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A study of the dynamics of high temperature brittle intergranular fracture by acoustic emission
Acla metal/. Vol. 36, No. 2. pp. 44-452, Printed in Great Britain
1988
Oool-6160/8X 163.00+ 0.00 Pergamon Journals Ltd
A STUDY OF THE DYNAMICS OF HIGH TEMPERATURE BRITTLE INTERGRANULAR FRACTURE BY ACOUSTIC EMISSION
Materials
C. A. HIPPSLEY, D. J. BUTTLE and C. B. SCRUBY Physics and Metallurgy Division, Harwell Laboratory, Didcot, Oxon OX11 ORA, England (Redoed
25 Murrh
1987)
Abstract-This paper presents a study of high temperature brittle intergranular fracture (HTBIGF) in which acoustic emission (AE) data and fractography were used together to make deductions about the dynamics of fracture. The AE event rate was proportional to the rate of overall crack growth, and the average crack increment per AE event correlated well with spacing of striations observed on the fracture surface. The AE from HTBIGF was also compared with ambient temperature brittle intergranular fracture and high temperature ductile fracture. The HTBIGF generated about 50% of the emission 01 fully brittle fracture, and at least an order of magnitude more than that from ductile fracture. The observation of discrete bursts of AE of this magnitude, coupled with the striations on the fracture surface, indicates that HTBIGF takes place in discrete brittle steps. This is consistent with the segregation of sutphur as an embrittling species to the region of the crack tip, promoting stepwise decohesion, rather than a continuous crack growth process. RCsumkNous presentons dans cet article une etude de la rupture intergranulaire fragile $ haute temperature (RIGFHT), od l’kmission acoustique (EA) et la fractographie ont Cte utilisees de pair pour obtenir des informations sur la dynamique de la rupture. La vitesse de I’EA est proportionnelle a la vitesse de croissance globale de la fissure, et I’accroissement moyen de la vitesse pour chaque EA correspond bien a l’espacement des stries observees sur la surface de rupture. Nous avons Cgalement compare I’EA provoquee par la RlGFHT a la rupture intergranu~aire fragile B la temp&ature ambiante, et i la rupture ductile a haute temperature. La RIGFHT g&n&e environ 50% de I’emission de la rupture entitrement fragile, et au moins dix fois plus que Yemission de ia rupture ductile. L’observation d’ondes disc&es d’EA de cette amplitude, jumelte a l’observation de stries sur la surface de rupture, montre que la RIGFHT a lieu par &tapes fragiles individuelles. Ce fait est en accord avec la segregation du soufre, espece fragilisante, vers l’extremite des fissures (ce qui provoque une d&cohesion par itapes successives) plutot qu’avec un mecanisme de croissance continue des fissures. Zus~~f~ung-In dieser Arbeit wird der intergranulare Hochtem~ratur-Spr~dbruch untersucht; hierzu werden Ergebnisse der akustischen Emission und fraktografische Beobachtungen zur Ermittlung der Dynamik des Bruchvorganges zusammen benutzt. Die Rate der akustischen Emissionsereignisse war proportional zur Gesamtrate des RiOfortschritts; die mittlere RiBvergroBerung pro akustischem Emissionsereignis korrelierte gut mit dem Abstand der auf der Bruchflkhe beobachteten Streifungen. Die akustische Emission des intergranularen Hochtemperatur-Sprodbruches wurde such mit intergranularem Sprodbruch bei Raumtemperatur und mit dem duktilen Hochtemperaturbruch verglichen. Beim intergranularen H~htem~ratur-Spr~dbruch wurde etwa 50% der akustischen Emission des volist~ndig spriiden Bruches erzeugt und wenigstens eine Gr~~nordnung mehr als h&m duktilen Bruch. Die Beobachtung diskreter Ausbriiche an akustischer Emission dieser Gr6Benordnung, gekoppeit mit den entsprechenden Streifungen an der Bruchoberflkhe. zeigt, da13 der intergranuiare HochtemperaturSprddbruch in diskreten spriiden Schritten ablauft. Diese Folgerung ist vertrtiglich mit der Segregation des Schwefels als verspriidendes Element im Bereich der R&pi&e, wodurch eine schrittweise Dekohasion gefijrdert wird, und nicht ein kontinuierlicher ProzeB des RiBwachstums. 1. fNTRODU~TION
High Temperature Brittle Intergranular Fracture (HTBIGF) has been recognised in recent years as a significant mechanism of failure in ferritic alloy steels [I-3]. It was first identified as one of two fracture mechanisms involved in stress-relief cracking of weld heat-affected zones 14-61, the other mechanism being intergranular microvoid coalescence [7]. Detailed studies of HTBIGF under isothermal conditions have shown that it occurs by slow crack propagation ( 10e9 to 10e5 ms-‘) in the presence of a stress concentrator at temperatures in the range 300~650°C. The rate of crack growth is a function of both the applied
stress-intensity and prior austenitising temperature (illustrated in 2fsCr 1Mo steel by Fig. l), (81 and also depends on the segregation of residual embrittling impurities to grain boundaries and to the crack tip region under stress [ 1,2]. Scanning Auger Microprobe (SAM) examination has revealed a general “background” segregation of phosphorus IS], together with local concentration of sulphur near the growing HTBIGF crack tip [ 1,2,9]. Dynamic segregation of sulphur to the crack tip region at elevated temperature under the influence of the local stress-field is envisaged, in a manner analogous to the more familar mechanism of hydrogen embrittlement at ambient temperature [IO]. 441
HIPPSLEY et al.: Austenltising .1300, o 1300, . 1300,
DYNAMICS OF HIGH TEMPERATURE
treatment
..
WQ FC 1200, WC’ FC 1100, WQ
/
.
I 0
I 20
Appkd
‘*
I 30
I 40
I 50
stress
henshy,
I 60
I 70
80
K, (MN rn-“‘)
Fig. 1. Example of crack growth behaviour for different final austenitising temperatures (tested in vacuum) [8].
The following models of the overall cess have been proposed:
fracture
transducers attached to the surface of the material. Broadly speaking, the amplitude of the detected AE depends upon the size and lifetime of each deformation or fracture event. Furthermore, AE is only emitted when a crack advances, and not when it is static [1 I]. Thus careful measurements of the emission activity as a function of stress intensity or time, for instance, can be used to give insights into the dynamics of fracture. Full characterisation of each source event has been demonstrated using carefully chosen specimen geometries and broadband detection systems [12, 131. For the present study the waveguide technique necessitated by the high temperatures precludes the use of this quantitative approach. Nevertheless, some semi-quantitative information can be obtained from the AE data by careful calibration of the detection system. Clearly, an indication of whether the crack grows continuously or in steps, and of the true degree of its brittle character relative to well characterised lower temperature brittle fracture, would help to clarify the mechanism of HTBIGF. The present study applies acoustic emission (AE) as a laboratory tool, in combination with fractographic analysis, to address these questions.
2. EXPERIMENTAL
pro2.1. Material
(i) Sulphur in solid solution is driven by the crack tip stress field under “pure-drift” conditions to concentrate at the stationary crack, until local grain boundary cohesion is reduced sufficiently to enable a discrete increment of crack growth into fresh, unembrittled material (stepwise cracking) [ 11. (ii) Small sulphides on the crack faces dissolve on exposure, freeing elemental sulphur. The crack is treated as a giant, acute cavity, and sulphur is swept into the tip by the cavity growth process [2,3]. This has the effect of either (a) reducing cohesion to produce stepwise growth as for (i), or (b) reducing the cavity tip llank-angle to accelerate continuous cavity growth. If there are sudden changes in the internal stress field in a material, caused for instance by the propagation of a crack of slip band, then some of the stored elastic energy is dissipated as elastic waves (stress waves). Depending upon their amplitude these can be detected as acoustic emissions (AE) by piezoelectric
Table 1. Composition
BRITTLE FRACTURE
and heat treatment
A commercial 2$r 1Mo steel was employed, as used for several previous SAM and fractographic investigations of HTBIGF. The bulk chemical composition is given in Table 1. Fracture testpieces were machined to a half-scale compact tension design ($T). Three batches of specimens were prepared for testing: the first, to be used for analysis of HTBIGF, was austenitised at 1300°C and water-queched (type WQ); the second, to provide a comparison with brittle intergranular fracture at ambient temperature, was again austenitised at 1300°C and waterquenched, but also tempered at 650°C for 500 h and temper-embrittled by means of a step-cooling treatment [14] (type TE); the final batch, to provide a comparison with ductile cracking by cavitation at elevated temperature, was left in the as-received condition, i.e. rolled plate (type AR). 2.2. Acoustic
emission and mechanical
testing
The apparatus used to monitor AE during hightemperature crack growth is illustrated schematically in Fig. 2. This will henceforth be referred to as system
of 2$r
1Mo steel by wt%
Cr
MO
Mn
s
Ni
CU
V
W
Nb
2.12 Sb to.005 C 0.13
0.95 Sn 0.010 N 0.006
0.53 P 0.015
0.17 S 0.021
0.20 Ti
0.15 Zr
<0.02 co
HIPPSLEY
er al.:
DYNAMICS
OF HIGH TEMPERATURE
BRITTLE
FRACTURE
443
Platinum
I
woveguide
1
Load Discriminator/ counter
~
ji+I
I
I
0 CT
oc;i-
_
Charge ampiifie
ir
r
Digitolage
ii
Constant current suPPlY
oscilloscope
Micro -volt meter
Fig. 2. Acoustic emission detection system A. A. Acoustic pulses from the specimen were transmitted to a broad-band transducer through a 2mm diameter platinum waveguide. The waveguide was screwed into the specimen, and gave a 60% transmission efficiency for a “burst” of acoustic emission as measured during calibration tests with an artificial acoustic source (pulsed neodymium-YAG laser [ 151). The transducer output signal was passed through a charge amplifier and a 0.1-l MHz filter before final amplification ( x 3100 gain) and processing by a r.m.s.d.c. converter. The r.m.s. signal pulse was recorded both directly, and via a pulse counter and a signal integrator, on a multi-channel Y/t chart recorder. The waveform of the acoustic burst, including the ringing associated with the specimen and waveguide, could also be recorded on a high-speed digital storage oscilloscope @SO). Crack growth during high-temperature testing was monitored by measuring the potential drop across the crack induced by passing a constant current through the fCT specimen. This arrangement is also illustrated in Fig. 2. The potential drop signal was recorded, after amplification, simultaneously with the AE signal on the Y/t chart recorder, and could be converted to values of crack length using a suitable calibration. The following experiments were performed to study fracture at elevated temperature: (if Tests at 400 and 500°C to assess detectability of AE during HTBIGF of WQ specimens, and to measure the rate of AE pulses together with the rate of crack growth as a function of applied stress-intensity.
