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The rate of fatigue crack propagation for Ti-6Al-6V-2Sn and Ti-6 A1-4V in aqueous environments has been measured as a function
of solution chemistry, frequency, and stress wave form. Depending on the specific encironment, three types of fatigue crack
growth rate behavior have been observed as a function of frequency. Crack growth rates increase with decreasing frequency
in distilled water, while addition of Na2SO4 produces frequency-independent behavior. In solutions containing chloride or bromide ions, a reversal in frequency-dependence
takes place at ΔKscc. Below this transition ΔK level, crack growth rates decrease with decreasing frequency due to passive film formation at the
crack tip. Above ΔKscc corrosion fatigue crack growth is due to SCC under cyclic loading. The ΔK transition in fatigue is lower than the static
stress corrosion threshold because of repeated rupture of the passive film at the crack tip, approaching KIsco only for very slow cycling frequencies.
This paper is based upon a thesis submitted by D. B. Dawson in partial fulfillment of the requirements of the degree of Doctor
of Science at Massachusetts Institute of Technology. 相似文献
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Crack Propagation in Flexural Fatigue of Concrete 总被引:1,自引:0,他引:1
Subramaniam V. Kolluru Edward F. O'Neil John S. Popovics Surendra P. Shah 《Canadian Metallurgical Quarterly》2000,126(9):891-898
In this paper the behavior of concrete subjected to flexural fatigue loading is studied. Notched concrete beams were tested in a three-point bending configuration. Specimens were subjected to quasi-static cyclic and constant amplitude fatigue loading. The cyclic tests were performed by unloading the specimen at different points in the postpeak part of the quasi-static loading response. Low cycle, high amplitude fatigue tests were performed to failure using four different load ranges. The crack mouth opening displacement was continuously monitored throughout the loading process. Crack propagation caused by quasi-static and fatigue loads is described in terms of fracture mechanics. It is shown that the crack propagation in the postpeak part of the quasi-static load response is predicted using the critical value of the mode I stress intensity factor (KIC). The ultimate deformation of the specimen during the fatigue test is compared with that from the quasi-static test; it is demonstrated that the quasi-static deformation is insufficient as a fatigue failure criterion. It is observed that crack growth owing to constant-amplitude fatigue loading comprises two phases: a deceleration stage when there is a decrease in crack growth rate with increasing crack length, followed by an acceleration stage where the rate of crack growth increases at a steady rate. The crack length where the rate of crack growth changes from deceleration to acceleration is shown to be equal to the crack length at the peak load of the quasi-static response. Analytical expressions for crack growth in the deceleration and acceleration stages are developed, wherein the expressions for crack growth rate in the deceleration stage are developed using the R-curve concept, and the acceleration stage is shown to follow the Paris law. It is observed that the crack length at failure for constant amplitude fatigue loading is comparable to that of the corresponding load in the postpeak part of the quasi-static response. Finally, a fracture-based fatigue failure criterion is proposed. 相似文献
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采用4点弯曲试样,对X65钢级UOE焊管焊缝接头进行NACE0177标准规定的SSCC(Sulfur Stress Corrosion Cracking,硫化物应力腐蚀开裂)试验,并采用应变计法完成目标应力加载。结果表明,通过应变计法可以有效地建立FPB(Four Point Bent,4点弯曲)试样加载应变-载荷-应力关系,并根据该关系准确确定目标加载应变量,从而实现目标应力的准确加载。采用该方法完成试验试样90%实际屈服强度的应力加载,并经H2S环境溶液浸泡720h后表面未见裂纹,试验材料表现出优异的抗硫化物应力腐蚀开裂性能。 相似文献
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《Canadian Metallurgical Quarterly》1997,36(3):157-167
Alloy 2219 has been evaluated under corrosion fatigue conditions. The effect of the micro-structures present in the T851 and T6 conditions on crack propagation rates has been determined. Tests were performed on compact tension specimens in air and in NaCI solutions at 23 and 70°C. The corrosion fatigue behavior of the material under these conditions was evaluated by studying the crack propagation kinetics and also crack fractography.The results of this study show that 2219 aluminum is resistant to corrosion fatigue in aqueous solutions containing up to 10% NaCI. However, the material is more resistant in the T6 than in the T851 condition. In the absence of stress corrosion cracking, the initial stages of the crack growth were characterised by ductile fracture while a mixed fracture mode dominated the final stages of the growth. At low loading frequencies (0. 1 Hz), crack branching and blunting were observed when tests were performed at 70°C. 相似文献
8.
