Fatigue Fracture Analysis Research Articles

EFFECTS OF TEMPERATURE AND STRAIN AMPLITUDE ON LOW-CYCLE FATIGUE PROPERTIES OF NICKEL-BASED SINGLE-CRYSTAL SUPERALLOY DD419

High-temperature and low-cycle fatigue tests were conducted on a nickel-based single-crystal superalloy DD419 under total strain-controlled conditions at 760 °C and 980 °C. The fatigue properties of the alloy are discussed by analysing the fatigue test data. Fracture morphology and dislocation structure were observed using scanning electron microscopy and transmission electron microscopy. At the same strain amplitude, the results indicate that the plastic deformation of the alloy is larger at 980 °C compared to 760 °C. This leads to a lower fatigue strength and shorter fatigue life, along with more severe damage. The value of the strain amplitude affects the cyclic stress response behaviour of the alloy. Under low strain amplitudes, the cyclic stress response behaviour differs between 760 °C and 980 °C. The hysteresis loop exhibits similar shapes at 760 °C and 980 °C, with an increase in the area as the strain amplitude rises. The fatigue fracture analysis indicates that micropores on the surface are the primary fatigue sources at 760 °C, while oxides on the surface are the main fatigue source at 980 °C, leading to cracking due to multiple sources. Moreover, transmission electron microscopy reveals that the deformation mechanism involving dislocations at 760 °C primarily occurs through plane slip and wave slip, whereas at 980 °C, dislocations mainly move through cross slip and climb.

Microstructural evolution and formation of fine grains during fatigue crack initiation process of laser powder bed fusion Ni-based superalloy

This study reports the fatigue failure mechanism at very-high-cycle fatigue (VHCF) regime of laser powder bed fusion (LPBF) GH4169 superalloy through a series of detailed microstructural characterizations, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), and energy dispersive spectrometer (EDS). Microstructural characterization of the solution and double-aging post-treated initial material exhibits process-induced imperfections (mainly pores), as well as γ′, γ′′ and δ precipitates, and carbide. The fatigue tests were performed using an ultrasonic fatigue tester (20 kHz). Fatigue fracture analysis suggests there exists a competitive fatigue failure mechanism with surface flaw initiation and internal pore initiation, corresponding to high-cycle fatigue (HCF) and VHCF regime, respectively. Focused ion beam (FIB) samples taken within the fatigue initiation area (FIA) revealed grain refinement and precipitate dissolution behavior. Based on the characterization results, the fatigue crack initiation mechanism was hypothesized: During numerous cyclic loading, the mobile dislocations shear γ′ and γ′′ precipitates, causing their dissolution and local chemical and mechanical alterations near internal pores. This enables twinning and promotes sub-grains formation. Sub-grains refine via localized continuous dynamic recrystallization (CDRX), forming a fine-grained layer that leads to crack initiation. This work reveals how precipitate dissolution contributes to VHCF crack initiation in LPBF GH4169 superalloy, highlighting the potential for extending alloy lifespan by adjusting precipitates and LPBF defects and incorporating these factors into fatigue life predictions for enhanced accuracy.

Enhancement of fatigue properties of selective laser melting fabricated TC4 alloy by multiple shot peening treatments

This study investigated the impact of multiple shot peening (SP) treatments on the surface morphology, microstructure, residual stress and fatigue performance of TC4 alloy manufactured by selective laser melting (SLM). The multiple SP treatments were conducted sequentially using cast steel beads, ceramic beads and glass beads at different peening intensities. In comparison to the conventional single SP process, multiple SP treatments reduced surface roughness and stress concentration coefficient while further increasing the residual compressive stress on the alloy surface. SP significantly enhanced the fatigue life of the alloy by 6.4 to 9.6 times, with a more pronounced improvement observed with multiple SP. Fatigue fracture analysis revealed that the untreated alloy showed multiple fatigue sources which all located on the surface, with surface printing defects acting as potential sites for crack initiation. After SP treatments, all fatigue cracks were initiated beneath the surface. The improved fatigue performance of SLM-formed TC4 alloy could be mainly attributed to the SP-induced surface microstructure refinement and residual compressive stresses. Furthermore, results indicated that, for subsurface crack initiation, the initial cracks primarily stemmed from the fracture of α/α' martensite in the primary β columnar grains.