(ii) Tests at 500°C to verify the source of AE pulses by (a) examining in detail the pulse waveform recorded on the DSO, and (b) comparing AE from HTBIGF in WQ specimens with AE from a fracture process which is known to be acoustically quiet (i.e. ductile cavity growth [16] in the AR specimens). The $T specimens were pre-fatigued before testing at a maximum stress intensity of 30MN rnw3j2at room tem~rature. All the high temperature tests were performed under constant load conditions using a Mayes dead-weight creep testing machine. This technique produced an applied stress-intensity which rose with crack growth from 40 to 80MNm-3/“. Following the elevated temperature tests, some further tests were performed on the embritted (TE) material at ambient temperature. Although use of a waveguide precludes fully quantitative AE measurements, an empirical comparison between AE during HTBIGF and AE from well characterised brittle intergranular fracture at lower temperature is possible. A second AE detection system (system B) was used (Fig. 3) which excluded the furnace and potential drop equipment, and employed three ampliiication channels for the AE signal covering a range of gain factors ( x 1, x 13 and x 680). The charge amplifier was also modified to reduce its own gain by a factor of 3.85. This difference in charge amplifier gain between the two systems was verified by producing an acoustic event of repeatable magnitude on the $t specimen (i.e. a laser pulse), and measuring the output of each charge amplifier in turn. The range of amplifier gains in the second system was necessary in
HIPPSLEY
444
et al.:
DYNAMICS
OF HIGH
TEMPERATURE BRITTLE FRACTURE
Platinum
Amplifier
/
R”,‘.,
DC
Load
0 CT 0
~
RMS -DC unit
Speci men
:
Chart recorder
Digital storage oscilloscope
Fig. 3. Acoustic emission detection system B.
order to capture the emissions from isolated brittle “pop-in” events in the TE specimens, whose magnitude could potentially vary widely, depending on the fracture event area. The !$T specimens were again pre-fatigued, but required charging with hydrogen- to produce brittle intergranular fracture on loading at ambient temperature. This was achieved using an electrolytic cell containing 10% sulphuric acid together with 1 gl-’ of thiourea as a hydrogen recombination poison. The specimens were tested in an Instron screw-driven machine at a constant cross-head displacement rate until the first pop-in event was detected. They were later broken open by fatigue loading, and the extent of the brittle pop-in measured optically In order to accurately compare the wide range of AE signals detected by systems A and B (Fig 2 and 3), calibration of both systems by a known input signal was required. This was accomplished by injecting a burst of sinusoidal waveform into the 0.1-l MHz filter of a similar frequency and duration as a typical AE pulse (plus ringing of specimen and waveguide) from a fracture event, i.e. 500 kHz over 10 ms. The peak-to-peak amplitude of the input pulse could then be varied to produce a range of r.m.s. pulses on the chart recorder, covering the signal amplitudes observed during each type of fracture test. The extended duration of some elevated temperature tests made use of the signal integrator impractical due to drift caused by variations in the r.m.s. signal background. Hence the direct r.m.s. pulse height, as measured from the chart recorder trace, was employed exclusively.
2.3. Fract~grup~y Specimens from all three conditions were broken open after testing, and examined in an Hitachi S570 scanning electron microscope (SEM) to verify the mechanism of fracture. Specimens given a similar heat treatment to the WQ condition, but tested in uucuflltz at 500°C as part of a parallel investigation of HTBIGF[8] were also examined in the SEM. Fine details of the fracture process were preserved in these specimens, and could be correlated with AE observations. Some of these fracture surfaces were nickelplated and sectioned in a plane normal to that of fracture and the crack growth direction. They were then pofished and etched in Nital for optical and SEM examination.
3. RESULTS 3. I. High te~p~r~t~re tests Specimens in the WQ condition (likely to manifest HTBIGF) tested at both 400 and SOO”C emitted detectable AE events during crack growth. Figure 4 shows a sample of a typical trace recording r.m.s. pulses. The following parameters were determined after successive increments of crack growth in each test: stress-intensity (K) crack increment (Au) time increment (At) event count per inurement (A~) crack growth rate (dda/dt)
HIPPSLEY et al.: DYNAMlCS
OF HIGH TEMPERATURE
/ 70
r
1
’ 1
?.
L
5
/ I ldd J
i 75
! 85
I 80
tog (K)
Fig. 5. Crack growth rate (da/dt) and AE event rate (dN/dt) as a function of applied stress intensity (K) for two tests at 400 and 500°C.
1
0.9mm Crack
445
BRITTLE FRACTURE
~
increment
Fig. 4. Example of acoustic emission detected HTBIGF in material WQ.
during
event count rate (d~Jd~) mean crack increment per event (Aa/A~) mean time increment per event (At/AN). Table 2 gives a detailed breakdown of these parameters for a test performed at 500°C. The increments of crack growth (Au) were chosen arbitrarily. It is clear that the rate of emission of events greater than the noise threshold (dNjdt) increased with crack growth rate and applied stress-intensity. Figure 5 plots log(dN/dt) and log(da/dt) vs log(K) for a test at 400 and 500°C. The approximately linear relationship between dN/dt and da/dt is illustrated in Fig 6 (one test at 4oo’C and two tests at 500°C). The waveform recorded for a typical event during crack growth in a sample of condition WQ is shown in Fig. 7(a). This includes the initial pulse from the fracture event and subsequent ringing in the s~cimen/rig~waveguide system which dies away exponentially with time. The waveform therefore has a
I
I
-7
-6 log
I -5
(do/dt)
Fig. 6. AE event rate (dN/dt) as a function of crack growth rate (dn/dt) for three tests at 400 and SOO~‘C.
duration of approximately 10 ms. Figure 7(b-d) compares waveforms on the same timescale. which were derived from (b) an artificial event on the specimen (a pencil lead break), (c) specimen grip noise induced by rapid changes in applied load and (d) electrical noise from the creep machine ham-ievelling device.
Table 2. Summary of data for one HTBIGF crack growth test at SO0 C K
(MN
=)
56.0
58.0 60.5 63.0 65.1 69.5
All (pm)
A.r (s)
AN (events)
da idt (urns I)
dN/dt levents s ‘)
Au,lAN (urn event I)
AI /AN (sevent ‘I
380 360 350 350 360 320
684 480 312 150 80 SO
43 41 41 44 45 41
0.56 0.75 1.1 2.3 4.5 6.4
0.063 0.085 0.131 0.293 0.562 0.820
8.8 8.8 8.5 X.0 X.0 7.a
16 12 7.7 3.5 1.8 1.3
HIPPSLEY et ai.:
446
‘I I’
10ms
DYNAMICS OF HIGH TEMPERATURE BRITTLE FRACTURE
I
?Oms
(a)
.
1Oms
_,
(b)
IOms
I
(c)
.
(d)
Fig. 7. Total AE waveforms obtained from (a) HTBIGF, @) pencil lead break, (c) specimen grip noise, (d) electrical interference. There is clearly a similarity between the true and artificial fracture events [Fig. 7(a) and (b)] and a difference between these and both types of noise [Fig. 7(c) and (d)] in the waveform shape and rise time. In two WQ tests at 500°C the waveform characteristics were monitored, using the larger events to trigger the DSO. Of these, more than 75% were similar in character to the fracture event waveform. Testing of specimens in the AR condition (likely to manifest ductile cavitation) produced crack growth rates an order of magnitude less than those observed in the WQ specimens tested over the same K and Au range. This is consistent with the greater ductility of the fracture mechanism in these specimens. However, this resulted in a greater contribution of spurious noise to the overall AE signal in AR specimens than in WQ specimens. In order to correct for this effect, and enable an accurate comparison of the true AE from each fracture mode to be made, the noise signal alone was measured during a period when both specimens were under load at temperature, but no crack growth was detected. These signals were scaied to the time period over which the comparison was made, and subtracted from the overall signal obtained during crack growth. A graphical illustration of the result of this process is given in Fig. 8. The AE energy measured from crack growth in the AR specimen was signi~cantly smaller than that measured from the same extent of cracking in the WQ
Cavity
growth
HTBIGF
$
3 .?i
E
0 8
a
4
1,
@37mm Crack
0.37mm
J
increment
Fig. 8. Comparison of AE from cavity growth and HTBIGF over the same crack increment at 500°C.
