Location Of Crack Initiation Research Articles

Mechanical Behaviors and Fracture Characteristics of Sandstone Combinations with Different Pre-crack Angles

Layered rock masses contain many pre-existing defects, and investigating their failure mechanism is crucial for controlling layered rock mass instability. In order to understand the layered rock fracture mechanism, this study develops sandstone combination models containing cracks of different angles using the Particle Flow Code software (PFC2D) to conduct the uniaxial loading tests. In addition, it analyzes the mechanical properties and fracture characteristics of the models. The pre-crack angles and the composite materials determine the mechanical properties and crack propagation of the sandstone combination (SC) models. The results indicated that the peak stress and elastic modulus of the SC models increase as the pre-crack angle rises. The force chain concentration and the crack initiation location change with the different pre-crack angles, and the initiation stress improves gradually. The entire acoustic emission (AE) process is divided into three phases per axial stress and AE event number. The AE active level significantly correlates with the pre-crack angle change. The “V” type trend is shown in the crack number distribution, and the tensile crack is the predominant mode with some shear cracks. The micro-crack concentration and macroscopic damage zones are more prevalent in the low-strength parts, and the pre-crack angles define the influence weight of the pre-crack angle and the composite material strength.

Ultimate strength and fatigue of stiffened welded tubular joints in floating energy production structures

Monotonic and cyclic loading experiments are performed on 1/6-scale stiffened steel joints representing the welded connection between the tubular braces and the vertical cylinders of a prototype offshore floating structure for combined wind-wave energy production. The structural behavior of the welded joints is investigated to determine their ultimate strength under monotonic loading and their fatigue life. The brace-to-cylinder welded connection is full penetration and semi-automatic, whereas the stiffeners are welded to the inner surface of the cylinder with fillet welds. High-frequency mechanical impact post-weld treatment is applied to five specimens. Experimental observations indicate two critical locations for fatigue crack initiation: the crown of the brace-to-cylinder weld and the fillet weld of the central stiffener. These locations are also verified by finite element simulations and fractographic examination of failed specimens. Post-weld treatment improves the fatigue resistance of the welded joints in the high-cycle fatigue domain but has negligible influence on the low-cycle fatigue response of the connection and its monotonic ultimate strength. Finally, one fatigue-cracked specimen is tested under monotonic loading exhibiting structural performance similar to the intact specimens.

Phase-field modeling of fracture in fused filament fabricated thermoplastic parts and experimental validation

In this work, a phase-field fracture model is formulated specifically for thermoplastic parts manufactured by fused filament fabrication (FFF). Due to their layered nature, FFF parts present an orthotropic fracture behavior, with their interfaces representing energetically favorable crack paths. The proposed phase-field model relies on the introduction of two damage variables, one for cross-layer and one for inter-layer crack propagation, allowing to capture the distinct fracture behavior of the thermoplastic filaments and the interfaces. The model is applied to the three-point bending of single-edge notched bending (SENB) FFF specimens with six different material orientations. The comparison with the experimental results shows that the proposed phase-field model can predict the experimentally observed peak load for the six material orientations with a maximum error of ≈ 11%. Furthermore, the model can also accurately predict the crack path and the softening behavior observed experimentally. To validate the capacities of the phase-field model, an experimental campaign on FFF compact-tension (CT) specimens modified with holes was performed. Comparing the numerical prediction with the experimental results for the modified CT specimens shows that, although the model features some limitations, it is still able to accurately capture the location of crack initiation and predict the peak load with a relative error of ≈ 2%.

