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1.
H. F. Lopez R. Bharadwaj J. L. Albarran L. Martinez 《Metallurgical and Materials Transactions A》1999,30(9):2419-2428
In this work, the role of the microstructure in the stress sulfide cracking (SSC) resistance of an API X-80 steel was investigated
by exposure of as-received and heat-treated specimens to a H2S-saturated aqueous National Association of Corrosion Engineers (NACE) solution. It was found that for similar corrosive environments
and applied stress intensity factors of 30 to 46 MPa√m, crack growth in LEFM (linear elastic fracture mechanics) compact specimens
is strongly influenced by heat treating. In the as-received alloy, crack growth in the direction normal to rolling was controlled
by metal dissolution of the crack tip region in contact with the corrosive environment, with crack growth rates of the order
of 1/W(da/dt)∼8.3×10−4 h−1. Alternatively, crack growth in the direction parallel to the rolling direction did not show metal dissolution, but instead
hydrogen embrittlement along segregation bands. In this case, crack growth rates of the order of 1.2×10−3 h−1 were exhibited. In the martensitic condition, the rate of crack propagation was relatively fast (1/W(da/dt)∼4.5×10−2 h−1), indicating severe hydrogen embrittlement. Crack arrest events were found to occur in water-sprayed and quenched and tempered
specimens, with threshold stress intensity values (K
ISSC) of 26 and 32 MPa√m, respectively. Apparently, in the water-sprayed condition, numerous microcracks developed in the crack
tip plastic zone. Crack growth occurred by linking of microcracks, which were able to reach the main crack tip. In particular,
preferential microcrack growth occurred across carbide regions, but their growth was severely limited in the ferritic matrix.
Quenching and tempering (Q&T) resulted in a tempered martensite microstructure characterized by fine distribution carbides,
most of which were cementite. In this case, the crack path continually shifted to follow the ferrite interlath boundaries,
which contained mostly fine cementite precipitates. As a result, the crack was tortuous with numerous bifurcations along ferrite
grain boundaries. Most of the tests were carried out in NaCl-free NACE solutions; the only exception was the as-received condition
where 5 wt pct NaCl was added to the sour environment. In this case, crack growth did not occur after exposing the specimen
to the salt-free NACE solution for 30 days, but addition of 5 pct NaCl promoted crack propagation. 相似文献
2.
Masaaki Tabuchi Yuji Nakasone Toshio Ohba Koichi Yagi Tatsuhiko Tanabe 《Metallurgical and Materials Transactions A》1995,26(2):383-389
Creep crack growth tests were conducted on Ni-26 pct Cr-17 pct W-0.5 pct Mo alloys with different carbon contents in air and
in helium gas environment at 1273 K using the compact-type (CT) specimen, and the effects of carbon content and environment
on creep crack growth rate are discussed. Creep crack growth rateda/dt is evaluated by theC* parameter. Theda/dt is faster in higher-carbon alloys than in lower-carbon alloys in each environment. This effect of carbon content is attributed
to the lower creep ductility due to the increase of fine trans-granular carbides in higher-carbon alloys. The environmental
effect on theda/dt vs C* relations is scarcely observed in higher-carbon alloys. In the 0.003 pct C alloy, however,da/dt is much lower in the He gas environment than in air. Carburization is observed ahead of the crack tip in the He gas environment
at 1273 K. The intergranular carbides precipitated due to carburi-zation have a granular configuration and are considered
to prevent the grain boundary sliding in lower-carbon alloys. 相似文献
3.
