Fatigue Failure Mechanism Research Articles

Fatigue behavior of 316L stainless steel fabricated via Material Extrusion Additive Manufacturing

Material Extrusion Additive Manufacturing (MEAM) is a novel technique that widens the field of additive manufacturing applications in producing metal parts. There is limited knowledge about the process-induced properties of the metal parts obtained with such a process, especially when dealing with cyclic loading. The present work aims to assess the fatigue life of 316L components fabricated via MEAM, investigating the fatigue failure mechanisms. The resulting S-N curve shows a lower fatigue limit compared to wrought 316L. Internal defects are detrimental to the fatigue life since crack initiation sites are observed from the pores resulting from the deposition strategy of the material during the printing phase.

Method for Evaluating Bolt Competitive Failure Life Under Composite Excitation

In this study, the competitive failure mechanism of bolt loosening and fatigue is elucidated via competitive failure tests on bolts under composite excitation. Based on the competitive failure mechanism, the mode prediction model and “load ratio—life prediction curve” (ξ–N curve) of the bolt competitive failure are established. Given the poor correlation of the ξ–N curve, an evaluation model of the bolt competitive failure life is proposed based on Miner’s linear damage accumulation theory. Based on the force analysis of the thread surface and simulation of the bolt connection under composite excitation, a theoretical equation of the bolt competitive failure life is established to validate the model for evaluating the bolt competitive failure life. The results reveal that the proposed model can accurately predict the competitive failure life of bolts under composite excitation, and thereby, it can provide guidance to engineering applications.

Fatigue life and failure mechanism of nylon 66 cord/rubber composites under wide temperature range

This work simulates the real harsh service conditions of Fiber-reinforced polymer composites (FRPC), and the dynamic adhesion and failure mechanism of nylon 66 (PA66)-reinforced natural rubber (NR) composites under large deformation and wide temperature range are deeply studied by comprehensive analysis of the interfacial adhesion evolution, microstructure and failure damage location of the composites, interfacial heating rate, and modulus ratio of PA66/NR. The failure mechanism of PA66/NR composites is interfacial debonding at −20 °C, 25 °C and 80 °C, whereas that at 120 °C is matrix cracks. Interestingly, at −20 °C, the crack expansion at the interface is slow, and the samples show two-stage damage during fatigue, resulting in the highest fatigue life. At 80 °C, the interface heating rate and modulus ratio of the PA66/NR composites is the lowest, and thus the fatigue life is higher than that at 25 °C and 120 °C. Keywords: fiber reinforced polymer composites, fatigue life, failure mechanism.

The role of defect structure and residual stress on fatigue failure mechanisms of Ti-6Al-4V manufactured via laser powder bed fusion: Effect of process parameters and geometrical factors

Undoubtedly, one of the greatest revolutions in the last few decades has been the paradigm shift in design and manufacturing of components using additive manufacturing (AM) technologies. Despite the numerous benefits of metal AM, the presence of volumetric defects and the development of residual stress during manufacturing pose significant obstacles for producing load-bearing components critical for missions. Laser powder bed fusion (LPBF) has many process parameters, each potentially affecting volumetric defects and residual stress, which may lead to varying fatigue performance. In this paper, we comprehensively investigate the criticality of volumetric defects and residual stress on fatigue properties of AMed Ti-6Al-4V. Using X-ray computed tomography (XCT) and X-ray diffraction (XRD), we characterized volumetric defects and measured residual stress of the specimens, respectively. We performed extensive statistical analysis to correlate XCT, residual stress, and fatigue data, allowing us to identify the factors impacting components' fatigue life. Our observations indicate that residual surface stress plays a crucial role in the fatigue performance of components in the absence of defects at the “critical location”. Furthermore, we found that the pore location has a more significant impact than the defect size on fatigue fracture.

Contact fatigue performance and failure mechanisms of Fe-based small-module gears fabricated using powder metallurgy technique

Gears fabricated using powder metallurgy technique (PMT) have the potential to replace traditional wrought gears due to their very high forming efficiency and relatively low cost. However, their practical applications are restricted due to the lack of understanding of their fatigue properties and failure mechanisms. This work investigates the contact fatigue performance and corresponding failure mechanism of a typical Fe-based small-module gears fabricated by PMT. The fatigue tests were conducted under grease lubrication condition, using a power-opened small-module gear testing rig. The experimental results show that the current PMT fabricated small-module gears exhibit contact fatigue properties comparable to those of traditional wrought steel gears. It has been found that the subsurface cracks induced from pores are the dominant cause of spalling of PMT fabricated small-module gears during rolling contact. The cracks tend to propagate between adjacent pores, playing a vital role in the overall fatigue life. Thus, this study suggests that PMT can be a successful method for fabricating Fe-based small-module gears with excellent contact fatigue property if formation of pores during fabrication is well inhibited.

