Fatigue behaviour of a commercial aluminium alloy in sea water at different temperatures

Engineering Fr4crwe Medmies F’riatcd in Great Britain.

Vol. 45, No. 3,

pp.297-307, 1993

@x3-7944/93

wM + 0.00

Pergamon Press Ltd.

FATIGUE BEHAVIOUR OF A COMMERCIAL ALUMINIUM ALLOY IN SEA WATER AT DIFFERENT TEMPERATURES MOHAMED BAGAB BAYOUMJt Department of Mechanical Engineering, Faculty of Engineering, Assiut University, Assiut, Egypt Abstract-Fatigue tests of a commercial cynic alloy are conducted in sea water at different temperatures using a specially designed experimental environmental chamber installed on a standard rotating bending fatigue testing machine. The tests are carried out in air at 20°C to establish the a-Ncurve for this alloy as a reference curve, while tests in sea water are at 20, 30, 50 and 80°C to investigate the role of temperature on the fatigue behaviour. Changing the medium from air to sea water during fatigue results in a significant decrease in the fatigue endurance limit. Increasing the temperature from 20 to 80°C reduces the endurance limit from 0.58 to 0.3 uy. In a general&d empirical estimation formula, the present study suggests an incorporation of an environmental factor (C,) which depends on both the working medium and the temperature. Fracture surface examination on the scanning electron microscope indicates a great dependence of the pitting and/or intergranular corrosion fatigue cracks on the testing temperature and the applied cyclic stress levels.

1. INTRODUCTION fatigue is an important form of localized corrosion. Several studies have shown that the cost due to corrosion amounts to 2-4% of the gross national product. In addition, several surveys of corrosion failures in different companies and industries have shown that 2040% of all the failures experienced are due to corrosion fatigue [l, 21. Corrosion fatigue may be defined as the combined action of an aggressive environment and a cyclic stress leading to premature failure of metals by cracking [3,4& The first corrosion fatigue obse~ations were reported by Haigh [s] on steel cables in a sea water environment. Corrosion fatigue involves material properties, fracture mechanics and electrochemistry. In general, resistance of a metal to corrosion fatigue is associated more with its inherent corrosion resistance than with high mechanical strength. Numerous investigations through classical experiments have shown that corrosion fatigue cracks start from corrosion pits [6-81. Commonly, corrosion fatigue occurs in two stages [9]; in the first stage pitting and crack formation take place, while in the second stage propagation of the cracks initiated at the base of the pits takes place. Different mechanisms have been proposed to explain the corrosion fatigue behaviour [IO-l 33. One of the most popular m~ha~sms illustrates that corrosion pits on the fatigue specimen surface act as stress con~ntration sites which lead to the reduction of fatigue life in corrosive environments, The precise effect of various corrosion mechanisms can be broadly class&d into four categories, namely pitting, preferential dissolution of induced anodic material, protective fihn destruction and surface energy reduction. The most interesting aspect of corrosion fatigue is the fact that both mechanical and enviromnental elements contribute to the process, acting jointly to produce a result that is more severe than either of these acting alone. It is worth noting that pit formation in metals and alloys in aggressive solutions undoubtedly leads to a reduction in fatigue life. Investigations have reported that crack initiation from pits is responsible for reduction in fatigue life of aluminium alloy in 3.5% NaCl solution [14,15f. However, the corrosion fatigue phenomena also occur in enviro~ents where pitting does not take place 1161.Further evidence that corrosion-induced pitting may not always be responsible for early crack initiation has been shown by Duquette and Uhlig[l7J Nevertheless, many workers have recently reported pitting as an important element in corrosion and thereby in corrosion fatigue cracking of materials containing second phase particles [ 18-2 11. CORROSION

tPresent address: Department of Production Engineering and Mechanical Systems Design, King Abdulazix University, P.O. Box 9027, Jeddah 21413, Saudi Arabia. 291