X. J. Wu W. Wallace A. K. Koul M. D. Raizenne 《Metallurgical and Materials Transactions A》1995,26(11):2973-2982
Near-threshold fatigue crack growth was studied in 8090-T8771 Al-Li alloy tested in moist laboratory air. The testing was
conducted using (1) the ASTM E-647 load-shedding procedure, (2) a power-law load-shedding procedure, and (3) a constant-amplitude
(CA) loading procedure. Crack closure in the three procedures was analyzed. In reconciling fatigue crack growth rates (FCGRs)
with different crack closure levels under identical testing parameters, the conventional ΔK
eff (=K
max —K
op) fails to correlate the test data and the modified ΔK
eff (=K
max
-
χKop, where χ is the shielding factor, defined by an energy approach) is proven to be the true crack driving force. A parallel
slip-rupture model is proposed to describe the mechanism of near-threshold fatigue crack growth in this alloy. The model explains
the mode transition from crystallographic slip band cracking (SBC) to subgrain boundary cracking (SGC)/brittle fracture (BF)
in terms of a microstructure-environment synergy. The transition is related to the material’s short-transverse grain size. 相似文献
4.
Fracture mechanics and surface chemistry studies of fatigue crack growth in an aluminum alloy 总被引:3,自引:0,他引:3
R. P. Wei P. S. Pao R. G. Hart T. W. Weir G. W. Simmons 《Metallurgical and Materials Transactions A》1980,11(1):151-158
Fracture mechanics and surface chemistry studies were carried out to develop further understanding of the influence of water
vapor on fatigue crack growth in aluminum alloys. The room temperature fatigue crack growth response was determined for 2219-T851
aluminum alloy exposed to water vapor at pressures from 1 to 30 Pa over a range of stress intensity factors (K). Data were also obtained in vacuum (at < 0.50 μPa), and dehumidified argon. The test results showed that, at a frequency
of 5 Hz, the rate of crack growth is essentially unaffected by water vapor until a threshold pressure is reached. Above this
threshold, the rates increased, reaching a maximum within one order of magnitude increase in vapor pressure. This maximum
crack growth rate is equal to that obtained in air (40 to 60 pct relative humidity), distilled water and 3.5 pct NaCl solution
on the same material. Parallel studies of the reactions of water vapor with fresh alloy surfaces (produced either byin situ impact fracture or by ion etching) were made by Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS).
The extent of surface reaction was monitored by changes in the oxygen AES and XPS signals. Correlation between the fatigue
crack growth response and the surface reaction kinetics has been made, and is consistent with a transport-limited model for
crack growth. The results also suggest that enhancement of fatigue crack growth by water vapor in the aluminum alloys occurs
through a “hydrogen embrittle ment” mechanism. 相似文献
5.
Fatigue crack growth rates were measured at room temperature in dry air for three 7075-T6 aluminum alloys with different inclusion
content. Volume fractions of inclusions were determined for each alloy by the point count method with two different automated
systems. Plots of the fatigue crack growth rate (da/dN) vs the stress-intensity-factor range (ΔK) show a well defined change of slope at the transition between plane strain and plane stress fracture. This transition is
associated with a marked increase in the amount of fracture by void growth around inclusions. The volume fraction and mean
spacing of voids within the cyclic plastic zone have been determined as a function of ΔK by quantitative fractography. Fracture by voids is important when the mean spacing of such voids is approximately equal to
the width of the cyclic plastic zone in the plane of the crack. It is concluded that the inclusion content increases the fatigue-crack
growth rates only within the plane stress range, that is for values of the stress-intensity-factor range ΔK \s> 20 kpsi√in. 相似文献
6.
《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
7.
Morihiko Nakamura Ei-ichi FurubaYashi 《Metallurgical and Materials Transactions A》1983,14(3):717-726
The crack propagation behavior in delayed fracture was investigated at room temperature under pressure up to 784 kPa of hydrogen
gas for high strength steels with the tensile strength of 1500 or 2000 MPa. For specimens with the tensile strength of 1500
MPa, the crack propagation rateda/dt increased with C content from 0.21 to 0.42 wt pct, and then it decreased with increasing C content up to 0.53 wt pct.da/dt increased rapidly with Mn content from 0.009 to 0.84 wt pct, and it increased gradually with Mn content from 0.84 to 2.13
wt pct. The permeation flow of hydrogen from the crack tip surface could be qualitatively estimated from the dependence ofda /dt upon the hydrogen pressure based on the simple assumption. In addition to the grain boundary embrittlement caused by tempering
and/or hydrogen, the permeation flow of hydrogen was used to explain qualitatively the dependence ofda/dt upon C or Mn content. 相似文献
8.
