共查询到20条相似文献,搜索用时 15 毫秒
1.
B. London D. V. Nelson J. C. Shyne 《Metallurgical and Materials Transactions A》1988,19(10):2497-2502
Fatigue crack growth in compact tension samples of high purity 4140 steel quenched and tempered to various strength levels
was investigated. Tempering temperatures of 200, 400, 550, and 700 °C produced yield strengths from 1600 to 875 MPa, respectively.
Crack propagation and crack closure were monitored inK-decreasing tests performed underR = 0.05 loading conditions in laboratory air. Results indicated that as the yield strength increased the crack growth rate
increased at a given ΔK and ΔKth decreased. Threshold values varied from 2.8 MPa m1/2 (200 °C temper) to 9.5 MPa m1/2 (700 °C temper). Cracks in the 200 °C tempered samples grew by an intergranular mechanism following prior austenite grain
boundaries probably caused by hydrogen embrittlement or tempered martensite embrittlement. Tempering above 200 °C produced
transgranular fatigue crack growth. The level of crack closure increased with tempering temperature and with crack propagation
in a given tempered condition. Crack closure was caused by a combination of plasticity-induced and oxide-induced mechanisms.
The use of an effective stress intensity range based on crack closure consolidated the fatigue crack growth curves and the
threshold values for all tempering temperatures except 200 °C.
Formerly Graduate Research Assistant, Department of Materials Science and Engineering, Stanford University, Stanford, CA.
Formerly Professor, Department of Materials Science and Engineering, Stanford University, Stanford, CA. 相似文献
2.
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
3.
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
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.
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
6.
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
7.
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
8.
Small surface fatigue crack growth in specially designed cantilevered bending samples of high-purity 4140 steel quenched and
tempered to various strength levels was investigated. Tempering temperatures of 200 °C, 400 °C, 550 °C, and 700 °C were used
to produce a range of yield strengths and microstructures. Crack propagation and crack closure were monitored with a surface
acoustic wave ultrasonic technique. The small crack results were compared to those of long cracks in compact tension samples.
Small cracks in the 200 °C and 400 °C tempers grew at ΔK levels above their long crack thresholds. Small cracks in the 550
°C tempers grew at ΔK levels slightly below the long crack threshold. The surface cracks in the 700 °C temper grew well below
the long crack threshold showing the “small crack effect.” Small crack growth occurred in a very narrowda/dN- ΔK scatter band showing much less variation with microstructure than the long crack data. The differences between the long and
small crack data were due to the high, relatively similar closure values for the small cracks and the variation of long crack
growth with microstructure.
Formerly Graduate Research Assistant, Department of Materials Science and Engineering, Stanford University, Stanford, CA. 相似文献
9.
Ravichandran K. S. Dwarakadasa E. S. 《Metallurgical and Materials Transactions A》1990,21(12):3171-3186
An attempt has been made to systematically investigate the effects of microstructural parameters, such as the prior austenite
grain size (PAGS), in influencing the resistance to fatigue crack growth (FCG) in the near-threshold region under three different
temper levels in a quenched and tempered high-strength steel. By austenitizing at various temperatures, the PAGS was varied
from about 0.7 to 96 μm. The microstructures with these grain sizes were tempered at 200 °C, 400 °C, and 530 °C and tested
for fatigue thresholds and crack closure. It has been found that, in general, three different trends in the dependence of
both the total threshold stress intensity range, ΔK
th
, and the intrinsic threshold stress intensity range, ΔK
eff, th
, on the PAGS are observable. By considering in detail the factors such as cyclic stress-strain behavior, environmental effects
on FCG, and embrittlement during tempering, the present observations could be rationalized. The strong dependence of ΔK
th
and ΔK
eff, th
on PAGS in microstructures tempered at 530 °C has been primarily attributed to cyclic softening and thereby the strong interaction
of the crack tip deformation field with the grain boundary. On the other hand, a less strong dependence of ΔK
th
and ΔK
eff, th
on PAGS is suggested to be caused by the cyclic hardening behavior of lightly tempered microstructures occurring in 200 °C
temper. In both microstructures, crack closure influenced near-threshold FCG (NTFCG) to a significant extent, and its magnitude
was large at large grain sizes. Microstructures tempered at the intermediate temperatures failed to show a systematic variation
of ΔKth and ΔKeff, th with PAGS. The mechanisms of intergranular fracture vary between grain sizes in this temper. A transition from “microstructure-sensitive”
to “microstructure-insensitive” crack growth has been found to occur when the zone of cyclic deformation at the crack tip
becomes more or less equal to PAGS. Detailed observations on fracture morphology and crack paths corroborate the grain size
effects on fatigue thresholds and crack closure.
