Fatigue Crack Tip Research Articles

The Corrosion Fatigue Behavior and Mechanism of AerMet 100 Steel in 3.5% NaCl at Room Temperature.

AerMet 100 steel is a new type of double-hardened high-strength steel, which is often used as landing gear material in amphibious aircraft. In the present paper, the corrosion fatigue behavior and mechanism of AerMet 100 high-strength steel in a 3.5% NaCl solution was studied by stress-controlled fatigue tests and a series of subsequent characterizations of the fracture surface, microstructure, and cracks. The results indicated that the fatigue life of AerMet 100 high-strength steel decreased with a decrease in the stress level in a 3.5% NaCl solution, satisfying the relationship lgN = -2.69 × 10-3 σ + 6.49. The corrosion fatigue crack usually initiated from the corrosion pit and propagated across the martensitic flat noodles. Meanwhile, the corrosion fatigue crack tip was filled with Cr2O3, Fe2O3, and amorphous material; it propagated in the transgranular mode by a slip dissolution mechanism. This study provides some engineering significant for the fatigue performance of AerMet 100 steel in a 3.5% NaCl solution.

Effect of Fatigue and Precorrosion Fatigue on Fatigue Crack Propagation and Fracture Failure Behavior of 5B70 Aluminum Alloy

Aluminum alloys are ideal lightweight structural materials for spacecraft, but they are susceptible to local corrosion during service. Herein, a systematic investigation is carried out to determine the fatigue behavior of 5B70 aluminum alloy under salt spray precorrosion. The mechanism of salt spray corrosion, fatigue life, crack propagation, and failure fracture mechanism of 5B70 aluminum alloy are studied. Under the salt spray corrosion, 5B70 aluminum alloy mainly undergoes pitting corrosion via a galvanic corrosion mechanism. After precorrosion, the formation of macroscopic fatigue cracks is accelerated, which significantly reduces its fatigue life. Fracture occurs due to single crack initiation and propagation under fatigue conditions. The fatigue crack tip undergoes a mixture of intergranular and transgranular modes, but mainly transgranular mode. Multiple strain localization phenomena occur during precorrosion fatigue, and corrosion pits on the surface provide a tortuous crack propagation path; the fatigue crack tip is mainly transgranular mode.

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.

Deformation behavior at fatigue crack tip in nickel-based alloy

The microstructure and deformation behavior around the fatigue crack tip can significantly affect the early stage of the fatigue crack propagation, but currently, the evolution and deformation mode of the microstructure in the fatigue crack tip at the nanoscale are still unclear. Therefore, it is important to study the microstructure and deformation behavior at the fatigue crack tip in nanoscale to further reveal the fatigue failure mode of metal materials. In this study, the microstructure evolution and deformation mode of the fatigue crack tip of nickel-based alloy are studied by HRTEM technique. The results show that a large number of dislocation, stacking fault and micro/nano-twin are formed around the fatigue crack tip. The formation of these microstructures can promote the release of strain in the matrix during cyclic deformation. The main deformation mode of the fatigue crack tip includes slip, twinning, and dislocation shearing micro/nano-twin.

Fatigue crack extension mechanism and mode-type analyses of martensitic steels for proposing fatigue strength evaluation method: Example of 18% Ni BCC martensitic steel

The fatigue properties of martensitic steels tend to differ significantly from those of low- and medium-strength steels. Even if the hardness is the same, the fatigue strength varies substantially depending on the microstructure. Thus, for using martensitic steel rationally and developing materials with high fatigue strength, it is desirable to establish a fatigue strength evaluation method that can consider the microstructural effects on fatigue properties. For this purpose, 18% Ni bcc martensitic steel was used as a model metal to conduct rotating-bending fatigue tests in this study. The fatigue crack behavior on the smooth specimen surface was examined, and the fracture surface morphology on both sides of the broken specimen and the microplastic strain distribution near the fatigue crack tip were analyzed. The results clarified the mechanism underlying fatigue crack extension (FCE). The analytical results revealed three types of FCE mechanisms dependent on the stress amplitude, stress state (i.e., plane stress or plane strain), and martensitic microstructure. Considering these mechanisms, a unique stress–life curve was predicted, and a particular fatigue crack shape and fractography were confirmed. The material indices were speculated to be the representative properties of martensitic steels.

