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Materials Science Forum
ISSN: 1662-9752, Vol. 889, pp 143-147
doi:10.4028/www.scientific.net/MSF.889.143
© 2017 Trans Tech Publications, Switzerland
Submitted: 2016-10-31
Accepted: 2016-11-03
Online: 2017-03-20
Investigation on Mode I Propagation Behavior of Fatigue Crack
in Precipitation-Hardened Aluminum Alloy with Different Mg Content
S.F. Anis1,2, a, M. Koyama2,b and H. Noguchi2,c *
1
Department of Mechanical Engineering, University of Technology, Malaysia, Jalan Sultan Yahya
Petra, 54100 Kuala Lumpur, Malaysia
2
Department of Mechanical Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka,
819-0395, Japan
a
sfa800@gmail.com, bkoyama@mech.kyushu-u.ac.jp, cnoguchi.hiroshi.936@m.kyushu-u.ac.jp
Keywords: Al6061-T6 alloy, Rotating bending fatigue test, Fatigue striation, Mode I crack,
Dynamic strain aging.
Abstract. The influence of excess Mg on the Mode I propagation of fatigue crack was examined in
newly developed precipitation-hardened Al alloy containing Zr and excess Mg. The aim of this
study was to evaluate the underlying factor affecting fatigue crack growth rate in the stage II region.
For this purpose, the rotating bending fatigue tests were performed in constant amplitude loading,
and replication technique with an optical microscope was used to measure the crack growth in the
Al alloys. Through analyses of the crack propagation on the specimen surface and striation
formation of the fracture surface, the effects of excess Mg in the Al alloys were clarified to promote
the occurrence of mode I fatigue crack, and decelerate the fatigue crack propagation. These facts
suggest that the dynamic strain aging of Mg induces the formation of fatigue striation and reduce
the driving force of the crack propagation. The findings were supported by the fractographic
observations in the fatigue crack propagation region.
Introduction
Aluminum alloy 6061 is commonly used in engineering application such as automotive parts and
aircraft structure because of their excellent corrosion resistance, light weight, and good weldability.
However, there are still undesired characteristics of this alloy such as no fatigue limit, less fatigue
resistance and a large scatter in fatigue life [1] compared to steel [2]. Therefore, fatigue behavior of
this alloy has been subjected to numerous studies to improve fatigue material properties [3] [4]. For
instance, several researchers [5] [6] have developed a new 6061-based alloy with an additional
content of Mg, and they have successfully confirmed the ability to generate fatigue limit in facecentered cubic (FCC) metals via fatigue test and coaxing effect test. However, the effect of excess
Mg on fatigue crack propagation behavior in the Al6061-T6 alloy has not been investigated
sufficiently.
Generally, the fatigue crack growth can be divided into two stages; stage I and stage II [7]. The
cracks begin to grow across several grains in the stage I are strongly affected by slip characteristics
and material surface condition such as microstructure and surface roughness. Meanwhile, in stage
II, a crack tends to propagate perpendicular to the loading direction, and the fatigue crack growth
rate (FCGR) depends on the crack growth resistance of the material. An important characteristic of
this stage is the presence of fatigue striation formation that represents Mode I fatigue crack growth
[8].
The aim of this study was to investigate the influence of excess Mg on fatigue crack propagation
behavior in the stage II region. The investigation has not limited to the observation of fatigue crack
growth on the specimen surface, but also included the fractographic examination of the fracture
surface. The implications of the results for the fatigue crack propagation and the striation formation
were then discussed.
