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Effect of structural factors on corrosion crack resistance parameters
Materials Science and Engineering, A 130 (1990) 29-35
29
Effect of Structural Factors on Corrosion Crack Resistance Parameters YU. A. KRUPIN and 1. K. KISELEV 031, Moscow Institute of Steel and Alloys, Leninsky Prospekt 4, Moscow 117936 (U.S.S.R.) (Received September 21, 1989; in revised form April 26, 1990)
Abstract
The stress corrosion cracMng of flat specimens of high strength steel was studied. Data on the nucleation and kinetics of the growth of primary cracks are obtained by analysis of the cumulative peak amplitude of acoustic emission signals and changes in sample compliance. 1. Introduction Investigations of stress corrosion cracking (SCC) of high strength steel with a low temperature tempered martensite structure by standard methods leave some unresolved questions connected with structural peculiarities of the material. First, the kinetic diagrams of cracking (KDCs) describe the process ambiguously; this is connected with the different initial conditions of loading (i.e. the Km value at which the investigation of crack growth in precracked samples begins) [1]. Second, the correctness of definition of the threshold value of the stress intensity factor KISCC (below which no crack growth occurs) depends greatly on the choice of the time base of tests and the crack-growth-monitoring method. Third, the standard methods [2] record the crack growth on a macrolevel, when many structural elements are involved in the process. Fourth, it is impossible to study the nucleation processes of surface corrosion defects in precracked specimens. In tests for SCC susceptibility, flat samples loaded by constant displacement [3] are widely used; to perform a test, the sample is exposed to the corrosive medium and the time to failure or the time to the appearance of the first visible crack is measured. Localization of the contact of the corroding solution with the sample surface enables us not only to record acoustic emission (AE) signals without inserting the measuring system into the solution, but also to measure the 0921-5093/90/$3.50
mechanical parameters during the cracking process so that a higher number of tests on one sample can be carried out. 2. Material and technique The samples of steel AISI 4340 (2 m m x 8 m m x 100 ram) oil quenched from 950°C and tempered for 2.5 h at 250 °C were tested in fourpoint bending at constant displacement in contact with an aqueous solution of 20% H2SO 4 plus 30 g of NaCI per litre at a cathodic current density of 20 mA c m -2 (Fig. 1). The stress on the sample surface was calculated from 3L o = 2 - ~ (P0- AP)
(1)
where L is the load, 2b = 8 mm is the sample width, h = 2 mm is the sample thickness, AP is the decrease in load due to changes in sample compliance because of surface crack growth and P0 = 880 N is the initial load, which corresponds to a surface stress value of 1300 MPa. The A E signals with further peak detection [4] were continuously monitored by a non-resonant piezoelectric transducer. An A E "event" was
az•
2
I
x \ \ N N \ \ \'~ ",'\ \ x \
Fig. 1. Experimental set-up: 1, specimen; 2, dynamometer; 3, AE transducer; 4, bath with corrosive medium and platinum anode. © Elsevier Sequoia/Printed in The Netherlands
30
defined to be a single pulse exceeding the noise level by 20 dB. These pulses were characterized by the amplitude referred to the input A i (measured in millivolts) and time ti (in seconds) of appearance. For each test, the time dependence of the cumulative amplitude Acum(t ) w a s determined by summing (over a time interval of 50 s) the amplitude of each signal Ag(t~) recorded before a time t:
ted bath, a single crack was always nucleated. After the appearance of this crack, samples were unloaded (failure was never reached), dried and then broken to reveal semielliptic cracks on the fracture surface (Fig. 2). These cracks had a width 2c on the surface and a depth a (Fig. 3). The aspect ratio of the cracks is constant with an uncertainty of less than 5% determined by the Fisher criterion
Acum(t) = ~, Ai (ti) ,,<,
a _ k 2c h 2b
(2)
The experimental set-up made it possible to amplify the A E signals by 50 dB in frequency band 0.05-3 MHz and to record the peak amplitudes in a dynamic band of 70 dB. The minimum resolution time was determined by the critical frequency of the recording instrument (a high speed plotter) which was 10 ms.
