Engineering FailureAnalysis, Vol. 3, No. 2, pp. 137-147, 1996
Pergamon
Copyright © 1996 Elsevier Science Ltd
Printed in Great Britain. All rights reserved
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PII:S1350-6307(96)00006-4
COMPARISON OF FATIGUE CRACK RETARDATION METHODS
ZELJKO D O M A Z E T
Department of Mechanical Engineering, FESB-University of Split, 21000 Split, R. Bo~kovi6a bb, Croatia
(Received 2 November 1995)
Abstract--The efficiency of several fatigue crack retarding methods are examined. Al-alloy flat
specimens with a central "key hole" starter notch were tested in a closed-loop servo hydraulic test
machine. After the cracks had propagated to a predefined value, the following crack retarding methods
were undertaken: repair welding, welded metal reinforcements, adhesively bonded CFRP patches, single
peak tensile overloads and fatigue crack arrest holes. Fatigue test programmes were performed with
constant load amplitudes and spectrum tensile loads. The results show that expanded and filled crack
arrest holes, and CFRP patches applied to both sides, are the best fatigue crack retarding methods with
all load conditions. Copyright (~ 1996 Elsevier Science Ltd.
Keywords: fatigue crack retardation, repair welding, CFRP patches, arrest holes
1. N O M E N C L A T U R E
a
ao
E
EI
E2
G
KT
N
R
Sa
Sa
SE
v
half length of crack, mm
crack length at repair, mm
modulus of elasticity, MPa
longitudinal Young's modulus, MPa
transverse Young's modulus, MPa
shear modulus, MPa
stress concentration factor
number of cycles
cyclic stress ratio
maximum spectrum amplitude ratio
cyclic stress amplitude, MPa
spectrum maximum stress amplitude, MPa
residual stress, MPa
Poisson's ratio
2. I N T R O D U C T I O N
Examinations of structural failures reveal that many such failures are due to fatigue fracture
[1, 2]. A fortunate circumstance with many fatigue failures is a relatively long crack
propagation period from the original defect to final failure. Hence, the crack can be
discovered easily and one of the following actions can be undertaken:
(a) unloading the system and replacing the cracked component or the whole structure;
(b) reducing the external loads and continuing careful crack growth control;
(c) retarding, stopping or eliminating the crack.
As conventional repairs involving complete replacement can be time-consuming and expensive, and reduction of service loads with existing fatigue cracks is questionable, quick and
simple crack retardation methods seem to be the best solution. The real question is which
crack repair method should be applied in any particular case. Because there are numerous
repair methods, the aim of this research was to find out the most effective methods and to
define the parameters for their optimal performance. The best methods were determined by
comparing the respective fatigue lives of cracked specimens which had been repaired in
different ways and loaded with constant and variable cyclic loads.
137
138
ZELJKO D O M A Z E T
3. EXPERIMENTAL PROCEDURE
3.1. Specimens
Fatigue crack propagation panels, 250 mm long and 80 mm wide, were cut from a single
sheet of 10 mm thick AICuMg2 (DIN 1725 or 2024-T3) aluminum alloy. The chemical
composition and mechanical properties are given in Tables 1 and 2, respectively.
The longitudinal axis of the specimens was aligned with the rolling direction of the sheet.
The free length of the panel between the grips was twice the panel width and central
10 m m x 0.35 mm notches ("key hole") were machined in the transverse direction to provide
a crack starter (Fig. 1).
3.2. Test procedure
Cycle fatigue tests were performed at a room temperature of 20 to 22 °C and a relative
humidity of 45 to 50%, with a 250 kN closed-loop servo hydraulic, computer controlled,
testing machine ("Schenck" type) at the Fraunhofer Institut ftir Betriebsfestigkeit (LBF) in
Darmstadt. Nominal stress amplitudes were Sa = +72MPa (R = - 1 ) with constant load
amplitude, and Sa = +_72 MPa (R = 0) with load spectrum, calculated for the load carrying
cross-section of the specimen. This programme was carried out using an eight-block variable
loading spectrum, based on the Gaussian type distribution. Spectrum size was 105 cycles, with
a frequency of 1-60 Hz, depending on the load level. The crack length measurements were
obtained using a panoramic microscope (magnification 20x). After the crack had propagated
to a predefined value (ao = 12 ram), different crack retarding methods were undertaken.
After the specimens were treated, they were again loaded under the same load conditions.
The best methods were determined by comparing the fatigue lives (number of cycles). All test
conditions were carried out with three specimens.
4. CRACK RETARDING METHODS
The following crack retarding or crack stopping methods were used.
