Fatigue Crack Propagation Research Articles

Analysis of the crack initiation and short crack propagation of laser cladding IN718 enduring fretting fatigue at 650 ℃

In order to reveal the behaviors of crack initiation and short fatigue crack propagation in laser cladding IN718 under high temperature conditions, very high cycle fretting fatigue (VHCFF) experiments were conducted at 650 ℃ with a transverse clamping force of P=400N and different axial stress loading conditions to obtain S_N data. The surface wear morphology and fracture morphology characteristics were analyzed by scanning electron microscope (SEM) and white light interferometer. Additionally, the microstructure characteristics of grains in the short fatigue crack region were examined through electron back scatter diffraction (EBSD). Parameters such as equivalent grain size, grain elastic modulus, Schmid factor, geometric compatibility factor, twist angle, and tilt angle were considered in the analysis of the characteristics of fatigue crack initiation and propagation. The results indicate that fretting fatigue cracks tend to initiate on grains with higher Schmid factors, larger equivalent grain size, and lower grain elastic modulus on the fretting contact surface. Moreover, the short fatigue cracks tend to propagate along the grains with the microstructural features of the high Schmid factor, the geometric compatibility factor of m′>0.4, the twist angle of α<50°, and the tilt angle of β<50°.

An adaptive multi-patch isogeometric phase-field model for fatigue fracture

Fatigue fracture is one of the main causes of structural failure under cyclic loading, so accurate evaluation of fatigue fracture is very important for the design of structures subjected to cyclic loading. In this work, we propose an adaptive fatigue phase-field model within the framework of multi-patch isogeometric analysis to simulate crack nucleation and propagation in brittle materials under cyclic loading. Multiple non-uniform rational B-splines (NURBS) patches are used to exactly represent complex structures, and the Nitsche’s method is adopted to enforce the compatibility of the displacements, stresses and phase-field variables between two patches. The coupled governing equations of fatigue phase-field model are established based on the Nitsche’s method, and the selection of Nitsche’s parameters are determined from numerical experiments. A refinement-correction adaptive approach based on locally refined non-uniform rational B-splines (LR NURBS) structured mesh refinement strategy is developed to alleviate the computational burden caused by the requirement of small element size around crack surface and the large number of fatigue loading. The proposed multi-patch isogeometric phase-field model can deal with the coupling between equal/unequal length interfaces, thus the fatigue crack propagation simulation in arbitrary complex structure can be implemented. Several numerical examples are investigated to verify the practicality of the multi-patch isogeometric phase-field model and the effectiveness of the adaptive scheme.

Fatigue Reliability Assessment for Orthotropic Steel Decks: Considering Multicrack Coupling Effects

Multiple fatigue cracks are generally present in practical engineering due to the existence of welding; the size and number of cracks of orthotropic steel bridge decks are greatly uncertain. The component failure conditions caused by these cracks may have correlations. Currently, it is still a challenging issue to develop a physical model of multiple fatigue crack propagation in bridge decks and perform a fatigue reliability assessment, which is also the motivation that drives the innovation of this study. A fatigue reliability evaluation method is presented for orthotropic steel bridge decks, considering the coupling effect of multiple cracks and the randomness of vehicle loading. A numerical simulation method for multifatigue crack growth is developed by combining the ABAQUS and FRANC3D programs. The equivalent crack depth under different spacing and depths of collinear cracks is calculated by using numerical simulation and the multicrack equivalent characterization method. The critical damage accumulation function of multiple fatigue cracks is established using linear elastic fracture mechanics. Subsequently, the critical damage accumulation function of multiple fatigue cracks is established based on linear elastic fracture mechanics. In order to solve the time-consuming problem of traditional Monte Carlo method, the iHL-RF method and AK-MCS method are developed for fatigue reliability analysis. The results show that compared with the single-crack model, the fatigue reliability of orthotropic steel deck will be crucially reduced considering the coupling effect of double cracks. The MCS, iHL-RF and AK-MCS methods can effectively solve the fatigue reliability analysis problem. Compared with the MCS method, the reliability calculation time based on AK-MCS method is significantly reduced. The AK-MCS method-based method reduces the time for calculating the reliability of orthotropic steel decks by 50% compared with the iHL-RF method. The reliability analysis of orthotropic steel deck bridge based on AK-MCS method is proved to be efficient and accurate.