HIPPSLEY
et 01.:
DYNAMICS
OF HIGH TEMPERATURE
BRITTLE
FRACTURE
447
/Pop-in
System
0
IO
20
30
Simulated
40 i/p
50
60
(mV p-p
1
50
40 -
70
x(b)
x / x
30 -
System B
/ x
20Pop-in
A
event IO -
J x'
0
500
I 1000 Simulated
Fig.
Displacement
Fig. 9. Example of load/displacement plot for ambient temperature test of material TE, together with typical AE waveform from brittle intergranular pop-in (inset).
specimen. A repeat of this comparative gave a similar outcome.
measurement
I 1500 i/p
I 2000
I 2500
? 00
(mV p-p)
10. Calibrations of r.m.s. output (chart recorder) vs simulated AE source input for systems A and B.
and the nominal stress-intensity at fracture (i.e. not necessarily valid according to BS 5447), is given in Table 3 for six tests. The relatively large size of these events made saturation of the AE detection system (type B, Fig. 3) a possibility. However, examination of the waveform captured on the DSO from the lowest gain channel in system B showed no evidence of saturation (Figure 9).
3.2. Ambient temperature tests
3.3. Calibration of AE detection systems
Gradual loading of embrittled TE specimens, charged with hydrogen, produced a pop-in of brittle crack growth at ambient temperature preceding or coincident with the maximum achievable load. A sample load vs displacement plot is shown in Fig. 9. This type of test gave rise to a single large AE event, in contrast to the many smaller events observed during HTBIGF. The event size is expressed as the r.m.s. peak height measured from the Y/t recorder and normalised to the calibration source (vide 3.3). This, together with the corresponding fractured area
The amplitude of the artificial electrical source (500 kHz gated cw pulse of 10ms duration) was adjusted to produce a response on the Y/t recorder covering the range of experimental observations obtained from high ambient temperature tests using systems A and B. Plots of source peak-to-peak (pp) voltage vs recorded r.m.s. peak voltage are given in Fig. 10 for system A (a) and the lowest gain channel of system B (b). This calibration was used to normalise all high temperature data (system A) and ambient temperature data (system B) to the
Table 3. Summary of data normalised to artificial calibration source for ambient temperature tests (TE material-Kq refers to nominal stress-intensity at pop-in) Kq (MN mm3’2) 88 110 88
15 95 70
r.m.s. peak (mV) 5.0 8.3 17.7 15.7 22.0 26.5
Artificial source equivalent (mV)
Pop-in area (mm*)
AE per unit area (mV mmm2)
735 910 1315 1230 1445 1595
37.3 45.1 40.9 58.0 63. I 66.0
19.7 20.2 32. I 21.2 22.9 24.2
HIPPSLEY
448
el 01.:
Table 4. Summary
DYNAMICS
of data normalised
Material/Test WQ-HTBIGF at WQ-HTBIGF at WQ-HTBIGF at WQ-HTBIGF at WQ-HTBIGF at WQ-HTBIGF at AR-cavity growth
OF HIGH
500°C 500°C 500°C 500°C 500°C 500°C at 500°C
TEMPERATURE
to artificial
calibration
BRITTLE
source for high temperature
K (MN m-3i2)
Increment in crack area (mm2)
ZAE (artificial source equivalent i 3.85 mV)
56.0 58.0 60.5 63.0 65.7 56-70 56-70
4.86 4.61 4.48 4.48 4.61 24.3 24.3
49.9 51.4 52.6 46.1 58.8 170 g20.5
equivalent artificial source amplitude. For a given increment of crack growth in the AE detectability study (Table 2) and the comparison of WQ and AR cracking (Fig. 8) at SOOC, each r.m.s. pulse recorded by system A was converted to an equivalent artificial source p-p value (also taking into account the additional factor of 3.85 between system A and B charge amplifier gains). The sum of these values over each crack increment gave an average magnitude of AE emission per cracked area for the high temperature fracture mechanisms (summarised in Table 4). This could be compared directly with the normalised individual event values for the ambient temperature tests given in Table 3.
FRACTURE tests
AE per unit area (mVmmm2) 10.3 12.5 II.5 10.3 12.7 7.0 $0.8
ations on the intergranular facets which were orien ted normal to the direction of crack growth (Fig. 12). I ‘he striation spacing was in the range 2-9pm, and tdid not vary noticeably with applied stress-intensity or crack growth rate. Observation of these features in
3.4. Fractography Inspection of fracture surfaces from the high temperature tests performed in air confirmed that the WQ specimens exhibited HTBIGF, while the AR specimens exhibited ductile cavitation [Fig. 1l(a, b)]. The TE specimens charged with hydrogen and tested at ambient temperature exhibited 100% brittle intergranular fracture in the pop-in region [Fig. 11(c)]. The high resolution study of HTBIGF produced at 500°C in a vacuum [8] revealed the presence of stri-
Fig.
Il. SEM fractographs (b) AR-cavity growth
illustrating fracture modes in AE specimens: (a) WQ-HTBIGF at SOO”C, at 5OO”C, (c) TE-brittle intergranular pop-in at ambient temperature.
HIPPSLEY ef al.:
DYNAMICS OF HIGH TEMPERATURE BRITTLE FRACTURE
449
section (Fig. 13) indicated that they were cusps in the order of 1 pm in height. The cusp position on the facets did not correspond to any obvious microstructural feature, such as a lath or packet boundary, and neither did the cusp spacing reflect the lath width (0.08 pm), packet size (100pm) or grain size (250 v m) ]8]. 4. DISCUSSION 4.1. Verification of AE measurements
It is clear from Fig. 4 that significant levels of AE were detected during fracture of WQ specimens at 400 and 500°C. This is consistent with previous observations of AE during stress-relief cracking and creep crack growth in low alloy steels [17,18],
Fig. 12. SEM fractographs showing HTBIGF from vacuum test at 500°C: (a) general intergr~uIar, (b) detail of grain boundary facet with striations.
Fig. 13. Micrographs of HTBIGF in section: (a) optical micrograph of Ni-plated fracture surface showing cusps, (b) and (cf SEM micrographs of (a) showing detail of cusps.
450
HIPPSLEY et al.: DYNAMICS OF HIGH TEMPERATURE BRITTLE FRACTURE
although the authors are unaware of any study to date aimed specifically at the high temperature brittle inter~anular fracture mechanism, HTBIGF. Three specific results point to the AE detected originating with the HTBIGF fracture process, rather than spurious noise sources, giving confidence to its use as an analytical tool. First, Figs 5 and 6 show that the AE count rate (d~/dt) correlated closely with the rate of crack propagation (da/dt). The gradients of the log(dhr/dt) vs log(da/dt) plots in Fig. 6 varied between 0.85 and 1.05, indicating that, within experimental error, the count rate was directly proportional to the crack growth rate. This would be expected if the AE was produced by a series of discrete fracture events, which together resulted in the overall crack growth. Secondly, most of the waveforms observed during HTBIGF [Fig. 7(a)] were typical of a fracture event, rather than the various possible sources of spurious noise [Fig. 7(c,d)]. Although propagation through the waveguide tends to obscure source differences, it is also interesting to note that the AE signals from HTBIGF were similar in character to those from brittle intergranular fracture at ambient temperature[Figs 7(a) and 91. Finally, a comparison of AE from HTBIGF with that from a fracture process which is known to be of low acoustic energy (i.e. cavity growth f16]), showed an approximately tenfold increase in magnitude of emissions from the HTBIGF process. This comparison was made under the same conditions of stress-intensity and crack increment, and therefore must reflect intrinsic differences in the two crack growth mechanisms. 4.2, Correlation fractography
between
AE
observations
and
The primary implication of the detection of discrete AE bursts is that HTBIGF occurs by the summation of many small fracture events. This is consistent with the step-wise models of crack growth [l, 21. If it is assumed that each emission corresponds to one unit of crack growth across the specimen, the average step size may be equated to Aa/AN in Table 2. In this manner, the AE observations predict a step size of ~8 pm which does not vary significantly with stress-intensity or crack growth rate. The SEM observations of HTBIGF facets showed striations after testing in vacuum which were 2-9 pm apart. The striation spacing also did not change significantly with stress-intensity or crack growth rate [8]. There is therefore a close correlation between the AE and SEM info~ation, which strongly supports the step-wise models of HTBIGF. 4.3. Implications for the mechanism of HTBIGF The evidence discussed above, supporting step-wise growth, indicates that the steady state cavity growth mechanism, suggested by Chen [3], is unlikely in
log( Au /At
f
Fig. 14. Average time between events At/AN) and average crack increment per event (Au/AN) as a function of crack growth rate (da/dr), for a test at 500°C.