4.1. Repair welding
The most frequent repair method for all weldable metals is repair crack grinding and
welding. The entire fatigue crack was ground out as a preparation for an X-form weldment.
The welding procedures were performed at the SLV Institute in Mannheim to ensure the best
repair weld quality. The welding parameters were as follows:
Welding process
Welding current
Shielding gas
Gas flow
Filler material
MAG
250 A
Argon
12 lmin -~
SG-AIMg4.5MnZr (DIN 1732)
Table 1. Chemical composition of AICuMg2 alloy
Wt%
Si
Fe
Cu
Mn
Mg
Cr
Zn
0.11
0.23
4.62
0.58
1.62
0.01
0.01
Table 2. Mechanical properties of A1CuMg2 alloy
Tensile strength
Yield strength (0.2% offset)
Modulus of elasticity
Elongation
Poisson's coefficient
486 MPa
396 MPa
73,300 MPa
14.7%
0.33
Comparison of fatigue crack retardation methods
;
]
,
139
,
thickness 10 mm
80
%=12 %=12
8
L
10
In the grips
Fig. 1. Specimen configuration.
In half of these specimens, a new starter notch was machined to ensure the same crack origin
and crack propagation conditions (Fig. 2).
4.2. Metal reinforcements
As a possible solution for fatigue crack retardation, strengthening panels are bolted or
welded. In this research metal reinforcements (metal patches) were welded over the cracked
area of the specimens. Patches were applied to one and both sides (Fig. 3). The patch
material was the same as the specimen panel, while patch shape and size were chosen to
replace the cracked portion of the specimen cross-section.
4.3. CFRP patches
The repair of fatigue cracked specimens with adhesively bonded carbon fibre reinforced
plastic (CFRP) patches was investigated. The method offers several advantages over otheI
ZELJKO DOMAZET
140
!
I
I
i
2
tJ
::::J
;o
i
4
I
new s t a r t e r notch
//
/
f
i
70
r
Fig. 2. Fatigue crack grinding and welding (with and without new starter).
techniques [3, 4]; for example speed of repair, absence of new holes or welds which might act
as crack initiators, minimum increase in weight, stiffening and sealing the area around the
crack, curved and double curved surface application etc.
The laminate material studied was a unidirectional composite, consisting of carbon fibres
and epoxy matrix, 0.12 mm thick. The laminates were cured in an autoclave at 175 °C and at
a pressure of 8 MPa. The 20-layer symmetric composite (02/90/0/+45/0/90/02) was chosen as
the standard patch type (2.5 mm thick). This CFRP composite is of good quality, which is
illustrated by its excellent mechanical properties as shown in Table 3 [4]. For the investigations covered in this paper, "3M" XA 9323 adhesive was used. Composite patches were glued
over the crack area on one and both sides (Fig. 4).
4.4.
Single peak overloads
In this part of the investigation, the fatigue crack growth rate following a single tensile
overload was studied. The overload normally caused a slight increase in the crack growth rate
followed by a significant decrease in the growth rate and then a gradual increase back to the
steady state growth rate. During this reduced growth rate period, a significant delay may
occur [5, 6]. If the overload is sufficiently large, complete crack arrest can occur. The single
peak overloads were performed by constant amplitude tests in two different ways:
(a) 50% tensile overloads every 10% of crack length propagation, and
(b) 100% tensile overloads every 3 mm of crack length propagation.
Comparison of fatigue crack retardation methods
141
5
lO
//A-,~
i so
///'1- ~1
" / A ,"
Fig. 3. Welded metal patches.
Table 3. Mechanicalproperties of applied composites
LongitudinalYoung's modulus, E1 (MPa)
Transversal Young's modulus, E2 (MPa)
Shear modulus, G12 (MPa)
Poisson's coefficient,v12
Laminate properties
Patch properties
140,000
9700
4600
0.31
93,200
41,900
10,900
0.22
4.5. Crack arrest holes
Crack arrest holes, one of the most successful methods of fatigue crack repair, was analysed
extensively. After the fatigue crack had propagated to the predefined value, crack arrest holes
were made at the crack tips as follows:
(a) simple crack arrest holes (q~ 8 mm H7);
(b) 5% enlarged (cold-worked) crack arrest holes (q~ 8.4 ram);
(c) 5% enlarged crack arrest holes filled with steel pins (q~ 8.45 mm).
In spite of the extensive literature dealing with these techniques, there are few theoretical
explanations of the improvements [1, 7, 8]. In order to explain the differences between the
fatigue behaviour of the virgin specimen and specimens with differently treated holes, the
stress distributions caused by external loads, and residual stresses caused by mandrelizing and
inserting oversized pins, have to be determined. Therefore, numerical and analytical methods
of stress and strain determination in the annular material volume around the hole were
carried out. Numerical analysis was performed by the finite-element method (FEM) on a PC
ZELJKO D O M A Z E T
142
80
~/oJ~2
/
/
/ !