K.S. RAVICHANDRAN, formerly Research Scholar, Department of Metallurgy, Indian Institute of Science 相似文献
10.
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
11.
In an attempt to analyze the behavior of physically “short” cracks, a study has been made of the development, location, and
effect of crack closure on the behavior of fatigue cracks arrested at the “long” crack threshold stress intensity range, ΔK
TH
, in underaged, peak aged, and overaged microstructures in a 7150 aluminum alloy. By monitoring the change in closure stress
intensity,K
cl, during thein situ removal of material left in the wake of arrested threshold cracks, approximately 50 pct of the closure was found to be confined
to a region within ∼500 μm of the crack tip. Following wake removal, previously arrested threshold cracks recommenced to propagate
at low load ratios even though nominal stress intensity ranges didnot exceed ΔK
TH
, representing the behavior of physically short cracks emanating from notches. No such crack extension at ΔK
TH
was seen at high load ratios. With subsequent crack extension, crack closure was observed to redevelop leading to a deceleration
in growth rates. The development of such closure was found to occur over crack extensions comparable with microstructural
dimensions, rather than those associated with local crack tip plasticity. Such results provide further confirmation that the
existence of a fatigue threshold and the growth of physically short cracks are controlled primarily by crack closure, and
the data are discussed in terms of the micro-mechanisms of closure in precipitation hardened alloy systems.
Formerly Graduate Student with the Department of Mechanical Engineering, University of California, Berkeley 相似文献
12.
The continuous variations in crack shape or aspect ratio, a/c (a is the crack depth and c is the halfsurface length), of small surface cracks, induced by grain boundaries, have been investigated during the fatigue
crack growth of small cracks in a titanium (Ti-8Al) alloy. The significance of the aspect ratio variations in explaining the
“anomalous” small-crack behavior was evaluated. The aspect ratio data were determined from the measurements of crack compliance,
made using a laser interferometric system, and the measurements of surface crack length (2c), made using a photomicroscopic system. The variations in aspect ratio were found to be large at small crack sizes of the
order of a few grain diameters. The experimental a/c data were compared with the patterns of crack aspect ratio variation, obtained from the oretical simulations. The simulated
data were generated by assuming alternate crack propagation at the surface and at the depth, the details of which are presented
in Part I of the study accompanying this article. A good agreement was found between the simulated and the experimentally
observed variations. After incorporating the a/c variations in ΔK calculations, the scatter in the growth data of small cracks was significantly reduced and was found to be of the same order
as in large cracks. Additionally, it is shown in this study that the conventional methods of analysis of small-crack data,
performed with an assumption of a/c=1, can result in significant errors in ΔK calculation and an increased level of scatter in small-crack growth data. Small cracks also were found to exhibit low closure
levels relative to large cracks. The results of the study strongly indicate that characteristics of small cracks, often referred
to as anomalous, are due to the assumption of a/c=1 in situations of large variations in aspect ratio, the use of conventional methods of data analysis, and the lower levels
of crack closure found naturally in small cracks. 相似文献
13.
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
14.
