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In a wind energy system, the safety and stability of substructures plays an important role during the in service of offshore structure. Offshore structures are continuously subjected to high cyclic fatigue loads and may experience fatigue cracks due to the continuous accumulation of plastic strain and stress concentrations at welded joints. The fatigue life of welded tubular joints is one of the most important factors determining the life of offshore structures. In this study, the fatigue analysis of tubular joints of the tripod and jacket support structure was performed using 3D fatigue FEM to estimate the fatigue life and predict the positions of crack initiation. The 3D fatigue FE is based on constitutive equations and continuum damage mechanics. The welding state of tubular joints were reproduced to calculate the welding residual stresses and welding deformation. The residual stresses and weld deformation were used as input together with cyclic loading in the 3D fatigue FE to calculate the fatigue life and predict the crack initiation positions. The S-N curve calculated by the 3D fatigue FE analysis were compared with the SN curves of Eurocode 3. The results show that 3D fatigue FE analysis is an effective tool to analyze large and complex structures before installation to ensure the safety and stability of the structure. [ABSTRACT FROM AUTHOR]

Offshore wind power became an important source of renewable energy that is rapidly growing over the past several years. Large offshore wind turbines require strong and large support structures. The support substructures are exposed to diverse dynamic and static loads such as waves, snow, wind ice, earthquakes, and tides. These unstable, destructive repeated loads lead to material degradation or damage accumulation which in turn leads to the failure of offshore structures. Fatigue is considered one of the significant modes of failure in offshore structures. The constant exposure of welded multiplanar tubular joints of tripod structure under cyclic loading or the presence of flaws due to fabrication and construction induces high-stress concentration at these joints. The material degradation or damage accumulation at welded joints gives rise to the formation or propagation of cracks, and ultimately leads to a collapse of giant steel structures. This research aimed at investigating the origin of crack positions and fatigue life in tripod offshore structures. The 3-D fatigue FE analysis was carried out in two steps, Firstly, the welding deformation and welding residual stress were calculated in thermal and mechanical analysis. Secondly, 3D fatigue crack analysis based on cyclic hysteresis constitutive equation and fatigue damage theory was used to calculate the fatigue life and crack initiation in tripod offshores structure. Results obtained from the analysis were compared with the results from hot-spot stress method and Eurocode 3. From the results, it was considered that 3D-fatigue FEM is a useful method to estimate fatigue life and crack initiation. [ABSTRACT FROM AUTHOR]

The mechanism of oxide inclusions in fatigue crack initiation in the very-high cycle fatigue (VHCF) regime was clarified by subjecting bearing steels deoxidized by Al (Al-deoxidized steel) and Si (Si-deoxidized steel) to ultrasonic tension-compression fatigue tests (stress ratio, R = −1) and analyzing the characteristics of the detected inclusions. Results show that the main types of inclusions in Si- and Al-deoxidized steels are silicate and calcium aluminate, respectively. The content of calcium aluminate inclusions larger than 15 µm in Si-deoxidized steel is lower than that in Al-deoxidized steel, and the difference observed may be attributed to different inclusion generation processes during melting. Despite differences in their cleanliness and total oxygen contents, the Si- and Al-deoxidized steels show similar VHCF lives. The factors causing fatigue failure in these steels reveal distinct differences. Calcium aluminate inclusions are responsible for the cracks in Al-deoxidized steel. By comparison, most fatigue cracks in Si-deoxidized steel are triggered by the inhomogeneity of a steel matrix, which indicates that the damage mechanisms of the steel matrix can be a critical issue for this type of steel. A minor portion of the cracks in Si-deoxidized steel could be attributed to different types of inclusions. The mechanisms of fatigue fracture caused by calcium aluminate and silicate inclusions were further analyzed. Calcium aluminate inclusions first separate from the steel matrix and then trigger crack generation. Silicate inclusions and the steel matrix are closely combined in a fatigue process; thus, these inclusions have mild effects on the fatigue life of bearing steels. Si/Mn deoxidation is an effective method to produce high-quality bearing steel with a long fatigue life and good liquid steel fluidity. [ABSTRACT FROM AUTHOR]

