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1.
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
2.
Ken Gall Nancy Yang Mark Horstemeyer David L. McDowell Jinghong Fan 《Metallurgical and Materials Transactions A》1999,30(12):3079-3088
Constant-amplitude high-cycle fatigue tests (σmax=133 MPa, σmax/σy=0.55, and R=0.1) were conducted on cylindrical samples machined from a cast A356-T6 aluminum plate: The fracture surface of the sample
with the smallest fatigue-crack nucleating defect was examined using a scanning electron microscope (SEM). For low crack-tip
driving forces (fatigue-crack growth rates of da/dN<1 × 10−7 m/cycle), we discovered that a small semicircular surface fatigue crack propagated primarily through the Al-1 pct Si dendrite
cells. The silicon particles in the eutectic remained intact and served as barriers at low fatigue-crack propagation rates.
When the semicircular fatigue crack inevitably crossed the three-dimensional Al-Si eutectic network, it propagated primarily
along the interface between the silicon particles and the Al-1 pct Si matrix. Furthermore, nearly all of the silicon particles
were progressively debonded by the fatigue cracks propagating at low rates, with the exception of elongated particles with
a major axis perpendicular to the crack plane, which were fractured. As the fatigue crack grew with a high crack-tip driving
force (fatigue-crack growth rates of da/dN>1 × 10−6 m/cycle), silicon particles ahead of the crack tip were fractured, and the crack subsequently propagated through the weakest
distribution of prefractured particles in the Al-Si eutectic. Only small rounded silicon particles were observed to debond
while the fatigue crack grew at high rates. Using fracture-surface markings and fracture mechanics, a macroscopic measure
of the maximum critical driving force between particle debonding vs fracture during fatigue-crack growth was calculated to be approximately K
max
tr
≈6.0 MPa √m for the present cast A356 alloy. 相似文献
3.
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
4.
The present study compares the fatigue and fracture properties of the high-strength β titanium alloy β-Cez with the conventional α+β titanium alloy Ti-6Al-4V, because of increasing interest in replacing α+β titanium alloys with β titanium alloys for highly stressed airframe and jet engine components. This comparison study includes the Ti-6Al-4V alloy
in an α+ β-processed condition (for a typical turbine blade application) and the β-Cez alloy in two distinctly different α+β-processed and β-processed conditions (optimized for a combination of superior strength, ductility, and fracture toughness). The comparison
principally showed a much lower yield stress for Ti-6Al-4V (915 MPa) than for both β-Cez conditions (1200 MPa). The Ti-6Al-4V material also showed the significantly lower high-cycle fatigue strength (resistance
against crack initiation) of 375 MPa (R=−1) as compared to the β-Cez alloy (∼600 MPa, R=−1). Particularly in the presence of large cracks (>5 mm), the fatigue crack growth resistance and fracture toughness of
the Ti-6Al-4V material is superior when compared to both β-Cez conditions. However, for small crack sizes, the conditions of both the alloys under study show equivalent resistance
against fatigue crack growth. For the β-Cez material, where microstructures were optimized for high fracture toughness (conventional large crack sizes) by thermomechanical
processing, maximum K
Ic-values of 68 MPa√m of the β-processed β-Cez condition (tested in the longitudinal direction) decreased by ∼50 pct in the presence of small cracks (1 mm). A similar
decrease in fracture toughness was obtained by loading the β-processed β-Cez condition perpendicular to the flat surfaces of the pancake-shaped β grain structure (tested in the short transverse direction). These results were discussed in terms of the effectiveness of
the crack front geometry in hindering crack propagation. Further, the results of this study were considered for alloy selection
and optimized microstructures for fatigue and fracture critical applications. Finally, the advantage of the α+β-processed β-Cez condition in highly stressed engineering components is pointed out because of its overall superior combination of fatigue
crack initiation and propagation resistance (especially against small fatigue cracks). 相似文献
5.
The effects of microstructure and specimen size on the fatigue crack growth rate of an annealed 0.42 C steel were investigated
under uniaxial fatigue loading in air. Although a dramatic fluctuation of crack growth rate was found in the propagation process
of microstructurally small cracks, the mean value of crack growth rate can be evaluated by a simple mechanical parameter,
σ
a
n
l (l, crack length; n, constant), under high stress levels where small-scale yielding conditions are exceeded. This parameter is also effective
for cracks larger than 1 to 2 mm under high stress levels, as long as the finite boundary effect of a specimen on the driving
force of crack propagation is considered. The crack growth rate of the alloy was described as a function of stress amplitude
and crack length in terms of two mechanical parameters, σ
a
n
l and ΔK. The applicable conditions of the two parameters were discussed and manifested. 相似文献
6.
