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1.
K. S. Ravichandran H. C. Venkata Rao E. S. Dwarakadasa C. G. Krishnadas Nair 《Metallurgical and Materials Transactions A》1987,18(6):865-876
Near threshold fatigue crack growth behavior of a high strength steel under different tempered conditions was investigated.
The important aspect of the study is to compare the crack growth behavior in terms of the closure-free component of the threshold
stress intensity range, ΔK
th,eff While a systematic variation in the absolute threshold stress intensity range with yield strength was observed, the trend
in the intrinsic ΔK
th or ΔK
th,eff exhibited a contrasting behavior. This has been explained as due to the difference in fracture modes during near threshold
crack growth at different temper levels. It is shown that in a high strength and high strain hardening microstructure, yielding
along crystallographic slip planes is difficult and hence it exhibited a flat transgranular fracture. In a steel with low
strain hardening characteristics and relatively low strength, a tendency to crystallographic planar slip is observed consequently
resulting in high ΔK
th. Occurrence of a predominantly intergranular fracture is shown to reduce intrinsic ΔK
th drastically and increase crack growth rates. Also shown is that crack closure can occur in high strength steels under certain
fracture morphologies. A ‘transgranular planar slip’ during the inception of a ‘microstructure sensitive’ crack growth is
essential to promote intergranular and faceted fracture. The occurrence of a maximum in the fraction of intergranular fracture
during threshold crack growth corresponds to the ΔK value at which the cyclic plastic zone size becomes equal to the prior austenitic grain size. 相似文献
2.
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
3.
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
4.
Saburo Matsuoka Hiroyuki Masuda Masuo Shimodaira 《Metallurgical and Materials Transactions A》1990,21(8):2189-2199
The fatigue threshold and low-rate crack propagation properties for a carbon steel, two high-strength steels, and two stainless
steels were investigated in a 3 pct sodium chloride aqueous solution at frequencies between 0.03 and 30 Hz. Tests were conducted
in a manner designed to avoid crack closure. Under freely corroding conditions, the effective values of the threshold stress
intensity factor range, ΔKth,eff, were lower than in air for all of the steels. In particular, the ΔKth,eff values for the carbon and high-strength steels were almost equal to the theoretical ΔKth value of about 1 MPa m1/2 calculated on the basis of the dislocation emission from the crack tip. At a given ΔK level higher than the threshold, the
fatigue crack propagation rates accelerated with decreasing frequency for all of the steels. Under cathodic protection, the
threshold and fatigue crack propagation properties were coincident with those in air regardless of material and frequency.
The observed fatigue crack propagation behavior in a 3 pct NaCl solution was closely related to the corrosion reaction of
the bare surface formed at the crack tip during each loading cycle. 相似文献
5.
T. J. Sutherland P. B. Hoffman J. C. Gibeling 《Metallurgical and Materials Transactions A》1994,25(11):2453-2460
The fatigue crack propagation properties of a rapidly solidified aluminum alloy are compared with those of a metal matrix
composite (MMC) made of the same base alloy with the addition of 11.5 vol pct SiC particulate. The high-temperature base material,
alloy 8009 produced by Allied-Signal, Inc. (Morristown, NJ), is solidified and processed using powder metallurgy techniques;
these techniques yield a fine-grained, nonequilibrium microstructure. A direct comparison between the fatigue crack propagation
properties of the reinforced and unreinforced materials is possible, because alloy 8009 requires no postprocessing heat treatment.
As a consequence, this comparison reflects the influence of the SiC particulate and not differences in microstructure that
could arise during processing and aging. The experimental data demonstrate that the SiC-reinforced material exhibits modestly
superior fatigue crack propagation properties: slower crack growth rates for a given ΔK, at near-threshold crack growth rates.
Even when the data are corrected for crack closure using an effective stress intensity factor, ΔKeff, the composite exhibits lower crack propagation rates than the unreinforced matrix alloy. Microscopic evidence shows a rougher
fracture surface and a more tortuous crack path in the composite than in the base alloy. It is argued that the lower crack
growth rates and higher intrinsic threshold stress intensity factor observed in the composite are associated with crack deflection
around SiC particles.
Formerly Graduate Research Assistant, University of California-Davis 相似文献
6.