Influence of Loading Waveform on the Fatigue Life of 34CrNi3MoVA Steel

Mechanical components often experience fatigue loading from various waveform conditions during their operational lifespan. However, the underlying mechanisms through which variations in loading waveform affect the fatigue life of components remain unclear. Thus, this study conducted tension–compression fatigue experiments on 34CrNi3MoVA steel specimens under the same stress amplitude with different waveforms (cosine, triangular, sawtooth, and reverse sawtooth) to investigate the effects of loading waveform variations on the cyclic strain hardening behaviors, the fatigue fracture failure, and the fatigue life. The results indicated that specimens under different waveforms all exhibited cyclic strain hardening. The fatigue cyclic hardening level progressively increased in the order of cosine, triangular, and sawtooth waveforms, resulting in a continuous increase in cyclic saturation strain amplitude. The analysis of fatigue fractures demonstrated a consistent increase in both the initiation and propagation zone areas in the order of cosine, triangular, and sawtooth waveforms, and the boundary between the propagation and final fracture zones gradually shifted from a straight to a curved shape. The influence mechanisms of cyclic loading waveforms on the fatigue life of specimens were analyzed based on the energy dissipation, leading to the development of a universal fatigue life prediction model applicable to different waveform conditions, the model was then verified with the reverse sawtooth wave specimens and resulted in a prediction error less than 15%. The study is expected to serve as a significant guide for predicting and evaluating the fatigue life of mechanical components under various fatigue loading conditions.

Effect of Heat Treatment on Interface Behavior and Fatigue Property of 316L/2Cr13 Multilayered Steel

In this study, multilayered stainless steel (MLS) composed of the austenitic stainless steel 316 L and the martensitic stainless steel 2Cr13 is prepared by the cumulative rolling method. Specifically, changes in interface and fatigue properties before and after the heat treatment are verified by scanning electron microscope, tensile test, and fatigue test. In the results, it is revealed that common grains appear at the interface after the heat treatment. Although the overall mechanical properties are improved, the interfacial bonding strength is increased and the plasticity is greatly improved after heat treatment, the fatigue properties are greatly decreased simultaneously. The main cause is that during the rolling process, large deformation leads to grain fragmentation as well as the generation of a large number of subgrains, resulting in higher dislocation density and increased deformation resistance. Therefore, the fatigue performance of MLS in as‐rolled state is better than that of MLS in the heat‐treatment state. In addition, fatigue fracture analysis indicates that cracks usually initiate at the 2Cr13 layer or at the interface between the 316 L layer and the 2Cr13 layer. Ultimately, in this study, the quadratic relationship between tensile strength and fatigue limit is analyzed through fatigue ratio to predict fatigue limit.

Fatigue fracture analysis of the 42CrMo4 crankshaft used for mining dump truck

A failure investigation was performed into the fractured 42CrMo4 crankshaft used for wide-body mining dump truck in this paper. Fracture morphology, chemical composition, microstructure and mechanical properties of the crankshaft material were studied to investigate the premature failure mechanism. All the crankshaft broke at the 4th or 6th necks and a high-cycle bending fatigue type in this case could be identified by the observation of beach marks and fatigue striations in the propagation area according to fractography results. The fatigue source is underneath the hardening layer of R fillet and the root cause could be identified as the designed inadequate thickness of quenching layer at the fillets obtained through medium frequency induction quenching (MFIQ), which could not provide adequate fatigue strength upon cyclic loadings for long term. Fatigue validation test indicated that the bending moment increased with the increasing thickness of the quenching layer, and the appropriate thickness was recommended about 3.5 mm based on overall consideration.

