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1.
C. Fukuoka Y. G. Nakagawa J. J. Lance R. N. Pangborn 《Metallurgical and Materials Transactions A》1996,27(12):3841-3851
The objective of this work is to evaluate the damage induced below and above the fatigue limit (Δσ
t
=360 MPa) in pressure vessel steels, such as SA508. Fatigue damage was induced in samples taken from an SA508 steel plate
by various loading histories in order to examine the influence of prior cyclic loading below the fatigue limit. Cell-to-cell
misorientation differences were measured by the selected area diffraction (SAD) method. Surface cracking was also studied
by the replication method. Small cracks were observed after precycling both below and above the fatigue limit. It was, however,
found that fatigue test bars had a longer lifetime after precycling below the fatigue limit, while precycling above the fatigue
limit caused other specimens to fail even when subsequently cycled below the fatigue limit. Cell-to-cell misorientation usually
increases with accumulation of fatigue damage, but it was found that the misorientations measured after precycling below the
fatigue limit decreased again at the beginning of the subsequent cycling above the fatigue limit. It should be noted that
the misorientation at failure was always about 4 to 5 deg, regardless of loading histories. Misorientation showed good correlation
with the fatigue lifetime of the samples. 相似文献
2.
H. J. Maier R. G. Teteruk H. -J. Christ 《Metallurgical and Materials Transactions A》2000,31(2):431-444
A microcrack propagation model was developed to predict thermomechanical fatigue (TMF) life of high-temperature titanium alloy
IMI 834 from isothermal data. Pure fatigue damage, which is assumed to evolve independent of time, is correlated using the
cyclic J integral. For test temperatures exceeding about 600 °C, oxygen-induced embrittlement of the material ahead of the advancing
crack tip is the dominating environmental effect. To model the contribution of this damage mechanism to fatigue crack growth,
extensive use of metallographic measurements was made. Comparisons between stress-free annealed samples and fatigued specimens
revealed that oxygen uptake is strongly enhanced by cyclic plastic straining. In fatigue tests with a temperature below about
500 °C, the contribution of oxidation was found to be negligible, and the detrimental environmental effect was attributed
to the reaction of water vapor with freshly exposed material at the crack tip. Both environmental degradation mechanisms contributed
to damage evolution only in out-of-phase TMF tests, and thus, this loading mode is most detrimental. Electron microscopy revealed
that cyclic stress-strain response and crack initiation mechanisms are affected by the change from planar dislocation slip
to a more wavy type as test temperature is increased. The predictive capabilities of the model are shown to result from the
close correlation with the microstructural observations. 相似文献
3.
4.
D. Eylon J. A. Hall C. M. Pierce D. L. Ruckle 《Metallurgical and Materials Transactions A》1976,7(12):1817-1826
To establish correlations between microstructure and mechanical properties for the Till alloy, twelve different combinations
of hot die forging and heat treatment, in the α+β and β phase regions, were investigated. The resulting heat treated forgings
were classified into four distinct categories based on their microstructural appearance. The room temperature tensile, post-creep
tensile, fracture toughness and fatigue crack propagation properties were measured along with creep and low cycle fatigue
at 566°C. The creep, tensile, fatigue crack propagation and fracture toughness properties, grouped in a manner similar to
the microstructural categories. The fracture appearance and behavior of the cracks during propagation in fatigue and in fracture
toughness tests were examined, and correlations with the microstructure discussed. In the case of the fully transformed acicular
microstructure, it was found that the size and the orientation of colonies of similarly aligned α needles are dominant factors
in the crack behavior. 相似文献
5.
