第一工業大学研究報告第26号（2014）pp.35－39

35

Effect of Moisture on Fatigue Crack Growth of
Age-Hardened Al Alloy

Effect of Moisture on Fatigue Crack Growth of
K. KARIYA and N. KAWAGOISHI
Age-Hardened
Al Alloy
12K. KARIYA1 and N. KAWAGOISHI2
Department of Mechanical systems Engineering,Daiichi Institute of Technology,kirishima 899-4395,Japan
E-mail 1k-kariya@daiichi-koudai.ac.jp
2n-kawagoishi@daiichi-koudai.ac.jp

ABSTRACT: The effect of moisture on the growth behavior of a fatigue crack was investigated for an
extruded bar of age-hardened Al-Zn-Mg-Cu alloy (7075-T6) in relative humidity of 25% and 85% at
frequencies of 50 Hz and 6 Hz under rotating bending. Fracture in high humidity was a shear mode inshort life region and a tensile one in long life region at 50 Hz, though all the fractures were a tensile
one in low humidity. Fracture surface was covered with many slip planes and voids in the shear mode
fracture, while a few brittle facets were observed in addition to striations in high humidity. On theother hand, at 6 Hz in high humidity, fracture occurred in the tensile mode with striations and brittle
facets even in short life region. The difference in fracture mechanism depending on the fatigue life andfrequency was explained from the difference in amount of hydrogen generated by the reaction ofaluminum with water vapor.
Keywords: Extruded 7075-T6, Moisture, Shear mode crack, Tensile mode crack

1. INTRODUCTION
High strength Al alloys have no definite fatigue limit
and their fatigue strengths are very low in comparison
with their high static strengths [1]. In addition, theirfatigue strength are largely influenced by ambient air,even though the humidity is mild for low and medium
strength metals [2]. Therefore it is important to know
the effect of humidity on fatigue properties.
In the present study, rotating bending fatigue tests
were carried out in relative humidity of 25% and 85%
at frequency of 50 Hz and 6 Hz for an extruded bar ofage-hardened Al-Zn-Mg-Cu alloy to investigate theeffect of moisture on the growth behavior of a fatigue
crack.
2. MATERIAL AND EXPERIMENTAL
PROCEDURE
Material used was an extruded bar of age-hardened
Al-Zn-Mg-Cu alloy, 7075-T6. The chemicalcomposition (mass %) of the alloy was 0.09 Si, 1.47Cu, 0.25 Fe, 0.03 Mn, 2.56 Mg, 0.19 Cr, 5.46 Zn, 0.03
Zr, 0.03 Ti, and the remainder Al. The mean grain size
was about 13 μm. The alloy had a marked texture of a(111) plane at the cross section of the bar. The

mechanical properties were 527 MPa of 0.2% proof
stress, 673 MPa of tensile strength, 712 MPa of true
fracture strength and 11.3% of reduction of area,respectively.
Fatigue specimen was a smooth one with 8 mm indiameter. The observation of fatigue damage and themeasurement of crack length were conducted under ascanning electron microscope (SEM) or under an
optical microscope by using the plastic-replica
technique. Surface crack length, ℓ, was measured in thecircumferential direction of the specimen in bothpropagations of a tensile mode and a shear one. Fatigue
tests were carried out using a rotating bending fatigue
testing machine operated at 50 Hz and 6 Hz repetitions
in relative humidity (RH) of 25% and 85%. The
deviation of humidity was RH±5% and temperature inambient air was not controlled but was 25±3°C.
3. EXPERIMENTAL RESULTS ANDDISCUSSIONFigure 1 shows S-N curves in constant humidity of
25% and 85% at 60 Hz. In the figure, typical crackmorphologies at low and high stress levels are alsoshown. Fatigue strength was decreased by highhumidity. Fatigue strength at 107 in 85% decreased toabout 70% in comparison with that in 25%.

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Stress amplitude a (MPa)300
σa=260MPa250

σa=230MPa

200
150
100

RH25%
RH85%?

50

σa=260MPa05

σa=180MPa
61071081010

Number of cycles to failure Nf (cycle)
Fig. 1: S-N curves and macroscopic appearance of fracture at 50 Hz(a) In 25%(σa=240MPa, striations)(b-1) At high stress level(σa=260MPa, striations)(b-1) At high(a) In 25%stress level(σa=240MPa)(σa=260MPa)(b-2) At low stress level(b-3) At low stress level(σa=200MPa, striations)(σa=180MPa, brittle facets)(b) In 85%
Fig. 2: Fracture surfaces2(b-2) At lowstress level(σa=200MPa)(b) In 85%Fig. 3: Fracture surfaces

KARIYA and KAWAGOISHI：Effect of Moisture on Fatigue Crack Growth of Age-Hardened Al Alloy

37350

Stress amplitude σa (MPa)

300
250

σa=240MPa

200
150

RH25% (6Hz)
RH85% (6Hz)
RH25% (50Hz)
RH85% (50Hz)

100
500?

