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The fatigue process consists, from the engineering point of view, of three stages: crack initiation, fatigue crack growth, and the final failure. It is also known that the fatigue process near notches and cracks is governed by local strains and stresses in the regions of maximum stress and strain concentrations. Therefore, the fatigue crack growth can be considered as a process of successive crack increments, and the fatigue crack initiation and subsequent growth can be modeled as one repetitive process. The assumptions mentioned above were used to derive a fatigue crack growth model based, called later as the UniGrow model, on the analysis of cyclic elastic–plastic stresses–strains near the crack tip. The fatigue crack growth rate was determined by simulating the cyclic stress–strain response in the material volume adjacent to the crack tip and calculating the accumulated fatigue damage in a manner similar to fatigue analysis of stationary notches. The fatigue crack growth driving force was derived on the basis of the stress and strain history at the crack tip and the Smith–Watson–Topper (SWT) fatigue damage parameter, D = σmaxΔε/2. It was subsequently found that the fatigue crack growth was controlled by a two-parameter driving force in the form of a weighted product of the stress intensity range and the maximum stress intensity factor, ΔK p K max 1?p . The effect of the internal (residual) stress induced by the reversed cyclic plasticity has been accounted for and therefore the two-parameter driving force made it possible to predict the effect of the mean stress including the influence of the applied compressive stress, tensile overloads, and variable amplitude spectrum loading. It allows estimating the fatigue life under variable amplitude loading without using crack closure concepts. Several experimental fatigue crack growth datasets obtained for the Al 7075 aluminum alloy were used for the verification of the proposed unified fatigue crack growth model. The method can be also used to predict fatigue crack growth under constant amplitude and spectrum loading in various environmental conditions such as vacuum, air, and corrosive environment providing that appropriate limited constant amplitude fatigue crack growth data obtained in the same environment are available. The proposed methodology is equally suitable for fatigue analysis of smooth, notched, and cracked components. 相似文献
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The fatigue crack growth behavior of MAR-M200 single crystals was examined at 982 °C. Using tubular specimens, fatigue crack
growth rates were determined as functions of crystallographic orientation and the stress state by varying the applied shear
stress range-to-normal stress range ratio. Neither crystallographic orientation nor stress state was found to have a significant
effect on crack growth rate when correlated with an effective ΔK which accounted for mixed-mode loading and elastic anisotropy. For both uniaxial and multiaxial fatigue, crack growth generally
occurred normal to the principal stress direction and in a direction along which ΔK
II vanished. Consequently, the effective ΔK was reduced to ΔKI and the rate of propagation was controlled by ΔK
I only. The through-thickness fatigue cracks were generally noncrystallographic with fracture surfaces exhibiting striations
in the [010], [011], and [111] crystals, but striation-covered ridges in the [211] specimen. These fracture modes are contrasted
to crystallographic cracking along slip bands observed at ambient temperature. The difference in cracking behavior at 25 and
982 °C is explained on the basis of the propensity for homogeneous, multiple slip at the crack tip at 982 °C. The overall
fracture mechanism is discussed in conjunction with Koss and Chan’s coplanar slip model. 相似文献
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George Sunny Vikas Prakash John J. Lewandowski 《Metallurgical and Materials Transactions A》2013,44(10):4644-4653
In the present study, dynamic fracture experiments are performed on fully amorphous Liquidmetal-1 (LM-1), a Zr-based BMG, to better understand fracture initiation and propagation in notched specimens. Experiments are conducted on notched (110 μm notch radius) four-point bend specimens using an instrumented modified split-Hopkinson pressure bar apparatus. The results of these experiments suggest that the critical dynamic stress intensity factor achieved by the notched LM-1 specimens is ~110 MPa m1/2, which is similar to the fracture toughness determined from previous quasi-static fracture experiments. This insensitivity of the fracture toughness to crack tip loading rate suggests negligible loading-rate sensitivity on the dynamic fracture initiation toughness in LM-1. In situ high-speed camera images of the notched sample during the dynamic loading process show multiple fracture initiation attempts and subsequent arrests prior to catastrophic fracture initiation. Controlled stress wave loading experiments designed to induce sub-critical levels of damage in the notched specimens show extensive deformation banding extending 150 to 200 μm outward from the notch. The deformation bands, nominally perpendicular to each other, run along the direction of the notch and perpendicular to it. They are consistent with slip-line fields in notched samples of elastic perfectly plastic materials. Subsequent loading of the damaged specimen again shows several attempts at crack initiation followed by blunting; the initial sub-critical damage in the region around the notch is understood to increase the energy required for catastrophic specimen failure and is consistent with an increase in the effective notch radius due to preexisting damage. 相似文献
5.
