C. ZHANG et al.: EFFECT OF STOP-HOLE-INDUCED MATERIAL REMOVAL ON THE FATIGUE PROPERTIES ...
457–465
EFFECT OF STOP-HOLE-INDUCED MATERIAL REMOVAL ON
THE FATIGUE PROPERTIES OF CRACKED DT4C STEEL
VPLIV ODSTRANITVE MATERIALA ZARADI IZDELAVE
IZVRTINE ZA UPO^ASNITEV NAPREDOVANJA UTRUJENOSTNE
RAZPOKE NA UTRUJANJE JEKLA DT4C
Chunguo Zhang
1,*
, Rongwei Liu
2
, Qiankun Liu
3
, Cuiping Ren
4
1,2,3,4
Key Laboratory of Road Construction Technology and Equipment, MOE, Chang’an University, Xi’an 710064, China
2
Center of Industrial Technology, Chengde Petroleum College, Chengde 067000, China
Prejem rokopisa – received: 2019-01-25; sprejem za objavo – accepted for publication: 2019-02-14
doi:10.17222/mit.2019.022
There is a lack of a quantitative study on the appropriate value of material removal Lr from the crack-tip front induced by
drilling a stop-hole. The effect of Lr variation on the fatigue life and post-initiation fatigue crack growth rate (da/dN)o fa
cracked specimen was studied for two different stop-hole geometries (circle and ellipse). Three different values of Lr = 0.3, 0.6
and 0.9 mm were selected for the parametric study by changing the location of the stop-hole center in relation to the initial
fatigue crack-tip. The results show that Lr = 0.6 mm results in the longest fatigue life and lowest post-initiation da/dN for all the
specimens repaired by either circular or elliptical stop-holes. In addition, the fatigue-life extension and post-initiation da/dN
reduction resulted from the elliptical stop-hole are significantly more than those of the circular stop-hole for the same Lr.
Keywords: stop hole, material removal, fatigue crack growth, fatigue life extension, DT4C steel
Do sedaj je bilo izvedenih zelo malo kvantitativnih {tudij ovrednotenja vpliva odstranitve materiala Lr z vrha konice utruje-
nostne razpoke, ki nastane z vrtanjem izvrtine za zaustavitev (upo~asnitev) napredovanja razpoke med utrujanjem materiala.
Avtorji opisujejo {tudijo vpliva variiranja Lr na trajno dinami~no trdnost oz. dobo trajanja in hitrost rasti utrujenostne razpoke
(da/dN) E-CT preizku{ancev s predhodno izdelanimi zarezami, utrujenostnimi razpokami in izvrtinami dveh razli~nih geometrij
(kro`no in elipti~no). Avtorji so izbrali tri razli~ne vrednosti Lr (0,3 mm, 0,6 mm in 0,9 mm) za parametri~no {tudijo. Pri tem so
spreminjali polo`aj sredine izvrtine v odvisnosti od lege konice utrujenostne razpoke. Rezultati so pokazali, da imajo pri Lr =
0,6 mm vsi izdelani preizku{anci iz izbranega jekla najdalj{o dobo trajanja in najni`jo hitrost napredovanja razpoke da/dN,ne
glede na geometrijo izvrtine (kro`no ali elipti~no). Nadalje so ugotovili, da je podalj{anje dobe trajanja in zmanj{anje hitrosti
napredovanja razpoke da/dN pri elipti~nih izvrtinah znatno ve~je kot pri kro`nih izvrtinah z enakim Lr.
Klju~ne besede: izvrtine za upo~asnitev napredovanja utrujenostne razpoke, odstranitev materiala, rast utrujenostne razpoke,
podalj{anje dobe trajanja, DT4C jeklo
1 INTRODUCTION
Fatigue cracks are often generated in metallic struc-
tural components during their service lives, and propa-
gate to final failure or fracture.
1
The investigation of
crack-growth retardation is a significant subject for
increasing the remaining lives of cracked structural
components.
2
There are a number of methodologies of
delaying or arresting crack growth to extend the fatigue
lives of cracked components, such as welding repair,
3,4
tensile overload(s),
5–7
shot peening,
8–12
indentation,
13,14
and deposition.