24 Cr 1MO steel. The two mechanisms in which grain boundary cohesion is reduced by the dynamic segregation of sulphur, enabling the crack to propagate a discrete distance, are more consistent [1,2]. These models differ regarding the source of sulphur and the mechanism of segregation, and are discussed in detail in Ref. [9]. The observation of a constant step-size (Table 2) means that the average time between steps (Atlas) must vary with stress intensity and crack growth rate (Table 2). This is illustrated in Fig. 14, which plots both step size and step time interval as a function of crack growth rate for a test at 500°C. The range of step time intervals of approximately 1-20s has important application to the modelling of stress-driven sulphur segregation to crack tips. The “pure-drift” model of Hippsley et al. [l] assumed originally a constant value of 1s. This choice now appears to be physically reasonable in magnitude, although the step time interval should be considered a variable in future refinements of the model. An estimate of the degree of brittleness associated with HTBIGF steps may be obtained by comparison with AE from the ductile cavity growth mechanism (AR specimens) and the truly brittle intergranular pop-in fracture in TE specimens at ambient temperature. The acoustic energy emitted from a fracture event depends on the velocity of crack growth [12]. Brittle intergranular fracture involves very limited plasticity, and takes place at a velocity approaching that of sound in the fractured material (i.e. N 1000 ms-t). Cavity growth requires extensive plastic deformation and /or diffusion, and occurs cor~spondin~y slower at < 10 ms-’ [12]. Figure 15 summarises the amplitude of AE per unit area of crack growth, normalised to the artificial source amplitude, for each of the fracture mechanisms observed. Only an upper limit to the cavity growth output could be given due to the low signal to noise ratio obtained. This is approximately two orders of magnitude below the signal given by the
HIPPSLEY et al.:
DYNAMICS
Brittle intergranular fracture (ambient) -- I
HTBIGF
__
OF HIGH
--
_.
(500°C) I
I
Cavity growth (500°C)
TEMPERATURE BRITTLE FRACTURE
451
phosphorus, reduces grain boundary cohesion in the crack tip region to a level below which crack propagation can occur under the prevailing crack tip stress conditions. The crack then grows at a velocity of some hundreds of metres per second into fresh material, which has not been enriched by sulphur and is therefore tougher, and is arrested. This cycle is repeated to produce step-wise HTBIGF. It is un~rtain at present whether the new crack nucleates in front of the old crack or at the crack tip. If nucleation takes place in front of the crack tip, the new crack would grow backwards to coalesce with the old, forming cusps at the point of coalescence [Fig. 16(c)], as observed in some sections (Fig. 13). This process has been observed on a larger scale in studies of HTBIGF crack nucleation under blunt notches [2, 191.Alternatively, if crack nucleation took place at the old crack tip, a ledge-like fracture morphology would result [Fig. 16(d)]. Some examples of this morphology have also been observed on fracture surfaces [Fig. 12(b)]. Kameda 1201 has reported a recent AE/fractographic study of slow intergranular crack growth by hydrogen embrittlement mechanisms in “pure” and phosphorus-doped NiCr steel alloys. The
Fig. 15. Summary of AE amplitude Per unit area detected from each type of fracture process. (a)
brittle intergranular fracture at ambient temperature, consistent with their relative crack propagation velocities. Although not fully equivalent to the ambient brittle intergranular fracture, the output from HTBIGF is much closer to that from the brittle mechanism (i.e. ~50%) than that from the ductile mechanism. A more quantitative analysis would require the geometry of crack growth (e.g. single or multiple initiation sites), the crack tip stress conditions and the crack acceleration to be taken into account 1121. However, at this stage it is apparent that the AE observations from HTBIGF describe a step-wise mode of cracking in which each step has a high degree of brittle character. Consideration of the AE results discussed above, together with detailed fractographic and surface chemical analyses from parallel studies [8,9], enable the following mechanism for HTBIGF to be proposed. The fracture process begins with a stationary, stressed, and therefore blunted, intergranular crack [Fig. 16(a)]. The grain boundary is already enriched homogeneously by phosphorus, which segregates under thermal activation. Dynamic segregation of sulphur occurs under the influence of crack tip stress, producing a concentration of sulphur which is increased locally on the crack faces and in front of the crack tip over a distance of a few microns [Fig. 16(b)]. The sulphur enrichment increases with time until its embrittling effect, added to that of the pre-segregated
Crack
Grain-
boundary
‘~General phosphorus enrichment c
(b)
Dynamic sulphur enrichment m
(cl JJ
cusps
id) U I / Ledges
Fig. 16. Schematic diagram (not to scale) illustrating the possible mechanisms of HTBIGF and related fractographic features.
452
HIPPSLEY ef al.:
DYNAMICS
OF HIGH TEMPERATURE
phosphorus-doped alloy exhibited a high AE signal and featureless facets. It was concluded that crack growth occurred by bursts of fracture across whole facets, and was an essentially brittle process. The “pure” alloy exhibited an order of magnitude less AE and facets covered with cusps, The cusps were similar in appearance to those observed on HTBIGF facets, but were more closely spaced (i.e. < 1 pm apart), and were not necessarily aligned normal to the direction of crack growth. An examination of the underlying microstructure showed that these cusps were probably associated with martensitic laths. Microcracks were considered to nucleate in front of the main crack and coalesce in an essentially ductile process. The examination of HTBIGF in this study has revealed some similarities to the hydrogen embrittlement observations, but also the following important differences: (i) The HTBIGF mechanism was acoustically active, giving approximately 50% of the truly brittle fracture signal per unit area, and was therefore of a relatively high brittle character. However, cusps were also observed on the intergranular fracture facets, suggesting a stepwise mode of crack propagation of unit increment less than the facet diameter. (ii) The cusps observed on HTBIGF facets were not associated with any microstructural feature, but were controlled by the extent of grain boundary embrittlement local to the crack tip. Hence although the HTBIGF process displays some features of both hydrogen emb~ttlement mechanisms described by Kameda, it appears to be a distinct mechanism in its own right. The next phase of this study will compare acoustic emission data from a range of slow crack growth processes, including HTBIGF, hydrogen embrittlement and stresscorrosion cracking in order to clarify further the mechanisms of fracture. 5. CONCLUSIONS 1. High temperature brittle intergranular fracture (HTBIGF) generated detectable acoustic emissions. The rate at which acoustic events were produced was directly proportional to the rate of overall crack propagation. 2. The average crack increment per acoustic event correlated well with the spacing of striations observed on the fracture surface, indicating that HTBIGF occurred by a step-wise m~hanism. 3. The HTBIGF crack appeared to be stationary for a discrete period, Al, before jumping by an
BRITTLE FRACTURE
increment, Aa. The crack increment was relatively constant, while the delay, At, was inversely proportional to the overall crack velocity. 4. A semi-quantitative analysis of the magnitude of acoustic emissions from various fracture processes confirmed that HTBIGF was of a brittle character when compared first with brittle intergranular fracture at ambient temperature, and secondly with ductile cavity growth at high t~~rature. HTBIGF generated about 50% of the emission from ambient temperature brittle intergranular fracture, and at least an order of magnitude more than that from ductile cavity growth at high temperature. 5. These observations were consistent with the models of HTBIGF in which the crack is enabled to propagate in discrete, brittle steps by the dynamic, local segregation of sulphur to the crack tip region. Acknowledgement-Work described in this paper was undertaken as part of the Underlying Research Programme of the UKAEA. The authors arc grateful to Drs J. A. Hudson and S. G. Druce for helpful discussions.
REFERENCES C. A. Hippsley, H. Rauh and R. Bullough, Acta metall. 32, 1381 (1984). 2. J. Shin and C. J. McMahon Jr, Acfa mefall. 32, 1535 (1984). 3. I. W. Chen, Aeta metall. 34, 1335 (1986). 4. J. C. Ritter and R. McPherson, Metals Tech. 1, 506 (1974). 5. C. A. Hippsley, J. F. Knott and B. C. Edwards, Acta metall. 28, 869 (1980). 6. J. Shin, J. Kameda and C. J. McMahon Jr, Micro and Macro techie of Crack Growth (edited by Sadamenda et al.), p. 149. T.M.S.-A.I.M.E., Philadelphia, Pa (1981). I. A. G. Vinckier and A. W. Pense, Weld. Res. Council Bull. 197 (1974). 8. P. Bowen and C. A. Hippsley, Acta metall. To be nublished. 9. C, A. Hippsiey, Acta met&. 35, 2399 (1987). IO. A. R. Troiano. Tram. Am. Sot. me&Is 52. 54 (1960). Il. C. B. Scruby and H. N. G. Wadley, Prog. ~uc~. kner& 11, 275 (1985). 12. C. B. Scruby, Research Techniques in Nondestructive Testing (edited by R. S. Sharpe), Vol. 8, p. 141 (1985). 13. C. B. Scruby, K. A. Stacey and G. R. Baldwin, J. Phys. D 19, 1597 (1986). 14. A. E. Powers, J. Iron. Steel. inst. 186, 323 (1957). 15. C. B. Scruby, H. N. G. Wadley, R. J. Dewhurst, D. A. Hutchins and S. B. Palmer. Mater. Evai. 39. 1250 (1981). 16. H. N. G. Wadley, D. Furze, C. B. Scruby and B. L. Eyre, Mends Sri. J. 13, 451 (1979). 17. J. N. Clark CEGB Rep. R/M/R279 (1979). 18. S. Kirihara, T. Yoshioka, M. Siga and M. Sukekawa, J. foe. Muter. Sci. Jan. 33, 371 (1984). 19. J. J. Lewandowski, C.-A. Hippsley, MI B. D. Ellis and J. F. Knott, Actu metall. 35, 593 (1987). 20. J. Kameda, AC~Q meiull. 34, 1721 (1986). I.
