Stage Of Fatigue Crack Growth Research Articles

Structural, thermal and acoustic aspects of crack propagation in titanium alloys

The aim of this work was to investigate the correlation between structural, acoustic emission and thermal characteristics of the fatigue crack growth in titanium alloys. Cluster analysis of the acoustic emission signals revealed two different types of signals recorded during the development of a fatigue crack. It has been shown experimentally that the evolution of the stored energy tends to an asymptotic value at the final stage of fatigue crack growth and it correlates with the intensification of the twinning process in titanium alloy Ti Grade 2. A correlation was assumed between the stages of change in the heat flux, the cumulative energy of the first cluster of AE signals and the crack length.

Regulate the microstructure, tensile properties and fatigue crack growth behavior of an Al–Zn–Mg–Cu alloys fabricated by laser powder bed fusion via post-heat treatment

An Al–Zn–Mg–Cu–Si–Zr–Er alloy was fabricated by laser powder bed fusion technology. A unique cellular structure consisting of α-Al, Mg2Si, and Al2CuMg phases was observed in both as-printed (AP) and direct aging (DA) alloys. Direct aging treatment at 150 °C × 4 h can improve the yield strength and hardness of the alloy, and precipitation strengthening and fine grain strengthening are dominant mechanisms. The strength of the regression re-aging (RRA) heat treated alloy decreased, while the elongation was obviously improved, and the grain boundary was continuous without precipitates free zones. DA could not significantly hinder the fatigue crack growth rate of the as-printed alloy, but the improvement of plasticity, elimination of residual stress, and the effect of precipitates in RRA alloy extend the life of the fatigue crack growth stage, thereby improving the overall fatigue life of the alloy from 1.17 × 105 cycles to 3.93 × 106 cycles.

Study on fatigue crack propagation behavior of 2050 Al[sbnd]Li alloy during non-isothermal aging

The fatigue crack propagation behavior of 2050AlLi alloy at different stages during non-isothermal aging (NIA) was tested. It was found that when the aging temperature was reduced to 150 °C, the alloy exhibited higher fatigue crack growth resistance and lower fatigue crack growth rate and the fatigue crack growth path was mainly transgranular propagation in different stages of NIA. In addition, in the first stage of fatigue crack growth, the size and quantity of precipitates in the grain are the main factors affecting the crack growth. Because the tiny T1 phase can be sheared by dislocation without damage accumulation, it can effectively prevent crack propagation. In the second stage of fatigue crack growth, the strength difference between grain boundary and intragranular will directly affect the fatigue crack growth resistance of the alloy. The smaller the strength difference is, the more uniform the deformation of crack tip in the process of crack growth, and it is not easy to produce stress concentration and damage accumulation at the grain boundary. In this case, the alloy exhibits the better the fatigue crack growth resistance and the slower the crack growth rate.

Front face strain compliance for quantification of short crack growth in fatigue testing

Short fatigue crack growth investigation is of considerable scientific interest as it comprises a significant portion of the total fatigue life of a structure. It is very challenging to accurately quantify this stage of fatigue crack growth experimentally. In this article, a novel front face strain compliance technique for single-edge notched specimens subjected to four-point bending is proposed. Finite element analysis is performed to determine the correlation between crack length and strain change near the crack. This relationship is then validated by experiments in which strains are measured by strain gauges attached near the short crack, and crack length is quantified by examining beachmark lines at the fracture surfaces. Based on the numerical and experimental results, it is concluded that the strain measured near the notch allows quantifying short crack growth for normalised crack lengths in the range 0.01 ≤ a/W ≤ 0.06 ( a/W being the ratio of crack length over specimen width). A compliance equation based on the front face strain is finally presented.

Characterization of Fatigue Crack Growth Based on Acoustic Emission Multi-Parameter Analysis.

