Fatigue Crack Propagation Characteristics Research Articles

The Review of Research on Fatigue Crack Propagation in Metallic Materials

In recent years, with the development of industry and scientific technology, the demand for materials has been increasing. Fatigue crack propagation is an important issue in the field of materials, and developing relevant theories and methods, understanding the mechanisms of fatigue cracks, and making scientific predictions on fatigue crack propagation are hot topics both domestically and internationally. Fracture mechanics, as a discipline studying the strength and propagation of cracks in materials, has developed a relatively complete theoretical system and engineering application methods over more than 100 years of continuous development. This article focuses on the effects of factors such as temperature, stress ratio, and load frequency on the fatigue crack propagation characteristics of metallic materials, and summarizes the common theories and methods of fatigue crack propagation.

Fatigue crack propagation characteristics of aluminum/steel laser-metal inert gas fusion-brazed joints in different micro-zones

Research on fatigue damage of aluminum/steel dissimilar joints is critical to ensure the reliability of multi-material lightweight structures. In this study, the fatigue crack propagation (FCP) characteristics of the interface, weld, and heat-affected zone (HAZ) of aluminum/steel laser-metal inert gas (MIG) fusion-brazed joints were investigated. Additionally, the influencing role of the microstructure of different micro-zones on the FCP behaviors was discussed. Single-edge notched tensile (SENT) specimens of the interface region exhibited a higher crack growth rate and crack threshold than SENT-weld and SENT- HAZ specimens due to the highly hard and brittle nature of the intermetallic compounds (IMC) at the aluminum/steel interface. Moreover, the fatigue fracture surface of SENT-interface specimens showed a brittle fracture pattern without fatigue striation characteristics, which was different from aluminum/steel solid-state and traditional aluminum alloy welded joints. Electron backscatter diffraction (EBSD) analysis results revealed that the synergistic effects of grain size, sub-grain boundaries, and micro-performance all contributed to the crack deflection of SENT-weld at a large angle to the HAZ, thereby resulting in more optimized crack resistance in the weld zone.

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°.

Effect of Heat Treatment on Fatigue Properties of TA15 Titanium Alloy Repaired by Laser Deposition

Laser‐deposition repair technology is employed to repair damaged TA15 titanium alloy forgings. Heat treatment is used to control the microstructure, strength, and plasticity of these repaired forgings. The fatigue properties and crack initiation and propagation characteristics of the repaired parts are studied. Optical microscopy and electron microscopy are used to analyze the microstructure, fatigue fracture, and longitudinal section near initiation. In the results, it is indicated that the tensile and fatigue properties of repaired parts after 900 °C heat treatment are superior to those treated at other temperatures. This is attributed to the strong tissue sensitivity of the repaired specimens’ performance. With increasing heat‐treatment temperature, the secondary α phase in the dimorphic structure tissue thickens. Additionally, the restoration zone develops in layered α phases and becomes truncated, leading to shortening and thickening. The heat‐affected zone undergoes a transformation from a transitional structure to a basket organization. After heat treatment at 1000 °C, the microstructure becomes relatively slender, cracks propagate readily, and the resistance is weakened.

Failure analysis of large and complex engine blades based on experimental research

On a certain day in December 2022, a GE90 engine equipped by an airline experienced a surge during flight. Ground inspection revealed broken blades of the 9-stage high-pressure compressor. To identify the cause of its fracture failure, the macroscopic fracture surface was first analyzed. Obvious fatigue crack propagation characteristics were found at the blade cross-section. Small pits formed by the impact were observed at the crack source location, indicating that the blade was first formed into a circular arc-shaped impact wound after being impacted by an external object. In the subsequent service process, under continuous vibration, fatigue cracks are formed at the impact point as the crack source and gradually propagate, ultimately leading to blade fracture and failure. To further explore the source of foreign objects causing impact damage, the surface scanning method using energy spectrum analysis was used. An enriched region of C element was found near the impact point. However, due to the lack of conditions for the formation of C combustion products at the compressor location, it can be considered that the foreign object may have come from external sources.

A prediction method for fatigue crack growth under high stress levels using accumulative plastic damage properties

The objective of this research is to elucidate the characteristics of low-cycle fatigue crack propagation from the perspective of accumulative plastic damage, proposing a reliable prediction model for assessing the crack growth in EH-36 steel subjected to high stress levels. The plastic damage of the crack-tip is measured by the plastic energy at the critical distance point from the crack-tip. And, a prediction program is developed to calculate the fatigue damage accumulation and fatigue crack growth rates. In order to verify the validity of the prediction method, a sequence of quantitative loading experiments is conducted on specimens consisting of cracked plates and stiffened plates (made of EH-36 steel). The findings show that the prediction model effectively represents the fatigue crack growth behavior in high stress conditions. Increasing the mean stress and stress amplitude is beneficial to increase plastic energy increments, thereby accelerating the progression of fatigue damage towards the critical threshold.