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.

Mechanistic model for hydrogen accelerated fatigue crack growth in a low carbon steel

Fracture by hydrogen accelerated fatigue crack growth is a severe type of environmental failure. Although fatigue crack growth has been the subject of intense investigation over several decades, a complete mechanistic and predictive model is still lacking. Such a crack growth model is even more rare in the case of hydrogen in view of the lack also of constitutive models for material deformation under cycling loading that account for the hydrogen effect. In this study, we present a model for fatigue crack propagation induced by alternating crack tip plastic blunting and re-sharpening, which in the presence of hydrogen can be accelerated by hydrogen enhanced dislocation motion and generation. The Chaboche constitutive model, which is a nonlinear kinematic hardening model capable of capturing many features of material behavior under cyclic loading, is used for the calculation of the stress and strain fields at the propagating crack tip. The Chaboche model is calibrated using a sequence of experimental data from uniaxial strain-controlled cyclic loading tests and uniaxial stress-controlled ratcheting tests with a low carbon steel, JIS SM490YB, in the absence and presence of hydrogen. The numerical simulation results indicate that the proposed crack propagation model can predict Paris law behavior and can successfully demonstrate acceleration of fatigue crack growth in the presence of hydrogen. Significantly, the profiles of the steady-state opening stress and strain ahead of the fatigue crack tip in a compact tension (C(T)) specimen were found to have sections over which they vary as ln(1/r) with distance r from the crack tip, consistent with the crack-tip strain field for a non-stationary crack.

Fatigue Crack Growth Rates and Crack Tip Opening Loads in CT Specimens Made of SDSS and Manufactured Using WAAM.

Additive manufacturing offers greater flexibility in the design and fabrication of structural components with complex shapes. However, the use of additively manufactured parts for load-bearing structural applications, specifically involving cyclic loading, requires a thorough investigation of material fatigue properties. These properties can be affected by many factors, including residual stresses and crack tip shielding mechanisms, which can be very different from those of conventionally manufactured materials. This research focuses on super duplex stainless steels (SDSSs) fabricated with wire arc additive manufacturing (WAAM) and investigates their fatigue crack growth rates and the net effect of crack tip shielding mechanisms. Using the compliance-based method, we measured crack tip opening loads in compact tension (CT) specimens with cracks propagating longitudinally and transversely to the WAAM deposition direction. It was found that fatigue crack growth rates were very similar in both directions when correlated by the effective stress intensity factor range. However, the differences in crack tip opening loads explain a quite significant influence of the deposition direction on the fatigue life.

An innovative approach to planar mixed-mode fatigue crack growth study

Determining fatigue crack growth parameters under mixed-mode loading is crucial for damage tolerance design, particularly in assessing a component's residual life with defects to prevent disasters. This study introduces a novel methodology for determining fatigue crack growth rate. A digital microscope is utilized to track the fatigue crack tip with respect to local coordinate systems established on the specimen surface. A grid is drawn and subdivided into units with proposed nomenclature. Compact Tension and Shear (CTS) specimens, extracted from AA-7085 plates, undergo fatigue loading in a servo-hydraulic universal testing machine. A fixture proposed by Richard et al. induces planar mixed-mode loading cases in the specimen. During experiments, the digital microscope tracks the crack tip using grid unit corners as local origins. The local coordinates of the crack tip are transformed into the crack tip coordinates with respect to the pre-crack tip. Crack evolution data are used to determine the fatigue crack growth rate by analyzing tangents of a three-parameter fitted curve. The equivalent SIF ranges are computed at different crack lengths in an XFEM script written in Ansys. The modified Paris parameters are found and experimentally observed crack deflection angles are compared with the Maximum Tangential stress, Strain energy density, and Richard’s criterion.

