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Subcritical crack growth and its effect upon the fatigue characteristics of structural alloys
Errpr,zre.ri,lp FrocrrrrrMe
PergamonPress Printedin Great Britain
SUBCRITICAL CRACK GROWTH AND ITS EFFECT UPON THE FATIGUE CHARACTERISTICS OF STRUCTURAL ALLOYS-t W. Westinghouse
Research
G.
CLARK,
Laboratories,
JR. Pittsburgh.
Pennsylvania
Abstract-The subcritical crack growth behavior noted during the fracture toughness testing of several structural alloys was investigated and compared with the crack propagation characteristics observed under fatigue loading conditions. The stress intensity factor, which describes the stress conditions necessary to initiate monotonic loading subcritical crack growth, was found to correspond to that associated with an accelerated fatigue crack growth rate. From these observations, it appears that irregularities in the fatigue growth characteristics at higher K levels may be the direct result of subcritical crack growth occurring in
conjunction with fatigue damage.
INTRODUCTION SUBCRITICAL monotonic
crack loading
growth fracture
or crack toughness
extension testing
prior is not
to final failure during conventional an uncommon
phenomenon
and
has been noted in a variety of high-strength alloys[ l-31. The present concept of fracture behavior attributes the stable nature of subcritical crack growth to the effect of plastic deformation in the vicinity of an advancing crack front[3]. Krafft et af.[4] distinguish between two categories of plastic flow which resist the occurrence of fast fracture: (I) localized plastic yielding at the crack tip, and (2) the possible restraining effects of free surface boundary stresses. Consideration of these two catgeories of plastic flow can in turn be used to differentiate between two types of subcritical crack growth which may be encountered under direct tension loading in a neutral environment. For the case of a finite plate containing a sharp defect subjected to opening-mode tension stresses, crack growth is accompanied by the formation of a crack tip plastic zone[3,4]. As the stress continues to increase the plastic zone size also increases, thereby resisting unstable crack extension; as a result, crack growth is essentially self-limiting and will continue only with increasing stress. This subcritical crack growth behavior can be considered representative of the inherent fracture characteristics of the material. Upon increasing the load, the stresses eventually become sufficiently high to overcome the restraining effect of the crack tip plastic zone and rapid failure may occur (plane strain failure). However, if the specimen size and geometry are such that the free surface boundary stresses provide additional restraining effects, the crack may grow only with increasing load and fast fracture will not occur until the free surface stresses are overcome (non-plane strain failure). This type of subcritical crack growth does not represent a true material property. but instead is size dependent. The consideration of both categories of plastic flow and their effect upon crack propagation is very important from the fracture mechanics point of view and provides the basis for adequate specimen design and the evaluation of rising load fracture toughness testing[5]. The stress intensity concept of linear elastic fracture mechanics provides a singleterm parameter (K) which describes the crack tip stress state, and within limits is *Presented at June
19-21.
1967.
the National
Symposium
on Fracture
Mechanics.
Lehigh
University.
Bethlehem,
Pa.,
386
W. G. CLARK,
JR.
independent of how the state of stress is achieved[6]. Therefore, since crack propagation is dependent upon the stresses at the crack tip, it is reasonable to suspect that subcritical crack growth which occurs under rising load conditions may influence fracture behavior under cyclic loading conditions; in particular, the cyclic crack growth rate. Many attempts have been made in recent years to establish a satisfactory relationship between engineering design parameters and cyclic crack growth rates[7, Xi, Generally, these proposed relationships indicate that the fatigue crack growth rate is dependent upon the alternating stress level and the crack length. This consideration has led Paris to develop a relatively simple exponential function which provides an empirical relationship between sinusoidal crack growth and the corresponding stress intensity[6]. This relationship is expressed as: daldN = C*AK” where daldN is the crack growth rate; C is a constant which depends upon the material, the relative mean load, and the frequency; and AK is the cyclic change in stress intensity. The value of the exponent n as first determined by Paris was 4; however. recent investigations indicate that n can vary from 1 to 6 depending upon the material and stress level [91. The interpretation of a considerable amount of cyclic crack growth data in terms of the exponential function indicates that a linear relationship (with a slope = n) between log du/dN and log AK does exist at lower K levels [6]. However, as the value of AK approaches the critical stress intensity factor, deviation from the initial linear relationship to an accelerated crack growth rate is observed[6, IO]. Carman and Katlin have demonstrated that failure to account for this change in slope in a design situation involving a high-strength steel can lead to a very serious overestimation of the number of cycles required for an existing flaw to grow to the critical size for catastrophic failure [lo]. Thus, it is obvious that irregularities observed during the generation of cyclic crack growth data must be explained and accounted for before such an empirical relationship can become a useful design tool. Since the occurrence of subcritical crack growth during rising load toughness testing has also been observed at relatively high K levels, it may be possible that the accelerated crack growth rate associated with cyclic loading is the result of a combination of subcritical growth and fatigue damage. The purpose of this investigation was to evaluate the subcritical crack growth behavior of several structural alloys and to relate this behavior to the cyclic crack growth characteristics. MATERIALS The materials involved in this investigation were selected to provide a wide spectrum of structural alloys ranging from a typical high-strength, low-toughness aluminum alloy to a rather tough, low-strength steel. Aluminum alloy 7079-T6 was selected as representative of a typical high-strength, low-toughness material. Alloy steel HP 9-4-25 and aluminum 5456-H321 were selected to represent materials of intermediate strength and toughness and a Ni-MO-V forging grade steel was selected as representative of a high-toughness, low-strength alloy. The 7079-T6 aluminum and HP 9-4-25 steel alloys were supplied in the form of 3 in. thick plate and the 5456-H321 aluminum
Subcritical crack growth
387
as a 1 in. thick plate. The Ni-MO-V steel alloy was supplied as a large disc removed from a generator rotor forging. The alloys were treated by conventional techniques to the strength levels consistent with the normal requirements for the particular alloy and product form. Table 1 gives the composition and room temperature tensile properties of the alloys under investigation.
was supplied
EXPERIMENTAL PROCEDURE All fracture toughness tests under both monotonic and cyclic loading conditions were conducted at room temperature with 1 in. thick Wedge-Opening-Loading (WOL) type toughness specimens of the 1T geometry illustrated in Fig. 1. The aluminum specimens were modified slightly (5 per cent deep side notching) in an attempt to minimize the anisotropic crack behavior commonly encountered in these alloys. The conventional direct tension tests were conducted at a head motion rate of O-040 in./min on a universal hydraulic test machine. The load-displacement characteristics of the specimen under test were measured with a clip gage calibrated to 40,uin. of strain per 1 ml of displacement and recorded on an autographic X-Y recorder. In addition. crack behavior during the application of load was monitored with a IO megahertz (MHz) ultrasonic transducer used in conjunction with conventional ultrasonic maw-detention instrumentation[ 1 I]. A sensitivity level capable of detecting crack growth on the other of 0.005 in. was used. The combination of displacement gage and ultrasonic instrumentation provided a monitoring technique capable of distinguishing between crack growth and general yiefding at the crack tip prior to rapid failure. The IT type WOL specimens used to establish fracture behavior under cyclic loading conditions were side notched (45” included angle, 0.010 in. root radius) to a depth of 0.050 in. on each side prior to fatigue cracking in order to ensure crack propagation perpendicular to the loading direction. All cyclic testing was conducted on a constant-load universal fatigue machine under sinusoidal loading conditions (0 to maximum load) at a frequency of 1800 c/min. The maximum alternating load was held constant throughout each individual test resulting in a AK equivalent to K,,,. An ultrasonic flaw-detection technique involving the use of a 10 MHz, 3/g in.-diameter ceramic transducer was used to measure and record the extent of fatigue crack propagation encountered[ 111. As a result, a continuous record of crack length versus number of elapsed cycles was available for evaluation. EXPERIMENTAL RESULTS Figures 2-5 show typical room temperature load-displacement records obtained for each alloy as a result of conventional rising load fracture toughness testing. The accompanying ‘ultrasonic’ crack behavior records indicated that crack growth occurred prior to specimen failure in the HP 9-4-25 alloy steel and in both aluminum alloys !7079-T6 and 545GH321). No crack growth was observed in the Ni-MO-V alloy steel and the test specimens exhibited a considerable amount of gross plastic yielding prior to failure. Table 2 presents the value of the stress intensity factor associated with deviation from linearity (D), initiation of crack growth (I), and specimen failure (U) as determined from the interpretation of the load displacement and crack growth records. The expressions and method involved in the determination of the stress intensity factors are readily available in the Iiterature[ 12,131. In each case, the K values were based on the initial fatigue crack length measured on the fractured specimens.
