Influence of environment on fatigue crack growth in the threshold region

Acta Metalluroica Vol. 29, pp. 21 to 32 Pergamon Press Ltd 198t. Printed in Great Britain

0001-6160/81/0101-0021$02.00/0

I N F L U E N C E OF E N V I R O N M E N T ON FATIGUE CRACK G R O W T H IN T H E T H R E S H O L D R E G I O N S. S T A N Z L I t and E. T S C H E G G 2 t tUniversity, Institute of Solid State Physics, A-1090, Boltzmanng. 5. Austria 2Technical University, Institute of Applied Physics, A-1040 Vienna, Karlsplatz 13, Austria (Received 10 March 1980; in revised form 22 July 1980)

Abstract--Low fatigue crack growth rates (down to 4 x 10-t, m/cycle) were produced using a high frequency 20 kHz ultrasonic fatigue testing machine. The influence of non corrosive (silicone oil) and corrosive (3.59~ sodium chloride solution) environments was compared. Down to crack propagation rates of some 10- to m/cycle which corresponds to a crack growth rate of one lattice space per cycle no difference of crack growth rates was found. However, below this rate there seems to exist for non corrosive environments a threshold cyclic stress intensity, below which crack growth becomes diminishingly small, whereas no threshold was found for the corrosive environment. In the first case crack propagation is controlled by plastic deformation processes, in the second case these processes are markedly restricted. For this region, a transition in fracture mode from ductile transcrystalline to intergranular cracking was found.

Rtsum6--De faiblcs vitesse de fissuration (jusqu'/t 4 x 10-14 m/cycle) ont 6t~ produites fi I'aide d'une machine de fatigue ultrasonique ~ haute frequence, 20 kHz. L'influence d'environements non corrosif (huile silicone) et corrososif (solution de chlorure de sodium fi 3,50/0) fur compar6e. Aucune difference ne rut not6e jusqu'~ des vitesses de fissuration de 10- to m/cycle, ce qui correspond A un taux d'avancement de la figure d'une espacement interatonique par cycle. Cependant, en dessous de cette vitesse et darts le cas du milieu non corrosif, il semble qu'il existe un scull de facteur d'intensit6 de contrainte cyclique en dessous duquel la vitesse de fissuration devient n6gligeable. Par contre, aucun seuil ne rut observ~ darts I'environement corrosif. Darts le premier cas, la propagation de la fissure est control6e par les m6chanismes de d6formation plastique alors que darts le second cas, la plasticit6 est notablement r6duite. Une transition du mode de rupture, de ductile transcrystalline/t intergranulaire, rut observ6e darts le cas du milieu corrosif.

Ztr~amrnenfmmng--Sehr niedere ErmiidungsriBwachstumsgeschwindigkeiten (bis 4 x 10-t4m/Las twechsel) wurden mit Hilfe einer 20 kHz Ultraschall Resonanzmaschine erzeugt. Der EinfluB yon nichtkorrosivem Medium (Silicontil), beziehungweise korrosivem Medium (3,55o Salzwasser) auf die RiBausbreitungsgeschwindigkeit wurde untersucht. Bis hinunter zu Ril3wachstumsgeschwindigkeiten yon einigen 10- xo m/Lastwechsel war kein Unterschied auf die Wachstumsrate festzustellen. Unterhalb dieses Wertes, der einem RiBwachstum yon einem Gitterabstand pro Lastwechsel entspricht, scheint for nicht korrosive Umgebung ein Schwellwert f'tir den Spannungsintensitiitsbereich, unterhalb welchem die RiBausbreitungsgeschwindigkeit verschwindend klein wird, zu existieren, nicht abet f'tir korrosive Umgebungen. Im ersten Fall wird die RiBausbreitung durch plastische V.erfor.mungsprozesse bestimmt, wohingegen dies¢ im zweiten Fall stark eingeschr~inkt sind. Auch wurde eine Anderung des Bruchmodus yon duktil transkristallinem zu interkristallinem Brechen beim Unterschreiten yon einigen 10-1o m/Lastwechseln in korrosiver Umgebung festgestellt, die auf das Zusammenwirken mehrerer Prozesse zuriickzuftihren sein diirfte.