15
Beyond that, drilling a round hole in
the vicinity of fatigue crack-tip, often named the stop-
hole method, is one of the most effective and the easiest
crack-growth arresting or retardation techniques for
cracked components,
16,17
which is more effective than
infiltration and safer than applying an overload.
18
To avoid any detrimental influence and/or to increase
the potential beneficial influence resulted from drilling
stop hole(s), a number of studies have been carried out to
assess the effects of stop-hole geometry, size and confi-
guration on fatigue properties and remaining lives of
cracked metals. According to Song et al.,
19
a larger hole
diameter can give rise to a longer crack initiation and
total fatigue lives as the stop-hole diameter ranges from
2 mm to 3 mm. Cold expansion induced by inserting pins
into stop-holes can further increase the fatigue initiation
life from the hole-edge due to the incorporation of com-
pressive residual stress around the holes.
20–22
In addition
to the wide attentions over the geometry, size and confi-
guration of the stop-hole, Chen
23
innovatively considers
the fatigue-behavior influence of the cracked structural
component on the appropriate stop-hole size. Most re-
cently, double stop-hole methodology and its optimized
configuration relative to the distance between the two
centers of the holes have been demonstrated to be con-
ducive to the further enhancement of the fatigue life
extension of the cracked specimens.
17,18
Drilling a stop-hole at the fatigue crack-tip turns the
sharp crack into a blunt notch. This methodology can
provide a longer fatigue initiation life from the stop-hole
edge at the expense of material loss. As is well known,
Materiali in tehnologije / Materials and technology 53 (2019) 4, 457–465 457
UDK 620.178.3:669.018 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 53(4)457(2019)
*Corresponding author's e-mail:
zcguo2008@163.com

the location of the micro crack-tip can be detected accur-
ately even under dynamic loading conditions.
24,25
With
different emphases to those aforementioned studied on
stop-hole methods, the primary objective of the present
investigation is to study the effect of material removal L
r
from crack-tip front induced by drilling a hole on the
fatigue properties of cracked specimens by measure-
ments of the total remaining life, initiation life after drill-
ing the stop-hole, and the post-initiation crack-growth
rate behavior. The different L
r
values were achieved by
changing the position of stop-hole center relative to the
original fatigue crack-tip.
2 EFFECT OF DRILLING A STOP-HOLE ON
THE STRESS SINGULARITY AT THE FATIGUE
CRACK-TIP
In almost every structural component, stresses vary
from point to point during their service lives, and failure
will occur in locations where the stresses are the highest
or a singularity. In fact the stress concentration is a domi-
nant factor affecting the mechanical failures of a cracked
component,
26
especially fatigue failure. A stress concen-
tration has two main causes: loading mode and geometry
change. The effect of geometric features, e.g., hole,
crack and notch, is more severe compared with loading
modes such as bending and torsion. The influence of a
fatigue crack on the stress contribution is much more
severe than that of a hole or a notch.
Drilling a stop-hole in the vicinity of the fatigue
crack-tip turns the sharp crack into a blunt round-notch.
As illustrated in Figure 1, the stress magnitude is unex-
pectedly high in the immediate vicinity of a crack tip,
and rapidly decreases with the distance from the crack
tip along the crack growth path for a cracked specimen.
In fact, the stress at the fatigue crack-tip is theoretically
infinite or singular because the crack-tip radius is almost
zero. If we replaced the sharp crack tip with a stop hole
without changing the original crack length, however, the
stress magnitude in the near-stop-hole region could be
significantly reduce in comparison with that in the origi-
nal near-crack-tip zone. Thus, drilling a stop-hole at the
fatigue crack tip can increase the remaining fatigue life
of the cracked components due to the significant de-
crease of the stress concentration.
3 EXPERIMENTAL PART
A commercial DT4C steel widely used for engineer-
ing structures subjected to fatigue loading was selected
in this study. Its tensile strength is 610 MPa, and elon-
gation is 11 %. A standard extended-compact tension
(E-CT) specimen with a U-notch and with thickness of
10 mm was adopted for conducting the fatigue crack
growth testing to validate the methodology. All the
specimens were sliced using wire-electrode cutting from
the same plate to avoid any variation of the mechanical
properties of the tested specimens.