The role of hydrogen in promoting differences in fatigue crack growth rates observed in Type 304/304L stainless steel at elevated temperature

Hydrogen has been postulated to affect fatigue crack growth rates (FCGRs) in Type 304/304L stainless steel during exposure to high purity water (HPW) environments at elevated temperature. To assess this mechanism, FCGR testing was performed at elevated temperature (260°C) in air and pressurized hydrogen gas. Initial results indicate the measured FCGRs in hydrogen are up to 8x lower than those measured in air under comparable fatigue loading conditions. Post-test evaluation of compact tension specimens indicates crack mouth opening displacements and fracture surface morphologies generated in hydrogen that are consistent with those generated in HPW environments. Observations of crack tip blunting suggest the reduced FCGRs in elevated temperature hydrogen occur due to a decrease in the effective ΔK. Finite element plasticity modeling, using a hydrogen-dependent nonlinear kinematic hardening approach, supports the hypothesis that crack tip blunting and retarded FCGRs are due to a hydrogen-based change in cyclic behavior (e.g. cyclic creep) occurring ahead of the fatigue crack tip.

Multi-scale characterization of fatigue crack tip deformation and propagation in QP steel

A novel measurement method for multi-scale characterization of fatigue crack tip deformation and propagation behaviors in high strength and plasticity, multi-phase, fine-grain Quenching and Partitioning (QP) steel was proposed. The fatigue crack propagation (FCP) test system with the macro/micro images acquisition devices was built to measure the FCP rate and crack tip deformation simultaneously. First, at the mesoscale, the evolution characteristics of the crack tip deformation fields, plasticity induced crack closure (PICC) behaviors and plastic zone during the FCP process were analyzed using the sub-image stitching and matching digital image correlation (DIC) method. Then, the FCP models were established with special emphasis on the model considering the PICC effect based on the obtained crack opening load at the mesoscale. Moreover, the corresponding microscale surface crack and fracture morphologies, microstructure phase transformation and deformations were observed and analyzed by electron backscatter diffraction (EBSD) and scanning electron microscope (SEM) techniques.

Effects of pressurized water reactor environment and cyclic loading parameters on the low cycle fatigue behavior of 304L stainless steel

Austenitic stainless steels used in light water reactor coolant environments are susceptible to environmentally assisted fatigue due to non-monotonic loading conditions, primarily associated with load-follow, thermal transients, or intermittent plant shutdowns and start-ups. This study investigates the effects of a high-temperature pressurized water reactor (PWR) water environment and cyclic loading parameters on the low cycle fatigue behavior of austenitic 304L stainless steel. Prolonged exposure to a PWR environment and cyclic loading conditions such as a lower strain rate or a higher fraction of slow strain rate enhances the initiation and accelerates the crack growth rate of fatigue cracks, resulting in decreased fatigue life. The deformation-induced α'-martensite is observed in proximity to fatigue crack tips primarily in specimens tested in simulated PWR primary water, while cellular dislocation structures are more frequently observed near crack tips in specimens tested in high-temperature air. The deformation-induced martensitic transformation from γ-austenite to α'-martensite, occurring via the precursor ε-martensite phase, contributes to the accelerated fatigue crack growth rate in a PWR environment with hydrogen.

Study of cyclic plastic zone ahead of the fatigue crack tip using Digital Image Correlation system

Structural components are susceptible to cyclic loadings during their operation. They are more prone to failure due to localized plastic deformation. The plastically deformed region ahead of the crack tip due to fatigue loading is the important factor responsible for fatigue crack propagation. The plastically deformed region is separated into monotonic and cyclic plastic zones. The cyclic plastic zone (CPZ) is approximately 1/3rd to 1/4th of the monotonic plastic zone (MPZ). The existing analytical expressions to calculate the size of the CPZ are based on elastic perfectly plastic analysis. In this paper, an experimental technique, namely 3D-Digital image correlation (DIC), is used to evaluate the size of the CPZ during the fatigue crack growth rate (FCGR) tests on CT specimens. The DIC system having two cameras is used to capture high-resolution images of crack region of CT specimen surface during the FCGR tests. The FCGR tests are carried out on nuclear piping material steel SA333Gr6 under constant load range and constant load ratio. The post-processing of DIC images revealed an asymmetric butterfly-type shape of the cyclic plastic zone. The DIC evaluated CPZ was compared with the CPZ obtained by a calibrated cyclic plasticity material model-based finite element (FE) analysis. It was observed that the average difference between DIC evaluated experimental CPZ and CPZ calculated by plane stress-based FE analysis is about 5%.