Effect of ultrasonic rolling on crack propagation behavior of Ti6Al4V titanium alloy laser welded joints

This study investigated the effect of ultrasonic rolling on the fatigue resistance of Ti6Al4V titanium alloy laser welded joints, with an emphasis on crack growth rate. Multiple analyses were conducted on samples subjected to different numbers of rolling passes, including analysis of surface roughness, morphology, cross-section structure, hardness, fatigue fracture, and residual stress. Ultrasonic rolling was found to considerably enhance the life cycle of the sample. One, three, and five rolling passes resulted in fatigue life increases of 17.14%, 46.38%, and 66.61%, respectively, when compared with untreated samples. Treated samples exhibited rough and intricate crack propagation paths on fracture surfaces, along with numerous secondary cracks. Fatigue striation width decreased by 48.6%, 62.5%, and 69.4% for samples receiving one, three, and five rolling passes, respectively. These fatigue striations, which were narrower and more densely distributed, indicate that ultrasonic rolling effectively increases fatigue life. The ultrasonic rolling treatment enhanced fatigue performance and reduced the rate of crack propagation due to changes in surface compressive residual stress, microstructure, microhardness, and improved surface quality. Crack retardation primarily occurred in initial stage of crack propagation and stable propagation stages, whereas inhibitory effects weakened during the final fracture stage due to superposition and offset of the higher driving force with compressive residual stress. In summary, the study demonstrates that treatment via ultrasonic rolling is an effective method for improving fatigue life and fracture toughness in Ti6Al4V titanium alloy laser-welded joints.

In vitro fatigue behavior and in vivo osseointegration of the auxetic porous bone screw

The incidence of screw loosening, migration, and pullout caused by the insufficient screw-bone fixation stability is relatively high in clinical practice. To solve this issue, the auxetic unit-based porous bone screw (AS) has been put forward in our previous work. Its favorable auxetic effect can improve the primary screw-bone fixation stability after implantation. However, porous structure affected the fatigue behavior and in vivo longevity of bone screw. In this study, in vitro fatigue behaviors and in vivo osseointegration performance of the re-entrant unit-based titanium auxetic bone screw were studied. The tensile-tensile fatigue behaviors of AS and nonauxetic bone screw (NS) with the same porosity (51%) were compared via fatigue experiments, fracture analysis, and numerical simulation. The in vivo osseointegration of AS and NS were compared via animal experiment and biomechanical analysis. Additionally, the effects of in vivo dynamic tensile loading on the osseointegration of AS and NS were investigated and analyzed. The fatigue strength of AS was approximately 43% lower while its osseointegration performance was better than NS. Under in vivo dynamic tensile loading, the osseointegration of AS and NS both improved significantly, with the maximum increase of approximately 15%. Preferrable osseointegration of AS might compensate for the shortage of fatigue resistance, ensuring its long-term stability in vivo. Adequate auxetic effect and long-term stability of the AS was supposed to provide enough screw-bone fixation stability to overcome the shortages of the solid bone screw, developing the success of surgery and showing significant clinical application prospects in orthopedic surgery. Statement of significanceThis research investigated the high-cycle fatigue behavior of re-entrant unit-based auxetic bone screw under tensile-tensile cyclic loading and its osseointegration performance, which has not been focused on in existing studies. The fatigue strength of auxetic bone screw was lower while the osseointegration was better than non-auxetic bone screw, especially under in vivo tensile loading. Favorable osseointegration of auxetic bone screw might compensate for the shortage of fatigue resistance, ensuring its long-term stability and longevity in vivo. This suggested that with adequate auxetic effect and long-term stability, the auxetic bone screw had significant application prospects in orthopedic surgery. Findings of this study will provide a theoretical guidance for design optimization and clinical application of the auxetic bone screw.