V. B. Dutta S. Suresh R. O. Ritchie 《Metallurgical and Materials Transactions A》1984,15(6):1193-1207
microstructures with maximum resistance to fatigue crack extension while maintaining high strength levels. A wide range of
crack growth rates has been examined, from ~10-8 to 10-3 mm per cycle, in a series of duplex microstructures of comparable yield strength and prior austenite grain size where intercritical
heat treatments were used to vary the proportion, morphology, and distribution of the ferrite and martensite phases. Results
of fatigue crack propagation tests, conducted on “long cracks” in room temperature moist air environments, revealed a very
large influence of microstructure over the entire spectrum of growth rates at low load ratios. Similar trends were observed
at high load ratio, although the extent of the microstructural effects on crack growth behavior was significantly less marked.
Specifically, microstructures containing fine globular or coarse martensite in a coarse-grained ferritic matrix demonstrated
exceptionally high resistance to crack growth without loss in strength properties. To our knowledge, these microstructures
yielded the highest ambient temperature fatigue threshold stress intensity range ΔK0 values reported to date, and certainly the highest combination of strength and ΔK0 for steels (i.e., ΔK0 values above 19 MPa√m with yield strengths in excess of 600 MPa). Such unusually high crack growth resistance is attributed
primarily to a tortuous morphology of crack path which results in a reduction in the crack driving force from crack deflection
and roughness-induced crack closure mechanisms. Quantitative metallography and experimental crack closure measurements, applied
to currently available analytical models for the deflection and closure processes, are presented to substantiate such interpretations.
Formerly Lecturer and Research Engineer in the Department of Materials Science and Mineral Engineering, University of California 相似文献
15.
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
16.
B. L. Averbach Bingzhe Lou P. K. Pearson R. E. Fairchild E. N. Bamberger 《Metallurgical and Materials Transactions A》1985,16(7):1267-1271
The growth rates of fatigue cracks propagating through the case and into the core have been studied for carburized X-2M steel
(0.14 C, 4.91 Cr, 1.31 Mo, 1.34 W, 0.42 V). Fatigue cracks were propagated at constant stress intensities, ΔK, and also at
a constant cyclic peak load, and the crack growth rates were observed to pass through a minimum value as the crack traversed
the carburized case. The reduction in the crack propagation rates is ascribed to the compressive stresses which were developed
in the case, and a pinched clothespin model is used to make an approximate calculation of the effects of internal stress on
the crack propagation rates. We define an effective stress intensity, Ke = Ka
+ Ki, where Ka is the applied stress intensity, Ki = σid
i
1/2
, σi is the internal stress, and di is a characteristic distance associated with the depth of the internal stress field. In our work, a value of di = 11 mm (0.43 inch) fits the data quite well. A good combination of resistance to fatigue crack propagation in the case and
fracture toughness in the core can be achieved in carburized X-2M steel, suggesting that this material will be useful in heavy
duty gears and in aircraft gas turbine mainshaft bearings operating under high hoop stresses. 相似文献
17.
H. Abdel-Raouf T. H. Topper A. Plumtree 《Metallurgical and Materials Transactions A》1979,10(4):449-456
A high purity Al-4 pct Cu alloy has been overaged for two different times at 400°C giving interparticle spacings (λ) of about
0.53 and 1.37 μm. Cyclic plasticity of the alloy with the smaller interparticle spacing can be explained in terms of plastic
deformation behavior controlled by the structure whereas that for the alloy with the larger interparticle spacing is controlled
by the matrix. The fatigue lives of the weaker alloy (λ = 1.37 μm) may be accurately predicted using the models of Coffin-Manson
and Tomkins, however, these models are not applicable to the stronger alloy (λ = 0.53 μm). It was found that the crack tip
opening displacement at the threshold stress intensity range (ΔKth) was equivalent to the interparticle spacing. ΔKth is related to the cyclic yield stress, σcy and the interparticle spacing in the following manner: ΔKth ≈ (2 Eλσcy)1/2, whereE is the modulus of elasticity. In the present case, the term λσcy is constant, giving the impression that ΔKth is independent of the mechanical properties and microstructure. At very low growth rates, however, the fatigue crack growth
is independent of these parameters and also the method of cyclic deformation. A transition to higher crack growth rates occurs
when the plastic zone size reaches approximately one-seventh of the specimen thickness, allowing a nonplanar crack front to
be developed. The value of the stress intensity range (ΔKT) at this transition was found to be dependent upon the interparticle spacing according to the relation: ΔKTλ = 9.6 Pa-m3/2.
