Crack Propagation Region Research Articles

High cycle fatigue behavior and strengthening mechanisms of laser powder bed fusion pure tantalum

The fatigue behavior and strengthening mechanisms of the laser powder bed fusion pure tantalum (LPBF-Ta), coupled with a fatigue life prediction model, were originally explored. The fatigue strength of LPBF-Ta fabricated at the optimal laser energy density (154.16 J/mm3) is 348 MPa (106 cycles), showing a remarkable improvement of about 80 % and 50 % compared with that of the annealed and cold worked commercially pure tantalum, respectively. The microstructural analysis identifies that cellular structures averaging 300 nm in size contribute 438.2 MPa to strengthening, serving as the main contributor to fatigue strength. The breaching of cell walls by dislocations, along with the activation of cross-slip, ensures plasticity and ultimately enhances fatigue strength. Besides, the analysis of the defects reveals that the internal lack of fusion pore can induce the secondary fatigue crack, resulting in a region of brittle crack propagation. Furthermore, a finite life prediction model is established by originally incorporating the size of the crack source defect, and it significantly improves the prediction accuracy of the fatigue life. These results provide guidance for the development of fatigue strengthening strategies for LPBF-Ta.

Lattice Defects Present beneath Crack Initiation, Propagation, and Final Fracture Regions on the Same Hydrogen Embrittlement Fracture Surface of Martensitic Steel

Lattice Defects Present beneath Crack Initiation, Propagation, and Final Fracture Regions on the Same Hydrogen Embrittlement Fracture Surface of Martensitic Steel

Effect of Mode II in the mixed-mode on the fatigue crack growth behaviour for SAPH440 material

In this study, the effect of mixed-mode loading on fatigue crack growth under varying applied loads for SAPH440 material was experimentally investigated. Previous research has either examined the effect of mixed-mode loading at a fixed angle or its effect on slanted cracks. This paper uses a CTS specimen and a loading device to apply varying angles of load during Mode I loading conditions. Initially, a numerical analysis was performed to validate existing equations for the stress intensity factor, which quantifies the crack driving force in the experiments. In the crack initiation region, load shedding techniques and load increasing techniques were used to confirm fatigue cracking considering mixed-mode effects. The change in the threshold value according to the loading angle was also evaluated. In the crack propagation region, a multi-level loading technique was employed to assess the mixed-mode effects under varying loading angles. This process verified the mixed-mode effect under conditions where the applied load angle varies. The fatigue crack fracture surfaces were examined using digital camera capture and SEM. It was found that when mixed-mode loading occurs during Mode I loading, the fatigue crack propagation rate varies depending on the mixed-mode loading angle.

Mechanical-thermal coupling fatigue failure of CoCrFeMnNi high entropy alloy

High entropy alloys exhibit excellent performance under different-temperature conditions. In order to investigate the effect of temperature on the mechanical-thermal coupling fatigue failure of classical CoCrFeMnNi high entropy alloy to accelerate the application of high entropy alloys, the uniaxial tensile and fatigue tests at temperatures ranging from RT to 500 °C were conducted, and statics simulation analysis of fatigue specimens under fatigue loading conditions was conducted. The results of the uniaxial tensile tests illustrated that the yield stress and tensile strength of the alloy decreased with increasing temperature. In contrast, the elongation after the fracture of the alloy increased with temperature. The fatigue test results revealed that the fatigue life decreased as the temperature increased. The fracture morphology of the fatigue specimens and the flank morphology and dislocation distribution near the crack nucleation zone were collected to support the investigation. Accordingly, the reasons for the high-temperature-induced decrease in fatigue crack nucleation and propagation lives were systematically investigated. The main reasons for the higher-temperature-induced decrease in crack nucleation lives were a decrease in the yield stress of the alloy, more easily triggered dislocation motion and stress homogenization. The reasons for the decrease in fatigue crack propagation lives induced by higher temperatures were the more prone occurrence of dislocation motion, a decrease in yield stress, and a decrease in the area of the crack propagation region.

Fatigue Behavior and Fracture Surface Analysis of Corroded High-Strength Bridge Cable Wires.

