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1.
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
2.
Wu-Yang Chu Che-Mei Hsiao Yu-Sheng Zhao 《Metallurgical and Materials Transactions A》1988,19(4):1067-1073
The stress intensityK
II
of a Mode II specimen was calculated using a finite element methodvia theJ integral. The site, direction, and the threshold value for crack initiation from the notch under cyclic Mode II loading in
air, in water, and under dynamic charging with hydrogen were investigated. The results showed that the Mode II fatigue crack
in a high strength steel initiated at or close to the site of the maximum principal stress, rather than at the site of the
maximum shear stress, and the subsequent crack growth was oriented approximately normal to the direction of the maximum principal
stress. The site and direction of crack initiation in water and under dynamic charging with hydrogen were similar but different
from that in air. The threshold values for crack initiation in air, in water, and under dynamic charging were 28.8, 12.3,
and 10.2 MPa m1/2, respectively. The fracture surface of a corrosion fatigue crack in water and under dynamic charging consisted of intergranular
facets at low ΔK
II
values but of quasi-cleavage at higher ΔK
II
values and were different from those in air. 相似文献
3.
R. M. Horn 《Metallurgical and Materials Transactions A》1975,6(9):1525-1533
The relationships between microstructure and fatigue crack propagation behavior were studied in a 5Mo-0.3C steel. Microstructural
differences were achieved by varying the tempering treatment. The amounts, distribution, and types of carbides present were
influenced by the tempering temperature. Optical metallography and transmission electron microscopy were used to characterize
the microstructures. Fatigue fracture surfaces were studied by scanning electron microscopy. For each heat treatment the fatigue
crack growth properties were measured under plane strain conditions using a compact tension fracture toughness specimen. The
properties were reported using the empirical relation of Paris [da/dN = CoΔKm]. It was found that secondary hardening did influence the fatigue crack growth rates. In particular, intergranular modes
of fracture during fatigue led to exaggerated fatigue crack growth rates for the tempering treatment producing peak hardness.
Limited testing in a dry argon atmosphere showed that the sensitivity of fatigue crack growth rates to environment changed
with heat treatment. 相似文献
4.
Charpy V‐notch (CVN) impact‐test values are widely used in toughness specifications for AISI H11 hot‐work tool steel, even though the fracturing energy is not directly related to the tool design. KIc, the plain‐strain stress‐intensity factor at the onset of unstable crack growth, can be related to the tool design; however, KIc test values are not widely used in toughness specifications. This is surprising since to the designer KIc values are more useful than CVN values because the design calculations for tools and dies of high‐strength steels should take into account the strength and the toughness of materials in order to prevent the possibility of rapid and brittle fracture. An investigation was conducted to determine whether standardized fracture‐toughness testing (ASTM E399‐90), which is difficult to perform reliably for hard materials with a low ductility, could be replaced with a so far non‐standard testing method. A particular problem is that the manufacture of the fatigue crack samples is difficult and expensive, and this has promoted the search for alternative fracture‐toughness testing methods. One of the most promising methods is the use of circumferentially notched and fatigue‐precracked tensile specimens. With this technique the fatigue crack in the specimen is obtained without affecting the fracture toughness of the steel, if it is obtained in soft annealed steel, i.e., prior to the final heat treatment. The results of this investigation have shown that using the proposed method it was possible to draw, for the normally used range of working hardness, combined tempering diagrams (Rockwell‐C hardness ‐ Fracture toughness KIc ‐ Tempering temperature) for some AISI H11 hot‐work tool steel delivered from three steel plants. On the basis of the combined tempering charts the influence of the processing route on the mechanical properties was investigated. In the same way, vacuum‐heat‐treated tool steels were assessed and their properties expressed as a ratio of the fracture toughness to the hardness (KIc/HRc). 相似文献
5.
