A STUDY OF DEFOR~ATrON
AND FRACTURE
PROCESSES
IN A LOW-ALLOY
STEEL
BY ACOUSTIC
EMISSION
TRANSIENT
ANALYSIS
H. N. G. WADLEY*
* Metallurgy
Division.
and t Materials
and C. B. SCRUBYt
Physics
Division.
A.E.R.E.,
Harwell.
England
Abstract--A
broad-band
acoustic emission detection
system has been used to record the transient
surface displacements
due to the release of elastic waves during deformation
and fracture of EN3OA.
a low-alloy steel. Acoustic emission signals were recorded from material tested in the quenched,
tempered and temper-embrittled
conditions:
Tests were also performed
upon material which had been
hydrogen charged prior to testing.
The specimens which fractured
by ductile mechanisms
generated
acoustic emission transients
with
shorter measured risetimes on average than those which fractured by a brittle intergranular
mechanism
induced by hydrogen.
The risetimes of the transient
displacements
have been tentatively
interpreted
in terms of source lifetimes. whilst their amplitudes
have been related to source arcas, and hence
their time derivatives
to source velocities. The results suggest that the emission from the quenched
material is related to the fast shear of interinclusion
ligaments. while the source during intergranular
fracture is most probably
brain-boundary
decohesion
over areas of one to five grain facets. Source
identification
in the temper-embrittled
material is less certain because of secondary intergranular
cracking in an otherwise ductile fracture.
RCsumk--Nous avons utilisi’ un systkme de dttection d%mission acoustique $ large bande pour enregistrer les dGplacements
superficiels
transitoires
dus a I’tmission
d’ondes ilastiques
au tours de la
deformation
et de la rupture d’un acier f~libiement allid EN30A. Nous avons enregistre
les signaux
d’emission acoustique
dans le cas de rnat~riallx trempi-s. revenus. ou fragilis&. Nous avons egalement
etleciut des essais sur des mati-riaux pr&ablement
charges en hydrogene.
Les temps de montke des 6missions acoustiques
transitoires
des Cchantillons
dont la rupture ttait
ductile &Gent infirieurs en moyennc B ceux des echantillons
dent la rupture intergranulaire
fragile
6tait causee par I’hydrogkne. On a essaq& d’interpretcr
les temps de montCe des dCplacements
transitoires
2i partir de la duree de vie des sources. alors que I’on a relit! leur amplitude
;i la surface des sources.
et done leur dCriv6e par rapport
au temps ;I la vitesse des sources. Les r&hats
donnent d penser
que l’6mission dans materiaux
tremptis provient du cisaillement
rapide de ligaments reliant les inclusions. alors que dans le cas de la rupture intergranulaire
elle provient vraisemblablement
de la d&oh&
sion intergranulaire
suivant une B cinq facettes. identification
des sources dans les matCriaux fragilists
est moins sire, B cause de la fiss~lration intergranulaire
secondaire
dans un matiriau
yui prksenterait
~lutrement tine rupture ductile.
Zusammeufassung--Mit einem breibandigen
System zur Aufnahme akustischer
Erscheinungen
wurden
die transienten
OberflBchenverschiebungen.
erzeugt durch elastische Wellen wHhrend der Verformung
und des Bruches in niedrig legiertem Stahl EN30A. untersucht.
Die Signale wurden an Materinlien
aufgenommen,
die abgeschreckt.
getempert
und temperverspriidet
waren. Versuche wurden such an
Material durchgefiihrt,
welches vor der Messung mit Wasserstoff beladen worden war.
Proben,
die iiber duktile
Mechanismen
brechen. erzeugten
transiente
akustische
Ereignisse
mit
kiirzeren mjttleren Anstiegszeiten
als diejenigen. die iiber einen vom Wasserstoff
verursachten
intergranularen
SprGdbruch brachen. Die Anstiegszeiten
der Verschieb~jngstransienten
werden versuchsweise
mit Lebensdailern
von Quellen interpretiert.
wohingegen
deren Amplituden
auf die Quellbereiche
bezogen wet-den. demzufoige
deren Zeitableitungen
auf die Quellgeschwindigkeiten.