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans
Tech Publications, www.scientific.net. (#103369317, University of Auckland, Auckland, New Zealand-12/11/17,10:33:01)
144
Engineering and Innovative Materials V
Material characterization
The present study deals with two types of newly developed precipitation-hardened Al6061-T6
alloys with an additional element of Zirconium (Zr) in both Al alloys and the presence of excess
Magnesium (Mg) in the latter, in which we denote as Al6061-Zr and Al6061-Zr-Mg. The addition
of Zr to Al alloys has exhibited positive effects on microstructure via grain refinement and
improvement in the mechanical properties [9]. On the other side, the influence of Mg in Al alloys
has been investigated to promote dynamic strain aging (DSA) [10] [11] and to assist nonpropagation of a crack in the fatigue limit regime [5] [6]. Details about the chemical composition
for both Al alloys are listed in Table I.
The manufacturing processes for both Al alloys were as follows. The billets of these alloys with
a diameter about 155 mm were obtained by a semi-continuous casting method and subjected to a
homogenization treatment at 823 K for 14,400 s. The billets were extruded to form the round bars
with a diameter of 23 mm at 773 K. The extruded bars were then solid solution treated at 813 K for
3600 s in an air furnace and immediately water-quenched. Subsequently, T6 aging was carried out
at 463 K for 14,400 s.
Table 1. Chemical composition of the precipitation-hardened Al alloys (wt%)
Element
Si
Fe
Cu
Mn Mg
Cr
Ti
Al
Zr Excess Mg
Al6061-Zr alloy
0.55 0.21 0.24 0.09 0.9 0.26 0.02 bal. 0.16
Al6061-Zr-Mg alloy 0.55 0.2 0.23 0.09 1.39 0.26 0.02 bal. 0.14
0.49
Experimental Procedure
Smooth specimens of the alloys were used for rotating bending fatigue test with the geometry
shown in Fig. 1(a). All specimens were mechanically polished using fine emery papers with the grit
number up to 3000 at the central of the specimens to remove scratches on the surface, and
subsequently electro-polished at 30 V and 323 K in a solution of 40 g of gelatin, 40 g of oxalic acid
dehydrate and 2000 ml of phosphoric acid with a concentration of 85% to eliminate the work
hardened layer.
Fatigue tests were carried out by using Ono-type rotating bending fatigue test machine in room
temperature at the stress ratio R was equal to -1 and frequency f of 60 Hz (sinusoidal waveform).
The replica technique with a microscope was used to observe the crack growth on the specimen
surface. A schematic illustration of crack length measurement is depicted in Fig. 1(b). The
fractographic observation of the fracture surfaces was performed by a scanning electron microscope
(SEM). To investigate the behavior of the Mode I crack, the area of fatigue striation was measured
at seven different regions, located at about 500, 1000, 1500, 2000, 2500, 3000 and 3500 µm from
the crack initiation site, as shown in Fig. 2(a). Fig. 2(b) shows the size of the examination area in
each region. The striation area was calculated as striation ratio of the striation area to the
observation area.
(a)
(b)
Fig. 1. Schematic illustration: (a) rotating bending fatigue specimen, and (b) fatigue crack
measurement.
Materials Science Forum Vol. 889
145
Fig. 2. SEM images of the fatigue fracture surfaces: (a) seven regions of fatigue striation evaluation,
and (b) size of observation area for each region.
Results and discussion
S-N curves. Fig. 3 compares the S-N diagrams of the two Al alloys. The fatigue tests were
performed at four different stress amplitudes. It was found that the cracks were easily coalesced
each other at high stress amplitude of 250 MPa due to the easiness of crack initiation. At finite life
regime, even with additional content of Zr, the fatigue strength of Al6061-Zr-Mg alloy
demonstrates similar result from the previous studies [5] [6] with a distinct fatigue limit, which is
absent in the Al6061-Zr alloy. This phenomenon was believed to be due to the addition of a strain
aging capability that can be strengthening the material in the vicinity in front of the crack tip. This
finding implies that the ability of Al alloy with additional Mg content resists against small crack
growth. In contrast, for Al alloy without excess Mg, the fatigue failure occurred even in the range of
fatigue limit (107 ~ 108). To minimize the tendency of occurrence crack coalescence, the
investigation on Mode I fatigue crack growth behavior is focused on the stress amplitude of