Effect of pad rotation angle and coefficient of friction on fretting fatigue crack nucleation behavior

Fretting fatigue may occur on surfaces that are in contact, subjected to a normal load, and where there is a cyclic small relative movement between them. It could occur in press-fitted shafts, bolted and dovetail joints, and many other engineering components. It is recognized that the coefficient of friction of the contacting bodies is a primary variable in the fretting process. Also, a recent study indicates that its behavior might be greatly affected by the possible rotation of the pad. Consequently, the aim of this investigation is to evaluate the impact of diverse angles of rotation of the pad and different coefficients of friction on crack initiation angle, location, and life. For this purpose, a numerical model based on a finite element model of a cylinder-plane test configuration is constructed. The Fatemi-Socie damage parameter for multiaxial fatigue is used to evaluate initiation behavior. Firstly, different angles of rotation are applied to the pad, in order to model a lack of stiffness of the test rig, while the coefficient of friction is kept constant and equal to 0.75. Secondly, several coefficients of friction between the pad and the specimen are utilized, while considering a cyclic rotation of the pad between 0.19° and – 0.19°. For the case studied, the results indicate that an increase of the coefficient of friction (μ) may cause a substantial reduction of crack initiation life. However, a variation of μ in the range 0.75 ≤ μ ≤ 0.85 does not produce a significant change in life estimation. This suggests that it cannot be stated that an increase in the coefficient of friction always leads to a reduction in nucleation life. Regarding the angle of initiation of the crack, the results show that an increase of μ causes an approximately linear reduction of the angle. In the case of an increase of the pad angle of rotation, this causes a longer nucleation life and a smaller crack nucleation angle. Even the rotation angle of 0.23° produces an increase of initiation life from 16.6 to 154 thousand cycles. It is concluded that there is a major impact on initiation behavior of the coefficient of friction and the rotation of the pad, which may be due to the lack of rigidity of the test setup. Therefore, controlling the stiffness of the test rig and measuring accurately the coefficient of friction are important factors to consider in fretting fatigue research.

Thermomechanical modeling and experimental study of a multi-layer cast iron repair welding for weld-induced crack prediction

Large-scale components such as hubs in wind turbines are often made of cast iron to minimize the production costs. One of the common challenges in the casting process of such large-scale components is manufacturing defects. However, repair welding will induce residual stress which can initiate cracks in the repaired structure, especially since cast iron is not as tough as steel. The current study addresses developing a thermo-mechanical model of the cast iron repair weld validated with experiments to predict thermal and residual stresses and to identify critical locations for crack initiation. A thermo-mechanical weld model is developed, and the predicted temperature and residual stress distribution are validated against experimental data. Two repair weld experiments, one manual and one automated are carried out and are simulated using the developed thermo-mechanical model. The regions with maximum principal residual stresses are calculated by the thermo-mechanical model and the maximum principal stress method is used to predict the location and direction of the developed cracks in the repair weld. A comparison with the repair weld experiments shows good correlation with the observed cracks in the welded specimens. The outcome of this research provides a basis for repair weld optimization of large-scale cast iron components in order to reduce the carbon footprint caused by their reproduction.

Influence of bedding plane on the tensile properties and crack propagation of soft and hard laminated rock-like under Brazilian test

To investigate the tensile properties and crack propagation of anisotropic rock-like specimens, soft and hard laminated rock-like specimens were made for the Brazilian splitting test. Five bedding plane inclination angles (θ = 0°, 30°, 45°, 60° and 90°) and three thickness ratios of soft and hard layers (1:1, 1:3 and 3:1) were taken into consideration for the rock-like concrete specimens. The effects of different θ and rock layer thickness ratios on the tensile strength, brittleness and energy characteristics were analyzed. The variation in tensile strain and crack opening displacement of tensile fracture were obtained by the digital image correlation (DIC) technique for the specimens. The crack initiation position and propagation path were determined. It was found that the fracture process zone appeared before the initiation of macroscopic cracks owing to the accumulation of tensile strain. The crack initiation position occurs near the two loading points regardless of the different angle θ. When θ = 0° and 90°, the crack propagates toward the center of the specimen along the loading direction. However, when θ = 30°, 45° and 60°, the cracks tend to propagate along the bedding plane. The influence of the crack propagation path on the final fracture pattern was analyzed. The results indicated that the angle θ has a strong influence on tensile crack initiation and propagation under the Brazilian test. The fracture process zone appears at the location of crack initiation and the expanding cracking path before the formation of macroscopic cracks. The absorbed energy increases with increasing tensile strength, and it decreases as θ increases. The brittleness index of the specimen increases gradually with increasing θ.

