Copyright(c)JCPDS-International Centre for Diffraction Data 2000,Advances in X-ray Analysis,Vol.43
Study of the Fatigue Fracture Surface Regions of Steels
Using Microbeam Synchrotron X-Ray Diffraction
Y.Yoshioka*, K.Akita**, H.Suzuki**, T.Sasaki***
*Musashi Institute of Technology, Tokyo, Japan
**Tokyo Metropolitan
***Kanazawa
University, Tokyo, Japan
University, Kanazawa, Japan
ABSTRACT
A cyclic plastic zone area near fatigue-fractured surface was studied using a back-reflection
camera and high intensity, microbeam, synchrotron X-ray source (SR) in order to measure the
variation of the stress intensity factor range (AK) along the fatigue fracture.
The existence of subgrains within the cyclic plastic region was shown by the appearance of
small spots in the Debye-Scgerrer rings of the imaging plate. This effect did not exist outside
of the cyclic plastic area. The size of the cyclic plastic zone below the surface was estimated
from the relation between the number of spots in a Debye-Scherrer ring and the depth from
the fracture surface. AK was measured from this relation. A synchrotron X-ray source made
these measurements possible within a high positional resolution and a reasonable period of
exposure time.
INTRODUCTION
It has been shown that a good correlation exists between the maximum stress intensity factor
(Kmax ) and the distribution of residual stress or the X-ray line broadening of a diffraction
profile near the fractured surface1)‘4). However, it is complicated to evaluate the stress
intensity factor range AK on a fatigue fracture from such parameters. A microbeam X-ray
diffraction technique is a powerful tool for use in such a study. We attempted to detect the
cyclic plastic zone near the fatigue-fractured surface with a conventional X-ray source5)6)7).
The results showed that the existence of a cyclic plastic zone was observable but it was
difficult to quantitatively estimate the zone size because of insufficient positional resolution of
microbeam X-rays from the conventional X-ray source.
In this study, we used microbeam X-rays from a synchrotron radiation source (SR) to obtain a
high-resolution X-ray beam and Debye-Scherrer patterns near the fracture surface. The
relation between the number of spots in a Debye-Scherrer ring and the size of the cyclic
plastic area was measured. The possibility of estimating AK was also discussed.
EXPERIMENTAL
Microbeam X-rays from Synchrotron Radiation Source
382
This document was presented at the Denver X-ray
Conference (DXC) on Applications of X-ray Analysis.
Sponsored by the International Centre for Diffraction Data (ICDD).
This document is provided by ICDD in cooperation with
the authors and presenters of the DXC for the express
purpose of educating the scientific community.
All copyrights for the document are retained by ICDD.
Usage is restricted for the purposes of education and
scientific research.
DXC Website
– www.dxcicdd.com
ICDD Website
- www.icdd.com
Copyright(c)JCPDS-International Centre for Diffraction Data 2000,Advances in X-ray Analysis,Vol.43
16.2 m
---,i
Slit
20.2 m
Horjzontally
focusing
Is
Collimating mirror
I Double crystal
monochromator
-’
23.2 m
Vertically focusing
383
-c
27.5 m
Slit
\
Focusing mirror
I
Specimen
Beam size: 0.55 in vertical, 1.5 in horizontal
Divergence: 1.2 in vertical, 12.0 in horizontal (mrad)
Fig. 1 Schematic layout of beam line 3A at Photon Factory of KEK.
The synchrotron radiation system at the Photon Factory (PF) of the High Energy Accelerator
Research Organization, KEK, in Tsukuba, Japan was used as the X-ray source. The beam line
used was BL-3A, and the optical layout used is shown in Fig. 18’. It consists of a double Si 111
crystal monochrometer, collimating mirror and focusing mirror. X-rays between h=0.25 nm
and h=O. 12 nm are available with the optical system. The divergence angle of the beam
source was 1.2 mrad in vertical direction and 12 n-n-adin horizontal direction. The beam size
was 0.55 mm (vertical) and 1.5 mm (horizontal) on the specimen.
Figure 2 shows a schematic
60
layout of the microbeam X-ray
back reflection camera used in
this study. Specimen is to be
Monochromatic
located at the focusing position
X-ray beam
of beam line, and a 0.2 mm
sdiameter pinhole was set 140
I
mm up stream from the
Single
specimen position. We used an
Pinhole
imaging plate (IP) as an area
detector,
and the
X-ray
distance between the IP and
Imaging Plate Video microscope
the specimen was 80 mm. The
wavelength of X-rays was
adjusted so that the Bragg Fig.2 Optical layout of microbeam x-ray back- reflection
angle 28 of an hkl diffraction camera.
appears at 154 degrees. In the
present study, since aFe 211 diffraction was used, the wavelength used was 0.2280 nm, and
the diameter of the Debye-Scherrer ring was about 78 mm. The beam size on the specimen
was about 250 pm@ and the positional resolution was calculated as 0.41 pm. This positional
resolution was superior to that possible with a conventional laboratory X-ray source’). When
the same optical layout was employed using a laboratory source, the positional resolution was
about 2.5 pm.