Ni-MO-V Steel
5456H321 Aluminum
HP9-4-25 Steel
7079-T6 Aluminum
Alloy
0.29
Mn
0.50
0.14
C
0.23
0.009
P
0.08
Cu
Fe
Si
P
Mn
0.008
Si
0.010
S
Mg
Cr
8.41
Ni
0.10 0.25
Cr
3.4
Ni
0.08
0.21
Si
0.01
S
3.8 4.8
Zn
5.17
0.29 0.37
Mg
0.68
0.10 0.30
Mn
0.008
0.3
Si
0.33
Mn
0.4
Fe
0.26
c
0.4 0.8
Cu
0.08
Cr 0.30
MO
0.03
0.03
0.48
MO
0.07
V
0.07
V
(nominal)
Ti
0.40
Cr
0.10
Ti
Zn
Composition (wt. %)
3.9
Co
84
37
176
65
0.2% Yield strength (ksi)
Table I. Composition and tensile properties of test materials
102
52
186
76
Ultimate strength (ksi)
53
40
64
27
Red. in area (%)
21
12
14
II
Elong. (%)
Subcritical crack growth
389
0.700 Dia.
0.625-18 Thd CL3 x.625DP
Fig. 1. Wedge opening loading type fracture toughness specimen (IT Geometry).
The results of the cyclic loading portion of this investigation are summarized graphicatly in Figs. 6-9. Note that the log crack growth rate-log stress intensity relationships for the 7079-T6 aluminum and the Ni-MO-V alloy steel are essentially linear (slope n = 3) throughout the stress intensity range represented (Figs. 6 and 9). However, the HP 9-4-25 steel and the 54%H321 aluminum ahoy exhibit a distinct slope transition to an accelerated growth rate (Figs. 7 and 8). The stress intensity level at which the accelerated crack growth begins is presented in Table 2.
u
DI PIU -
Deviation Cmck initiation Pop in Uttimote lood
Specimen identiiicotim Testing temperature Deviation bod (lb) Pop in lood (lb) Utimo* load (lb)
Fig.
2.
Typical room temperature
load-displacement
curve for 70’79-T6 aium~num.
390
W. G. CLARK.
JR.
D-D&ah I - Crack initiath PI- Pap in U - Ultimation
Specimen idsntifiiatian Bsting &mpemhn Deuialtanbad(lb) Pap-inload Ulttmation (lb)
HP3T-8 t5 75°F 21*0 =4DO 2V50
Dispkxement
Fig. 3. Typical
room temperature
load-displacement
curve for HP 9-4-25 alloy steel
In order to establish the state of stress associated with the various portions of this investigation, the plane strain measurement capacity of the IT WOL toughness specimen with respect to the various alloys was determined. According to the latest ASTM tentative recommendations for plane strain toughness testing of high-strength alloys, the maximum plane strain stress intensity factor which can be determined with
D-Deviation I- Cmck irhttion U-Ultimateload
I
:: 0
I/
/
%d7lSl, &"tifiKOtiW, Spacinartypa
lbsting teqmmtm Dwiath load hp in loodUb Ultlmok load ( lb)
5456L- 83 wOL0-f) 75T 3950 7700
V Displacement
Fig. 4. Typical
room temperature
load-displacement
curve for 5456-H32
I aluminum.
Subcriti~l
crack growth
391
D - Deviitioo U - Ultimate land
Specimen id~ti~~ation Testing tempemtum Deviation load (lb) f%p-in lood (lb) Ultimate iood (lb)
Fig. 5. Typical
room temperature
looo~
800-
I
28,900
load-displacement
1,
curve for Ni-Mo-V
,,,I
1
steel.
i
7079T6 Al~min~rn
600-
0.2% YieldStrength= 65ksi TestTemp. = 75V
400.
TestFrequency = l&N cpm Max. Cyclicbad 0 -14Lw '-2coo# n -3oMM l -4@w
zw-
BDE- 8105 75’F 12,900
f-
I
-1 6 -4-
2-
I1
2,
/
3
I ,////I
45678910 StressInfenSity Factor,KI,
Fig. 6. Crack
growth
rate as a function
/
/
20
30
ksi&
of stress intensity
: 40 30"
for 7079-T6 aluminum.
392
W. G. CLAKK.JK.
.I
1 10
l/I
!
20 40 60 80100 StressIntensityFactor,KI, ksifi
2w1
Fig. 7. Crack growth rate as a function of stress intensity for HP 9-4-25 steel.
the 1T WOL specimen is approximately equivalent to 0.63 times the 0.2 per cent offset yield strength of the material under test[5]. This value is presented in Table 2 for the various materials. DISCUSSION Examination of the stress intensity data presented in Table 2 for the HP 9-4-25 alloy steel indicates a definite correlation between the stress intensity factor associated with monotonic loading crack initiation (107 ksi (in.)1’2) and that corresponding to the cyclic crack growth slope transition (100 ksi (in.)‘12). A similar correlation is apparent for the 7079-T6 aluminum alloy (Fig. 6), although the narrow stress intensity range, over which subcritical crack growth initiates and final failure occurs, makes it difficult to establish a distinct slope transition. Based upon the limited amount of crack growth data available at a stress intensity factor above 30 ksi (in.) 1/Zit appears that a transition does exist. Note the absence of a cyclic crack growth slope transition for the Ni-MO-V alloy steel which also failed to exhibit subcritical crack growth under monotonic loading conditions (Fig. 9). From these observations, it appears that the occurrence of subcritical crack growth does, in fact, influence the cyclic crack growth rate of some structural alloys.
397
Subcriticalcrackgrowth
54%H321Aluminum 0.2%Yield Strength = 37 ksl Test Temp.= 75Y Test Frequency= Bf~Ocpm Max.Cyclic Load . - 15w P -2cw 0 -4iw 0 -5m v -55cw
v I “= 5.2
1
I_-, 50 60 Fig. 8. Crack growth rate as a function of stress intensity for 5456-H321 aluminum (longitudinal orientation).
