Fatigue Crack Initiation Stage Research Articles

Crack initiation life of aviation gear pair

Abstract The gear transmission system is an important part of the aero-engine. The most serious fault is tooth root fatigue fracture. This paper mainly discusses the time of fatigue crack initiation stage of aviation gear tooth root position. The crack initiation state can be evaluated by analyzing the root stress. The static contact analysis is solved by using the generated aviation gear pair model. The maximum stress in the bending direction of gear teeth is calculated. The changing pattern of tooth root stress with meshing position is described in detail. According to the multi-axial stress fatigue theory, the surface damage parameters and fatigue life of the gear tooth root position are calculated, and the location of the crack initiation is obtained. Fatigue cracks often appear at half of the gear tooth width and near the tooth root circle.

High-cycle fatigue behavior of Haynes 282 superalloy subjected to accelerated ageing

The room temperature high-cycle fatigue behavior of Haynes 282 subjected to accelerated ageing at 750, 800 and 850 °C for different durations was studied. Ageing promotes the growth of γ’ precipitate and the formation of M6C and M23C6 carbides, topologically closed packed µ phase, but has limited influence on the grain growth. Although, fatigue strength increases with increasing ageing duration, at both 750 °C and 800 °C, contrasting variations in the same were observed with increasing ageing temperature. By measuring the fatigue striation spacings on fractured surfaces and using the results of a finite element model developed for unnotched bend specimens available in the literature, the Paris slopes, m, of aged and unaged samples were determined to be in the range of 3–4. The fatigue crack initiation stage, which is > 99 % of the total fatigue life, increases with increasing size of γ’, except for alloys aged at 800 °C and 850 °C. Deviations in the trend are attributed to the evolution of M6C carbides at temperatures > 800 °C at the MC/γ interface. Implications of these results are discussed in the context of predicting the fatigue strength of Haynes 282 utilized in service for long durations.

Enhancing fatigue performance of TiN-coated Ti-6Al-4V by N/Ti ratio modulation

In this paper, we propose an approach to enhance fatigue properties of TiN-coated Ti-6Al-4V for aero-engine compressor blades by N/Ti ratio modulation of TiN. TiN coatings with N/Ti ratios of 0.70, 0.85, and 0.92 were prepared by modifying the nitrogen flow during coating deposition. The resulting coatings were evaluated for surface roughness, phase structure, microstructure, hardness and residual stress. Furthermore, the rotating bending fatigue test indicated that TiN coatings at different N/Ti ratios negatively affected the fatigue properties of Ti-6Al-4V. Specifically, the fatigue limit of the substrate dropped from 531 MPa to 480 MPa, 510 MPa, and 440 MPa when coated with TiN coatings at N/Ti ratios of 0.70, 0.85, and 0.92, respectively. However, the high-temperature insulation process during coating preparation enhanced the substrate's fatigue performance. The reduced fatigue performance of TiN coated Ti-6Al-4V was attributed to the cracking of TiN, led to the fatigue crack initiation on the substrate surface and accelerated fatigue failure. In the fatigue crack initiation stage, the TiN coating's higher stress concentration factor accelerated crack initiation by inducing stress concentration, reducing initiation cycles. During fatigue crack growth, the coating's residual compressive stress hindered propagation and reduced crack length caused by coating fracture in the substrate.

Crack Initiation Mechanism of AISI 4340 Steel for High‐Cycle Torsional Fatigue Loading

The nucleation of microstructural fatigue cracks in AISI 4340 steel is investigated. The pre‐electropolished specimen is employed for the observation of the very early stage of fatigue crack initiation correlated with the microstructure. It is found that there are precipitate‐free zones (PFZs) around the lath and packet boundary, and the strength of PFZ is lower than that of the lath due to the lack of second‐phase strengthening. Therefore, the fatigue crack would first initiate within the PFZ. Besides, the initiated crack plane usually is the maximum shear stress plane. However, the MnS inclusions would cause stress concentration so that the lath boundary (LB) deviating from the maximum shear stress plane could also have sufficient driving force to crack. In contrast, when the LB is vertical to the cracked MnS inclusion, it would prevent the crack from propagating into the matrix because of the anisotropy of lath martensite. The effects of the stress concentration caused by the MnS inclusion on the maximum deviation angle and the S‐N curve are discussed.

