Short Fatigue Crack Propagation Research Articles

Critical damage events of 3D printed AlSi10Mg alloy via in situ synchrotron X-ray tomography

Fish-scale-like melt pool structures and internal defects are unique characteristic in additively manufactured (AM) metals. These features play a critical role in the damage and fracture processes under different service conditions, governing the material's performance. However, the relationship between these damage features and loading conditions, as well as the spatial interactions between melt pool structures and internal defects remains incompletely understood. By in situ time-lapse synchrotron X-ray tomography and diffraction, we identify the initiation and growth events of life-limiting damage under tensile, low cycle fatigue (LCF), and high cycle fatigue (HCF) loading. A novel transition from meso-structure insensitive, defect-dominated short fatigue crack propagation to a meso-structure sensitive mechanism occurs as the plastic zone expands of a growing crack. Under tension and LCF, the damage accumulation gradually increases and micro-voids nucleate at the melt pool boundaries (MPBs) after which the crack path follows the MPBs. In contrast, under HCF, surface defects initiate fatigue cracking and the MPBs have a very limited effect on the crack propagation path. Finally, physics-informed machine learning method is introduced to develop a novel methodology for predicting fatigue life by including three-dimensional features of defects in AM parts.

Short fatigue crack growth and retained austenite in steels processed via quenching and partitioning

Previous research has consistently found that introducing metastable retained austenite (RA) as a second phase retards the failure of steel under fatigue. However, the reasons for this benefit are not understood. Accordingly, the properties of RA most advantageous to resist fatigue are not known. Within this context, this paper examines the interaction between second-phase RA and short fatigue crack growth in a steel processed via quenching and partitioning, using quasi in situ electron backscatter diffraction experiments. Results show that most RA transforms into martensite under the plastic strain surrounding the crack. They also reveal various mechanisms whereby RA transformation delays short fatigue crack propagation; transformation-induced crack closure (TICC), crack deflection and branching, and roughness-induced crack closure (RICC). Crack deflection and branching are driven by a tendency of cracks to propagate towards transformed RA, which is against the previous assumptions in the literature. Furthermore, the impact of crack deflection/branching on retardation is more powerful than that of TICC acting alone. Microstructures including second-phase RA should avoid RA-lean areas and promote elongated RA grains, with relatively large size, and major axis normal to the preferential crack growth direction. Untransformed RA within the plastic zone (i.e. overstabilized) does not contribute to crack retardation.

Effect of loading conditions on short fatigue crack propagation behaviours in cast Al[sbnd]Si alloys

The effect of loading conditions on short crack propagation behaviours in cast AlSi alloys was investigated by a combination of photo-microscopy monitoring and post mortem analysis. The findings suggest that the path of short cracks is minimally influenced by loading conditions, whereas loading conditions significantly affect the fatigue crack growth rate (FCGR) and its statistical characteristics. Crack closure effect is observed under zero and negative stress ratio conditions. Roughness-induced crack closure occurs due to the coarse grain microstructure of cast AlSi alloys and the Stage-I mode growth of cracks within grains in the microstructural short crack regime. The high level of plasticity around the crack tip under negative stress ratio conditions has a significant impact on FCGR, resulting in a negative crack opening stress. When the stress ratio exceeds approximately 0.3, short cracks exhibit no crack closure. The load-level effect is observed only at high load conditions with negative stress ratios. Finally, a probabilistic short crack growth model was established using a Bayesian linear regression model to analyse the effect of stress ratio on the uncertainty of FCGR. After considering the crack closure effect, the difference in distributions of FCGR model parameters decreases significantly among different stress ratio conditions.