In engineering structures that are subject to cyclic loading, monitoring and assessing fatigue crack growth (FCG) plays a crucial role in ensuring reliability. In this study, the acoustic emission (AE) technique was used to monitor the FCG behavior of 2.25Cr1Mo0.25V steel in real-time. Specifically, an AE multi-parameter analysis was conducted to qualitatively assess the crack growth condition and quantitatively correlate the crack growth rate with AE. Various AE parameters were extracted from AE signals, and the performances of different AE parameters were analyzed and discussed. The results demonstrated that four stages of FCG, which correspond to macrocrack initiation, stable crack growth with low crack growth rate, stable crack growth with high crack growth rate, and unstable crack growth, are distinctly identified by several AE time domain parameters. The sudden and continuous occurrence of many AE signals with high count (>100) and high energy (>40 mV·ms) can provide early and effective warning signs for accelerated crack growth before final failure occurs. Moreover, linear correlations between crack growth rate and different AE parameters are established for quantifying crack growth. Based on the AE multi-parameter analysis, it was found that the count, energy, and kurtosis are superior AE parameters for both qualitatively and quantitatively characterizing the FCG in 2.25Cr1Mo0.25V steel. Results from this research provide an AE strategy based on multi-parameter analysis for effective monitoring and assessment of FCG in engineering materials.

The effect of porosity defects on the mid-cycle fatigue behavior of directed energy deposited Ti-6Al-4V

This paper investigates the mid-cycle fatigue (MCF) behavior of as-built DED Ti-6Al-4V. Fatigue tests in groups at three stress levels were completed in a laboratory environment. The results show that internal pore initiation mode is more common for this material. The porosity-induced crack initiation mechanism involves a mixture of cleavage and intergranular fracture. Pore size classification method is quantitatively developed, which in turn elucidate the potential relationships among microstructure, pore size, fatigue behavior, and fatigue crack growth stages in high vacuum. A quantitative study of pore sizes and fine granular area (FGA) shows that the FGA stress intensity factor range ΔKFGA in the MCF regime is not a material constant. In addition, three key factors (i.e., stress level, pore size and pore-to-surface distance) affecting the fatigue behavior and performance of the material are discussed.

The Effects of Pits Size and Shape on Fatigue Crack Initiation

Generate and monitor the growth of corrosion pits on the surface of 316L stainless steel using the scanning droplet cell technique and assess the effects of varies factors such as pits size, pits shape, flow rate, and applied stress on pit initiation and growth. Configure of a test system to measure the pit and short fatigue crack growth behaviour, to investigate each stage of the corrosion fatigue damage process occurring at the material surface and compare this with the modified film surface using alternating voltage passivation process (AVPP). Understand the effects of corrosion on cracking behaviour during the different stages of fatigue crack growth.

Циклическая трещиностойкость высокопрочных сталей, склонных к водородному охрупчиванию, в коррозионной среде

High-strength steels (yield point 500 MPa and above) have long been used in deepwater technology. Nowadays they are becoming increasingly popular in shipbuilding and offshore engineering for certain highly-loaded structural members. Stronger steel means higher absolute stresses in structure, so it becomes necessary to analytically estimate fatigue strength, not only high-cycle, but low-cycle, too, where fatigue crack growth stage comprises a considerable share of service life. Presently, literature offers only scarce data on fatigue cracking toughness of these steels because until quite recently these materials were required only for certain specific applications. Meanwhile, this kind of materials features a number of peculiarities mostly related to the effect of corrosive environment upon their behavior: specifically, hydrogen embrittlement that might be caused only by static tension load but by cyclic loading as well. This paper presents the experimental data on how high-strength steel proneness to hydrogen embrittlement (the main mechanism of corrosive cracking under stress) affects fatigue crack growth rate in corrosive environment. The studies were performed on the specimens of Cr-Ni-Mo-Va steel alloy (rated yield point at least 780 MPa). It is shown that steels with greater susceptibility to corrosive cracking under stress feature 2–2,5 higher growth rate in corrosive environment. In extreme cases, fatigue crack toughness curves not taking into account potential susceptibility of material to hydrogen embrittlement and actual loading rate might lead to considerably overstated strength estimates. The study also yielded limit coefficients for fatigue cracking toughness curves that enable sufficiently conservative assessments.