The crack growth behavior of a gamma titanium aluminide alloy, K5S, was investigated at 650 °C and 800 °C under constant load
conditions in a nearly fully-lamellar microstructural form. Crack growth at both temperatures ensues at stress intensities
(K) much higher than anticipated from the R curves. At 650 °C, creep crack extension occurs through the formation of microcracks (interlamellar (IL) separation) and
their joining to the main crack tip through ligament fracture. This results in a mainly transgranular (TG) fracture with occasional
IL separation. This process features a rapid initial crack growth but at decreasing growth rate, followed by a nearly no-growth
stage. At 800 °C, crack extension is accompanied by extensive plastic deformation and consists of an initial rapid transition
period and a subsequent steady state. For similar K’s, crack extension and growth rate are greater at 800 °C than at 650 °C, but even these are very slow processes for this
alloy. The resistance to crack propagation at 650 °C is explained in terms of work hardening that arises during the extended
primary creep deformation occurring ahead of the crack tip. Increased crack propagation at 800 °C is accredited to grain boundary
and lamellar-interface weakening and extensive post primary creep damage in the plastic zone. The resulting fracture at 800
°C is mainly boundary fracture, which consists of IG fracture involving formation and coalescence of voids, and IL separation. 相似文献
9.
R. Pilkington D. A. Miller D. Worswick 《Metallurgical and Materials Transactions A》1981,12(2):173-181
The effect has been investigated of prior damage on the creep crack propagation characteristics of 0.5 pct Cr, 0.5 pct Mo,
0.25 pct V steel at 823 K. On a macroscopic basis, the parametersK
1 andC
* both appear to correlate withda/dt although the parameterC
* is unable to distinguish between virgin and damaged specimens. Rupture lives in the predamaged specimens are reduced by up
to 60 pct when compared to virgin samples. Microscopically, it is found that the nature of the cavitation damage suggests
that surface and grain boundary diffusion processes may have a minimal part to play, crack growth being controlled by the
growth of cavities which is in turn controlled by the deformation of the surrounding matrix. A number of microscopic models
are compared with the experimental data and it is suggested that a model which gives the best correlation with results is
one proposed on the basis of matrix deformation.
Formerly of the Department of Metallurgy, Manchester University
Formerly of the Department of Metallurgy, Manchester University 相似文献
10.
The creep crack growth rates (CCGR) of PM/HIP René-95 were measured from 10-9 m per second to 10-4 m per second in air and in high purity argon at 760°C and 650°C using single edge notched (SEN) specimens. The crack length
was monitored by the D.C. potential difference technique. The data were reported asda/dt vs the elastic stress intensity factor,K,, since PM/HIP René-95 is a creep-brittle material. The CCGR were shown to be strongly environment sensitive. The CCGR were
up to 1000 times faster in air than in argon for a given value ofK
I
. The temperature andK
I
dependence of CCGR in air were shown to correlate with a modified formulation of the Larson-Miller parameter. Notched stress
rupture (NSR) tests were performed at 650°C in air in order to study the effect of notch root radius on the time to initiate
a creep crack. A comparison of the NSR data with SEN data for the same values of initialK, shows that the crack initiation times are a strong function of the notch root radius. It was observed that ninety percent
of the rupture time is spent in crack initiation when the notch root radius is finite, while no incubation time was observed
for creep crack growth from fatigue precracked specimens.
Formerly with Massachusetts Institute of Technology, Cambridge, MA 相似文献
11.
Wu-Yang Chu Chi-Mei Hsiao Wen-Xue Li Yan-Guo Zhang 《Metallurgical and Materials Transactions A》1984,15(11):2087-2092
The aqueous stress corrosion behavior of cast steel and forged steel of the same heat has been examined. It was shown that
the activation energy of crack growth of both cast and forged steels was identical,i.e., Q = 5540 cal/mol, and was comparable with the apparent diffusion activation energy of hydrogen in the steel. Theda/dt for cast and forged steel increased under both cathodic and anodic polarization conditions. Correspondingly, the steady-state
hydrogen permeation flux increased steeply with the increase of polarizing current under either cathodic or anodic polarization.