1. I N T R O D U C T I O N

influence on the fatigue crack threshold of low alloy steel, using air, distilled water, and dry hydrogen gas, and testing frequencies of 120 to 180 Hz. Results given by Paris et al. [8] show, that for growth rates less than 2.5 x 10-s m/cycle distilled water crack growth rates fall slightly below those of room air for pressure vessel steel and forged steel, and that the threshold value of AK in distilled water is about 10% higher than that established in room air. A small difference in AKth between air and acidic chloride solution is reported by Moskowitz and Pell o u x [ 9 ] for a high strength steel. Mautz and Weiss [10] find no effects of humid argon and room air as compared to dry argon environment on the

A considerable amount of research work has been done on the influence of corrosive environments on fatigue crack growth behavior. The studies by Pelloux et al. [I], Speidel et al. [2, 3"I, Kraft and Cullen [4"I, Hudson and Sewald[5"l and recently Verkin and Greenberg [6] summarize the main results. The studies in this field show, however, that the results, especially in the range of the threshold stress intensity are scarce and not conclusive, e.g. Bucci et aL [7] find, that there is no essential environmental t O n leave at: Mechanical Engineering Department, MIT, Cambridge, MA 02139, U.S.A. during 1980-1981. 21

22

STANZL and TSCHEGG: FATIGUE CRACK GROWTH IN THE THRESHOLD REGION

threshold of an ultrahigh strength steel and a high strength aluminium alloy, whereas environmental effects were noted in the near threshold region. Results for aluminium-, magnesium-, and titaniumalloys are compiled by Speidel et al. [2]. No environmental influence is detected for these alloys in the region of low AK ranges. In addition, studies on a Ti-6AI-4V alloy of various heat treatments reveal that there is little or no environmental influence of air or dry argon on AKth [l 1]. On the contrary, according to Irving and Beevers[12] and Ebara et al.[13] in Ti-6A1-4V alloys the crack growth rates are substantially higher in air than in vacuum and the threshold level is markedly reduced. In the case of aluminium alloys Feeney et al. [14] find, that the environmental influence of wet air, distilled water, and 3.5~o sodium chloride solution is more pronounced at lower levels of cyclic stress intensity. Pook [l 5] even recognizes a dramatic effect on the threshold of mild steel in 3% NaC1 solution. The threshold is much less than one lattice spacing per cycle. Various attempts at interpreting the mechanisms of environmental influence discuss the chemical reactions at the crack tip [3, 14-18], diffusion of gas from crack surfaces to the interior of the metal [19], especially hydrogen embrittlement [3, 9,19-21], the transport of hydrogen by dislocations [9, 22-25], and the absorption of oxygen at the freshly generated crack tip [10, 20, 26, 27]. Also crack tip blunting [28], formation of clean metal surfaces [29] and wedging effects [30,31] as well as oxide-film rupture processes [18, 32] may account for environmentally influenced crack growth behavior. The environmental influence becomes more effective, when different mean stresses are used. To explain this phenomenon it was assumed, that e.g. corrosive products would change the amount of crack closure, thus also changing the local R-value or the residual compressive loads normal to the fracture surface, respectively [33-39]. Electron scanning microscopy studies of the fracture surfaces [4, 9, 20, 27, 36, 40-45] as well as studies on plastic zone sizes and plastic deformation [39, 42, 46, 47] are most helpful tools in studying the kinetics of environmentally assisted crack propagation behavior. It has been supposed that even in the case of corroding environments "a stress intensity (AK~cD exists, below which no corrosion fatigue crack growth can be observed within a reasonable time" [2]. To investigate this AK range, where crack growth rates become extremely low, long testing times are needed. For this reason it was considered advantageous to use a time saving fatigue testing method. This method is carried through with the help of ultrasonic equipment which has already proved to be a useful tool of investigation [48-50]. First 2.3 Hertz results of Speidel show that there is no frequency