The effects of various microconstituents on crack initiation and propagation in high-cycle fatigue (HCF) were investigated
in an aluminum casting alloy (A356.2). Fatigue cracking was induced in both axial and bending loading conditions at strain/stress
ratios of −1, 0.1, and 0.2. The secondary dendrite arm spacing (SDAS) and porosity (maximum size and density distribution)
were quantified in the directionally solidified casting alloy. Using scanning electron microscopy, we observed that cracks
initiate at near-surface porosity, at oxides, and within the eutectic microconstituents, depending on the SDAS. When the SDAS
is greater than ∼ 25 to 28 μm, the fatigue cracks initiate from surface and subsurface porosity. When the SDAS is less than
∼ 25 to 28 μm, the fatigue cracks initiate from the interdendritic eutectic constituents, where the silicon particles are
segregated. Fatigue cracks initiated at oxide inclusions whenever they were near the surface, regardless of the SDAS. The
fatigue life of a specimen whose crack initiated at a large eutectic constituent was about equal to that when the crack initiated
at a pore or oxide of comparable size. 相似文献
6.
D. Eylon J. A. Hall C. M. Pierce D. L. Ruckle 《Metallurgical and Materials Transactions A》1976,7(11):1817-1826
To establish correlations between microstructure and mechanical properties for the Ti-ll alloy, twelve different combinations
of hot die forging and heat treatment, in the a + 8 and Β phase regions, were investigated. The resulting heat treated forgings
were classified into four distinct categories based on their microstructural appearance. The room temperature tensile, post-creep
tensile, fracture toughness and fatigue crack propagation properties were measured along with creep and low cycle fatigue
at 566‡C. The creep, tensile, fatigue crack propagation and fracture toughness properties, grouped in a manner similar to
the microstructural categories. The fracture appearance and behavior of the cracks during propagation in fatigue and in fracture
toughness tests were examined, and correlations with the microstructure discussed. In the case of the fully transformed acicular
microstructure, it was found that the size and the orientation of colonies of similarly aligned α needles are dominant factors
in the crack behavior.
Formerly a National Research Council Associate, Air Force Materials Laboratory
Formerly with AFML 相似文献
7.
The thermal fatigue behavior of the combustor alloys In 617 and HAYNES 230 before and after welding 总被引:1,自引:0,他引:1
F. Meyer-Olbersleben N. Kasik B. Ilschner F. Rézaï-Aria 《Metallurgical and Materials Transactions A》1999,30(4):981-989
The nickel-base alloys IN 617 and HAYNES 230 for welded high-temperature components have been subjected to thermal fatigue
(TF) loading. In a series of TF tests in air, single wedge specimens were induction heated and compressed-air cooled at the
leading edge for various temperature cycles between 200 °C and either 850 °C, 950 °C, or 1050 °C. The test rigs permitted
simultaneous measurements of temperature and total strain along the edge of specimen during TF cycling. Both materials have
been tested in conditions relevant for hot path components in the gas turbines, e.g., “as delivered,” “welded,” and “welded + notched”. Under identical temperature cycles and thermal gradients, HAYNES 230 showed
a higher TF strength than IN 617 in the as-delivered condition. It is suggested that this advantage of HAYNES 230 is primarily
related to its lower value of the relevant combination of properties of this alloy: coefficient of thermal expansion, thermal
conductivity, elastic modulus, ultimate tensile strength, taken at maximal operating temperature. In addition, the advantage
of the HAYNES 230 is described by a lower plastic strain, which is induced at the wedge region during TF loading. Moreover,
microstructural details of crack initiation, crack propagation, and reactions with the gaseous environment play an important
role. Both alloys investigated in the present work showed plastic deformation with a maximum in the central zone of the wedge
tip. In this zone, slip bands and grain distortion occurred, whereas both ends of the wedge tip free of visible plastic deformation.
The TF cycles led to multiple transgranular crack initiation and propagation. In welded specimens of IN 617 and HAYNES 230,
cracks appeared first in the center of the weld. The susceptibility of welds to TF cracking depends considerably on the weld
filler and the surface quality. It was shown for HAYNES 230 that a mismatched weld could reduce the TF life to less than 50
pct of non-welded specimens. The lower TF-fatigue strength of the welded specimens can be explained by the difficulty of the
cast alloy in the welded zone to accommodate the repeated thermal shocks by plastic deformation. Notches introduced in the
heat-affected zone (depth about 0.1 mm) reduced the TF life of both alloys by a factor as high as 4. The thermal fatigue strength
of the welded material can almost reach the values of the base alloy provided the use of matching electrodes, post-weld heat
treatment, and grinding off the weld beads is carefully executed. 相似文献
8.