Die Ergebnisse
legen nahe, da8 die Schaliemission
der abgeschreckten
Proben mit der schnellen Scherung van BBndern
zwischen Einschliissen
zusammenhingt.
wohingegen
die Quellen wlhrend
des intergranularen
Bruchs
sehr wahrscheinlich
in Abliisung en an Korngrenzen
ijber Bereiche von I bis 5 Korngrenzfdcetten
betehen. Die Bestimmung
der Quellart in dem tempervcrspriideten
Material ist weniger sicher wegen
der sekundlren
intergranularen
RiBbildung beim sonst duktilen Bruch.
INTRODUCTION
waves [l-5]
which radiate from regions of rapid
strain relaxation. When these elastic waves reach the
surface of a body they cause displacements
that can
be detected by sensitive transducers
and are called
acoustic emissions.
There have been many studies of acoustic emission
During the deformation
and fracture of crystalline
materials. the elastic strain energy, stored within a
solid under load, can be released in a number of ways.
One part is dissipated
in the form of elastic
613
WADLEY
614
AND
SCRUBY:
DEFORMATION
over a period of about 20 years (for a review see
Ying [6]) and the emission detected has been qualitatively related to a number of micromechanisms
of
deformation
and fracture in metals. Recently, more
systematic studies [7-91 have shown that acoustic
emission can be a sensitive function of material composition and metallurgical
structure. This is considered to be because only the most abrupt and energetic processes of deformation
and fracture generate
detectable emission and these are very dependent on
metallurgical variables.
During the mechanical testing of metals, several different processes may contribute to the total acoustic
emission activity. It would clearly be an important
advance if each of the acoustic emission waveforms
detected could be analysed in terms of the source process dynamics. The way would then be open for
acoustic emission to be used both for the dynamic
study of deformation and fracture micromechanisms.
and for assessing the severity of defects generating
acoustic emission in important engineering structures
such as pressure vessels. This has been noted in a
number of recent papers[10-153
in which attempts
have been made to develop diagnostic
techniques
based on spectral or amplitude analysis.
The problems
encountered
in attempting
to
measure acoustic emission waveforms which can be
theoretically related to the source process, or in looking for changes in waveforms from different processes,
are severe [16,17]. However, a recent analysis has
shown that the use of a displacement-sensitive
broadband detection system, together with a specimen satisfying certain geometrical requirements, should enable
basic parameters, defining at least part of the source
processes, to be recorded [ I7f. Preliminary
experiments by these authors using a capacitance
transducer and a ‘Yobell’ specimen geometry have demonstrated that such measurements
can be made for the
more energetic emissions.
This paper describes the results of a series of experiments in which the same broad-band system was used
to record acoustic emission waveforms from a range
of fracture mechanisms.
It is shown that the parameters which define the measured waveforms are
sensitive to changes in microstructure
and’ fracture
mechanism, and a simple interpretation
of the data
is presented.
EXPERIMENTAL
PROCEDURE
The experiments were performed on a commercial
low-alloy steel designated
EN30A, supplied in the
form of 70 mm dia. bar. Its chemical composition,
as determined
by X-ray fluorescence
and emission
AND FRACTURE
STUDY
Plate sum&d
above
2-4
specimen
Cwn
(Dimensionsin millimetres)
Fig. 1. The testing configuration showing a ‘Yobell’ specimen in the grips and a capacitance transducer mounted
on the top face of the specimen.