Analysis of the crack initiation and short crack propagation of laser cladding IN718 enduring fretting fatigue at 650 ℃

In order to reveal the behaviors of crack initiation and short fatigue crack propagation in laser cladding IN718 under high temperature conditions, very high cycle fretting fatigue (VHCFF) experiments were conducted at 650 ℃ with a transverse clamping force of P=400N and different axial stress loading conditions to obtain S_N data. The surface wear morphology and fracture morphology characteristics were analyzed by scanning electron microscope (SEM) and white light interferometer. Additionally, the microstructure characteristics of grains in the short fatigue crack region were examined through electron back scatter diffraction (EBSD). Parameters such as equivalent grain size, grain elastic modulus, Schmid factor, geometric compatibility factor, twist angle, and tilt angle were considered in the analysis of the characteristics of fatigue crack initiation and propagation. The results indicate that fretting fatigue cracks tend to initiate on grains with higher Schmid factors, larger equivalent grain size, and lower grain elastic modulus on the fretting contact surface. Moreover, the short fatigue cracks tend to propagate along the grains with the microstructural features of the high Schmid factor, the geometric compatibility factor of m′>0.4, the twist angle of α<50°, and the tilt angle of β<50°.

Microstructural insights into fatigue short crack propagation resistance and rate fluctuation in a Ni-based superalloy manufactured by laser powder bed fusion

The microstructural sensitivity of fatigue short crack path and its propagation rate in a Ni-based superalloy GH4169 manufactured by laser powder bed fusion (LPBF) was investigated at room temperature. In-situ digital image correlation (DIC) observation and post-mortem microstructural analysis around the crack path were performed. The results show that the intragranular cracks developed in the shear cracking mode are closely aligned along the activated slip bands in the γ-matrix grains with the crystallographic characteristics of parallel to the γ-{111} slip planes. Multiple slip was also activated, causing the crack retardation or deflection. Low-angle grain boundaries and subgrain boundaries were shown to cause deflections of intragranular cracking, while high-angle grain boundaries significantly arrested the short crack propagation. Moreover, the resistance of grain boundaries to short cracking was assessed using combined metrics including the crystallographic and microstructural parameters of twist angle, the Schmid factor and the geometrical compatibility factor. These site-specific microstructural analyses around the crack path provide insights into the microstructural origins of resistance to the short crack propagation as well as an interpretation of the observed significant fluctuations in the crack propagation rate.

Fatigue behaviors of ultrasonic surface rolling processed AISI 1045: The role of residual stress and gradient microstructure

In the study, AISI 1045 steel were treated by ultrasonic surface rolling processing (USRP) with different parameters in terms of coverage rate. The gradient structures characterized with refined polished surface, the sub-surface nano-lamellar microstructure with a maximum compressive residual stress of 453.4 MPa was obtained, which shall be responsible to the superior fatigue properties to the pristine. Furthermore, the comparative investigation of fatigue behavior demonstrates that the surface roughness has the pronounce effects on the resistance to crack initiation, though the excessive USRP with the surface roughing may result in the premature crack growth; in contrast to the gradient microstructure, the compressive residual stress may delay effectively the incubation and increase the growth resistance of the physically short crack. Besides, the benefit of gradient microstructure is relatively limited in the resistance of fatigue short crack propagation.

Microstructure relevant fatigue short crack propagation behavior of Al0.3CoCrFeNi high entropy alloy: in-situ SEM study

The microstructure effects on fatigue short crack propagation behavior of Al0.3CoCrFeNi high entropy alloys (HEAs) were studied by in-situ fatigue testing under scanning electron microscope (SEM). The experimental results indicated that fine grain (FG) specimens had better fatigue crack propagation resistance and higher fatigue life compared with coarse-grain (CG) specimens. Fatigue short crack propagation followed transgranular mode, associated with octahedral slip. The fatigue small crack growth is primarily microstructurally sensitive, showing evident fluctuations related to the blocking effects of grain boundaries and twin boundaries. Furthermore, a modified fatigue crack growth model has been proposed, which can successfully evaluate the crack growth rates of HEAs with different grain microstructures under different fatigue stresses.