M. A. Daeubler A. W. Thompson I. M. Bernstein 《Metallurgical and Materials Transactions A》1990,21(3):925-933
This work examined the influence of microstructure on the surface fatigue crack propagation behavior of pearlitic steels.
In addition to endurance limit or S(stress amplitude)-N(life) tests, measurements of crack initiation and growth rates of
surface cracks were conducted on hourglass specimens at 10 Hz and with aR ratio of 0.1. The microstructures of the two steels used in this work were characterized as to prior austenite grain size
and pearlite spacing. The endurance tests showed that the fatigue strength was inversely proportional to yield strength. In
crack growth, cracks favorably oriented to the load axis were nucleated (stage I) with a crack length of about one grain diameter.
Those cracks grew at low ΔK values, with a relatively high propagation rate which decreased as the crack became longer. After
passing a minimum, the crack growth rate increased again as cracks entered stage II. Many of the cracks stopped growing in
the transition stage between stages I and II. Microstructure influenced crack propagation rate; the rate was faster for microstructures
with coarse lamellar spacing than for microstructures with fine lamellar spacing, although changing the prior austenite grain
size from 30 to 130 jμm had no significant influence on crack growth rate. The best combination of resistance to crack initiation
and growth of short cracks was exhibited by microstructures with both a fine prior austenite grain size and a fine lamellar
spacing.
Formerly with Carnegie Mellon University 相似文献
9.
Wearandshellingaretwoimportantformsofdamageforrailsteelssothatmanyresearchesareaimingatthem[1~4].Manyoftheseresearcheswerelimitedtodeterminingthewearandtosurveyingthetopographyofthesamplesurfacesandonlyafewoftheminvestigatedthemicrostructures,themorphol… 相似文献
10.
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
11.
Y. G. Nakagawa H. Yoshizawa M. E. Lapides 《Metallurgical and Materials Transactions A》1990,21(7):1989-1996
Fatigue test bars fabricated from an SA508 class 3 low-carbon steel plate were cyclically deformed at 300 °C (constant low-cycle
fatigue, total strain range Δε = 0.78 pct and 0.48 pct) to crack initiation (100 pct cumulative damage, CD) and to the factors
75, 50, and 25 pct CD. The test bars were cut perpendicular to the stress axis at the center of the gage length. The X-ray
diffraction line-broadening (XRD) was performed on the cross sections created by the cuts. Thin foils (∼0.1-μm thick) were
prepared from each cross section and used for the transmission electron microscope (TEM) and selected area diffraction (SAD)
study. The half-value line breadth change measured by the XRD increased with the CD increase up to 50 pct, beyond which a
significant reduction was observed for the 75 and 100 pct CD sample regardless of the incident X-ray beam angle. By the TEM,
the undamaged material (0 pct CD) was characterized by high-angle boundaries, small carbide precipitates, and dislocation
cell networks in grains. These characteristics did not show any appreciable changes in all of the samples with fatigue damage
of the respective levels. Micro-orientation changes of the dislocation cells studied by the SAD of the foils and a statistical
data analysis clearly demonstrated that the mean orientation difference in the cells and its standard deviation increased
gradually as the CD increased. 相似文献
12.