3. Results and discussion 25 SCC tests had been carried out. In 18 tests, a round bath 5 mm in diameter (and about 20 m m 2 in area) was used to hold the corrosion medium in contact with the sample surface, while in seven tests a slotted bath of 0.3 mm width (the area of which was 1.5 mm:) oriented perpendicular to the applied stress was used. Under the slot-
Fig. 2. The semielliptic crack in the sample cross-section.
(3)
where the form factor k = 0.71 + 0.04. In tests using the round bath, nine samples were cracked by growth of a single crack, in other cases the number of cracks reached between five and 10 (Fig. 4). From the results of tests with single cracks, the calibration curve of log(Acum) vs. log(R) was plotted (Fig. 5), where R = (n/4) a2c is the area of a semielliptic crack (R is in square millimetres and Acu m is in millivolts). The log(Acum) dependence upon log(R) corresponds to the regression equation log,0(A cum)= D0 + Ol l°gl0(R)
(4)
with the coefficients D 0 = 1 . 4 4 8 + 0 . 0 0 5 and D l = 0 . 9 5 + 0 . 0 8 . The coefficient DI does not (with an uncertainty of less than 5% by Student's
31
0.75
I
(in Fig. 6, the parallel scale R / R o is given; R 0 = 16 mm 2 is the sample cross-section). For example, the area of the first microcrack determined by the mean amplitude A 1 of the first A E signal in 25 tests of 0.140_+0.040 mV gives R ( t l ) / R o = 0.0003. If the form of the first microcrack may be assumed to be semielliptic and to satisfy eqn. (3), then its parameters are
2~
0.25
a=34_+3/~m 2 c = 192 + 69/~m
O.OO
090
0.50 '
025 '
0'.75
C -ff
Fig. 3. T h e g r o w t h law of the s u r f a c e semielliptic crack.
criterion) statistically differ from 1; so the calibration relationship (eqn. (4)) can be simplified to A ...... = 2S.05+I',:
R
(5)
For all tests using the round bath the time dependences of the load P ( t ) and the logarithm log{A~um(t)i of the cumulative amplitude were similar (Fig. 6). The time at which the load dropped by 0.2% characterizes the incubation period T L= t~,,,.. The first A E signal is usually recorded at roughly the same time ft. This signal corresponds to nucleation of the first crack: ]tinc - t l l < 100 s (the difference may be explained by the inaccuracy in the graphical determination of the initial load drop by the low speed chart paper). The time t~xp corresponds to the start of exponential growth of crack. For this regime, the linear dependence log(Acorn) vs. time is characteristic. This stage is preceded by a period of stable crack growth given by Tz=texp-tin c. Exponential crack growth finishes at the time to failure tf where T~ = t f - t~xp. The parameters T 1 ( T~ = t~,~= t~), T3 and tf are distributed lognormally (with an uncertainty of 5% by the Kolmogorov-Smirnov criterion): T~ = J-~"J'J'J n + 8-~51() 5 s; T~=375 +tH~ -S0 s; If =')220+580 . . . . . . a60 S. The incubation periods for tests using the round bath is TE 1330~s15510s; the T 1 values of . . . . 5~0 s for tests using the slotted bath are not statistically different (with an uncertainty of less than 5% by Student's criterion). The same is true for times to 54o s and, with failure: with one crack, tr = 1, .O'gll . . . + 43o many cracks, tj=2580+~7~ s for tests using the round bath. Using the calibration curve (eqn. (5)) the crack area R (or, more precisely, the total area of all currently existing cracks) may be determined for any value of cumulative amplitude of A E signals
(6)
(this value is marked by an arrow in Fig. 3). The prior austenite grain mean size determined by 1000 measurements was 31 + 1 /~m for this material, which does not (with an uncertainty of less than 5% by Student's criterion) statistically differ from the depth of the first microcrack determined by the first A E signal in multiple tests. Stable crack growth begins when the crack area reaches an R(ti,,c)/Ro value of 0.015, and exponential growth when the area exceeds R ( t c x p ) / R o = 0 . 0 2 2 which, referred to the prior austenite grain area ( R ~ , = 7 . 5 5 x 1 0 4 ram2), corresponds to 300 Ra and 450 R~,. The stress intensity factor for a crack of the size in eqn. (6) and for a given loading condition gives (the calculations are carried out by the algorithm in ref. 5), at the intersection of the crack with surface, KIB = 8.1 _+4.2 MPa m ~/2 and, at the greatest depth, K I A = 12.2 +4.1 MPa m l/-~. The stress intensity factor does not statistically (with an uncertainty of less than 5% by Student's criterion) differ for different points along crack front and thus, on the average, Ki0 = 10.2 -+ 4.1 MPa m J/2. The equations of linear elastic fracture mechanics are valid only if the plastic zone size % is small compared with the crack length a [6]:
rp ~ ~
I (7>.]
