Figure 6 crack propagation curves in electropolished specimen. Introduction since its discovery by elber1, fatigue crack closure has been an intensely studied parameter associated with fatigue crack growth behavior. High strength al alloys had a strong tendency for planar slip due to the high density of coherent and shearable precipitates in the alloys. Fractographic analysis of the overload effect on fatigue. In recent years, many studies have focused on the fatigue behaviour of al alloys in the corrosive environment.
However, must be observed that those factors causing improvement in fatigue crack propagation resistance, very often tend to have a detrimental influence on fatigue crack initiation. The influence of excess mg on the mode i propagation of fatigue crack was examined in newly developed precipitationhardened al alloy containing zr and excess mg. Introduction since its discovery by elber1, fatigue crack closure has been an in tensely studied parameter associated with fatigue crack growth behavior. The smooth specimens were fatigued at room temperature under constant maximum stress control when stress ratio r is 0. Comparison of microstructure and mechanical properties of. In addition, the influences of two holes in different locations on the fatigue small crack initiation and propagation characteristics of cast mgal alloys were discussed and the difference of fatigue life both cast am60b and az91 alloys under the same condition is an equal proportion of the difference of fatigue crack growth rate both cast. Fatigue crack propagation prediction of a pressure vessel. The residuals stresses induced by plastic preload in tension were calculated numerically by finite element method fem. Fatigue crack initiation and propagation behavior were investigated on a tubular specimen of al 7075 alloy subjected to multiaxial mixed mode bendingtorsional loadings. The data for the alloys fit into a single scatter band of. Effect of compressive residual stress on fatigue crack growth rate fcgr behavior was investigated on two alalloys. Influence of pitting corrosion on fatigue and corrosion.
It was found that the crack populations were a weibull function of the applied maximum stress. Porosity and other material defects played respective important roles in determining the fatigue crack initiation and propagation behavior. Experimental investigation of crack initiation and. Synergistic effect of environmental media and stress on the. Effect of compressive residual stress on fatigue crack growth rate fcgr behavior was investigated on two al alloys. Effect of saltwater fog on fatigue crack nucleation of al. Pdf fatigue crack initiation in aerospace aluminum alloys.
Aluminum and its alloys in the very high cycle fatigue regime. In addition, the fatigue multicracks were amalgamated for the samples with titled 45 of two small holes of cast mgal alloys when the hole distance. Fatigue crack propagation under the effect of overload peak. Jun 10, 2016 in recent years, many studies have focused on the fatigue behaviour of al alloys in the corrosive environment. Fatigue crack propagation in aluminumbase copper alloys. Highcycle fatigue crack initiation and propagation in an equiatomic crmnfeconi highentropy alloy were investigated using smooth specimens. Fatigue crack propagation ratesdadn in binary al alloys with 3. Fatigue crack growth resistance and crack closure behavior.
Recommended citation li, jinxia, the effect of microstructure and texture on high cycle fatigue properties of. Fatigue tests are performed on an allicu cp271 alloy in air and indifferent corrosive solutions on smooth and notched specimens to determine the crack initiation and crack propagation properties. While on the subject of dislocations, it is appropriate to briefly discuss fatigue. Effect of humidity on fatigue properties of shotpeened high.
Fatigue crack initiation site and propagation paths in. Fatigue is one of the primary reasons for the failure of structural components. The fatigue crack initiation and early propagation behavior of 2a97 alli alloy was studied. Discussion of fatigue crack initiation mechanism for cast. Citeseerx the effect of microstructure and texture.
The fatigue crack initiation position of cast magnesium alloys depends strongly on the radius of curvature of through hole or stress concentration factor at the closed edges of two through holes. Materials free fulltext fatigue and fracture behavior of. Short communication laser shock peening on fatigue. In this paper fatigue crack initiation and propagation in notched plate under constant amplitude loading through tensile residual stress field of 2024 t351 alalloy plate were investigated. Short communication laser shock peening on fatigue behavior. Some of these alli alloys, such as allizr and almglizr alloys are more resistant to stress corrosion cracking scc than more conventional alloys subjected. In the case of a low stress level and a low loading frequency, pits may play a inajor role in short fatigue crack initiation and propagation. Sep 29, 2017 fatigue crack growth fcg rates have traditionally been formulated from fracture mechanics, whereas fatigue crack initiation has been empirically described using stresslife or strainlife methods. T6 aluminium alloy based on scanning electron microscopy sem in situ observation. The preceding results on fatigue crack initiation and propagation showed that for pure alloy the whole life time is determined only by the crack propagation period since the nucleation period is restricted to the first few cycles of the experiment. Crack initiation fatigue crack initiation at an inclusion.