spectroscopy, is given in Table 1. Seventeen cylindrical geometry
‘Yobell’ specimens,
with dimensions
shown in Fig. 1, were machined from the bar with
their tensile axis along the rolling axis. They were
then subjected to one of three types of heat treatment
to produce specimens in the quenched, tempered and
temper-embrittled
conditions (Table 2). All the specimens were electropoiished
to remove the brittle oxide
Layer and, in addition, the central area of the 60mm
dia. end of the specimen was lapped flat and polished
to iOS@m. Nine of the specimens, labelled DI to
D9, were tested in the as-heat-treated
condition; the
remainder, labelled HI to H8, were hydrogen embrittled before testing. Hydrogen was introduced into
the gauge region of the specimen by cathodic precharging in a 0.5 M sulphuric acid solution, poisoned
Table 1. The composition of EN30A (wt. ppm or wt.%)
Element
Concentration
Si
P
C
s
0.281,$ 0.037; 0.2u/, 130
Ni
4.24%
As
240
Sn
100
Cu
700
Cr
1.167;
Mn
0.49%
MO
0.07:;
Nb
<O.Ol%
Sb
<0.01%
WADLEY
Designation
Quenched
AND
SCRUBY:
Table
2. Heat
Heat
treatment
treatment
1 h.930
Oil quench
Oil quench >
D4
Tempered
DS
D6
D7
D8
D9
Temperembrittled
Specimen identity
(a) HZ free
(h) Hz precharged
DI
D?. D3
HI
D5
H3
I h, 1000 C
i- I h. 650 C
+24h.500
C
Oil yucnch
Oil quench
Water quench
D6
t-14. HS. Hh
Hl. HX
Oil quench
Oil quench
Water quench
Table
Quenched
615
identities
Oil quench
Oil quench
Water quench
with arseneous oxide, at a current density of 1 mA
mm--’ for 24 h. An identical treatment resulted in a
bulk hydrogen concentration
of about 4 wt. ppm in
sheet specimens of EN30A.
The Yobell specimens were tested in tension using
an lnstron model 1195 screw-driven
machine at a
constant crosshead displacement
rate. and the strain
rates cdlcuiated for each test (Table 3) were based
upon a 2mm gauge length. Special grips. shown in
Fig. 1. were designed to ensure that specimens were
electrically insulated from the testing machine. This
had the disadvantage of softening the machine so that
a part of the crosshead displacement
caused elastic
distortion
at the grips rather than strain in the
sample. During each test the load on the specimen
was monitored
as a function of crosshead displacement. and it was assumed that the total crosshead
Dl
D?
D3
STUDY
1 h.93O’C
+ I h, 650 C
+24h. 500 C
Zh,lOOOC
Heat
treatment
method
Oil quench
Oil quench
C
FRACTURE
and specimen
I h.930 C
2 h. 1000 C
+ 1h, 650 C
+24h.500’C
Specimen
identity
AND
conditions
Quench
+ I h. 6% C
Tcmperembrittled
DEFORMATION
Strain
rate/s-’
4 x lo-4
8 x 1om4
8 x 10m4
D7. DX, D9
displacement
represented the extension of the specimen gauge length.
The acoustic emission recording system is shown
schematically in Fig. 2. The normal surface displacement of the central area of the polished-end face of
the specimen (i.e. the epicentre) was monitored using
an air gap capacitance
transducer. The end of the
specimen acted as one plate of the capacitor. the other
was a 6 mm dia. disc positioned 2 4trm above the
specimen. When the transducer was operated with a
constant voltage across its plates (about 35 V) the
transducer sensitivity wds given by
6Y
fis=-7
t-VA
(11
where
rSy = change in charge on the plate
3. Mechanical
properties
Yield stress/MNm-’
1380
1220
I220
U.T.S./MNm-’
Ductility/mm
1830
1510
1530
0.22
0.27
0.30
8 x 1o-4
850
1030
0.79
4
8
8
8
8
X06
700
672
693
707
1030
876
834
898
862
0.55
0.88
0.90
0.64
0.85
0.12
x
x
x
x
x
lo-‘+
lo-4
10-4
1o-4
IO-*
Hl
Quenched
4 x lo-5
1410
1630
H2
Tempered
4 x 10-s
690
842
0.44
Temper
embrittled
4
4
4
4
4
4
919
962
933
2601
> 820
1170
1230
>636
1240
0.00
0.00
0.05
0.09
0.00
0.35
H3
H4
H5
H6
H-l
H8
x
x
x
x
x
x
10-s
10-s
10-s
10-s
10-s
10-S
616
WADLEY
AND
SCRUBY:
DEFORMATION
AND
FRACTURE
STUDY
--_---__-_
I
i
I
,
I
CHARGE
AMPLIFIER
h,
f
‘(-
AMPLIFIER
.
I
AND
FILTERS
TRANSIENT
RECORDER
TRANSIENT
ANALYSIS
Fig. 2. The broad-band acoustic emission detection system. The risetime of the transient recording
system was determined experimentally to be 4011s. A high-pass filter removes noise below 35 kHz.