where o !, is the yield stress. Numerical calculation for o ! = 1500 MPa and KI = Ki0 = 10.2 MPa m l/-~ yields rp = 5 /~m. This value is the upper limit because the effective value of yield stress is slightly higher than that determined from a deformation diagram owing to the embrittling effect of hydrogen and grain boundary impurity segregation; so in this c a s e rp "~ a. The absence of plastic deformation traces on grain facets after SCC (compare for example ref. 7) also confirms the
32
Fig. 4. Cracks on sample surface in the region of contact with corrosion environment:(a) fracture with one crack; (b) fracture with manycracks. validity of linear fracture mechanics. So the Ki0 value is a threshold K~scc for a given material under given conditions--a crack of smaller depth cannot appear owing to the grain boundary mechanism of cracking--and within an order of magnitude coincides with the values obtained by standard methods for martensitic steel in aqueous environments (8-16 MPa m 1/2) [1]. During the
relief of the initial stress down to 1200 MPa in 6 h, neither A E signals nor a decrease in load was recorded in six analogous tests. Investigation of the kinetics of the further growth of surface cracks was carried out by correlating the crack size with the actual load for constant initial sample deformation. The depth a and width 2c of a developing single semielliptic
33 ¢ Q
A cure, m.V 500
C
b o - ~
t/t
"~
'~00
o.o
~;
¢'o
50
,@, IV
Fig. 7. The dependence of the semielliptic surface crack size upon decrease in load.
A'p,
,,P,
N/s O. 05
'
~.
~
.~
,'o
,'5 2'0 R,,,,,,,'
Fig. 5. The calibration dependence of cumulative A E amplitude upon the crack area: e, complete cross-section fracture.
0.03 _
/
•-
R
ij
J 0.0~
t
Acum(~)//~ll
639 0.t
,b0
2'00
0.00
Fig. 8. The t!me dependence of the load decrease AP and its derivative AP.
il
0.0f
j
\b
\
If O.(]lt
T,
'1 ~ 7~ I
tit~
r3
t~p
I I
~÷
Fig. 6. The time dependence of cumulative AE signals A cum ( o r crack area referred to sample crosssection R o, i.e. R/Ro)and of sampleload P. amplitude
crack measured on non-fractured samples are linearly correlated to load decrease (Fig. 7), and with an uncertainty of less than 5% by the Fisher criterion: c
~ = (0.29_+ 0.03) + (0.0286 _+0.0033) AP
The values of the crack growth parameters and d = d a / d t were calculated from eqns. (7) and (8) by numerical differentiation of the time dependence of relief AP(t) obtained by averaging "load-time" diagrams (Fig. 8, nine measurements per point; the time was counted from the start of relief). Using this approach to calculate the parameters c and a, the values of stress intensity factor were calculated in deep (A) and surface (B) crack points (Fig. 3). The dependences of the semielliptic surface crack growth rate upon the actual value of stress intensity factor (KDC) for points A and B are different (Fig. 9) and with an uncertainty of less than 5% by the Fisher criterion, are given by =dc/dt
(7)
(OC KIB2.6-+ 0.5 OCKIA 10'9-+ 1.3
a
= (0.31 _+0.14)+ (0.0088 + 0.0014) AP where AP is in newtons.