σa=260MPa
510

67

10

10810

Number of cycles to failure Nf (cycle)
Fig. 4: S-N curves and macroscopic appearance of fracture at 6 Hz
Macroscopic appearances of fractures are a shear
mode at high stress levels in high humidity, thoughthose are a tensile mode in other conditions ofhumidity and stress level.
Figure 2 shows fracture surfaces yielded by theshear mode crack and the tensile one. Fracture surface
was covered with many slip planes and voids in theshear mode fracture, while it was with striations in thetensile mode fracture. In addition, a few brittle facets
are also observed at low stress levels in high humidity.
The fracture surface yielded by the shear mode crack
was a (100) plane which was confirmed by etch pit
method.
Figure 3 shows morphologies of the tensile crackmode and the shear mode one. The macroscopic
growth direction of the shear mode crack is about 35°
to the specimen axis and the shear mode crack isstraighter than the tensile mode crack, meaning thatthe deformation of the shear mode crack are localized
at the crack tip.
Figure 4 shows S-N curves and typical fracture
morphologies at 6 Hz. In the figure, S-N curves at 50
Hz are also indicated by lines only. The tests at low
frequency were carried out to investigate the effect ofexposure time in high humidity. As well known, there
is no or little influence of frequency on fatigue life in
low humidity. While, in high humidity, fatigue life at
low frequency was larger than that at high frequency
in spite of decrease in crack initiation life due tolonger exposure in high humidity, i.e. promotion of
crack initiation by anodic dissolution. In addition, thespecimen fractured in the tensile mode.
Figures 5 and 6 show morphologies of cracks andfracture surfaces in high humidity at stress level where
the shear mode fracture occurred at high frequency. In
case of the low frequency test, a crack propagates in azigzag manner and a few brittle facets are observed inaddition to striations at fracture surface similar to theresults at low stress levels in high humidity at 50 Hz
as shown in Fig.2 (b-3).
From the results mentioned above, the difference infracture mechanism depending on fatigue life andfrequency in high humidity may be explained asfollows. High humidity is a sever environment morethan in hydrogen gas. Moreover, in high humidity,hydrogen atoms are generated by the reaction ofaluminum with water vapor [3] and diffuse into thematrix through the transportation of hydrogen atoms by3

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dislocations [4].

第26号（2014）

In short life region, ductile fracture occurred and thecrack growth was accelerated in high humidity. Inaddition, the deformation around a crack was localized.
These properties are explained by Hydrogen enhanced
localized plasticity (HELP) mechanism [4]. Moreover,in the growth process, a crack propagated to thespecified direction, 35° inclined to the specimen axis,
because the alloy had a marked texture. That is, thegrowth direction of crack is corresponding to the angle
composed by the fracture surface, (100) plane, and thetexture, (111) plane. This is a reason for the growth ofmacroscopic shear mode crack. On the other hand, thetensile mode crack in long life region may be explained
as follow. Fatigue life at 6 Hz was hardly decreased byhigh humidity, meaning that the crack growth rate waslower in high humidity than that in low humidity,
because the crack initiation was accelerated by highhumidity due to anodic dissolution [5], [6]. In addition,a crack propagated in the tensile mode and a zigzag
manner. These results suggest that the shear mode
crack to the specified direction was suppressed. Longer
exposure in high humidity at low frequency may
supply many hydrogen atoms. It is reported that theinteraction of hydrogen atoms with dislocations has not
only a strengthening effect but also a weakening effectinversely depending on the hydrogen content [4], [7].
Consequently, a crack causing by multiple slips
propagates because of the suppression of slip to the one
direction due to many hydrogen atoms. Moreover, thegrowth of a zigzag crack is suppressed by roughness
induced crack closure. These may be reasons for thatfracture occurred by the tensile mode and fatigue life
was not reduced in high humidity at low frequency inspite of reduction of the crack initiation life.

Fig. 5: Crack morphology in high humidity(6 Hz, σa=220MPa)(a) Striations

4. CONCLUSIONSIn the present study, rotating bending fatigue tests
were carried out in relative humidity of 25% and 85%
and at frequencies of 50 Hz and 6 Hz for an extruded
bar of age-hardened Al-Zn-Mg-Cu alloy (7075-T6) toinvestigate the effects of moisture and stress level on
the propagation behaviour of a fatigue crack.
Macroscopic appearances of fractures are a shear mode
at high stress levels in high humidity at high frequency,
though those are the tensile mode in other conditions ofhumidity, stress levels and frequency. The macroscopic
growth direction of the shear mode crack was about 35°
to the specimen axis and the shear mode crack in highhumidity was straighter than the tensile mode crack in
low humidity. In high humidity, fatigue life at low
frequency was larger than that at high frequency inspite of decrease in crack initiation life due to longer(b) Brittle facets
Fig. 6: Fracture surfaces in high humidity(6 Hz, σa=280MPa)

4

KARIYA and KAWAGOISHI：Effect of Moisture on Fatigue Crack Growth of Age-Hardened Al Alloy

exposure time in high humidity, though there is no or
little influence of frequency on fatigue life in lowhumidity.
AKNOWLEDGEMENT
Authors express their thanks to Professor Q. Chen ofKumamoto University for valuable discussion.
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the Society of Materials Science, 1977; 26(281):143-148[3] Ricker R.E, Duquette D.J: The Role of Hydrogenin Corrosion Fatigue of High Purity Al-Zn-Mg
ExposedtoWater
Vapor
MetallurgicalTransactions, Vol.19A , 1988; 1775-1783[4] Birnbaum H. K, Sofronis P: Hydrogen-enhanced
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Morino K: Effect of microstructure on fatigue
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