The effectiveness of MIG welding with Argo‐shield gas & ER70S‐6 electrode in joining LRS (Grade‐B) steel was investigated through structure–property correlation of the joint region. Microstructure, tensile and fatigue properties, and mode of fracture (SEM fractograph) were correlated. Fatigue behavior has been investigated in air and sea water with thin specimen at near‐endurance stress amplitude up to 105 cycles. The crack growth rate (da/dN) maintained a non‐linear relationship with logarithm of stress intensity factor range (logΔK) for the near‐threshold values of ΔK. Considerable hardness and microstructural variation was observed across the weldment. Weld with more pearlite content was found to possess higher hardness and strength than the parent steel. Though, both in weld and in parent steel, either in air or in sea water, fatigue crack propagated at very slow rate with significant intermittent crack arrest, weld provided much higher resistance to crack growth in air. However, sea water accelerated the crack growth in weld and brought it closer to that in the parent steel. The morphologically complex microstructure of weld suffered much faster crack propagation in sea water than in air. While fatigue fracture in parent steel (both in air and sea water) and weld in air was found to occur through dimple rupture via microvoid coalescence, weld in sea water exhibited a mixed mode of failure. 相似文献
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J. T. Al-Haidary A. A. Wahab E. H. Abdul Salam 《Metallurgical and Materials Transactions A》2006,37(11):3205-3214
The fatigue crack propagation rate (FCPR) in 316L austenitic stainless steel (ASS) and its weldments was investigated, at
two loading amplitudes, 7 and 8.5 kN, under tension-tension mode. Two welding techniques, submerged arc welding (SAW) and
manual arc welding (MAW), have been used. Magnetic δ-ferrite, depending upon Ni and Cr content in the metal, in the weld zone
upon solidification was considered. The ferrite number (FN) of δ-ferrite formed in the SAW zone was much higher (maximum 9.6)
compared to the corresponding value (maximum 0.75) in the MAW zone. A fatigue starter notch was positioned at different positions
and directions with respect to the weld zone, in addition to the heat-affected zone (HAZ). Regions of high and low FCPRs as
the fatigue crack propagated through and across the weld zone have been noticed. This is related to the direction of the tensile
residual stresses present in weld zone, resulting from solidification of the weld metal. The FCPR was higher along through
the HAZ and weld zone because of the microstructural change and direction and distribution of tensile residual stresses. The
FCPR was much lower when crack propagated perpendicular to the weld zone, particularly in the case of SAW in which higher
δ-ferrite volume fraction was noticed. A lower FCPR found across the weld zone, in both SAW and MAW, was accompanied by rubbed
areas in their fractures. 相似文献
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E-Wen Huang Soo Yeol Lee Wanchuck Woo Kuan-Wei Lee 《Metallurgical and Materials Transactions A》2012,43(8):2785-2791
Quantitative determination of the stress fields around the crack tip is a challenging and important subject to understand the fatigue crack-growth mechanism. In the current study, we measured the distribution of residual stresses and the evolution of the stress fields around a fatigue crack tip subjected to the constant-amplitude cyclic loading in a 304L stainless steel compact-tension (CT) specimen. The three orthogonal stress components (i.e., crack growth, crack opening, and through thickness) of the CT specimen were determined as a function of distance from the crack tip with 1-mm spatial resolution along the crack-propagation direction. In-situ neutron-diffraction results show that the enlarged tensile stresses were developed during loading along the through-thickness direction at a localized volume close to the crack tip, resulting in the lattice expansion in all three orthogonal directions during P max. The current study suggests that the atypical plane strainlike behavior observed at the midthickness position might be the reason for the mechanism of the faster crack-growth rate inside the interior than that near the surface. 相似文献
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The growth of short fatigue cracks in a NiCrMoV steel forging was examined, under constant applied stress intensity range
(ΔK = 31 MPa-m1/2) in deaerated deionized water and 0.3 M Na2SO4 solution, as a function of frequency and temperature. Measurements were also made of the kinetics of electrochemical reactions
of bare steel surfaces with the deaerated 0.3 M Na2SO4 solution, under free corrosion, to provide for comparison and correlation. Fatigue crack growth rate increased with reductions
in frequency and with increases in temperature. The maximum amount of crack growth enhancement by the different environments
appeared to be equal, although the crack growth response in deionized water appeared to be consistent with a faster reaction
rate. The temperature and frequency dependence for corrosion fatigue crack growth corresponded directly with that for charge