Characterization of fatigue crack propagation of pitting-corroded rebars using weak magnetic signals

In this study, mode-I high-cycle fatigue tests were conducted on smooth standard specimens manufactured from HRB400 steel bars, prefabricated with a semi-ellipsoid corrosion pit. Fatigue crack growth (FCG) behavior was predicted using the FCG model, based on linear elastic fracture mechanics (LEFM). During the fatigue process, an online piezomagnetic signals was detected, while the residual magnetic field (RMF) distribution near the corrosion pit was recorded after the specimen was unloaded. The range of the magnetic tangential induction ΔBt and the gradient of the normal component of the RMF distribution Gmax were selected as the characteristic weak magnetic parameters, which exhibited a three-stage evolution law with fatigue progression. Next, the evolution mechanism of the weak magnetic field during the FCG process of the pitting-corroded rebars was comprehensively explored. Subsequently, quantitative relationships between the stress intensity factor (SIF) range ΔK and ΔBt as well as the fatigue crack length a and Gmax were separately established. The errors between the predicted and validation experimental results of ΔK, a, and FCG life were all within 20%. Furthermore, the influence of the pit geometry and stress ratio on the variation of ΔBt versus ΔK, and Gmax versus a was investigated. The increments of ΔBt and Gmax in different 1000-cycle intervals during various FCG stages were recorded, which could help estimate FCG behavior of pitting-corroded rebars in practical applications.

Internal Crack Initiation and Growth Starting from Artificially Generated Defects in Additively Manufactured Ti6Al4V Specimen in the VHCF Regime.

The aim of the present work was to investigate the ‘fine granular area’ (FGA) formation based on artificially generated internal defects in additively manufactured Ti6Al4V specimens in the early stage of fatigue crack growth in the ‘very high cycle fatigue’ (VHCF) regime. Fatigue tests were performed with constant amplitude at pure tension-compression loading (R = −1) using an ultrasonic fatigue testing setup. Failed specimens were investigated using optical microscopy, high-resolution ‘scanning electron microscopy’ (SEM), and ‘focused ion beam’ (FIB) techniques. Further, the paper introduces alternative proposals to identify the FGA layer beneath the fracture surfaces in terms of the ‘cross section polishing’ (CSP) technique and metallic grindings with special attention paid to the crack origin, the surrounding microstructure, and the expansion of the nanograin layer beneath the fracture surface. Different existing fracture mechanical approaches were applied to evaluate if an FGA formation is possible. Moreover, the results were discussed in comparison to the experimental findings.

Fracture surface formation of notched 2017A-T4 aluminium alloy under bending fatigue

The effect of cyclic loading on facture surface topology in notched components made by aluminium alloys is not completely clear. Fractography and fracture mechanics can help to understand this interdependency. This paper aims to study the distribution of the fracture surface roughness of notched 2017A-T4 aluminium alloy after bending fatigue using an optical focus-variation surface measurement technique by applying the fracture zone concept. The effects of stress level at the notch root and the load ratio on fatigue crack growth and fracture surfaces are analysed. Profile and areal surface parameters of four fracture surface regions were investigated at incremental crack lengths of the specimens. Studies have confirmed that the surface areas associated with the main stages of fatigue phenomenon (i.e. crack initiation, crack propagation, and final rupture) have significant differences in roughness which can be explained by the different loading scenarios. Overall, fatigue fracture surfaces have smallest roughness values at the crack initiation stage and a gradual increase during the fatigue crack growth stage.