Bridge cable wires suffer from alternating stress and environmental erosion, leading to premature failure prior to its design life. This paper investigates the fatigue and mechanical behaviors of corroded bridge cable wires with a zinc-aluminum (Zn-Al) alloy coating. Based on the salt spray corrosion test and microstructure analysis, the anti-corrosion resistance and corrosion appearance characteristics of the Zn-Al alloy coating and galvanized coating were investigated. The Zn-Al alloy coating was superior in resistance to corrosion fatigue for the improvement in toughness and the generation of anti-corrosion Zn-Al and Fe-Zn-Al phases. Equations of the accelerated corrosion depth of the steel wires had been regressed to roughly estimate the corrosion life of the Zn-Al alloy coating, which can reach 29.1 years with a thickness of 70 μm. The fatigue and mechanical properties of the bare wires after the salt spray test were further studied based on tensile tests and fatigue tests. The fatigue properties of the bridge cable wire would decrease with the corrosion degree due to the deterioration and embrittlement of materials, where ductility characterized by the elongation rate was the most affected. Fracture surfaces of the wires were captured and analyzed based on a method for recognizing graphical contours. Insufficient fatigue life may occur in the steel wires after corrosion and increase with the degree of corrosion. The pit depth logarithmically weakened the fatigue life of steel wires for the weakening of fatigue toughness and the bearing area. The flat fracture was more common with a single fatigue source, while multiple fatigue sources led to step-like fractures for the generation of multiple dispersed crack propagation regions. Corrosion fatigue was more sensitive to the existence of fatigue sources than the reduction. Multiple initiation sources significantly reduced the fatigue life due to the cracking facilitation of the joint effect of multiple pits. The electrochemical reactions of corrosion can lead to material embrittlement and a reducing effect on the fracture toughness of the steel wires.

Plastic hinge behavior of rectangular CFRP-confined RC columns: Meso-scale modelling and formulation

The ultimate rotation capacity, depending on the plastic hinge length, is a vital factor in the seismic design of flexural members. This paper focuses on the investigation of the plastic hinge length and size effect of rectangular carbon fiber-reinforced polymer (CFRP)-confined reinforced concrete (RC) columns. Firstly, considerable scatter and evident bias of the calculation results were found after evaluating existing models for calculating the plastic hinge length of FRP-confined RC columns. Accordingly, a meso-scale numerical approach was developed, with concrete heterogeneity considered, to investigate the influences of the confinement ratio and corner radius on the plastic hinge length and size effect of rectangular CFRP-confined RC columns. Results show that columns having different cross-sectional sizes exhibit similar failure patterns, and the region of lateral crack propagation for columns with lower confinement ratios is larger than that for columns with higher confinement ratios. The actual plastic hinge length is determined based on the yielding zone of reinforcement, the crushing zone of concrete, and the localization zone of curvature, while the localization zone of curvature is employed to determine the real plastic hinge zone. The size effect is found to exist in the plastic hinge length ratio, and it is weakened with the increase of the corner radius, which can be attributed to the fact that columns with higher corner ratios have more effective confinement. Taking into account the size effect, a new model for calculating the ultimate drift ratio of CFRP-confined RC columns was proposed based on an assumption of curvature distribution and presented accurate predictions.

Effect of Hydrogen on Fatigue Life and Fracture Morphologies of TRIP-Aided Martensitic Steels with Added Nitrogen

The effects of hydrogen on the tensile properties, fatigue life, and tensile and fatigue fracture morphologies of nitrogen-added ultrahigh-strength transformation-induced plasticity (TRIP)-aided martensitic (TM) steels were investigated. The total elongation and number of cycles to failure (Nf) of the hydrogen-charged TM steels decreased with the addition of nitrogen; in particular, adding 100 ppm of nitrogen decreased the total elongation and Nf of the TM steels. The quasi-cleavage cracking around the AlN occurred near the sample surface, which is the crack propagation region, although dimples appeared at the center of the fracture surface in the tensile samples. The initial fatigue crack initiated at the AlN precipitate or matrix/AlN interface, located at the notch root. During crack propagation, new cracks were initiated at the AlN precipitates or matrix/AlN interfaces, while quasi-cleavage crack regions were observed around the AlN precipitates. The decrease in the total elongation and Nf of the hydrogen-charged TM steel with 100 ppm of added nitrogen might be attributable to the crack initiation around the AlN precipitates formed by a large amount of hydrogen trapped at the AlN precipitates and matrix/AlN interfaces, and to the dense distribution of AlN, which promoted crack linkage.