Flexural fatigue behavior of butt-welded circular concrete-filled double skin steel tube (CFDST)：Experimental study and numerical modeling

Detailed understandings on the fatigue behavior of the concrete-filled double skin steel tubes (CFDSTs) under multiaxial stress states are essential to promote their applications in marine structures. A systematical investigation consisting both the experimental study and the according numerical modeling has been conducted. Physical tests were carried out to investigate the flexural fatigue behavior of the butt-welded hollow steel tubes (HSTs), concrete-filled steel tubes (CFSTs) and CFDSTs, in which the development of fatigue cracks and the fatigue life were captured. The feasibility of applying the existing S–N curves originally obtained from the HSTs to the constitutive steel tubes within the CFDSTs has been consequently verified. A two-stages simulation method was developed to analyze the full range development of fatigue cracks based on both the damage mechanics and the extended finite element method (XFEM). The influence of the multiaxial stress states on the fatigue behavior for the constitutive steel tubes was studied quantitatively, considering the offshore application scenarios where the steel tubes within the CFDSTs were subjected to larger external hydrostatic pressure or internal transmitted content pressure. The results show that the existence of the infilled concrete can effectively improve the fatigue behavior of the steel tubes. The life prediction models for both the fatigue crack initiation stage and propagation stage have been proposed, where the crack initiation life of the steel tube may reduce by 30% when its stress triaxiality increases from 0.36 to 0.48.

Research on the Fatigue Performance of Composite Girders with CSW and ST Truss

To study the fatigue performance of a composite girder with corrugated steel web (CSW) and steel tube (ST) truss, based on an engineering object, a test and finite element (FE) analysis were carried out. The fatigue failure mode and the sensitivities of the hot spot stress to the geometric parameters of the composite girder above were studied. The position with the maximum hot spot stress and the fatigue crack position were discussed and determined. Furthermore, a fatigue life evaluation method was discussed. It was found that linear extrapolation was applicable to finding the hot spot stress. The fatigue performance evolution comprised three stages, including initiation, expansion, and failure of the fatigue crack. The fatigue crack initiation stage accounted for about 95.6% of its fatigue life, and the fatigue crack was opening type I. The maximum hot spot stress emerged at the endpoint of the inclined web of CSW (point S). The main reason for the behavior was that the inclined web had a small out-of-plane flexural stiffness but had to bear an out-of-plane bending moment. Moreover, for the same section of the CSW, the hot spot stress on the molding side was less than that on the non-molding side. The hot spot stress exhibited negative correlations with the chord diameter, chord thickness, and bend radius, meanwhile, it had the highest sensitivity to the chord diameter. The existing S-N curves were not suitable to evaluate the fatigue life of the composite girder with CSW and ST truss. Since the intersecting weld between the CSW and the chord was located in longitudinal welded joints, the hot spot stress method suggested adopting the basic fatigue strength of 100 MPa, corresponding to 2,000,000 load cycles. Meanwhile, an empirical calculation model of the S-N curve for the composite girder with CSW and ST truss was created.

Research on Fatigue Properties and Fracture Mechanism of δ-TRIP Steel

In this work, a new type of δ-TRIP steel was designed. The fatigue limit and S-N curve were measured by the fatigue testing machine. The fatigue fracture was observed by the scanning electron microscope (SEM), and influencing factors of fatigue crack initiation were analyzed. The results show that the fatigue limit of δ-TRIP steel is 361MPa, and fatigue cracks are initiated on the surface of the sample. In the initial stage of fatigue crack initiation, the crack growth rate of the specimen is relatively slow under the action of stress. As the crack continues to expand, the magnitude and direction of the stress on the crack tip change and the stress situation becomes more complicated. The crack begins to branch and grows in different directions The metastable retained austenite in δ-TRIP steel undergoes phase transformation under the action of external force, which will absorb a lot of energy. At the same time, there are many large angle grain boundaries in the steel hindering the crack propagation. These two characteristics make the δ-TRIP steel has excellent fatigue performance.

Research on high cycle fatigue damage characterization of FV520B steel based on the nonlinear Lamb wave

In this study, high-strength steel FV520B sheets were subjected to high-cycle tensile-tension fatigue experiments at room temperature in order to obtain fatigue-damaged specimens. Then Lamb waves were used to perform nonlinear ultrasonic testing on them to obtain the normalized relative nonlinear coefficients, β′/ β0. The corresponding relationship between β′/ β0 and the percentage of fatigue life was obtained. Finally, the microstructural changes of the damaged samples were observed by a scanning electron microscope in order to explore the correlation mechanism between β′/ β0 and the degree of micro-defects and fatigue damage. The experimental results showed that as the number of fatigue cycles increased, β′/ β0 first slowly rose, then quickly rose to reach a peak, and finally declined, which was consistent with the generation and propagation of dislocations and cracks during fatigue damage. As the length and number of microcracks increased, β′/ β0 also increased, especially in the initial stage of fatigue crack initiation. β′/ β0 was very sensitive to the size of fatigue cracks, so the change of β′/ β0 can be used to detect the degree of early fatigue damage of the material.