Fretting damage analysis is carried out for several 3D contact geometries (cylinder-on-plane and punch-on-plane) under different slip regimes. A robust numerical environment is implemented to simulate the consequences of fatigue-fretting, in terms of wear or fatigue crack initiation. The numerical model allows to characterize the abrasive wear phenomena observed experimentally, and introduces specific damage post-processing to predict crack initiation. The numerical process includes an automatic mesh generation for updating the geometry due to wear effect. The difference between 2D and 3D cases is highlighted. Cases where a 3D numerical approach appears to be mandatory to obtain a quantitative agreement between simulation and experiment are mentioned. [ABSTRACT FROM AUTHOR]

The interacting effects of internal defects and corrosion pits on the stress concentration factors in hourglass-shaped specimens were investigated through a finite element method investigation. Very high cycle fatigue tests were performed to confirm the shape of the internal defects and the depth of the corrosion pit. According to the experimental observations, a series of three-dimensional models containing internal voids and corrosion pits were established. Stress distribution around the internal void and the corrosion pit under uniaxial tensile loading were obtained. Based on the finite element analysis, the stress concentration factors around the defects were found fully dependent on the structure parameters of the internal void and the corrosion pit. Stress concentration factors increased with the depth-to-length ratios Rd for models with a single corrosion pit. For models with a single internal void, the stress concentration factors increased dramatically when the internal void is present near the surface. Parameters of distance-to-radius ratio (D2 − h)/a and depth-to-length ratio Rd play important roles in the stress distribution for a model with both an internal void and a corrosion pit. Severe stress concentration would be introduced when the parameter (D2 − h)/a < 2. For a deep corrosion pit (i.e., Rd ≥ 1), the stress concentration factors were determined by two parts which were dominated by parameters of (D2 − h)/a and Rd, separately. For a shallow corrosion pit (i.e., Rd < 1), the stress concentration factors were dominated by the interaction of (D2 − h)/a and Rd. Two very close corrosion pit and void were most likely to be an initiation position and gave rise to devastating effects on the very high cycle fatigue performance of the specimens. Empirical equations were proposed to evaluate the relationship between the stress concentration factors and the structural parameters of defects. The obtained quantitative information would be useful for understanding of crack initiation and fatigue life prediction of engineering components. [ABSTRACT FROM AUTHOR]

To understand the influence of recrystallization on the low-cycle fatigue behavior of directionally solidified nickel-based DZ4 superalloy, crystal plasticity-based finite element (CPFE) analysis was carried out to study the cyclic plastic behaviors. Using the design of experiments (DOE) technique, a series of CPFE simulations were designed and performed for DZ4 superalloy with recrystallized grains to examine the cyclic plastic shear deformation at the carbide/matrix interface, at the recrystallized grain boundary/matrix, and in the matrix. Seven influence factors, i.e., applied strain, strain ratio, carbide modulus, dwell time, misorientation angle, carbide aspect ratio, and carbide size, were considered, and the effects of which were quantitatively evaluated by the maximum accumulated plastic shear strains via CPFE-based DOE analysis. The results showed that carbide and recrystallized grain boundary had competitively significant effects on fatigue crack initiation. Applied strain was the most influential parameter among all the studied parameters. [ABSTRACT FROM AUTHOR]

In order to improve reliability of an automotive engine, a fatigue design method was studied based on the fatigue mechanisms in flake graphite cast iron. In the present study, fatigue test specimens were cut from an actual engine component. The material used showed nonlinear behavior in stress-strain relationship from lower stress level during tensile testing. Rotating bending fatigue testing and axial load fatigue testing were performed. Fatigue crack initiation and propagation behavior were observed during the fatigue tests by a replication technique. Fatigue cracking initiated from the tip of flake graphite in early stage of the total fatigue life when the applied stress was higher than the fatigue limit. On the other hand, fatigue cracking was not observed in the specimen tested with applied stress below the fatigue limit. The threshold stress intensity factor obtained with a small surface crack was lower than that obtained with a through-thickness crack. Fatigue limits under various loading conditions such as bending loading and axial loading were predicted well based on fracture mechanics using the area parameter. Threshold stress intensity factor for a small surface crack and the maximum graphite size assumed as an initial crack were used for the prediction. Axial load fatigue tests with different stress ratios were performed. The fatigue limit diagram obtained by the fatigue tests did not follow the modified Goodman relation. On the other hand, fatigue limits prediction based on fracture mechanics approach showed good agreement with fatigue limit obtained with fatigue tests in a wider range of mean stresses σmean varying from − 150 to 50 MPa, in negative stress ratio. In case of σmean higher than 50 MPa, in a positive stress ratio, a smaller effect of stress ratio in the fatigue limit diagram was observed. It was considered that this phenomenon could be explained by the lower effect on the crack closure in a small surface crack initiated from the tip of graphite flake and also influence of local plastic behavior of the matrix around the tip of graphite flake. It can be summarized that the fracture mechanics approach is an effective way to predict fatigue limits of an engine component produced by flake cast iron in different loading conditions and in wider range of mean stress conditions. [ABSTRACT FROM AUTHOR]