Firstly, a fatigue pre-crack of 1 mm from the U-notch
root in each E-CT specimen was introduced with a grad-
ually increasing K (stress intensity factor range) so that
the initial total crack length after introducing the fatigue
pre-crack was 5.8 mm from the loading line. Then, the
fatigue crack growth tests were performed on the cracked
E-CT specimens with and without stop-holes using a
servo-hydraulic fatigue-test machine with a capacity of
100 kN. For the specimens containing stop-holes, six
circular or elliptical stop holes were drilled in each
cracked E-CT specimen using wire-electrode cutting
when the crack propagated to a = 6.3 mm from the load-
ing line (or 1.5 mm from the U-notch root), 8 mm,
9.6 mm, 11.2 mm, 12.8 mm and 14.4 mm, respectively.
Figure 2 shows the geometry and dimensions of the
E-CT specimen, and the geometry and locations of the
circular/elliptical stop-holes (e.g., elliptical hole, and
material removal L
r
= 0.6 mm) drilled in each cracked
E-CT specimen.
The stop-hole-induced material removal L
r
was de-
fined by a measurement of the length from the original
fatigue crack-tip to the new circular/elliptical hole edge,
as illustrated in Figure 2.
There are a total 14 cracked E-CT specimens with a
1-mm fatigue pre-crack or initial crack a = 5.8 mm from
the loading line: (1) two as-received specimens without
drilling any stop-hole; (2) six specimens repaired by
circular stop-holes, i.e., two identical specimens for each
group of L
r
= 0.3 mm, L
r
= 0.6 mm, and L
r
= 0.9 mm; (3)
six specimens containing elliptical stop-holes, i.e., two
identical specimens for each group of L
r
= 0.3 mm, L
r
=
0.6 mm, and L
r
= 0.9 mm. The radius of circular
stop-hole is 2 mm, and the major semi-axis and minor
semi-axis of the elliptical stop-hole are respectively
2 mm and 1 mm and the direction near the crack is
illustrated in Figure 2c. For convenience, the specimen
containing six circular stop-holes with L
r
= 0.3 mm is
called "C-0.3", and so on, the other five groups of speci-
mens are called C-0.6, C-0.9, E-0.3, E-0.6 and E-0.9,
respectively.
C. ZHANG et al.: EFFECT OF STOP-HOLE-INDUCED MATERIAL REMOVAL ON THE FATIGUE PROPERTIES ...
458 Materiali in tehnologije / Materials and technology 53 (2019) 4, 457–465
Figure 1: Stress distribution as a function of the distance from the
crack tip or the hole edge where is the point of maximum stress, to-
gether with that of plain tension, indicating the effect of the geometric
features on the stress distribution

All the cracked specimens were tested under the
same fatigue-loading conditions: constant amplitude
axial fatigue loading with haversine waveform at a freq-
uency of 2 Hz, and a load ratio R 0.02. The maximum
load was set to be 23 kN for all of the specimens. The
a–N curves (a is crack length from the loading line, and
N is the number of fatigue cycles) and da/dN- K curves
are plotted in the forthcoming sections, where da/dN is
the fatigue crack growth rate and K is calculated by the
following K-formula.
4
K
P
BW dW
== ⋅
+
−−
⋅
⋅+− +
2
11
115 094 248 295
15
2
  
()(/)
(. . . .
.

34
124 − .
(1)
where P = applied load, B = thickness, W = width of
specimen,  = a/(W-d), a = crack length from loading
line, d = distance from sample edge to loading line, and
0.1  (a+d)/W  1.
4 RESULTS
The material removal L
r
from the fatigue crack-tip
front due to drilling a stop hole was considered as a
variable parameter for both circular and elliptical stop-
hole methods. For each group of cracked E-CT speci-
mens, two identical samples were measured to check the
consistency and reliability of the experimental results.
Figure 3 displays the a – N curves of specimens contain-
ing stop-holes with different geometries (circle and
ellipse) and different material removal (L
r
= 0.3–0.9 mm),
which are to illustrate the effect of L
r
variation on the
remaining fatigue life of the cracked specimen, and
compared with those of the as-received cracked samples.