The behavior of anisotropic fatigue short crack initiation and propagation for a directionally solidified superalloy CM247LC at room temperature

AbstractThe effects of mechanical and microstructural anisotropy on short fatigue crack initiation and propagation behaviors of a directionally solidified superalloy have been studied. An unusual result was found where the fatigue lives of specimens with grains longitudinally aligned along the loading direction fail at lower lifetimes than specimens with transversely loaded grains when the applied stress is close to the yield stress. This is mainly attributed to the lower Young's modulus of the longitudinal specimen, which induces more local plastic strain (at stress concentration features) leading to earlier crack initiation and faster crack propagation under the applied test stress.

Microstructural effects on short fatigue crack propagation in cast Al-Si alloys

The role of microstructure on the short fatigue crack behaviour of cast Al-Si alloy under high cycle loading conditions is investigated by tracking the progress of cracks through fine, medium and coarse microstructured samples. Generally, in the early stage of crack growth, short cracks mainly grow transgranularly, with cracks growth close to low index crystallographic planes (mainly {100}) in some grains and frequently inhibited at grain boundaries. In the late stage, the crack growth mode gradually shifts from crystallographic stage-I to non-crystallographic stage-II. The short cracks grow faster and fluctuate significantly according to local microstructure. Our results show that finer microstructures offer greater crack nucleation and propagation resistance.

Comparative study of the effects of conventional shot peening and ultrasonic shot peening on very high cycle fatigue properties of GH4169 superalloy

This study investigates the effects of two different shot peening techniques, conventional shot peening (CSP) and ultrasonic shot peening (USP), on the very high cycle fatigue (VHCF) properties of the GH4169 superalloy. The study evaluates and compares surface roughness, X-ray diffraction patterns, microhardness, and residual stress in specimens treated with CSP and USP. The VHCF tests reveal that both CSP and USP treatments improve the VHCF properties of the GH4169 superalloy. However, while the USP-treated specimens have deeper work hardening and compress residual stress layers, their VHCF lives are significantly shorter than those of the CSP-treated specimens. Additionally, the study finds that the stress triaxiality, which inhibits dislocations at the crack tip, is higher closer to the center of the specimen. The transmission electron microscopy (TEM) results further confirm that the reduced dislocation density and increased stress concentration in the plastic zone of the high stress triaxiality region promote fatigue short crack propagation and reduce the fatigue properties of the material.

Retarding effect of submicron carbides on short fatigue crack propagation: Mechanistic modeling and Experimental validation

High-strength steels generally contain a large number of spherical carbides with the size ranging from several hundred nanometers to several micrometers, which have an important impact on fatigue performance. Most studies believe that the carbides, similar to inclusions as fatigue crack initiators, should be refined as much as possible to improve fatigue performance. However, when the carbide size reaches submicron scale (several hundred nanometers to one micrometer), the traditional theory regarding the effect of carbides on fatigue performance may not be applicable. In this paper, it is found that smaller carbides are not always better, and an optimal submicron carbide size exists. By means of experiment and simulation, the mechanism as to how submicron carbides affect the fatigue performance of high-strength steels is systematically studied. It is found that a kind of microstructural transition occurs around carbides as a result of local stress concentration. This leads to the formation of Effective Strengthening Layers (ESLs), which force the short fatigue cracks to propagate along the ESL-matrix interface and decelerate crack propagation. The stress concentration required to generate ESL decreases with carbide size. Therefore, the competition between increasing the specific area of ESL and decreasing stress concentration with regard to decreasing carbide size yields an optimum carbide size. Based on this finding, a novel quantitative fatigue performance evaluation model with experimental validation is proposed for high-strength steels, providing a theoretical guidance for the microstructure design of submicron carbides for fatigue performance improvement.

Study on the short fatigue crack initiation and propagation behavior of 42CrMo

A micro-tensile fatigue test of 42CrMo alloy steel was carried out. The result shows that the crack initiation and propagation of the material could be characterized with randomness and localization. There existed two sorts of short fatigue crack propagation behaviors for the tested material, that is, self-propagation and joint propagation with other short cracks. A short fatigue crack growth rate function was built considering the propagation characteristics. Moreover, a simulation model of short fatigue crack evolution was built. Total three samples were tested and simulated. The experimental result was in good consistent with that from simulation, both showing that the crack growth rate accelerated first and then decelerated during propagation.