D. A. Jablonski J. V. Carisella R. M. Pelloux 《Metallurgical and Materials Transactions A》1977,8(12):1893-1900
The fatigue crack growth rates of three solid solution strengthened superalloys were measured at 25°C and from 538 to 871°C
over a range of frequencies varying from 0.01 to 10.0 Hz. The three alloys were respectively nickel base, cobalt base and
iron base alloys with approximately the same chromium content. The plots of crack growth ratevs AK, the range of the stress intensity factor, show three different regimes. At low ΔAK the crack growth rates are frequency
independent and the fracture is strongly crystallographic. In the medium range of ΔK the fatigue crack growth rates are frequency
and waveform dependent, indicating a strong creep-oxidation time dependent fracture mechanism. At high ΔK, nearK
c, the growth rates are again frequency independent and fracture proceeds by a void coalescence mechanism. The correlations
between the fractographic features as seen in the SEM and the measured crack growth rates provide a good basis for the understanding
of fatigue crack growth at elevated temperatures in the elastic-plastic range. 相似文献
7.
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
8.
Masazumi Tanaka 《Metallurgical and Materials Transactions A》1996,27(9):2678-2685
The influence of aqueous environments on fatigue crack propagation behavior was investigated for two types of structural steel
(SB42 and HT80) in pure and 3 pct NaCl water under freely corroding conditions. In the intermediate to high ΔK region, fatigue crack propagation rates were higher in both aqueous environments and in 1 atm hydrogen than in air for both
types of steel, and the acceleration effect increased power functionally with decreasing frequency from 5 to 0.0005 Hz. Such
a crack growth acceleration property was explained by the mechanism of cyclically induced hydrogen embrittlement, as shown
by the brittle striations formed on the fracture surfaces. On the other hand, in the lower ΔK region, both aqueous environments inversely suppressed crack growth and enhanced the threshold stress intensity factor range
ΔK
th just above the ΔK
th in air, while only in aerated 3 pct NaCl water was the crack observed to grow even under the condition below the ΔK
th in air, not showing the threshold. Probable mechanisms for such fairly complex environmental effects were also suggested. 相似文献
9.
A. Yuen S. W. Hopkin G. R. Leverant C. A. Rau 《Metallurgical and Materials Transactions B》1974,5(8):1833-1842
Stage fatigue crack propagation in Ti-6A1-4V has been studied as a function of various fracture mechanics parameters, including
the stress intensity range (ΔK) and both positive and negative ratios of the minimum to maximum stress (R). It was found that the fracture surface appearance undergoes a transition from cyclic cleavage to striations at a ΔKeff of approximately 13 MNm-3/2 (11.8 ksi√in.). It was also observed that the measured striation spacings are generally within a factor of two of the optically
measured crack growth rates. Both of these results can be particularly useful for determining unknown component cyclic loadings
during failure analysis. The criterion for the cyclic cleavage to striation transition is considered to be a change from primarily
single to multiple slip within the individual grains at the crack tip. This occurs when the cyclic plastic zone size becomes
approximately equal to the α grain size. 相似文献
10.
J. McKittrick P. K. Liaw S. I. Kwun M. E. Fine 《Metallurgical and Materials Transactions A》1981,12(8):1535-1539
Measurement of the threshold for fatigue macrocrack propagation, ΔKo, in a number of aluminum alloys has shown an increase with grain size and decrease with increase in strength as with steels.
The results are not primarily due to environmental enhancement of fatigue crack growth because an even larger variation in
ΔKo with microstructural change is noted at 77 K than at 300 K. In particular, ΔKo of high purity 2124-T4 increases much more on cooling from 300 to 77 K than does ΔKo of 2024-T4. It is suggested that ΔKo is determined by the stress necessary to operate a dislocation source near the crack tip. A Frank-Read type source is proposed
for 2024-T4 with constituent particles acting as pinning points while double cross-slip, a thermally activated process, is
proposed for the source in high purity 2124-T4. 相似文献
11.
Fatigue crack propagation in high-strength A286 steel was studied by comparing crack growth rates determined from: (1) conventional
long-crack propagation tests, (2) closure-free long-crack tests at constant Kmax, and (3) small-crack propagation tests. Small-crack growth rates were measured by following the growth of surface cracks
in samples cycled from near-zero stress to 0.5 or 0.8σy. While most of the surface cracks became dormant shortly after nucleation, some grew into long cracks, and some of these
propagated at cyclic stress intensities below the long-crack threshold, ΔKth (or ΔK
th
eff
, the threshold cyclic stress intensity after crack closure effects have been removed). Surface cracks grew more rapidly than
long cracks at the same ΔKor ΔKeff. The small-crack effect disappeared when the crack-tip plastic zone size became greater than the grain size. The results
show that the absence of crack closure is only one of several factors that influence short-crack growth in A286 steel. Both
peak stress and microstructural effects are important. Microstructural effects are apparently responsible for subthreshold
crack growth; the cracks that grow at ΔK < ΔK
th
eff
form and grow in statistically weak regions of the microstructure. 相似文献
12.