6x =
C’=
A=
E=
change in plate separation. x
applied voltage
plate area
dielectric constant of air.
The charge from the transducer was fed into a
charge-sensitive preamplifier before band-pass fihering between 35 kHz and 45 MHz, followed by further
amplification prior to monitoring with an oscilloscope, and recording with a Biomation 8100 transient
recorder. This had a IO ns sampling interval, and was
linked with a PDPEI/E minicomputer. A small component of the signal was also monitored with a
Hewlett-Packard power meter whose output was
displayed on a chart simultaneously with the load-displacement curve. This enabled detectable emissions
to be located on the load-displacement curve and
their approximate magnitude to be recorded.
For the tests with specimens D2, D3, D6, D7 and
D8 the transducer was calibrated before the start of
the test so that the relation between q and x was
known. In such tests the amplitude of the acoustic
emission waveform recorded on the Biomation transient recorder could be related to the surface displacement of the specimen in absolute units, thus enabling
the amplitudes of emission events from different tests
to be compared. Unfortunately the majority of tests
reported here were performed using an uncalibrated
transducer and only qualitative comparisons of
amplitudes could then be made. However, it was stili
possible to measure, and quantitatively compare, the
time dependence of the emission waveforms. After
testing, all the specimens were examined using optical
and scanning electron microscopy, both to characterise the microstructures and to ascertain the fracture
mode.
RESULTS
The mechanical properties of specimens tested to
fracture are shown in Table 3. The material tested
in the quenched condition had a high yield and
ultimate tensile stress. The variation in strength level
between Dl, D2 and D3 could have been due to a
slightly larger prior austenite grain size in specimens
D2 and D3, as a cornsequence of the higher austenitising temperature used during their heat treatment.
The quenched material had a low ductility, giving a
total plastic extension of 0.2-0.3mm. However, this
was not the same as the extension of the gauge length
as it also included the component due to the distortion of the grips.
Tempering at 650°C led to the expected reduction
in yield and ultimate tensile stresses and a recovery
of ductility. Further ageing at 500°C to produce
material in the temper-embrittled condition led to
only a slight further reduction in strength and little
change in room temperature ductility. The addition
of hydrogen before testing caused a reduction in ductility but had little systematic effect upon other
mechanical properties. All the specimens underwent
extensive electropolishing to remove the oxide formed
during heat treatment. The resulting variations in
gauge diameter (assumed to be 3 mm in calculating
the stresses) or slight variations in tempering temperature may explain the apparently higher yield stresses
of specimens H5, H6 and H8 compared with D5-9.
The material had an apparent prior austenite grain
size of about 80pm, which was difficult to measure
accurately due to irreproducible
etching. The
quenched condition consisted of acicular-martensite
within the prior austenite grains. There was evidence
of banding and manganese sulphide stringers were
present, both oriented along the testing axis of the
specimens. Tempering at 650°C caused the precipitation of carbides, whilst further ageing at 500°C had
little discernible effect upon microstructure.
The specimen fracture modes were markedly
affected by both heat treatment type and hydrogen charging treatment. The fracture appearance of
specimens tested in the uncharged condition are
WADLEY
Fig. 3. The fracture
AND
SCRUBY:
appearance
near
DEFORMATION
AND
the centre of the fracture
(hydrogen free).
FRACTURE
STUDY
face for each
heat-treated
617
condition
618
WADLEY
AND
SCRUBY:
DEFORMATION
AND FRACTURE STUDY
Fig. 4. The fracture appearance near the outside of the fracture face for each heat-treated
(hydrogen free).
condition
WADLEY
Fig. 5. The fracture
AND
SCRUBY:
appearance
DEFORMATION
AND
of EN30A when tested after hydrogen
condition.