《钢铁研究学报(英文版)》2015,(7)
Very high cycle fatigue behavior(107-109 cycles)of 304 Laustenitic stainless steel was studied with ultrasonic fatigue testing system(20kHz).The characteristics of fatigue crack initiation and propagation were discussed based on the observation of surface plastic deformation and heat dissipation.It was found that micro-plasticity(slip markings)could be observed on the specimen surface even at very low stress amplitudes.The persistent slip markings increased clearly along with a remarkable process of heat dissipation just before the fatigue failure.By detailed investigation using a scanning electron microscope and an infrared camera,slip markings appeared at the large grains where the fatigue crack initiation site was located.The surface temperature around the fatigue crack tip and the slip markings close to the fracture surface increased prominently with the propagation of fatigue crack.Finally,the coupling relationship among the fatigue crack propagation,appearance of surface slip markings and heat dissipation was analyzed for a better understanding of ultrasonic fatigue damage behavior. 相似文献
13.
The high cycle fatigue (HCF) and cyclic crack growth rate (CCGR) properties of the dispersion strengthened ODS-alloy MA 6000
were investigated with smooth bars and with fracture mechanics samples at 850 °C. The material was very coarse grained with
the grains elongated in the rolling direction. Fatigue crack initiation and crack propagation were studied parallel and perpendicular
to the rolling direction and a pronounced influence of orientation was found. The fatigue limit of sam-ples cut parallel to
the grain elongation direction (p-samples) was almost a factor of 2 higher than the one of samples cut transverse to the elongation
direction (t-samples). Inclusions were found to be responsible for crack initiation. For p-samples a reasonable agreement
between particle size, fatigue limit, and crack growth behavior was found. For t-type samples such an agreement also exists
provided differences in the crack growth behavior of short cracks and long cracks are taken into consideration. The low fatigue
strength of t-samples could be linked with low Young's modulus in this direction. The crack propagation rate of long cracks
is lower in t-samples than in p-samples due to crack branching along the grain boundaries. HCF-strength of MA 6000 is high
compared to conventional cast alloys mainly because of reduced size of crack nucleation sites and higher fatigue threshold
stress intensity range ΔKth, as a result of higher Young's modulus. 相似文献
14.
An experimental investigation was undertaken to study the relationship between mechanical properties and low stress fatigue
crack propagation. Attention was focused on the “fatigue” or “reversed plastic zone” at the crack tip, since it was felt that
material properties in this region were of prime importance in the crack propagation process. An effort was made to simulate
this region through fully reversed strain-cycling tests on tensile specimens. Mechanical properties obtained from a number
of materials before and after strain cycling were correlated with crack propagation data from the same materials. Evidence
indicated that while monotonic tensile properties are inadequate for correlation purposes, the cyclic strain-hardening coefficient,
the cyclic yield strength, and the elastic modulus appear to be important parameters. This was felt to be an indication of
the importance of strain cycling in the reversed plastic zone in influencing the rate-governing mechanisms in fatigue crack
growth.
Formerly Research Assistant, Department of Metallurgy and Materials Science, Lehigh University, Bethlehem, Pa. 相似文献
15.
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
16.
Energy-based models for predicting the low-cycle fatigue life of high-strength structural steels are presented. The models are based on energy dissipation during average of cycles, cycles to crack propagation and total cycles to failure. Plastic strain energy per cycle was determined and found as an important characteristic for initiation and propagation of fatigue cracks for high-strength structural steels. Fatigue strain-life curves were generated using plastic energy dissipation per cycle (loop area) and compared with the Coffin-Manson relation. Low cycle fatigue life was found similar from both methods. The material showed Masing-type behavior. The cyclic hysterisis energy per cycle was calculated from cyclic stress-strain parameters. The fracture surfaces of the fatigue samples were characterized by scanning electron microscope and the fracture mechanisms were discussed. 相似文献
17.
This article reports research on the initiation and growth of small fatigue cracks in a nickel-base superalloy (produced commercially
by INCO as INCOLOY* 908) at 298 and 77 K. The experimental samples were square-bar specimens with polished surfaces, loaded
in fourpoint bending. The crack initiation sites, crack growth rates, and microstructural crack paths were determined, as
was the large-crack growth behavior, both at constant load ratio (R) and at constant maximum stress intensity (K
max). Small surface cracks initiated predominantly at (Nb,Ti)xCy, inclusion particles, and, less frequently, at grain boundaries. Small cracks grew predominantly along {111} planes in individual
grains and were perturbed or arrested at grain boundaries. For values of ΔK above the large-crack threshold, ΔK
th, the average rate of smallcrack growth was reasonably close to that of large cracks tested under closure-free conditions.