Estimating crack tip position in adhesively bonded joints subjected to mode II quasi‐static loading

AbstractThis study aimed at estimating crack tip position in adhesive bonded joints under mode II quasi‐static loading using experimental and numerical approaches. Experimental techniques were utilized and compared, including optical backscatter reflectometry, visual testing, and a novel strategy based on digital image correlation. Additionally, a finite element analysis was employed to identify the numerical crack tip position and the extent of damage within the bondline. This analysis revealed that a significant portion of the crack propagation region in the adhesive is occupied by the fracture process zone. Moreover, optical backscatter reflectometry shows the potential to detect this process zone within the adhesive that the other methods may not detect. This capability is particularly beneficial for detecting damage at early stages.

The Cold-Brittleness Regularities of Low-Activation Ferritic-Martensitic Steel EK-181

The behavior of the EK-181 low-activation ferritic-martensitic reactor steel (Fe–12Cr–2W–V–Ta–B) in the states with different levels of strength and plastic properties after traditional heat treatment (THT) and after high-temperature thermomechanical treatment (HTMT) in the temperature range from −196 to 25 °C, including the range of its cold brittleness (ductile–brittle transition temperature, DBTT) is studied. The investigations are carried out using non-destructive acoustic methods (internal friction, elasticity) and transmission and scanning electron microscopy methods. It is found that the curves of temperature dependence of internal friction (the vibration decrement) of EK-181 steel after THT and HTMT are similar to those of its impact strength. Below the ductile–brittle transition temperature, it is characterized by a low level of dislocation internal friction. The temperature dependence curves of the steel elastic modulus increase monotonically with the decreasing temperature. In this case, the value of Young’s modulus is structure-sensitive. A modification of the microstructure of EK-181 steel as a result of HTMT causes its elastic modulus to increase, compared to that after THT, over the entire temperature range under study. The electron microscopic studies of the steel microstructure evolution near the fracture surface of the impact samples (in the region of dynamic crack propagation) in the temperature range from −196 to 100 °C reveal the traces of plastic deformation (increased dislocation density, fragmentation of the martensitic structure) at all of the temperatures under study, including those below the cold brittleness threshold of EK-181 steel.

Fatigue crack growth rate and propagation mechanisms of SiC particle reinforced Al alloy matrix composites

PurposeThis study aims to investigate the fatigue crack propagation behavior of SiC particle-reinforced 2124 Al alloy composites under constant amplitude axial loading at a stress ratio of R = 0.1. For this purpose, it is performed experiments and comparatively analyze the results by producing 5, 10, 15 Vol.% SiCp-reinforced composites and unreinforced 2124 Al alloy billets with powder metallurgy (PM) production technique.Design/methodology/approachWith the PM production technique, SiCp-reinforced composite and unreinforced 2124 Al alloy billets were produced at 5%, 10%, 15% volume ratios. After the produced billets were extruded and 5 mm thick plates were formed, tensile and fatigue crack propagation compact tensile (CT) samples were prepared. Optical microscope examinations were carried out to determine the microstructural properties of billet and samples. To determine the SiC particle–matrix interactions due to the composite microstructure, unlike the Al alloy, which affects the crack initiation life and crack propagation rate, detailed scanning electron microscopy (SEM) studies have been carried out.FindingsOptical microscope examinations for the determination of the microstructural properties of billet and samples showed that although SiC particles were rarely clustered in the Al alloy matrix, they were generally homogeneously dispersed. Fatigue crack propagation rates were determined experimentally. While the highest crack initiation resistance was achieved at 5% SiC volume ratio, the slowest crack propagation rate in the stable crack propagation region was found in the unreinforced 2124 Al alloy. At volume ratios greater than 5%, the number of crack initiation cycles decreases and the propagation rate increases.Originality/valueAs a requirement of damage tolerance design, the fatigue crack propagation rate and fatigue behavior of materials to be used in high-tech vehicles such as aircraft structural parts should be well characterized. Therefore, safer use of these materials in critical structural parts becomes widespread. In this study, besides measuring fatigue crack propagation rates, the mechanisms causing crack acceleration or deceleration were determined by applying detailed SEM examinations.

