Fatigue Crack Extension Research Articles

Quantitative assessment of compression fatigue history effect on the subsequent tension fatigue limit of strain localized material

This study conducts compressive fatigue tests with an extended notch on a strain-localized material for quantitative evaluation of damage during compression fatigue and the corresponding effect of loading history on subsequent tensile fatigue limits. Hence, fatigue crack “growth” and “propagation” of two types are found in damage accumulation (DA) mode. The former features several simultaneous multi-crack initiations and independent extensions. Contrarily, the latter features coalescence between the main and secondary cracks. Moreover, the near-crack-tip mechanics causing crack extension and non-propagation in the respective fatigue crack extension types are discussed. Furthermore, a method for subsequent tensile fatigue limit prediction considering the compression fatigue effect is proposed by studying the non-propagating crack length, Vickers hardness, and residual stress in the DA mode during compression fatigue, corresponding to Murakami–Endo’s equation parameters for a mechanically small crack. Thus, this study is anticipated to hold great significance for understanding fatigue damage caused by different load blocks and improving Miner’s rule.

Fracture behavior analysis and fatigue assessment of the spring clip in heavy‐haul railway

AbstractThis paper proposes a novel method based on a numerical simulation approach and fatigue fracture theory to analyze the fracture behavior and assess the fatigue life of spring clips. To this end, the finite element (FE) model of the vehicle‐track‐foundation system was developed including the key parts of the type II fastening system. Meanwhile, the spring clip FE model was validated through the natural vibration characteristics. Subsequently, the numerical simulations were performed under various working conditions to obtain the fatigue characteristics of the spring clips. The fatigue crack initiation life and damage were analyzed using FE results for the spring clips with ε‐N curve and Brown‐Miller criterion. Moreover, based on the initial crack geometry of the spring clips, the maximum circumferential stress criterion with the Paris equation was employed to analyze the crack extension behavior and assess the fatigue crack extension life.

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.

Prediction of fatigue crack extension life and assessment of load reduction

Prediction of fatigue crack extension life and assessment of load reduction

Mechanism of Microstructural Alterations of M50 Bearing Steel during Rolling Contact Fatigue under High Loads

Rolling contact fatigue (RCF) of vacuum induction melted–vacuum arc remelted (VIM-VAR) M50 bearing steel under high loads was carried out, using a three-ball-rod RCF tester. Dark etching regions (DER) and butterflies were found in the subsurface region below the raceway of the RCF-tested sample. The DER appeared in the region of maximum shear stress located at a depth of 30 μm to 170 μm below the raceway. Carbon atoms migrated through high-density dislocations, and part of the martensite plates was transformed into cellular ferrites, due to the redistribution of dislocations during the deformation of martensite under the action of cyclic shear stress. Butterflies appeared in the region of maximum shear stress located at a depth of 20 μm to 314 μm below the raceway. Butterflies were initiated in the primary carbides, with length values ranging from 5 μm to 15 μm. The plate martensite in the butterfly wings was transformed into nanocrystalline ferrites, due to the increase in the dislocation density and rearrangement of dislocations during the extension of fatigue cracks from the primary carbides to the matrix under cyclic shear stress.

Experimental and Theoretical Study on the Fatigue Crack Propagation in Stud Shear Connectors.

Steel-concrete composite girder bridges are subjected to reciprocal cyclic loading from vehicles, and the stud shear connectors are the key components for transmitting shear forces. Thus, it is necessary to study the fatigue performance of the stud shear connectors. At present, there are few studies on the fatigue crack propagation process of studs, and the variation curve of the crack depth of studs with the number of fatigue loading cycles is not clear. In this study, the degradation law of fatigue properties and the fatigue crack propagation law of stud shear connectors in steel-concrete composite structures are examined under fatigue loading. The fatigue properties, i.e., failure mode, the dynamic slip-fatigue number curve, cross-sectional characteristics, and the residual bearing capacity of the stud specimens, are first systematically studied through ten standard push-out specimen tests. The test results show that the relative value of the fatigue crack extension area increases, while the relative value of the residual bearing capacity of the studs decreases approximately linearly. Then, the expression of the relationship between the fatigue crack depth and the residual load-bearing capacity of the stud is proposed, based on the fatigue crack theory of fracture mechanics. Finally, combined with the ABAQUS and FRANC3D software, a fatigue crack propagation finite element analysis (FEA) model of the stud is established. The FEA results showed that the trends in the number of cyclic loads and the fatigue crack depth of studs are basically the same for the simulation curve, test curve and theoretical calculation curve.