transfer between the “bare” and “filmed” metal surfaces under free corrosion. The results showed that shortcrack growth in
the aqueous environments is controlled by the rate of electrochemical reactions, and is thermally activated with an apparent
activation energy of about 40 kJ/M. 相似文献
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Elena Garlea Hahn Choo Gongyao Y. Wang Peter K. Liaw Bjørn Clausen Donald W. Brown Jungwon Park Philip D. Rack Edward A. Kenik 《Metallurgical and Materials Transactions A》2010,41(11):2816-2828
The hydride-phase formation and its influence on the fatigue behavior of a Zircaloy-4 alloy charged with hydrogen gas are investigated. First, the microstructure and fatigue crack propagation rate of the alloy in the as-received condition are studied. Second, the formation and homogeneous distribution of the delta zirconium hydride in the bulk and its effect on the fatigue crack propagation rate are presented. The results show that in the presence of hydrides, the zirconium alloy exhibits reduced toughness and enhanced crack growth rates. Finally, the influence of a preexisting fatigue crack in the specimen and the subsequent hydride formation are examined. The residual lattice strain profile around the fatigue crack tip is measured using neutron diffraction. It is observed that the combined effects of residual strains and hydride precipitation on the fatigue behavior are more severe leading to propagation of the crack under near threshold loading. 相似文献
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M. Valsan D. Sundararaman K. Bhanu Sankara Rao S. L. Mannan 《Metallurgical and Materials Transactions A》1995,26(5):1207-1219
A comparative evaluation of the low-cycle fatigue (LCF) behavior of type 316LN base metal, 316 weld metal, and 316LN/316 weld
joints was carried out at 773 and 873 K. Total strain-controlled LCF tests were conducted at a constant strain rate of 3 ×
10−3 s−1 with strain amplitudes in the range ±0.20 to ±1.0 pct. Weld pads with single V and double V configuration were prepared by
the shielded metal-arc welding (SMAW) process using 316 electrodes for weld-metal and weld-joint specimens. Optical microscopy,
scanning electron microscopy (SEM), and transmission electron microscopy (TEM) of the untested and tested samples were carried
out to elucidate the deformation and the fracture behavior. The cyclic stress response of the base metal shows a very rapid
hardening to a maximum stress followed by a saturated stress response. Weld metal undergoes a relatively short initial hardening
followed by a gradual softening regime. Weld joints exhibit an initial hardening and a subsequent softening regime at all
strain amplitudes, except at low strain amplitudes where a saturation regime is noticed. The initial hardening observed in
base metal has been attributed to interaction between dislocations and solute atoms/complexes and cyclic saturation to saturation
in the number density of slip bands. From TEM, the cyclic softening in weld metal was ascribed to the annihilation of dislocations
during LCF. Type 316LN base metal exhibits better fatigue resistance than weld metal at 773 K, whereas the reverse holds true
at 873 K. The weld joint shows the lowest life at both temperatures. The better fatigue resistance of weld metal is related
to the brittle transformed delta ferrite structure and the high density of dislocations at the interface, which inhibits the
growth rate of cracks by deflecting the crack path. The lower fatigue endurance of the weld joint was ascribed to the shortening
of the crack initiation phase caused by surface intergranular crack initiation and to the poor crack propagation resistance
of the coarse-grained region in the heat-affected zone. 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
15.
Kolluru V Subramaniam John S. Popovics Surendra P. Shah 《Canadian Metallurgical Quarterly》2002,128(6):668-676
In this paper cyclic quasi-static and constant amplitude fatigue responses of concrete subjected tensile compression–tension (C–T) biaxial stress are presented. In the tensile C–T region within the biaxial stress space, magnitude of the principal tensile stress is larger than or equal to that of the principal compressive stress. An experimental program consisted of subjecting hollow, cylindrical concrete specimens to torsional loading. Failure in both quasi-static and fatigue is due to crack propagation. It is shown that the crack propagation resulting from the biaxial loading can be predicted using Mode I fracture parameters. The fatigue crack growth is observed to be a two-phase process: an acceleration stage that follows a deceleration stage. The crack length where the rate of crack growth changes from deceleration to acceleration is shown to be equal to the crack length at the quasi-static peak load. Analytical expressions for crack growth in the deceleration and acceleration stages are developed in terms of the mechanisms that influence quasi-static crack growth. The model parameters obtained from uniaxial fatigue tests are shown to be sufficient for predicting the biaxial fatigue response. Finally, a fracture-based fatigue-failure criterion is proposed, wherein the fatigue failure can be predicted using the critical Mode I stress intensity factor. 相似文献
16.