Fatigue crack propagation characteristics of segmented egg-shaped pressure shells

This study focused on the fatigue crack propagation and fatigue life of an egg-shaped pressure shell fabricated by welding and hydroforming a six-segment frustum. First, the segmented egg-shaped pressure shell was designed, and the hydroforming was analysed by the finite element method (FEM). Moreover, the numerical simulation of the egg-shaped shell under uniform external pressure and welding residual stress was carried out, and the stress distribution at the weld was obtained. Second, the equivalent specimens were designed to have structures with maximum stress in the weld. Equivalent specimens with different initial crack shape ratios were tested, and the unified crack propagation rate model of 18Ni (250) was obtained by fitting the data. The fatigue life of the equivalent specimen was calculated by ABAQUS and FRANC3D software and compared with the test results. Finally, the crack propagation rate model was used to predict the fatigue life of the egg-shaped pressure shell. The results of this study provide a reference for prediction of fatigue damage and assessment of the safe operation of egg-shaped pressure shells.

Fatigue crack growth behavior of 42CrMo high-strength steel with tempered sorbite/bainite microstructures: Roles of grain and constituent in microstructures

Microstructural differences through the thickness have always been an attention focus for large-scale steel structures, since they could significantly influence fatigue crack propagation (FCP) characteristics. Here, the effects of microstructure in FCP behaviors of 42CrMo steel were analyzed with emphasis on two different tempered sorbite/bainite (TS/B) microstructures: (i) fine TS/B structures (M42); (ii) coarse TS/B structures (M52). The results show that the FCP rate is relatively slower for M52 than for M42 in the regime through the later stage of near-threshold (I-L) to the early stage of Paris-regime (Ⅱ-E) with low ΔK, but is fairly similar for the two cases in the other ΔK regimes. Under relatively low ΔK, the FCP path of M52 is more tortuous with numerous micro-cracks in B and larger-sized crack deflections at the carbides and boundaries compared to M42, especially in the I-L where fatigue micro-cracks can deflect at the single coarse carbide, thereby resulting in roughness-induced crack closure and crack retardation behavior. Theoretical analysis shows that FCP behaviors are closely related to the cyclic plastic region size (Δrp) at the crack-tip. In the early stage of near-threshold region, Δrp is restricted within a grain-size scale for both steels, thus leading to a similar FCP rate. However, from the I-L to the Ⅱ-E, coarse B/ferrite-grains in M52 are always larger than the Δrp of fatigue crack-tip, and thereby lead to numerous micro-cracks formation and crack deflections at the carbides and B boundaries, inducing crack closure. In the later stage of Paris region, B/ferrite-grains in the two cases are much smaller than Δrp, so that FCP behaviors depend on the microstructure group effects, or overall strength and fracture toughness of materials, which cause the close FCP rate.

Numerical evaluation on fatigue crack growth and life predictions of an FPSO mooring system

This study aims to systematically study the fatigue crack propagation characteristics of the mooring system for a floating production storage and offloading vessel (FPSO) through the fracture mechanics-based approach. The remaining fatigue life of the mooring lines is predicted by a self-integrated program through the simulation of the crack growth behavior. Additionally, parametric studies are performed to investigate the influence of initial crack status (location/size/shape) on mooring crack propagation characteristics. Results illustrate that the remaining fatigue life at the uppermost position of mooring lines at the wave-ward side is similar, which is approximately 178 years longer than that of mooring lines at the wave-back side. Fatigue issues are the severest in the crown region, where the critical fatigue life is about 49% of that in the bend region and 19.9% of that in the straight region. With the increase of the initial crack size and aspect ratio, the remaining fatigue life experiences a remarkable decrease with amplitudes up to 47.8% and 30.1% respectively. The integrated evaluation program is a promising tool for the damage tolerance design of mooring system due to the excellent capability in predicting the remaining fatigue life and charactering the crack growth behavior.

Experimental and modeling study on small fatigue crack initiation and propagation behavior of Inconel 617

Small fatigue crack initiation and propagation behavior of Inconel 617 was studied by in-situ SEM observation, electron back-scattered diffraction (EBSD), digital image correlation (DIC), crystal plasticity finite element method (CPFEM) and extended finite element method (XFEM) in this paper. The experimental results indicated that different surface corrosion methods used to display surface microstructure will affect the small fatigue cracking mode of Inconel 617. The local micro-scale strain field of Inconel 617 near the notch and crack tip was quantitatively measured by a new grain scale DIC method. The CPFEM-XFEM simulation model of small fatigue crack initiation and propagation characteristics of Inconel 617 was established. A new framework for small fatigue crack path and lifetime prediction at the polycrystalline scale was constructed, which includes the influences of slip system and grain boundary on small fatigue crack initiation and growth rate.