Effect of Impact Velocity and Angle on Impact Wear Behavior of Zr-4 Alloy Cladding Tube.

In the pressurized water reactor nuclear power plant, 316L SS chips were captured by the support grid and continued to affect the Zr-4 cladding tube, causing the fuel rods to wear and perforate. In this work, a 60° acute angle cone of 316L SS was used to simulate the cyclic impact of debris on a Zr-4 alloy tube with different initial impact velocities and impact angles. Results showed that increasing the initial impact velocity will generate a wear debris accumulation layer with a wear-reducing effect, but also promote the extension and expansion of fatigue cracks, resulting in the delamination of Zr-4 alloy tubes. The inclination of the impact angle increases the energy loss. The energy loss rate of the 45° impact is as high as 69.68%, of which 78% is generated by the impact-sliding stage. The normal force is mainly responsible for the wear removal and plastic deformation of Zr-4 alloy tubes. Tangential forces cause severe cutting in Zr-4 alloys and pushes the resulting wear debris away from the contact surfaces.

Interaction analysis between strain concentration and strain localization in cracked body

AbstractThe appearance of strain localization (SL) in front of the crack tip is a factor that promotes transition toward the fatigue crack extension mode. Therefore, to predict fatigue crack extension behavior in case multiple modes occur, it is necessary to evaluate the degree of SL. In this study, the following simulations were performed on a single‐crystal twinning‐induced plasticity (TWIP) steel: (a) SL characterization by crystal plasticity finite element method (CPFEM) analysis of a smooth specimen under tension, (b) strain concentration (SC) characterization by EP‐FEM analysis of a cracked specimen under tension, and (c) characterization of interaction results between SC and SL of a cracked specimen under tension by CPFEM analysis. A comparison of the above results found that the interaction between SC and SL was large for long cracks and a significant strain field, different from the Hutchinson, Rice, Rosengren (HRR) singular field, was formed in front of the crack tip. Therefore, a parameter was proposed to quantify the SL properties in a smooth specimen. Subsequently, a fracture‐mechanics‐like method was proposed in which the results of (b) and the introduced parameters control the results of (c).

Modeling and Analysis of Acoustic Emission Generated by Fatigue Cracking

The acoustic emission (AE) method is a popular and well-developed method for passive structural health monitoring of metallic and composite structures. The current study focuses on the analysis of one of its processes, sound source or signal propagation. This paper discusses the principle of plate wave signal sensing using piezoelectric transducers, and derives an analytical expression for the response of piezoelectric transducers under the action of stress waves, to obtain an overall mathematical model of the acoustic emission signal from generation to reception. The acoustic emission caused by fatigue crack extension is simulated by a finite element method, and the actual acoustic emission signal is simulated by a pencil lead break experiment. The results predicted by the mathematical model are compared with the experimental results and the simulation results, respectively, and show good agreement. In addition, the presence of obvious S0 mode Lamb waves is observed in the simulation results and experimental results, which further verifies the correctness of the analytical model prediction.

FEM assessment of the procedures providing for a uniform crack shape in specimens for fracture toughness tested in full thickness

Acceptance tests of base metal and welded joints include the evaluation of critical CTOD values as required in product specifications. Tests have to be done in natural (full) thickness. For blanks made of pipes, quantitative criteria of satisfactory straightening and the fixture dimensions are developed. Satisfactory fatigue precrack front linearity can be attained with additional treatment. Local side compression appears efficient for the examined materials. Sequential FEM calculations of welding, side compression and specimen notching allow constructing the residual stress curves to predict fatigue crack extension. The optimum compression displacement is found. The above sequence of operations is proved to be preferable in comparison with side compression performed after notching.

Notch shape dependence of fatigue crack extension in equiatomic CrMnFeCoNi high-entropy alloy

Fatigue tests were conducted on an equiatomic CrMnFeCoNi high-entropy alloy with three different micro-stress concentration sources. A fatigue crack behavior analysis using a replica of a specimen surface, fractographic analysis in a fracture surface, fatigue crack shape analysis, and strain analysis under the fracture surface were performed. It was found that the fatigue crack extension mode was transited by the severity of the strain concentration from the damage accumulation mode to the plastic deformation mode.