The low cycle fatigue property, damage mechanism, and life prediction of additively manufactured Inconel 625: Influence of temperature

AbstractSymmetrical high‐temperature low‐cycle fatigue tests were performed in this study to examine the influence of temperature on the fatigue failure mechanism of the additively manufactured Inconel 625. According to the fracture analysis, cracks initiate from the crystallographic plane at room temperature. At 600°C, oxidation of carbides leads to crack initiation. The strengthening effect of γ″ precipitation prevents crack propagating within the matrix. At 750°C, the crack becomes transgranular propagating, as a consequence of γ″‐δ transformation. Geometrically necessary dislocations distribution on the fracture supports this result. Furthermore, a modified life prediction formula considering effect of temperature and strain amplitude was established. It was observed that the modified model predicted the fatigue life of the studied materials well in both test conditions.

Microstructure-based interior cracking mechanisms and life prediction of additively manufactured Ni-based superalloy with temperature effect

The localized microstructure related fatigue cracking of additively manufactured (AM) Ni-based superalloy has been attracting increasing attention. Understanding the connection between the inhomogeneous microstructure and the fatigue characteristics with small deformation is crucial. Herein, the microstructure-related high cycle fatigue and very high cycle fatigue properties of an Ni-based AM superalloy were examined. The fatigue tests were conducted with stress ratio of 0.1 at room (25 ℃) and elevated (650 ℃) temperatures. To support the modeling of the internal fatigue failure mechanism, additional studies on microstructure features and mechanical properties were carried out in conjunction with electron backscatter diffraction, energy dispersive spectrometry, and monotonic tensile tests. Results of 2D fracture surface observation showed that the interior facet-induced fatigue crack nucleated under relatively low applied stress at both temperatures, which were assisted by the interior abnormal structure. Furthermore, the interior crack nucleation region consisting many facets was formed under shear stress and strain based on 3D restructure of fracture surface. Combined with evaluation of threshold stress intensity factor, the transition lengths from small crack to long cracks for surface failure at 25 ℃ and interior failure at 650 ℃ were evaluated. Based on the above analysis, the interior failure mechanisms were summarized related with microstructure characteristics. Finally, fatigue index parameters based on different failure modes were calculated to predict the fatigue life. The anticipated and experimental data are determined to be in satisfactory agreement.

Multi-field coupling multiple nonlinear vibration model and fatigue failure mechanism of deep-ocean mining hydraulic lifting pipe

Multi-field coupling multiple nonlinear vibration model and fatigue failure mechanism of deep-ocean mining hydraulic lifting pipe

Fatigue strength and life prediction of lower limb venous stents under three-stage loading conditions

After the implantation of lower limb artery stents, the complex loading conditions imposed on the limb can lead to fatigue failure, which may induce inflammation and restenosis. To investigate the effect of multi-axial loading conditions on the fatigue performance of stents, five stents, namely APsolute Pro (APbott Vascular, USA), Complete SE (Medtronic, USA), Protégé EverFlex (PE3, USA), Pulsar-35 (Biotronik, Germany), and E-luminexx-B (Bard, USA), were analyzed based on the finite element method (FEM). Besides, their fatigue strength was determined under three levels of loading conditions, including tension-bending-torsion and compression-bending-torsion. Based on that, the fatigue life of these stents was predicted. The results showed that based on the nominal stress method, tension-bending-torsion loading had a more significant impact on the fatigue life of stents than compression-bending-torsion loading. Besides, two different types of initial cracks were analyzed by the fracture mechanics method. The results suggested that both the initial crack and the external load were the main causes of stent fatigue fractures. Compared with the loading nature, the influence of the initial crack on stent fatigue life was more significant. Under the same loading condition, the APsolute Pro stent had the longest fatigue life, while the E-luminexx-B stent had the shortest. Moreover, the mechanism of stent fatigue failure was revealed by exploring the fatigue performance and life prediction of stents under complex loading conditions. These findings have important implications for improving the structural design of stents and their clinical selection.