1988
Oool-6160/8X 163.00+ 0.00 Pergamon Journals Ltd
A STUDY OF THE DYNAMICS OF HIGH TEMPERATURE BRITTLE INTERGRANULAR FRACTURE BY ACOUSTIC EMISSION
Materials
C. A. HIPPSLEY, D. J. BUTTLE and C. B. SCRUBY Physics and Metallurgy Division, Harwell Laboratory, Didcot, Oxon OX11 ORA, England (Redoed
25 Murrh
1987)
Abstract-This paper presents a study of high temperature brittle intergranular fracture (HTBIGF) in which acoustic emission (AE) data and fractography were used together to make deductions about the dynamics of fracture. The AE event rate was proportional to the rate of overall crack growth, and the average crack increment per AE event correlated well with spacing of striations observed on the fracture surface. The AE from HTBIGF was also compared with ambient temperature brittle intergranular fracture and high temperature ductile fracture. The HTBIGF generated about 50% of the emission 01 fully brittle fracture, and at least an order of magnitude more than that from ductile fracture. The observation of discrete bursts of AE of this magnitude, coupled with the striations on the fracture surface, indicates that HTBIGF takes place in discrete brittle steps. This is consistent with the segregation of sutphur as an embrittling species to the region of the crack tip, promoting stepwise decohesion, rather than a continuous crack growth process. RCsumkNous presentons dans cet article une etude de la rupture intergranulaire fragile $ haute temperature (RIGFHT), od l’kmission acoustique (EA) et la fractographie ont Cte utilisees de pair pour obtenir des informations sur la dynamique de la rupture. La vitesse de I’EA est proportionnelle a la vitesse de croissance globale de la fissure, et I’accroissement moyen de la vitesse pour chaque EA correspond bien a l’espacement des stries observees sur la surface de rupture. Nous avons Cgalement compare I’EA provoquee par la RlGFHT a la rupture intergranu~aire fragile B la temp&ature ambiante, et i la rupture ductile a haute temperature. La RIGFHT g&n&e environ 50% de I’emission de la rupture entitrement fragile, et au moins dix fois plus que Yemission de ia rupture ductile. L’observation d’ondes disc&es d’EA de cette amplitude, jumelte a l’observation de stries sur la surface de rupture, montre que la RIGFHT a lieu par &tapes fragiles individuelles. Ce fait est en accord avec la segregation du soufre, espece fragilisante, vers l’extremite des fissures (ce qui provoque une d&cohesion par itapes successives) plutot qu’avec un mecanisme de croissance continue des fissures. Zus~~f~ung-In dieser Arbeit wird der intergranulare Hochtem~ratur-Spr~dbruch untersucht; hierzu werden Ergebnisse der akustischen Emission und fraktografische Beobachtungen zur Ermittlung der Dynamik des Bruchvorganges zusammen benutzt. Die Rate der akustischen Emissionsereignisse war proportional zur Gesamtrate des RiOfortschritts; die mittlere RiBvergroBerung pro akustischem Emissionsereignis korrelierte gut mit dem Abstand der auf der Bruchflkhe beobachteten Streifungen. Die akustische Emission des intergranularen Hochtemperatur-Sprodbruches wurde such mit intergranularem Sprodbruch bei Raumtemperatur und mit dem duktilen Hochtemperaturbruch verglichen. Beim intergranularen H~htem~ratur-Spr~dbruch wurde etwa 50% der akustischen Emission des volist~ndig spriiden Bruches erzeugt und wenigstens eine Gr~~nordnung mehr als h&m duktilen Bruch. Die Beobachtung diskreter Ausbriiche an akustischer Emission dieser Gr6Benordnung, gekoppeit mit den entsprechenden Streifungen an der Bruchoberflkhe. zeigt, da13 der intergranuiare HochtemperaturSprddbruch in diskreten spriiden Schritten ablauft. Diese Folgerung ist vertrtiglich mit der Segregation des Schwefels als verspriidendes Element im Bereich der R&pi&e, wodurch eine schrittweise Dekohasion gefijrdert wird, und nicht ein kontinuierlicher ProzeB des RiBwachstums. 1. fNTRODU~TION
High Temperature Brittle Intergranular Fracture (HTBIGF) has been recognised in recent years as a significant mechanism of failure in ferritic alloy steels [I-3]. It was first identified as one of two fracture mechanisms involved in stress-relief cracking of weld heat-affected zones 14-61, the other mechanism being intergranular microvoid coalescence [7]. Detailed studies of HTBIGF under isothermal conditions have shown that it occurs by slow crack propagation ( 10e9 to 10e5 ms-‘) in the presence of a stress concentrator at temperatures in the range 300~650°C. The rate of crack growth is a function of both the applied
stress-intensity and prior austenitising temperature (illustrated in 2fsCr 1Mo steel by Fig. l), (81 and also depends on the segregation of residual embrittling impurities to grain boundaries and to the crack tip region under stress [ 1,2]. Scanning Auger Microprobe (SAM) examination has revealed a general “background” segregation of phosphorus IS], together with local concentration of sulphur near the growing HTBIGF crack tip [ 1,2,9]. Dynamic segregation of sulphur to the crack tip region at elevated temperature under the influence of the local stress-field is envisaged, in a manner analogous to the more familar mechanism of hydrogen embrittlement at ambient temperature [IO]. 441
HIPPSLEY et al.: Austenltising .1300, o 1300, . 1300,
DYNAMICS OF HIGH TEMPERATURE
treatment
..
WQ FC 1200, WC’ FC 1100, WQ
/
.
I 0
I 20
Appkd
‘*
I 30
I 40
I 50
stress
henshy,
I 60
I 70
80
K, (MN rn-“‘)
Fig. 1. Example of crack growth behaviour for different final austenitising temperatures (tested in vacuum) [8].
The following models of the overall cess have been proposed:
fracture
transducers attached to the surface of the material. Broadly speaking, the amplitude of the detected AE depends upon the size and lifetime of each deformation or fracture event. Furthermore, AE is only emitted when a crack advances, and not when it is static [1 I]. Thus careful measurements of the emission activity as a function of stress intensity or time, for instance, can be used to give insights into the dynamics of fracture. Full characterisation of each source event has been demonstrated using carefully chosen specimen geometries and broadband detection systems [12, 131. For the present study the waveguide technique necessitated by the high temperatures precludes the use of this quantitative approach. Nevertheless, some semi-quantitative information can be obtained from the AE data by careful calibration of the detection system. Clearly, an indication of whether the crack grows continuously or in steps, and of the true degree of its brittle character relative to well characterised lower temperature brittle fracture, would help to clarify the mechanism of HTBIGF. The present study applies acoustic emission (AE) as a laboratory tool, in combination with fractographic analysis, to address these questions.
2. EXPERIMENTAL
pro2.1. Material
(i) Sulphur in solid solution is driven by the crack tip stress field under “pure-drift” conditions to concentrate at the stationary crack, until local grain boundary cohesion is reduced sufficiently to enable a discrete increment of crack growth into fresh, unembrittled material (stepwise cracking) [ 11. (ii) Small sulphides on the crack faces dissolve on exposure, freeing elemental sulphur. The crack is treated as a giant, acute cavity, and sulphur is swept into the tip by the cavity growth process [2,3]. This has the effect of either (a) reducing cohesion to produce stepwise growth as for (i), or (b) reducing the cavity tip llank-angle to accelerate continuous cavity growth. If there are sudden changes in the internal stress field in a material, caused for instance by the propagation of a crack of slip band, then some of the stored elastic energy is dissipated as elastic waves (stress waves). Depending upon their amplitude these can be detected as acoustic emissions (AE) by piezoelectric
Table 1. Composition
BRITTLE FRACTURE
and heat treatment
A commercial 2$r 1Mo steel was employed, as used for several previous SAM and fractographic investigations of HTBIGF. The bulk chemical composition is given in Table 1. Fracture testpieces were machined to a half-scale compact tension design ($T). Three batches of specimens were prepared for testing: the first, to be used for analysis of HTBIGF, was austenitised at 1300°C and water-queched (type WQ); the second, to provide a comparison with brittle intergranular fracture at ambient temperature, was again austenitised at 1300°C and waterquenched, but also tempered at 650°C for 500 h and temper-embrittled by means of a step-cooling treatment [14] (type TE); the final batch, to provide a comparison with ductile cracking by cavitation at elevated temperature, was left in the as-received condition, i.e. rolled plate (type AR). 2.2. Acoustic
emission and mechanical
testing
The apparatus used to monitor AE during hightemperature crack growth is illustrated schematically in Fig. 2. This will henceforth be referred to as system
of 2$r
1Mo steel by wt%
Cr
MO
Mn
s
Ni
CU
V
W
Nb
2.12 Sb to.005 C 0.13
0.95 Sn 0.010 N 0.006
0.53 P 0.015
0.17 S 0.021
0.20 Ti
0.15 Zr
<0.02 co
HIPPSLEY
er al.:
DYNAMICS
OF HIGH TEMPERATURE
BRITTLE
FRACTURE
443
Platinum
I
woveguide
1
Load Discriminator/ counter
~
ji+I
I
I
0 CT
oc;i-
_
Charge ampiifie
ir
r
Digitolage
ii
Constant current suPPlY
oscilloscope
Micro -volt meter
Fig. 2. Acoustic emission detection system A. A. Acoustic pulses from the specimen were transmitted to a broad-band transducer through a 2mm diameter platinum waveguide. The waveguide was screwed into the specimen, and gave a 60% transmission efficiency for a “burst” of acoustic emission as measured during calibration tests with an artificial acoustic source (pulsed neodymium-YAG laser [ 151). The transducer output signal was passed through a charge amplifier and a 0.1-l MHz filter before final amplification ( x 3100 gain) and processing by a r.m.s.d.c. converter. The r.m.s. signal pulse was recorded both directly, and via a pulse counter and a signal integrator, on a multi-channel Y/t chart recorder. The waveform of the acoustic burst, including the ringing associated with the specimen and waveguide, could also be recorded on a high-speed digital storage oscilloscope @SO). Crack growth during high-temperature testing was monitored by measuring the potential drop across the crack induced by passing a constant current through the fCT specimen. This arrangement is also illustrated in Fig. 2. The potential drop signal was recorded, after amplification, simultaneously with the AE signal on the Y/t chart recorder, and could be converted to values of crack length using a suitable calibration. The following experiments were performed to study fracture at elevated temperature: (if Tests at 400 and 500°C to assess detectability of AE during HTBIGF of WQ specimens, and to measure the rate of AE pulses together with the rate of crack growth as a function of applied stress-intensity.