(8)
(9) (10)
Therefore the crack growth along the surface occurs at a rate an order of magnitude higher than that into the bulk (Fig. 9). Actual K~ values at
34
(i,(~, m__~m s 2
n
~6
J
~0-~
-3
Io
0.5
~s
~'o
3'~
,;o
':5 ~', M~I .m ~/2
Fig. 9. The surface crack growth kinetics.
points A and B are roughly equal; so is the environment effect due to high hydrogen diffusivity. Nevertheless, the growth rates of the crack fronts are different. The amplitude distribution of A E pulses plotted for different stages of SCC using the whole set of recorded signals (Fig. 10) enables us to judge cracking mechanism changes; while during the stable growth stage the most probable area for the major part of the crack steps is (5-50)R a and 250R a is the maximum, during exponential growth the total number of steps increases by two orders of magnitude along with substantial widening of the interval--from 5R a to 1000Ra. Thus, while the values of K~scc agree with those measured using the standard procedure, the kinetics of growth of a real surface crack cannot be determined by the usual method; the discrepancy in growth rates measured at various points of a crack front at nearly equal values of Kt may lead to incorrect life estimations obtained by integration of the KDCs for a construction having surface cracks. 4. Conclusions
(1) In the SCC of high strength steel, the first surface crack recorded by A E signals is formed
t
5
~0
A~mV
Fig. 10. The distribution of A E pulse amplitudes A (or areas of crack steps referred to the austenite grain area Ra): ×, stable crack growth stage; o, exponential crack growth stage. The total interval is 0.07-13 mV, and the histogram has 15 columns of 0.89 mV. no = 5237 is the total number of A E pulses recorded in 18 tests. The left boundary of the first column (70 pV) is not drawn.
in less than 10 ms and has the appearance of a defect with the depth of one prior austenitic grain facet and the width of five to six prior austenitic grain facets. (2) The time before nucleation of the first crack (the incubation period) remains unchanged when the area of local contact of the corrosion environment with the sample surface changes by one order of magnitude, and the number of cracks nucleated does not affect the time to fracture. (3) KISCC determination using the first AErecorded crack is independent of uncertainties connected with the choice of time base and the crack growth rate measurement method, because it is calculated from the crack of minimum size which may be determined experimentally. (4) The ambiguity of the KDC is due not only to the difference in K~0 values; at K~0 = Kiscc the KDCs are different for different crack front points. (5) The duration of the exponential growth of surface cracks accounts for only 16% of the total time before fracture but, during this period, 98% of the sample cross-section is cracked because of an increase in total number of crack steps and an
35
increase of two orders of magnitude of interval in their areas.
References 1 O.N. Romaniv and G. N. Nikiforchin, Mekhanika Korrosionnogo Razrusheniya Konstruktsionnykh Splavov, Metallurgiya, Moscow, 1986,294 pp. (in Russian). 2 Standard test method for plane-strain fracture toughness of metallic materials, ASTM Stand. E 399, in Annual Book of ASTM Standards, ASTM, Philadelphia, PA, 1984, pp. 710-730.
3 D. O. Sprowls, Tests for stress-corrosion cracking, in Metals" Handbook, Vol. 8, American Society for MetaLs, Metals Park, OH, 1985, pp. 495-536. 4 V. G. Khanzhin, Kolichestvennyi analis protsessov razrusheniya metodom akusticheskoy emissii, ('and. Sci. (Phys.) Thesis, Moscow Institute of Steel and Alloys, 1988 (in Russian). 5 J. C. Newman and I. S. Raju, An empirical stress-intensity factor equation for the surface crack, Eng. Fract. Mech., 15 /1-2)(1981) 185-192. 6 K. Hellan, Introduction to Fracture Mechanics, McGrawHill, New York, 1984. 7 Fractography and atlas of fractographs, Metals Handbook, Vol. 9, American Society for Metals, Metals Park, OH, 1974, 8th edn.
