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1.
G. P. Zhang Ph.D. Student Z. G. Wang G. Y. Li S. D. Wu 《Metallurgical and Materials Transactions A》1997,28(3):665-672
The effect of crystallographic orientation on the fatigue-crack initiation and propagation in Ni3Al(CrB) single crystal was studied using a compact-tension specimen. Stage I crystallographic cracking and cleavage fracture
were observed. Crystallographic cracking can occur on two or more {111} slip planes simultaneously. It was shown that the
threshold stress intensity for crack initiation from the notch root exhibits a dependence on crystallographic orientation.
In addition, an effect of orientation on microcracking behavior was also shown. The number of {111} planes intersecting with
each other determines the different microscopic features on the cleavage fracture surface. 相似文献
2.
K. S. Chan J. E. Hack G. R. Leverant 《Metallurgical and Materials Transactions A》1986,17(10):1739-1750
The effects of crystallographic orientation and stress state on the multiaxial fatigue behavior of MAR-M200* single crystals
were examined. Using notched tubular specimens subjected to combined tension/torsion cyclic loads, crack growth rates were
determined at ambient temperature as functions of stress intensity range, the shear stress range-to-normal stress range ratio,
and crystallographic orientation. Comparison of crack growth data at the same effective ΔK reveals a weak dependence of the
crack growth rate on both the tube axis and the notch orientation. For a given set of tube axis and notch orientation, the
crack growth rate might or might not vary with the applied stress state, depending on whether roughness-induced crack closure
is present. In most cases, subcritical cracking occurs either along a single 111 slip plane or on ridges formed with two 111
slip planes. Neither fracture mode is altered by a change in the applied stress state. This complex crack growth behavior
will be discussed in terms of the crack-tip stress field, slip morphology, and crack closure.
Formerly with Southwest Research Institute 相似文献
3.
Fatigue crack propagation rates at very low cyclic stress intensity levels (1 to 3 MNm-372) have been measured in cube-oriented, planar slip nickel-base superalloy monocrystals using a high frequency (20 kHz) resonant
fatigue testing technique. It is found that crack propagation is entirely along the crystallographic slip planes and the crack
growth rate does not drop off into a threshold behavior but follows a power law with a power law exponent close to 4, which
is similar to the functional dependency observed at higher cyclic stress intensity levels in similar superalloys. The observed
behaviors are discussed with respect to a new theory on threshold and the effects of strong crystallographic constraints on
crack propagation behavior. 相似文献
4.
Deformation structure and subsurface fatigue crack generation in austenitic steels at low temperature 总被引:2,自引:0,他引:2
In order to progress in the understanding of fatigue crack generation for high-strength alloys, the subsurface fatigue crack
initiation sites were characterized and the deformation structure was investigated for the solution-treated 24Cr-15Ni-4Mn-0.3N
and 32Mn-7Cr-0.1N austenitic steels. High-cycle fatigue tests of those steels were carried out at 4, 77, and 293 K. Subsurface
crack initiation was detected in the lower-peak stress and/or in the longer-life range at the three temperatures. The subsurface
crack initiation sites were intergranularly formed. The localized deformation and/or strain concentration by dislocation arrays
of the (111)–〈110〉 system assisted intergranular cracking due to incompatibility at grain boundaries. Dislocation movements
were restricted to their slip planes. Even at the lower stress level, dislocations had generated in more than one slip system
and piled up to a grain boundary. The peak cyclic stress was lowered with the increasing size of the subsurface crack initiation
site. The dependence of the subsurface crack size on the peak cyclic stress was discussed. 相似文献
5.
The effects of crystallographic orientation on the fatigue crack growth behavior of MAR-M200* single crystals were examined.
Using compact-tension specimens tested at 20 Hz, fatigue crack growth rates were determined at ambient temperature at minimum
stress to maximum stress ratios,R, of 0.1 and 0.5. In most cases, subcritical crack growth occurred either along a single {111} slip plane or a combination
of {111} planes. The mode of cracking was generally mixed and contained mode I, II, and III components. Considerable crack
deflection and branching were also observed. Some fracture surfaces were found to contain a significant amount of asperities
and, in some specimens, black debris. Based on Auger spectroscopic analyses and the fracture surface appearance, it appears
that the black debris represented oxides formed due to rubbing of the fracture surfaces. Using stress intensity solutions
obtained based on the Boundary-Integral-Equation technique, an effective ΔK was successfully used for correlating the crack
growth rate data. The results indicate that the effect of crystallographic orientation on crack growth rate can be explained
on the basis of crack deflection, branching, and roughness-induced crack closure.