Fatigue life prediction using critical distance on aluminum alloy wire containing indentation produced marks

The present study aims to predict fatigue life in aluminum alloy 6201-T81 wires containing defects produced by hardness indentation procedures. The wires tested were obtained from a 900 MCM All Aluminum Alloy Conductor (AAAC 900 MCM) used in power transmission lines. The stress field in the vicinity of the defect and the determination of the crack initiation site (hotspot) were obtained by applying elastic–plastic simulations via the Finite Element Method (FEM). Such a simulation considered both the indentation process, and the fatigue loading on the wire. The Smith-Watson-Topper multiaxial fatigue criterion applied to an equivalent stress computed by the Volume Method (MV) was used to estimate life. Challenges presented by the incorporation of the residual stress field and the choice of the best calibration procedures were addressed. Comparison between stress gradients in simulations and failures observed in fractographic analyses of specimens subjected to fatigue testing suggested similar potential crack propagation regions. A new and wide experimental campaign showed that presence of indentation defects reduced the fatigue life of the wires. 84% of the lives computed by the proposed methodology were within a factor of 3 compared with tests, being the worst estimate within a factor of 5 .

Effect of microstructure heterogeneity on the cryogenic fracture toughness of the dissimilar joint between SA645 and 304L made by laser welding

The cryogenic fracture toughness of the SA645/304L dissimilar weld, produced using continuous wave IPG fiber laser welding with a 0.3 mm beam offset towards the 304L side, and the microstructural effects on the fracture behavior were systematically investigated. The weld metal consists of ∼89% columnar dendritic martensite and ∼11% retained austenite (RA), where the ununiform distribution of constituent phases as well as the distinctions between martensite and RA in terms of morphology and mechanical properties lead to the microstructure heterogeneity of weld metal. Pop-in phenomenon appears on the Force-Displacement (F-V) curve during the crack tip opening displacement (CTOD) test for the weld. The CTOD value for the weld is ∼0.057 mm, about 8 times less than that when pop-in effect is ignored (∼0.563 mm). Rapid propagation of the pre-fatigue crack tip along the center of the weld leads to the pop-in phenomenon. Fracture surface in pre-fatigue crack tip (PCT) region shows quasi-cleavage features, while fracture surface in stable crack propagation (SCP) region presents a ductile-fractured surface with dimples. The crack propagation path in SCP region is frequently deflected. Inhibition effect of grain boundaries on crack propagation and TRIP effect of retained austenite (RA) result in the improvement in fracture toughness of the SCP region.

Experimental and numerical studies on the mechanical properties and behaviors of a novel wood dowel reinforced dovetail joint

To improve the mechanical strength of dovetail joints, resisting and postponing the crack propagation, a novel method of enhancing the mechanical strength of the dovetail joint using a dowel-reinforced method was proposed. The connection technical parameters of the dowel-reinforced dovetail joint (DRDJ) were investigated and optimized based on experimental and numerical methods. The results showed that there were three typical mechanical behaviors of the DRDJ characterized, and each of them can be separated into three regions, i.e., elastic region, crack propagation region, and failure region, by the defined crack initiation point and critical fracture point. The effects of dowel inserting distance and depth on mechanical properties of the DRDJ was significant. The proposed method was validated experimentally to be an efficient approach to improving the crack initiation load, critical fracture load, and elastic stiffness, and delay crack propagation. The connection technical parameters of the DRDJ were analyzed based on response surface method and experimentally validated to be the distance of 18.56 mm and depth of 32.19 mm. Tensile strength of the DRDJ was seriously influenced by wood dowel species and dowel fit, and increased with the increase of dowel fit and mechanical strength of wood dowel based on numerical analysis. In conclusion, the dowel-reinforced method is capable of improving the mechanical strength of dovetail joints, resisting and postponing the crack propagation, and has an advantage on the risk of pre-warning of wood structure failure.