effect on crack growth rates between 5.10 -~° and 10 - 6 m/cycle detectable, when tests are performed in non corrosive environments [in 49]. In corrosive environments, however, a different behaviour at low and high frequency testing might be expected as the environmental sensitivity decreases slightly with increasing test frequency [14]. This problem will be treated in a forthcoming study. In this paper crack growth rates within the threshold region are compared for corrosive and non corrosive environmental fatigue stressing, at a constant frequency level of 20 kHz. Our research concentrates on the question whether there exists a threshold stress intensity range for corrosive and non corrosive environment. 2. EXPERIMENTAL PROCEDURES The fatigue testing equipment was an ultrasonic resonance system which produces push-pull loads (R = - 1 ) w i t h a frequency of 20kHz. The samples were band shaped with a length equal to the half wave length 2/2 of ultrasound (iron and steel 2/2 = 125mm, copper 2/2 = 100mm). Maximum strain is attained half way along the length of the samples and is calculated from displacement measurements (details see [49]). The sample width was between 4 and 10 mm and the thickness 1 to 5 ram. The thickness chosen guaranteed plain strain conditions for all used cyclic stress intensity levels. The samples were single edge notched in the place of maximum strain with wedge shaped notches of radius 0.5 mm and a depth of about 1 ram. In addition, a small fatigue crack was introduced before annealing and testing in order to eliminate notch effects. Three materials have been used: 1. Mild steel with a composition (wt.?/o) as follows: 0.036 C, 0.01 Si, 0.08 Mn, 0.012 P, 0.008 S, 0.002 A1, 0.005 N, 0.0015 O. The yield stress was 275 MN m -a, the tensile stress 335 MN m - 2 and the tensile strain was 35~o. The samples were recrystallized at 700°C for l h and furnace cooled, thus giving grain sizes of about 15/~m. 2. Austenitic 18.8 Cr-Ni steel (AISI Type 304) with the following composition (wt.~o): 0.06 C, 18.5 Cr, 9.5 Ni. The yield stress was 230 MN m-2, tensile stress about 700 MN m - 2 and tensile strain about 55°4. The steel was annealed at 1050°C for 30rain and quenched in water of 10°C, thus producing austenite with small amounts of ferrite. 3. Copper of 99.9~o. Yield stress was 69 MN m-2, tensile stress 240MN m -2 and tensile strain about 45~o. It was annealed at 600°C for 90 min and furnace cooled. Grain size was about 20/~m. All samples were polished mechanically and electrolytically in order to produce optimal conditions for the identification of crack paths. To study environmental effects two liquids have been used, inert silicone oil and 3.5To sodium chloride

STANZL and TSCHEGG:

FATIGUE CRACK GROWTH IN THE THRESHOLD REGION

solution. Silicone oil was used instead of vacuum though there is some question as to whether it is completely inert. Liquid cooling was necessary in order to avoid heating effects due to damping which may become considerably large at the high frequency of 20kHz. Experiments are prepared to compare crack growth behavior in vacuum and silicone oil. Crack propagation was observed by a video attachment [50] which was capable of refining the events to 1/30th of a second. Observation of sample surfaces was based on a magnification about 100. The displacement amplitude was measured by a magnetic Coil and was used for cyclic stress intensity calculation. It was synchronously introduced into the recording by an a.c./d.c, electronic converter. In the same way the number of cycles was introduced. Thus it was possible to correlate the number of cycles to the crack length and cyclic stress intensity at every moment of stressing. For calculating cyclic stress intensities AK only the tensile part of the push-pull stress was used, assuming that compressive loads do not contribute to crack growth. This means that the effect of not fully closed cracks at zero stress [51-53,38] is neglected. AK = Kmax was obtained as Kmax = o ' x / a ' Y . The term a is notch depth plus fatigue pre-crack length plus crack length. Y was calculated according to