(ii) Tests at 500°C to verify the source of AE pulses by (a) examining in detail the pulse waveform recorded on the DSO, and (b) comparing AE from HTBIGF in WQ specimens with AE from a fracture process which is known to be acoustically quiet (i.e. ductile cavity growth [16] in the AR specimens). The $T specimens were pre-fatigued before testing at a maximum stress intensity of 30MN rnw3j2at room tem~rature. All the high temperature tests were performed under constant load conditions using a Mayes dead-weight creep testing machine. This technique produced an applied stress-intensity which rose with crack growth from 40 to 80MNm-3/“. Following the elevated temperature tests, some further tests were performed on the embritted (TE) material at ambient temperature. Although use of a waveguide precludes fully quantitative AE measurements, an empirical comparison between AE during HTBIGF and AE from well characterised brittle intergranular fracture at lower temperature is possible. A second AE detection system (system B) was used (Fig. 3) which excluded the furnace and potential drop equipment, and employed three ampliiication channels for the AE signal covering a range of gain factors ( x 1, x 13 and x 680). The charge amplifier was also modified to reduce its own gain by a factor of 3.85. This difference in charge amplifier gain between the two systems was verified by producing an acoustic event of repeatable magnitude on the $t specimen (i.e. a laser pulse), and measuring the output of each charge amplifier in turn. The range of amplifier gains in the second system was necessary in
HIPPSLEY
444
et al.:
DYNAMICS
OF HIGH
TEMPERATURE BRITTLE FRACTURE
Platinum
Amplifier
/
R”,‘.,
DC
Load
0 CT 0
~
RMS -DC unit
Speci men
:
Chart recorder
Digital storage oscilloscope
Fig. 3. Acoustic emission detection system B.
order to capture the emissions from isolated brittle “pop-in” events in the TE specimens, whose magnitude could potentially vary widely, depending on the fracture event area. The !$T specimens were again pre-fatigued, but required charging with hydrogen- to produce brittle intergranular fracture on loading at ambient temperature. This was achieved using an electrolytic cell containing 10% sulphuric acid together with 1 gl-’ of thiourea as a hydrogen recombination poison. The specimens were tested in an Instron screw-driven machine at a constant cross-head displacement rate until the first pop-in event was detected. They were later broken open by fatigue loading, and the extent of the brittle pop-in measured optically In order to accurately compare the wide range of AE signals detected by systems A and B (Fig 2 and 3), calibration of both systems by a known input signal was required. This was accomplished by injecting a burst of sinusoidal waveform into the 0.1-l MHz filter of a similar frequency and duration as a typical AE pulse (plus ringing of specimen and waveguide) from a fracture event, i.e. 500 kHz over 10 ms. The peak-to-peak amplitude of the input pulse could then be varied to produce a range of r.m.s. pulses on the chart recorder, covering the signal amplitudes observed during each type of fracture test. The extended duration of some elevated temperature tests made use of the signal integrator impractical due to drift caused by variations in the r.m.s. signal background. Hence the direct r.m.s. pulse height, as measured from the chart recorder trace, was employed exclusively.
2.3. Fract~grup~y Specimens from all three conditions were broken open after testing, and examined in an Hitachi S570 scanning electron microscope (SEM) to verify the mechanism of fracture. Specimens given a similar heat treatment to the WQ condition, but tested in uucuflltz at 500°C as part of a parallel investigation of HTBIGF[8] were also examined in the SEM. Fine details of the fracture process were preserved in these specimens, and could be correlated with AE observations. Some of these fracture surfaces were nickelplated and sectioned in a plane normal to that of fracture and the crack growth direction. They were then pofished and etched in Nital for optical and SEM examination.
3. RESULTS 3. I. High te~p~r~t~re tests Specimens in the WQ condition (likely to manifest HTBIGF) tested at both 400 and SOO”C emitted detectable AE events during crack growth. Figure 4 shows a sample of a typical trace recording r.m.s. pulses. The following parameters were determined after successive increments of crack growth in each test: stress-intensity (K) crack increment (Au) time increment (At) event count per inurement (A~) crack growth rate (dda/dt)
HIPPSLEY et al.: DYNAMlCS
OF HIGH TEMPERATURE
/ 70
r
1
’ 1
?.
L
5
/ I ldd J
i 75
! 85
I 80
tog (K)
Fig. 5. Crack growth rate (da/dt) and AE event rate (dN/dt) as a function of applied stress intensity (K) for two tests at 400 and 500°C.
1
0.9mm Crack
445
BRITTLE FRACTURE
~
increment
Fig. 4. Example of acoustic emission detected HTBIGF in material WQ.
during
event count rate (d~Jd~) mean crack increment per event (Aa/A~) mean time increment per event (At/AN). Table 2 gives a detailed breakdown of these parameters for a test performed at 500°C. The increments of crack growth (Au) were chosen arbitrarily. It is clear that the rate of emission of events greater than the noise threshold (dNjdt) increased with crack growth rate and applied stress-intensity. Figure 5 plots log(dN/dt) and log(da/dt) vs log(K) for a test at 400 and 500°C. The approximately linear relationship between dN/dt and da/dt is illustrated in Fig 6 (one test at 4oo’C and two tests at 500°C). The waveform recorded for a typical event during crack growth in a sample of condition WQ is shown in Fig. 7(a). This includes the initial pulse from the fracture event and subsequent ringing in the s~cimen/rig~waveguide system which dies away exponentially with time. The waveform therefore has a
I
I
-7
-6 log
I -5
(do/dt)
Fig. 6. AE event rate (dN/dt) as a function of crack growth rate (dn/dt) for three tests at 400 and SOO~‘C.
duration of approximately 10 ms. Figure 7(b-d) compares waveforms on the same timescale. which were derived from (b) an artificial event on the specimen (a pencil lead break), (c) specimen grip noise induced by rapid changes in applied load and (d) electrical noise from the creep machine ham-ievelling device.
Table 2. Summary of data for one HTBIGF crack growth test at SO0 C K
(MN
=)
56.0
58.0 60.5 63.0 65.1 69.5
All (pm)
A.r (s)
AN (events)
da idt (urns I)
dN/dt levents s ‘)
Au,lAN (urn event I)
AI /AN (sevent ‘I
380 360 350 350 360 320
684 480 312 150 80 SO
43 41 41 44 45 41
0.56 0.75 1.1 2.3 4.5 6.4
0.063 0.085 0.131 0.293 0.562 0.820
8.8 8.8 8.5 X.0 X.0 7.a
16 12 7.7 3.5 1.8 1.3
HIPPSLEY et ai.:
446
‘I I’
10ms
DYNAMICS OF HIGH TEMPERATURE BRITTLE FRACTURE
I
?Oms
(a)
.
1Oms
_,
(b)
IOms
I
(c)
.
(d)
Fig. 7. Total AE waveforms obtained from (a) HTBIGF, @) pencil lead break, (c) specimen grip noise, (d) electrical interference. There is clearly a similarity between the true and artificial fracture events [Fig. 7(a) and (b)] and a difference between these and both types of noise [Fig. 7(c) and (d)] in the waveform shape and rise time. In two WQ tests at 500°C the waveform characteristics were monitored, using the larger events to trigger the DSO. Of these, more than 75% were similar in character to the fracture event waveform. Testing of specimens in the AR condition (likely to manifest ductile cavitation) produced crack growth rates an order of magnitude less than those observed in the WQ specimens tested over the same K and Au range. This is consistent with the greater ductility of the fracture mechanism in these specimens. However, this resulted in a greater contribution of spurious noise to the overall AE signal in AR specimens than in WQ specimens. In order to correct for this effect, and enable an accurate comparison of the true AE from each fracture mode to be made, the noise signal alone was measured during a period when both specimens were under load at temperature, but no crack growth was detected. These signals were scaied to the time period over which the comparison was made, and subtracted from the overall signal obtained during crack growth. A graphical illustration of the result of this process is given in Fig. 8. The AE energy measured from crack growth in the AR specimen was signi~cantly smaller than that measured from the same extent of cracking in the WQ
Cavity
growth
HTBIGF
$
3 .?i
E
0 8
a
4
1,
@37mm Crack
0.37mm
J
increment
Fig. 8. Comparison of AE from cavity growth and HTBIGF over the same crack increment at 500°C.