Nonlinear Properties Analysis of Metallic Fatigue Damage Process Based on the Combined Nonlinear Parameter

To effectively extract the nonlinear properties of fatigue crack extension and evaluate the fatigue damage of metallic specimens, the chaos and fractal theory is applied to analyze the nonlinear output signal in a nonlinear ultrasonic experiment. A combined nonlinear parameter using the Lyapunov exponent, Kolmogorov entropy, and correlation dimension is proposed to evaluate the nonlinearity of fatigue damage. The nonlinear ultrasonic experiment is carried out on specimens with different stages of fatigue life, wherein the ultrasonic nonlinear output signal is collected and its combined nonlinear parameters are calculated. With the increase of fatigue damage in metallic specimens, the combined nonlinear parameter increases monotonically and tends to become saturated when a macroscopic crack appears, which indicates that complexity and randomness gradually increase during the accumulation of fatigue damage. According to microcrack propagation, the combined nonlinear parameter is very sensitive to the evolution of fatigue damage in metallic specimens, especially earlier fatigue damage. The relationship between the combined nonlinear parameter and the fatigue damage of specimens is established, which can provide an effective analytical method for evaluating fatigue damage and predicting the service life of specimens.

Continuous texture evolution during staged solid solution of Al‐Cu‐Mg and the influence on fatigue resistance

AbstractContinuous texture evolution of the Al‐Cu‐Mg alloy during the staged solid solution and corresponding influence on fatigue property has been studied by means of X‐ray diffraction, electron back scattered diffraction and scanning electron microscopy. The characteristic texture transition rate XCGB and orientation streamline approach was introduced to further investigate the behind mechanisms. The results show that the two‐step solid solution could significantly enhance the XCGB, thereby contributes to reduce the fatigue crack growth rate of Al‐Cu‐Mg T3 sheet during stage Π. The near Cube texture with high Schmid factor (SF) and low Taylor factor (TF) tends to induce significant crack deflection, crack closure and strengthened planar slip. The potential 60°<1, −1, 1> twin relation and mutual transformation existed between Goss and P texture would further reduce the fine random orientation grain and inter‐granular fatigue crack extension. Such above effects would be responsible for the excellent correspondence between XCGB and fatigue resistance.

Microstructure evolution and wear behavior of titanium alloy under cryogenic dry sliding wear condition

Microstructure evolution and wear behavior of Ti6Al4V alloy under cryogenic dry sliding wear condition was investigated. An external cryogenic device was designed to produce a steady cryogenic environment (−172 °C) during the wear process. We found that the near surface microstructure of Ti6Al4V alloy exhibited obvious accumulation of dislocations in the grain boundaries of α phase, as well as the refinement of α and β phase grains during cryogenic temperature wear. On the contrary, the plastic deformation was predominated by twins during room temperature wear. Under cryogenic temperature wear, little plastic deformation of microstructure was observed in the near surface and subsurface of specimens, while severe deformation was found to be prevalent under room temperature wear. Worn surfaces of specimens after cryogenic wear tests were smoother with lower block spalling and friction coefficient. The cryogenic wear behavior was considered to be fracture spalling of surface induced by the initiation and extension of fatigue cracks, unlike the severe oxidative and delamination wear under room temperature. Meanwhile, as compared to room temperature wear, wear rate of Ti6Al4V alloy was reduced by 49.7%, 57.2% and 36.6% under the loading force of 100 N, 150 N and 200 N respectively after cryogenic temperature wear.

Numerical investigation on fatigue crack growth of fracturing pump head using cohesive zone model

This paper concerns with the fatigue crack growth of the fracturing pump head. The bi-linear cohesive zone model (CZM) with irreversible damage evolution equation was employed to simulate the fatigue crack growth of the fracturing pump head. Through comparing the data of the fracture toughness tests and fatigue crack tests with numerical results, the selected cohesive zone model parameters for alloy steel 30CrNi2MoV were reliable. In order to reduce dependence of computational resources a simplified method of the fracturing pump head was proposed to predict the fatigue crack growth. The fatigue crack extension along the crack front on the fracturing pump head are discussed in detail. The variation of the fatigue crack growth rate da/dN along the crack front is periodicity. The results show that cohesive zone model is suitable for prediction of the fatigue crack growth on the fracturing pump head.