FRACTURE
embrittling
STUDY
for each heat-treated
619
620
WADLEY
AND
SCRUBY:
DEFORMATION
shown in Figs. 3 and 4. Each fracture had two
regions: a central area that fractured approximately
normal to the stress axis (Fig. 3) and an outer region
of shear (Fig. 4). In the quenched condition the central area of the fracture had an undulating appearance [Fig. 3(a)] and was covered in an array of
shallow cusp-shaped dimples that often contained
small, _ 1 pm dia. inclusions that were found to be
rich in manganese and sulphur. Tempering at 650°C
led to the disappearance of the undulating fracture
[Fig. 3(b)] and to a more conventional cup and cone
fracture. Additional tempering at 500°C in one case
caused an intergranular fracture, but more usually
had little effect upon the fracture mode in the central
area [Fig. 3(c)]. The appearance of the outer regions
of shear fracture were more sensitive to heat treatment than the central areas. In the quenched condition the fracture consisted of elongated dimples
[Fig. 4(a)] whilst the tempered material had steps on
the shear lip [Fig. 4(b)]. The temper-embrittled
material that fractured by a predominantly ductile
mode did show regions of faceting which were consistent with secondary intergranuIar cracking at the
inner edge of the shear zone [Fig. 4(c)].
The effect of hydrogen embrittlement was to cause
a change in fracture mode to partially or fully intergranular fracture in all specimens (Fig. 5). In both
the quenched and tempered conditions. only isolated
areas of intergranular fracture covering -5% of the
fracture were observed [Fig. S(a) and (b)] and optical
metallography revealed that in the quenched material
this was often associated with the presence of carbonrich bands. The remainder of the fracture in both
materials was reminiscent of the uncharged material.
The temper-embrittled material always underwent intergranular fracture with only isolated areas of tearing
linking regions of intergranular cracking. Extensive
secondary cracking (Fig. 6) was always observed in
specimens tested in this condition (e.g. Fig. 6 for
specimen H4).
-The load-extension and acoustic-emission power
curves for hydrogen-free specimens tested in each
condition are shown in Fig. 7(a), (b) and (c) and can
be compared with similar material tested after cathodic precharging [Fig. 7(d), (e) and (f)]. The acoustic
emission power recorded by the technique used here
has no fundamental
significance and was only
recorded to mark, on the load-extension curves, the
position and approximate magnitude of the individual
emission events. In all tests the majority of the emission occurred during the period of plastic instability
just prior to final fracture (Fig. 7). Sometimes groups
of transients could not be drawn individually in
Fig. 7, and where this occurred the number of overlapping transients is indicated.
The aim of the experiment was to record the waveforms of individual acoustic emission transients with
as little distortion as possible, and an example of one
from the failure of hydrogen-free temper-embrittled
material is shown in Fig. 8. Transients similar to this
AND FRACTURE
STUDY
Fig. 6. An example of secondary cracking in hydrogenembrittled EN30A tested after ageing at 500°C for 24 h.
were recorded from every test and the average risetime (risetime is the time for the amplitude to go from
10 to 90% of its maximum) of those within the dynamic range of the system is given in Table 4. It is
clear from this table that the average risetime varies
with the condition of the material. A better way of
displaying this data is to plot a risetime histogram
(Fig. 9) for each fracture mode. Unfortunately, insufficient data was obtained from the material in the
tempered condition for a histogram, and even the
other histograms are based on rather limited data.
The histograms have a lower limit of 40ns imposed
by the recording system, and the risetime averages
shown in Table 4 must therefore be slightly higher
than those of the actual surface displacements. Nevertheless, it is clear that the shape of the risetime distribution shows considerable variation with fracture
process.
In a few of the tests a calibrated transducer was
used and this important development allowed the
emission amplitude to be measured, thus characterising the source more fully and permitting comparisons
to be made between results from different tests.
Examples of amplitude distributions for hydrogenfree material in the quenched and the temper-embrittled conditions are shown in Fig. 10. The dynamic
range of the distributions were severely limited by
the eight-bit precision of the recording system.
DISCUSSION
The results of the previous section show that the
risetimes of acoustic emission transients during fracture are dependent on the metallurgical condition of
the steel. The shape of individual acoustic emission
transients (Fig. 8), and their relation to individual
source events, has been discussed in a previous
paper [17]. Firstly, the risetime of the leading edge
of an acoustic emission is related to the lifetime of
the source event, i.e. the time for which a crack event
WADLEY
AND
DUCTILE
SCRUBY:
DEFORMATION
FRACTURE
AND
FRACTURE
HYDROGEN
621
STUDY
- ASSISTED
FRACTURE
-06
APPROX6
TRANSIENTS
-03
-0-S
-lx
-0-3
i?