Saburo Matsuoka Etsuo Takeuchi Satoshi Nishijima Arthur J. McEvily 《Metallurgical and Materials Transactions A》1989,20(4):741-749
Near-threshold fatigue crack growth properties were investigated for a low-alloy steel 1Cr-1Mo-0.25V and a stainless steel
SUS403 (13Cr) in the temperature range from 25 to 550°C. Fatigue tests were conducted at frequencies of 0.5, 5, and 50 Hz,
in a manner designed to avoid crack closure. The effective value of threshold stress intensity range increased with increasing
temperature and with decreasing frequency for the Cr−Mo−V steel, whereas the effective threshold stress intensity range was
independent of temperature and frequency in the case of the SUS403 steel. At a given ΔK value, the fatigue crack growth rates accelerated with increasing temperature and with decreasing frequency for the Cr−Mo−V
steel. However, although the rate of fatigue crack growth was independent of frequency at a given temperature for the SUS403
steel, the rate did increase with temperature. The observed threshold levels and crack growth behavior were closely related
to the oxidation process of the bare surface formed at the crack tip during each load cycle. 相似文献
7.
Small crack size accelerates corrosion fatigue propagation through high strength 4130 steel in aqueous 3 pct NaCl. The size
effect is attributed to crack geometry dependent mass transport and electrochemical reaction processes which govern embrittlement.
For vacuum or moist air, growth rates are defined by stress intensity range independent of crack size (0.1 to 40 mm) and applied
maximum stress (0.10 to 0.95 Φys). In contrast small (0.1 to 2 mm) surface elliptical and edge cracks in saltwater grow up to 500 times faster than long (15
to 40 mm) cracks at constant δK. Small cracks grow along prior austenite grain boundaries, while long cracks propagate by
a brittle transgranular mode associated with tempered martensite. The small crack acceleration is maximum at low δK levels
and decreases with increasing crack length at constant stress, or with increasing stress at constant small crack size. Reductions
in corrosion fatigue growth rate correlate with increased brittle transgranular cracking. Crack mouth opening, proportional
to the crack solution volume to surface area ratio, determines the environmental enhancement of growth rate and the proportions
of inter- and transgranular cracking. Small cracks grow at rapid rates because of enhanced hydrogen production, traceable
to increased hydrolytic acidification and reduced oxygen inhibition within the occluded cell. 相似文献
8.
R. O. Ritchie F. A. McClintock H. Nayeb-Hashemi M. A. Ritter 《Metallurgical and Materials Transactions A》1982,13(1):101-110
To provide a basis for estimating fatigue life in large rotating generator shafts subjected to transient oscillations, a study
is made of fatigue crack propagation in Mode III (anti-plane shear) in torsionally-loaded spheroidized AISI4340 steel, and
results compared to analogous behavior in Mode I. Torsional S/N curves, determined on smooth bars containing surface defects,
showed results surprisingly close to expected unnotched Mode I data, with lifetime increasing from 104 cycles at nominal yield to 106 cycles at half yield. Fatigue crack growth rates in Mode III, measured on circumferentially-notched samples, were found to
be slower than in Mode I, although still power-law related to the alternating stress intensity(△K
III) for small-scale yielding. Mode III growth rates were only a small fraction (0.002 to 0.0005) of cyclic crack tip displacements(△CTD
III) per cycle, in contrast to Mode I where the fraction was much larger (0.1 to 0.01). A micromechanical model for Mode III
growth is proposed, where crack advance is considered to take place by a Mode II coalescence of cracks, initiated at inclusions
ahead of the main crack front. This mechanism is consistent with the crack increment being a small fraction of △CTDIII per cycle.
Formerly with Massachusetts Institute of Technology, Cambridge, MA
Formerly with M.I. T. 相似文献
9.
The microscopic fatigue damage characteristics and short fatigue crack growth of an unnotched SiC(SCS-6) fiber-reinforced
Ti-15-3 alloy composite were investigated in tension-tension fatigue tests (R = 0.1) carried out at room temperature for applied maximum stress of 450, 670, and 880 MPa.In situ observation of the damage-evolution process was done using optical and scanning laser microscopies, which were attached in
the fatigue machine. The first damage for the composite started from a cracking of the reaction layer followed by fiber fracture.