Fatigue crack propagation experiment and numerical simulation of 42CrMo steel

42CrMo steel is widely used in ultrahigh-strength structures such as low-speed heavy-duty gears. Mastering the fatigue crack propagation law has important significance for predicting structural fatigue life. Firstly, the fatigue crack propagation experiment is used to obtain the upper and lower thresholds value of type I fatigue crack propagation of 42CrMo steel compact tensile specimen under the alternating load of stress ratio R = 0.1. The Paris formula describing the relationship between the fatigue crack propagation rate and the crack tip stress intensity factor between the upper and lower thresholds value is obtained. Scanning electron microscopy was used to observe the microscopic features of different stages of fatigue fracture. The results show that the twin boundary can provide a place for crack initiation; the defects in the material can promote the initiation and extension of fatigue cracks. The fatigue crack propagation of 42CrMo steel compact tensile specimens was numerically simulated by the finite element method. The relationship between the crack tip stress intensity factor and the crack length was obtained. The analysis results show that the crack tip stress intensity factor calculated by the plane finite element method differs slightly from the experimental results during the stable extension stage. After correction, the correlation coefficient between the numerical simulation correction value and the crack tip stress intensity factor value obtained by the experiment is 0.9926. Finally, the fatigue crack propagation rate corresponding to the crack tip stress intensity factor in the finite element results is calculated by the Paris formula and briefly analyzed. Compared with the experimental results, it shows that the numerical simulation is consistent with it, indicating the accuracy of the numerical simulation method, which can effectively predict the initiation and propagation of fatigue cracks in 42CrMo steel compact tensile specimens.

Fatigue crack extension of an interfacial crack between molding resin and metallic substrate in a power module under thermal and mechanical cycle loads

Fatigue crack extension of an interfacial crack between molding resin and metallic substrate in a power module under thermal and mechanical cycle loads

Deformation and Vibration-Induced Stress Intensity of a High-Temperature Turbine Rotor with a Breathing Transverse Crack

The vibrations of a double-seat weighty rotor of the steam turbine with a breathing transverse crack are examined in the field of operating environment temperatures. The 3D vibration model for the rotor with a breathing transverse crack is applied. The variable two-dimensional temperature field is found from the solution of nonstationary heat transfer problem. Its effect on the contact of crack edges on rotor vibrations was evaluated. The distribution of stress intensity factors along the crack front was established for different rotor positions. Fatigue crack extension modes are assessed.

Compressive fatigue damage and failure mechanism of fiber reinforced cementitious material with high ductility

The fiber reinforced cementitious material with high ductility has potential use in particular environments and structures that undergo repeated or fatigue loads. In this study, a series of monotonic and fatigue tests were performed to investigate the compressive fatigue behavior of this material. It is found that the fatigue life of this material is higher than that of plain concrete and steel fiber reinforced concrete under the same stress level. In addition, the failure deformation of fiber reinforced cementitious material with high ductility under fatigue load was larger than the monotonic envelope, while the envelope coincides with the monotonic loading curve for concrete or fiber reinforced concrete. The failure surface and damage process were investigated and a new failure mode of polyvinyl alcohol fiber with crushed end was discovered. The fatigue failure surface could be divided into three regions, including fatigue source region, transition region and crack extension region.

Dynamic response of a riser under excitation of internal waves

In this paper, the dynamic response of a marine riser under excitation of internal waves is studied. With the linear approximation, the governing equation of internal waves is given. Based on the rigid-lid boundary condition assumption, the equation is solved by Thompson-Haskell method. Thus the velocity field of internal waves is obtained by the continuity equation. Combined with the modified Morison formula, using finite element method, the motion equation of riser is solved in time domain with Newmark-β method. The computation programs are compiled to solve the differential equations in time domain. Then we get the numerical results, including riser displacement and transfiguration. It is observed that the internal wave will result in circular shear flow, and the first two modes have a dominant effect on dynamic response of the marine riser. In the high mode, the response diminishes rapidly. In different modes of internal waves, the deformation of riser has different shapes, and the location of maximum displacement shifts. Studies on wave parameters indicate that the wave amplitude plays a considerable role in response displacement of riser, while the wave frequency contributes little. Nevertheless, the internal waves of high wave frequency will lead to a high-frequency oscillation of riser; it possibly gives rise to fatigue crack extension and partial fatigue failure.