Failure mechanism and life correlation of Inconel 718 in high and very high cycle fatigue regimes

High and very high cycle fatigue (HCF and VHCF) behaviours and failure mechanism of Inconel 718 were investigated via ultrasonic fatigue tests and microscopic characterization techniques. It is shown that the S-N curve continuously decreases in the HCF regime, while forming a plateau for VHCF. Cracks initiate from a single site on the surface or subsurface. Site-specific focused ion beam sampling and transmission electron microscopy characterization reveal that the crack initiation is governed by slip-induced localized plastic deformation. Microcracks initiate near non-metallic inclusions or slip band-grain boundary site. Plus, a life correlation model considering the size of micro-defect was proposed to better correlate the fatigue life.

Metallurgical evaluation and failure analysis of turbo expander Tie-bolt

Failure in a turbo-expander Tie-Bolt (stud) made of superalloy A-286 was investigated. The evaluation of the chemical composition and tensile properties confirmed compliance with the relevant standard. However, the coarsening of γ′ due to over aging condition, the non-uniform grain size and banded coarse MC carbide particles, the segregation of phosphorus to grain boundaries and the growth of carbide particles at the grain boundaries led to a decrease in fatigue strength of the alloy. Fractography studies showed that fracture occurred due to primary axial and secondary bending loads. Multiple ratchets and tire tracks on the fracture surface were consistent with low-cycle fatigue (LCF) failure mechanism. Fatigue striations spacing and micro hardness profile of the threads were used to calculate the stress amplitude and estimate the fatigue strength of Tie-bolt. Neuber’s rule was used to estimate the local stress and strain in the thread root. The calculation results showed that the failure occurred due to the application of cyclic loads at a stress level lower than fatigue strength of the stud. A large amount of sulfur detected at the multiple cracks initiation areas by EDS analysis, indicated the presence of H2S in the environment, which has caused cathodic hydrogen charge and reducedthe steel ductility and accelerated fatigue crack nucleation and growth.

Fatigue failure mechanism of Ti60 titanium alloy in HCF and VHCF regime at different temperatures

The failure mechanism of Ti60 titanium alloy in the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) at different temperatures was investigated. The results showed that, at different temperatures, there are two different fatigue failure mechanisms, i.e., the oversized facet and oxide shedding failure mechanisms. At room temperature, most of the fatigue cracks initiate at the oversized facet. At 300℃, the crack initiates at the oversized facet for the test with small applied stress, while the crack initiates at the oxide shedding site for the test with large applied stress. At 400℃, all the fatigue cracks initiate at the oxide shedding site. Higher temperature and stress will be conducive to the failure mechanism of oxide shedding to become dominant.

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.

Effects of intrinsic mechanical properties on the dynamic bonding strength of GLC films undergoing ball bearing contact fatigues

Wear-resistant films, which are widely applied to extend the service life of the gears and bearings, usually surfer from dynamic bonding fatigue failure induced by the Hertz-like contact. However, there is still lack of research on the influence of the main mechanical parameters of films on the dynamic bonding fatigue failure mechanism. In this work, graphite-like carbon (GLC) films with a low friction coefficient and different Ti doping ratios were deposited on the surface of stainless steel via direct current magnetron sputtering (DCMS) technology. The microstructure and mechanical properties of the films before and after vacuum annealing were investigated by SEM, Raman, and nanoindentation, and the dynamic bonding failure life of the films were evaluated by the ball-bearing-disc method with dynamic contact fatigue loads. The results revealed that the critical dynamic bonding strength exhibited a significant function dependence on the values of hardness-to-elastic modulus (H3/E2) parameters of the GLC films. The film failure mechanism showed a transition from the film/substrate interface spalling mode to the near-surface cohesion shear spalling mode with the increase of H3/E2.

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.

Experimental study on thermal fatigue damage and failure mechanisms of basalt exposed to high‐temperature treatments

AbstractUnderstanding the effects of thermal fatigue damage on the failure mechanisms of rocks is a key concern in underground engineering. The effects of high temperature on the physical–mechanical behaviors and the failure mechanism of basalt under uniaxial compression are investigated with a combination of acoustic emission (AE), computed tomography (CT), and scanning electron microscope (SEM). The high temperature heavily affects the physical–mechanical properties of basalt but has no effect on the mineral compositions. The evolution characteristics of inter‐event time function F(τ) and cumulative AE energy can be employed to characterize the fracture process of thermally damaged basalt. The damage mechanisms of thermal fatigue are attributed to the occurrence of intergranular cracks, intragranular cracks, and transgranular cracks and irregular holes within basalt. The failure mechanisms of basalt change from shear fracture to mixed tensile–shear fracture and finally to tensile fracture based on the statistical characteristics of low and high dominant frequencies.