The effect of residual stresses and loading frequencies on corrosion fatigue crack growth behavior under synthetic seawater
with a free corrosion potential was examined using center-cracked tension (CCT) and single edge-cracked tension (SECT) specimens
machined from mild steel butt-welded joints and the parent material. A series of fatigue crack growth tests were carried out
with a sinusoidal loading wave form at a stress ratio of 0.05 with a loading frequency of 0.017 to 6.7 Hz. The results show
that the crack growth resistance of a weld metal in the SECT specimen is higher than that in the CCT specimen regardless of
testing conditions. The discrepancy is attributed to the differences in residual stress distribution at the crack tip in the
two specimen geometries. The crack growth rate of the weld metal in the CCT specimen in seawater increased with decreasing
loading frequency. The acceleration of the crack growth rate may be related to the occurrence of brittle striation or cleavage
due to hydrogen embrittlement. It was found that the corrosion fatigue crack growth rate of a welded joint with tensile residual
stress can be predicted using the effective stress intensity factor range, which takes into account both the residual stress
and the loading frequency effects. 相似文献
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《Acta Metallurgica Materialia》1991,39(3):255-262
The stress intensity range ΔK below which no cyclic plastic deformation at the crack tip, and hence, no fatigue crack propagation occurs is investigated. The emission of dislocations from the crack tip is assumed as mechanism for the dislocation generation. For a mode III crack, a computer simulation is carried out to study the influence of the number of dislocations, the friction stress and the critical stress intensity, ke, to emit a dislocation. If during loading only one dislocation is emitted, the return of this dislocation to the crack tip and the emission of a dislocation with an opposite sign and the recombination with the first dislocation are possible during unloading. The ΔK necessary for both mechanisms is about 2ke. If during loading more than one dislocation is emitted, during unloading at first a certain number of disclocations return to the crack tip before a dislocation of opposite sign is emitted. The necessary ΔK to move one dislocation back to the crack tip during unloading decreases with increasing number of dislocations and reaches a constant values of about 1.1ke. This value of ΔK then is roughly independent of the friction stress and ke. 相似文献
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K. T. Venkateswara Rao S. C. Siu R. O. Ritchie 《Metallurgical and Materials Transactions A》1993,24(3):721-734
Micromechanisms influencing crack propagation in a unidirectional SiC-fiber (SCS-8) continuously reinforced Al-Mg-Si 6061
alloy metal-matrix composite (SiCf/Al-6061) during monotonie and cyclic loading are examined at room temperature, both for the longitudinal (0 deg or L-T) and
transverse (90 deg or T-L) orientations. It is found that the composite is insensitive to the presence of notches in the L-T
orientation under pure tension loading due to the weak fiber/matrix interface; notched failure strengths are ∼1500 MPa compared
to 124 MPa for unreinforced 6061. However, behavior is strongly dependent on loading configuration, specimen geometry, and
orientation. Specifically, properties in SiCf/Al in the T-L orientation are inferior to unreinforced 6061, although the composite does exhibit increasing crack-growth
resistance with crack extension (resistance-curve behavior) under monotonie loading; peak toughnesses of ∼16 MPa√m are achieved
due to crack bridging by the continuous metal phase between fibers and residual plastic deformation in the crack wake. In
contrast, such bridging is minimal under cyclic loading, as the ductile phase fails subcritically by fatigue such that the
transverse fatigue crack-growth resistance is superior in the unreinforced alloy, particularly at high stress-intensity levels.