29
Effect of Structural Factors on Corrosion Crack Resistance Parameters YU. A. KRUPIN and 1. K. KISELEV 031, Moscow Institute of Steel and Alloys, Leninsky Prospekt 4, Moscow 117936 (U.S.S.R.) (Received September 21, 1989; in revised form April 26, 1990)
Abstract
The stress corrosion cracMng of flat specimens of high strength steel was studied. Data on the nucleation and kinetics of the growth of primary cracks are obtained by analysis of the cumulative peak amplitude of acoustic emission signals and changes in sample compliance. 1. Introduction Investigations of stress corrosion cracking (SCC) of high strength steel with a low temperature tempered martensite structure by standard methods leave some unresolved questions connected with structural peculiarities of the material. First, the kinetic diagrams of cracking (KDCs) describe the process ambiguously; this is connected with the different initial conditions of loading (i.e. the Km value at which the investigation of crack growth in precracked samples begins) [1]. Second, the correctness of definition of the threshold value of the stress intensity factor KISCC (below which no crack growth occurs) depends greatly on the choice of the time base of tests and the crack-growth-monitoring method. Third, the standard methods [2] record the crack growth on a macrolevel, when many structural elements are involved in the process. Fourth, it is impossible to study the nucleation processes of surface corrosion defects in precracked specimens. In tests for SCC susceptibility, flat samples loaded by constant displacement [3] are widely used; to perform a test, the sample is exposed to the corrosive medium and the time to failure or the time to the appearance of the first visible crack is measured. Localization of the contact of the corroding solution with the sample surface enables us not only to record acoustic emission (AE) signals without inserting the measuring system into the solution, but also to measure the 0921-5093/90/$3.50
mechanical parameters during the cracking process so that a higher number of tests on one sample can be carried out. 2. Material and technique The samples of steel AISI 4340 (2 m m x 8 m m x 100 ram) oil quenched from 950°C and tempered for 2.5 h at 250 °C were tested in fourpoint bending at constant displacement in contact with an aqueous solution of 20% H2SO 4 plus 30 g of NaCI per litre at a cathodic current density of 20 mA c m -2 (Fig. 1). The stress on the sample surface was calculated from 3L o = 2 - ~ (P0- AP)
(1)
where L is the load, 2b = 8 mm is the sample width, h = 2 mm is the sample thickness, AP is the decrease in load due to changes in sample compliance because of surface crack growth and P0 = 880 N is the initial load, which corresponds to a surface stress value of 1300 MPa. The A E signals with further peak detection [4] were continuously monitored by a non-resonant piezoelectric transducer. An A E "event" was
az•
2
I
x \ \ N N \ \ \'~ ",'\ \ x \
Fig. 1. Experimental set-up: 1, specimen; 2, dynamometer; 3, AE transducer; 4, bath with corrosive medium and platinum anode. © Elsevier Sequoia/Printed in The Netherlands
30
defined to be a single pulse exceeding the noise level by 20 dB. These pulses were characterized by the amplitude referred to the input A i (measured in millivolts) and time ti (in seconds) of appearance. For each test, the time dependence of the cumulative amplitude Acum(t ) w a s determined by summing (over a time interval of 50 s) the amplitude of each signal Ag(t~) recorded before a time t:
ted bath, a single crack was always nucleated. After the appearance of this crack, samples were unloaded (failure was never reached), dried and then broken to reveal semielliptic cracks on the fracture surface (Fig. 2). These cracks had a width 2c on the surface and a depth a (Fig. 3). The aspect ratio of the cracks is constant with an uncertainty of less than 5% determined by the Fisher criterion
Acum(t) = ~, Ai (ti) ,,<,
a _ k 2c h 2b
(2)
The experimental set-up made it possible to amplify the A E signals by 50 dB in frequency band 0.05-3 MHz and to record the peak amplitudes in a dynamic band of 70 dB. The minimum resolution time was determined by the critical frequency of the recording instrument (a high speed plotter) which was 10 ms.