Experimental and numerical study of the fatigue properties of stress-corroded steel wires for bridge cables

In this paper, the fatigue tests of stress-corroded steel wires were carried out to investigate the effects of corrosion age, stress level, stress range on the failure mode, fatigue life and S-N curve of steel wires. Then, the fatigue fracture and crack growth rate of steel wires with different corrosion degrees were analyzed by SEM scanning, revealing the crack initiation and propagation mechanism of corroded steel wires. Finally, a numerical analyzing method for the fatigue life of steel wires considering the corrosion surface was proposed, and the relationship between the surface stress distribution and fatigue life was studied. The results shows that the fatigue life of steel wires decreased gradually with the increase of corrosion degree. When the corrosion degree was 5.0%, 10% and 20%, the fatigue life of steel wires decreased by 32.67%, 48% and 67.84%, respectively. The influence of corrosion degree on the fatigue life of steel wire was first sensitive and then insensitive. Meanwhile, compared with the unstressed corrosion state, the fatigue life of stress-corroded steel wires decreased by a maximum of 56.45%. In addition, stress corrosion affected the crack initiation location and shortened the crack propagation depth. The higher corrosion depth and the sharp surface were more likely to be the fatigue crack source. With the increase of corrosion degree, the striation spacing and crack growth rate of fatigue fracture increased, and the proportion of crack initiation and propagation zone decreased from 45% of the diameter to 28% of the diameter of the cable. Finally, the proposed numerical analysis method considering the real corroded surface can effectively reveal the fatigue failure mechanism and evaluate the residual fatigue life of corroded steel wires.

Fatigue strength of tubular welded connections in offshore floating platforms

AbstractThe work reported in the present paper is part of a European research program aimed at developing a hybrid tension‐leg floating platform, suitable for combined offshore wind and wave energy exploitation. Eight (8) fatigue tests on welded tubular joint specimens are performed, which represent the full penetration brace‐to‐cylindrical column welded connections of the project prototype structure. The large‐diameter columns are reinforced internally with a grid of longitudinal and transverse stiffeners fillet welded to the shell. The specimens are approximately 1/6‐scale physical models made of S355 grade steel, and subjected to cyclic loading under constant load amplitude histories. The brace‐to‐cylinder welds are semiautomatic, and high‐frequency mechanical impact treatment is applied in four specimens. The experiments are supported by post‐fracture metallographic examination and finite element analyses. The results indicate two locations for fatigue crack initiation: brace‐to‐cylinder crown and cylinder‐to‐stiffener connection.

Effect of Initial Crack Position on Crack Propagation Behaviors of Heavy-Duty Transmission Gear.

The tooth bending fatigue fracture is caused by the alternating loads for the heavy-duty transmission gears. The crack initiation and propagation are the two major parts in the failure process. The crack propagation behavior is mainly affected by initial crack position except for the load and material properties. In this paper, the crack propagation model of a gear is established under the considering of crack initiation location by using extended finite element method (XFEM). The model accuracy is verified by testing results of strain and fractography by conducting the single-tooth bending fatigue experiment. The influence of crack initiation locations on subsequent crack propagation behavior is analyzed. The crack length in the tooth width direction and depth direction is faster when the initial crack is located in the middle of root surface. The crack growth rate is lower for the initial crack located in the surface close to the end surface of the gear.

Antiviral/antibacterial and mechanical properties of HAp/TiO2 porous composite

This study aims at observing damage behavior of porous components under compression using multi modal measurement combining Acoustic Emission (AE) and IR.uniform porous structures were fabricated using a 3D printer in order to observe the damage process during the compressive loading. Then, the influence of the crack-connecting behavior among pores on mechanical properties was investigated experimentally. By using AE and IR method, high agreement between the onset of AE signal and temperature change at the surface of the porous material was revealed. The crack initiation location of porous materials with spherical and elliptical pores is found to be near the estimated location of the sudden AE emission source.