The data presented in Table 2 for the 54.56H32 1 aluminum alloy is not consistent with the behavior noted above. The stress intensity factor associated with the initiation of subcritical crack growth (30 ksi (in.)z’2) is considerably higher than that corresponding to the inflection on the cyclic crack growth curve (23 ksi (in.)1’2). Note, however. that the stress intensity factor at which accelerated cyclic crack growth is observed for the 5456-H321 aluminum alloy corresponds to the maximum plane strain measurement capacity of the iT WOL specimen (23 ksi (in.)““). A similar observation is apparent for the case of the HP 9-4-25 steel. From these observations, one might logically conclude that the slope transition is not the result of the influence of subcritical crack growth but rather is due to a change in the state of stress from predominantly plane strain conditions to non-plane strain conditions. If this were the case, however, one would expect some type of inflection in the cyclic crack growth rate curve for the Ni-MO-V alloy corresponding to the stress intensity at which deviation from linearity (plastic yielding) was observed on the load displacement curve (68 ksi (in.Y?. No such injection is apparent. In addition, the absence of a distinct discontinuity in the slope of the log dulcW vs. log AK curve corresponding to the fracture mode transition, has been noted for other structural alloys[l4, 151. Also, a considerable amount of published crack growth rate data indicates that the cyclic crack growth rate appears independent of specimen thickness[7,8, 161. The reported data cover a range
394
NiMoV Steel 9.2% Yield Strength = 84.5 ksi Test Temp. = 759 Test Frequency = 1800 cpm Max. Cyclic Load l -4im# c - 5ca.x l -6DW 4 -7m D -8lm!# 0 - 92006 a-99oM
-
Fig. 9. Crack
growth
20 40 60 85 1w Stress Intensity Factor, K,, ksi,K
rate as a function
of stress
intensity
-7
for Ni-MO-V
steel
of thicknesses involving both plane strain and non-plane strain conditions and no significant difference in the crack growth rate is apparent. A limited amount of cyclic crack growth data established on 2 in.-thick WOL specimens (plane strain measurement capacity 0% cr&t prepared from a similar heat of 5456-H32 1 aluminum indicates a slope transition corresponding to that noted for the 1 in.-thick specimens. Therefore. the accelerated cyclic crack growth behavior noted in this investigation cannot be attributed directly to a change in the state of stress. In view of the fact that the cyclic crack growth rate appears independent of specimen thickness, it is reasonable to assume that the free surface boundary stresses generally associated with fracture toughness testing are relieved to some extent as a result of repeated loading conditions. This assumption leads to the conclusion that the subcritical crack growth associated with the inherent plane strain fracture behavior of a material may constitute a primary effect upon the cyclic crack growth data. Further examination of the stress intensity data summarized in Table 2 indicates that the tougher the material, the greater the range between the K value at deviation from linearity (D) and that at the initiation of crack growth (I). This behavior is to be expected since the amount of plastic yielding which occurs prior to significant crack extension is proportional to the relative toughness of the material. Consideration of the recommended plane strain measurement capacity indicates
7079-T6 Aluminum HP 9-4-25 Steel 5456-H32 I Aluminum Ni-MO-V Steel
Material 30 105 23 68
(in.)“‘)
At dev.
. (ksi
Table
32 107 30 None
(ksi (in.)?
crack growth
of subcritical
Af I-initiation
2. Summary
35 130 46 175
(ksi (in.)l ‘)
load
At ultimate
of stress intensity
capacity
41 110 23 53
(ksi (in.)’ “)
specimen
At recommended
(K,) data
of
31 100 24 None
(ksi (in.)?
crack growth rate
cyclic
At initiation accelerated
396
W. G. CLARK,
JR.
that the initiation of crack extension under sustained loading conditions falls within the capacity limits for the 7079-T6 aluminum and the HP 9-4-25 alloy steel. Since the amount of stable crack extension, which occurs prior to plane strain failure is relatively small (assumed to be equivalent to one plastic zone-O.020 in. for HP 9-4-25 steel and 0.010 in. for the 7079-T6 aluminum), the stress intensity factor associated with the initiation of crack growth can be considered a realistic value of the actual plane strain critical stress intensity factor (K,,). Note that the K value associated with the ultimate load is considerably higher and cannot be used as a satisfactory K,, design parameter. Since crack extension in the 5456-H321 aluminum alloy occurred above the measurement capacity, it is possible that the free surface boundary stresses resisted the initiation of crack growth at the specimen center; thus, in this case, it would be erroneous to conclude that the critical stress intensity factor corresponds to the initiation of crack growth. Fracture toughness tests must be conducted on thicker test specimens to establish a realistic K,, value for the 5456-H32 1 aluminum. In view of the gross plastic yielding observed in the Ni-MO-V steel, it is obvious that a much larger test specimen is required to establish a value K,,. Many factors aside from the stress intensity level at the crack tip are known to affect the propagation characteristics noted for a given structural alloy[2]. Prominent among these factors are loading rate and environmental effects. The loading rate associated with the fatigue portion of this investigation was considerably higher than that used in the rising load tests (approximately lo4 to 1); therefore, it is reasonable to suspect that the subcritical crack growth observed in the HP 9-4-25 alloy would occur at a lower K level under fatigue conditions since the increased loading rate tends to decrease the measured K,, value. In addition, possible corrosion effects are aggravated by cyclic loading; thus, it is not surprising that subcritical crack extension in the 5456-H321 aluminum alloy was noted at a lower K level under repeated loading conditions. One can easily visualize the brittle protective oxide coating at the crack tip being progressively destroyed during the cyclic loading. The effect of monotonic loading material characteristics upon fatigue crack growth behavior has been demonstrated by Watts and Burns[l7]. Their data clearly indicate that the creep properties of Perspex (Plexiglas) observed under static loading conditions contribute to an accelerated fatigue crack growth rate at a K level corresponding to the initiation of creep. Macroscopically, the fatigue properties of Perspex are considered very similar to those found in metals and these data tend to substantiate the observations involved in this investigation. In order to reveal the nature and appearance of subcritical crack growth, several 5456-H32 1 aluminum specimens were subjected to tension loads above that at which crack extension occurred, unloaded, and then subjected to cyclic loading. The cyclic loading served to induce fatigue striations which clearly marked the extent of prioi crack growth. Figure 10 shows the resulting fracture appearance. Note the rough texture and bowed-out crack front associated with the subcritical crack growth. This observation is consistent with the appearance of crack growth behavior reported in a variety of structural alloys [ 1, 181. Figure 11 presents the results of an electron microscopic eXaminatiOn Of the fracture behavior Observed in aluminum alloy 5456-H32 1. The typical fracture appearances associated with fatigue and subcritical crack growth are shown in Figs. 1 l(a) and 1 I(b). respectively. No variation in the fracture appearance associated with the subcritical crack growth and fast fracture was apparent. Figure 1 I(C) illustrates the
flat
r
Additional
fatigue cracking
-+
Subcritical
crack growth
-
Fig. 10. Fracture
appearance
fracture
of a 5456-H321 aluminum WOL specimen followed by cyclic loading to failure.
lniiiol fatigue cmck
subjected
to tension loading
[Facing page 3961
Mag. 8400 Y A Fractograph Illustrating Typical Low Stress Intensity Fatigue
Mag. 2520 ” B Fractograph of Subcritical Crack Growth
Mag. 1470
c Fractograph Fig. 11. Fractographic
at Initiation of Accelerated Crack Growth appearance
of aluminum alloy 5456H321.