Fatigue properties of an Al-Si-Cu cast alloy were investigated with life up to very high cycle fatigue regime. Particular attention was given to specimen size effect and fatigue crack initiation mechanism. Uniaxial fatigue tests with constant amplitudes were carried out by using ultrasonic fatigue testing machine operating at 20 kHz, compared to the results obtained by using conventional hydraulic fatigue machine at 35 Hz. In order to evaluate the size effect, two sets of specimens with different dimensions were used. The results show that the fatigue strength of the studied alloy decreases with the increase in specimen size. Scanning electron microscopy observation of fracture surfaces revealed that most fatigue cracks initiated from microstructural defects such as porosity located on specimen surface or in subsurface. Weibull statistical analysis of fatigue data accounting for size effect was performed to predict the fatigue life of the Al-Si-Cu alloy in the very high cycle regime. Good agreement was found between the fatigue life prediction and the experimental data. [ABSTRACT FROM AUTHOR]

General features of the damage evolution in cyclic loading of structural materials are summarized. The attention is paid to the comparison of the damage mechanisms in materials that are used for service both at room and at elevated temperatures, namely austenitic stainless steel Sanicro 25. Principal mechanisms leading to fatigue fracture at room and at elevated temperature are documented. While cyclic slip localization is a decisive process in the initiation of fatigue cracks at room temperature, the localized oxidation plays an important role in isothermal high temperature cyclic loading. Specific mechanisms of the early fatigue damage in thermomechanical cyclic loading are studied. The in-phase thermomechanical loading leads to the intergranular crack initiation due to preferred grain boundary oxidation and intergranular crack growth. The out-of-phase thermomechanical loading results in oxide cracking and localized oxidation of the metal in the area of cracked oxides and to transgranular crack growth. The damage mechanisms can explain differences in fatigue life under various loading and temperature conditions. [ABSTRACT FROM AUTHOR]

The purpose of the study is to experimentally investigate the fatigue crack initiation and propagation characteristics of nuclear power plant steam generator tube materials, Inconel 600 alloy, under fretting fatigue conditions. In addition, this study aims to enhance the overall reliability of nuclear power plant equipments by developing fretting fatigue life and integrity evaluation methods. In order to achieve these purposes, a fretting fatigue experiment apparatus is developed and fatigue crack growth experiments are conducted to evaluate the characteristics of Inconel 600 alloy under various stress conditions. Finally, microstructures are observed in accordance with fretting fatigue and contact pressure conditions and fretting fatigue crack initiation mechanism is examined. [ABSTRACT FROM AUTHOR]

AISI type M3 class 2 tool steel (or in German designation DIN: HS6-5-3 tool steel) is most commonly used in tooling industry, and also in some engine parts. Those components are usually subjected to cyclic stresses and mostly fail by fatigue. Fatigue crack initiation in this material occupies large fraction of total lifetime and strongly depends on microstructural features of primary and eutectic carbides, such as shape, shape ratio, volume fraction, the distribution of carbides as well as load ratio. To model fatigue initiation mechanisms of forged M3:2 tool steel, McDowell's model was modified and developed for different length-scales. For fatigue crack formation and short crack growth, a hierarchical approach was used and the life time of these stages were estimated based on the local cyclic plasticity. Through this relation the effect of microstructural features on both fatigue crack formation and short crack growth in the material were identified. The results of the proposed model have explicitly reflected the influence of microstructural features on both fatigue crack formation and propagation in forged M3:2 tool steel. Moreover, the model can be used for improving the fatigue resistance of a tool steel component. [ABSTRACT FROM AUTHOR]