Effects of microstructure and temperature on short fatigue crack propagation behaviour of powder metallurgy superalloy FGH4098 in vacuum

Short fatigue crack propagation (SFCP) behaviour of a powder metallurgy (PM) superalloy (i.e. FGH4098) for aeroengine turbine disc application was investigated at room temperature and 650 °C in vacuum (10−3 pa) by in-situ fatigue test. The SFCP mechanism was analyzed by the microstructural characterization and assessment of the SFCP rate. The results show that the SFCP resistance of fine grained (FG) FGH4098 is slightly better than that of coarse grained (CG) FGH4098 at room temperature due to the hindering effects of grain boundary, and close to that of CG FGH4098 at 650 °C. The temperature exerts more evident influence on SFCP behaviour of FG FGH4098 due to the grain boundary oxidation compared with that of CG FGH4098. In the scenario which grain boundary oxidation is absent or insignificant, SFCP shows transgranular feature and is closely related to slip bands linked to the slip systems with high Schmid factor and/or geometric compatibility factor. Grain boundary oxidation still occurs in both FG and CG FGH4098 alloy and mainly forms Cr oxides at 650 °C, which results in intergranular or mixed-inter-transgranular SFCP and promotes SFCP to some extent for FG FGH4098 alloy.

In-situ SEM investigation on the fatigue behavior of Ti–6Al–4V ELI fabricated by the powder-blown underwater directed energy deposition technique

Underwater directed energy deposition (UDED) is a promising technology for on-site maintenance and repair of Ti–6Al–4V alloy. The microstructurally short fatigue crack initiation and growth behaviors of Ti–6Al–4V repaired by UDED were investigated through in-situ SEM fatigue testing. The results were compared with those of the Ti–6Al–4V repaired by in-air directed energy deposition (DED) and a substrate. The experimental results show that the microstructure of the Ti–6Al–4V repaired by UDED was dominated by α′ martensite. The α′ martensite in the Ti–6Al–4V repaired by in-air DED decomposed into α+β due to the intrinsic heat treatment. Both the strength and microhardness of the Ti–6Al–4V repaired by UDED were higher than those of the Ti–6Al–4V repaired by in-air DED. Compared with the Ti–6Al–4V repaired by in-air DED, the Ti–6Al–4V repaired by UDED presented a short micro crack initiation time and a poor fatigue crack propagation resistance. The short fatigue crack initiation and propagation behaviors of the as-deposited Ti–6Al–4V were primarily controlled by the structures and morphologies of the microstructures rather than the residual stresses. In addition, the prior-β boundaries in the Ti–6Al–4V repaired by UDED had a resistance effect on the fatigue crack growth rate. However, the resistant effect of prior-β boundaries in the Ti–6Al–4V repaired by in-air DED was not obvious and the local colony α+β played a dominant role in the resistance of short fatigue crack propagation.

Effects of Microstructure and Temperature on Short Fatigue Crack Propagation Behaviour of Powder Metallurgy Superalloy Fgh4098 in Vacuum

Effects of Microstructure and Temperature on Short Fatigue Crack Propagation Behaviour of Powder Metallurgy Superalloy Fgh4098 in Vacuum

The Role of Crystallographic Texture and Basal Plane Slip on Microstructurally Short Fatigue Crack Initiation and Propagation in Forged Billet and Rolled Bar Ti-6Al-4V Alloy

The Role of Crystallographic Texture and Basal Plane Slip on Microstructurally Short Fatigue Crack Initiation and Propagation in Forged Billet and Rolled Bar Ti-6Al-4V Alloy

Crack initiation and propagation from geometric microdefects: Experiment and transition fatigue behavior