The role of microstructure and local crystallographic orientation near porosity defects on the high cycle fatigue life of an additive manufactured Ti-6Al-4V

Titanium alloys such as Ti-6Al-4V built by most of the additive manufacturing processes are known to contain process induced defects, non-conventional microstructure and strong crystallographic texture; all of which can affect the fatigue strength. In this study we evaluated the effect of crystallographic orientation of α and α lath width around gas pore defects on the high cycle fatigue life of Wire + Arc Additive Manufactured Ti-6Al-4V by means of Electron Back Scattered Diffraction. Here we show that variations in crystallographic orientation of α lath and its width in the vicinity of the crack initiating defect were the main reasons for the considerable scatter in fatigue life. Pyramidal slip systems with high Schmid factor active around the defects resulted in longer fatigue life compared to pyramidal slip with lower Schmid factor. In the absence of pyramidal slip, cracks initiated from active prismatic slip systems. When considering the influence of the microstructure, a higher number of smaller α laths around the defect resulted in longer fatigue life, and vice versa. Overall, the fatigue crack initiation stage was controlled collectively by the complex interaction of porosity characteristics, α lath width and its crystallographic orientation at the crack initiation location.

Metal Magnetic Memory Testing of the Drilling Riser Pipeline Steel Based on Pulsating-Impact-Fatigue Test

To know the fatigue damage state of X80 pipeline steel which is used for the drilling riser and prevent the occurrence of fatigue fracture accidents. Pulsating-impact-fatigue tests were conducted in laboratory, magnetic memory signals were measured during the whole fatigue process. The distribution of stress and magnetic field vector with different crack depth was analyzed by using finite element (FE) method. During fatigue crack initiation stage, magnetic signals increased slowly with fatigue loading cycles because of the effect of stress-magnetization. When fatigue crack appeared, magnetic signals were mainly affected by magnetic flux leakage (MFL) which was induced by fatigue crack. Magnetic memory characteristic parameters had a good linear relationship with fatigue crack depth. Magnetic memory signals were able to reflect the change of fatigue damage status.

Improvement mechanisms of fatigue strength in weld joints by hammer peening

The hammer peening process is well known as one of the methods to improve fatigue lives in welded joints. Peening induces a compressive residual stress field near the weld toe in which fatigue crack usually initiates. When this method is applied to large scale welded strictures, the effects of the induced compressive residual stress on fatigue crack initiation stage and propagation stage should be separately comprehended because the balance of both stages in total fatigue life will be changed for different structure scales. In this study, the improvement mechanisms in fatigue crack initiation and propagation lives due to a compressive residual stress field was clarified by experimental observations of crack propagation behavior. The morphologies of propagating surface crack front were quantified in the specimens of plate and gusset welded joint with and without the hammer peening process. The experimental results suggested that the deeper aspect ratio of an elliptical surface crack just after crack initiation and propagation in the peened specimens induced fatigue crack retardation. Fatigue crack retardation in the peened specimens was discussed to clarify the fatigue crack life improvement mechanisms from the viewpoint of the stress intensity factor range suppression due to the crack aspect ratio.

The early stage of fatigue crack initiation in a 12%Cr martensitic steel

The objective of the paper is to understand the mechanism of early stage crack formation in the low cycle fatigue of a martensitic steel with a complex and hierarchic microstructure. As a result of interrupted test at a total strain range Δεt=1.2% at room temperature, the evolution of deformation relief represented by extrusions and intrusions was characterized by Atomic Force Microscopy and Transmission Electron Microscopy on the surface and in the bulk of specimen. A mechanism of extrusion-intrusion creation based on the vacancy diffusion induced redistribution of matter and plane slipping was proposed. The extrusion-intrusion pair leads to the martensitic steel cracking in the interfaces of microstructure.