I
CFRP
i i'
~v
,,.,,.,-,r,~~///////2////////~//~
T
ii/
/ i\
]
AI CuMg 2
~
~//,b'/////////////////////~
CFRP
~////////////////~
AI Cu~gg 2
_
~t
CFRP
II
2~0
il
ili
<b
coo
I
Ji/i;'
il
{(
I
i!
i
8O
Fig. 4, C F R P patches.
using the NONSAP program (Fig. 5). Plane stress conditions were assumed. The material was
defined as elastic-ideally hardening, following the von Mises yield condition [9].
An experimental analysis was performed by measuring the strains in the longitudinal and
transversal directions by means of 46 strain gauges [1, 10! (Fig. 6). Because of plastic
deformation and the high gradient in the critical area, these strains had to be recalculated by
Benning's semi-empirical method [11].
Analytical methods, according to H s u - F o r m a n (plane stress conditions) and RichImpellizzeri (plane strain conditions) were carried out for the sake of comparison, and as the
simplest solutions in practice [1, 12, 13].
5. TEST RESULTS
The test results are summarized in Fig. 7 (constant load amplitude) and Fig. 8 (spectrum
load amplitudes) where the fatigue life is plotted in terms of cycles to failure.
The a - N curves are plotted for all fatigue crack retarding methods as well as for crack
propagation without any repair procedure. At both load conditions, the best results were
obtained with CFRP patches bonded to both sides and cold-worked crack arrest holes filled
with pins. The results are reviewed as follows.
Comparison of fatigue crack retardation methods
143
J
11
x\\\\\~
Fig. 5. Finite-element mesh (l/4 of specimen).
Fig. 6. Strain gauges.
5.1. Repair weldments
Repair grinding and welding represent the removal of one large defect, but the possible
introduction of small welding defects in a critical area of the component. Hence, the fatigue
behaviour of repaired components is strongly influenced by the weld shape and weld quality.
According to [1, 2, 14, 15], only the best welding methods and welding parameters are
permitted in welding repair jobs. Given these precautions, the fatigue test results on
AICuMg2 specimens are not encouraging. Fatigue lives of welded specimens with new starter
notches were practically the same or even less than the fatigue lives of unrepaired specimens.
5.2. Metal patch reinforcements
Improvements in fatigue lives of specimens with one-sided welded patches were insignificant, mostly because of additional out-of-plane bending in the specimen. Multiple improvements were obtained with two-sided welded patches, primarily because of the great reduction
144
ZELJKO DOMAZET
All I
i{?s,?5,]{>?
' L I {,
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1
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.
~8,',
-" [
,
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E
~
ANi/ANc
!-
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0 without crack retardation
v
0
L
30C
%a : ' 72 MPo
50% overtoads
R :
2
repair weld and new starter
1
()
20
L(
7 - ~ i m p i e crack arrest holes
///iJ)
A{ CuMg 2
.J
//3
?
Y
tO-
.
.
metat patch-one side
10
.
.
r
"
I
i
t
,
i
3
metal patches -both sides
11
100% overloads
5
CFRP patch - one side
8
expended crack arrest holes
1
welded wltt~out slarter
6
CJ RP patches - both s~de',
9
expended hotes with pins
J
I
i
J
I
i
l
I
i
i
I
I
105
10A
J
I
i
I
I
]
I
1
i
r
r
r
i
f
107
106
Number d cycles
(to 9 )
Fig. 7. Results of different crack retardation methods on specimens loaded with constant amplitude,
R=-I.
No=I4 002
_
AN
_
__ Ii !
~.o <<9i~o._o
2~.2
80.% >120 >163
@ 1i9¢®¢ 4> I
0~
I
"N
0 without crack retardation
!
l, metat patch - one side
E
Sa : z 72 MPa
0
F~--O
~ 20-
2 welded and ne,N starter
I
% CFRP patch- one side
H : 1051p :0.2
crack arrest holes
At Cu Mg 2
welded without starter
#
()
expended crack arres toles
meta{ patches b~h sides
CFRP patches~both sides
o o : 12 mrn
expended hotes and pins
Y
. . . .
10~
jli
T
T
i
f
r
~
r
i ]
10 5
I
i
i
i
i
i
106
Number of cycles
(log)
Fig. 8. Results of different crack retardation methods on specimens loaded with variable amplitudes,
R=0.
of stress at the location of the fatigue crack. A new critical location appears at the fillet weld
toe between the reinforcement and the specimen, in the load direction. With both constant
and variable load amplitudes, new fatigue cracks originated from these locations and, through
semi-elliptical crack propagation, led to final failure.