Effect of rivet arrangement on fatigue performance of electromagnetic riveted joint with [formula omitted]10 mm diameter rivet

Electromagnetic riveting (EMR) can effectively form large-diameter rivets and produce high-quality joints. In this paper, the effect of rivet arrangement on fatigue performance of electromagnetic riveted joint was investigated. Quasi-static shear tests, fatigue tests, numerical simulation and microstructure observation were carried out. The results showed that the rivet arrangement had little effect on the shear performance and had a great impact on the fatigue performance of the EMR joint. Specifically, the difference between the average maximum shear load of the V-joints (60.49 kN) and the H-joints (60.41 kN) was small. The fatigue life of the H-joints was higher than that of the V-joints at high fatigue load levels (Fmax>30.225 kN), while the result was opposite at low fatigue load levels (Fmax<30.225 kN). Furthermore, the fatigue fracture analysis showed that there were two typical failure modes for the joints: (I) the rivet fracture, and (II) the sheet fractured on the manufactured head side. The V-joints mainly displayed the coexistence of failure mode I and mode II, while the H-joints displayed failure mode II at all fatigue load levels. At high fatigue load levels, the significant stress concentration in both rivets resulted in the rivet fracture of the V-joints within a relatively short time. Due to multiple extension directions and a smaller extension zone, the fatigue life of H-joints was lower than that of the V-joints at low fatigue load levels. Therefore, the fatigue performance of the component could be improved by choosing a different rivet arrangement.

Crack initiation mechanisms and life prediction of GH4169 superalloy in the high cycle and very high cycle fatigue regime

In this study, the high cycle and very high cycle fatigue behavior of the GH4169 superalloy was investigated. The results of the fatigue tests demonstrated that failures in the high cycle fatigue regime were initiated from the material surface, whereas failures in the very high cycle fatigue regime had two competing modes of crack initiation: surface and internal. The fatigue fracture analysis revealed a quasi-cleavage fracture mode with facets for internal crack initiation. Additionally, the electron backscatter diffraction (EBSD) results indicated that the dislocations located at the internal twin boundaries of the material were preferentially activated under fatigue cyclic loading, resulting in significant strain localization at the twin boundaries. The {111} <110> slip systems with high Schmid factors had a higher probability of activation. The stress intensity factor (SIF) estimated based on rough area (RA) and facet sizes correspond to the fatigue long crack and short crack growth thresholds of the material, respectively. Finally, a modified FIP fatigue life prediction model was proposed based on the microstructure of crystallographic facets, and the results from experiment and the model prediction were found to be in good agreement.

Retracted: Mechanical Strength and Fatigue Fracture Analysis on Al-Zn-Mg Alloy with the Influence of Creep Aging Process

Retracted: Mechanical Strength and Fatigue Fracture Analysis on Al-Zn-Mg Alloy with the Influence of Creep Aging Process

Experimental study of fatigue behaviour of Q690qENH bridge weathering high-strength steel welded joints

The Q690qENH weathering high-strength steel has a considerable potential for bridge engineering due to its superior mechanical strength, toughness, and corrosion resistance. To evaluate its performance under fatigue loads, the high-cycle fatigue behaviour of Q690qENH base metal and its welded joints is studied. A total of 68 specimens are tested to obtain their stress–fatigue life (S−N) curves. Results show that the fatigue strengths of the base metal, butt-welded joint, and cross-welded joint are greater than those specified in the ANSI/AISC 360-16, Eurocode 3-2005, and BS 7608-2014 design standards. The most suitable S−N curve for evaluating the fatigue properties of the Q690qENH base metal, butt-welded joint, and cross-welded joint was deemed to be that given by ANSI/AISC 360-16. New S−N curves were proposed for the base metal and butt-welded joint, whereas the S−N curve for the cross-welded joint remained unchanged as per the existing codes. Additionally, the cumulative fatigue damage behaviour of the butt-welded and cross-welded joints was analysed, and a safety threshold based on the ratcheting strain rate was established to provide early warning at 85%–95% of fatigue life. Through digital image correlation and fatigue fracture analysis, the cause of fatigue fracture in the welded joints was determined to be caused by stress concentration at the weld toe.