Experimental Study of Fatigue Crack Initiation and Strain Evolution around a Micro-Void by <i>In Situ</i> SEM and Digital Image Correlation

Fatigue crack initiation stage occupies a large proportion of total fatigue life in modern engineering materials and structures which are often designed under lower service loading conditions. In this paper, the fatigue crack initiation behavior from a micro-void in a small-scale specimen was studied in-situ in SEM. Surface morphologies were monitored in-situ and images were taken during interrupted tests at selected number of cycles, and displacement and strain map around the void was calculated based on digital image correlation (DIC) technique. The results indicated that the strain evolution near the micro-void could be divided into stages, before crack initiation. The strain increasing rate was fast in the early stage and slower in the second stage. A critical cyclic strain value for fatigue crack initiation from the micro-void was obtained around 9%, and was believed to be the dominant factor for early stage of fatigue damage.

Rotating bending fatigue behaviour of a Ni3Al-based single crystal alloy IC6SX at 760°C

The high cycle fatigue behaviour at 760°C of a Ni3Al-based single crystal alloy IC6SX has been studied in the present investigation. The specimens for the fatigue tests were prepared by screw selection crystal method in a directional solidification furnace. The rotating bending fatigue tests of the specimens was carried out at the temperature of 760°C in air, the stress ratio of R (σmax/σmin) was − 1, and the rotating speed of the fatigue tests was 6500 r min− 1 (108 Hz). The stress–fatigue life cycle (S − Nf) curve was obtained based on the fatigue tests, and the fracture surfaces were examined using scanning electron microscopy (SEM). The results showed that the fatigue cracks initiated from the surface defects of the specimens and the cracks propagated along the (111) crystal plane. The fatigue fracture was composed of three different characteristic regions corresponding to the three fatigue stages, including fatigue crack initiation stage, steady-state propagation stage and the fast propagation stage. The research results also showed that the rotating bending fatigue performance of alloy IC6SX is superior to that of the second-generation single crystal nickel-based superalloys containing 3 wt-% Re.

Degradation of sensing properties of fiber Bragg grating strain sensors in fatigue process of bonding layers

Bonding layers, serving as the strain transmission mediums, may bring undesirable effects to the sensing properties of fiber Bragg grating (FBG) strain sensors during their fatigue process. To analyze the strain sensitivity and the reflected spectrum of FBG strain sensors in different fatigue stages of bonding layers, their strain sensitivities were derived according to strain transfer models. Resorting to T-matrix formalism, the affected reflected spectra were stimulated. Theoretical analysis results show that there is a gradual decline in the strain sensitivity during the stage of fatigue crack initiation, and that significant distortions emerge in the reflected spectrum during the stage of fatigue crack propagation. In addition, a cyclic loading fatigue test was conducted and the phenomenon observed in the test showed a good agreement with the theoretical prediction.

Partition of Cyclic Plasticity in an Austenitic-Ferritic Duplex Stainless Steel and its Significance for Fatigue Crack Initiation under High and Very High Cycle Fatigue Loading Conditions

The present study documents that at loading amplitudes close to the fatigue limit, cyclic irreversible plastic deformation in form of slip band generation in the austenitic-ferritic duplex stainless steel X2CrNiMoN22-5-3 (318 LN) mainly takes place in few austenite grains without any microcrack initiation in these grains. This was shown by means of focused ion beam (FIB) cutting in combination with high resolution scanning electron microscopy (SEM) at pronounced extrusion-intrusion-pairs in several austenite grains. Investigations by means of confocal laser scanning microscopy (CLSM) revealed that the slip band density in these grains increases with the number of loading cycles and remains constant in the very high cycle fatigue (VHCF) regime. Under such loading conditions, fatigue cracks frequently initiate in the ferrite phase due to anisotropy stresses which are strongly superimposed by stress intensifications at the tip of austenite slip bands. TEM investigations revealed that austenite slip bands, which are piling up against phase boundaries, cause localized dislocation generation and motion in neighboring ferrite grains. The cyclic irreversible motion of these dislocations on several parallel slip planes is correlated with the stage of fatigue crack initiation. A crystal plasticity model based on a finite element program, which considers anisotropic elasticity, allows for the determination of crack initiation sites in real microstructures according to the above mentioned mechanisms. Crystallographic orientations, measured by means of the electron back scatter diffraction (EBSD) technique, serve as input parameters for the calculations regarding microcrack initiation as well as for the analysis of the subsequent short fatigue crack propagation, which is strongly affected by microstructural barriers such as grain and phase boundaries.