E
iti
3
2
w
4
-02
EXTENSION
(mm
EXTENSION
(al
/mm
id)
OFF _SCtLE
10 -
-06
e-
-0-S
$
f
EXTENSION
/mm
EXTENSION
(el
(bl
c
2
0
I mm
APPROX
10 TRANSIENTS
01
02
04
0.6
0.6
1.0
EXTENSION
/mm
0
EXTENSION
/mm
(f)
ICI
Fig. 7. Load and acoustic emission power as a function of extension for EN30A tested in the (a)
quenched. (b) tempered and (c) tempered embrittled
conditions,
in which the fracture mode was predominantly
ductile. together with the data for material tested after hydrogen embrittling of(d) quenched.
(e) tempered and (f) temper-embrittled material.
lasted. Secondly. the amplitude of the leading edge
can be related, as a first approximation,
to the magnitude F of a force-step-function
source using a relation
developed by Pekeris [ 181 and Pekeris and Lifson
[I91 and which can also be extracted from a more
general solution due to Willis [Z].
ci=k-
F
h/l
(2)
where d is the amplitude of the leading edge vertical
surface displacement
measured at the epicentre (i.e.
vertically above the source), h is the depth of the
source below the specimen surface at the position of
the transducer, /1 is the shear modulus. F is the magnitude of the force step and k is a constant determined
by the elastic properties of the propagation
medium.
Sinclair [20] has recently calculated k for steel and
obtained a value of 4.7 x IO-‘. Thirdly, the delay
622
WADLEY
ANU
SCRUBY:
DEFORMATION
AND FRACTURE
STUDY
SHEAR
LONGITUDINAL iNTERMEDiATE
STRUCTURE COMZNENT
COMPONENT
,
h
.*
\
P
,
h
-II-
RISE
TIME
I
3
I
4
TIME
AFTER
I
8
EVENT/ps
Fig. 8. A typical transient measured with the calibrated broad-band
referred to in the text.
of the shear-wave arrival at the epicentre provides
a method for determining
the depth of the source
below the epicentre. These three variables do not fully
specify the shape of actual acoustic emission transients and work is in progress attempting to relate
more fully other features of the transient
shape to
the micromechanisms
of fracture.
The risetimes of individual acoustic emission tran-
I
7
I
6
1
5
system. showing the parameters
sients varied (Fig. 9) and it was not possible to characterise a fracture mode by means of a single risetime
measurement.
When the risetime distributions
for
materials fracturing by the same mode were plotted
it was possible to observe a difference in distribution
(Fig. 9). Firstly, the risetimes of the hydrogen-free
material tended to be shorter than those from cathodically precharged specimens and this is most obvious
Table 4. Fracture modes and acoustic emission risetime data
Specimen
Condition
Dl
D2
D3
Quenched
D4
Tempered
No. transients
in dynamic range
Fracture mode
45’ shear with central
area
undulating
Cup and cone
67
11
predominantly ductily cup + coneisolated areas of brittle mode
13
28
24
25
76
57
47
63
56
13
87
3
106
19
10
106
75
t
3
131
t
t
Temperembrittled
HI
Quenched
45- shear with central undulating
area + 5% intergranular
HZ
Tempered
Cup and cone with
intergranular
Temperembrittled
-5”j,
Intergranular + isolated areas of
tearing
HI
H8
t N.B. H5. H7 and H8 generated acoustic
or overloaded the transient recorder.