Fatigue Behavior of M20 Torque Shear High-Strength Bolts under Constant-Amplitude Loading

Torque-shear high strength bolts were developed and widely used recently, and such high-tensile bolts may fracture in practical engineering due to the frequent complex loads, resulting in economic losses and even casualties. However, the fatigue performance of M20 torque shear high-strength bolts under constant-amplitude loading has not been investigated yet, and there are no specific design provisions for determining the constant-amplitude fatigue performance of such bolts. Hence, a total of 10 constant-amplitude fatigue tests were conducted using an MTS fatigue testing machine. For comparison, five different stress amplitudes were investigated. The fatigue performance, stress concentration and fracture analysis were analyzed. The scanning electron microscope images of fatigue failure were obtained to analyze the fatigue fracture characteristics of high-tensile bolts. A finite element model was established to analyze the stress distribution and the hot-spot stress of the bolts. The results suggested that the allowable nominal stress amplitude of M20 torque-shear type high-strength bolts was 96.371 MPa, while the allowable hot-spot stress amplitude was 283.296 MPa. Finally, the test results were compared against the existing design provisions. Upon comparison, the existing design formulas in GB 50017(2017), ANSI/AISC 360-16 (2010) and Eurocode 3 (2003) were found to be generally conservative. The S-N curve of torque-shear high strength bolts under constant-amplitude loading was proposed using the hot-spot stress amplitudes.

The in‐service fatigue fracture mechanisms for the I‐stage low‐pressure compressor disk of the aircraft engine D30KU‐154

AbstractThis paper contains the in‐service fatigue fracture analysis for the first stage low‐pressure compressor disk of the aircraft engines D30KU‐154. Based on the results of the fractographic investigation on collapsed compressor disks the fatigue crack initiation and propagation principles were established. It is shown that the crack initiation in the rim part of the compressor disk is due to a high frequency loading that leads to very high cycle fatigue fracture. The total fatigue lifetime of the compressor disk is determined by simultaneous action of low amplitude loading due to blades vibration, and high amplitude loading due to flight cycle (centrifugal forces). Study of in‐service fracture of the compressor disk was based on the evaluation of numerical simulations of the stress state in the damaged zone under corresponding loading conditions. The estimation of fatigue lifetime and crack path predictions were performed based on the multiregime fatigue fracture model developed by the authors.

Experimental investigation of ultra-low cycle fatigue behaviors of laminated rubber bearings in spatial grid structures

Ultra-low-cycle fatigue failure of laminated rubber bearings in steel grid structures is likely to occur due to an earthquake, which leads to a failure or even complete collapse. The present paper investigates the effects of different variables on the hysteretic behavior, skeleton curves, and stiffness degradation of bearings and anchor bolts. The variables include the vertical force and diameter of anchor bolts. For these purposes, ultra-low-cycle fatigue tests on laminated rubber bearings were carried out under constant vertical loading and horizontal reciprocating variable loading. In addition, the morphological analysis of fatigue fractures was performed to investigate the failure mechanism of anchor bolts in the bearings. It was found that the anchor bolt is the weakest part of the bearings during strong earthquakes. Cracks originate at the root of the anchor bolt and expand as the stress concentration increases, and a typical ultra-low-cycle fatigue fracture micro-morphology appears. Also, it was also found that increasing the vertical pressure can increase the peak value of the horizontal reaction force of the specimen and increase the energy consumption. It means that increasing the diameter of the anchor bolt can increase the stiffness of the bearing, increase the peak value of the horizontal reaction force, and slow down the stiffness degradation. Furthermore, a formula for the relationship between horizontal stiffness degradation and cumulative damage of a bearing is established. This formula can provide a reference for the seismic capacity evaluation of laminated rubber bearings used in spatial grid structures.