Interfacial fatigue fracture of elastomer bilayers under cyclic large deformation

Applications of stretchable polymeric soft materials, such as elastomers and hydrogels, in fields of biomedicine and soft devices frequently require strong bonding between these materials or these materials and other substrates. Methods of achieving the strong interfacial bonding have been well developed, but the fatigue behaviors of these bonding interfaces under cyclic large deformation have been rarely explored in both experiments and theories. In this work, an experimental methodology is presented to study the interfacial fatigue fracture of firmly bonded elastomer bilayers enabled by topological entanglements under cyclic large deformation. The relationship between the interfacial fatigue crack propagation speed v and the energy release rate G is obtained. Three regions are identified in this relationship: crack initiation, stable crack propagation, and catastrophic crack propagation regions. The threshold of energy release rate G0 for the crack initiation under cyclic large deformation is about 35 times smaller than the critical energy release rate Gc for the catastrophic crack propagation. Both in the crack initiation and catastrophic crack propagation regions, the logarithmic v increases with increasing the logarithmic G nonlinearly. While in the stable crack propagation region, the experimental points of logarithmic v at various logarithmic G yield a linear relationship with a positive slope. It is demonstrated that by designing the structure of the bonding interface or the bonding edge, the interfacial fatigue fracture of elastomer bilayers can be alleviated. The experimental methodology presented in this work can be utilized to study the interfacial fatigue fracture between stretchable materials of other types. The findings in this work help to reveal the mechanism of interfacial fatigue fracture between stretchable materials and the development in anti-interfacial fatigue bonding methods.

Low-temperature fatigue behavior and damage analysis of Al-7Zn-2Mg-1.5Cu alloy based on hysteretic energy

The fatigue performance of Al-7Zn-2Mg-1.5Cu alloy was investigated using strain-controlled low-cycle fatigue (LCF) tests at various low temperatures focusing on the effects of hysteresis energy on fatigue life and the analysis of corresponding parameters. The results show that the alloy exhibits different cyclic hardening ratio depended on the applied total strain amplitude. The cyclic hysteresis energy displays cyclic stabilization at different temperatures. The initiation of fatigue crack occurs on the surface of specimens and many fatigue striations can be found on fatigue crack propagation region. In addition, the hysteresis energy model can be used to evaluate fatigue life at different temperatures. Wherein, intrinsic fatigue toughness (W0) decreases obviously with the decrease of temperature, while damage transition exponent (β) increases slightly. Comparing with low temperature tensile test results, Wo is related to the static toughness and β has a relation with the yield strength ratio. The relationships between the dislocation configuration, fatigue crack propagation mode and two fatigue parameters are discussed from microscopic viewpoint.

High cycle fatigue life prediction model based on crystal plasticity and continuum damage mechanics for Ni-based single crystal superalloys under a multiaxial stress state

A high cycle fatigue life prediction model based on continuum damage mechanics, crystal plasticity and critical distance theory is developed. A novel damage evolution law for anisotropic Ni-based single crystal materials built on continuum damage mechanics is proposed. The crystal plasticity framework was introduced to take into account the particular characteristics of high cycle fatigue. The critical distance theory was innovatively applied to the structure of film cooling holes to predict the lifetime for materials under a multiaxial stress state. High cycle fatigue tests considering drilling technology, stress ratio and creep time were performed using Ni-based single crystal superalloy plates with film cooling holes. The constitutive equations were implemented into a finite element framework to simulate the high cycle fatigue properties of Ni-based single crystal superalloys, and the results of numerical calculations were compared with experimental observations. It was demonstrated that the constitutive formulations proposed in the present study can be used to model the mechanical behavior of Ni-based single crystal superalloys under a multiaxial stress state. The life prediction errors are found to be within the factor-of-three scatter band. The fatigue life of the specimen with ultrafast laser drilled film cooling holes is larger than that with electrostream machined film cooling holes under the same test conditions. The fatigue strength of the specimen with film cooling holes at a stress ratio R = 0.95 is higher than that at a stress ratio R = -1. The life of the specimen with film cooling holes is found to decrease with increasing creep time before high cycle fatigue. The γ/γ'-microstructure evolution in the specimens with film cooling holes is related to the kinetics of the stress redistribution during the fatigue process. A minimum stress level is essential for rafting to occur. The damage distribution for the plate model is in line with the crack initiation location and propagation region of the specimen.