It is clear that the results from two separate tests are very
consistent with each other, which therefore provides a
reliable estimate to study the L
r
-variation influences for
the two different stop-hole geometries.
As shown in Figure 3, the remaining fatigue lives are
significantly increased for all the specimens repaired by
either circular or elliptical stop-holes in comparison with
those of the as-received samples. It is more important for
the specimens containing stop-holes, the number of
fatigue cycles first increases then decreases along with
the increasing of L
r
from 0.3 mm to 0.9 mm, and that the
maximum fatigue life occurs in the case of L
r
= 0.3 mm
for both circular and elliptical stop-hole cases. That is
the magnitude of the fatigue life extension due to the
incorporation of stop holes increases when the L
r
value
increases from 0.3 mm to 0.6 mm, but drops instead if L
r
further increases from 0.6 mm to 0.9 mm.
To illustrate the influence of drilling a stop-hole, the
a – N curve obtained from an as-received specimen was
C. ZHANG et al.: EFFECT OF STOP-HOLE-INDUCED MATERIAL REMOVAL ON THE FATIGUE PROPERTIES ...
Materiali in tehnologije / Materials and technology 53 (2019) 4, 457–465 459
Figure 2: Fatigue samples: a) E-CT specimen (in mm), b) specimen
containing a fatigue pre-crack of 1 mm, and c) a cracked specimen
repaired by six elliptical stop-holes
Figure 3: Effect of L
r
variation from 0.3 mm to 0.9 mm on the
remaining fatigue lives of the cracked specimens repaired by:
a) circular stop-holes, b) elliptical stop-holes

chosen as the reference property and compared with that
of the specimen containing elliptical stop-holes with L
r
=
0.6 mm shown in Figure 4a. Clearly, the slope of the
a – N curve after drilling each stop-hole is much higher
than the corresponding slope of the as-received specimen
for the same crack length, indicating the rate of fatigue-
cycles rise was significantly increased by drilling a stop
hole. To emphasize the hole geometry’s influence on the
fatigue life, the a – N curves obtained from the speci-
mens with circular and elliptical stop-holes are directly
compared in Figure 4b to 4d for the cases of L
r
= (0.3,
0.6 and 0.9) mm, respectively. The remaining fatigue
lives of the specimens repaired by the elliptical stop-
holes are higher than those of the samples with circular
stop-holes for the same L
r
, especial for the cases of L
r
=
0.6 mm and 0.9 mm. Obviously, a smaller curvature or a
larger radius around the ellipse vertex is expected to
result in a higher fatigue initiation life compared with the
case of a circular stop-hole where the curvature is larger.
It should be noted that the da/dN variation after
drilling each stop-hole, not recognizable in the a – N
curve, is easily identifiable in the Paris curve. The da/dN
–  K curves from two separate fatigue-crack-growth
tests are shown in Figure 5 for the as-received samples,
and in Figure 6 for the specimens containing circular
and elliptical stop-holes. Obviously, the experimental
results from the two separate tests are very consistent in
each group of specimens, which therefore provide
reliable results to study the effects of the stop-hole
geometry and the L
r
value on the subsequent da/dN after
drilling each hole in the cracked specimens.
To make a clearer comparison between the as-
received samples and the specimens containing stop-
C. ZHANG et al.: EFFECT OF STOP-HOLE-INDUCED MATERIAL REMOVAL ON THE FATIGUE PROPERTIES ...
460 Materiali in tehnologije / Materials and technology 53 (2019) 4, 457–465
Figure 5: The da/dN –  K curves obtained from the two as-received
cracked specimens
Figure 4: The a – N curves obtained from: a) the specimen repaired by elliptical stop-holes with L
r
= 0.6 mm and the as-received sample, and the
specimens containing circular and elliptical stop-holes with: b) L
r
= 0.3 mm, c) L
r
= 0.6 mm and d) L
r
= 0.9 mm