298

M. R. BAYOUMI Table 1. Chemical composition of the aluminium alloy used in the exmrimental Drosvamme (weight%) Element

Si

Cu

Mn

Mg

Fe

Al

Amount

0.85

0.05

0.48

0.72

0.25

Balance

Corrosion fatigue occurs in all materials exposed to a corrosive environment and subjected to fatigue-type stresses. As in stress corrosion cracking [22] and fatigue [23], there are several aspects of the problem arising from mechanical, environmental and metallurgical properties. In other words, many mechanical, environmental and metallurgical variables have an effect on corrosion fatigue susceptibility. Ideally, it is desirable to characterize the corrosion fatigue properties of a material in terms of all of these variables in order to represent either the fatigue strength (a,) or the fatigue life (Nf) as a function of these variables. However, such characterization is very expensive and cannot be justified; the experimental task will be limited to examining only specific variables according to need. Temperature variations lead to great differences in the degree of corrosion attack, and cause the penetration of aggressive substances such as chloride ions and carbon dioxide to proceed more rapidly. Studies carried out on the effect of temperature and humidity on corrosion processes showed that when the conditions are such that corrosion can occur, its rate is increased by high temperature and high humidity [24,25]. It was reported that the rate of corrosion appears to be sharply increased by an increase in temperature in the range of 20-4O”C, especially at high humidity. While a reasonable amount of work has been done on the role of temperature during corrosion processes, a study of the effect of temperature on both fatigue and corrosion fatigue is lacking. Thus the present study is aimed at investigating the dependence of fatigue strength as well as fatigue mechanisms on the temperature of sea water as an aggressive corrosive working medium. The present research is also intended to establish quantitative considerations for the endurance limits which have to be implemented in design against fatigue in corrosive environments under different temperatures. 2. EXPERIMENTAL

PROCEDURE

2.1. Material characterization A commercial aluminium alloy is used in this investigation. Chemical analysis was conducted on a sample of this alloy and gave the chemical composition reported in Table 1. For microstructure observation, polishing and etching were carried out on a specimen of the aluminium alloy under consideration, and then the specimen was examined using scanning electron microscopy. 2.2. Determination of mechanical properties

Tensile specimens were machined from the supplied rods, such that the gauge lengths in the axial direction coincided with the axis of the supplied rods. Three round tensile specimens were machined to dimensions and tolerances required by ASTM standard E8-82 [26]. The tensile specimen geometry is indicated in Fig. 1. The testing procedures to determine the mechanical properties, namely the yield stress, the ultimate stress, the percentage elongation, the percentage reduction in area and the strain hardening exponent, are in accordance with the ASTM ES-82 standard.

Dimmsions

are

in

MMS

Fig. 1. Geometry of the tension test specimen.

Fatigue behaviour of a commercial ahmkium alloy

61.9

r

26.61

.I.

Dimensions

err

6t.S

I-

ts6.t in

299

MMS

f

Fig. 2. Dimensions and geometry of the fatigue specimen.

2.3. Fatigue tests under d#ereet corrosive co~d~tio~~ Approxitnately 100 fatigue specimens having the geometry and dimensions as shown in Fig. 2 were machined from the supplied round bars. A rotating bending fatigue testing machine was used to conduct the fatigue tests at a frequency of 4000 rpm. A specially designed, const~ct~ and manufactured environmental testing chamber suitable for use on the rotating bending fatigue testing machine was installed. The main constituents of the chamber are a corrosive medium tank, a heater, a temperature control circuit which controls the temperature in the range of 25-IOO”C, and a circulating pump and level control for safety purposes. This set-up is used to adjust the corrosive medium (sea water in this study) temperature as required, and to circulate the corrosive medium around the fatigue test specimens. A ~agrammatic drawing of this set-up is shown in Fig. 3a while the electrical circuit used to operate and control this installation is presented in Fig. 3b. Stress (0) versus number of cycles to failure (N) curves were obtained; first in air at room temperature (20°C) and secondly in sea water at different temperatures starting from 20°C and

A&POWER

(4

SOPPLY

@I

Fig. 3. Layout of the designed environmental testing chamber installed on the rotating bending fatigue machine. (a) Diagrammatic drawings of corrosion fatigue testing. (b) Electrical circuit used in the corrosion fatigue set-up.