Sensitivity assessment of the crack propagation behavior in welded stiffened panels

ABSTRACT Welded stiffened panels play an integral role in maintaining the strength, stability, and lightweight characteristics of marine vessels. The stiffeners also affect the service life of the structure under propagating cracks. In this context, properly quantifying the effect of various input parameters on the fatigue crack propagation behavior is crucial to accurately predict the service life. This paper quantifies the influence of relevant input parameters, covering geometric and mechanical properties, on the crack propagation and fatigue service life of welded stiffened panels. Three-dimensional finite element analysis, artificial neural networks, and an elastic-plastic crack advancement rule are integrated to predict the crack propagation. Variance-based sensitivity analysis is conducted to evaluate the effect of variability in the considered input parameters on the variance of the fatigue service life of these panels. It was found that neglecting the uncertainties associated with the geometric parameters can lead to unconservative estimates of the fatigue reliability.

Effect of environmental media on the growth rate of fatigue crack in TC4 titanium alloy: Seawater and air

The fatigue crack growth rate of TC4 alloy in seawater is significantly faster than that in air. In order to analyze the behind reasons, the crack propagation path are investigated by scanning electron microscopy in conjunction with electron backscattering diffraction. The results indicates that the complex ionic environment in seawater can diminish the resistance of the fatigue crack growth. Especially, the damage of the α/β interphase induced by hydrogen embrittlement and Cl- in the seawater accelerate the fatigue crack propagation along the phase interface, although the presence of α/β interphase can reduce the crack propagation rate in air.

Metal magnetic memory characterization of fatigue crack propagation of Q345qD bridge steel under the influence of stress ratio

Fatigue fracture failure has been one of the major causes of damage in steel bridges. To better use fracture mechanics to study this failure behavior, the fatigue crack growth behavior of bridge steel plates under different stress ratios was investigated by metal magnetic memory nondestructive testing technique. The changes of magnetic signals during fatigue crack growth were recorded and analyzed, and then the magnetic signal characterization method of fatigue crack growth length was discussed both theoretically and experimentally. The stress ratio was incorporated into the model to establish a quantitative relationship equation between the magnetic signal characteristic and the crack growth length. Subsequently, the proposed relationship was applied to the fatigue crack growth remaining life prediction study and verified by experimental results. The results showed that the magnetic signal gradient is suitable as a characteristic parameter to quantitatively evaluate the fatigue crack growth length, and the relative errors between the prediction results and the test results were all within 15 %.

Fatigue crack growth and slow crack growth of HDPE pipes under internal pressure and flat plate compression

High-Density Polyethylene (HDPE) pipes are widely used in city gas and water systems as buried pipelines for their excellent performance. However, in addition to the internal pressure, it also suffers loads from traffic vehicles, foundation settlements, and illegal buildings. In this paper, fatigue crack propagation of HDPE pipes with a longitudinal external crack was experimentally studied under different load ratios in two cases. Pulsatile internal pressure was implemented in the first case. In the other one, pulsatile flat plate compression and constant internal pressure were applied. The crack mouth opening displacements were recorded by the digital image correction technique. The crack depth was obtained with compliance calibration. It shows that the fatigue crack growth rate decreases as the load ratio increases. In both cases, the slow crack growth kinetics were gained by extrapolating the load ratio to 1.0, representing constant internal pressure or flat plate compression.