Wavefield imaging of nonlinear ultrasonic Lamb waves for visualizing fatigue micro-cracks

The traditional nonlinear ultrasonic technique, as typified by the second-harmonic generation and the frequency mixing response, can be employed to identify and characterize the micro-damage. However, the research on micro-damage characterization using nonlinear Lamb wave imaging technique remains an ongoing challenge and is rarely reported. A method called standardized amplitude difference is proposed for nonlinear feature enhancement, and further for fatigue crack imaging based on the wavefield data. Wavefield data contain abundant information on the spatial and temporal variation of propagating waves in the damaged structure. The nonlinearity index β′ of the signal difference under the high and low incident wave amplitudes is calculated for fatigue crack imaging. Two scanning methods, including local scanning and global scanning, are introduced to image the fatigue crack tip and visualize the wave field of the harmonics respectively. The experimental validation, based on the imaging results of an aluminum alloy plate specimen with a barely visible fatigue crack and a steel plate with a blind hole, manifests that the proposed method can be used to enhance and extract the nonlinear features and suppress the fundamental frequency, so as to improve the signal-to-noise ratio (SNR) of the micro-damage imaging results.

Modelling of hydrogen distribution at the fatigue crack tip of austenitic stainless steels in internal and external hydrogen tests

A hydrogen diffusion model was developed, which accounts realistically for the kinetics of hydrogen adsorption, absorption and their reverse processes at the gas-solid interface. Using the model, the hydrogen distribution at the fatigue crack tip of austenitic stainless steels (304, 316 L) in internal and external hydrogen tests were analyzed. Results indicate that for stable austenitic stainless steel, the hydrogen concentration at the fatigue crack tip is much higher in internal hydrogen test than that in external hydrogen test, which is attributed to the extremely low adsorption, desorption and diffusion rates of γ phase. For metastable austenitic stainless steel, hydrogen rapidly enters into strain-induced α′ martensite around the fatigue crack tip, forming an enrichment peak at the α′/γ interface in external hydrogen test. While significant hydrogen outgassing occurs in α′ phase in internal hydrogen test, leading to a much lower crack tip concentration compared with external hydrogen test.

Residual fracture toughness and fracture energy of concrete with different strengths after fatigue loading

To evaluate the residual fracture toughness and fracture energy of concrete with different strengths after fatigue loading, the static, fatigue, and fatigue-static tests were conducted on 264 three-point bending (TPB) beams, respectively. The test results showed that the residual fracture toughness and fracture energy decreased rapidly at first and then steadily with the increase of the fatigue crack tip opening displacement and the number of fatigue cycles. For a given fatigue load level, the decrease rate of the residual fracture toughness increased with an increase in the concrete strength, but the decrease rate of the residual fracture energy decreased with increasing concrete strength. Finally, the degradation models for the residual fracture toughness and fracture energy of concrete were presented. When the tensile strength of concrete, the number of fatigue cycles, and the fatigue crack tip opening displacement are known, the residual fracture toughness and fracture energy of concrete after fatigue loading are determined. The research results are helpful to assess the safety of concrete structures in-service after fatigue loading.

Experimental study on fatigue damage of continuous steel–concrete composite beam by acoustic emission