Conversely, fatigue cracks are bridged by unbroken SiC fibers in the L-T orientation and exhibit marked crack deflection and
branching; the fatigue crack-growth resistance in this orientation is clearly superior in the composite. 相似文献
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为探究不同加载角度下A7085铝合金Ⅰ-Ⅱ复合型疲劳裂纹扩展机理,在MTS疲劳试验机上采用紧凑拉伸剪切试件(CTS)对A7085铝合金进行不同加载角度的疲劳实验;用有限元分析计算不同裂纹扩展长度的裂纹尖端应力强度因子,通过七点递增多项式法对数据进行处理,计算出A7085铝合金Paris公式中的参数C和m.结果表明不同加载角度的裂纹基本沿着与外载荷垂直的方向扩展,裂纹扩展路径近似为一条直线,裂纹扩展角测量结果基本符合最大环向拉应力理论;Ⅰ-Ⅱ复合型裂纹一旦发生扩展,Ⅱ型应力强度因子KⅡ所占比例急剧减小,Ⅰ型应力强度因子KⅠ不断增大,此后KⅡ远远小于KⅠ,有效应力强度因子(KⅠ和KⅡ的组合)基本等于KⅠ,相当于裂纹扩展主要受Ⅰ型应力强度因子控制,研究结果有助于对Ⅰ-Ⅱ复合型疲劳裂纹扩展机理的理解. 相似文献
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《Acta Metallurgica Materialia》1993,41(8):2317-2335
Finite element analyses were conducted with the objective of determining the cyclic near-tip fields ahead of a stationary tensile fatigue crack lying along the interface between two dissimilar solids. The model systems analyzed are (i) a metal-metal bimaterial whose components have the same elastic properties but different plastic deformation characteristics, and (ii) a metal-ceramic bimaterial. In both cases, monotonic loading to the peak tensile load results in a predominantly mode I field ahead of the crack. However, unloading in one or both components of the bimaterial with prior plastic deformation generates mixed-mode conditions at the crack tip. The mode mixity persists during a significant portion of the next loading phase and is gradually removed upon reloading to the peak stress. The cyclic plastic zone size directly ahead of the fatigue crack is approximately one-fifth the size of the monotonic plastic zone. The residual compressive stresses within the cyclic plastic zone are computed. The implications of reversed yielding and mixed-mode near-tip fields to constant-amplitude and variable-amplitude fatigue fracture are discussed. 相似文献
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A series of high-temperature fatigue crack growth experiments was conducted on a continuous-fiberreinforced SM1240/TIMETAL-21S
composite using three different temperatures, room temperature (24 °C), 500 °C, and 650 °C, and three loading frequencies,
10, 0.1, and 0.02 Hz. In all the tests, the cracking process concentrated along a single mode I crack for which the principal
damage mechanism was crack bridging and fiber/matrix debonding. The matrix transgranular fracture mode was not significantly
influenced by temperature or loading frequency. The fiber debonding length in the crack bridging region was estimated based
on the knowledge of the fiber pullout lengths measured along the fracture surfaces of the test specimens. The average pullout
length was correlated with both temperature and loading frequency. Furthermore, the increase in the temperature was found
to lead to a decrease in the crack growth rate. The mechanism responsible for this behavior is discussed in relation to the
interaction of a number of temperature-dependent factors acting along the bridged fiber/matrix debonded zone. These factors
include the frictional stress, the radial stress, and the debonding length of the fiber/matrix interface. In addition, the
crack growth speed was found to depend proportionally on the loading frequency. This relationship, particularly at low frequencies,
is interpreted in terms of the development of a crack tip closure induced by the relaxation of the compressive residual stresses
developed in the matrix phase in regions ahead of the crack tip during the time-dependent loading process. 相似文献
13.