C. ZHANG et al.: EFFECT OF STOP-HOLE-INDUCED MATERIAL REMOVAL ON THE FATIGUE PROPERTIES ...
Materiali in tehnologije / Materials and technology 53 (2019) 4, 457–465 461
Figure 7: Comparison of da/dN – K curves between the as-received sample and the specimens repaired by different stop-holes with different L
r
:
a) C – 0.3, b) C – 0.6, c) C – 0.9, d) E – 0.3, e) E – 0.6, f)E–0.9
Figure 6: The da/dN – K curves obtained from two separate tests for each group of specimens repaired by different stop-holes with different L
r
:
a) C – 0.3, b) C – 0.6, c) C – 0.9, d) E – 0.3, e) E – 0.6, and f) E – 0.9.

holes, only one sample in each group (e.g., the first
specimen in Figures 5 and 6) was selected to study the
influence of the L
r
variation on the subsequent crack-
growth behavior after drilling the stop-hole. The direct
comparisons between the specimens repaired by diffe-
rent stop-hole geometries with different L
r
and the
as-received specimen are shown in Figure 7 following
the da/dN measurements. For the first experimental point
of the da/dN measurement after drilling each stop-hole,
the "fatigue-crack growth rate" was named as (da/dN)
IP
.
It should be mentioned that the "N"of"(d a/dN)
IP
"isde -
fined as the summation of fatigue cycles related to the
crack initiation from the stop-hole edge and the sub-
sequent crack propagation, and the "da" is the increment
of the fatigue crack from the hole edge.
It is clear that the value of (da/dN)
IP
was obviously
reduced with an increasing L
r
value from 0.3 mm to 0.6
mm, but there was no notable difference between L
r
=
0.6 mm and L
r
= 0.9 mm for both the circular and ellip-
tical stop-hole cases. Additionally, the value of (da/dN)
IP
obtained from the specimens repaired by the elliptical
stop holes is lower than that from the samples containing
circular holes for the same L
r
value, indicating that an
elliptical stop-hole can further decrease the (da/dN)
IP
compared with the circular stop-hole. In Figure 7, the
first point (da/dN)
IP
was followed by a gradual approach
to the normal or steady-state crack growth rate, and it be-
came more evident in the cases of L
r
= 0.6 mm in com-
parison with that of L
r
= 0.3 mm for both the circular and
elliptical stop-holes. If the L
r
was further increased to 0.9
mm, however, the phenomenon of post-initiation
crack-growth retardation was greatly reduced, especially
for the specimen with circular stop holes. It has been
proved that a similar phenomenon (i.e., post-initiation
crack-growth retardation) appears in a cracked CT
sample repaired by a single circle stop-hole with various
diameters.
20
The drop of the post-initiation da/dN values
is mainly due to the following two reasons: (i) the incor-
poration of stop-hole built compressive residual stresses
in the near-stop-hole region;
20
(ii) the change of geo-
metry in the near-stop-hole region generates an obvious
stress concentration and thus decreases significantly the
stress magnitude in the immediate vicinity of the post-
initiation crack tip.
5 DISCUSSION
For the cracked specimens repaired by stop-holes and
the as-received cracked specimens, the fatigue life is
defined as the total remaining fatigue lives from the
pre-crack of 1 mm (or initial a = 5.8 mm) to the final
failure under the same fatigue loading conditions. This is
that the fatigue life of a specimen repaired by circular or
elliptical stop-holes is the summation of the fatigue
cycles related to the crack initiation from each stop-hole
edge and subsequent crack propagation. As described
earlier, the material removal L
r
from the fatigue crack-tip
was considered as a variable parameter, which ranges
from 0.3 mm to 0.9 mm in the study for two different
stop-hole geometries.