HIPPSLEY
et 01.:
DYNAMICS
OF HIGH TEMPERATURE
BRITTLE
FRACTURE
447
/Pop-in
System
0
IO
20
30
Simulated
40 i/p
50
60
(mV p-p
1
50
40 -
70
x(b)
x / x
30 -
System B
/ x
20Pop-in
A
event IO -
J x'
0
500
I 1000 Simulated
Fig.
Displacement
Fig. 9. Example of load/displacement plot for ambient temperature test of material TE, together with typical AE waveform from brittle intergranular pop-in (inset).
specimen. A repeat of this comparative gave a similar outcome.
measurement
I 1500 i/p
I 2000
I 2500
? 00
(mV p-p)
10. Calibrations of r.m.s. output (chart recorder) vs simulated AE source input for systems A and B.
and the nominal stress-intensity at fracture (i.e. not necessarily valid according to BS 5447), is given in Table 3 for six tests. The relatively large size of these events made saturation of the AE detection system (type B, Fig. 3) a possibility. However, examination of the waveform captured on the DSO from the lowest gain channel in system B showed no evidence of saturation (Figure 9).
3.2. Ambient temperature tests
3.3. Calibration of AE detection systems
Gradual loading of embrittled TE specimens, charged with hydrogen, produced a pop-in of brittle crack growth at ambient temperature preceding or coincident with the maximum achievable load. A sample load vs displacement plot is shown in Fig. 9. This type of test gave rise to a single large AE event, in contrast to the many smaller events observed during HTBIGF. The event size is expressed as the r.m.s. peak height measured from the Y/t recorder and normalised to the calibration source (vide 3.3). This, together with the corresponding fractured area
The amplitude of the artificial electrical source (500 kHz gated cw pulse of 10ms duration) was adjusted to produce a response on the Y/t recorder covering the range of experimental observations obtained from high ambient temperature tests using systems A and B. Plots of source peak-to-peak (pp) voltage vs recorded r.m.s. peak voltage are given in Fig. 10 for system A (a) and the lowest gain channel of system B (b). This calibration was used to normalise all high temperature data (system A) and ambient temperature data (system B) to the
Table 3. Summary of data normalised to artificial calibration source for ambient temperature tests (TE material-Kq refers to nominal stress-intensity at pop-in) Kq (MN mm3’2) 88 110 88
15 95 70
r.m.s. peak (mV) 5.0 8.3 17.7 15.7 22.0 26.5
Artificial source equivalent (mV)
Pop-in area (mm*)
AE per unit area (mV mmm2)
735 910 1315 1230 1445 1595
37.3 45.1 40.9 58.0 63. I 66.0
19.7 20.2 32. I 21.2 22.9 24.2
HIPPSLEY
448
el 01.:
Table 4. Summary
DYNAMICS
of data normalised
Material/Test WQ-HTBIGF at WQ-HTBIGF at WQ-HTBIGF at WQ-HTBIGF at WQ-HTBIGF at WQ-HTBIGF at AR-cavity growth
OF HIGH
500°C 500°C 500°C 500°C 500°C 500°C at 500°C
TEMPERATURE
to artificial
calibration
BRITTLE
source for high temperature
K (MN m-3i2)
Increment in crack area (mm2)
ZAE (artificial source equivalent i 3.85 mV)
56.0 58.0 60.5 63.0 65.7 56-70 56-70
4.86 4.61 4.48 4.48 4.61 24.3 24.3
49.9 51.4 52.6 46.1 58.8 170 g20.5
equivalent artificial source amplitude. For a given increment of crack growth in the AE detectability study (Table 2) and the comparison of WQ and AR cracking (Fig. 8) at SOOC, each r.m.s. pulse recorded by system A was converted to an equivalent artificial source p-p value (also taking into account the additional factor of 3.85 between system A and B charge amplifier gains). The sum of these values over each crack increment gave an average magnitude of AE emission per cracked area for the high temperature fracture mechanisms (summarised in Table 4). This could be compared directly with the normalised individual event values for the ambient temperature tests given in Table 3.
FRACTURE tests
AE per unit area (mVmmm2) 10.3 12.5 II.5 10.3 12.7 7.0 $0.8
ations on the intergranular facets which were orien ted normal to the direction of crack growth (Fig. 12). I ‘he striation spacing was in the range 2-9pm, and tdid not vary noticeably with applied stress-intensity or crack growth rate. Observation of these features in
3.4. Fractography Inspection of fracture surfaces from the high temperature tests performed in air confirmed that the WQ specimens exhibited HTBIGF, while the AR specimens exhibited ductile cavitation [Fig. 1l(a, b)]. The TE specimens charged with hydrogen and tested at ambient temperature exhibited 100% brittle intergranular fracture in the pop-in region [Fig. 11(c)]. The high resolution study of HTBIGF produced at 500°C in a vacuum [8] revealed the presence of stri-
Fig.
Il. SEM fractographs (b) AR-cavity growth
illustrating fracture modes in AE specimens: (a) WQ-HTBIGF at SOO”C, at 5OO”C, (c) TE-brittle intergranular pop-in at ambient temperature.
HIPPSLEY ef al.:
DYNAMICS OF HIGH TEMPERATURE BRITTLE FRACTURE
449
section (Fig. 13) indicated that they were cusps in the order of 1 pm in height. The cusp position on the facets did not correspond to any obvious microstructural feature, such as a lath or packet boundary, and neither did the cusp spacing reflect the lath width (0.08 pm), packet size (100pm) or grain size (250 v m) ]8]. 4. DISCUSSION 4.1. Verification of AE measurements
It is clear from Fig. 4 that significant levels of AE were detected during fracture of WQ specimens at 400 and 500°C. This is consistent with previous observations of AE during stress-relief cracking and creep crack growth in low alloy steels [17,18],
Fig. 12. SEM fractographs showing HTBIGF from vacuum test at 500°C: (a) general intergr~uIar, (b) detail of grain boundary facet with striations.
Fig. 13. Micrographs of HTBIGF in section: (a) optical micrograph of Ni-plated fracture surface showing cusps, (b) and (cf SEM micrographs of (a) showing detail of cusps.
450
HIPPSLEY et al.: DYNAMICS OF HIGH TEMPERATURE BRITTLE FRACTURE
although the authors are unaware of any study to date aimed specifically at the high temperature brittle inter~anular fracture mechanism, HTBIGF. Three specific results point to the AE detected originating with the HTBIGF fracture process, rather than spurious noise sources, giving confidence to its use as an analytical tool. First, Figs 5 and 6 show that the AE count rate (d~/dt) correlated closely with the rate of crack propagation (da/dt). The gradients of the log(dhr/dt) vs log(da/dt) plots in Fig. 6 varied between 0.85 and 1.05, indicating that, within experimental error, the count rate was directly proportional to the crack growth rate. This would be expected if the AE was produced by a series of discrete fracture events, which together resulted in the overall crack growth. Secondly, most of the waveforms observed during HTBIGF [Fig. 7(a)] were typical of a fracture event, rather than the various possible sources of spurious noise [Fig. 7(c,d)]. Although propagation through the waveguide tends to obscure source differences, it is also interesting to note that the AE signals from HTBIGF were similar in character to those from brittle intergranular fracture at ambient temperature[Figs 7(a) and 91. Finally, a comparison of AE from HTBIGF with that from a fracture process which is known to be of low acoustic energy (i.e. cavity growth f16]), showed an approximately tenfold increase in magnitude of emissions from the HTBIGF process. This comparison was made under the same conditions of stress-intensity and crack increment, and therefore must reflect intrinsic differences in the two crack growth mechanisms. 4.2, Correlation fractography
between
AE
observations
and
The primary implication of the detection of discrete AE bursts is that HTBIGF occurs by the summation of many small fracture events. This is consistent with the step-wise models of crack growth [l, 21. If it is assumed that each emission corresponds to one unit of crack growth across the specimen, the average step size may be equated to Aa/AN in Table 2. In this manner, the AE observations predict a step size of ~8 pm which does not vary significantly with stress-intensity or crack growth rate. The SEM observations of HTBIGF facets showed striations after testing in vacuum which were 2-9 pm apart. The striation spacing also did not change significantly with stress-intensity or crack growth rate [8]. There is therefore a close correlation between the AE and SEM info~ation, which strongly supports the step-wise models of HTBIGF. 4.3. Implications for the mechanism of HTBIGF The evidence discussed above, supporting step-wise growth, indicates that the steady state cavity growth mechanism, suggested by Chen [3], is unlikely in
log( Au /At
f
Fig. 14. Average time between events At/AN) and average crack increment per event (Au/AN) as a function of crack growth rate (da/dr), for a test at 500°C.