Subcriticalcrack growth
397
typical fracture appearance at the crack length corresponding to the initiation of the accelerated fatigue crack growth rate. Note the occurrence of both fatigue crack growth and ductile fracture mechanisms. From this observation, it is apparent that the fracture behavior is a combination of two distinct mechanisms. However, Hertzberg and Paris have made a similar observation corresponding to a transition from plane strain to nonplane strain fatigue crack propagation and no abrupt change in the slope of the da/dN vs. AK relationship was observed[l4]. Therefore, it does not appear that a distinction can be made between the fracture appearance associated with subcritical crack growth, and non-plane strain fatigue failure. Aside from the observations associated directly with the major objective of this investigation, the data reveal several other interesting conclusions. Consideration of the fatigue crack growth characteristics of structural alloys in terms of more conventional engineering parameters rather than fracture mechanics indicates that a change in the net section stress from the elastic to the plastic region (below and above the established yield strength) may be the cause of the marked change in the fatigue crack growth rate[9]. The ratio of net section stress (a,) to yield strength (a,,,) at the point of accelerated crack growth indicates that the stresses were well below the established yield strength of the respective materials-the (a,/~& ratio was 0.44, 0.57, and 0.59 for the 7079-T6 aluminum, HP 9-4-25 steel, and the 5456-H32 1 aluminum alloys, respectively. Thus, it is apparent that the net section stresses need not be above the yield strength to induce a change in the fatigue crack growth rate. Another pertinent observation associated with this investigation concerns the wide range of maximum alternating loads used to develop the cyclic crack growth data. Note that no significant effect of alternating load on the cyclic crack growth ratestress intensity relationship is apparent for the alloys investigated.
CONCLUSIONS crack growth which occurs under rising load conditions represents inherent fracture behavior which in turn affects the fatigue crack growth properties; in particular, the fatigue crack growth rate. (2) The change in slope (to an accelerated crack growth rate) of the log daldN vs. log AK relationship can be the result of the influence of subcritical crack growth on fatigue behavior. (3) The stress intensity factor associated with the initiation of significant crack growth under predominantly plane strain conditions may be used to provide a realistic estimate of the material K,,. (4) A change in the state of stress from plane strain to non-plane strain conditions does not appear to affect the cyclic crack growth characteristics of structural alloys. (5) The maximum alternating cyclic load does not appear to affect the fatigue crack growth rate-stress intensity relationship under sinusoidal loading conditions. (I ) Subcritical
Ac,knol(,lrd~etnents -The author is indebted to his many colleagues who contributed 1~1the preparation of this paper. In particular the author would like to thank Mr. E. T. Wessel for his helpful suggestions and review of the work. Mr. L. J. Ceschini for development of the fatigue cracking techniques, &knd Mr. R. C. Hates for the interpretation of the electron photomicrographs.
REFERENCES [)I E. A. Steigerwald
and G. L. Hanna, Initiation of slow crack propagation in high-strength materials, P~oc. Am. Si>c. Test. Muter. 62 (I 962). 121 G. G. Hancock and H. H. Johnson, Subcritical crack growth in AM350 stcel.Mtrr<>,. R~~,~,Slnnd. 6.
398
W. G. CLARK. JR.
[31 The slow growth and rapid propagation of cracks, Second Report of a Special ASTM Committee, Mater. Res. Stand. 1,389-393 (1961). [41 J. M. KratTt, A. M. Sullivan, R. W. Boyle, Effect of dimensions on fast fracture instability of notched sheets. Proc. Crack Propagation Symp. Vol. I, pp. 8-28. College of Aeronautics, Cranfield. England (1961). 151 W. F. Brown and J. E. Srawley, Current status of plane strain crack toughness te\ting. A.S.T.M. .~pe(. Tech. Publ. No. 4 10
(1967).
[61 P. C. Paris, Fatigue crack growth. Presented at the Second Annual Workshop in FI-actare Mechanics. Denver Research Institute (1965). [71 A. J. Brothers andS. Yukawa, Fatique crack propagation in low-alloy heat-treated steels, Presented at the Metals Engng. Conf. Cleveland, Ohio: A.S.M.E. Paper No. 66-Met-2 (1966). PI B. Cotterell, An interpretation of the mechanics of crack growth by fatigue. J. has. Engng 230-236 (1965).
[91 H. W. Liu, Fatigue crack propagation
and the stresses
and strains in the vicinity of a crack. rll>p/
Mater. Res. 229 (I 964). [lOI C. M. Carman and J. M. Katlin, Low cyclic fatigue crack propagation steels. A.S.M.E. Paper No. 66-Met-3 (1966).
characteristics
of high-strength
1111 W. G. Clark, Jr., Ultrasonic detection of fracture initiation and extension in the W.O.L. type frdctllrc toughness specimen. Mater. Eualuution. 2.5. 185- I90 (1967). [I21 W. K. Wilson, Westinghouse Research Laboratories, unpublished data. [I31 E. T. Wessel, W. G. Clark and W. K. Wilson, Engineering methods for the design and selection of materials against fracture U.S. Army Tank and Aatomotiue Center Rep., Contr. No. DA-30-069. AMC602(T) (1966). [I41 R. W. Hertzberg and P. C. Paris, Application of electron fractography and fracture mechanics to fatigue crack propagation, Proc. 1st Int. Conf. on Fracture Vol. 1.p. 466 (I 965). 1151 C. M. Hudson, Investigation of fatigue crack growth in Ti-8AI-lMo-1V (duplex-annealed) specimens having various widths. NASA Tech. Note No. D-3879 (I 967). 1161 N. E. Frost and K. Denton, Effect of sheet thickness on the rate of growth of fatigue cracks in mild steel. J. Mech. Engng Sci. 3 ( I96 I ). Proc. 22nd Au. /I71 N. H. Watts and D. J. Burns, Fatigue crack propagation in polymethylmethacrylate, Tech. Conf S.P.E. (Session XIV-2), Montreal (1966). 1181 W. F. Brown, Jr., Some observations of pop-in determination of K7,. Presented at the AH. .Mri,i A.S.M.E.
(1963). (Received
25 February
1967)
Resume- Le comportement de la croissance sous-critique de rupture remarque pendant les essais de duretc a la rupture de plusieurs alhages structurels, a ete etudii et compare aux caracteristiques de la propagation de la rupture observtes dans des conditions de charge de fatigue. Le facteur d’intensite de tension, qui d&it les conditions de tension necessaires pour initier une croissance sous-critique de rupture a charge monotonique, correspondait a celui associe au taux de croissance de rupture avec fatigue acceleree. A partir de ces observations, il apparait que des irregularites dans les caracteristiques de croissance de rupture par la fatigue a des niveaux de K superieurs, peuvent &tre le resultat direct de la croissance sous-critique de rupture survenant en conjonction avec le dommage cause la fatigue. Zusammenfassung- Das Risswachstum im subkritischen Bereich, das bei der Bruckzahigkeitspriifung verschiedener Baulegierungen beobachtet worden war, wurde untersucht und mit den unter ermiidungsbedingungen festgestellten Fissfortpflanzungs eigenschaften verglichen. Es wurde gefunden, dass de7 Spannungsintensitatsfaktor, der die zum Einleiten subkritischen Risswachstums bei monotoner Belastung erforderlichen Spannungsbedingungen beschreibt, dem mit einem beschleunigten ribivachstum in Ermiidungs bruch verbundenen Faktor entspricht. Diese Beobachtungen deuten darauf hin, dass Unregelmlssigkeiten in den ribivachstums eigenschaften im Ermiidungs bruch bei hoheren K-werten das unmittelbare Ergebnis von subkritischem Risswachstum im Zusammeiihang mit Ermtidungschaden sein konnten.