3. Results and discussion 25 SCC tests had been carried out. In 18 tests, a round bath 5 mm in diameter (and about 20 m m 2 in area) was used to hold the corrosion medium in contact with the sample surface, while in seven tests a slotted bath of 0.3 mm width (the area of which was 1.5 mm:) oriented perpendicular to the applied stress was used. Under the slot-
Fig. 2. The semielliptic crack in the sample cross-section.
(3)
where the form factor k = 0.71 + 0.04. In tests using the round bath, nine samples were cracked by growth of a single crack, in other cases the number of cracks reached between five and 10 (Fig. 4). From the results of tests with single cracks, the calibration curve of log(Acum) vs. log(R) was plotted (Fig. 5), where R = (n/4) a2c is the area of a semielliptic crack (R is in square millimetres and Acu m is in millivolts). The log(Acum) dependence upon log(R) corresponds to the regression equation log,0(A cum)= D0 + Ol l°gl0(R)
(4)
with the coefficients D 0 = 1 . 4 4 8 + 0 . 0 0 5 and D l = 0 . 9 5 + 0 . 0 8 . The coefficient DI does not (with an uncertainty of less than 5% by Student's
31
0.75
I
(in Fig. 6, the parallel scale R / R o is given; R 0 = 16 mm 2 is the sample cross-section). For example, the area of the first microcrack determined by the mean amplitude A 1 of the first A E signal in 25 tests of 0.140_+0.040 mV gives R ( t l ) / R o = 0.0003. If the form of the first microcrack may be assumed to be semielliptic and to satisfy eqn. (3), then its parameters are
2~
0.25
a=34_+3/~m 2 c = 192 + 69/~m
O.OO
090
0.50 '
025 '
0'.75
C -ff
Fig. 3. T h e g r o w t h law of the s u r f a c e semielliptic crack.
criterion) statistically differ from 1; so the calibration relationship (eqn. (4)) can be simplified to A ...... = 2S.05+I',:
R
(5)
For all tests using the round bath the time dependences of the load P ( t ) and the logarithm log{A~um(t)i of the cumulative amplitude were similar (Fig. 6). The time at which the load dropped by 0.2% characterizes the incubation period T L= t~,,,.. The first A E signal is usually recorded at roughly the same time ft. This signal corresponds to nucleation of the first crack: ]tinc - t l l < 100 s (the difference may be explained by the inaccuracy in the graphical determination of the initial load drop by the low speed chart paper). The time t~xp corresponds to the start of exponential growth of crack. For this regime, the linear dependence log(Acorn) vs. time is characteristic. This stage is preceded by a period of stable crack growth given by Tz=texp-tin c. Exponential crack growth finishes at the time to failure tf where T~ = t f - t~xp. The parameters T 1 ( T~ = t~,~= t~), T3 and tf are distributed lognormally (with an uncertainty of 5% by the Kolmogorov-Smirnov criterion): T~ = J-~"J'J'J n + 8-~51() 5 s; T~=375 +tH~ -S0 s; If =')220+580 . . . . . . a60 S. The incubation periods for tests using the round bath is TE 1330~s15510s; the T 1 values of . . . . 5~0 s for tests using the slotted bath are not statistically different (with an uncertainty of less than 5% by Student's criterion). The same is true for times to 54o s and, with failure: with one crack, tr = 1, .O'gll . . . + 43o many cracks, tj=2580+~7~ s for tests using the round bath. Using the calibration curve (eqn. (5)) the crack area R (or, more precisely, the total area of all currently existing cracks) may be determined for any value of cumulative amplitude of A E signals
(6)
(this value is marked by an arrow in Fig. 3). The prior austenite grain mean size determined by 1000 measurements was 31 + 1 /~m for this material, which does not (with an uncertainty of less than 5% by Student's criterion) statistically differ from the depth of the first microcrack determined by the first A E signal in multiple tests. Stable crack growth begins when the crack area reaches an R(ti,,c)/Ro value of 0.015, and exponential growth when the area exceeds R ( t c x p ) / R o = 0 . 0 2 2 which, referred to the prior austenite grain area ( R ~ , = 7 . 5 5 x 1 0 4 ram2), corresponds to 300 Ra and 450 R~,. The stress intensity factor for a crack of the size in eqn. (6) and for a given loading condition gives (the calculations are carried out by the algorithm in ref. 5), at the intersection of the crack with surface, KIB = 8.1 _+4.2 MPa m ~/2 and, at the greatest depth, K I A = 12.2 +4.1 MPa m l/-~. The stress intensity factor does not statistically (with an uncertainty of less than 5% by Student's criterion) differ for different points along crack front and thus, on the average, Ki0 = 10.2 -+ 4.1 MPa m J/2. The equations of linear elastic fracture mechanics are valid only if the plastic zone size % is small compared with the crack length a [6]:
rp ~ ~
I (7>.]