The effects of heat treatment and of the presence of primary carbides on the fracture toughness,K Ic and the fatigue crack growth rates,da/dN, have been studied in M-2 and Matrix II high speed steels. The Matrix II steel, which is the matrix of M-42 high speed steel, contained many fewer primary carbides than M-2, but both steels were heat treated to produce similar hardness values at the secondary hardening peaks. The variation of yield stress with tempering temperature in both steels was similar, but the fracture toughness was slightly higher for M-2 than for Matrix II at the secondary hardening peaks. The presence of primary carbides did not have an important influence on the values ofK Ic of these hard steels. Fatigue crack growth rates as a function of alternating stress intensity, ΔK, showed typical sigmoidal behavior and followed the power law in the middle-growth rate region. The crack growth rates in the near threshold region were sensitive to the yield strength and the grain sizes of the steels, but insensitive to the sizes and distribution of undissolved carbides. The crack growth rates in the power law regime were shifted to lower values for the steels with higher fracture toughness. SEM observations of the fracture and fatigue crack surfaces suggest that fracture initiates by cleavage in the vicinity of a carbide, but propagates by more ductile modes through the matrix and around the carbides. The sizes and distribution of primary carbides may thus be important in the initiation of fracture, but the fracture toughness and the fatigue crack propagation rates appear to depend on the strength and ductility of the martensite-austenite matrix. 相似文献
6.
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
7.
Robert O. Ritchie 《Metallurgical and Materials Transactions A》1977,8(7):1131-1140
Interactions between hydrogen embrittlement and temper embrittlement have been examined in a study of fracture and low growth
rate (near-threshold) fatigue crack propagation in 300-M high strength steel, tested in humid air. The steel was investigated
in an unembrittled condition (oil quenched after tempering at 650°C) and temper embrittled condition (step-cooled after tempering
at 650°C). Step-cooling resulted in a severe loss of toughness (approximately 50 pct reduction), without loss in strength,
concurrent with a change in fracture mode from micr ovoid coalescence to inter granular. Using Auger spectroscopy analysis,
the embrittlement was attributed to the cosegregation of alloying elements (Ni and Mn) and impurity elements (P and Si) to
prior austenite grain boundaries. Prior temper embrittlement gave rise to a substantial reduction in resistance to fatigue
crack propagation, particularly at lower stress intensities approaching the threshold for crack growth(x0394;K
o). At intermediate growth rates (10-5 to 10-3 mmJcycle), propagation rates in both unembrittled and embrittled material were largely similar, and only weakly dependent
on the load ratio, consistent with the striation mechanism of growth observed. At near-threshold growth rates (<10−5 to 10−6 mmJcycle), embrittled material exhibited significantly higher growth rates, 30 pct reduction in threshold ΔKo values and intergranular facets on fatigue fracture surfaces. Near-threshold propagation rates (and ΔKo values) were also found to be strongly dependent on the load ratio. The results are discussed in terms of the combined influence
of segregated impurity atoms (temper embrittlement) and hydrogen atoms, evolved from crack tip surface reactions with water
vapor in the moist air environment (hydrogen embrittlement). The significance of crack closure concepts on this model is briefly
described. ntmis]formerly with the Lawrence Berkeley Laboratory, University of California in Berkeley.
Formerly with the Lawrence Berkeley Laboratery, University of California in Berkeley. 相似文献
8.
Elmar K. Tschegg Christian Tauschitz Stefanie E. Stanzl 《Metallurgical and Materials Transactions A》1982,13(8):1483-1489
Fatigue crack growth curves(Δa/ΔN =f(K
max
)) were measured with 2.5 mm thick sheets of electron beam welded iron base superalloy A286. Fatigue testing frequency was
21 kHz,R = −1 (mean stress zero) and the environment was noncorrosive silicone oil at 20 °C. Two series of samples with different
welding conditions were tested. One series was welded perfectly, whereas the second contained microcracks within the weld
and the heat affected zone. It was shown that the crack growth rate in the base metal is slower than in the weld. The threshold
stress intensity factorK
th
of the base metal is 14 MNm-3/2 and that of the weld, 10 MNm
-3/2
. However, at higherK
max
values, the crack grows more rapidly in the weld than in the base metal; for example, the crack growth rate is 16 times higher
at Kmax = 20 MNm
-3/2
. Microcracks introduced by an imperfect welding process do not influence the fatigue cracking behavior in the threshold regime;
atK
max
= 15 MNm-3/2, however, the crack growth rates differ by an order of magnitude. Fractographic examination shows considerable differences
in the fracture appearance of weld, heat affected zone, and base material. Weld and base metal display ductile fracture surfaces
and the heat affected zone is characterized by crystallographic fracture facets. 相似文献
9.