Formerly with Southwest Research Institute 相似文献
6.
The effect of orientation, applied stress state, environment, and temperature on the crack growth and crack-path behavior
of single-crystal specimens of UDIMET 720 has been examined. Stage I cracking has been promoted by mixed-mode loading, plane
stress, and vacuum conditions; increasing the test temperature to 600 °C does not suppress stage I crack growth in vacuum.
Consideration of the local resolved shear-stress intensity and local resolved normal-stress intensity for each slip system
as it intersects the nominal crack-growth plane allows the prediction of stage I crack paths, clarifying the importance of
secondary single-crystal testing orientation (i.e., nominal crack-growth direction as well as effective tensile axis). A combination of both opening and shearing are found to promote stage I crack growth, and boundary conditions have been established within
which stage I cracking is promoted. Highly deflected stage I cracking gives rise to significant shielding effects, but under
suitable mixed-mode loading, highly oriented, coplanar stage I crack growth can be produced. Intrinsic stage I cracking under
mixed-mode loading appears to be greatly accelerated compared with mode I-dominated stage II crack growth for comparable stress-intensity
levels. 相似文献
7.
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
8.
Carbon-free single crystals of Mar-M200 were tested in pulsating tension, stress-controlled fatigue at temperatures and frequencies
ranging from 1033 to 1255°K and 0.033 to 1058 Hz, respectively. The axis of loading was parallel to [001], the natural growth
direction for directionally-solidified nickel-base alloys. Except for the lowest frequency at the higher temperatures where
creep damage was extensive, crack initiation occurred at subsurface microporosity. Cracks initiated and propagated in the
Stage I mode (crystallographic cracking on the {111} slip planes) at the lower temperatures and higher frequencies, whereas
Stage (perpendicular to the principal stress axis) crack initiation and propagation was found at the higher temperatures and
lower frequencies. Often a transition from Stage II to Stage I crack propagation was observed. It was established that Stage
I cracking occurred under conditions of heterogeneous, planar slip and Stage II cracking under conditions of homogeneous,
wavy slip. A thermally activated recovery process with an activation energy of 368 KJ/mole (88 Kcal/mole) determined the instantaneous
slip character,i.e., wavy or planar, at the crack tip. In addition, it was found that an optimum frequency existed for maximizing fatigue life.
At frequencies below the optimum, creep damage was detrimental, while at frequencies greater than the optimum, intense, planar
slip was detrimental. The optimum frequency increased with increasing temperature. 相似文献
9.
Large grain specimens with average grain size of 0.0127 m made from commercial purity titanium were subjected to a torsional
cyclic strain at two different amplitudes: ±0.008 and =0.003. Fatigue damage was studied by scanning electron microscopy and
crystal orientations were determined by X-ray diffraction and surface trace analysis. It was found that cyclic strain amplitude
influenced the deformation mode and the nature of the macroscopic crack propagation. At high strain amplitudes the normal
slip processes were observed and microcracking was observed on the (0001), and {1100} slip planes. The macroscopic crack propagation
was dominated by the Stage I shear mode; however, some Stage II tensile mode propagation was observed after extensive Stage
I propagation. At low strain amplitude twin plane cracking was observed on the {1011}, {1010}, and {1123} planes in addition
to normal slip plane cracking, and the macroscopic crack propagation was dominated by the Stage II tensile mode. However,
microscopic examination showed the macroscopic tensile mode cracks to be composed of microscopic shear mode cracks along slip
planes and twin planes. At both low and high strain amplitudes cracking was observed on the {1120} plane which is neither
a slip or twin plane in titanium. It is proposed that this cracking mode was a result of a dislocation reaction forming sessile
dislocations on the {1120} plane. 相似文献
10.
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
11.
A model for the initiation of hydrogen embrittlement cracking at notches in gaseous hydrogen environments 总被引:1,自引:0,他引:1
A model for the initiation of hydrogen embrittlement cracking in gaseous hydrogen environments is presented. The model is
based on the stress-induced diffusion of hydrogen atoms to the regions of high triaxial stress ahead of a plastically strained
notch. The influence of yield stress and notch geometry on the apparent threshold stress intensity for embrittlement are considered
and derived analytically. The time dependence for crack initiation for apparent stress intensities above the threshold is
derived from a simple diffusion model. The results of the model are in agreement with reported hydrogen embrittlement phenomena. 相似文献
12.