Improving the strength-ductility synergy and corrosion resistance of Inconel 718/316L dissimilar laser beam welding joint via post-weld heat treatment

Inconel 718/316L dissimilar welding is widely used in nuclear power plants, which suffers from the deterioration of mechanical properties and corrosion resistance after welding, attributed to the formation of brittle phase and the existence of residual stress. In this paper, the impact of post-weld heat treatment (PWHT) on Inconel 718/316L welding joints is systematically investigated. By conducting PWHT after welding, notable reductions in residual stress in as-welded joints are significantly reduced and the microstructure is improved, resulting in an enhanced strength-ductility synergy and corrosion resistance. The as-welded joints exhibit the generation of brittle Laves phase and welding residual stress in the fusion zone. After PWHT at 650 °C, a substantial increase in the quantity of fine-scale γ″ strengthening phases with a uniform dispersion distribution in weldment is observed, along with the release of residual stress. This combination yields an optimal strength-ductility synergy and corrosion resistance of the joint. However, the δ phase precipitates along in Laves phase boundary after PWHT at 850 °C, which exhibits a semi-coherent interface with γ matrix, and a non-coherent interface with the Laves phase, leading to severe stress concentration at the δ phase interface. This severe stress concentration could be a preferred location for crack initiation and propagation, resulting in a substantial loss in mechanical behavior. Additionally, the increase in the phase boundary of γ/γ″, γ/Laves, Laves/δ and γ/δ reduces the corrosion resistance of the joint. The findings of this study provide an effective strategy for improving the performance of Inconel 718/316L dissimilar laser beam welding joint.

Crack propagation life of high-speed aviation gear

The most common fault in aviation power transmission systems is the fatigue fracture of gears, which seriously affects the reliability and safety of the structure. The aim of this study is to analyze the crack propagation path (CPP) to predict the crack propagation life (CPL) of gears. The location of crack initiation is determined by statics analysis and building a gear pair model in ANSYS software. A FEM model of the gear pair with a crack is created, the Stress Intensity Factor (SIF) at the crack tip is calculated, and the influence of the initial state of the crack on the SIF is analyzed. FRANC3D is adopted to obtain the CPP and CPL. Based on the obtained results, the impact of gear structure parameters on CPL is discussed. With the reduction of flange thickness, web thickness, and root radius, the CPL gradually decreases.

Review of Fatigue Crack Initiation Mechanisms Development and Monitoring in the Very High Cycle Fatigue Regime

Abstract Fatigue devices working at high frequency (20 kHz) allow now to explore the fatigue domain with a total number of cycles much higher than 107 cycles in reasonable times. In that case, lifetime belongs to the very high cycle fatigue (VHCF) domain. As for low and high cycle fatigue (LCF and HCF), the fracture begins by microstructural damage because of microplasticity, before leading to the initiation stage (stage I), and then the short and long crack propagation stages (stage II). It is now admitted that in VHCF the initiation stage is higher than 90 % of the total life. The main difference with LCF and HCF is the crack initiation location. For materials without internal defects (type I), the initiation is always on the specimen surface, whereas for materials with internal defects (inclusions, porosities, etc.) named type II, the microstructure appears in subsurface on defects leading to a typical “fish eye” on fractographies. If the crack initiation mechanisms on surface are well accepted to be related to dislocations gliding in well-oriented grains, the mechanisms in subsurface are not very clear and many hypotheses on the fish-eye development are suggested by different authors. In the fish-eye center, it appears a fine granular area more or less large, the origin of which is debated. In the first part, this article proposes a review on these crack initiation mechanisms (surface and subsurface). Microplasticity produces a temperature increase in fatigue tests. It has been demonstrated that the temperature recording during tests is a mean to determine the number of cycles corresponding to crack initiation and crack propagation. In the second part, through two examples on Armco iron (surface initiation) and a low alloy steel (subsurface crack initiation), the method to determine the number of cycles during the initiation and propagation stages is exposed.