The aim of this study was to examine the effect of grain size, regulated by means of austenitizing temperature, on the process of hot-work tool steel cracking. Fracture toughness (Klc) of this steel depended on the ratio of the average linear intercept of grain (L?) and the diameter of the plastic strain zone formed ahead of the propagating crack (dy). Investigations were supplemented by the fractographic analysis. The dependence of the Klc one the ratio L?:dy has been confirmed. In the subcritical range, when L?:dy<1, the grain growth causes rapid decrease of fracture toughness. In an overcritical range, when L?:dy>1, grain growth resulting from the increase of austenitizing temperature causes stoppage of the decrease of fracture toughness and can even lead to increase. Thus, tools requiring the highest hardness can be quenched from possibly high temperatures without fear of any disadvantageous influence upon the toughness of even a strong grain growth. In the critical range, when L?:dy≈1, fracture toughness of tool steel reached minimum. 相似文献
10.
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
11.
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
12.
The fatigue crack propagation characteristics of a friction stir welded Al‐Mg‐Si alloy, 6082, have been investigated. The electrical potential drop method was used for measurements. A low and a high load ratio (R) level were tested. At low load ratio (R=0.1) and a low stress intensity δK the propagation rate in the weld was higher than in the parent material by a factor of 3 to 5. However, the propagation rates were approaching each other close to fracture. At high load ratio (R=0.8) the propagation rate was similar in the parent material and weld. The weld crack growth rate was about the same at low and high R (except close to fracture), while the parent material growth rate increased at high R. Paris law was used to describe the measured crack propagation rates in the weld. In the case of the parent material, showing an R‐dependence, Forman's law was used. 相似文献
13.
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
14.
J. Toribio 《Metallurgical and Materials Transactions A》1997,28(1):191-197
A fracture mechanics approach to hydrogen-assisted microdamage in eutectoid steel is presented. Fractographic analysis revealed
micromechanical effects of hydrogen in the form of tearing topography surface (TTS). The progress of this microdamage is modeled as a macroscopic crack that extends the original fatigue precrack and
involves linear elastic fracture mechanics principles. In this case, the change from hydrogen-assisted microdamage (TTS) to
cleavagelike topography takes place when a critical stress intensity factor (K
H) is reached, and this value depends on the amount of hydrogen which penetrated the vicinity of the actual crack tip (the
fatigue precrack plus the TTS area). It is shown that the value K
H depends on experimental variables—mainly on the fatigue precracking regime—and its value may be associated with a characteristic
level of stress intensity factor in the crack growth kinetics curve. 相似文献
15.
High frequency (123 Hz) fatigue crack propagation studies were conducted under rising ΔK conditions (R-ratio = 0.22) on single edge notch specimens of austenitic stainless steel (type 316L) that contained an annealed precrack.
Tests were conducted in near neutral (pH 5.5) solutions of 1 M NaCl and 1 M NaCl + 0.01 M Na2S2O3 under potentiostatically controlled conditions and in desiccated air. Attention was directed primarily to the near threshold
behavior and the stage I (crystallographic) region of cracking. Good mixing between the crack solution and bulk solution was
obtained and crack retardation and arrest effects, due to surface roughness induced closure, were minimized at high anodic
potentials by electrochemical erosion. Thermodynamic considerations showed that hydrogen played no role in fatigue crack propagation.
Analysis of the results in terms of the estimated effective cyclic stress intensity, ΔK
eff, showed a systematic effect of potential on the average crack growth increment per cycle,da/dN. Anodic dissolution processes were considered to make an insignificant contribution toda/dN. A model was proposed for stage I fatigue cracking based on the effect of oxide nucleation rate on restricted slip reversal.