70
72
78
902, intergranular
D5
D6
D7
D8
D9
H3
H4
H5
H6
12
5
2
Average
risetime/ns
emission
transients
that
were either
too low in amplitude to be detected
WADLEY
AND SCRUBY:
DUCTILE
FRACTURE
AND
DEFORMATION
FRACTURE
HYDROGEN-ASSISTED
la)
FRACTURE
idI
OLJENCHED
QUENCHED
50
623
STUDY
100
RISE
150
TIME
I
_I
200
250
50
100
RISE
I ns
5
TEMPER
c
200
250
200
250
Ins
(e)
TEMPER
EMBRITTLED
I DUCTILE
150
TIME
EMBRITTLED
1
l-
0
RISE
TIME
50
50
100
RISE
distributions
150
TIME
Ins
200
1
250
Ins
of acoustic emission transients
recorded
steel in different conditions.
when the distributions
for the temper-embrittled
material are compared, [Fig 9(b) and (e)]. Secondly.
the material condition affected the risetime distributions of the hydrogen-free material leading to shorter
transients
on average from the temper-embrittled
TIME
EMBRITTLED
I INTERGRANULAR
0
150
RISE
TEMPER
Fig. 9. Risetime
100
Ins
during
the fracture
of EN3OA
material (b) compared with the quenched material (a).
The distribution for the hydrogen-embrittled
material
tested in the quenched condition (d) appeared to be
similar to that of the unembrittled
material (a) with
the exception of a few much longer risetime tran-
624
WADLEY
AND
SCRUBY:
System
tower
Limit
FRACTURE
STUDY
$$t
I
‘System
Lower
Limit
System
,Upper
;Li mit
0.2
1
s
._
2
&
AND
System
z
0.3 *I
(u
5
6
DEFORMATION
Llr
f
‘I
0.1
I Ampiitu~
t
I
0
0
5
Surface
10
Displacement
2
15
/pm
Ial
5
Surface
10
I pm
(b)
Fig. IO. The distribution of leading edge amplitudes for EN30A tested in the (a) quenched
temper-embrittled
conditions in the absence of hydrogen.
sients. Finally,
the material tested in the temperembrittled condition which underwent intergranuIar
fracture in the absence of hydrogen (c), give a distribution which was different from material in the same
condition which underwent either a ductile fracture
(b) or a hydrogen-assisted
intergranular
fracture (e).
The data is too sparse to attempt to define the parameters governing the shape of these distributions or
to make detailed comparisons.
Comparing
Fig. 9(c)
and (e) shows that hydrogen-assisted
intergranular
fracture generates transients
with longer risetimes
than those from hydrogen-free intergranular
fracture.
These in turn are longer than those from hydrogenfree ductile fracture in temper-embrittled
material
[Fig. 9(b)], i.e. the events that generated the emission
during ductile fracture are faster than those generated
during hydrogen-assisted
intergranular
brittle fracture.
Consider now the particular fracture and deformation processes which act as sources of emissions in
this steel. This is a complex problem as there is a
range of poorly understood deformation and fracture
processes operating during the later stages of each
test, and it is not possible to relate, unambiguously,
one of these processes to the emission activity. In the
quenched specimens, extensive ductile dimple formation occurred. but it is unfikely that the formation
of each dimple generated an acoustic emission transient simply because a large number of uniformly
sized dimples were observed whilst only a few energetic acoustic emission events were generated.
A
second possible source could have been the fracture
or decohesion
of inclusions, but this can be discounted as it would be expected to occur close to
yield rather than during final fracture, and it is unlikely that the transients generated could be so sensitive to heat treatment. A third possible mechanism
could be a form of fast shear fracture similar to that
proposed by Clark and Knott [21]. Such fractures are
often observed in materials with low work-hardening
:
15
Displacement
and (b)
capacity, where they cause a zig-zag fracture path due
to the fast shear linkage of interinclus~on
voids
[21-241. The low-alloy steel used in the present study
had a low work-hardening
capacity, which is manifested as a yield stress to ultimate tensile stress ratio
between 0.75 and 0.8. and examination
of Fig. 3(a)
shows the fracture path to have an undulating
appearance
in the guenched condition.
Additional
support for the fast shear model can be obtained from
measurements
of the amplitude
of the transients
recorded from the quenched material [specimens D2,
D3, Fig. 10(a)]. These were typically lo-” m and
using equation (2) might correspond to a force magnitude of 0.3 N. As a first approximation,
if a local
stress of I.5 x 109NmW2 during crack growth was
present than the area of crack that would lead to
a 0.3 N force pulse would be about 450~1m* which
might correspond
to an area of about 30 x 1.5ym
in Fig. 3(a).