The matrix cracking initiated near the broken fiber when the microhardness of the matrix just to the side of the fracture
fiber reached ≈6 GPa, and the number of cycles for the initiation of this cracking decreased with the increase of applied
stress. The slope of the relation of surface crack growth lengthvs number of cycles fell into two characteristic stages; in the first stage, the rate was lower than the second stage and accelerated.
The surface crack growth rate,d(2c)/dN,vs surface crack length relation also fell into two stages (stages I and II). With the increase in surface crack length, the
crack-growth rate,d(2c)/dN, decreased in stage I and increased in stage II. The transition from stage I to stage II occurred due to the fracture of
fibers located around the first fractured fiber. It was concluded that the fatigue crack growth resistance of the composite
in the short-crack region was controlled by the fiber fracture and matrix work hardening near the fractured fiber. When the
fiber fracture occurred, the surface crack growth rate was accelerated and became faster than that of the monolithic matrix. 相似文献
10.
K. T. Venkateswara Rao W. Yu R. O. Ritchie 《Metallurgical and Materials Transactions A》1988,19(3):563-569
A study has been made of the mechanics and mechanisms of fatigue crack propagation in a commercial plate of aluminum-lithium
alloy 2090-T8E41. In Part II, the crack growth behavior of naturallyoccurring, microstructurally-small (2 to 1000μm) surface cracks is examined as a function of plate orientation, and results compared with those determined in Part I on
conventional long (≳5 mm) crack samples. It is found that the near-threshold growth rates of small cracks are between 1 to
3 orders of magnitude faster than those for long cracks, subjected to the same nominal stress intensity ranges (at a load
ratio of 0.1). Moreover, the small cracks show no evidence of an intrinsic threshold and propagate at ΔK levels as low as 0.7 MPa{ie563-01}, far below the long crack threshold ΔKTH. Their behavior is also relatively independent of orientation. Such accelerated small crack behavior is attributed primarily
to restrictions in the development of crack tip shielding (principally from roughness-induced crack closure) with cracks of
limited wake. This notion is supported by the close correspondence of small crack results with long crack growth rates plotted
in terms of ΔKeff
(i.e., after allowing for closure above the effective long crack threshold). Additional factors, including the different statistical
sampling effect of large and small cracks with microstructural features, are briefly discussed. 相似文献
11.
M. A. Daeubler A. W. Thompson I. M. Bernstein 《Metallurgical and Materials Transactions A》1988,19(2):301-308
The fatigue behavior of the iron-base superalloy A-286 was studied at room temperature in air for three aging conditions:
underaged, peak aged, and overaged. A fatigue strength at 107 cycles of about 200 MPa, independent of aging condition, was measured for an applied load ratio ofR =0.1. Surface crack initiation and propagation were measured using hourglass specimens. Surface cracks were invariably initiated
in slip bands orientated between 45 and 55 deg to the load axis, and an average ratio of crack depth to crack length of about
0.45 for these semi-elliptical cracks was measured. These earliest observable short surface cracks grew at an accelerated
propagation rate in the near-threshold regime but were retarded in a transition stage, resulting in a minimum in crack growth
rate. This behavior was correlated to the interaction of the crack with specific microstructure features. Following this minimum,
the crack growth accelerated again with increasing ΔK and appeared to converge with the crack growth behavior expected for long through cracks. The crack propagation rate at fixed
ΔK was lowest in underaged, compared to peak aged and overaged microstructures. The minimum and trends in crack growth rate
appeared to depend on the development of roughness-induced closure.
M. A. DAEUBLER, formerly with Carnegie Mellon University 相似文献
12.
Mixed-mode, high-cycle fatigue-crack growth thresholds are reported for through-thickness cracks (large compared to microstructural
dimensions) in a Ti-6Al-4V turbine blade alloy in both lamellar and bimodal microstructural conditions. Specifically, the
effect of combined mode I and mode II loading, over a range of phase angles (β=tan −1 (ΔK
II/ΔK
I) from 0 to 62 deg (ΔK
II/ΔK
I∼0 to 1.9), is examined at a load ratio (ratio of minimum to maximum loads) of R = 0.1 and a cyclic loading frequency of 1000 Hz in ambient-temperature air. When the mixed-mode, crack-driving force is characterized
in terms of the strain-energy release rate (ΔG), incorporating contributions from both the applied tensile and shear loading, the threshold for fatigue-crack growth is
observed to increase significantly with the applied mode-mixity (ΔK
II/ΔK
I) for both microstructures, an effect attributed to enhanced crack-tip shielding. The pure mode I threshold, in terms of ΔG
TH, is observed to be a lower bound (worst case) with respect to mixed-mode (I + II) behavior. For large crack sizes, the threshold
fatigue-crack growth resistance of the lamellar structure is observed to be superior to that of the bimodal material for all
phase angles investigated. Consideration of mode I fatigue-crack growth thresholds for small cracks in these same microstructures
suggests that this rank ordering of mixed-mode fatigue resistance may not hold for crack lengths that are comparable to microstructural
size scales. Examination of the fatigue-crack wake indicates that, for the lamellar microstructure, the path of crack extension
is significantly influenced by the local microstructure over length scales on the order of the relatively coarse lamellar
colonies (∼500 μm). Comparatively, the crack path in the bimodal material is more strongly influenced by the applied crack-driving force.