PergamonPress Printedin Great Britain
SUBCRITICAL CRACK GROWTH AND ITS EFFECT UPON THE FATIGUE CHARACTERISTICS OF STRUCTURAL ALLOYS-t W. Westinghouse
Research
G.
CLARK,
Laboratories,
JR. Pittsburgh.
Pennsylvania
Abstract-The subcritical crack growth behavior noted during the fracture toughness testing of several structural alloys was investigated and compared with the crack propagation characteristics observed under fatigue loading conditions. The stress intensity factor, which describes the stress conditions necessary to initiate monotonic loading subcritical crack growth, was found to correspond to that associated with an accelerated fatigue crack growth rate. From these observations, it appears that irregularities in the fatigue growth characteristics at higher K levels may be the direct result of subcritical crack growth occurring in
conjunction with fatigue damage.
INTRODUCTION SUBCRITICAL monotonic
crack loading
growth fracture
or crack toughness
extension testing
prior is not
to final failure during conventional an uncommon
phenomenon
and
has been noted in a variety of high-strength alloys[ l-31. The present concept of fracture behavior attributes the stable nature of subcritical crack growth to the effect of plastic deformation in the vicinity of an advancing crack front[3]. Krafft et af.[4] distinguish between two categories of plastic flow which resist the occurrence of fast fracture: (I) localized plastic yielding at the crack tip, and (2) the possible restraining effects of free surface boundary stresses. Consideration of these two catgeories of plastic flow can in turn be used to differentiate between two types of subcritical crack growth which may be encountered under direct tension loading in a neutral environment. For the case of a finite plate containing a sharp defect subjected to opening-mode tension stresses, crack growth is accompanied by the formation of a crack tip plastic zone[3,4]. As the stress continues to increase the plastic zone size also increases, thereby resisting unstable crack extension; as a result, crack growth is essentially self-limiting and will continue only with increasing stress. This subcritical crack growth behavior can be considered representative of the inherent fracture characteristics of the material. Upon increasing the load, the stresses eventually become sufficiently high to overcome the restraining effect of the crack tip plastic zone and rapid failure may occur (plane strain failure). However, if the specimen size and geometry are such that the free surface boundary stresses provide additional restraining effects, the crack may grow only with increasing load and fast fracture will not occur until the free surface stresses are overcome (non-plane strain failure). This type of subcritical crack growth does not represent a true material property. but instead is size dependent. The consideration of both categories of plastic flow and their effect upon crack propagation is very important from the fracture mechanics point of view and provides the basis for adequate specimen design and the evaluation of rising load fracture toughness testing[5]. The stress intensity concept of linear elastic fracture mechanics provides a singleterm parameter (K) which describes the crack tip stress state, and within limits is *Presented at June
19-21.
1967.
the National
Symposium
on Fracture
Mechanics.
Lehigh
University.
Bethlehem,
Pa.,
386
W. G. CLARK,
JR.
independent of how the state of stress is achieved[6]. Therefore, since crack propagation is dependent upon the stresses at the crack tip, it is reasonable to suspect that subcritical crack growth which occurs under rising load conditions may influence fracture behavior under cyclic loading conditions; in particular, the cyclic crack growth rate. Many attempts have been made in recent years to establish a satisfactory relationship between engineering design parameters and cyclic crack growth rates[7, Xi, Generally, these proposed relationships indicate that the fatigue crack growth rate is dependent upon the alternating stress level and the crack length. This consideration has led Paris to develop a relatively simple exponential function which provides an empirical relationship between sinusoidal crack growth and the corresponding stress intensity[6]. This relationship is expressed as: daldN = C*AK” where daldN is the crack growth rate; C is a constant which depends upon the material, the relative mean load, and the frequency; and AK is the cyclic change in stress intensity. The value of the exponent n as first determined by Paris was 4; however. recent investigations indicate that n can vary from 1 to 6 depending upon the material and stress level [91. The interpretation of a considerable amount of cyclic crack growth data in terms of the exponential function indicates that a linear relationship (with a slope = n) between log du/dN and log AK does exist at lower K levels [6]. However, as the value of AK approaches the critical stress intensity factor, deviation from the initial linear relationship to an accelerated crack growth rate is observed[6, IO]. Carman and Katlin have demonstrated that failure to account for this change in slope in a design situation involving a high-strength steel can lead to a very serious overestimation of the number of cycles required for an existing flaw to grow to the critical size for catastrophic failure [lo]. Thus, it is obvious that irregularities observed during the generation of cyclic crack growth data must be explained and accounted for before such an empirical relationship can become a useful design tool. Since the occurrence of subcritical crack growth during rising load toughness testing has also been observed at relatively high K levels, it may be possible that the accelerated crack growth rate associated with cyclic loading is the result of a combination of subcritical growth and fatigue damage. The purpose of this investigation was to evaluate the subcritical crack growth behavior of several structural alloys and to relate this behavior to the cyclic crack growth characteristics. MATERIALS The materials involved in this investigation were selected to provide a wide spectrum of structural alloys ranging from a typical high-strength, low-toughness aluminum alloy to a rather tough, low-strength steel. Aluminum alloy 7079-T6 was selected as representative of a typical high-strength, low-toughness material. Alloy steel HP 9-4-25 and aluminum 5456-H321 were selected to represent materials of intermediate strength and toughness and a Ni-MO-V forging grade steel was selected as representative of a high-toughness, low-strength alloy. The 7079-T6 aluminum and HP 9-4-25 steel alloys were supplied in the form of 3 in. thick plate and the 5456-H321 aluminum
Subcritical crack growth
387
as a 1 in. thick plate. The Ni-MO-V steel alloy was supplied as a large disc removed from a generator rotor forging. The alloys were treated by conventional techniques to the strength levels consistent with the normal requirements for the particular alloy and product form. Table 1 gives the composition and room temperature tensile properties of the alloys under investigation.
was supplied
EXPERIMENTAL PROCEDURE All fracture toughness tests under both monotonic and cyclic loading conditions were conducted at room temperature with 1 in. thick Wedge-Opening-Loading (WOL) type toughness specimens of the 1T geometry illustrated in Fig. 1. The aluminum specimens were modified slightly (5 per cent deep side notching) in an attempt to minimize the anisotropic crack behavior commonly encountered in these alloys. The conventional direct tension tests were conducted at a head motion rate of O-040 in./min on a universal hydraulic test machine. The load-displacement characteristics of the specimen under test were measured with a clip gage calibrated to 40,uin. of strain per 1 ml of displacement and recorded on an autographic X-Y recorder. In addition. crack behavior during the application of load was monitored with a IO megahertz (MHz) ultrasonic transducer used in conjunction with conventional ultrasonic maw-detention instrumentation[ 1 I]. A sensitivity level capable of detecting crack growth on the other of 0.005 in. was used. The combination of displacement gage and ultrasonic instrumentation provided a monitoring technique capable of distinguishing between crack growth and general yiefding at the crack tip prior to rapid failure. The IT type WOL specimens used to establish fracture behavior under cyclic loading conditions were side notched (45” included angle, 0.010 in. root radius) to a depth of 0.050 in. on each side prior to fatigue cracking in order to ensure crack propagation perpendicular to the loading direction. All cyclic testing was conducted on a constant-load universal fatigue machine under sinusoidal loading conditions (0 to maximum load) at a frequency of 1800 c/min. The maximum alternating load was held constant throughout each individual test resulting in a AK equivalent to K,,,. An ultrasonic flaw-detection technique involving the use of a 10 MHz, 3/g in.-diameter ceramic transducer was used to measure and record the extent of fatigue crack propagation encountered[ 111. As a result, a continuous record of crack length versus number of elapsed cycles was available for evaluation. EXPERIMENTAL RESULTS Figures 2-5 show typical room temperature load-displacement records obtained for each alloy as a result of conventional rising load fracture toughness testing. The accompanying ‘ultrasonic’ crack behavior records indicated that crack growth occurred prior to specimen failure in the HP 9-4-25 alloy steel and in both aluminum alloys !7079-T6 and 545GH321). No crack growth was observed in the Ni-MO-V alloy steel and the test specimens exhibited a considerable amount of gross plastic yielding prior to failure. Table 2 presents the value of the stress intensity factor associated with deviation from linearity (D), initiation of crack growth (I), and specimen failure (U) as determined from the interpretation of the load displacement and crack growth records. The expressions and method involved in the determination of the stress intensity factors are readily available in the Iiterature[ 12,131. In each case, the K values were based on the initial fatigue crack length measured on the fractured specimens.