Formerly Lecturer and Research Associate, Department of Mechanical Engineering, University of Waterloo 相似文献
18.
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
19.
B. D. Worth J. M. Larsen S. J. Balsone J. W. Jones 《Metallurgical and Materials Transactions A》1997,28(13):825-835
Fatigue crack growth studies have been conducted on a two-phase alloy with a nominal composition of Ti−46.5Al−3Nb−2Cr−0.2W
(at. pct), heat treated to produce duplex and lamellar microstructures. Fatigue crack growth tests were conducted at 23°C
using computer-controlled servohydraulic loading at a cyclic frequency of 20 Hz. Several test methods were used to obtain
fatigue crack growth rate data, including decreasing-load-range-threshold, constant-load-range, and constant-K
max increasing-load-ratio crack growth control. The lamellar microstructure showed substantial improvement in crack growth resistance
and an increase in the threshold stress intensity factor range, ΔK
th, when compared with the behavior of the duplex microstructure. The stress ratio had a significant influence on crack growth
behavior in both microstructures, which appeared to be a result of roughness-induced crack closure mechanisms. Fractographic
characterization of fatigue crack propagation modes indicated a highly tortuous crack path in the fully lamellar microstructure,
compared to the duplex microstructure. In addition, limited shear ligament bridging and secondary cracking parallel to the
lamellar interfaces were observed in the fully lamellar microstructure during fatigue crack propagation. These observations
were incorporated into a model that analyzes the contribution of intrinsic vs extrinsic mechanisms, such as shear ligament bridging and roughness-induced crack closure, to the increased fatigue crack
growth resistance observed for the fully lamellar microstructure.
S.J. BALSONE, formerly with the United States Air Force, Wright Laboratory, Materials Directorate 相似文献
20.
B. D. Worth J. M. Larsen S. J. Balsone J. W. Jones 《Metallurgical and Materials Transactions A》1997,28(3):825-835
Fatigue crack growth studies have been conducted on a two-phase alloy with a nominal composition of Ti-46.5Al-3Nb-2Cr-0.2W
(at. pct), heat treated to produce duplex and lamellar microstructures. Fatigue crack growth tests were conducted at 23 °C
using computer-controlled servohydraulic loading at a cyclic frequency of 20 Hz. Several test methods were used to obtain
fatigue crack growth rate data, including decreasing-load-range-threshold, constant-load-range, and constant-K
max increasing-load-ratio crack growth control. The lamellar microstructure showed substantial improvement in crack growth resistance
and an increase in the threshold stress intensity factor range, ΔK
th
, when compared with the behavior of the duplex microstructure. The stress ratio had a significant influence on crack growth
behavior in both microstructures, which appeared to be a result of roughness-induced crack closure mechanisms. Fractographic
characterization of fatigue crack propagation modes indicated a highly tortuous crack path in the fully lamellar microstructure,
compared to the duplex microstructure. In addition, limited shear ligament bridging and secondary cracking parallel to the
lamellar interfaces were observed in the fully lamellar microstructure during fatigue crack propagation. These observations
were incorporated into a model that analyzes the contribution of intrinsic vs extrinsic mechanisms, such as shear ligament bridging and roughness-induced crack closure, to the increased fatigue crack
growth resistance observed for the fully lamellar microstructure. 相似文献