Alloy design and environmental chemistry have significant effects on corrosion fatigue crack initiation and growth. The resistance to micro crack growth, mainly associated with crack plane twist at the particlematrix interface, also influenced the growth behaviors of the microcracks at the particles on the surface. Abstract fatigue crack propagation rates dadn in binary al alloys with 3. Fracture surface characterization suggested that the fatigue crack propagation mechanism was a mixture of transgranular and intergranular cracking. It was found that the fatigue strength of nugget zone decreased continuously even beyond 10 7 cycles with no traditional fatigue limits. The microstructural deformation characteristics, i. The resistance to microcrack growth, mainly associated with crack plane twist at the particlematrix interface, also influenced the growth behaviors of the microcracks at the particles on the surface.
Fatigue crack closure is used to explain the different behavior of the compared alloys. Comparative fatigue study of age hardening alalloys under. Fatigue crack growth fcg rates have traditionally been formulated from fracture mechanics, whereas fatigue crack initiation has been empirically described using stresslife or strainlife methods. The experimental method could be employed to characterize the fatigue crack initiation behaviors and their anisotropy in wrought and cast al alloys such as aa8090 alli, aa2026, aa7075 and a7 alloys. They find a wide spectrum of failure modes, including zero failure fatigue runouts, initiation from pores, and observations of extensive coarse transgranular facets on the fracture surfaces. Quantification of fatigue weaklinks in high strength al alloys. Comparison between cast alsi10mg from the view of fatigue properties. Fatigue properties and cracking behavior of colddrawn commercially pure aluminum wires cpaws widely used as the overhead transmission conductors were investigated. For the commercial alloy the total fatigue life is the same for its both versions commercial alloys. Mpa m12 p a a critical stress for crack propagation s c and crack length a the three modes of crack surface displacement a mode i, opening or tensile mode plane strain fracture toughness b mode ii, sliding mode c mode iii, tearing mode k. Moreover, the method has been developed and applied in numerous structural components, such as steam pipes and pressure vessels, and in the research areas of rocks, composite materials, and metals. Fatigue tests were conducted on notched specimens of 2024t3 and 7075t6 al alloys. Quantitative analysis of crack formation life ni, microstructurally small crack. In addition, the influences of two holes in different locations on the fatigue small crack initiation and propagation characteristics of cast mg al alloys were discussed and the difference of fatigue life both cast am60b and az91 alloys under the same condition is an equal proportion of the difference of fatigue crack growth rate both cast.
Both of initiation and propa gation of a crack, especially in the early propagation process, are accelerated in high humidity by corrosion as shown in fig. Sep, 2018 the fatigue crack initiation position of cast magnesium alloys depends strongly on the radius of curvature of through hole or stress concentration factor at the closed edges of two through holes. Dislocations play a major role in the fatigue crack initiation phase. However, pitting still occurs below ep in some al alloys 2. In an attempt to better understand the fatigue crack propagation behaviour of aili alloys investigations were performed on 25mm plate material of the first of the new generation of lowdensity al alloys, 8090. Acoustic emission is a technique to monitor defect formation and failures in structural materials used in services or laboratories. Sem online investigation of fatigue crack initiation and propagation in cast magnesium alloy article pdf available in journal of materials science 397. Texture was a key factor controlling the crack initiation and propagation.
Fatigue crack growth resistance and crack closure behavior in. Fatigue responses of three aa 2000 series aluminum alloys. The resistance to fatigue crack initiation may be better than it is for in homogeneously deforming microstructures, but the crack propagation resistance can be lower in this case. The behavior of fatigue crack initiation and propagation. Fatigue crack initiation site and propagation paths in high.