Ductile failure of Inconel 718 during flow forming process and its numerical investigation

Understanding the fracture mechanisms and predicting failure in forming processes is of great interest to increase formability and numerical methods have been proven effective. Incremental forming processes such as flow forming possess challenges due to material experiencing complex stress states and highly non-linear deformation histories during forming. Although some of the ductile failure criteria integrated into finite element (FE) simulations are found to be in better correspondence with the experimental findings, they mostly lack sufficient accuracy in predicting formability limits for flow forming and spinning processes. In this context, this work is concerned with the analysis and prediction of ductile failure of nickel-based super alloy Inconel 718 (IN718) in the flow forming process through FE analysis. Failure is modeled with three different models, namely Cockcroft–Latham (CL), Johnson–Cook (JC) and modified Mohr–Coulomb (MMC), implemented as a user subroutine in explicit FE solver Abaqus. Tensile tests are conducted with four different specimen geometries in order to identify parameters of the plasticity and damage models. Then, the developed framework is applied to a FE simulation of the flow forming process to predict formability limits and perform process parameter optimization to improve formability. Flow forming tests conducted at three thickness reduction values reveal that the forming limit of the material is at about 50% thickness reduction where radial cracks are observed on the outer surface. It is found that the CL model is able to predict formability limits and crack initiation locations while the MMC and JC models failed in both with significant over prediction of damage. Furthermore, the effect of temperature and process parameters such as the ratio of feed rate to revolution speed and roller axial and radial offsets are further examined with the developed FE frameworks. It has been shown that process parameter optimization requires accurate modeling of ductile damage since the behavior of different models is diverse.

Effect of Holding Time on Microstructure and Mechanical Properties of Dissimilar γ'‐Strengthened Superalloys TLP Joint

To achieve efficient connectivity of dissimilar γ'‐strengthened superalloy, transient liquid‐phase (TLP) diffusion bonding of 718Plus and Ni3Al‐based superalloys is carried out using BNi‐2 interlayer at a constant temperature of 1100 °C with holding times of 3, 15, 30, and 60 min. It is shown in the results that when the holding time is insufficient, isothermal solidification transforms into athermal solidification. Due to the enrichment of B and Cr elements in liquid phase, ternary eutectic consisting of γ, Ni3B, and CrB is formed at the joint center. Ternary eutectic with higher hardness (≈600 Hv) is poorly bonded to the matrix, which can easily become crack initiation location and significantly deteriorated the tensile properties. In addition, microhardness of the joint becomes more uniform and tensile strength gradually increases due to the reduction of eutectic with holding time increasing. Isothermal solidification completes and a joint without eutectic structure is formed at 1100 °C for 60 min. Diffusion of Al atoms leads to the formation of γ' with gradient size and tensile strength is comparable to that of base metal which is about 690.93 MPa.

Effect of double crack on fatigue crack growth life of 3D printing compressor impeller

A compressor impeller with pressure ratio less than 3 is taken as the research object. The 3D printing residual stress was introduced into damage tolerance assessment of blades, and the effect of double crack on Fatigue Crack Growth Life (FCGL) was investigated. The impeller was printed by selective laser melting technology and then heat treated. Finite element method was applied to numerically simulate 3D printing process, heat treatment process and actual working process of impeller, and mechanical quantities such as stress and deformation were completely analyzed. Numerical simulation results agreed well with the experimentally observed crack location in the middle of the blade. Forman–Newman–de Koning model and linear elastic fracture mechanics theory were applied to give the growth law of impeller under single and double crack conditions. The result shows that crack initiation locations of impeller during printing and operating are respectively middle part and root part of blade. The stress intensity factor at the initial crack tips of single and double cracks exhibits a quadratic relationship with rotational speed when only external load is considered. The crack growth paths of single crack in middle and root are radial and axial respectively, and the FCGL of single crack in root is lower than that of single crack in middle. With the increase of rotational speed, the critical crack length decreases linearly and the cycles decreases nonlinearly in both single and double crack conditions. The FCGL of double crack is between two single cracks at low speed and lower than two single cracks at high speed. The critical crack length of double crack is lower than that of corresponding single crack. At 60000rpm, the dominant crack changes from middle crack to root crack.