Ni-MO-V Steel
5456H321 Aluminum
HP9-4-25 Steel
7079-T6 Aluminum
Alloy
0.29
Mn
0.50
0.14
C
0.23
0.009
P
0.08
Cu
Fe
Si
P
Mn
0.008
Si
0.010
S
Mg
Cr
8.41
Ni
0.10 0.25
Cr
3.4
Ni
0.08
0.21
Si
0.01
S
3.8 4.8
Zn
5.17
0.29 0.37
Mg
0.68
0.10 0.30
Mn
0.008
0.3
Si
0.33
Mn
0.4
Fe
0.26
c
0.4 0.8
Cu
0.08
Cr 0.30
MO
0.03
0.03
0.48
MO
0.07
V
0.07
V
(nominal)
Ti
0.40
Cr
0.10
Ti
Zn
Composition (wt. %)
3.9
Co
84
37
176
65
0.2% Yield strength (ksi)
Table I. Composition and tensile properties of test materials
102
52
186
76
Ultimate strength (ksi)
53
40
64
27
Red. in area (%)
21
12
14
II
Elong. (%)
Subcritical crack growth
389
0.700 Dia.
0.625-18 Thd CL3 x.625DP
Fig. 1. Wedge opening loading type fracture toughness specimen (IT Geometry).
The results of the cyclic loading portion of this investigation are summarized graphicatly in Figs. 6-9. Note that the log crack growth rate-log stress intensity relationships for the 7079-T6 aluminum and the Ni-MO-V alloy steel are essentially linear (slope n = 3) throughout the stress intensity range represented (Figs. 6 and 9). However, the HP 9-4-25 steel and the 54%H321 aluminum ahoy exhibit a distinct slope transition to an accelerated growth rate (Figs. 7 and 8). The stress intensity level at which the accelerated crack growth begins is presented in Table 2.
u
DI PIU -
Deviation Cmck initiation Pop in Uttimote lood
Specimen identiiicotim Testing temperature Deviation bod (lb) Pop in lood (lb) Utimo* load (lb)
Fig.
2.
Typical room temperature
load-displacement
curve for 70’79-T6 aium~num.
390
W. G. CLARK.
JR.
D-D&ah I - Crack initiath PI- Pap in U - Ultimation
Specimen idsntifiiatian Bsting &mpemhn Deuialtanbad(lb) Pap-inload Ulttmation (lb)
HP3T-8 t5 75°F 21*0 =4DO 2V50
Dispkxement
Fig. 3. Typical
room temperature
load-displacement
curve for HP 9-4-25 alloy steel
In order to establish the state of stress associated with the various portions of this investigation, the plane strain measurement capacity of the IT WOL toughness specimen with respect to the various alloys was determined. According to the latest ASTM tentative recommendations for plane strain toughness testing of high-strength alloys, the maximum plane strain stress intensity factor which can be determined with
D-Deviation I- Cmck irhttion U-Ultimateload
I
:: 0
I/
/
%d7lSl, &"tifiKOtiW, Spacinartypa
lbsting teqmmtm Dwiath load hp in loodUb Ultlmok load ( lb)
5456L- 83 wOL0-f) 75T 3950 7700
V Displacement
Fig. 4. Typical
room temperature
load-displacement
curve for 5456-H32
I aluminum.
Subcriti~l
crack growth
391
D - Deviitioo U - Ultimate land
Specimen id~ti~~ation Testing tempemtum Deviation load (lb) f%p-in lood (lb) Ultimate iood (lb)
Fig. 5. Typical
room temperature
looo~
800-
I
28,900
load-displacement
1,
curve for Ni-Mo-V
,,,I
1
steel.
i
7079T6 Al~min~rn
600-
0.2% YieldStrength= 65ksi TestTemp. = 75V
400.
TestFrequency = l&N cpm Max. Cyclicbad 0 -14Lw '-2coo# n -3oMM l -4@w
zw-
BDE- 8105 75’F 12,900
f-
I
-1 6 -4-
2-
I1
2,
/
3
I ,////I
45678910 StressInfenSity Factor,KI,
Fig. 6. Crack
growth
rate as a function
/
/
20
30
ksi&
of stress intensity
: 40 30"
for 7079-T6 aluminum.
392
W. G. CLAKK.JK.
.I
1 10
l/I
!
20 40 60 80100 StressIntensityFactor,KI, ksifi
2w1
Fig. 7. Crack growth rate as a function of stress intensity for HP 9-4-25 steel.
the 1T WOL specimen is approximately equivalent to 0.63 times the 0.2 per cent offset yield strength of the material under test[5]. This value is presented in Table 2 for the various materials. DISCUSSION Examination of the stress intensity data presented in Table 2 for the HP 9-4-25 alloy steel indicates a definite correlation between the stress intensity factor associated with monotonic loading crack initiation (107 ksi (in.)1’2) and that corresponding to the cyclic crack growth slope transition (100 ksi (in.)‘12). A similar correlation is apparent for the 7079-T6 aluminum alloy (Fig. 6), although the narrow stress intensity range, over which subcritical crack growth initiates and final failure occurs, makes it difficult to establish a distinct slope transition. Based upon the limited amount of crack growth data available at a stress intensity factor above 30 ksi (in.) 1/Zit appears that a transition does exist. Note the absence of a cyclic crack growth slope transition for the Ni-MO-V alloy steel which also failed to exhibit subcritical crack growth under monotonic loading conditions (Fig. 9). From these observations, it appears that the occurrence of subcritical crack growth does, in fact, influence the cyclic crack growth rate of some structural alloys.
397
Subcriticalcrackgrowth
54%H321Aluminum 0.2%Yield Strength = 37 ksl Test Temp.= 75Y Test Frequency= Bf~Ocpm Max.Cyclic Load . - 15w P -2cw 0 -4iw 0 -5m v -55cw
v I “= 5.2
1
I_-, 50 60 Fig. 8. Crack growth rate as a function of stress intensity for 5456-H321 aluminum (longitudinal orientation).