Effect of high-low frequency superimposed loading on the fatigue crack propagation of longi-web connection joint

The superposition of low-frequency (LF) wave loads and high-frequency (HF) loads resulting from springing and whipping will cause coupling damage due to the interaction of various frequency load components. In this study, a series of fatigue tests under different high-low frequency superimposed loading (called ‘‘superimposed loading’’ hereafter) conditions were carried out, and the co-simulation of FRANC3D and ABAQUS was utilized to explore the influence of high-low frequency coupling damage on the fatigue life and crack propagation characteristics of the typical welded joint of hull structures. The results indicated that the coupling damage has a significant influence on the decrease of fatigue life, especially on the crack propagation life. In parallel, the intermittent HF loads could also lead to fatigue life reduction and should be paid attention to. Moreover, based on the simulation, a correction coefficient of effective stress intensity factor considering the influence of coupling damage was proposed.

Environmental Damage Behavior of 2A12 Aluminum Alloy under Coupling Action of Marine Atmospheric Environment and Tensile-Compressive Load

The coupling test of marine atmospheric environment and tensile-compressive fatigue load for 2A12 aluminum alloy was carried out in outdoor environment of Hainan station by coupling test equipment of mechanical load. The coupling effect of marine atmospheric environment corrosion and tensile-compressive fatigue load on the environmental damage behavior of 2A12 aluminum alloy was discussed from the aspects of corrosion morphology, tensile properties and fracture morphology. The results show that the corrosion characteristics of 2A12 aluminum alloy in coupling test and static exposure test are consistent. The corrosion depths of the static exposure test for 6 months and the coupling test for 35 days are 71 μm and 121 μm, respectively. The results of tensile properties show that the decrease rate of tensile properties of 2A12 aluminum alloy are accelerated by the coupling effect. And the accelerated ratio of the coupling test was more than 10 times that of the static exposure test. The fracture morphology shows that the coupling fracture crack of 2A12 aluminum alloy starts from the corrosion surface, and the crack source and propagation zone are quasi-cleavage fatigue crack propagation characteristics. The coupling test results are closer to the actual situation of equipment, which has important reference value for the design and material selection of bearing structure.

Determination of fatigue crack propagation thresholds using small-scale specimens

The damage tolerance approach is widely used in the design and estimation of inspection intervals of safety-relevant metallic components subject to fatigue loading. The approach relies on the knowledge of the fatigue crack propagation characteristics, wherein a relevant role is played by the fatigue crack propagation threshold. Nevertheless, the use of material data determined by testing on conventional specimens is not straightforward in the case of thin-walled components such as turbine blades or additively manufactured parts, in which the local variation of material properties in highly stressed regions must be considered. In these cases, the possibility of investigating the fatigue crack propagation properties on a limited portion of material is crucial. For this purpose, a new test procedure has been developed for small-scale specimens which allows the determination of the intrinsic fatigue crack propagation threshold and the near-threshold regime. The validity and limitations of the method are demonstrated on the high strength steel S960QL, along with a comparison with data determined by testing on conventional geometries.

Fatigue Crack Growth Behavior and Fracture Toughness of EH36 TMCP Steel.

The fatigue crack growth behavior and fracture toughness of EH36 thermo-mechanical control process (TMCP) steel were investigated by fatigue crack growth rate testing and fracture toughness testing at room temperature. Scanning electron microscopy was used to observe the fracture characteristics of fatigue crack propagation and fracture toughness. The results indicated that the microstructure of EH36 steel is composed of ferrite and pearlite with a small amount of texture. The Paris formula was obtained based on the experimental data, and the value of fracture toughness for EH36 steel was also calculated using the J-integral method. The observations conducted on fatigue fracture surfaces showed that there were a lot of striations, secondary cracks and tearing ridges in the fatigue crack propagation region. Additionally, there existed many dimples on the fracture surfaces of the fracture toughness specimens, which indicated that the crack was propagated through the mechanism of micro-void growth/coalescence. Based on the micromechanical model, the relationship between the micro-fracture surface morphology and the fracture toughness of EH36 steel was established.