This disparity in behavior is attributed primarily to the relatively heterogeneous crack-growth resistance of the coarse lamellar
microstructure. 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.
J. P. Campbell R. O. Ritchie K. T. Venkateswara Rao 《Metallurgical and Materials Transactions A》1999,30(3):563-577
Ambient-temperature fracture toughness and fatigue crack propagation behavior are investigated in a wide range of (γ+α
2) TiAl microstructures, including single-phase γ, duplex, coarse lamellar (1 to 2 mm colony size (D) and 2.0 μm lamellar spacing (λ)), fine lamellar (D ∼ 150 μm, λ=1.3 to 2.0 μm), and a powder metallurgy (P/M) lamellar microstructure (D=65 μm, λ=0.2 μm). The influences of colony size, lamellar spacing, and volume fraction of equiaxed γ grains are analyzed in terms of their
effects on resistance to the growth of large (>5 mm) cracks. Specifically, coarse lamellar microstructures are found to exhibit the best cyclic and monotonic crack-growth
properties, while duplex and single-phase γ microstructures exhibit the worst, trends which are rationalized in terms of the salient micromechanisms affecting growth.
These mechanisms primarily involve cracktip shielding processes and include crack closure and uncracked ligament bridging.
However, since the potency of these mechanisms is severely restricted for cracks with limited wake, in the presence of small (<300 μm) cracks, the distinction in the fatigue crack growth resistance of the lamellar and duplex microstructures becomes far less
significant. 相似文献
15.
B. L. Averbach Bingzhe Lou P. K. Pearson R. E. Fairchild E. N. Bamberger 《Metallurgical and Materials Transactions A》1985,16(7):1253-1265
Fatigue cracks were propagated through carburized cases in M-50NiL (0.1 C,4 Mo, 4 Cr, 1.3 V, 3.5 Ni) and CBS-1000M (0.1 C,
4.5 Mo, 1 Cr, 0.5 V, 3 Ni) steels at constant stress intensity ranges, ΔK, and at a constant cyclic peak load. Residual compressive
stresses of the order of 140 MPa (20 Ksi) were developed in the M-50NiL cases, and in tests carried out at constant ΔK values
it was observed that the fatigue crack propagation rates,da/dN, slowed significantly. In some tests, at constant peak loads, cracks were stopped in regions with high compressive stresses.
The residual stresses in the cases in CBS-1000M steel were predominantly tensile, probably because of the presence of high
retained austenite contents, andda/dN was accelerated in these cases. The effects of residual stress on the fatigue crack propagation rates are interpreted in
terms of a pinched clothespin model in which the residual stresses introduce an internal stress intensity, Ki where Ki, = σid
i
1/2
(σi = internal stress, di = characteristic distance associated with the internal stress distribution). The effective stress intensity becomes Ke
= Ka
+ Ki where Ka is the applied stress intensity. Values of Ki were calculated as a function of distance from the surface using experimental measurements of σi and a value of di
= 11 mm (0.43 inch). The resultant values of Ke were taken to be equivalent to effective ΔK values, andda/dN was determined at each point from experimental measurements of fatigue crack propagation obtained separately for the case
and core materials. A reasonably good fit was obtained with data for crack growth at a constant ΔK and at a constant cyclic
peak load. The carburized case depths were approximately 4 mm, and the possible effects associated with the propagation of
short cracks were considered. The major effects were observed at crack lengths of about 2 mm, but the contributions of short
crack phenomena were considered to be small in these experiments, since the two steels were at high strength levels, and short
cracks would be expected to be of the order of 10 μm. Also, the two other steels behaved differently and in a way which followed
the residual stress patterns. Both M-50NiL and CBS-1000M have a high fracture toughness, with Klc = 50 MPa · m1/2 (45 Ksi · in1/2), and the carburized cases exhibit excellent resistance to rolling contact fatigue. Thus, M-50NiL, carburized, may be useful
for bearings where high tensile hoop stresses are developed, since fatigue cracks are slowed in the case by the residual compressive
stresses, and fracture is resisted by the relatively tough core. 相似文献
16.