Ken Gall Huseyin Sehitoglu Yavuz Kadioglu 《Metallurgical and Materials Transactions A》1996,27(11):3491-3502
The results of a systematic investigation involving forward and reversed plastic zones for a growing fatigue crack under plane-strain
double-slip conditions are presented. The study focuses on plastic-deformation fields outside the small-scale yielding regime.
The size of the macroscopic forward plastic zone is found to be nearly proportional to the square of the applied-stress intensity
over the critical resolved shear stress. The size of the forward plastic zone is also found to depend on the angles of the
two microscopic slip directions with respect to the crack line. When the microscopic slip directions are kept symmetric about
the crack-line normal, and the angle between them is varied, the forward plastic zone sizes hardly vary. However, when the
angle between the slip lines is kept constant, and both planes are simultaneously rotated, the forward plastic zone sizes
vary by a factor of three. The ratio of the reversed plastic zone size to the forward plastic zone size is also found to be
dependent on the orientation of the microscopic slip planes. The ratio varies when the angle between the microscopic slip
planes is changed, or when the orientations of both planes are rotated simultaneously. Stationary cracks are generally found
to have larger reversed plastic zones than fatigue cracks, and the difference is attributed to crack closure. 相似文献
13.
Carbon-free single crystals of Mar-M200 were tested in pulsating tension, stress-controlled fatigue at temperatures and frequencies ranging from 1033 to 1255°K and 0.033 to 1058 Hz, respectively. The axis of loading was parallel to [001], the natural growth direction for directionally-solidified nickel-base alloys. Except for the lowest frequency at the higher temperatures where creep damage was extensive, crack initiation occurred at subsurface microporosity. Cracks initiated and propagated in the Stage I mode (crystallographic cracking on the {111} slip planes) at the lower temperatures and higher frequencies, whereas Stage (perpendicular to the principal stress axis) crack initiation and propagation was found at the higher temperatures and lower frequencies. Often a transition from Stage II to Stage I crack propagation was observed. It was established that Stage I cracking occurred under conditions of heterogeneous, planar slip and Stage II cracking under conditions of homogeneous, wavy slip. A thermally activated recovery process with an activation energy of 368 KJ/mole (88 Kcal/mole) determined the instantaneous slip character,i.e., wavy or planar, at the crack tip. In addition, it was found that an optimum frequency existed for maximizing fatigue life. At frequencies below the optimum, creep damage was detrimental, while at frequencies greater than the optimum, intense, planar slip was detrimental. The optimum frequency increased with increasing temperature. 相似文献
14.
《Acta Metallurgica》1987,35(4):981-987
The effect of cross slip on cracking along coplanar slip bands in anisotropic single crystals is examined. By considering the activation of slip due to the stress field at a crack tip, it is demonstrated that:
- 1.(1) cross slip would not be effective in relaxing elastic normal stresses ahead of a crack propagating along coplanar slip bands, and
- 2.(2) the unrelaxed elastic normal stresses can be plastically relaxed by out-of-plane noncoplanar slip only.
15.
16.
The low cycle fatigue (LCF) properties of a single-crystal nickel-base superalloy, René* N4, have been examined at 760 and
980 °C in air. Specimens having crystallographic orientations near [001], [01l], [111], [023], [236], and [145] were tested
in fully reversed, total-strain-controlled LCF tests at a frequency of 0.1 Hz. At 760 °C, this alloy exhibited orientation
dependent tension-compression anisotropies of yielding which continued to failure. Also at 760 °C, orientations exhibiting
predominately single slip exhibited serrated yielding for many cycles. At 980 °C, orientation dependencies of yielding behavior
were smaller. In spite of the tension-compression anisotropies, cyclic stress range-strain range behavior was not strongly
orientation dependent for either test temperature. Fatigue life on a total strain range basis was highly orientation dependent
at both 760 and 980 °C and was related chiefly to elastic modulus, low modulus orientations having longer lives. Stage I crack
growth on (111) planes was dominant at 760 °C, while Stage II crack growth occurred at 980 °C. Crack initiation generally
occurred at near-surface micropores, but occasionally at oxidation spikes in the 980 °C tests. 相似文献
17.