Fracture and fatigue experiments have been conducted on liquid phase bonded (LPB) and solid-state bonded (SSB) aluminum-alumina
interfaces. The LPB interfaces contain voids and dendritic FeAl3 precipitates, whereas SSB interfaces are relatively defect-free. These precipitates result in local embrittlement, yet both
interfaces are strong and tough. Upon cyclic loading, mode 1 cracks in both systems grow alternately along the interface and
within the A1. The development of a tortuous crack path elevates the apparent fatigue threshold through crack closure. Under
mixed mode loading, fatigue cracks approaching SSB interfaces propagate through the A1 rather than along the interface. Conversely,
for LPB interfaces, mixed mode cyclic crack growth along the interface occurs in preference to propagation in the A1. Correlation
between the striation spacing and the crack tip opening displacement suggests a growth mechanism based on crack tip blunting. 相似文献
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《Acta Metallurgica Materialia》1990,38(11):2327-2336
Transient subcritical crack-growth behavior following abrupt changes in the applied load are studied in transformation-toughened ceramics. A mechanics analysis is developed to model the transient nature of transformation shielding of the crack tip Ks, with subcritical crack extension following the applied load change. Conditions for continued crack, growth crack growth followed by arrest, and no crack growth after the load change, are considered and related to the magnitude and sign of the applied load change and to materials properties such as the critical transformation stress. The analysis is found to provide similar trends in Ks compared to values calculated from experimentally measured transformation zones in a transformation-toughened Mg-PSZ. In addition, accurate prediction of the post load-change transient crack-growth behavior is obtained using experimentally derived steady-state subcritical crack-growth relationships for cyclic fatigue in the same material. 相似文献
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A failure diagram that combines the thresholds for failure of a smooth specimen to that of a fracture mechanics specimen,
similar to the modified Kitagawa diagram in fatigue, is presented. For a given material/environment system, the diagram defines
conditions under which a crack initiated at the threshold stress in a smooth specimen becomes a propagating crack, by satisfying
the threshold stress intensity of a long crack. In analogy with fatigue, it is shown that internal stresses or local stress
concentrations are required to provide the necessary mechanical crack tip driving forces, on one hand, and reaction/transportation
kinetics to provide the chemical potential gradients, on the other. Together, they help in the initiation and propagation
of the cracks. The chemical driving forces can be expressed as equivalent mechanical stresses using the failure diagram. Both
internal stresses and their gradients, in conjunction with the chemical driving forces, have to meet the minimum magnitude
and the minimum gradients to sustain the growth of a microcrack formed. Otherwise, nonpropagating conditions will prevail
or a crack formed will remain dormant. It is shown that the processes underlying the crack nucleation in a smooth specimen
and the crack growth of a fracture mechanics specimen are essentially the same. Both require building up of internal stresses
by local plasticity. The process involves intermittent crack tip blunting and microcrack nucleation until the crack becomes
unstable under the applied stress. 相似文献
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《Acta Metallurgica Materialia》1991,39(6):1367-1376
Hydrogen assisted subcritical cleavage of the ferrite matrix occurs during fatigue of a duplex stainless steel in gaseous hydrogen. The ferrite fails by a cyclic cleavage mechanism and fatigue crack growth rates are independent of frequency between 0.1 and 5 Hz. Macroscopic crack growth rates are controlled by the fraction of ferrite grains cleaving along the crack front, which can be related to the maximum stress intensity, Kmax. A superposition model is developed to predict simultaneously the effects of stress intensity range (ΔK) and K ratio (Kmin/Kmax). The effect of Kmax is rationalised by a local cleavage criterion which requires a critical tensile stress, normal to the {001} cleavage plane, acting over a critical distance within an embrittled zone at the crack tip. 相似文献
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Fatigue crack growth mechanisms of long cracks through fields with low and high residual stresses were investigated for a
common structural aluminum alloy, 6061-T61. Bulk processing residual stresses were introduced in the material by quenching