The influences of the polarization upon theda/dt and the hydrogen permeation flux were similar. TheK
ISCC
of the cast steel was larger than that of the forged steel. This may be due to the observation that the steady-state permeation
flux for the forged steel was twice as large as that of the cast steel. For both cast and forged steels the fracture modes
were clearly dependent uponK
I
at the crack tip, and a transition from dimple to quasi-cleavage or intergranular was observed on the fracture surfaces with
decreasingK
I
.
Formerly Students at Beijing University of Iron and Steel Technology 相似文献
12.
The effect of high-pressure hydrogen and temperature on crack growth was studied in wedge-opening-load (WOL) samples of a
low-carbon steel. At temperatures above 280 ‡C, a hydrogen pressure of 3 ksi gave an increasing amount of acceleration in
crack growth. These conditions approached but were below that needed to give hydrogen attack (HA) in the surrounding matrix.
The value ofda/dt increases exponentially with temperature, andQ is roughly equal to that for grain boundary diffusion. The growth is absent atK
1
= 0 but varies little withK
1
above 15 MPa√m. The value ofda/dt increases steadily with hydrogen pressure in the range of 3 to 21 MPa.
Formerly Visiting Scholar, The Ohio State University. 相似文献
13.
D. L. Davidson K. S. Chan A. McMinn G. R. Leverant 《Metallurgical and Materials Transactions A》1989,20(11):2369-2378
A study has been made of fatigue crack growth through the magnesium alloy ZE41A and a composite of this alloy reinforced with
alumina fibers. Crack growth rates were measured and failure mechanisms characterized for specimens with fibers parallel to
the loading axis and for two off-axis orientations. Crack opening displacements and matrix and fiber strains in the vicinity
of the crack tip were measured using the stereomaging technique. Crack growth rates through the composite were retarded by
the fibers. For the composite with fibers at 22.5 deg to the loading axis, fibers were found to fracture in the composite
at the same stress as measured for the fibers alone. Fiber fracture was the dominant growth-controlling mechanism for fibers
oriented on and 22.5 deg to the loading axis, and little fiber pullout was observed. However, for crack growth through material
with fibers oriented at 45 deg to the loading axis, crack growth was found to exist principally through the interface. Driving
forces for cracks in interfaces were determined to be smaller than the applied δK. It was found that approximate fatigue crack growth rates through the composites could be predicted from those through the
matrix by adjusting the tensile modulus. The upper and lower bounds of fatigue crack growth rate were also computed for the
composite using a micromechanics-based model that incorporated observed failure mechanisms.
A. McMINN, formerly with Southwest Research Institute, is with Failure Analysis Associates, Washington, D.C. 相似文献
14.
D. C. Slavik C. P. Blankenship E. A. Starke R. P. Gangloff 《Metallurgical and Materials Transactions A》1993,24(8):1807-1817
The influences of microstructure and deformation mode on inert environment intrinsic fatigue crack propagation were investigated
for Al-Li-Cu-Mg alloys AA2090, AA8090, and X2095 compared to AA2024. The amount of coherent shearable δ (Al3Li) precipitates and extent of localized planar slip deformation were reduced by composition (increased Cu/Li in X2095) and
heat treatment (double aging of AA8090). Intrinsic growth rates, obtained at high constantK
max to minimize crack closure and in vacuum to eliminate any environmental effect, were alloy dependent;da/dN varied up to tenfold based on applied ΔK or ΔK/E. When compared based on a crack tip cyclic strain or opening displacement parameter (ΔK/(σys
E)1/2), growth rates were equivalent for all alloys except X2095-T8 which exhibited unique fatigue crack growth resistance. Tortuous
fatigue crack profiles and large fracture surface facets were observed for each Al-Li alloy independent of the precipitates
present, particularly δ, and the localized slip deformation structure. Reduced fatigue crack propagation rates for X2095 in
vacuum are not explained by either residual crack closure or slip reversibility arguments; the origin of apparent slip band
facets in a homogeneous slip alloy is unclear. Better understanding of crack tip damage accumulation and fracture surface
facet crystallography is required for Al-Li alloys with varying slip localization. 相似文献
15.