3D characterization of closure of surface and internal fatigue cracks in nodular cast iron using Digital Volume Correlation of laboratory X-ray tomography images

Digital volume correlation (DVC) of laboratory X-ray tomography images has been used to characterize the closure of artificial internal fatigue cracks in nodular graphite cast iron. DVC allows to produce crack opening displacement (COD) map from 3D tomography images at the sub-voxel scale thanks to the graphite nodules which can be used as internal natural markers. Fatigue samples containing controlled internal defects have been produced in a nodular cast iron by diffusion bonding plates containing laser-machined sharp notches. Specimens with internal cracks and surface cracks were respectively characterized by laboratory tomography at different loads during fatigue crack propagation tests. The local stress intensity factors (SIFs) have been extracted from the measured displacement fields by using Williams series, based on those results the closure levels of internal cracks and surface cracks were comparatively analyzed and discussed. Crack closure levels were found to be linearly related to the location of the surface crack front, and the level of internal crack closure was comparable to that of the surface crack in bulk.

Fatigue performance and alternate coupling mechanism of ZL114A aluminum alloy under corrosion-fatigue alternating

This paper investigates the fatigue performance of ZL114A cast aluminum alloy through laboratory corrosion-fatigue alternating test conducted under simulated tropical marine humid-hot atmosphere. The study examines the impact of different alternating frequency on the fatigue properties of the aluminum alloy. At the same time, the micro morphology of the specimen surface and fatigue fracture were observed, and the damage mechanism under the alternating action of corrosion-fatigue was analyzed. The concept of ‘fatigue impact factor’ in alternating test was proposed to reflect the effect of fatigue on corrosion in alternating process. The results show that the increase of alternating frequency leads to an extended fatigue life for the alloy, following a power function relationship. Pitting is identified as the primary form of corrosion damage on specimen surfaces, with multi-crack initiation observed in fatigue fractures. It is found that the fatigue impact factor exponentially increases with each alternating cycle during testing. In early alternating cycles of test, the presence of fatigue inhibits corrosion by promoting passivation of corrosion pits; however, in later alternating cycles, propagation of fatigue cracks accelerates growth in size for these corrosion pits which subsequently promotes further corrosion.

On the influence of secondary branches on crack propagation in rolling contact fatigue

Gear and bearing failures are most often caused by rolling contact fatigue (RCF). Understanding the growth of a main surface-initiated crack into the depth of the piece, as well as the growth of subsurface-initiated branches towards the surface is necessary, in order to improve fatigue life prediction and reduce fatigue damage repair or component replacements before catastrophic failure. The purpose of this work is to analyze, based on 3D finite element simulations, the initiation and mode I growth of secondary branches in a case hardened gear, their influence on the main crack growth in mode II, and the competition between mode I and mode II crack growth. The residual stress field issued by the case hardening is taken into account and its role is analyzed. The reasons why branch cracks develop only from the upper main crack face and why only those initiated when the main crack is still shallow can reach the surface, inducing spalling are explained.

Study on fatigue damage tolerance of rail steel materials using peridynamics

With railways transitioning into the maintenance era, this study proposes a method to ensure structural safety and control maintenance costs. We develop a Peridynamics (PD) fatigue crack growth prediction model to determine the safe crack threshold for rail steel material under cyclic loading, known as the fatigue damage tolerance size. By conducting fatigue tests, we obtain the PD parameters for the rail steel and rigorously validate the model's reliability. Using both the compact tensile (CT) specimen model and a transient wheel-rail contact model, we analyze factors influencing crack propagation and fatigue damage tolerance. The results reveal that higher cyclic loads and smaller stress ratios result in shorter fatigue life and smaller damage tolerance sizes. Additionally, larger initial crack angles, faster train speeds, and heavier axle loads reduce fatigue crack propagation life and damage tolerance sizes. Ultimately, based on damage tolerance analysis, it is recommended to limit crack propagation in U71Mn rail steel within a 3.8-millimeter safety range for reliable operation.