Experimental Investigation of Fatigue Crack Growth Behavior in Banded Structure of Pipeline Steel

The effect of a ferrite/mixed phase banded structure on the fatigue crack growth behavior of pipeline steel has been investigated in this work. Constant stress-controlled fatigue tests were carried out on unilateral notch specimens in the longitudinal direction (LD) and the transverse direction (TD). The strain accumulation was measured by digital image correlation (DIC) method and the fatigue fracture morphology was analyzed. The results show that the banded structure in the LD would cause a wide range of crack deflections, especially in the early stage of fatigue crack growth, which results in a lower fatigue crack growth rate compared to the TD. At the later stage, due to the increase of the plastic zone of the crack tip, more ferrite in the crack growth path of the TD specimen is also beneficial for crack tip blunting, thereby slowing down the increase of the crack growth rate.

Shear band deformation and fatigue striations during three-point bending fatigue crack growth in brittle metallic glass

Shear band plays a key role in fatigue damage and fracture behaviors in metallic glass (MG). The effect of the shear band on fatigue crack growth of a brittle Ti32.8Zr30.2Ni5.3Cu9Be22.7 (ZT3) MG with superior fatigue resistance was investigated by three-point bending fatigue testing in this work. The fatigue crack growth process is classified into three stages based on the shear band feature and crack profile. It is found that the crack path is straight and smooth with no or several shear bands near it in the former stage of fatigue crack growth, but transformed to be staircase-like with extensive shear-band proliferation in the latter stage. The transformation of crack growth path is mainly determined by three factors, i.e., the inclined shear band with respect to mode I crack plane direction, maximum energy release rate along the mode I crack plane direction and the asymmetric shear-band attribution on two sides of mode I crack plane direction. On the other hand, the fatigue fracture surface shows three regions with different fatigue striation features, which match well with three stages on the sample surface. The shear band theory including shear-band formation, propagation and cracking is utilized to well interpret the mechanism of fatigue striation formation. The present findings reveal the relation between shear band and fatigue crack growth and may provide instructions for future design of MGs with excellent fatigue resistance through controlling shear banding behavior.

An online‐offline prognosis model for fatigue life prediction under biaxial cyclic loading with overloads

AbstractThis paper presents a robust online‐offline model for the prediction of crack propagation under complex in‐phase biaxial fatigue loading in the presence of overloads of different magnitudes. The online prognosis model comprises a combination of finite element analysis and data‐driven regression to predict the crack propagation under constant loading, while the offline model is trained using experimental data to inform the post‐overload crack growth retardation behavior to the online model. The developed methodology is validated by conducting biaxial fatigue experiments using aluminum AA7075‐T651 alloy cruciform specimens. A close correlation is observed between the experimental results and model predictions. The results show that the model successfully predicts the crack retardation behavior under the influence of overloads with different magnitudes occurring at different stages of fatigue crack growth. Error analysis is conducted to investigate the sensitivities of the number of training points and crack increments to the prediction accuracy. In addition, the error propagation with respect to the crack length is studied, which provides constructive suggestions for further model improvement.

The mode I fatigue behaviours of notched freestanding Al-3Mg foils

The effect of the thickness on fatigue crack propagation behaviour of rolled and annealed Al-3Mg alloy foils was investigated using notched rectangular specimens with thicknesses of 30µm and 100µm and elastic finite element method analysis. A specialized apparatus in combination with a force sensor and a video graphics array digital camera to record the crack growth was employed to investigate the mode I fatigue behaviour of freestanding Al-3Mg alloy foils. Tests were conducted at ambient temperature, with a loading frequency of 2Hz and a loading ratio R of 0.1. The foil thickness was found to play a distinct role in the fatigue properties of micron-foil metals. Compared to the 100-μm-thick foil, the 30-μm-thick foil exhibited higher average yield stress (σ0.2), but lower elongation, fracture toughness and fatigue life. The crack propagation rate da/dN of both foils as a function of the stress intensity factor ΔK followed the Paris relationship. The Paris constant m, increased slightly, from 4.6 to 5.6, as the foil thickness decreased from 100µm to 30µm. A smaller plastic zone size at the crack tip and a higher value for the self-similarity parameter z were found in the thinner foils, resulting in the higher Paris constant m value. With typical fatigue striations, the 30-μm-thick foils demonstrated failure in a brittle mode during the stable fatigue crack growth stage. However, the 100-μm-thick foils had abnormal striations and dimples in the stable fatigue crack growth stage, they failed in a mixed ductile and brittle mode.