200 MPa.
Fatigue crack growth behavior. In both Al alloys, the fatigue crack was initiated at defect on
the specimen surface due to high local stress concentration, and Fig. 4 compares the fatigue crack
growth curves of the main cracks at stress amplitude of 200 MPa. Generally, fatigue crack in the
Al6061-Zr-Mg alloy was initiated earlier than that in the Al6061-Zr alloy at the beginning of
initiation life. The easiness of the crack initiation in the Al6061-Zr-Mg alloy is attributed to the
existence of DSA in this alloy. It is known that strain localization associated with DSA can assist
fatigue crack initiation [12]. However, the fatigue crack in Al6061-Zr-Mg alloy propagates more
slowly with increasing crack length than that in the Al6061-Zr alloy. It can be seen clearly after 2
105 cycles. This implies the different behavior of fatigue crack propagation in both Al alloys, and
might be influenced by the Mode I crack. To investigate these behaviors in more detail, we
observed the fracture surface by using SEM.
Influence of excess Mg on mode I propagation behavior. By fractographic observations, the
Mode I fatigue crack propagation could be examined based on the formation of fatigue striation on
the fracture surface. To examine the different behavior of Mode I crack, the striation ratios were
analyzed to determine the influence of excess Mg in generating Mode I crack in the stage II region.
It found that Al6061-Zr-Mg alloy is easier to generate Mode I crack with a larger fatigue striation
area compared to Al6061-Zr alloy, as shown in Table II. The retardation of the FCGR in Al6061Zr-Mg alloy might be due to the high tendency in generating striation formation. This phenomenon
possibly facilitates crack closure mechanism [13], which induces contact between mating fracture
surfaces during the loading cycle, resulting in the reduction of the local driving force for the crack
growth and deceleration of the FCGR.
146
Engineering and Innovative Materials V
Fig. 3. S-N fatigue data of precipitationhardened Al alloys.
Fig. 4. Crack length versus number of
cycles at the stress amplitude of 200 MPa.
Table 2. Effect of excess Mg on striation formation in Al alloys.
Distance from crack
Striation ratio, AS/AO (%)
initiation site (µm)
Al6061-Zr alloy
Al6061-Zr-Mg alloy
500
0.35
0.48
1000
0.66
1.32
1500
1.03
8.03
2000
6.30
17.03
2500
14.85
36.20
3000
34.40
60.95
3500
59.90
81.10
Conclusions
In this study, the fatigue crack propagation in stage II region was examined experimentally by
using smooth specimen. The influence of excess Mg on Mode I fatigue crack propagation has been
highlighted. Based on experimental results, the main conclusions obtained in this study can be
drawn as follows:
(1) The fatigue crack in Al6061-Zr-Mg alloy was initiated earlier and propagate slower in stage
II region than that in the Al6061-Zr alloy.
(2) The fatigue striation examination confirmed the enhancement of the Mode I crack
propagation in the Al alloy with additional Mg content, and it may be attributed to the DSA
effect in this alloy.
(3) The retardation of FCGR in Al6061-Zr-Mg alloy was believed to be due to the high
tendency in generating striation formation, which possibly facilitates crack closure
mechanism.
In addition, it is speculated that the DSA of Mg has a significant influence on FCG behavior in
Al6061-T6 alloy through restricting the motion of the dislocation, which requires more force to
cause the dislocations to facilitate slip along the crystal planes. Further research would be continued
to examine in detail the effect of DSA on Mode I fatigue crack growth. An understanding of the
crack growth behavior of Al6061-T6 alloys with excess Mg is necessary because the scatter of
FCGR might be influenced by the DSA phenomenon. These studies will be addressed in the phases
of future research.
Acknowledgement
One of the authors (S.F. Anis), would like to thank for the financial support provided to this study
by the Ministry of Higher Education, Malaysia (MOHE) and University of Technology, Malaysia.
Materials Science Forum Vol. 889
147
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