literature [54] to account for the finite sample width. The utmost value of a was 0.4 of sample width. 3. E X P E R I M E N T A L R E S U L T S In order to study the threshold range extensively, three modes of loading were used as depicted in Fig. 1. In the first case (Fig. la) stress is kept constant during the whole fatigue test, AK growing with increasing crack length. It is necessary to introduce the crack with a rather low amplitude in order to be able to find AK,h. In the second type of test (Fig. l b) AK was raised first, but then lowered to an assumed K,h and beneath, then raised again, lowered, and so on. This type of experiment allowed to test whether the stress sequence influences the crack growth rate and the threshold stress intensity K,h. It was found that there was little sequence effect in the case of mild steel and copper. Results on steel 304 were not conclusive. The third type of test was done as indicated by Fig. lc. Stress was lowered during crack propagation in such a way, that AK remained constant for a certain period. Then AK was raised to a slightly higher level and kept constant at this level, then raised and kept constant again, and so on. After that AK was lowered, kept constant, lowered again, and so on. This is another experimental arrangement to

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Fig. 1. Cyclic stress intensity sequence applied for testing fatigue crack growth in the threshold region in corrosive and non corrosive environments (schematic).

24

STANZL and TSCHEGG:

FATIGUE CRACK GROWTH 1N THE THRESHOLD REGION

study the influence of stress intensity sequence on crack growth rate and threshold. Further experiments using computer control which allows for any modification of stress and stress intensity and sequence as well as of the number of cycles are on the way. With this equipment it is also possible to run random loading tests. As our first experiments did not show a pronounced sequence effect, the results are summarized in one curve for each tested material. However, the scattering of single points of the Aa/AN =f(Kmax) curves in Figs 2, 3, and 4 may be attributed partly to a sequence influence, especially in the case of steel 304. The scattering of the data points is furthermore caused by the method of evaluation. The data points were obtained in this work from single crack growth measurements and not, as it is done usually, from the gradient of a curve (crack length vs number of cycles) at a constant loading stress. Crack growth rates were obtained from crack growth increments of 100/~m. Only in the range of extremely slow crack growth rates (10-13 to 4 x 10-~4 m/cycle) even shorter increments were used to spare time. We checked several times whether the propagation of these short cracks is continued within the same stress intensity range over longer distances. As this proved to be the case, the results of shorter crack increments were included in Figs 2-4. The accuracy of measurements was about 10/~m, therefore the

scattering in the very low crack growth range is greater than in the higher one. Points without arrows refer to propagated cracks, whereas points with arrows indicate non propagated cracks. The corresponding crack growth rates of these points with arrows were obtained by dividing the least resolvable length of 10/am by the applied number of cycles. For this reason a point with arrows indicates that a crack did not grow faster than with the indicated growth rate and, that this crack eventually grew slower or not at all. Furthermore the extremely low crack growth rates below 10-~0 m/cycle were obtained mostly by addition of crack growth rates Aal/ANI greater than 10-~°m/cycle and crack growth arrests (Aa = 0) during a certain number of cycles, N2. The total crack growth rate Aa3/AN3 was approximated by Aaa/AN 3 = (Aa~ + 0)/(AN~ + AN2). Especially in the case of oil environment the stops were very pronounced. When sodium chloride solution was used, cracks propagated much more uniformly. All results taken together show that scattering in Figs 2--4 is rather large for the whole crack growth range studied. This is partly due to the fact that we concentrated on short crack growth distances of 50-100/am. This limitation, however, made it possible to produce much more data. In Figs 2-4 the dependence of crack growth rates Aa/AN on the cyclic stress intensity, Kmax, i.e. the Aa/AN = f(K~,ax} curves for mild steel, steel 304, and

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STANZL and T S C H E G G :

F A T I G U E CRACK G R O W T H 1N THE T H R E S H O L D R E G I O N

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25

26

STANZI_ and TSCHEGG:

FATIGUE CRACK GROWTH IN THE THRESHOLD REGION

Fig. 5. Fracture surface of mild steel, typical for crack growth at 20 kHz stressing above 10- ~ m cy:clic in silicone oil. Ductile transcrystalline fatigue striations. copper are compared for silicone oil environment and 3.5°~o sodium chloride solution. These curves demonstrate two important results: 1. Down to crack growth rates of about 10-9m/cycle no corrosive enhancement of crack growth rate is obvious.