Monitoring fatigue crack growth using nonlinear ultrasonic phased array imaging

Nonlinear imaging techniques have recently emerged which have the potential to detect material degradation and challenging defects, such as closed cracks. This paper describes an investigation into the performance of nonlinear ultrasonic imaging (NUI) for the monitoring of the early stages of fatigue crack growth. This technique, in conjunction with conventional array imaging, is applied to the periodic monitoring of steel compact tension specimens subjected to high cycle fatigue loading. The detection limits of these techniques are investigated. Their abilities to localise and detect small cracks are further quantified with the aid of micrography. The results suggest that NUI is more sensitive than conventional ultrasonic imaging to the microscale changes occurring at the early stages of failure, i.e. detectability starts c. 15% of fatigue life. In addition to early detection, the potential for NUI to deliver accurate sizing of fatigue cracks and monitor crack propagation is also presented.

Influence of Technological and Operational Factors on Fatigue Resistance of 10GN2MFA Steel*

The paper presents the results of low-cycle fatigue tests for 10GN2MFA steel obtained by martensitic melting and electroslag remelting in air and in heat transfer medium at both room and elevated temperatures, as well as cyclic crack growth resistance in high-temperature aqueous medium. It is shown that the method of steelmaking exerts a strong influence on the fatigue crack growth stage.

Improvement on the Fatigue Performance of 2024-T4 Alloy by Synergistic Coating Technology.

In this paper, rotating bending fatigue tests of 2024-T4 Al alloy with different oxide coatings were carried out. Compared to the uncoated and previously reported oxide coatings of aluminum alloys, the fatigue strength is able to be enhanced by using a novel oxide coating with sealing pore technology. These results indicate that the better the coating surface quality is, the more excellent the fatigue performance under rotating bending fatigue loading is. The improvement on the fatigue performance is mainly because the fatigue crack initiation and the early stage of fatigue crack growth at the coating layer can be delayed after PEO coating with pore sealing. Therefore, it is a so-called synergistic coating technology for various uses, including welding thermal cracks and filling micro-pores. The effects of different oxide coatings on surface hardness, compressive residual stress, morphology and fatigue fracture morphology are discussed. A critical compressive residual stress of about 95–100 MPa is proposed.

Role of oxide thickening in fatigue crack initiation in LIGA nickel MEMS thin films

This paper presents the results of a combined experimental and theoretical study of the fatigue crack initiation in LIGA Ni thin films with a thickness of 270μm. The potential roles of surface oxide thickening and slip bands are explored for fatigue crack initiation. Surface oxides and roughness are characterized using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The underlying crack driving forces associated with pop-in in surface oxide layers are then compared to those associated with the early stages of fatigue crack growth. The implications of the results are discussed for the modeling of fatigue in LIGA Ni micro-electro-mechanical-systems (MEMS) structures.

Progress in Understanding the Fatigue Behavior of Ti Alloys

Titanium (Ti) alloys are used in critical, fatigue limited applications in aircraft and aircraft engines. Current design practices are, of necessity, conservative in order to minimize risk of unexpected failures. Among the sources of this conservatism are the inherent variations in the number of loading cycles the materials can withstand prior to fatigue crack initiation, the uncertainty in crack propagation lifetime prediction and the need to set safe minimum component life values. While the stochastic nature of fatigue is well-known, improved characterization methods have begun to provide a better understanding of the crack initiation process. This paper describes recent work designed to provide an improved understanding of the relationship between thermomechanical processing history, microstructure, texture and the fatigue behavior of α+β Ti alloys. Due to length limitations, the paper focuses on two important aspects of fatigue life variation: the effects of microstructural discontinuities on fatigue and the role of facet formation during crack initiation and the early stages of fatigue crack growth.