The combination of a video microscope and mirror was used to position the specimen, and
they were removed during measurement of X-rays.
Copyright(c)JCPDS-International Centre for Diffraction Data 2000,Advances in X-ray Analysis,Vol.43
Specimen, Fatigue Test and X-ray Measurement
Specimens were 0.15% low carbon steel and all were received with planer dimensions of the
ASTM standard 1 inch (25.4mm) thick compact tension type. However, the thickness
employed in this study was 12.7 mm. After being machined, the specimens were kept at 600
“C for 3 hours and furnace cooled. Average grain size was 20 pm@.
Fatigue tests were carried out under constant load control. Two kinds of stress ratio, R=O.O5
and 0.5, and the maximum load of lO.SkN were chosen as the experimental condition. The
tests were conducted in air at room temperature, and the test frequency was 30Hz.
Debye-Scherrer patterns of aFe 211 diffraction on the fracture surface were recorded on the
imaging plates. Several positions were chosen at stated intervals in the direction of crack
propagation for the measurement of the X-ray pattern. The distributions of X-ray parameters
beneath the fracture surface were again recorded on the new surfaces revealed by successive
electro-polishing.
RESULTS AND DISCUSSION
Observation of Debye-Scherrer
Patterns
The first fatigue test was carried out under the condition of a maximum load of 10.8 kN and
the stress ratio of 0.05. Figure 3a) shows a Debye-Scherrer Pattern recorded from fracture
surface at the position of AK=20 MPa m 1’2.Time required for exposure was only 2 minutes.
When the conventional microbeam X-ray generator was used, it would be more than 60
minutes for an imaging plate7) or 40 hours for an X-ray film@. The reduced exposure time
made this experiment practical.
A continuous ring is observed as shown in Fig3.a), but if this ring is partially enlarged as
shown in Fig.3b), the existence of fine spots is noticeable. This fact shows that crystallites in
this
region
would
be
polygonally de-formed, that is,
subgrains would be formed by
cyclic stressing. When a
conventional microbeam Xray source was used, such fine
spots in a ring were observed
beneath the fracture surface
region, but they were not
observed
at the
always
fracture surface because of
poor resolution5). Even if the
spots were observed beneath
the surface it was impossible
to quantitatively evaluate the
formation of substructure. In
4
this study, however, we were
b)
able to analyze on the surface Fig.3 Microbeam Debye-Scherrer patterns on fracture surface.
and in its vicinity.
AK=20 MPa I”. a) Whole pattern. b) Partially Enlarged pattern.
384
Copyright(c)JCPDS-International Centre for Diffraction Data 2000,Advances in X-ray Analysis,Vol.43
-?
24 pm
40 pm
84 pm
104 pm
Fig.4 Debye-Scherrer patterns from various depths from fracture surface.
Surface layers were successively removed by electro-polishing, and the X-ray patterns were
recorded at the position of same AK. Figure 4 shows patterns obtained from various positions
at depths of 24, 40, 84 and 104 pm. A continuous Debye-Scherrer ring was obtained in the
vicinity of fracture surface, but the number of spots in the ring decreases with an increase in
depth from fracture surface. The Debye-Scherrer ring gradually separates into several arcs and
fine spots in the depths of the fracture surface, and then such spots in the arcs almost
disappear at a depth of 104 mm. This indicates that the effect of cyclic stressing does not
reach this depth, that is, this region is outside of the cyclic plastic deformed zone.
There are several micobeam X-ray parameters such as total misorientation, micro lattice strain
and subgrain size. In the present study, fine spotty patterns were obtained. This indicates the
formation of substructure as described above. To measure the subgrain size, the number of
spots has to be counted and calculated using several assumptions. However, it is sufficient to
count the number of spots in order to measure the size of the plastic zone. The number of
spots in each Debye-Scherrer patterns was adopted as an X-ray parameter. We counted the
number of spots with the help of a computer aided image processing technique, and the
number of spots were plotted against the depth from the fracture surface. Results of the
fatigue test conducted under the conditions of a maximum load of 10.8 kN and a stress ratio
of 0.05 are shown in Fig.5 a). The number of spots at the position of AK=20 MPam”2
monotonically decreases with the depth below the fracture surface. Then it reaches a constant
at a depth of more than 100 pm. At a position of AK=40 MPam’“the depth is about 500 pm.
385
Copyright(c)JCPDS-International Centre for Diffraction Data 2000,Advances in X-ray Analysis,Vol.43
We estimated that such a depth
is equal to the cyclic plastic
zone size in this study.