In this study, in order to define a fatigue crack initiation unit, the relationship between the fatigue crack initiation process and the crystal structure in the pearlitic steel used for railroad rails was examined and fatigue tests, focusing on crack initiation, were performed. The fracture surfaces were analyzed using a scanning electron microscope (SEM) and electron backscatter diffraction (EBSD). The crystal structure of the pearlitic steel is composed of “pearlitic colonies” that have the same lamellar structure direction and “pearlitic blocks” that have the same ferrite crystal direction. The SEM and EBSD results revealed that the crack initiation depends on the pearlitic colonies. Therefore, we defined the characteristic dimension for fatigue crack initiation as the pearlitic colony size. However, for safety purposes, the pearlitic block size should be considered the engineering definition of the fatigue crack initiation unit, since decreasing the pearlitic block size should cause an improvement in the fatigue limit of pearlitic steel. [ABSTRACT FROM AUTHOR]

Two methods currently available for evaluating the probability of Multiple Site Damage (MSD) occurrence were studied in this paper. One of the methods is a probabilistic analysis approach based on the statistical theory and fatigue characteristics of each structural detail, and the other is an approach which defines the initial damage scenario by means of Monte-Carlo simulation, and multiple initial crack scenarios are randomly generated. A modified method based on the Monte-Carlo simulation was proposed in this paper, in which the random fluctuation of the stress was considered to give more accurate evaluation results. In the presented method, the probability of MSD occurrence in a structural element containing multiple details was calculated based on the Monte-Carlo simulation and the p-S-N curve of a single structural detail. Fatigue tests were accomplished using specimens containing 21-similar-details to obtain the fatigue life corresponding to MSD occurrence. Tests on single-detail specimens and static calibration tests were also conducted to get the basic fatigue properties of the material and the degree of stress fluctuation. The aforementioned three methods were compared and validated via the test results. The influence of the stress random fluctuation degree on the probability of MSD occurrence and influence of the distribution types on evaluating the MSD occurrence probability were discussed. [ABSTRACT FROM AUTHOR]

To predict fatigue crack initiation and fatigue crack growth lives in cold expanded double shear lap joints a numerical method has been employed. The total estimated fatigue lives were compared with available experimental fatigue test results for plain hole and cold expanded hole specimens of Al 2024-T3 in double shear lap joints. Three-dimensional finite element simulations have been performed in order to obtain the created residual stresses field due to cold expansion and subsequent far field longitudinal loading in the double shear lap joint. The obtained stress and strain distributions from the finite element analyses were employed to predict stress concentration factors to calculate fatigue crack initiation and fatigue crack growth lives using AFGROW computer code. The predicted fatigue lives demonstrate that there is a good agreement between the proposed method and experimental fatigue test results. [ABSTRACT FROM AUTHOR]

The fatigue and fracture behavior of nickel-based superalloy Inconel 718 was investigated up to the very high cycle regime under rotary bending tests at room temperature. It was found that this superalloy can still fracture after exceeding 10 cycles. Fractographic analysis revealed that there was a transition from fatigue crack initiation at multi-sites to single initiation with decreasing stress levels. The fracture surface can be divided into four areas according to the appearance, associated with fracture mechanics analysis of the corresponding stress intensity factors. The fracture mechanism dominant in each area was disclosed by scanning electron microscope examination and analyzed in comparison with those obtained from the crack growth tests. Subsequently, life prediction modeling was proposed by estimating the crack initiation and propagation stage respectively. It was found that Chan (2003)'s model for initiation life and the Paris law for growth life can provide comparable predictions against the experimental life. [ABSTRACT FROM AUTHOR]