Temperature-Moisture-Mechanical coupling fatigue behaviours of screwed Composite-Steel joints

This paper seeks to probe temperature-moisture-mechanical coupling fatigue behaviours of screwed composite-steel joints by using experimental and numerical methods. Novel temperature-moisture-mechanical (TMM) coupling constitutive model, fatigue failure criterion and progressive fatigue damage algorithm (PFDA) are first devised for simulating fatigue failure mechanisms and modes and for predicting fatigue lives of single-lap screwed composite-metal joints (CMJs) in arbitrary temperature-moisture environment. In order to validate the presented model and algorithm, tension–tension fatigue tests are then conducted on the CMJs in four representative environments: low temperature of −40 °C and dry (LTD), room temperature of + 23 °C and dry (RTD), room temperature of + 23 °C and wet (RTW) and elevated temperature of + 55 °C and wet (ETW), respectively. Finally, the TMM coupling fatigue behaviors of the CMJs are analyzed and discussed from the experiments and progressive fatigue damage modelling, and predicted results agree well with experimental findings, demonstrating the feasible and practical usage of the proposed model.

Mechanical fatigue of PLA in additive manufacturing

The manufacturing industry increasingly demands mechanical components, which must undergo different loads. Each industry has its own requirements and regulations, where the common demand is that the loads are repetitive according with the components functions. It is then well known that it is necessary to employ the fatigue assessment to evaluate if the component would not fail over its expected working life. In recent years, there has been an increasing interest in additive manufacturing due to it can be an alternative to manufacture designs more organics and enhanced shapes. However, the main weakness with this option is that the obtained components can fail under operational condition with repeated loads. Therefore, these components would not be used as a final product, so up to now this manufacturing process is not suitable for a large-scale production. It is also important to mention that any manufacturing process needs the fatigue prediction to estimate durability considering different sources of dispersion of properties such as the material and manufacturing process. The purpose of this investigation is to proposes an S-N life curve for PLA considering different printers and filament colours to include the scatter in the estimation. This S-N line curve can be used to understand the mechanism of fatigue failure, the structure of the failure was analysed. This curve is based on 5 regions of amplitude depending on the load levels. However, it was not possible to perform it as a percentage of ultimate resistance since it is presented a dispersion in quasi-static loads.

Parametric study on fatigue behavior of steel friction connections in shear

A parametric study on the fatigue behavior of steel friction connections was conducted in this paper. Fatigue failure mechanism of the connections and two periods of fatigue life were discussed in detail. The procedure to predict the fatigue life of steel friction connections was proposed using the finite element method, and was verified by the fatigue test results. A parametric study was then performed based on the prediction procedure. Considering the connection type, bolt related parameters, and plate related parameters, a total of ten cases were included in the parametric study. The results show that the fatigue strength of single lap connections is much lower than that of double lap connections, which is caused by the local bending of bolted plates and contact status change on the interface. Fatigue behavior of the single lap connections is greatly affected by the number of bolts, steel grade, and hole-making process, while the fatigue behavior of the double lap connections is greatly affected by the bolt preload, slip factor, main plate thickness, steel grade, and hole-making process.

Life assessment of thermomechanical fatigue in a woven SiC/SiC ceramic matrix composite with an environmental barrier coating at elevated temperature

In this work, the tension–tension fatigue behavior of a woven SiC/SiC ceramic matrix composite with an environmental barrier coating is investigated under elevated temperatures up to 1300 °C. Experimental investigations reveal the fatigue failure mechanism for SiC/SiC CMCs with EBCs under on-axis and off-axis loading conditions, and the materials undergo cyclic softening throughout the loading history. A mechanism-based framework is proposed to interpret the fatigue behavior of CMCs through detailed fatigue experiment observations. Based on the decrease in the elastic modulus of the composites, a new fatigue damage model is developed to characterize the fatigue damage evolution and assess the fatigue life of woven SiC/SiC CMCs with EBCs. The good agreement between the proposed model and experimental data indicates that the damage model has the potential to describe complex thermomechanical damage of SiC/SiC CMCs with EBCs under thermomechanical loading.