The effect of intermediate thermomechanical treatments on the fatigue properties of a 7050 aluminum alloy 总被引:2,自引:0,他引:2
A study was undertaken to determine the effect of microstructures produced by different ingot processing techniques on the
fatigue properties of a 7050 aluminum alloy. The different microstructures investigated were produced by hot-rolling to simulate
commercial processing (CP) methods or intermediate thermomechanical treatments (ITMT). Characterization of the microstructures
revealed that the CP 7050 material was partially recrystallized (<50 pct) due to the use of hot-rolling as the final deformation
step. The ITMT materials were examined in the as-recrystallized (AR) condition or in AR + hot rolled condition (AR + HR).
Results of the investigation showed thattotal fatigue life, both low and high cycle, were not greatly affected by the grain structures of the experimental materials. However,
metallographic studies indicated that crack initiation is probably more difficult in the fine-grained AR material. The results
of fatigue crack growth tests showed that higher crack growth rates observed at low ΔK values for ITMT {dy7050} were most likely due to the detrimental effects of undissolved Al2CuMg particles. These particles, which also contribute to low fracture toughness and higher crack growth rates at high ΔK levels, are formed during a furnace-cooling step in the ITMT processing schedule. 相似文献
13.
Kazuaki Shiozawa Shuming Sun R. L. Eadie 《Metallurgical and Materials Transactions A》2000,31(4):1137-1145
The corrosion fatigue crack propagation behavior of a squeeze-cast Al-Si-Mg-Cu aluminum alloy (AC8A-T6), which had been precracked
in air, was investigated at testing frequencies of 0.1, 1, 5, and 10 Hz under a stress ratio (R) of 0.1. Compact-toughness specimens were precracked about 6 mm in air prior to the corrosion fatigue test in a 3 pct saline
solution. At some near-threshold conditions, these cracks propagated faster than would be predicted by the mechanical driving
force. This anomalous corrosion fatigue crack growth was affected by the initial stress-intensity-factor range (ΔK
i), the precracking conditions, and the testing frequency. The initial crack propagation rate was as much as one order of magnitude
higher than the rate for the same conditions in air. This rapid rate was associated with preferential propagation along the
interphase interface in the eutectic structure. It is believed that a chemical reaction at the crack tip and/or hydrogen-assisted
cracking produced the phenomenon. Eventual retardation and complete arrest of crack growth after this initial rapid growth
occurred within a short period at low ΔK values, when the testing frequency was low (0.1 and 1 Hz). This retardation was accompanied by corrosion product-induced
crack closure and could be better explained by the contributory stress-intensity-factor range (ΔK
cont) than by the effective stress-intensity-factor range (ΔK
eff). 相似文献
14.
Mechanisms of Slow Fatigue Crack Growth in High Strength Aluminum Alloys: Role of Microstructure and Environment 总被引:1,自引:0,他引:1
S. Suresh A. K. Vasudévan P. E. Bretz 《Metallurgical and Materials Transactions A》1984,15(2):369-379
The role of microstructure and environment in influencing ultra-low fatigue crack propagation rates has been investigated
in 7075 aluminum alloy heat-treated to underaged, peak-aged, and overaged conditions and tested over a range of load ratios.
Threshold stress intensity range, ΔK0, values were found to decrease monotonically with increasing load ratio for all three heat treatments fatigue tested in 95
pct relative humidity air, with ΔK
0 decreasing at all load ratios with increased extent of aging. Comparison of the near-threshold fatigue behavior obtained
in humid air with the data forvacuo, however, showed that the presence of moisture leads to a larger reduction in ΔK0 for the underaged microstructure than the overaged condition, at all load ratios. An examination of the nature of crack morphology
and scanning Auger/SIMS analyses of near-threshold fracture surfaces revealed that although the crack path in the underaged
structure was highly serrated and nonlinear, crack face oxidation products were much thicker in the overaged condition. The
apparent differences in slow fatigue crack growth resistance of the three aging conditions are ascribed to a complex interaction
among three mechanisms: the embrittling effect of moisture resulting in conventional corrosion fatigue processes, the role
of microstructure and slip mode in inducing crack deflection, and crack closure arising from a combination of environmental
and microstructural contributions. 相似文献
15.