H. F. López R. Raghunath J. L. Albarran L. Martinez 《Metallurgical and Materials Transactions A》1996,27(11):3601-3611
Crack growth in an API X-80 exposed to sour gas environments was investigated using modified wedge-opening-loaded (MWOL) specimens.
The MWOL specimens were tested in the as-received condition and after annealing followed by water spraying to simulate improperly
welded regions. It was found that water-sprayed MWOL specimens were susceptible to stress sulfide cracking in a NaCl-free
NACE solution. Crack growth was relatively slow when subjected to an initially appliedK
I
of 30 MPa
. Under these conditions, crack growth rates continually decreased until crack arrest was exhibited at a thresholdK
I
(K
ISSC) of 26 MPa
. The exhibited crack growth rates were related to the facility with which nucleated microcracks joined the main crack front.
Apparently, preferential nucleation and growth of microcracks within the main crack tip plastic zone accounted for the exhibited
embrittlement. In particular, favorable microcrack growth followed a path consisting of fractured (cut) carbide regions, as
well as various interfaces, including globular inclusions and grain boundary precipitates. 相似文献
16.
R. M. Horn 《Metallurgical and Materials Transactions A》1975,6(9):1525-1533
The relationships between microstructure and fatigue crack propagation behavior were studied in a 5Mo-0.3C steel. Microstructural
differences were achieved by varying the tempering treatment. The amounts, distribution, and types of carbides present were
influenced by the tempering temperature. Optical metallography and transmission electron microscopy were used to characterize
the microstructures. Fatigue fracture surfaces were studied by scanning electron microscopy. For each heat treatment the fatigue
crack growth properties were measured under plane strain conditions using a compact tension fracture toughness specimen. The
properties were reported using the empirical relation of Paris [da/dN = CoΔKm]. It was found that secondary hardening did influence the fatigue crack growth rates. In particular, intergranular modes
of fracture during fatigue led to exaggerated fatigue crack growth rates for the tempering treatment producing peak hardness.
Limited testing in a dry argon atmosphere showed that the sensitivity of fatigue crack growth rates to environment changed
with heat treatment. 相似文献
17.
V. Dimopulos K. M. Nikbin G. A. Webster 《Metallurgical and Materials Transactions A》1988,19(4):873-880
Crack growth data under combined creep and fatigue loading conditions are presented on a nickel base superalloy and a brittle
and ductile low alloy steel. The main variables that have been examined are minimum to maximum load ratioR and frequency. It is shown that at high frequencies transgranular fatigue failure dominates and at low frequencies time dependent
mechanisms govern. Where fatigue processes control, it is demonstrated that crack growth/cycle can be described by the Paris
law and that the influence ofR ratio can be accounted for by crack closure caused by fracture surface roughness, oxidation, and creep and plastic strain
developed at the crack tip. At the low frequencies where time dependent processes dominate, it is shown that crack growth
can be characterized satisfactorily in terms of the creep fracture mechanics parameterC * using a model of crack extension based on ductility exhaustion in a creep damage zone at the crack tip. This model leads
to enhanced resistance to creep/fatigue crack growth with increase in material creep ductility.
This paper is based on a presentation made in the symposium “Crack Propagation under Creep and Creep-Fatigue” presented at
the TMS/AIME fall meeting in Orlando, FL, in October 1986, under the auspices of the ASM Flow and Fracture Committee. 相似文献
18.