The essential features of the model were considered to be relevant to cracking in aqueous environments and in desiccated air. 相似文献
16.
Fatigue threshold tests have been conducted on through-thickness and semielliptic cracks in laboratory air, vacuum, and salt
water at stress ratios(R = Kmin/Kmax
@#@) of 0.2 and 0.7. The effects of stress ratio are rationalized by crack closure concepts. Environmental effects are explained
by considerations of the irreversibility of slip at the crack tip and the role of debris on the fracture surfaces. Differences
in the fatigue crack growth rates in the three environments are attributed largely to the extent of the irreversibility of
slip due to the chemisorption of water/ water vapor at the crack tip. Debris in saltwater solutions is also shown to significantly
affect the near-threshold growth through its influence on crack closure and the transportation of environment to the crack
tip.
formerly Graduate Student, Department of Materials Science and Technology, Cambridge University
formerly with the Department of Materials Science and Metallurgy, Cambridge University 相似文献
17.
J. D. Almer J. B. Cohen R. A. Winholtz 《Metallurgical and Materials Transactions A》1998,29(8):2127-2136
The effects of residual microstresses and tensile residual macrostresses on fatigue crack propagation (FCP) are examined in
a high-carbon steel. Phase-specific diffraction measurements show that uniaxial deformation and radial cold expansion produce
predominantly microstress and tensile macrostress fields, respectively. Microstresses are found to have little effect on FCP
rates, while tensile macrostresses increase crack growth rates in a manner that depends systematically on ΔK. The increases are partly attributed to crack closure, which was found to be appreciable near the surface of control samples
but absent in the presence of tensile residual stresses. Both the ΔK dependence and absence of microstress effects were explored by X-ray microbeam measurements around propagating fatigue cracks
and found to stem from fading and/or redistribution of residual macrostresses and microstresses during fatigue crack growth. 相似文献
18.
The behavior of hydrogen induced slow crack growth in type 310 and type 16-20-10 stable austenitic stainless steels along
with type 321 unstable austenitic stainless steel were investigated. It was found that slow crack growth could occur in all
three types of stainless steels, and the threshold values wereK
H/Kc = 0.55, 0.7, and 0.78 for type 321, 310, and 16-20-10 stainless steel respectively, when charged under load. Slow crack growth
could also occur if the precharged specimens were tested under constant load in air. No slow crack growth occurred in the
precharged and then out-gassed specimens. This indicates that delayed cracking in stable austenitic stainless steels is induced
by hydrogen. Since there is no hydrogen induced α’ martensite in type 310 and 16-20-10 stainless steel, the existence of a’
martensite is not necessary for the occurrence of slow crack growth in the austenitic stainless steels, although it can facilitate
slow crack growth. The mode of hydrogen induced delayed fracture in either the stable or unstable austenitic stainless steel
is correlated with theK, value; the fracture surface is changed from ductile to brittle asK
1 is decreased. 相似文献
19.
《Acta Metallurgica Materialia》1991,39(5):793-806
A study of the influence of SiC-particulate reinforcement on ageing and subsequent fatigue crack growth resistance in a powder metallurgy 8090 aluminium alloy-SiC composite has been made. Macroscopic hardness measurements revealed that ageing at 170°C in the composite is accelerated with respect to the unreinforced alloy, though TEM studies indicate that this is not due to the enhanced precipitation of S'. Fatigue crack growth rates in the naturally aged condition of the composite and unreinforced matrix are similar at low to medium values of δK, but diverge above ≈ 8 MPa√m owing to the lower fracture toughness of the composite. As a result of the presence of the reinforcement, planar slip in the composite is suppressed and facetted crack growth is not observed. Ageing at or above 170°C has a deleterious effect on fatigue crack growth. Increased ageing time decreases the roughness of the fracture path at higher growth rates. These effect are though to be due to microstructural changes occurring at or near to the SiC/matrix interfaces, providing sites for static mode failure mechanisms to operate. This suggestion is supported by the observation that as δK increases, crack growth rates become Kmax dependent, implying the crack growth rate is strongly influenced by static modes. 相似文献
20.
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