The fatigue life extension resulting from drilling a
stop-hole is due to the sharp crack-tip being replaced by
a blunt round-notch, which removed the stress singula-
rity that occurred at the original crack-tip. Figure 8a
shows the effect of varying the L
r
value on the
fatigue-life extension for circular and elliptical stop-hole
cases. Obviously, the application of both circular and
elliptical stop-holes generates significant improvement in
the fatigue life of cracked specimens compared with the
as-received cracked specimens. For the two different
stop-hole geometries, the cracked specimens with L
r
=
0.6 mm have longer remaining fatigue lives compared
with the same stop-hole geometry with L
r
= 0.3. How-
ever, the improvement in the life extension began to
reduce if L
r
was further increased to 0.9 mm. The im-
provements in the remaining fatigue lives are quantified
by: (Average of stop-drilled total fatigue lives – average
of as-received total fatigue lives)/average of as-received
total fatigue lives. For the average of the fatigue lives, the
L
r
= (0.3, 0.6, and 0.9) mm stop-holes improved the
non-stop-drilled total fatigue life by respective 43 %,
159 % and 98 % for circular stop-holes, and a respective
78 %, 239 % and 195 % for the elliptical cases. Further-
more, the difference in the fatigue life improvements
coming from circular and elliptical stop-holes for the
same L
r
value increases gradually when L
r
increases from
0.3 mm to 0.9 mm.
For all the cases in this analyses, the fatigue-life
improvement coming from the elliptical stop-hole
models are more efficient than that of the circular
stop-holes under the same L
r
value. After drilling a
stop-hole in the vicinity of a crack tip, the fatigue crack
emanates along the initial fatigue crack growth path and
from the hole edge where the stress reaches its local
maximum value. A larger curvature radius of the oval
vertex, reducing the stress concentration, is expected to
result in a higher fatigue-crack initiation life compared
with the case of circular stop-holes where the curvature
radius is smaller, thus that will be a significant reason for
increasing the total fatigue life of specimens with
elliptical stop-holes. This conclusion was evidenced by
the experimental results as shown in Figures 8c and 8d,
especially for the case of L
r
= 0.6 mm. Therefore, from a
practical point of view, the elliptical stop-hole with L
r
=
0.6 mm is suggested as an appropriate scheme for the
drilling stop-hole methodology.
Figure 9 illustrates of the fatigue initiation life after
drilling each stop-hole for different hole geometries and
also for different L
r
values. It is evident that the fatigue
initiation life after each stop-hole gradually decreases
from the 1
st
to the 6
th
stop-holes for both circular and
elliptical cases, which because the applied  K conti-
nuously increases with an increase of the crack length
under constant amplitude fatigue loading. For both the
C. ZHANG et al.: EFFECT OF STOP-HOLE-INDUCED MATERIAL REMOVAL ON THE FATIGUE PROPERTIES ...
462 Materiali in tehnologije / Materials and technology 53 (2019) 4, 457–465

circular and the elliptical stop-hole specimens, fatigue
initiation lives from each stop-hole edge increase signifi-
cantly when L
r
increased from 0.3 mm to 0.6 mm. How-
ever, the initiation lives of the circular stop-hole samples
are approximately the same for L
r
= 0.6 mm and 0.9 mm
cases, and the elliptical stop-hole specimens with L
r
=
0.6 mm generate higher initiation lives compared with
the L
r
= 0.9 mm case. After drilling each stop-hole,
although the L
r
= 0.9 mm specimen has slightly higher
 K than the L
r
= 0.6 mm during the subsequent fatigue
loading, the comparative differences between the
 K-values of the circular and elliptical stop-hole sam-
ples are only in the 0.56–0.90 MPa m (1.59–1.69 %)
depending on the crack length. Similar situations also
occur in the L
r
= 0.3 mm and 0.6 mm cases.