where o !, is the yield stress. Numerical calculation for o ! = 1500 MPa and KI = Ki0 = 10.2 MPa m l/-~ yields rp = 5 /~m. This value is the upper limit because the effective value of yield stress is slightly higher than that determined from a deformation diagram owing to the embrittling effect of hydrogen and grain boundary impurity segregation; so in this c a s e rp "~ a. The absence of plastic deformation traces on grain facets after SCC (compare for example ref. 7) also confirms the
32
Fig. 4. Cracks on sample surface in the region of contact with corrosion environment:(a) fracture with one crack; (b) fracture with manycracks. validity of linear fracture mechanics. So the Ki0 value is a threshold K~scc for a given material under given conditions--a crack of smaller depth cannot appear owing to the grain boundary mechanism of cracking--and within an order of magnitude coincides with the values obtained by standard methods for martensitic steel in aqueous environments (8-16 MPa m 1/2) [1]. During the
relief of the initial stress down to 1200 MPa in 6 h, neither A E signals nor a decrease in load was recorded in six analogous tests. Investigation of the kinetics of the further growth of surface cracks was carried out by correlating the crack size with the actual load for constant initial sample deformation. The depth a and width 2c of a developing single semielliptic
33 ¢ Q
A cure, m.V 500
C
b o - ~
t/t
"~
'~00
o.o
~;
¢'o
50
,@, IV
Fig. 7. The dependence of the semielliptic surface crack size upon decrease in load.
A'p,
,,P,
N/s O. 05
'
~.
~
.~
,'o
,'5 2'0 R,,,,,,,'
Fig. 5. The calibration dependence of cumulative A E amplitude upon the crack area: e, complete cross-section fracture.
0.03 _
/
•-
R
ij
J 0.0~
t
Acum(~)//~ll
639 0.t
,b0
2'00
0.00
Fig. 8. The t!me dependence of the load decrease AP and its derivative AP.
il
0.0f
j
\b
\
If O.(]lt
T,
'1 ~ 7~ I
tit~
r3
t~p
I I
~÷
Fig. 6. The time dependence of cumulative AE signals A cum ( o r crack area referred to sample crosssection R o, i.e. R/Ro)and of sampleload P. amplitude
crack measured on non-fractured samples are linearly correlated to load decrease (Fig. 7), and with an uncertainty of less than 5% by the Fisher criterion: c
~ = (0.29_+ 0.03) + (0.0286 _+0.0033) AP
The values of the crack growth parameters and d = d a / d t were calculated from eqns. (7) and (8) by numerical differentiation of the time dependence of relief AP(t) obtained by averaging "load-time" diagrams (Fig. 8, nine measurements per point; the time was counted from the start of relief). Using this approach to calculate the parameters c and a, the values of stress intensity factor were calculated in deep (A) and surface (B) crack points (Fig. 3). The dependences of the semielliptic surface crack growth rate upon the actual value of stress intensity factor (KDC) for points A and B are different (Fig. 9) and with an uncertainty of less than 5% by the Fisher criterion, are given by =dc/dt
(7)
(OC KIB2.6-+ 0.5 OCKIA 10'9-+ 1.3
a
= (0.31 _+0.14)+ (0.0088 + 0.0014) AP where AP is in newtons.