Localized corrosion damage in Type 7075-T73 aluminum was investigated for a HNO3 + Fe2(SO4)3 deoxidizer solution which is frequently used for surface pretreatment prior to anodizing. The corrosion damage was quantified in the time domain using the electrochemical noise resistance ( Rn) and in the frequency domain using the spectral noise impedance ( Rsn). The Rsn was derived from an equivalent electrical circuit model that represented the corrosion cell implemented in the present study. These data are correlated to scanning electron microscopy (SEM) examinations and corresponding statistical analysis based on digital image analysis of the corroded surfaces. Other data used to better understand the corrosion mechanisms include the open circuit potential (OCP) and coupling-current time records. Based on statistical analysis of the pit structures for 600 and 1200 s exposures, the best fit was achieved with a 3-paramater lognormal distribution. It was observed for the 1200 s exposure that a small population of pits continued to grow beyond a threshold critical size of 10 μm. In addition, significant grain boundary attack was observed after 1200 s exposure. These data are in good agreement with the electrochemical data. Specifically, the Rn was computed to be 295 and 96 Ω-cm2 for 600 and 1200 s exposures, respectively. The calculated value of Rsn, theoretically shown to be equal to Rn in the low frequency limit, was higher than Rn for a 1200 s exposure period. However, better agreement between the Rn and Rsn was found for frequencies above 0.01 Hz. Experimental results on the measurement performance for potassium chloride (KCl) saturated double-junction Ag/AgCl and single-junction Hg/Hg2Cl2 reference electrodes in the low-pH deoxidizer solution are also compared. [ABSTRACT FROM AUTHOR]

Localized corrosion damage in Type 7075-T73 aluminum alloy was investigated for various anodizing pretreatment solutions. The postexposure surface corrosion was characterized by use of scanning electron microscopy (SEM) examination. In addition, SEM and energy dispersive spectroscopy (EDS) were used for second-phase (constituent) particle identification for those found to induce pitting corrosion during solution exposure. The pitting mechanisms were identified as circumferential where the particles are noble with respect to the matrix phase and by selective dissolution where they are anodic. The designated category-1 degreasing and category-2 inhibited alkaline solutions did not initiate localized corrosion after 1200 s exposures. However, the category-3 high-pH NaOH and category-4 low-pH HNO3 based solutions were found to initiate pitting attack, with the NaOH being significantly more aggressive. It was hypothesized that if the pits initiating during the pretreatment exposures were beyond a threshold size, on the order of 10-20 µm, a higher current density existed at these locations during subsequent electrochemical processes, thus resulting in larger and deeper pit structures. These surface defects are of primary concern with respect to accelerated fatigue crack nucleation. For smaller pits, on the order of 1-5 lm, the anodic process had a smoothing affect where the film growth tended to passivate the pits. [ABSTRACT FROM AUTHOR]

Rotating-bending uniaxial fatigue tests and micro-fatigue crack initiation tests were carried out using a permanent mold cast (PMC) and semi-solid die cast (SDC) with Al−7%Si−0.35%Mg composition in order to examine the relationship between solidification structures and fatigue behaviors. The crack length was measured using a replication method. Fatigue strength was improved in SDC, which was almost consistent with the predicted fatigue strength using the size of Si particle cluster. Resistance to fatigue crack initiation and fatigue strength were improved in SDC owing to the finer Si cluster and to higher ultimate tensile strength. Fatigue crack in PMC was preferentially initiated at pores. For SDC, the fatigue crack was initiated at the Si particle/matrix interface, and then sucessively grew along eutectic cell boundaries. [ABSTRACT FROM AUTHOR]

To investigate the effect of nitriding layer on both fatigue crack initiation and fatigue life, turbine rotor steel (lCr-IMo-0.25V steel) specimens were nitrided by the nitemper method and then put to a rotary bending fatigue test at room and elevated temperatures. In nitriding, temperature and time were controlled to obtain a different nitrided thickness. Microstructure analysis, micro-Vickers hardness test, and scanning electron microscope observation were carried out for evaluating experiments. In results, the fatigue cracks of nitrided specimens were initiated at inclusion near the interface between nitrided layer and substrate, which showed fish-eye type appearance in fractograph. The fatigue life of nitrided specimens at every temperature was prolonged compared to that of the non-nitrided. However, there was not observable improvement in fatigue characteristics with the increase of a nitrided thickness. [ABSTRACT FROM AUTHOR]

Fatigue crack initiation in Ti-6Al-4V has been investigated in high cycle fatigue (HCF) and low cycle fatigue (LCF) regimes at stress ratio R=0.1 using the replication technique. In all four tested α/β microstructures, the crack was initiated by fracture of equiaxed alpha grain. Fractured alpha grains are seen on the fracture surface as flat facets with features characteristics of cleavage fracture. In the regime of low stress amplitudes and in the absence of reverse loading, cleavage fracture contributes to crack initiation and early stages of crack growth in Ti-6Al-4V. This mechanism is discussed in relation to the anomalous mean stress fatigue behavior exhibited by this alloy. [ABSTRACT FROM AUTHOR]