P. K. Liaw J. Anello N. S. Cheruvu J. K. Donald 《Metallurgical and Materials Transactions A》1984,15(4):693-699
The Kmax-controlled near-threshold fatigue crack growth behavior was investigated on 422 stainless steel in a boiling NaCl solution.
During the test, there was a transition from corrosion fatigue to stress corrosion cracking. The transition occurred at very
high load ratios (R=-0.91) and at very lowAK levels (≤2.1 MPa√m). The characteristics of stress corrosion cracking (SCC) were manifested by time-based crack growth rather
than cycle-based crack growth, by crack extension under static loading, and by change in fracture mode. In corrosive environments,
the small ripple loading imposed on structural materials should be recognized for engineering designs and failure analyses. 相似文献
17.
Fatigue tests were performed on specimens containing weld heat affected zones at two orientations to the stress axis. Two
heat affected zones were studied, one in Ducol W30 (a low alloy steel) and the other in mild steel. Under conditions of constant
alternating and maximum stress intensity a fatigue crack only propagated at a uniform rate when it was remote from the heat
affected zone. A heat affected zone which was harder than either the parent plate or weld metal was found to reduce crack
propagation rates by a factor of up to 2 by restricting the plastic zone size around the crack tip. The changes in crack propagation
rate could not be related uniquely to the conditions of the material immediately adjacent to the crack tip. Furthermore, the
shape of the plastic zone was found to influence the direction of the propagation of a fatigue crack which always deviated
toward regions of lower flow stress. A crack was never found to follow the interface between the weld metal and the parent
metal heat affected zone because the flow stresses were not the same on either side of the interface. There was no difference
in crack propagation mechanism between the parent plate and its heat affected zone for the stress conditions imposed.
Formerly with Central Electricity Research Laboratories, Materials Division, Leatherhead, Surrey, England 相似文献
18.
Jixi Zhang Sergiy Kalnaus Majid Behrooz Yanyao Jiang 《Metallurgical and Materials Transactions A》2011,42(2):448-460
An experimental study of stress corrosion cracking (SCC) was conducted on 7075-T651 aluminum alloy in a chromate-inhibited,
acidic 3.5 pct sodium chloride aqueous solution using compact tension specimens with a thickness of 3.8 mm under permanent
immersion conditions. The effects of loading magnitude, overload, underload, and two-step high-low sequence loading on incubation
time and crack growth behavior were investigated. The results show that the SCC process consists of three stages: incubation,
transient crack growth, and stable crack growth. The incubation time is highly dependent on the load level. Tensile overload
or compressive underload applied prior to SCC significantly altered the initiation time of corrosion cracking. Transition
from a high to a low loading magnitude resulted in a second incubation but much shorter or disappearing transient stage. The
stable crack growth rate is independent of stress intensity factor in the range of 10 to 22 MPa
?{\textm} . \sqrt {\text{m}} . 相似文献
19.
D. B. Dawson 《Metallurgical and Materials Transactions A》1981,12(5):791-800
Differences in the corrosion fatigue crack growth behavior of anα–β titanium alloy in chloride-containing aqueous and methanol environments are reported, and discussed in relation to differences
in repassivation behavior for the two types of environments. Experiments have been conducted with various solution mixtures
of water (a passive film-forming environment) and methanol (a nonfilm-forming environment) to define the role of repassivation
in controlling fracture modes and crack growth rates at different frequencies. The critical event in determining whether the
repassivation process can suppress environmental fatigue fracture is the interaction between the rate of exposure of fresh
metal surfaces at the crack tip and the rate at which they can be repassivated. The out-come of this mechano-chemical interaction
is shown to be dependent on the frequency and stress intensity(ΔK) level as well as the chemistry of the environment. As a result, differences in repassivation behavior for methanol-water
solutions can be correlated with major differences in fatigue crack growth rates and fracture modes at low ΔK levels, whereas
repassivation differences have little effect at high ΔK levels. Based on these low-ΔK corrosion fatigue characteristics, methanol
solutions are concluded to be far more detrimental to titanium alloys than aqueous solutions. 相似文献
20.
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