The small fatigue crack growth behavior of an am60. In situ threedimensional study of corrosion fatigue crack. Some of these al li alloys, such as al lizr and al mglizr alloys are more resistant to stress corrosion cracking scc than more conventional alloys subjected. Several interesting points can be drawn from the results. Crack initiation fatigue crack initiation at an inclusion cyclic slip steps psb fatigue crack initiation at a psb. The deflection of cracks showed clear crystallographic features which in turn implies that grain orient, ation plays a significant role in controlling fatigue crack propagation in the alloy. The effect of initiation feature and environment on fatigue. Fatigue crack initiation and early propagation behavior of. K, the extrapolated rates for pure aluminum are more than 100 times greater than those in the three alloys at the same.
The fatigue crack lengths as a function of fatigue cycles for the laserpeened and unpeened 2024t3 al alloys with a fastener hole and stopholes are shown in fig. The normalized crack initiation life was almost the same regardless of the stress ratio and stress amplitude, indicating that twin deformation is not involved in the fatigue crack initiation mechanism because twinning occurs in fatigue tests under r. Before this analysis, it is necessary to present the fatigue life and the fatigue crack growth curves in order to analyze the effect of the overload on the fatigue crack propagation. The fatigue crack initiation life and fatigue life of both the laserpeened and unpeened specimens are summarized in table 1. They investigated sn characteristics of these alloys using ultrasonic fatigue testing at 20 khz which revealed that fatigue failure in alalloys can occur up to 109 cycles. Corrosion fatigue crack initiation in aluminum alloys can have significant effects on part life. Investigation on mode i propagation behavior of fatigue crack. Pdf sem online investigation of fatigue crack initiation. They investigated sn characteristics of these alloys using ultrasonic fatigue testing at 20 khz which revealed that fatigue failure in al alloys can occur up to 109 cycles.
A threedimensional quantitative understanding of short. Optical microscopy om and scanning electron microscopy sem were employed to capture a detailed view of the fatigue crack initiation and propagation of the samples as well. T6 show that the pore size and proximity to the specimen surface significantly influence fatigue crack initiation. Fatigue strength optimization of cast aluminium alloys requires an understanding of the role of micropores resulting from the casting process. Jang and coworkers present interesting results for fatigue tests taken from a gravity sand cast aluminum alloy cylinder block casting. Microstructure observations were examined by optical microscopy, transmission electron microscopy, scanning electron microscopy and electron back. Oct 29, 2009 jang and coworkers present interesting results for fatigue tests taken from a gravity sand cast aluminum alloy cylinder block casting. Influence of multiholes on fatigue behaviors of cast. Plane bending fatigue tests were conducted under stress ratios of.
With the help of a fourpointbend fatigue rig, highcycle fatigue tests were carried out on an aa2524 al alloy at room temperature, 15 hz and r. The current research provides insight into fatigue crack formation and progression in the poorly understood size regime that bridges safelife and damage tolerance approaches. An improved understanding of the mechanisms governing corrosion and corrosion fatigue damage of al alloys is necessary. The effect of porosity on the fatigue life of cast aluminium. Citeseerx the effect of microstructure and texture on high. Influential alloy design features include alloy chemistry. The aim of this study was to evaluate the underlying factor affecting fatigue crack growth rate in the stage ii region. Very high cycle fatigue crack initiation mechanism in nugget. The paper is divided into several related topics such as fatigue strength and lifetime analysis, fatigue crack initiation, fatigue crack propagation, fatigue durability, variable fatigue. Using acoustic emission in fatigue and fracture materials. Mechanisms and modelling of cracking under corrosion and. More recently, there has been efforts towards the use of the local stressstrain and similitude concepts to formulate fatigue crack growth rates. The life of a fatigue crack has two parts, initiation and propagation. The crack path and the possible minimum twist angles were measured using ebsd and calculated theoretically by a crystallographic model.
Fatigue crack initiation and propagation in alalloys. Role of eutectic silicon particles in fatigue crack initiation and propagation and fatigue strength characteristics of cast aluminum alloy a356. Crack propagation in oriented copper bicrystals with. Tests under pure bending and pure torsional loadings were also conducted to segregate the effect of multiaxiality. In this paper fatigue crack initiation and propagation in notched plate under constant amplitude loading through tensile residual stress field of 2024 t351 al alloy plate were investigated. It was found that the fracture surface of the cpaws shows an obvious fourstage fracture characteristic, i.
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