During fatigue loading, the non-uniformity of the
fatigue crack-tip from the surface to the middle of the
cracked specimen along thickness direction should be a
significant reason for explaining the influence of the L
r
variation on the fatigue life of the cracked specimens re-
paired by a stop-hole. To put the real crack-growth-
pattern into perspective, Figure 10 shows the macro-sec-
tion of a fatigue-fractured E-CT sample. It is conceivable
that the observed variation in the crack tip along the
thickness direction during the fatigue testing was formed
because of the different constraint from the surfaces to
the middle of the sample. For ductile materials like steel,
C. ZHANG et al.: EFFECT OF STOP-HOLE-INDUCED MATERIAL REMOVAL ON THE FATIGUE PROPERTIES ...
Materiali in tehnologije / Materials and technology 53 (2019) 4, 457–465 463
Figure 8: a) Effect of L
r
value on the remaining fatigue lives of cracked specimens repaired by different stop-holes (L
r
= 0 denotes the
as-received specimens), and fatigue-initiation-life comparisons between the circular and elliptical stop-hole cases for: b) L
r
= 0.3 mm, c) L
r
= 0.6
mm and d) L
r
= 0.9 mm
Figure 9: Fatigue initiation lives after drilling each stop hole with
different Lr-value: a) circular stop-hole, b) elliptical stop-hole

the most advanced and active crack-tip occurs in the
middle of sample, and the most underdeveloped and
sluggish crack tip is on the surfaces, which is the crack
length measured for the da/dN calculation. The maxi-
mum difference of the crack tip from the surfaces to the
center of the cracked specimen is approximately 0.6 mm.
It is a logical explanation that L
r
= 0.3 mm stop-hole
only removed the part of crack tip near the two surfaces
in which a new crack can easily form, L
r
= 0.6 mm
eliminated all the crack tips, and L
r
= 0.9 mm was greater
than the maximum difference of crack tip between
surfaces and the center of the specimen.
As is well known, stop-hole technique has both bene-
ficial and harmful influences on fatigue-life-extension of
a cracked specimen. That is, replacing the fatigue
crack-tip with a blunt stop-hole generates a longer
fatigue life for cracked specimen due to eliminating the
stress singularity at the original crack-tip, but drilling
hole-induced material removal from the crack-tip front
reduces the length of the fatigue crack propagation,
which removes the corresponding fatigue propagation
life. A key challenge with the stop-hole method is that it
is hard to balance them. To the best of our knowledge, it
is the first time that material removal L
r
is linked to the
fatigue life extension, specified by the total fatigue life,
initiation life after drilling each stop-hole and subsequent
crack growth rate.
Although the quantification is specific to DT4C steel,
the approach developed in this study can be applied to
the fatigue life extension of other metals and other types
of cracked components. Considering the extensive use of
the stop-hole method in engineering industries like air-
planes and high-speed rail, the elliptical stop-hole with
an appropriate L
r
(e.g., L
r
= 0.6 mm for DT4C steel plate
with 10 mm thickness in this study) is recommended as
an efficient approach for fatigue life improvement in
cracked metallic structural components.
6 CONCLUSIONS
The effect of material removal L
r
from the crack-tip
front on the fatigue properties of cracked E-CT speci-
mens repaired by either circular or elliptical stop-holes
was studied by measurements of the crack initiation life
and post-initiation da/dN after drilling each stop-hole,
and the remaining fatigue life. The major findings can be
drawn as follows:
For both circular and elliptical stop-hole methods,
drilling a stop-hole with different L
r
value ranged from
0.3 mm to 0.9 mm could provide a considerable fatigue
life extension and a dramatic reduction in the post-initi-
ation da/dN due to the incorporation of compressive
residual stresses in the near-stop-hole region and the
reduction of the stress magnitude in the immediate vici-
nity of the post-initiation crack tip.
Due to the non-uniformity of the crack-tip in the
sample thickness direction, material removal L
r
isakey
parameter affecting the fatigue-life extension of a
cracked specimen repaired by stop-holes.
The fatigue-life extension of cracked specimens is
more efficient for L
r
= 0.6 mm than any other L
r
value for
the stop-hole method. For average of the fatigue lives in
this study, the L
r
= (0.3, 0.6, and 0.9) mm stop-holes in-
creased the non-stop-drilled remaining fatigue life by a
respective 43 %, 159 % and 98 % for circular stop-holes,
and increased by a respective 78 %, 239 % and 195 %
for elliptical stop-holes.
The magnitude of the fatigue-life improvement for
the elliptical stop-hole specimens is significantly greater,
which resulted from the larger curvature radius of the
oval vertex, lowering the stress concentration, than that
for the samples containing circular stop-holes with the
same L
r
.
Acknowledgements
This work was supported by the National Natural
Science Foundation of China (No. 11672048 and
51405029), Shaanxi Province (No. 2017KJXX-11 and
2016JQ5020), and Chang’an University (No.
310825153510).
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