The data presented in Table 2 for the 54.56H32 1 aluminum alloy is not consistent with the behavior noted above. The stress intensity factor associated with the initiation of subcritical crack growth (30 ksi (in.)z’2) is considerably higher than that corresponding to the inflection on the cyclic crack growth curve (23 ksi (in.)1’2). Note, however. that the stress intensity factor at which accelerated cyclic crack growth is observed for the 5456-H321 aluminum alloy corresponds to the maximum plane strain measurement capacity of the iT WOL specimen (23 ksi (in.)““). A similar observation is apparent for the case of the HP 9-4-25 steel. From these observations, one might logically conclude that the slope transition is not the result of the influence of subcritical crack growth but rather is due to a change in the state of stress from predominantly plane strain conditions to non-plane strain conditions. If this were the case, however, one would expect some type of inflection in the cyclic crack growth rate curve for the Ni-MO-V alloy corresponding to the stress intensity at which deviation from linearity (plastic yielding) was observed on the load displacement curve (68 ksi (in.Y?. No such injection is apparent. In addition, the absence of a distinct discontinuity in the slope of the log dulcW vs. log AK curve corresponding to the fracture mode transition, has been noted for other structural alloys[l4, 151. Also, a considerable amount of published crack growth rate data indicates that the cyclic crack growth rate appears independent of specimen thickness[7,8, 161. The reported data cover a range
394
NiMoV Steel 9.2% Yield Strength = 84.5 ksi Test Temp. = 759 Test Frequency = 1800 cpm Max. Cyclic Load l -4im# c - 5ca.x l -6DW 4 -7m D -8lm!# 0 - 92006 a-99oM
-
Fig. 9. Crack
growth
20 40 60 85 1w Stress Intensity Factor, K,, ksi,K
rate as a function
of stress
intensity
-7
for Ni-MO-V
steel
of thicknesses involving both plane strain and non-plane strain conditions and no significant difference in the crack growth rate is apparent. A limited amount of cyclic crack growth data established on 2 in.-thick WOL specimens (plane strain measurement capacity 0% cr&t prepared from a similar heat of 5456-H32 1 aluminum indicates a slope transition corresponding to that noted for the 1 in.-thick specimens. Therefore. the accelerated cyclic crack growth behavior noted in this investigation cannot be attributed directly to a change in the state of stress. In view of the fact that the cyclic crack growth rate appears independent of specimen thickness, it is reasonable to assume that the free surface boundary stresses generally associated with fracture toughness testing are relieved to some extent as a result of repeated loading conditions. This assumption leads to the conclusion that the subcritical crack growth associated with the inherent plane strain fracture behavior of a material may constitute a primary effect upon the cyclic crack growth data. Further examination of the stress intensity data summarized in Table 2 indicates that the tougher the material, the greater the range between the K value at deviation from linearity (D) and that at the initiation of crack growth (I). This behavior is to be expected since the amount of plastic yielding which occurs prior to significant crack extension is proportional to the relative toughness of the material. Consideration of the recommended plane strain measurement capacity indicates
7079-T6 Aluminum HP 9-4-25 Steel 5456-H32 I Aluminum Ni-MO-V Steel
Material 30 105 23 68
(in.)“‘)
At dev.
. (ksi
Table
32 107 30 None
(ksi (in.)?
crack growth
of subcritical
Af I-initiation
2. Summary
35 130 46 175
(ksi (in.)l ‘)
load
At ultimate
of stress intensity
capacity
41 110 23 53
(ksi (in.)’ “)
specimen
At recommended
(K,) data
of
31 100 24 None
(ksi (in.)?
crack growth rate
cyclic
At initiation accelerated
396
W. G. CLARK,
JR.
that the initiation of crack extension under sustained loading conditions falls within the capacity limits for the 7079-T6 aluminum and the HP 9-4-25 alloy steel. Since the amount of stable crack extension, which occurs prior to plane strain failure is relatively small (assumed to be equivalent to one plastic zone-O.020 in. for HP 9-4-25 steel and 0.010 in. for the 7079-T6 aluminum), the stress intensity factor associated with the initiation of crack growth can be considered a realistic value of the actual plane strain critical stress intensity factor (K,,). Note that the K value associated with the ultimate load is considerably higher and cannot be used as a satisfactory K,, design parameter. Since crack extension in the 5456-H321 aluminum alloy occurred above the measurement capacity, it is possible that the free surface boundary stresses resisted the initiation of crack growth at the specimen center; thus, in this case, it would be erroneous to conclude that the critical stress intensity factor corresponds to the initiation of crack growth. Fracture toughness tests must be conducted on thicker test specimens to establish a realistic K,, value for the 5456-H32 1 aluminum. In view of the gross plastic yielding observed in the Ni-MO-V steel, it is obvious that a much larger test specimen is required to establish a value K,,. Many factors aside from the stress intensity level at the crack tip are known to affect the propagation characteristics noted for a given structural alloy[2]. Prominent among these factors are loading rate and environmental effects. The loading rate associated with the fatigue portion of this investigation was considerably higher than that used in the rising load tests (approximately lo4 to 1); therefore, it is reasonable to suspect that the subcritical crack growth observed in the HP 9-4-25 alloy would occur at a lower K level under fatigue conditions since the increased loading rate tends to decrease the measured K,, value. In addition, possible corrosion effects are aggravated by cyclic loading; thus, it is not surprising that subcritical crack extension in the 5456-H321 aluminum alloy was noted at a lower K level under repeated loading conditions. One can easily visualize the brittle protective oxide coating at the crack tip being progressively destroyed during the cyclic loading. The effect of monotonic loading material characteristics upon fatigue crack growth behavior has been demonstrated by Watts and Burns[l7]. Their data clearly indicate that the creep properties of Perspex (Plexiglas) observed under static loading conditions contribute to an accelerated fatigue crack growth rate at a K level corresponding to the initiation of creep. Macroscopically, the fatigue properties of Perspex are considered very similar to those found in metals and these data tend to substantiate the observations involved in this investigation. In order to reveal the nature and appearance of subcritical crack growth, several 5456-H32 1 aluminum specimens were subjected to tension loads above that at which crack extension occurred, unloaded, and then subjected to cyclic loading. The cyclic loading served to induce fatigue striations which clearly marked the extent of prioi crack growth. Figure 10 shows the resulting fracture appearance. Note the rough texture and bowed-out crack front associated with the subcritical crack growth. This observation is consistent with the appearance of crack growth behavior reported in a variety of structural alloys [ 1, 181. Figure 11 presents the results of an electron microscopic eXaminatiOn Of the fracture behavior Observed in aluminum alloy 5456-H32 1. The typical fracture appearances associated with fatigue and subcritical crack growth are shown in Figs. 1 l(a) and 1 I(b). respectively. No variation in the fracture appearance associated with the subcritical crack growth and fast fracture was apparent. Figure 1 I(C) illustrates the
flat
r
Additional
fatigue cracking
-+
Subcritical
crack growth
-
Fig. 10. Fracture
appearance
fracture
of a 5456-H321 aluminum WOL specimen followed by cyclic loading to failure.
lniiiol fatigue cmck
subjected
to tension loading
[Facing page 3961
Mag. 8400 Y A Fractograph Illustrating Typical Low Stress Intensity Fatigue
Mag. 2520 ” B Fractograph of Subcritical Crack Growth
Mag. 1470
c Fractograph Fig. 11. Fractographic
at Initiation of Accelerated Crack Growth appearance
of aluminum alloy 5456H321.