500
P=O. 15% carbon steel, lO.EKN, t&O.05
400
l
Similar results appeared on the
Debye-Scherrer patterns on the
specimen fractured under a
maximum load of 10.8.kN and
a stress ratio of 0.5, as shown
in Fig.5 b). The values are
-scattered by co.mparison to
those in the case of a stress
ratio 0.05 because the cyclic
plastic region is smaller.
However, the same tendency is
observable.
Estimation
of AK
2 300
2
B
%
2 200
E
s
4
0
1‘.; ,: ,; ,q(s /
100
0
0
(1)
400
300
200
100
500
600
Depth from fracture surface a (pm)
7””
0.15% carbon steel, P=lO.EKN, FkO.5
8
1i.O
6
ki
%
$200 -
2
SIOO
AK=lOMPa m112
l
0
o AK=lSMPa m112
0
A AK=2OMPam1j2
l
b)
s
0
l
2
-
A
0
A
A
l
0
co= C (AK/20J2
0
300 -o
2
0
The size of the cyclic plastic
zone at a crack tip is expressed
with the stress intensity factor
range AK and the yield stress
oY of materials as follows;
386
I0
0
50
80,.
A
100
4
150
Depth from fracture surface a @m)
Where C is a proportionality
constant, and this value is 0.15 Fig.5 Relation between number of spots in Debye-Scherrer
for a direction perpendicular to ring and the depth from fracture surface. a) R=O.O5,b) R=0.5.
the direction of crack propagation. Substituting a value of AK and 344 MFa for c+, the size of the plastic zone can be
calculated and a line in Fig. 6 shows this result. Open and closed circles in this figure indicate
the experimental depths determined from Fig.5. These values nearly agree with the
analytical line. We attempted such a plot on the experimental results by the use of a
conventional X-ray source, but it was quantitatively complicated to observe the agreement
between the theoretical value and experimental one, because the positional resolution of
diffraction pattern was poor. In the present study, however, the size of the cyclic plastic zone
can be measured from a change in the number of spots and then the stress intensity factor
range AK can be estimated.
CONCLUSIONS
The determination of the size of cyclic ,plastic zone at the tip of fatigue crack was attempted
by applying a microbeam X-ray diffraction technique with the use of a synchrotron radiation
X-ray source (SR). The positional resolution of an X-ray beam using a synchrotron radiation
source was superior to that possible when using a conventional microbeam X-ray generator.
Therefore, Debye-Scherrer diffraction patterns with many fine spots which indicates the
formation of substructure were obtained within the area of cyclic plastic zone. The following
results can be summarized:
Copyright(c)JCPDS-International Centre for Diffraction Data 2000,Advances in X-ray Analysis,Vol.43
1) The number of fine small spots in a
diffraction ring shows a maximum
value on the fatigue fracture surface,
but it gradually diminishes with depth
from the fracture surface, and such fine
spots disappear at a certain depth.
2) This depth approximately agrees with
the size of the cyclic plastic zone
calculated from fracture mechanics
theory, and thus it is possible to
estimate the stress intensity factor
range AK at a fracture surface.
3) The use of a synchrotron radiation
source enables the practical application
of such microbeam X-ray technique
from the view points of the accuracy of
estimated values of X-ray parameters
with the reduction of exposure time.
104 L
1 0.15% carbon steel
l
387
P=70.8KN, t?=O.5
o P=lO.L?KN, R=O.O+
.g
0)
IO
The research described
here was
performed on beam line 3A(BL-3A) in
10
Stress intensity factor range AK (MPa ml”)
the Photon Factory,
High
Energy
Research
Organization,
Accelerator
Fig.6 Relation between cyclic plastic zone size and
Tsukuba, Japan, under Contract No.
stress intensity factor range AK. Straight line
986265 and was sponsored by the Grantis theoretical value.
in-Aid for Scientific Research (C) of the
Ministry of Education, Science, Sports and Culture, Japan.
REFERENCES
1) Hirose,Y. & K.Tanaka; Adv. X-Ray Analysis, 29, pp 265270(1986)
2) Tanaka, K. & YAkiniwa; Role of Fracture Mechanics in Modern Technology, pp 735
746( 1987)
4) Lebmn, J.L. et al; Residual Stresses in Science & Technology, 1, pp 109-116(1987)
5) Yoshioka, Y. & B.Guimard; ICRS2, pp 852-857(1989)
6) Izumiyama, A., Y.Yoshioka & M.Terasawa; Adv. X-Ray Analysis, 22, pp 221-226(1979)
7) Yoshioka, Y., S.Ohya, K.Hasegawa & S.Yusa; MAT-TEC 93 Improvement of Materials,
pp 257-262( 1993)
8) Kawasaki, K. et al: Rev. Sci. Instrum, 63, pp 1023-1026(1992)