Reliability study for diesel engine cylinder head through fatigue failure analysis and structural optimization

Crack and failure are often captured in the cylinder head of diesel engine during high cycle fatigue test. In order to study the cause and improve the fatigue resistance, tensile test, metallographic observation and fracture analysis were carried out. In addition, the finite element method was applied to simulate the stress distribution and fatigue strength of the cylinder head. Meanwhile, the influence of load factors on the fatigue strength was analyzed. The structure was optimized on the finite element analysis. The results show that the material properties are not the main cause of cylinder head failure. High cycle fatigue failure of cylinder head is identified by the reason of working loads. Among the causes affect fatigue strength, gas force was determined as the dominant factor by changing stress amplitude, while thermal load as the secondary factor by changing mean stress. Additionally, the structural optimization measures for the cylinder head were put forward according to the failure causes, and the safety coefficient of the vulnerable parts was increased from 0.88 to 1.04, meeting the design requirements. This study can provide a reference for the engine reliability analysis.

Failure analysis of fatigue fracture for 60Si2Mn steel fastening clip in the track of high-speed railway

A 60Si2Mn steel fastening clip in the track of high-speed railway fractured abnormally during service, and its service life was 8 years. The microstructure and the fracture morphology of the fractured fastening clip were analyzed. In addition, the Finite element model was built to simulate and analyze the stress of the fastening clip in the service process. The results showed that the microstructure of the failed clip was normal, which was uniformly distributed tempered troostite and residual ferrite. The depth of decarburization layer and hardness of fractured clip were 115 μm and 44.7 HRC, respectively, which satisfied the standard requirements. The inclusions were also not found at the crack initiation on the fracture. However, significant rusting occurred at the fracture source and the anticorrosive coating was damaged in this position, which led to the initiation of fatigue cracks. The finite element simulation results of the stress showed that the stress on the fracture position was smaller as 600 MPa under the service condition. The maximum stress position of clip was 1459 MPa in another position, but the cracks was not formed in the position with the highest stress. Therefore, it was proved that the crack was caused by corrosion. The analysis of the normally fractured fastening clip in service showed that the position of fatigue fracture was consistent with the results of the finite element simulation. It was suggested to optimum and strictly control the process of spraying anticorrosive coating thus to obtain a uniform anticorrosive coating on the surface.

Vibration fatigue life and fracture analysis of 2024-T351 aluminum alloy treated by cryogenic laser peening.

A Nd:YAG nanosecond pulsed laser was used to carry out laser peening and cryogenic laser peening (CLP) experiments at room temperature (25°C) and cryogenic temperature (-100∘C), respectively, on the vibration fatigue life of 2024-T351 aluminum alloy. It was found that CLP induced a higher amplitude of compressive residual stress and improved the nano-hardness and the elastic modulus. Compared to the LP-ed samples, the fatigue life of the CLP-ed specimens was higher, and the initiation and propagation of fatigue cracks were more effectively inhibited. Therefore, CLP can effectively improve the fatigue performance of aluminum alloys.

Crack initiation and growth simulations of functionally graded dental implant subjected to cyclic axial loads

AbstractTwo essential characteristics in designing dental implants are having high fracture resistance and being compatible with jawbone. One of the best implant choices to achieve this aim is titanium‐hydroxyapatite functionally graded (FG) material. Also, fracture of implants under cyclic loads is one of the most common causes of destruction of dental implants. However, the crack initiation location and growth path as well as the optimum material distribution for the FG implants are questions that have not been answered yet. The aim of this study was introducing a new and powerful numerical approach to predict the crack initiation behavior and progressive damage in the FG dental implants, subjected to cyclic axial loadings. The approach is developed by employing extended finite element method (XFEM) in ABAQUS software, Visual Studio, and FORTRAN Compiler. Also, the Python code was modified for crack growth modeling in fatigue fracture analysis based on Paris law. The result showed that the micro‐cracks below the outer surface of FG implant are created, due to the application of cyclic load, which is invisible. This research shows that FG implants with power factor less than 1 show much more resistance to crack growth under pre‐tightening and cyclic loadings.

Substantiation of the Choice of Materials for Surfacing Based on the Fractographic Analysis of Fatigue Fractures

Substantiation of the Choice of Materials for Surfacing Based on the Fractographic Analysis of Fatigue Fractures