Fatigue life prediction and fracture mechanism of styrene-butadiene-styrene thermoplastic elastomer under uniaxial tension

The fatigue life and prediction of styrene-butadiene-styrene thermoplastic elastomer (SBS) under the action of uniaxial tension was investigated, and the fatigue fracture mechanism was analyzed. With the increases of amplitude and frequency, the fatigue life of SBS decreases. The fatigue life of SBS under the amplitude and frequency was predicted, and the shift factor was applied to predict the fatigue life of SBS at the other frequency and amplitude. With the increase of temperature, the fatigue life decreases first and then increases. The fatigue fracture surface presents crack source region, crack propagation region, and instantaneous fracture region. At – 40°C, the crack source region is rough, and the crack propagation region and instantaneous fracture region are rough with the rib morphology. With the increase of temperature at 23°C, the crack source region is relatively flat, and the crack propagation region presents the undulant surface. With the temperature further increased up to 50°C, the fracture surface is very flat, and the shell lines could be clearly seen. The crack growth rate of SBS increases with the increase of temperature. The gel structure of SBS is formed at high temperature through the chemical crosslinking.

Microstructures and Fatigue Properties of High-Strength Low-Alloy Steel Prepared through Submerged-Arc Additive Manufacturing

Submerged arc additive manufacturing (SAAM) is a viable technique for manufacturing large and complex specialized parts used in structural applications. At present, manufacturing high-strength low-alloy steel T-branch pipe through SAAM has not been reported. This paper uses this technology to manufacture low-alloy structural steel parts. The microstructures of the samples were characterized, which revealed that they were mainly composed of polygonal ferrites. The tensile properties in the horizontal and vertical directions of deposits were studied. Results show that the horizontal tensile strength of deposits was quite close to the vertical one, while the elongation rate in the vertical direction was obviously lower than that in the horizontal direction. Fatigue results indicate that the strain fatigue limit of high-strength low-alloy steel samples in vertical direction was 0.24%. The fatigue fractures of fatigue samples of deposits showed multi-source crack initiation characteristics and the crack propagation regions exhibited typical fatigue striations, so the final instantaneous fracture region showed a ductile fracture. Fatigue performance is very important for the safe service of structural parts, but there is a lack of relevant research on this additive manufacturing part. The results of this paper may support the popularization of the SAAM for high-strength low-alloy steel T-branch pipe.

Thermomechanical fatigue damage behavior and deformation mechanism of coke drum with Cr-Mo steel

ABSTRACT Damage behaviour and deformation mechanism of SA387Gr11CL2 (1.25Cr-0.5Mo) low alloy steel for coke drum in the thermomechanical fatigue regime are investigated. The experimental results indicate that a main macroscopic crack is detected in the gauge section of each testing specimen. Multiple crack initiation sources are observed in the fracture surface. There are a great deal of secondary cracks in the crack propagation region of the fracture surface. Longitudinal-section microcrack initiates on the specimen surface and its propagation path is predominantly transgranular. When the crack propagation arrives at the threshold with the cyclic number increase, crack bifurcation is obviously observed. Fatigue is the dominant damage of SA387Gr11CL2 low alloy steel under the thermomechanical fatigue loading. The dominant deformation mechanism of SA387Gr11CL2 low alloy steel during thermomechanical fatigue cycle is wavy slip of dislocations. The types and morphologies of carbides in SA387Gr11CL2 low alloy steel are not changed after plastic deformation.

Jointed Rock Failure Mechanism: A Method of Heterogeneous Grid Generation for DDARF

The original DDARF (discontinuous deformation analysis for rock failure) can only generate uniform grids, and the increase in the number of grids reduces the efficiency of calculation, which limits the use of DDARF in large-scale geotechnical engineering. This is a problem that needs to be solved in the original DDARF. A new method is proposed in this paper to optimize the generation of grids in DDARF, and the optimized DDARF can generate heterogeneous grids. The model of the Brazilian disc-splitting experiment was established by using the optimized DDARF, fine grids were generated in the crack propagation region of the model, andsparse grids were generated at the edge of the model. The simulation results show that the Brazilian disc-splitting experiment simulated by the optimized DDARF is more consistent with the physical experiment than the original DDARF. The optimized DDARF and the original DDARF were used to generate a heterogeneous grid model and a uniform grid model, respectively, to simulate the uniaxial compression experiment. Through the analysis of the experimental results, it can be concluded that the optimized DDARF is more accurate in simulating the cracking and propagation of joints in rock blocks, the results of optimized DDARF are more consistent with the simulation results of other software, and the computational efficiency of the optimized DDARF simulation experiment is much higher than that of the original DDARF at the same time.