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
17.
D. K. Benson J. C. Grosskreutz G. G. Shaw 《Metallurgical and Materials Transactions B》1972,3(5):1239-1248
The mechanisms of the fatigue of mill-annealed Ti-6A1-4V were studied at 600°F and room temperature. Early crack initiationN
0/N
f< 0.14) was found to occur in hcpα-grains by a slip-band mechanism under all but the least severe conditions of cyclic stress. Under cyclic stresses near the
fatigue limit at room temperature, fatigue cracks began much laterN
0/N
f ∼ 0.4) at the interface between hcpα and bccβ grains without detectable slip. Under all conditions, Stage I fatigue crack growth occupied 50 to 80 pct of the total life.
Although mechanical twins were produced in profusion near the growing Stage II fatigue cracks, they appeared to play no role
in crack initiation or Stage I crack growth; nor did they facilitate Stage II growth. None of the observations could be interpreted
as evidence for a metallurgical instability or strain-induced phase transformation which might be harmful to the fatigue resistance
of the alloy.
J. C. GROSSKREUTZ, formerly with Midwest Research Institute 相似文献
18.
Diana A. Lados Diran Apelian J. Fred Major 《Metallurgical and Materials Transactions A》2006,37(8):2405-2418
The fatigue crack growth behavior in Regions II and III of crack growth was investigated for hypoeutectic and eutectic Al-Si-Mg
cast alloys. To isolate and establish the mechanistic contributions of characteristic microstructural features (dendritic
α-Al matrix, eutectic phases, Mg-Si strengthening precipitates), alloys with various Si content/morphology, grain size level,
and matrix strength were studied; the effect of secondary dendrite arm spacing (SDAS) was also assessed. In Regions II and
III of crack growth, the observed changes in the fracture surface appearance were associated with changes in crack growth
mechanisms at the microstructural scale (from a linear advance predominantly through primary α-Al to a tortuous advance exclusively
through Al-Si eutectic Regions). The extent of the plastic zone ahead of the crack tip was successfully used to explain the
changes in growth mechanisms. The fatigue crack growth tests were conducted on compact tension specimens under constant stress
ratio,R=0.1, in ambient conditions. 相似文献
19.
D. L. Davidson K. S. Chan R. C. McClung 《Metallurgical and Materials Transactions A》1996,27(9):2540-2556
The fatigue characteristics of a Cu-bearing high-strength low-alloy (HSLA) steel were investigated in air, relative humidity
≈50 pct, as a function of microstructure, which was altered by heat treatments and welding. Small fatigue cracks (≈30-Μm long)
were naturally initiated from smooth specimens and grown past the transition length (≈200 Μm), where they exhibited the characteristics
of large fatigue cracks. The number of cycles to crack initiation depended on stress magnitude but not on microstructure,
although the site of initiation was microstructurally dependent. Small cracks in all microstructures grew at δK values below the large crack threshold. The as-received (polygonal ferrite) microstructure and one of the lath microstructures
that resulted from heat treatment exhibited the same growth rate correlation as large cracks in the linear (Paris) region,
and could be considered as an extension of the large crack growth region down to the point of initiation. Small cracks grew
at rates faster than expected through one of the heat-treated and the weld microstructures; therefore, the number of cycles
required for growth from initiation to the transition to large crack growth decreased about threefold, which is a potentially
important factor in predicting lifetimes of structures made from this steel. 相似文献
20.
Load-controlled fatigue tests were conducted on dual-phase X80 pipeline steel to investigate the effects of stress ratio (R-ratio) on the fatigue crack growth behaviour. Dual-phase X80 pipeline steel showed a non-linear relationship between fatigue crack growth rate (da/dN) and the stress intensity factor range (ΔK) at each R-ratio. Fatigue crack propagation curves of X80 pipeline steel were evaluated using the conventional Paris equation and a new exponential equation named αβ model. In addition, the electron back-scattered diffraction technique was used to study the effects of stress ratio on the fatigue crack growth behaviour. The results indicated that the corresponding ΔK of the transition point decreased with the increase of R-ratio. That was attributed to the variation of the crack path and the fracture mode because of the changes in the size of monotonic plastic zone and cyclic plastic zone at crack tip. Compared to the overall fitting, piecewise fitting by Paris equation and αβ model, piecewise fitting was the most accurate method, and αβ model is more convenient and efficient than the conventional Paris-based equations. 相似文献