However, short-crack growth rates varied widely, reflecting the local heterogeneity of the microstructure. The threshold cyclic
stress (Δσth) and the threshold cyclic stress intensity (ΔKσth) for small surface cracks were measured as functions of the crack size, 2c. The results suggest that a combination of the fatigue endurance limit and the threshold stress intensity for closure-free
growth of large cracks can be used to define a fatigue-safe load regime.
formerly with Lawrence Berkeley Laboratory 相似文献
18.
This contribution deals with the initiation and growth of short fatigue cracks in cyclic loaded notched specimens made of the 0.15 wt% carbon steel SAE1017. On the basis of experimentally determined data, a damage model based on cyclic crack growth has been developed, which accounts for the anomalous behaviour of short fatigue cracks. In this model the initiation and propagation of a critical crack is regarded as damage. This approach allows to calculate fatigue life for constant amplitude tests as well as for multilevel tests and irregular loading. Deviations from Miner's rule, which have been often observed for two‐level tests, are attributed to the varying fraction of crack initiation and propagation phase for different loads. The inaccuracy of Miner's rule deduced from two‐level tests is of secondary importance for service life calculation when compared with the negligence of amplitudes below the fatigue limit. The proposed model yields shorter service life than the elemental version of Miner's rule. 相似文献
19.
J. Wareing 《Metallurgical and Materials Transactions A》1975,6(8):1367-1377
Fatigue failure in metals and alloys occurs by the nucleation and controlled propagation of a surface crack. At ambient temperature
propagation is transgranular and is controlled, to a large extent, by continuum plasticity effects at the crack tip. At elevated
temperatures this simple process might be affected by oxidation and the tendency towards intergranular propagation. The elevated
temperature fatigue crack propagation behavior of a 20/25/Nb stainless steel and a type 316 stainless steel is studied by
optical measurement of the crack growth rate of artificially-induced notches under conditions where gross plastic straining
is present in the bulk of the material. Tests conducted at ambient temperature are included for comparison with elevated temperature
behavior. By reference to fatigue life data for smooth unnotched specimens, tested under identical conditions to the crack
growth tests, an attempt is made to rationalize the roles of crack initiation and propagation in the fatigue process and indicate
the relevance of crack growth data in predicting fatigue life data. 相似文献
20.
Fatigue experiments were conducted on polycrystalline and monocrystalline samples of a high purity Al, 5.5 wt pct Zn, 2.5
wt pct Mg, 1.5 wt pct Cu alloy in the peak-hardened heat treatment condition. These experiments were conducted in dry laboratory
air and in 0.5N NaCl solutions at the corrosion potential and at applied potentials cathodic to the corrosion potential. It has been shown
that saline solutions severely reduce the fatigue resistance of the alloy, resulting in considerable amounts of intergranular
crack initiation and propagation under freely corroding conditions for polycrystalline samples. Applied cathodic potentials
resulted in still larger decreases in fatigue resistance and, for poly crystals, increases in the degree of transgranular
crack initiation and propagation. Increasing amounts of intergranular cracking were observed when applied cyclic stresses
were reduced (longer test times). The characteristics of cracking, combined with results obtained on tensile tests of deformed
and hydrogen charged samples, suggest that environmental cracking of these alloys is associated with a form of hydrogen embrittlement
of the process zones of growing cracks. Further, it is suggested that stress corrosion cracking and corrosion fatigue of these
alloys occurs by essentially the same mechanism, but that the often observed transgranular cracking under cyclic loading conditions
occurs due to enhanced hydrogen transport and/or concentrations associated with mobile dislocations at growing crack tips. 相似文献