24 Cr 1MO steel. The two mechanisms in which grain boundary cohesion is reduced by the dynamic segregation of sulphur, enabling the crack to propagate a discrete distance, are more consistent [1,2]. These models differ regarding the source of sulphur and the mechanism of segregation, and are discussed in detail in Ref. [9]. The observation of a constant step-size (Table 2) means that the average time between steps (Atlas) must vary with stress intensity and crack growth rate (Table 2). This is illustrated in Fig. 14, which plots both step size and step time interval as a function of crack growth rate for a test at 500°C. The range of step time intervals of approximately 1-20s has important application to the modelling of stress-driven sulphur segregation to crack tips. The “pure-drift” model of Hippsley et al. [l] assumed originally a constant value of 1s. This choice now appears to be physically reasonable in magnitude, although the step time interval should be considered a variable in future refinements of the model. An estimate of the degree of brittleness associated with HTBIGF steps may be obtained by comparison with AE from the ductile cavity growth mechanism (AR specimens) and the truly brittle intergranular pop-in fracture in TE specimens at ambient temperature. The acoustic energy emitted from a fracture event depends on the velocity of crack growth [12]. Brittle intergranular fracture involves very limited plasticity, and takes place at a velocity approaching that of sound in the fractured material (i.e. N 1000 ms-t). Cavity growth requires extensive plastic deformation and /or diffusion, and occurs cor~spondin~y slower at < 10 ms-’ [12]. Figure 15 summarises the amplitude of AE per unit area of crack growth, normalised to the artificial source amplitude, for each of the fracture mechanisms observed. Only an upper limit to the cavity growth output could be given due to the low signal to noise ratio obtained. This is approximately two orders of magnitude below the signal given by the
HIPPSLEY et al.:
DYNAMICS
Brittle intergranular fracture (ambient) -- I
HTBIGF
__
OF HIGH
--
_.
(500°C) I
I
Cavity growth (500°C)
TEMPERATURE BRITTLE FRACTURE
451
phosphorus, reduces grain boundary cohesion in the crack tip region to a level below which crack propagation can occur under the prevailing crack tip stress conditions. The crack then grows at a velocity of some hundreds of metres per second into fresh material, which has not been enriched by sulphur and is therefore tougher, and is arrested. This cycle is repeated to produce step-wise HTBIGF. It is un~rtain at present whether the new crack nucleates in front of the old crack or at the crack tip. If nucleation takes place in front of the crack tip, the new crack would grow backwards to coalesce with the old, forming cusps at the point of coalescence [Fig. 16(c)], as observed in some sections (Fig. 13). This process has been observed on a larger scale in studies of HTBIGF crack nucleation under blunt notches [2, 191.Alternatively, if crack nucleation took place at the old crack tip, a ledge-like fracture morphology would result [Fig. 16(d)]. Some examples of this morphology have also been observed on fracture surfaces [Fig. 12(b)]. Kameda 1201 has reported a recent AE/fractographic study of slow intergranular crack growth by hydrogen embrittlement mechanisms in “pure” and phosphorus-doped NiCr steel alloys. The
Fig. 15. Summary of AE amplitude Per unit area detected from each type of fracture process. (a)
brittle intergranular fracture at ambient temperature, consistent with their relative crack propagation velocities. Although not fully equivalent to the ambient brittle intergranular fracture, the output from HTBIGF is much closer to that from the brittle mechanism (i.e. ~50%) than that from the ductile mechanism. A more quantitative analysis would require the geometry of crack growth (e.g. single or multiple initiation sites), the crack tip stress conditions and the crack acceleration to be taken into account 1121. However, at this stage it is apparent that the AE observations from HTBIGF describe a step-wise mode of cracking in which each step has a high degree of brittle character. Consideration of the AE results discussed above, together with detailed fractographic and surface chemical analyses from parallel studies [8,9], enable the following mechanism for HTBIGF to be proposed. The fracture process begins with a stationary, stressed, and therefore blunted, intergranular crack [Fig. 16(a)]. The grain boundary is already enriched homogeneously by phosphorus, which segregates under thermal activation. Dynamic segregation of sulphur occurs under the influence of crack tip stress, producing a concentration of sulphur which is increased locally on the crack faces and in front of the crack tip over a distance of a few microns [Fig. 16(b)]. The sulphur enrichment increases with time until its embrittling effect, added to that of the pre-segregated
Crack
Grain-
boundary
‘~General phosphorus enrichment c
(b)
Dynamic sulphur enrichment m
(cl JJ
cusps
id) U I / Ledges
Fig. 16. Schematic diagram (not to scale) illustrating the possible mechanisms of HTBIGF and related fractographic features.
452
HIPPSLEY ef al.:
DYNAMICS
OF HIGH TEMPERATURE
phosphorus-doped alloy exhibited a high AE signal and featureless facets. It was concluded that crack growth occurred by bursts of fracture across whole facets, and was an essentially brittle process. The “pure” alloy exhibited an order of magnitude less AE and facets covered with cusps, The cusps were similar in appearance to those observed on HTBIGF facets, but were more closely spaced (i.e. < 1 pm apart), and were not necessarily aligned normal to the direction of crack growth. An examination of the underlying microstructure showed that these cusps were probably associated with martensitic laths. Microcracks were considered to nucleate in front of the main crack and coalesce in an essentially ductile process. The examination of HTBIGF in this study has revealed some similarities to the hydrogen embrittlement observations, but also the following important differences: (i) The HTBIGF mechanism was acoustically active, giving approximately 50% of the truly brittle fracture signal per unit area, and was therefore of a relatively high brittle character. However, cusps were also observed on the intergranular fracture facets, suggesting a stepwise mode of crack propagation of unit increment less than the facet diameter. (ii) The cusps observed on HTBIGF facets were not associated with any microstructural feature, but were controlled by the extent of grain boundary embrittlement local to the crack tip. Hence although the HTBIGF process displays some features of both hydrogen emb~ttlement mechanisms described by Kameda, it appears to be a distinct mechanism in its own right. The next phase of this study will compare acoustic emission data from a range of slow crack growth processes, including HTBIGF, hydrogen embrittlement and stresscorrosion cracking in order to clarify further the mechanisms of fracture. 5. CONCLUSIONS 1. High temperature brittle intergranular fracture (HTBIGF) generated detectable acoustic emissions. The rate at which acoustic events were produced was directly proportional to the rate of overall crack propagation. 2. The average crack increment per acoustic event correlated well with the spacing of striations observed on the fracture surface, indicating that HTBIGF occurred by a step-wise m~hanism. 3. The HTBIGF crack appeared to be stationary for a discrete period, Al, before jumping by an
BRITTLE FRACTURE
increment, Aa. The crack increment was relatively constant, while the delay, At, was inversely proportional to the overall crack velocity. 4. A semi-quantitative analysis of the magnitude of acoustic emissions from various fracture processes confirmed that HTBIGF was of a brittle character when compared first with brittle intergranular fracture at ambient temperature, and secondly with ductile cavity growth at high t~~rature. HTBIGF generated about 50% of the emission from ambient temperature brittle intergranular fracture, and at least an order of magnitude more than that from ductile cavity growth at high temperature. 5. These observations were consistent with the models of HTBIGF in which the crack is enabled to propagate in discrete, brittle steps by the dynamic, local segregation of sulphur to the crack tip region. Acknowledgement-Work described in this paper was undertaken as part of the Underlying Research Programme of the UKAEA. The authors arc grateful to Drs J. A. Hudson and S. G. Druce for helpful discussions.
REFERENCES C. A. Hippsley, H. Rauh and R. Bullough, Acta metall. 32, 1381 (1984). 2. J. Shin and C. J. McMahon Jr, Acfa mefall. 32, 1535 (1984). 3. I. W. Chen, Aeta metall. 34, 1335 (1986). 4. J. C. Ritter and R. McPherson, Metals Tech. 1, 506 (1974). 5. C. A. Hippsley, J. F. Knott and B. C. Edwards, Acta metall. 28, 869 (1980). 6. J. Shin, J. Kameda and C. J. McMahon Jr, Micro and Macro techie of Crack Growth (edited by Sadamenda et al.), p. 149. T.M.S.-A.I.M.E., Philadelphia, Pa (1981). I. A. G. Vinckier and A. W. Pense, Weld. Res. Council Bull. 197 (1974). 8. P. Bowen and C. A. Hippsley, Acta metall. To be nublished. 9. C, A. Hippsiey, Acta met&. 35, 2399 (1987). IO. A. R. Troiano. Tram. Am. Sot. me&Is 52. 54 (1960). Il. C. B. Scruby and H. N. G. Wadley, Prog. ~uc~. kner& 11, 275 (1985). 12. C. B. Scruby, Research Techniques in Nondestructive Testing (edited by R. S. Sharpe), Vol. 8, p. 141 (1985). 13. C. B. Scruby, K. A. Stacey and G. R. Baldwin, J. Phys. D 19, 1597 (1986). 14. A. E. Powers, J. Iron. Steel. inst. 186, 323 (1957). 15. C. B. Scruby, H. N. G. Wadley, R. J. Dewhurst, D. A. Hutchins and S. B. Palmer. Mater. Evai. 39. 1250 (1981). 16. H. N. G. Wadley, D. Furze, C. B. Scruby and B. L. Eyre, Mends Sri. J. 13, 451 (1979). 17. J. N. Clark CEGB Rep. R/M/R279 (1979). 18. S. Kirihara, T. Yoshioka, M. Siga and M. Sukekawa, J. foe. Muter. Sci. Jan. 33, 371 (1984). 19. J. J. Lewandowski, C.-A. Hippsley, MI B. D. Ellis and J. F. Knott, Actu metall. 35, 593 (1987). 20. J. Kameda, AC~Q meiull. 34, 1721 (1986). I.