Effects of Cyclic Softening on Fatigue Crack Propagation Properties of Steel

Effects of Cyclic Softening on Fatigue Crack Propagation Properties of Steel

A strategy to improve the thermal fatigue resistance of solder joints based on amorphous Co-P coating to spontaneously build skeleton of CoSn3 laths

Integrated circuit systems are always suffering the temperature cycling environment caused by frequent power on and off. Mismatch in thermal expansion coefficients among chip package components will cause severe cyclic stress to the solder joints. During thermal fatigue, β-Sn grains tended to generate thermal fatigue cracks at the grain boundaries. This work proposed a strategy of using amorphous Co-15 at.% P coating as the interface layer to inhibit the initiation and propagation of thermal fatigue cracks and improve the shear strength of solder joints by numerous endogenous CoSn3 laths. Mechanistically, based on the amorphous structure of the Co-P coating, Co atoms would massively and rapidly diffuse into the molten solder during the liquid-solid diffusion stage of reflow soldering, and the eutectic solidification together with β-Sn spontaneously precipitated numerous laths of CoSn3 intermetallic compounds (IMC), which were evenly distributed and became the skeleton of the solders. The skeleton-like CoSn3 laths with high elastic modulus were the reinforcing phase of Sn-based solder, but also refined the grains of β-Sn. The refinement of β-Sn grains and the strengthening of CoSn3 laths played an important role in inhibiting the initiation and propagation of thermal fatigue cracks. In addition, a discontinuous Co-Sn IMC layer was formed between the amorphous Co-P and the solder. The alternating appearance of CoSn3 and relatively soft β-Sn at the interface avoided the generation of cracks at the interface during temperature cycling.

The Influence of Hard Coatings on Fatigue Properties of Pure Titanium by a Novel Testing Method.

This study investigates the impact of hard coatings on the fatigue properties of pure titanium. A specialized fatigue test which ensured machine equivalence was conducted to compare the fatigue behavior of coated and uncoated metals. The findings reveal that the application of coatings adversely affects the fatigue properties of pure titanium due to stress concentration from the coating, which accelerates fatigue crack propagation within the substrate material. Notably, zigzag fatigue cracks at the interface between the coating and substrate and multiple micro-cracks initiated within the coating are found.

Magnetic memory testing towards fatigue crack propagation of Q235 steel for remanufacturing

A crack propagation trial was performed on a Q235 steel M(T) sample to investigate the modifications in magnetic memory signals throughout the crack propagation procedure of ferromagnetic substances. Various detection lines were employed to gather and scrutinize magnetic memory signals under two-stage fatigue loading. The interrelation between the gradient value Kmax, the peak-to-peak value SP−Py, the stress intensity factor Ka, the extension of the sample 𝜀, crack length a, and the cyclic N was established. The findings indicate that the Hp(y) curves present a varied peak at the crack tip and in the notch, whereas the Hp(x) curve is linear. The magnetic signals display similar changes during two-stage fatigue loading, but the post-break state deviates. The fatigue process’s SP−Py shows three phases of fluctuation, escalation, and decline. Throughout the period of fatigue crack growth, Kmax and 𝜀 rise exponentially, Ka ascends linearly, and a shows linear changes. The characteristics of magnetic memory signals can measure harm after varying service periods and offer robust foundations for remanufacturing.

Notch structural stress theory: Part II predicting total fatigue lives of notched structures

Notch fatigue problems are among the most common topics for the reliability analysis and structural design of mechanical engineering components. In previous work, we proposed the Notch Structural Stress (NSS) method based on fracture mechanics for reflecting the notch effect on fatigue crack propagation lives. However, like other regular fatigue assessing procedures, NSS also cannot accurately predict the fatigue life of notched structures with slower stress gradients due to the inconsistent slope of their S-N curves. This paper presents the Theory of Notch Structural Stress (TNSS) for predicting total fatigue lives of notched structures, which overcomes the life prediction limitations caused by slope differences in S-N curves and constructs a life-predicting frame suitable for different notch types even including plain and sharp notches. Compared with the often-used stress-concentration-assessing procedures, TNSS provides a more effective consolidation of the S-N data from different notch types, mostly since it takes both Initiation and Propagation state resistance into account at the same time.