AbstractThe present work aims to study short fatigue crack initiation and propagation induced by geometric microdefects. An empirical approach is used to evaluate the microstructural influences on crack growth behavior. Small fatigue crack test on titanium alloy Ti‐6Al‐4V is performed to investigate the crack induced from a geometric microdefect. Analyses on crack initiation and fracture are respectively performed to study the microstructural influences on the stages of initiation and fracture. Subsequently, an empirical multiscale model is employed to predict the short fatigue crack propagation of alloys underlining the transition features. Grain size variation is adopted to reflect the main microstructural contribution to the short crack propagation. An alpha factor is defined to demonstrate the transition interim from microstructurally short crack to physically short crack stages. Furthermore, a relationship between transition factor and grain size is correlated. The results indicate that the value of transition factor tends to decrease with the augment of average grain size. The empirical approach is validated through a variety of fatigue experimental data of alloys including Ti‐6Al‐4V, 2024‐T3, and GH4169.

Cyclic Deformation Induced Residual Stress Evolution and 3D Short Fatigue Crack Growth Investigated by Advanced Synchrotron Tomography Techniques.

Diffraction and phase contrast tomography techniques were successfully applied to an austenitic–ferritic duplex stainless steel representing exemplarily a metallic material containing two phases with different crystal structures. The reconstructed volumes of both phases were discretized by finite elements. A crystal plasticity finite-element analysis was executed in order to simulate the development of the experimentally determined first and second order residual stresses, which built up due to the manufacturing process of the material. Cyclic deformation simulations showed the single-grain-resolved evolution of initial residual stresses in both phases and were found to be in good agreement with the experimental results. Solely in ferritic grains, residual stresses built up due to cyclic deformation, which promoted crack nucleation in this phase. Furthermore, phase contrast tomography was applied in order to analyze the mechanisms of fatigue crack nucleation and short fatigue crack propagation three-dimensionally and nondestructively. The results clearly showed the significance of microstructural barriers for short fatigue crack growth at the surface, as well as into the material. The investigation presented aims for a better understanding of the three-dimensional mechanisms governing short fatigue crack propagation and, in particular, the effect of residual stresses on these mechanisms. The final goal was to generate tailored microstructures for improved fatigue resistance and enhanced fatigue life.

An artificial neural network modeling approach for short and long fatigue crack propagation

Fatigue crack growth-based damage modeling approaches have received great interest due to its critical importance in the industry. However, a substantial deficiency of an explicit fatigue damage model to quantify the accurate fatigue crack growth behavior remains due to the complex fatigue crack growth behavior in different length scales. This complexity arises from the fact that fatigue crack growth (FCG) in different length scales depends on many damage controlling parameters. Machine learning-based fatigue damage modeling approaches have received noticeable attention for fatigue crack growth analysis due to their abilities to account for numerous damage parameters simultaneously. In the presented paper, a radial basis function artificial neural network (RBF-ANN) model has been developed to predict the FCG behavior, including the short and long crack regimes. The presented RBF-ANN model has been trained and verified by experimental data sets of Ti-6Al-4V titanium alloy, 2024-T3 and 7075-T6 aluminium alloys. The predictions showed that the RBF-ANN model has a good interpolation capability to predict the nonlinearity of both short and long crack growth behavior. However, the model shows poor extrapolation capability for accurate short crack growth predictions for cases that there are limited data sets in hand. The model effectiveness greatly depends on sufficient available input data.

Application of fracture mechanics to weld fatigue

The application of fracture mechanics to the determination of the fatigue behavior of weldments is discussed with the focus on classic fatigue, i.e., the overall fatigue life and the fatigue strength in terms of an S-N curve and the endurance limit. The following issues are addressed: specific features of short fatigue crack propagation, an adequate initial crack size, multiple crack propagation and its statistical treatment as well as welding residual stresses. As an example, an approach of the authors is applied to the determination of FAT classes for a butt weld with varying weld toe geometry.