A model of fatigue crack growth is proposed that utilizes the recent developments in notch analysis of fracture and a concept
of size effect that results from the changes in the critically stressed volume ahead of a crack tip. Accordingly, the fatigue
crack growth mechanism involves local stresses reaching the theoretical cohesive strength and causing brittle fracture of
atomic bonds at nominal stresses near the threshold, whereas slip-plane decohesion and plastic blunting and resharpening of
the crack tip process may occur at stresses above the threshold range. The model contains three material parameters σFF nF,
and ρF, that conveniently extend continuum analysis to situations where inhomogeneity of the material structure can influence
the behavior appreciably. The analytical expression from the model was found to correlate fatigue crack growth data reasonably
well in the low and intermediate stress ranges in Al 2024-T3, Al 7075-T6 and 250 grade maraging steel. The fracture modes
observed are in agreement with the predictions from the model. The same fatigue crack growth model can be extended to estimating
the threshold stress intensity factor range, ΔKo and fatigue notch sensitivity of different materials. 相似文献
16.
Fatigue cracking resistance of sintered steel as a function of temperature is characterized by crack growth rate vs the stress intensity range, ΔK. The stress ratio effects on fatigue crack propagation (FCP) are investigated from room temperature to 300 °C. The crack
closure effects on FCP are evaluated by both theoretical and experimental approaches. We found that the crack closure cannot
be fully responsible for the observed increase of fatigue resistance with low stress ratio. Experimental results support that
both K
max and ΔK control near-threshold crack growth. Fatigue crack resistance at high ΔK regime decreases with temperature. The apparent increase of fatigue resistance at the near-threshold regime at elevated temperatures
might be attributed to microcrack toughening. 相似文献
17.
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
18.
19.
W. A. Zinsser Jr. J. J. Lewandowski 《Metallurgical and Materials Transactions A》1998,29(6):1749-1757
The effects of changes in R ratio on the fatigue crack growth behavior of a Nb-10 at. pct Si composite as well as bulk Nb-1.24 at. pct Si were determined.
Fatigue crack growth experiments were performed over a range of ΔK levels at R ratios of 0.1 and 0.4. Qualitative and quantitative scanning electron microscopy studies were performed to characterize the
fatigue fracture features of the composites and alloys, in order to determine the factors controlling these fracture features.
The results of this work indicate that increases in R ratio reduce the observed threshold stress intensities in both materials. Somewhat higher fatigue thresholds were observed
in the Nb-Si
(ss)
compared to pure Nb in the literature. In contrast to the bulk Nb-Si
(ss)
alloy, which exhibited no evidence of cleavage fracture in fatigue at any R ratio or ΔK level, the Nb-Si
(ss)
constituent in the Nb-10 at. pct Si composite exhibited a distinct fracture mode transition from ductile tearing near threshold
and low ΔK to cleavage fracture with an increase in ΔK and K
max. Possible reasons for such observations are provided.
This article is based on a presentation made in the symposium “Fatigue and Creep of Composite Materials” presented at the
TMS Fall Meeting in Indianapolis, Indiana, September 14–18, 1997, under the auspices of the TMS/ASM Composite Materials Committee. 相似文献
20.
High frequency (123 Hz) fatigue crack propagation studies were conducted under rising ΔK conditions (R-ratio = 0.22) on single edge notch specimens of austenitic stainless steel (type 316L) that contained an annealed precrack.
Tests were conducted in near neutral (pH 5.5) solutions of 1 M NaCl and 1 M NaCl + 0.01 M Na2S2O3 under potentiostatically controlled conditions and in desiccated air. Attention was directed primarily to the near threshold
behavior and the stage I (crystallographic) region of cracking. Good mixing between the crack solution and bulk solution was
obtained and crack retardation and arrest effects, due to surface roughness induced closure, were minimized at high anodic
potentials by electrochemical erosion. Thermodynamic considerations showed that hydrogen played no role in fatigue crack propagation.
Analysis of the results in terms of the estimated effective cyclic stress intensity, ΔK
eff, showed a systematic effect of potential on the average crack growth increment per cycle,da/dN. Anodic dissolution processes were considered to make an insignificant contribution toda/dN. A model was proposed for stage I fatigue cracking based on the effect of oxide nucleation rate on restricted slip reversal.
The essential features of the model were considered to be relevant to cracking in aqueous environments and in desiccated air. 相似文献