2. In

the non corrosive oil environment the f(K,,,ax) curves become almost vertical. This behavior indicates the existence of a threshold stress intensity range [50]. (Low impurit.v levels of the silicone oil--possibly hydrogen bearing compounds-may be the reason for eventual crack gro~th in the threshold regime.) In the corrosive sodium chloride Aa/AN

=

Fig. 6. Fracture surface of mild steel after crack growth above 10 - 9 m/cyclic in 3.5",, NaC1 solution. Ductile transcrystalline striations as in Fig. 5.

STANZL and TSCHEGG:

FATIGUE CRACK GROWTH IN THE THRESHOLD REGION

27

Fig. 7. Ductile transcrystalline fracture of copper after 20 kHz fatigue cracking typical for growth rate b e t w e e n 10 - 9 and 10-~3 m/cycle in silicone oil. solution environment the drop off in Aa/AN with decreasing Km~x is less steep. No "threshold' behavior is observed. In order to explain these results, scanning electron micrographs were taken from different areas of the fracture surface. These micrographs were compared to the crack growth rates, which had been measured at

this area before. For the range of "higher" crack growth rates, i.e. more than 10-9 m,'cycle, two typical micrographs are shown for mild steel. The environment was silicone oil for Fig. 5 and sodium chloride solution for Fig. 6. These two fracture surfaces look similar, both exhibiting ductile fatigue striations. The same was found for copper and steel 304. This result is consistent with the result that crack growth rates in

Fig. 8. Intercrystalline fracture of copper after 20 kHz fatigue cracking typical for growth rates between 10 - 9 and 10-13 m/cyclic in 3.5°o NaC1 solution.

28

STANZL and TSCHEGG:

FATIGUE CRACK GROWTH IN THE THRESHOLD REGION

Fig, 9. Intercrystalline fracture of steel 304 typical for 20kHz fatigue cracking with growth rates between 10 -9 and 10- 13 m/cycle in 3.5°0 NaC1 solution. the range above some 10- 1o m/cycle are independent of the environment. Micrographs from areas with crack propagation rates lower than about l0 -9 re:cycle differ considerably for corrosive and non corrosive environments. Figure 7 is typical of crack growth rates between 4 x 1 0 - ' ° and some 10- 1o m/cycle for copper tested in oil environment. This micrograph shows a ductile

transcrystalline fracture. Figure 8, which characterizes fracture in sodium chloride solution exhibits intercrystalline cracks for all crack growth rates between 4 x 10 -14 and 10-1°m/cycle. Fracture of steel 304 is partl) intercrystalline for sodium chloride solution stressing, but also contains transcrystalline parts (Fig. 9). The fractures found for mild steel are strikingly similar to those for copper,

Fig. 10. Intercrystalline fracture of mild steel typical for 20kHz fatigue cracking with growth rates between 10 -9 and 10-13 m/cycle in 3,5°o NaCI solution.

STANZL andTSCHEGG:

FATIGUE CRACK GROW]H IN THE THRESHOLD REGION

i.e. grain boundary cracks, when stressed with low amplitudes in sodium chloride solution (Fig. I0). The marked difference in fracture appearance for non corrosive and corrosive fatigue cracking, respectively in the crack growth range of 4 × 10 -1'* and some 10-io m/cycle may serve as an explanation for the different threshold behaviour. 4. DISCUSSION It was found that crack propagation is discontinuous in silicone oil and at the lowest stress intensity ranges. The almost vertical curves indicate the existence of a threshold stress intensity range [50]. This result contrasts with our finding that crack propagation is much more uniform in sodium chloride solution environment. No cyclic threshold stress intensity seems to exist for sodium chloride solution environment. Our finding that Aa, AN =f(gmax) curves for sodium chloride solution environment deviate from that for silicone oil environment below about 10 -9 m/cycle is consistent with a marked change in fracture morphology, i.e. change from ductile transcrystalline to grain boundary cracking This result was reinforced by X-ray microbeum and recrystallization measurements[55]. These measurements show that the plastic zone size at the crack tip is smaller, when crack propagation takes place at the same stress intensity range in sodium chloride solution instead of silicone oil. First the question arises, why a corrosive influence is found at crack growth rates below 10-9m/cycle only. It is thought that the time dependence of surface diffusion and formation of a monolayer gas-molecules [56] on a metal surface explain this effect. First calculations [57] proved that at the ultrasonic frequency of 21 kHz only crack growth rates lower than 10 -9 m/cycle approximately give way to times, which are long enough for reaction. The next question is, how to assess for the existence of extreme crack growth rates in corrosive environments, which are smaller than one lattice spacing per cycle. One possibility is that the crack tip material dissolves chemicall), the whole process being assisted by the ultrasonic vibration of the liquid carrying away the dissolved atoms. This dissolution process could explain crack growth rates lower than one lattice space per cycle, provided one assumes that the dissolution of one atom layer needs more than one cycle. On the other hand, the changed fracture appearance (grain boundary cracks) at very low crack growth rates makes it reasonable to assume a different deformation mechanism for corrosive fatigue cracking. This type of cracking is characterized by less amounts of deformation and smaller plastic zones. These were found to be confined to areas equal to or smaller than one grain size at the crack tip [55]. A very similar result was obtained by Frandsen and

29

Marcus [58], who pointed out tha~ the amount of intergranular fracture was maximum, when the monotonic plastic zone was equal to the grain size. In this case, the efficiency of transport of hydrogen to the grain boundaries would be very high, accounting for an increase in crack growth rate. A similar mechanism seems to hold for our case of ultrasonic stressing of mild steel, copper, and steel 304. It is only at higher amplitudes that the plastic zone exceeds one grain size. In this case the crack may propagate transcrystalline by the mechanism of (multiple) slip. These findings favor the following answer to the problem of the existence of a threshold stress intensity range and a threshold crack growth rate for fatigue crack growth. In the case of polycrystals, where a 'ductile" deformation mechanism is effective, i.e. a crack growth mechanism, which works exclusively by a [multiple) slip process, a threshold exists. This threshold is reached, when the cyclic stress intensity is low enough that slip processes are confined to one grain or to a one-grain-layer, respectively, in front of the crack tip. Crack propagation only is possible, when the cyclic stress intensity is high enough to expand slip across the next grain boundary. Dislocation cell structures, which are formed b) multiple slip and still more grain boundaries act as deformation barriers [59]. To surmount these a minimum cyclic stress intensity, i.e. the threshold stress intensity is needed. Also Suzuki and McEvily pointed to a correlation of reversed plastic zone size, grain size, and threshold stress intensity [42]. But no threshold seems to exist, when another crack propagation mechanism becomes effective. In our case, this mechanism is grain boundary cracking. This mechanism occurs when material becomes embrittled at the crack tip and'or the grain boundary bonding is weakened by corrosive attack and when the plastic zone [especially the reverse plastic zone) is confined to areas smaller than one grain. Another example for the non-existence of a threshold was found for the case of monocrystals [56]. Corrosive attack must be reinforced by some deformation mechanism in order to be able to promote crack propagation. This fact is supported by the obserxation, that many grain boundaries show fatigue striations and sometimes even slip lines. One example is shown in Fig. 1 l, which is the fracture surface of a copper sample. These slip lines also may give way to environmentally enhanced intrusion-extrusion formation [1,60] at the grain boundary surfaces. This mechanism eventually makes possible further fatigue crack propagation at a stress intensity level, which would not be sufficient to propagate the crack tip across a grain boundary by plastic deformation alone [59]. In order to get more information about the mechanism of crack growth, the experimental results of crack growth rate were compared with calculated crack tip opening displacements, CTOD. The crack

30

STANZL and T S C H E G G :

F A T I G U E C R A C K G R O W T H IN T H E T H R E S H O L D R E G I O N

Fig. 11. Grain boundary crack in copper after fatigue crack growth with a rate of 10- 13 m cyclic in 3.5",, NaCI solution.