Influence of Chunky Graphite on Fatigue Crack Propagation Characteristics in Heavy Section Spheroidal Graphite Cast Iron

Influence of Chunky Graphite on Fatigue Crack Propagation Characteristics in Heavy Section Spheroidal Graphite Cast Iron

Safety assessment of the small welded pipe system by the test and FEA

Failures of small pipe systems continue to occur frequently at nuclear power plants and aerospace equipment resulting in degraded systems and unscheduled downtime. Currently, there is a lack of detailed technical requirements and evaluation criteria for the small pipe in the existing design codes. In the design code, if referring to the general fatigue evaluation method for metal parts, the results may be too conservative. It has been shown that fatigue tests based on structural parts can significantly extend the component life by recently studies. At the same time, for the small pipe, the influence of the crack propagation stage on the fatigue life (S–N curve) is usually ignored in the existing evaluation process. On the other hand, fatigue-related failures are generally detected as small cracks or leaks before major pressure boundary ruptures occur, so the “Leak-Before-Break” (LBB) can be also applied for the small pipe. This study is undertaken to improve the understanding of the fatigue life of the small pipe (comparing the pipe test results with the data in the code) and the characteristics of fatigue crack propagation. Also, the applicability of the LBB concept is discussed. The results show that the S–N curve obtained by the pipe test is higher than that gained by the standard specimen. The crack propagation life accounts about 3% of the fatigue life in the study case. The critical failure mode is dominated by the plastic collapse; it is therefore shown that the plastic collapse load or critical crack size can be increased by increasing the tensile strength of the materials. A safety factor of 9.25 for the LBB assessment is contained in the case; therefore, it is also useful to warn the final fracture of the pipe by installing a leak detection system.

Determination of fatigue crack propagation thresholds for steel in presence of environmental effects

The experimental determination of the resistance to fatigue crack propagation in steel, particularly in the so-called near-threshold regime, is a crucial issue for safety-relevant components which are designed to operate for a large number of loading cycles before undergoing periodic inspections. This work summarizes the most relevant results of extensive experimental campaigns conducted at BAM and IPM over the last years, which have been devoted to the determination of fatigue crack propagation characteristics on medium and high strength steels. The influence of environment, oxide-induced crack closure and test methodologies in the near-threshold regime are discussed.

Effect of shot peening on fatigue crack propagation of Ti6Al4V

In order to study the influence of shot peening on the fatigue crack propagation characteristics of Ti6Al4V (TC4) titanium alloy, under the condition of cyclic load amplitude 12 kN, stress ratio 0.1 and loading frequency 5 Hz, the uniaxial tensile fatigue tests were carried out on the original and shot peening specimens respectively, and the crack length and small crack propagation rate were calculated under different stress cycles. The results showed that the short crack propagation rate of shot peening TC4 titanium alloy decreased by 34 % ∼ 60 %, but the long crack propagation rate tended to increase. The overall performance was that the occurrence time of visible crack was delayed by 64.3 %, and the fatigue life was increased by 34.2 %, which showed that shot peening could effectively improve the fatigue performance of TC4 titanium alloy. This study can provide a theoretical reference for improving the fatigue performance of aviation materials and has a good engineering application value.

Numerical study on rolling contact fatigue in rail steel under the influence of periodic overload

Fatigue crack is one of the most common defects in railway system. Fatigue crack growth may lead to wheel and rail failure and damage to other components. In this paper, by applying the including service conditions of the railway system, first, a 3D nonlinear stress analysis model has been applied to estimate stress fields of the rail under the influence of periodic overloads. For this purpose, an UIC60 rail with accurate geometry using finite element method (FEM) is studied. Next, the results of this model are used for fatigue crack growth and also to estimate the fatigue life in rails using boundary element method (BEM). In this regard, first, the types of cracks and the corresponding factors which cause them were analyzed. In addition to performing stress analysis for different positions of the wheel, different values of the stress intensity factors (SIFs) were also obtained. Using these stress intensity factors, two characteristics of fatigue crack propagation namely, crack growth and direction of its development, were obtained. In this study, Paris Equation is used as a criterion of fatigue crack growth rate and the maximum tangential stress was considered as the crack growth direction.The numerical analysis results showed that the effect of periodic overloads on the fatigue life in rails was significant and could not be ignored.

Multiscale Analysis of Concrete Damage and Crack Propagation Under High Cycle Loading

Concrete is a typical multiphase composite material in which the initiation and propagation of cracks under fatigue load are mainly determined by its mesoscopic structure. In this paper, a concrete multiphase mesoscopic model which considers thickness of interfacial transition zone (ITZ) varying with aggregate’s size is established by the integrated scripting method. The model comprehensively contains the stochastic characteristics of concrete mesoscopic structure. According to the fatigue crack propagation characteristics in different stages, a multiscale method is proposed by establishing the interactive mesoscopic and macroscopic models to targetedly analyze the whole process of fatigue crack initiation, propagation and failure of concrete specimen. Based on the technique of cycle block, concurrent simulation of concrete damage and crack propagation under high-cycle fatigue load is realized. The analysis results show that fatigue cracks in concrete mainly born near the ITZ and gradually enter into the cement mortar matrix to formulate the cracks with finite size. These cracks influence each other until the dominant crack appears and insatiably propagates to result in the failure of specimen.