Comparison of fatigue crack retardation methods
145
5.3. Effect of single peak overloads
Examination of these results shows that performing periodical tensile overloads increases
the remaining fatigue lives. Increasing the overload ratio also increased the amount of delay,
which is primarily influenced by the plastic zone size and the residual stress in it. The
significant improvement was reached only in the case of 100% tensile overloads applied every
3 mm of crack growth. That means that the benefits of this popular method can mostly not be
used in practice. Namely, very few real structures could bear 100% overloads without any
consequences. Further investigations in this direction were therefore not continued.
5.4. Bonded CFRP patch repairs
The use of adhesively bonded composite patches as a fatigue crack repair method showed
very good results. In the case of application on both sides, the endurance limit was reached
with all load conditions. The main reasons are the large stress reduction (ca 50%) in the
cracked cross-section as well as the absence of any new stress concentrations. Fatigue life
improvements were encouraging with one-sided CFRP applications, primarily due to favourable stress redistribution in specimens with bonded composite reinforcement, caused by a
small patch thickness (Fig. 9) [4].
5.5. Fatigue crack arrest holes
Different results were obtained with different mechanical treatments of fatigue crack arrest
holes. An improvement was reached by boring the crack arrest holes in the tips of the cracks
(at both load conditions). This improvement is influenced by the crack size, hole diameter,
plate width (stress concentration) and surface quality of holes (roughness). Considering the
reduction of cross-section and high stress concentration factor (KT = 3), better results could
not be expected [Fig. 10(a)].
Multiple fatigue life extensions were obtained with 5% cold-worked holes bored in crack
tips. The main reason for such improvements are favourable compression residual stresses in
the critical material volume [Fig. 10(b)]. These compression residual stresses postpone the
I F=100 kN
Cross sections
A-A
I
B-B
i
- - E
in A[ - plate
Measured by
strain gauges
,I 2
( */o, )
in CFRP - patch
1.0
0
Dimensions
:
CFRP - patch
80x 50x 2.5
o
i
!
' I' '
-
=
-
a----I
B--'-I
Fig. 9. Strain distribution in specimen with bonded CFRP patch.
AICu Mg2 plate
245 x 80 x 10
146
ZELJKO D O M A Z E T
-.
6~
600
K-r=3
. ~,00
'00 k
d>J
ZOO
to
c;
f<
r:
I0
5
'
'
4
20 ~ (ram
SEt ~ S E 2 : 0
<n
/
to
KT= ]
3
10
1
'5
2()
"i
I
i
'
"%E2
r(m[n) i
4u~
-z:OO
t:2
~00
d
,;i ,,;
z0g
-
200
200
W)
to
~
0
0
N
- >/X
200' " \
• "~" S E 2
SE~
ZOOa)
SIt4PLE HOLES
4OO;
',~) b
%
'-;E 2
E N L A R O [ I) I < L E S
c ) 5°L ENLARGED HOLES
FILLED WITH PINS
Fig. 10. Residual stresses and external load stresses in all three cases of crack arrest holes.
origin of the new crack and, additionally, the plastic zone of surrounding material slows down
its propagation if the crack continues.
The best results of all crack arrest hole treatments, and of all the investigated crack
retardation methods, were obtained by crack arrest holes with 5% enlargement and filled with
1% oversized steel pins. In both load cases, the endurance limit was reached without any new
visible damage. The improvements consist of favourable residual stresses [Fig. 10(c)] as well
as a much lower stress concentration factor (KT = 1.5).
Residual stress and external load stress distributions (Fig. 10) showed very good agreement
with all three determinations [1, 10]: numerical, experimental and analytical.
6. C O N C L U S I O N S
(a) The best fatigue crack retardation methods for A1MgCu2 specimens are CFRP patches
applied on both sides and cold-worked crack arrest holes filled with steel pins.
(b) Large stress reductions and the absence of any new stress concentrations or out-of-plane
bending account for the results obtained with CFRP patches.
(c) Oversized pins in cold-worked crack arrest holes lead to the best results because of
favourable residual stresses, low stress concentration and load transfer.
(d) Other methods show variable results caused by material weldability, out-of-plane bending, and new stress concentrations.
Acknowledgements--The author is grateful to Professor Dr Vatroslav Grubigid for his helpful suggestions and to the
Fraunhofer Institut (LBF) in Darmstadt for making the experimental work possible.
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1.
2.
3.
4.
5.
6.
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