《Acta Metallurgica》1984,32(4):563-573
The double slip plane (DSP) crack model of Weertman, Lin and Thomson has been used to obtain crack growth equations for the mode II or III crack under a monotonically increasing stress (the R-curve) in this paper. [In a companion paper the growth under cyclic stress (the Paris fatigue crack growth equation) is determined.] The success of the analysis depends upon the fact that the DSP crack closely approximates a Bilby-Cottrell-Swinden (BCS) crack when the slip zone is large compared with the slip plane to crack plane spacing. Consequently the dislocation distribution on the slip planes approximates the BCS crack one ahead of the crack tip. Behind the crack tip a result fortuitous for the analysis is found that the gradient of the dislocation density on a slip plane is proportional to the shear stress on the slip plane. The results obtained are: if the friction stress on a slip plane is constant the crack never propagates catastrophically. Instead crack extension occurs which is proportional to K2 where rK is the stress intensity factor. Were the surface energy of a solid to be suddenly reduced, as it might be by the sudden introduction of an active environment, the distance the tip of a stressed stationary crack jumps is proportional to K. The distance jumped, for a large drop in surface energy, is smaller than the crack advance that occurs if the active environment were always present and the crack is monotonically loaded to the same value of K. If work hardening of the friction stress takes place stable crack growth takes place up to a critical Kc value. The R-curve equation is given by an integral which is simple in form but requires a numerical integration or series expansion. The critical Kc value is proportional to the critical Kcb stress intensity factor of a Griffith crack raised to the power (m + 1 )/2m where m is the power exponent of the simulated plastic stress-strain curve. We believe that this paper demonstrates the double slip plane crack model is the first crack model since the Griffith crack model and its variants that can give an explicit fracture equation starting from first principles. 相似文献
18.
R. B. Scarlin 《Metallurgical and Materials Transactions A》1976,7(10):1535-1541
High cycle fatigue crack growth rates have been measured in the cast nickel-base alloy IN 738 LC in directionally-solidified
form, at room and high temperature and for crack propagation both parallel and perpendicular to the solidification direction.
The resistance of this material to crack propagation has been compared with conventionally-cast material of the same composition.
The considerable differences in observed growth rates may be understood in terms of the effects of chemical segregation, crack
branching and crystallographic fracture. In particular, the high-temperature cyclic fracture toughness for crack growth perpendicular
to the solidification direction is higher than in conventionally-cast material as cracks tend to deviate along the segregated
interdendritic regions. However the room temperature threshold stress intensity amplitude is low because fatigue crack growth
occurs on definite crystallographic planes. 相似文献
19.
R. B. Scarlin 《Metallurgical and Materials Transactions A》1976,7(9):1535-1541
High cycle fatigue crack growth rates have been measured in the cast nickel-base alloy IN 738 LC in directionally-solidified
form, at room and high temperature and for crack propagation both parallel and perpendicular to the solidification direction.
The resistance of this material to crack propagation has been compared with conventionally-cast material of the same composition.
The considerable differences in observed growth rates may be understood in terms of the effects of chemical segregation, crack
branching and crystallographic fracture. In particular, the high-temperature cyclic fracture toughness for crack growth perpendicular
to the solidification direction is higher than in conventionally-cast material as cracks tend to deviate along the segregated
interdendritic regions. However the room temperature threshold stress intensity amplitude is low because fatigue crack growth
occurs on definite crystallographic planes. 相似文献
20.
Fretting fatigue studies were performed on quenched and tempered 4130 steel in laboratory air and in argon as functions of
relative slip displacement, normal pressure and applied cyclic stress. Significant reductions in fatigue resistance were observed
at all stress levels and increased with increasing normal pressures. However, a minimum in resistance was observed for relative
slip magnitudes of 20 to 30 μm. Inert environments improve fatigue resistance under fretting conditions. Metallographic observations
indicated that subsurface cracking was generally observed and that stress concentrations associated with this cracking resulted
in deviations to and away from the faying surfaces. Fretting cracks which deviated into the alloy become initiated fatigue
cracks. A mechanical model is proposed for fretting induced fatigue crack initiation which suggests that this phenomenon is
a simple extension of the basic fretting process. 相似文献