Fatigue Characteristics of Steel–Concrete Composite Beams

Fatigue in steel–concrete composite beams can result from cyclic loading, causing stress fluctuations that may lead to cumulative damage and eventual failure over an extended period. In this paper, the experimental findings from fatigue loading tests on composite beams with various arrangements are presented. Fatigue tests were performed up to 1,000,000 cycles using four-point loading, encompassing various ranges of shear stress at a consistent amplitude. Additionally, the effects of external post-tensioning and the strength of the shear connection were investigated. Static tests were run until failure to assess the enduring strength of the specimens subjected to fatigue. The cyclic mid-span deflections, slippages, and strains were measured during the testing. Based on the experimental findings, it was found that the damage region that the shear studs caused in the concrete slab, which resulted in a reduction in stiffness within the shear connection, grew as the loading cycles increased, leading to an increase in residual deflections and plastic slippages. Controlling the longitudinal fatigue cracks in the concrete slab was largely dependent on the strength of the shear connection between the steel beams and concrete slabs. Moreover, the applied fatigue loading range affected the propagation and distribution of fatigue cracks in the concrete slab. The strains in different parts of the composite specimens were significantly reduced by applying the external post-tensioning. With no signs of distress at the anchors, the tendons displayed excellent fatigue performance.

Investigating the impact of internal fatigue crack propagation in aluminum alloy plates repaired with a composite patch

Investigating the impact of internal fatigue crack propagation in aluminum alloy plates repaired with a composite patch

Effect of Y2O3 doping content on phase composition, mechanical properties and cavitation erosion performance of ZrO2 ceramics

Y2O3 stabilized ZrO2 (YSZ) is considered to be promising in resisting force-heat coupling damage caused by cavitation erosion (CE). However, the impact of doping Y2O3 on the microstructure and properties of ZrO2 is extremely complex, and there is still a lack of understanding about it, let alone the mechanism of these factors on the CE performance. In light of this, the present study investigated the effects of Y2O3 doping content (3 mol%, 7 mol% and 17 mol%) on the phase composition, grain size, microstructure, density, mechanical properties and CE performance of YSZ, and deeply analyzed the CE mechanism. The results indicated that the Y3+ entering the zirconia lattice effectively stabilized the high-temperature phases (t, t′ and c) and suppressed the formation of m-phase. With the increase of Y3+ concentration, the content of t continuously decreased while that of c continuously increased, so that only t + t′ phases were contained in the 3YSZ and only t′+c phases were contained in the 17YSZ. Meanwhile, the grain sizes and pores were constantly increasing, which obviously deteriorated the hardness, toughness and CE resistance. The t→m transformation under the action of cavitation load effectively absorbed impact energy and suppressed the propagation of fatigue cracks, thereby significantly delaying the fatigue spallation of materials. The 3YSZ with the densest microstructure, the best mechanical properties, the smallest grain size and the highest content of t-phase exhibited the best CE resistance. However, the continuous accumulation of m-phase at the grain edge, especially the amorphous structure formed by hydrolysis promoted by cavitation heat, provided a preferential channel for the propagation of fatigue cracks, and thus ultimately led to the exfoliation of the grains.

Time-dependent fatigue reliability estimation and optimal design of scraper chains under corrosive wear effect

Fatigue damage is one of the main failures of the scraper chain. Moreover, corrosive wear has a detrimental effect on fatigue crack propagation. It is so essential to study the fatigue reliability of the scraper chain considering the interaction of corrosive wear and fatigue. This paper is intended to propose a reliability-based estimation and optimal design of the scraper chain considering the performance degradation of the scraper chain under the interaction of corrosive wear and fatigue. First, the dynamic model of the scraper conveyor is established based on the finite element method. Second, Paris Low is used to empirically describe the occurrence of cracking with corrosion effects. Furthermore, the time-dependent reliability of scraper chains under the interaction of the corrosive wear and fatigue crack propagation is addressed by the fourth-moment saddlepoint approximation (FMSA) method, and the reliability-based design optimization (RBDO) of the scraper chain is then carried out. Finally, the applicability of the proposed method is verified by the SGZ1000/1400 scraper conveyor.