Crack growth in 2.25Cr-lMo steels exposed to 3000 psi hydrogen has been investigated in the temperature range 440 °C to 500
°C, using modified wedge-opening loaded specimens to vary stress intensity. Under conditions of temperature and hydrogen pressure,
where general hydrogen attack does not occur, the crack propagated by the growth and coalescence of a high density of methane
bubbles on grain boundaries, driven by the synergistic influence of internal methane pressure and applied stress. Crack growth
rates were measured in base metal, and the heat-affected zones (HAZs) of welds were tempered to different strength levels.
The crack growth rate increased with material strength. Above a threshold of about Kl = 20 MPa√m (at 480 °C), the crack growth rate increased rapidly with stress intensity, increasing as roughly Kl
6.5. Because of better creep resistance, stronger materials can sustain higher levels of stress intensity to drive crack growth
and nucleate the high density of voids necessary for crack growth. Stress relaxation by creep reduces the stress intensity,
and thus the growth rate, especially in weaker materials. The crack growth rate in the heat-affected zone was found to be
substantially faster than in the base metal of the welds. Analysis indicates that Kl rather than C* is the appropriate crack-tip loading parameter in the specimen used here and in a thick-walled pressure vessel.
The DC potential drop technique met with limited success in this application due to the spatially discontinuous manner of
crack growth and limited crack-tip opening displacement.
Formerly Graduate Student, Materials Science and Engineering Department, The Ohio State University 相似文献
19.
Fatigue crack propagation ratesda/dN in binary Al alloys with 3.6 wt pct Cu and 6.3 wt pct Cu and commercial 2024 aged at 21°C were compared with 99.95+ wt pct aluminum. Omitting an anomalous region at lowΔK, the extrapolated rates for “pure” aluminum are more than 100 times greater than those in the three alloys at the same ΔK. The data for the alloys fit into a single scatter band of a factor of three. It was suggested thatda/dN varies inversely with the square of the strength of the alloy but that another parameter related to the fatigue crack propagation
energy per unit area is also important. Theda/dN vs ΔK curves were determined for 3.6 wt pct Cu single crystals aged seven days at 21°C which containGP zones and two and seven days at 160°C which contain mixtures ofθ′ andθ′’. No systematic variation of (da/dN
Δ with crystallographic orientation was discerned, but the naturally aged specimen had a strong orientation dependence on crack
initiation. At low ΔK 21°C aged specimens gave the lowestda/dN while at high ΔK the warm aged specimens gave the lower values ofda/dN. Measurement ofda/dN vs ΔK curves were conducted on specimens of 3.6 wt pct Cu with 1 mm equiaxed grains aged for various times at 130°C, 160°C, and
190°C. All warm aged specimens experienced brittle intergranular fracture at sufficiently high ΔK. The transition ΔK where intergranular fracture first appears is inversely proportional to the aging temperature. The change of fracture mode
from intra to intergranular occurs gradually over a broad range of ΔK which shifts to lower ΔK with increase in aging temperature.
This research was supportd by U.S. Air Force Office of Scientific Research, Office of Aerospace REsearch, Grant No. AF-AFOSR-73-2431. 相似文献
20.
The crack growth behavior of several high temperature nickel-base alloys, under cyclic and static loading, is studied and
reviewed. In the oxide dispersion strengthened (ODS) MA 6000 and MA 754 alloys, the high temperature crack propagation exhibited
orientation dependence under cyclic as well as under static loading. The creep crack growth (CCG) behavior of cast nickel-base
IN-738 and IN-939* superalloys at 850 °C could be characterized by the stress intensity factor,K
1. In the case of the alloy IN-901 at 500 °C and 600 °C,K
1 was found to be the relevant parameter to characterize the creep crack growth behavior. The energy rate line integral,C*, may be the appropriate loading parameter to describe the creep crack growth behavior of the nickel-iron base IN-800H alloy
at 800 °C. The creep crack growth data of 1 Cr-Mo-V steel, with bainitic microstructure, at 550 °C could be correlated better
by C * than byK
1.
This paper is based on a presentation made in the symposium “Crack Propagation under Creep and Creep-Fatigue” presented at
the TMS/AIME fall meeting in Orlando, FL, in October 1986, under the auspices of the ASM Flow and Fracture Committee. 相似文献