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Fig. 12. Comparison of measured crack growth rates A a / A N in NaCI solution and crack tip opening displacements C T O D calculated from C T O D = I n (K,,~, El".

STANZL and TSCHEGG:

FATIGUE CRACK GROWTH IN THE THRESHOLD REGION

tip opening displacements were calculated from: C T O D = l~t(Kmax/E) 2 [2]. The results are shown in Fig. 12 and compared with the measured crack growth rates. It is interesting to note that crack growth rates, Aa/AN, and C T O D are in the same range of about 1 0 - 9 - 1 0 - ~ ° m at very low stress intensity ranges, when the C T O D are calculated according to 1/n(K .... /E) 2. CTOD results according to 1/n(K2ax/ar'E) would be about three orders higher than Aa/AN. The correspondence with the first equation seems to be accidental, especially since the exponent does not agree with the observed stress intensity dependence. Further investigations will be necessary to explain the mechanisms working on crack propagation at very low stress intensity ranges.

31

transcrystalline to intergranular fracture. Various mechanisms may account for environmentally assisted fatigue crack propagation within these very low stress intensity ranges. Reduction of plastic zone sizes, weakening of intercrystalline bonding by corrosive attack, enhanced intrusion, extrusion formation at grain boundaries, and rupture of oxide film at the crack tip may altogether be of significance for the processes described in this paper. Acknowledoements--The authors wish to thank Professor F. A. McClintoc and Professor R. O. Ritchie for helpful discussions. They thank Dr W. Schwarz, VEW Kapfenberg, Austria, for provision of steel specimens and the Fonds zur Frnderung der wissenschaftlichen Forschung in Osterreich for financial support. REFERENCES

5. C O N C L U D I N G

REMARKS

A corrosive influence on crack growth behavior is found only at crack growth rates below 10 - 9 m/cycle approximately when copper, iron, and steel 304 are stressed with 21 kHz ultrasound. As corrosion is a time dependent effect, it is expected that 21 kHz experiments cannot replace low frequency corrosion fatigue tests without further investigation. But the time-saving ultrasonic tests give some insight into possible crack growth mechanisms, especially in the threshold region. Ultrasonic tests also will be applicable to cases, where high frequency stressing occurs (wings of aeroplanes, parts of machines, e.g.). Furthermore strengthening effects due to the high strain rate of the 20 kHz experiments are expected to influence the fatigue cracking behavior. But former experiments[61-I surrendered no strengthening in mild steel at room temperature. Thus it seems that ultrasonic stressing may replace low frequency tests in this respect, though experiments on other materials [62] still are outstanding. Therefore the ultrasonic method is thought to complete low frequency tests on crack growth behavior especially in the threshold region as long as no corrosive effects are to be expected. 6. S U M M A R Y Fatigue crack propagation was performed with 20 kHz ultrasonic stressing with very low stress intensity ranges. It showed that a threshold stress intensity level seems to exist, below this value fatigue crack growth rates become diminishingly small, when polycrystalline mild steel, copper, and steel 304 samples are tested in non corrosive silicone-oil environment. When sodium chloride solution is used, crack propagation is observed also below this threshold. Furthermore, crack growth is more continuous in the sodium chloride solution environment than in non corrosive environment. Below this threshold stress intensity range the fracture mode of samples fatigued in corrosive environment, is completely changed from ductile

I. R. M. Pelloux, R. E. Stoltz and J. A. Moskowitz. Mater. Sci. Engng 25, 193 (1976).

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