Subcriticalcrack growth
397
typical fracture appearance at the crack length corresponding to the initiation of the accelerated fatigue crack growth rate. Note the occurrence of both fatigue crack growth and ductile fracture mechanisms. From this observation, it is apparent that the fracture behavior is a combination of two distinct mechanisms. However, Hertzberg and Paris have made a similar observation corresponding to a transition from plane strain to nonplane strain fatigue crack propagation and no abrupt change in the slope of the da/dN vs. AK relationship was observed[l4]. Therefore, it does not appear that a distinction can be made between the fracture appearance associated with subcritical crack growth, and non-plane strain fatigue failure. Aside from the observations associated directly with the major objective of this investigation, the data reveal several other interesting conclusions. Consideration of the fatigue crack growth characteristics of structural alloys in terms of more conventional engineering parameters rather than fracture mechanics indicates that a change in the net section stress from the elastic to the plastic region (below and above the established yield strength) may be the cause of the marked change in the fatigue crack growth rate[9]. The ratio of net section stress (a,) to yield strength (a,,,) at the point of accelerated crack growth indicates that the stresses were well below the established yield strength of the respective materials-the (a,/~& ratio was 0.44, 0.57, and 0.59 for the 7079-T6 aluminum, HP 9-4-25 steel, and the 5456-H32 1 aluminum alloys, respectively. Thus, it is apparent that the net section stresses need not be above the yield strength to induce a change in the fatigue crack growth rate. Another pertinent observation associated with this investigation concerns the wide range of maximum alternating loads used to develop the cyclic crack growth data. Note that no significant effect of alternating load on the cyclic crack growth ratestress intensity relationship is apparent for the alloys investigated.
CONCLUSIONS crack growth which occurs under rising load conditions represents inherent fracture behavior which in turn affects the fatigue crack growth properties; in particular, the fatigue crack growth rate. (2) The change in slope (to an accelerated crack growth rate) of the log daldN vs. log AK relationship can be the result of the influence of subcritical crack growth on fatigue behavior. (3) The stress intensity factor associated with the initiation of significant crack growth under predominantly plane strain conditions may be used to provide a realistic estimate of the material K,,. (4) A change in the state of stress from plane strain to non-plane strain conditions does not appear to affect the cyclic crack growth characteristics of structural alloys. (5) The maximum alternating cyclic load does not appear to affect the fatigue crack growth rate-stress intensity relationship under sinusoidal loading conditions. (I ) Subcritical
Ac,knol(,lrd~etnents -The author is indebted to his many colleagues who contributed 1~1the preparation of this paper. In particular the author would like to thank Mr. E. T. Wessel for his helpful suggestions and review of the work. Mr. L. J. Ceschini for development of the fatigue cracking techniques, &knd Mr. R. C. Hates for the interpretation of the electron photomicrographs.
REFERENCES [)I E. A. Steigerwald
and G. L. Hanna, Initiation of slow crack propagation in high-strength materials, P~oc. Am. Si>c. Test. Muter. 62 (I 962). 121 G. G. Hancock and H. H. Johnson, Subcritical crack growth in AM350 stcel.Mtrr<>,. R~~,~,Slnnd. 6.
398
W. G. CLARK. JR.
[31 The slow growth and rapid propagation of cracks, Second Report of a Special ASTM Committee, Mater. Res. Stand. 1,389-393 (1961). [41 J. M. KratTt, A. M. Sullivan, R. W. Boyle, Effect of dimensions on fast fracture instability of notched sheets. Proc. Crack Propagation Symp. Vol. I, pp. 8-28. College of Aeronautics, Cranfield. England (1961). 151 W. F. Brown and J. E. Srawley, Current status of plane strain crack toughness te\ting. A.S.T.M. .~pe(. Tech. Publ. No. 4 10
(1967).
[61 P. C. Paris, Fatigue crack growth. Presented at the Second Annual Workshop in FI-actare Mechanics. Denver Research Institute (1965). [71 A. J. Brothers andS. Yukawa, Fatique crack propagation in low-alloy heat-treated steels, Presented at the Metals Engng. Conf. Cleveland, Ohio: A.S.M.E. Paper No. 66-Met-2 (1966). PI B. Cotterell, An interpretation of the mechanics of crack growth by fatigue. J. has. Engng 230-236 (1965).
[91 H. W. Liu, Fatigue crack propagation
and the stresses
and strains in the vicinity of a crack. rll>p/
Mater. Res. 229 (I 964). [lOI C. M. Carman and J. M. Katlin, Low cyclic fatigue crack propagation steels. A.S.M.E. Paper No. 66-Met-3 (1966).
characteristics
of high-strength
1111 W. G. Clark, Jr., Ultrasonic detection of fracture initiation and extension in the W.O.L. type frdctllrc toughness specimen. Mater. Eualuution. 2.5. 185- I90 (1967). [I21 W. K. Wilson, Westinghouse Research Laboratories, unpublished data. [I31 E. T. Wessel, W. G. Clark and W. K. Wilson, Engineering methods for the design and selection of materials against fracture U.S. Army Tank and Aatomotiue Center Rep., Contr. No. DA-30-069. AMC602(T) (1966). [I41 R. W. Hertzberg and P. C. Paris, Application of electron fractography and fracture mechanics to fatigue crack propagation, Proc. 1st Int. Conf. on Fracture Vol. 1.p. 466 (I 965). 1151 C. M. Hudson, Investigation of fatigue crack growth in Ti-8AI-lMo-1V (duplex-annealed) specimens having various widths. NASA Tech. Note No. D-3879 (I 967). 1161 N. E. Frost and K. Denton, Effect of sheet thickness on the rate of growth of fatigue cracks in mild steel. J. Mech. Engng Sci. 3 ( I96 I ). Proc. 22nd Au. /I71 N. H. Watts and D. J. Burns, Fatigue crack propagation in polymethylmethacrylate, Tech. Conf S.P.E. (Session XIV-2), Montreal (1966). 1181 W. F. Brown, Jr., Some observations of pop-in determination of K7,. Presented at the AH. .Mri,i A.S.M.E.
(1963). (Received
25 February
1967)
Resume- Le comportement de la croissance sous-critique de rupture remarque pendant les essais de duretc a la rupture de plusieurs alhages structurels, a ete etudii et compare aux caracteristiques de la propagation de la rupture observtes dans des conditions de charge de fatigue. Le facteur d’intensite de tension, qui d&it les conditions de tension necessaires pour initier une croissance sous-critique de rupture a charge monotonique, correspondait a celui associe au taux de croissance de rupture avec fatigue acceleree. A partir de ces observations, il apparait que des irregularites dans les caracteristiques de croissance de rupture par la fatigue a des niveaux de K superieurs, peuvent &tre le resultat direct de la croissance sous-critique de rupture survenant en conjonction avec le dommage cause la fatigue. Zusammenfassung- Das Risswachstum im subkritischen Bereich, das bei der Bruckzahigkeitspriifung verschiedener Baulegierungen beobachtet worden war, wurde untersucht und mit den unter ermiidungsbedingungen festgestellten Fissfortpflanzungs eigenschaften verglichen. Es wurde gefunden, dass de7 Spannungsintensitatsfaktor, der die zum Einleiten subkritischen Risswachstums bei monotoner Belastung erforderlichen Spannungsbedingungen beschreibt, dem mit einem beschleunigten ribivachstum in Ermiidungs bruch verbundenen Faktor entspricht. Diese Beobachtungen deuten darauf hin, dass Unregelmlssigkeiten in den ribivachstums eigenschaften im Ermiidungs bruch bei hoheren K-werten das unmittelbare Ergebnis von subkritischem Risswachstum im Zusammeiihang mit Ermtidungschaden sein konnten.