (DIC) method and the fatigue fracture morphology was analyzed. The results show that the banded structure in the LD would cause a wide range of crack deflections, especially in the early stage of fatigue crack growth, which results in a lower fatigue crack growth rate compared to the TD. At the later stage, due to the increase of the plastic zone of the crack tip, more ferrite in the crack growth path of the TD specimen is also beneficial for crack tip blunting, thereby slowing down the increase of the crack growth rate.