Thermal Fatigue Crack Initiation of Laser Deposited High-temperature Titanium Alloy Ti60A in 20–700 °C

AbstractThermal fatigue damage of high-temperature titanium alloys is of great concern for severe temperature-fluctuating environment, and the thermal fatigue crack initiation stage plays a crucial role in thermal fatigue life. In present study, thermal fatigue tests keeping 55 seconds at 700 °C followed by water cooling 15 seconds at 20 °C were performed for the laser deposited high-temperature titanium alloy Ti60A (Ti5.54Al3.38Sn3.34Zr0.37Mo0.46Si). Thermal fatigue cracks initiate after 800 thermal fatigue cycles with a length of 20 µm. Subsequently numerous cracks grow to 500 µm and cause severe degradation after 1000 cycles. To investigate the crack initiation behavior, microstructural changes during thermal fatigue process were examined by OM, SEM, EPMA and TEM. Thermal fatigue cracks initiate preferably at grain boundaries, α/β interfaces, microvoids, and abnormal coarsened α produced by oxygen interstitial solution. Mechanisms of thermal fatigue crack initiation are related to compatibility of local deformation and microstructural changes during thermal fatigue process.

Insights beside assessments into small volume cyclic response of elastic plastic systems

In contrast to monotonic load studies, the engagement with a cyclic time dependent processes of small volume material response has been restrained. Crack tolerance in thin films almost by definition is restricted. Nevertheless, in terms of structural stability the fatigue resistance remains a critical concern regardless the scale. The cyclic displacement amplitudes can be enhanced by remote mechanical stress/strain or thermal origins resulted in similar outcome of cumulative irreversible micro-plasticity damage. Thus, the cyclic life in nano applications is mainly dominated by the crack nucleation controlled process. The current study is centered on the fatigue crack initiation stage as a first order design pillar and therefore require additional experimental input. Activities in elastic-plastic polycrystalline and single crystal systems in various crystal structures are described. Beside findings regarding the fatigue cracks initiation life, the role of compressive residual stress on the cyclic response has been also analyzed. The fatigue crack initiation criteria have been associated to other constitutive assessments that are related mainly to the crack stability equation elements. In addition, completely different insights are introduced into the concept of the critical film thickness, indicating the susceptibility to the crack formation. As addressed in heteroepitaxial layers, specific analogous characterization exists either in dislocation activities or by triggering crack formation. The physical aspects of threading dislocations growth and glide behavior and local decohesion ramifications are highly dependent on the layer thickness. Experimentally based, under strain control conditions at ambient temperatures the aforementioned fatigue crack initiation stage in small volume segments, obeyed a cumulative damage mechanism. It was also established that the low energy dislocation structures and the initiation life were dramatically dependent on the strain amplitude range. The role of compressive residual stress in prolonging the fatigue initiation life was also consistent. Even in silicon based materials, dislocation activities prevailed. It became apparent that categorical generalization in nano-silicon thin film fatigue behavior is beyond the susceptibility to the native oxide or environmental interaction. Issues like film processing methodology, film/substrate interfaces, residual stresses and micro-cracking feasibility occurrence as wall as critical sites still require further design insights.

A Practical Method for Fatigue Crack Initiation Detection Using an Ion-Sputtered Film

The development of fatigue in a machine element can be divided into the fatigue crack initiation stage and the crack growth stage. The effects of the surface conditions, such as roughness and surface qualities, on the fatigue strength are considered to be restricted to the fatigue crack initiation stage. Thus, these effects can be evaluated exactly by the investigation on the fatigue crack initiation life. In this research a practical method is presented for detecting fatigue crack initiation during fatigue tests using an ion-sputtered film. Ion-sputtered films were formed on acrylic and steel test pieces having a specially designed round notch. Three-point bending fatigue tests were performed and the instants of fatigue crack initiation on the notched surface of both acrylic and steel test pieces during the tests were identified clearly; thus the fatigue crack initiation life can also be determined experimentally using the presented method. In addition, since the system used to measure the electric resistance of the film, which indicates the crack initiation, is extremely simple, that is, it consists of a variable resistor, a DC power source, and a data recorder, this method is considered very practical for monitoring crack initiation.

Effect of magnetic treatment on fatigue life of AISI 4140 steel

The effect of magnetic field with different intensities on fatigue life of AISI 4140 steel has been investigated. For this purpose, unnotched and notched specimens were prepared from AISI 4140 steel. It was obtained that when the magnetic field especially was applied to the specimens at the stage of fatigue crack initiation, the fatigue life improved by reason of delaying formation time of slip bands due to regular direction and distribution of magnetic domains. And, it was observed that magnetic field was applied from the beginning of fatigue test up to occur the fatigue fracture, it was detrimental on the fatigue life because of over heating of the specimens.