Failure Mechanism Research on Bending Fretting Fatigue of 6061-T6 Aluminum Alloy by Experiment and Finite Element Method.

The fatigue failure mechanism of bending fretting for cyclic softening material 6061-T6 aluminum alloy was researched by experiment and finite element method. The influence of cyclic load on bending fretting fatigue was researched and the damage characteristics under different cycles was discussed experimentally though SEM images. In the simulation, a normal load transformation method was employed to obtain a simplified two-dimensional model used for simulating the bending fretting fatigue from a three-dimensional model. An advanced constitutive equation with the Abdel-Ohno rule and isotropic hardening evolution was transplanted into ABAQUS by UMAT subroutine to consider the ratchetting behavior and cyclic softening characteristics. The peak stain distributions under various cyclic loads were discussed. Additionally, the bending fretting fatigue lives and crack initiation locations referring to a critical volume method were estimated using the Smith-Watson-Topper critical plane approach and reasonable results were obtained.

The effect law of shell surface crack propagation under implosion loading

The fracture process of the metal cylindrical shell under the action of implosion is numerically simulated by AUTODYN finite element software for the expansion of cracks on the surface of the cases under the action of implosion loading. Numerical computations were used to determine the effect law of engineering factors such as the energy density of explosives, shell thickness, and shell material on crack-related parameters. The results show that the shell fracture radius along the axial direction remains nearly constant, and its value is less influenced by the energy density of explosives. The axial crack propagation velocity of the shell from the location of crack initiation to the non-exploding end of the shell shows a trend of change from high to low, fluctuating down. The axial crack propagation velocity and crack circumferential density both increase with the explosive energy density. The fracture density fluctuation range is between 0.4~0.73/mm. The law can be used to study the fracture damage of metal cylindrical shell structures under high impact.

Influence of the C Content on the Fatigue Crack Initiation and Short Crack Behavior of Cu Alloyed Steels

The mechanical properties of Cu alloyed steels are influenced significantly by the Cu content and the respective state of Cu precipitations as well as the C content. In this context, the effect of an increased C content on the fatigue crack initiation and growth of differently aged Cu alloyed steels with 0.005 (X0.5CuNi2-2: X0.5) and 0.21 wt.-% C (X21CuNi2-2: X21) was investigated in this study. Notched specimens were examined via SEM in interrupted fatigue tests to detect the location of crack initiation and growth. The results showed that fatigue crack initiation and growth occurred for both steels at grain boundaries, and within ferrite grains. However, a higher C content increased the incidence of crack initiation and growth at grain boundaries. This is caused by the smaller grains of X21 and especially by the presence of cementite on the grain boundaries. This explains why, in contrast to X0.5, no influence of the Cu precipitation state on the defect-based failure was observed for X21, as the precipitates are located within the ferrite grains and, thus, only have a minor impact on the fatigue failure mechanisms of X21.

Investigation of the heat energy and multi‐level load on subsurface fatigue damage evolution via multi‐scale method

AbstractThe relationship between heat energy and initial fatigue life of different metals under multi‐level loading stress and elevated temperature is introduced. The fatigue damage evolution model was established from the microscopic and mesoscopic perspectives to explore the fatigue damage and the crack initiation location at elevated temperature. Based on the method of continuous damage mechanics, the influence of multi‐level loads on fatigue cracking life and heat energy on strain energy of distribution were investigated. The mathematical model is verified by the experimental data. The model has important engineering significance for predicting the service life of parts at elevated temperature.