(8)
(9) (10)
Therefore the crack growth along the surface occurs at a rate an order of magnitude higher than that into the bulk (Fig. 9). Actual K~ values at
34
(i,(~, m__~m s 2
n
~6
J
~0-~
-3
Io
0.5
~s
~'o
3'~
,;o
':5 ~', M~I .m ~/2
Fig. 9. The surface crack growth kinetics.
points A and B are roughly equal; so is the environment effect due to high hydrogen diffusivity. Nevertheless, the growth rates of the crack fronts are different. The amplitude distribution of A E pulses plotted for different stages of SCC using the whole set of recorded signals (Fig. 10) enables us to judge cracking mechanism changes; while during the stable growth stage the most probable area for the major part of the crack steps is (5-50)R a and 250R a is the maximum, during exponential growth the total number of steps increases by two orders of magnitude along with substantial widening of the interval--from 5R a to 1000Ra. Thus, while the values of K~scc agree with those measured using the standard procedure, the kinetics of growth of a real surface crack cannot be determined by the usual method; the discrepancy in growth rates measured at various points of a crack front at nearly equal values of Kt may lead to incorrect life estimations obtained by integration of the KDCs for a construction having surface cracks. 4. Conclusions
(1) In the SCC of high strength steel, the first surface crack recorded by A E signals is formed
t
5
~0
A~mV
Fig. 10. The distribution of A E pulse amplitudes A (or areas of crack steps referred to the austenite grain area Ra): ×, stable crack growth stage; o, exponential crack growth stage. The total interval is 0.07-13 mV, and the histogram has 15 columns of 0.89 mV. no = 5237 is the total number of A E pulses recorded in 18 tests. The left boundary of the first column (70 pV) is not drawn.
in less than 10 ms and has the appearance of a defect with the depth of one prior austenitic grain facet and the width of five to six prior austenitic grain facets. (2) The time before nucleation of the first crack (the incubation period) remains unchanged when the area of local contact of the corrosion environment with the sample surface changes by one order of magnitude, and the number of cracks nucleated does not affect the time to fracture. (3) KISCC determination using the first AErecorded crack is independent of uncertainties connected with the choice of time base and the crack growth rate measurement method, because it is calculated from the crack of minimum size which may be determined experimentally. (4) The ambiguity of the KDC is due not only to the difference in K~0 values; at K~0 = Kiscc the KDCs are different for different crack front points. (5) The duration of the exponential growth of surface cracks accounts for only 16% of the total time before fracture but, during this period, 98% of the sample cross-section is cracked because of an increase in total number of crack steps and an
35
increase of two orders of magnitude of interval in their areas.
References 1 O.N. Romaniv and G. N. Nikiforchin, Mekhanika Korrosionnogo Razrusheniya Konstruktsionnykh Splavov, Metallurgiya, Moscow, 1986,294 pp. (in Russian). 2 Standard test method for plane-strain fracture toughness of metallic materials, ASTM Stand. E 399, in Annual Book of ASTM Standards, ASTM, Philadelphia, PA, 1984, pp. 710-730.
3 D. O. Sprowls, Tests for stress-corrosion cracking, in Metals" Handbook, Vol. 8, American Society for MetaLs, Metals Park, OH, 1985, pp. 495-536. 4 V. G. Khanzhin, Kolichestvennyi analis protsessov razrusheniya metodom akusticheskoy emissii, ('and. Sci. (Phys.) Thesis, Moscow Institute of Steel and Alloys, 1988 (in Russian). 5 J. C. Newman and I. S. Raju, An empirical stress-intensity factor equation for the surface crack, Eng. Fract. Mech., 15 /1-2)(1981) 185-192. 6 K. Hellan, Introduction to Fracture Mechanics, McGrawHill, New York, 1984. 7 Fractography and atlas of fractographs, Metals Handbook, Vol. 9, American Society for Metals, Metals Park, OH, 1974, 8th edn.