It appeared that the material tested in the tempered
condition generated less emission than the quenched
material, although only one specimen was tested. The
fracture did not have the undulating appearance so
typical of the quenched material but still had a low
work-hardening
capacity. A part of the loss in emission could have been due to a lower local shear stress
in the tempered material as the yield stress was about
850 MNm-’ (cf. 1300 MNm-’ for the quenched condition) so that crack increments of equal area to those
in the quenched material produced smaller amplitude
transients.
The emission activity returned after additional tempering at 500°C when the fracture was either a cup
and cone with isolated areas of what appeared to be
secondary intergranular cracking [Figs. 3(c) and 4(c)]
or almost completely intergranular.
The transients
generated in the former specimens had, on average,
shorter risetimes than those in the quenched material,
which suggests that the events which generated the
emission from temper-embrittled
material had shorter
WADLEY
AND
SCRUBY:
DEFORMATION
lifetimes. Examination of a typical amplitude distribution [Fig. IO(b)] shows that the initial surface displacements were _ I to 20 ppm which corresponds
to force magnitudes of 0.03 to 0.6 N or in the simplest
case crack increments of 40-750 pm’ assuming a local
stress of 800 MNm _ ‘. Observations
of the fractures
by scanning electron microscopy
revealed that the
chief difference between tempered
materials which
generated
little emission
and
temper-embrittled
material which was active was the presence of areas
of secondary cracking which covered areas of about
the same order as predicted
from the amplitude
measurements.
These could therefore have been the
main sources of emission in the temper-embrittled
condition.
One specimen tested in the temper-embrittled
condition underwent a predominantly
intergranular
fracture and generated transients with a different risetime
distribution from that of the cup and cone fractures
[Fig. 9(c)]. The change to intergranular
fracture led
on average to longer risetimes, which would be consistent with larger crack increments per event, assuming the crack velocity was about the same in both
materials. The specimens tested after cathodic charging underwent variable degrees of intergranular
fracture and this tended to increase the risetime of the
transients. Assuming that the emission source is intergranular decohesion. the longer risetimes of hydrogen-charged
material could be attributed
to an increased source area. perhaps extending over four or
five facets. If future tests use a calibrated
transducer.
it should be possible
to use amplitude
analysis
to aid
our understanding
of this fracture mechanism.
CONCLUSIONS
A broad-band acoustic emission technique has been
used to record acoustic emission transients generated
during the fracture of a low-alloy steel. It has been
found that:
1. The risetime of acoustic emission transients
generated
during
the fracture
of hydrogen-free
EN30A have, on average, shorter risetimes than those
from similar materials tested after a hydrogen-embrittling treatment.
2. The risetimes of acoustic emission transients.
even in the uncharged state, appear to be dependent
upon material condition. In particular the risetimes
of material in the temper-embrittled
condition are a
little shorter
than those from material
in the
quenched condition.
3. The risetime has been interpreted
initially in
terms of the source lifetime, and when a calibrated
transducer is used it is also possible to estimate the
approximate
crack area that generated the emission
transient.
AND
FRACTURE
STUDY
625
4. During ductile fracture of quenched material the
results are most consistent with the source of emission
being a localised shear fracture similar to that proposed by Clark and Knott [21].
5. In the temper-embrittled
condition the mechanism is not clear. but the transients could be related
to the onset of secondary intergranular
cracking for
those specimens which underwent a predominantly
cup and cone failure.
6. The emission source from hydrogen-embrittled
material was attributed to intergranular cracking, and
it seemed likely that this extended over several grain
facets.
Ack,lo~/edgrrtlrrlts-We should like to record our appreciation of the many helpful discussions of this study with
our colleagues
in the Metallurgy.
Materials
Physics and
Theoretical
Physics Divisions at Harwell and in particular
with Drs. B. L. Eyre, G. J. Curtis and R. Bullough.
In
addition. we have received considerable
assistance with the
theoretical aspects from Dr. J. Sinclair and with the practical aspects from A. Joinson and G. Shrimpton.
This study
was funded. in part. by the Ministry of Defence (Procurement Executive)
through
the Admiralty
Marine
Technology Establishment
(Holton Heath).
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