To investigate the developing process and mechanism of internal fatigue damage of continuous steel–concrete composite (CSCC) beams, the fatigue test of two-span CSCC beams under cyclic loading was conducted, and the acoustic emission (AE) testing method was used to monitor the fatigue damage accumulation during loading. Internal AE propagation characteristics and residual deflection were compared to analyze the overall damage process as well as the boundary points of each fatigue stage. The fatigue failure mechanism of CSCC beams was deduced from an internal damage point of view. The fuzzy C-means algorithm was used to cluster the AE signals and extract the signals of the concrete structure. According to the energy accumulation-release signal of the fatigue crack tip, a novel method for fatigue damage location identification of the concrete structure was proposed, and the propagation characteristics of the fatigue crack along with the variation law of the AE wave velocity within the concrete slab were further investigated. Control parameters for fatigue damage, similar to the relationship between magnitude and frequency of earthquakes, were calculated, and the warning signs of fatigue failure of CSCC beams were inferred. The AE cumulative energy-based fatigue damage model was proposed to predict the damage status and fatigue life of the composite beam. Results have shown that the AE detection method is capable of continuously and accurately monitoring the fatigue damage within the composite beams in real-time as opposed to inspections that are solely based on appearance. The fatigue damage energy model also confirmed the fact that the fatigue damage growth follows an exponential function. The fatigue damage energy model and the AE method proposed based on AE attributes provided a more feasible and stringent method for fatigue damage prediction and analysis for CSCC beams.

X-ray tomography of failure behaviors of arc welded AA2219 joints under tensile and cyclic loading

As one of the main structural materials widely used for the new generation of launch vehicles, the tensile properties and microstructural characterization of tungsten inert gas (TIG) welded 2219 aluminium alloy has been studied extensively. However, there are significant differences between fatigue fracture and tensile fracture modes. In this paper, the tensile and fatigue damage behaviors of arc-welded AA2219 joints were investigated in depth. Combined with the microstructure observation and fracture analysis done by using X-ray computed tomography, the main factors that affect the fatigue damage behavior are discussed. The results show that there are a large number of porosity defects in the weld zone, showing an aggregation distribution at the weld toe and weld pool line. The fine-grain zone around the fusion line is the softening zone of the welded joint. For tensile conditions, the cracks were initiated at the softening zone and extended along the fusion line to the final fracture. Under fatigue loading conditions, the weld reinforcement leads to the stress concentration at the weld toe, and at the same time, a lot of pore defects at the weld toe, which causes crack initiation at the pores in the weld toe. Due to the small plastic zone at the fatigue crack tip, the crack propagates almost perpendicular to the loading direction in the heat-affected zone, driven by a mode I crack. A large number of gas pores in the weld zone have no substantial effect on fatigue crack propagation.

Exploring the effect of complex hierarchic microstructure of quenched and partitioned martensitic stainless steels on their high cycle fatigue behaviour

Recent studies have demonstrated the viability of quenching and partitioning (Q&P) treatment for processing martensitic stainless steels showing an improved balance of high strength and sufficient ductility. However, to date, the fatigue behaviour of these materials has not been explored. This study examines the effect of their complex hierarchic microstructure on high cycle fatigue performance. Three steels with different alloying element contents underwent Q&P processing, resulting in multiphase microstructures rich in retained austenite. High cycle fatigue tests and analysis of fatigue fracture surfaces were performed using SEM and EBSD techniques. The results indicate satisfactory high cycle fatigue performance in Q&P treated martensitic stainless steels, surpassing traditional counterparts. Fatigue cracks predominantly form and propagate along martensite packet and block boundaries, while prior austenite grain boundaries and MnS inclusions have minimal influence on fatigue crack formation and growth. Microplastic deformation at the fatigue crack tip enhances local KAM values and triggers localized transformation of retained austenite grains. It is hypothesized that the developed Q&P treated martensitic stainless steels exhibit improved resistance to low cycle fatigue.

Grain-level residual stress distribution at dwell fatigue crack tips in a nickel-based superalloy

Dwell fatigue crack growth in nickel-based superalloys is influenced by a complex interplay between mechanical and chemical phenomena. The stress distribution at the crack tip is closely related to these phenomena and to the local microstructure. In this study we evaluate experimentally the microscale residual stress distribution ahead of crack tips in RR1000 alloy samples after interrupted high temperature dwell fatigue testing. In the crack opening direction the results show a compressive residual stress zone, the extent of which is linked to the plastic zone size and crack growth retardation behaviour. In the direction of crack propagation the residual stress is also influenced by crystallographic grain orientation. The results demonstrate a novel approach of experimentally evaluating crack tip stress distribution via focused ion beam – digital image correlation (FIB-DIC) ring-core measurements at the grain-level. This provides new insights into fatigue crack growth in microstructurally heterogeneous materials.