during heat treatment. Compact tension (CT) specimens were fatigue crack growth (FCG) tested at varying stress ratios to capture
the closure and K
max effects. The changes in fatigue crack growth mechanisms at the microstructural scale are correlated to closure, stress ratio,
and plasticity, which are all dependent on residual stress. A dual-parameter ΔK–K
max approach, which includes corrections for crack closure and residual stresses, is used uniquely to connect fatigue crack growth
mechanisms at the microstructural scale with changes in crack growth rates at various stress ratios for low- and high-residual-stress
conditions. The methods and tools proposed in this study can be used to optimize existing materials and processes as well
as to develop new materials and processes for FCG limited structural applications. 相似文献
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《Acta Metallurgica Materialia》1994,42(7):2353-2363
Fatigue crack growth under Mode I loading, and static fracture in both symmetrical and asymmetrical notched four point bend specimens, have been examined in SiC whisker reinforced 2009 aluminium alloy. In the fatigue tests a range of orientations of the starter notch, with respect to the extrusion direction, L, was examined. Slanted crack propagation was observed in all specimens except that in the T-L configuration. For the monotonic tests the specimen orientation (L-T) remained constant whilst the ratio of Mode I to Mode II loading was varied. Again crack deflection was observed in all cases apart from the L-T specimen under pure Mode I loading. Whisker debonding was found to be the dominant factor controlling crack deflection, independent of the mixity of the loading mode. Under mixed-mode static loading, the deflection angle was controlled by the average orientation of the whiskers subject to the asymmetrically distributed maximum principal stress. In fatigue loading, however, the crack tended to follow the most frequently observed whisker orientation. These contrasting observations are interpreted in terms of the different sampling volumes at the crack tip in monotonic fracture and in fatigue crack growth. 相似文献
19.
Fatigue crack growth behavior of a peak-aged Al2O3/Al-Cu composite was examined at 150 °C and compared to the behavior at room temperature (RT). At 150 °C, fatigue crack growth
rates showed strong dependence on loading time. At short loading time, when stress-intensity range was decreased to approach
fatigue threshold, crack growth rates at 150 °C were comparable to those measured at RT. Prolonged fatigue testing at near-threshold
crack growth rates resulted in oscillations of crack growth rate until the fatigue crack growth behavior was stabilized to
become similar to that in an overaged composite. Measurement of the matrix hardness at different distances from the crack
plane and transmission electron microscopy examination of the fatigue specimen have shown that the matrix microstructure at
the tip of the fatigue crack underwent overaging during prolonged testing in the near-threshold regime. Consequently, the
fatigue fracture mechanism was modified, a lower crack closure developed, and the fatigue threshold reduced to that of the
overaged composite. 相似文献
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Gaseous hydrogen-induced cracking of Ti-5Al-2.5Sn 总被引:2,自引:0,他引:2
The kinetics of hydrogen-induced cracking have been studied in the Ti-5Al-2.5Sn titanium alloy having a structure of acicular
α platelets in a β matrix. It was observed that the relationship between hydrogen-induced crack growth rate and applied stress
intensity can be described by three separable regions of behavior. The crack-growth rate at low stress-intensity levels was
found to be exponentially dependent on stress intensity but essentially independent of temperature. The crack-growth rate
at intermediate stress-intensity levels was found to be independent of stress intensity but dependent on temperature in such
a way that crack-growth rate was controlled by a thermally activated mechanism having an activation energy of 5500 cal per
mole and varied as the square root of the hydrogen pressure. The crack-growth rate at stress-intensity levels very near the
fracture toughness is presumed to be independent of environment. The results are interpreted to suggest that crack growth
at high stress intensities is controlled by normal, bulk failure mechanisms such as void coalescence and the like. At intermediate
stress-intensity levels the transport of hydrogen to some interaction site along the α-β boundary is the rate-controlling
mechanism. The crack-growth behavior at low stress intensities suggests that the hydrogen interacts at this site to produce
a strain-induced hydride which, in turn, induces crack growth by restricting plastic flow at the crack tip. 相似文献