M. R. BAYOUMI

300

ending at 80°C. It is worth noting that five different stress levels taken as fractions of the aluminium alloy yield stress are used in constructing the a-N curve, while three fatigue specimens are used to represent each experimental point on the a-N curve for each set of testing conditions. 2.4. Scanning electron microscopy observations Studies of the features of the fracture surface of the fractured specimens after fatigue tests were conducted using scanning electron microscopy, towards understanding and illustrating the differences in the fracture surface topography corresponding to the conditions of the corrosion fatigue tests at different temperatures. 3. RESULTS AND DEXXJSSlON The microstructure of the aluminium alloy used in this study as observed in the scanning electron microcope is shown in Fig. 4. The average tensile properties, namely, the yield stress, the ultimate tensile strength, the percentage elongation as well as the strain hardening exponent are determined from the tension test. The engineering stress versus engineering strain diagram and true stress versus true strain diagram are shown respectively in Fig. 5a and b. Since the well accepted and most simple method to evaluate the fatigue behaviour of engineering materials is to plot the

ENGINEERING

STRAIN

fi-

I

I

-.-30

0.026

I

o.czo

I

I 0.076

0. Ial

TRUESTFWIN (b)

True

streer-Trur

#train

curve.

Fig. 5. Tensile test diagrams.

I O.li5

I 0.160

Fatigue behaviour of a commercial aluminium alloy

Fig. 4. The microstructure of the aiuminium alloy as observed on the scanning electron microscope.

301

p/1. R. BAYOU~l

302

(a)AtT=20”Canda=:0.8ay

{c)AtT=50”Candu=0.40y

(e)AtT=80”Canda=0.5cry

(f’)AtT=805Canda=0.40y

Fig. 10. TypicaI scanning electron micrographs of the corrosion fatigue fracture surfaces at different sea water temperatures and applied cyclic stress levels.

Fatigue behaviour of a commercial alum&m

a2 by I 0

I 60

I I lo0 166 NUMBEROF CW.ES

I 200 x id

303

alloy

I 250

I 200

Fig. 6. V-N curves for the aluminium alloy tested under different conditions.

number of cycles to produce failure against the amplitude of an applied rotating bending stress, these plots lead to the characteristic a-N curves which ultimately give a very important fatigue design parameter called the fatigue strength or the endurance limit (a&. Thus, Fig. 6 shows the o-N curves for the aluminium alloy tested in air as well as in sea water at temperatures of 20, 30, 50 and 80°C. It is quite evident from Fig. 6 that there is a significant difference between the fatigue behaviour in air and the fatigue behaviour in sea water. Generally the a-N trend was pulled downwards upon changing the medium from air to sea water, while for the sea water as a working medium during fatigue, the a-N trends shifted downwards under the condition of increasing the temperature of the sea water. It is worth reporting that no significant differences were found between any of the tested conditions below a fatigue life of about 4 x IO5 cycles, which indicates that the combined drastic damaging effects of both the cyclically applied stress and the chemical action (corrosion) must operate simultaneously after a reasonable time factor. Above 4 x 10’ cycles there was a considerable reduction in the lives of the specimens tested in sea water compared with those of the specimens tested in air, and the severity of the reduction became steep upon increasing the temperature of the corrosive medium (sea water). Deterioration of alternating stress levels at constant fatigue life due to increasing the temperature of the sea water is clearly presented in Fig. 7a, while Fig. 7b demonstrates the role of sea water temperature increase on the fatigue endurance limit. If the factor resulting from the division of the fatigue endurance limit (a,) in sea water at a temperature (2’) by the fatigue endurance limit (0,) in air at temperature of 2O”C, is plotted against the temperature, it gives the relationship shown in Fig. 8. There is a great design need to obtain actual fatigue test data that pertain as closely as possible to the application involved, thus the safe life design methodologies [27,28] offer a generalized empirical formula to estimate the endurance limit. This formula uses three factors and is given by a, = c+&?~*C~*C~,

(1)

where a, is the designed fatigue endurance limit, S; is the endurance limit obtained from a completely reversed rotating bending test, CL is the load factor, C, is the gradient factor and C, is the surface factor. The values of the factors CL, C, and C, under different conditions are reported in refs 127,281. The findings of the present study suggest that another factor, which can be called an environmental factor, C,, has to be incorporated in eq. (1) to reflect the effect of working ~nditions on the estimate of the endurance limit. Then eq. (1) becomes (2)

304

M. R. BAYOUMI I

I

1

I

I

I

I

CORROSIVE MEOW4: SEA WTER

FOR CONSTANT LIFE OF 4 x 0” CVCLES

L ,

-20

30

FUt I 40

CONSTANT LIFE OF 3 x IO‘ i I I ._ 80 80 70

CYCLES I

I

80

80

I

I

TEMPERATURE l C

(b)

I

I

I

I

I

CORROSM

a8ffy

MEDluw * SE& WATER

8 g 0,wy s

I

h

o.4uy -

Iid t

a2qy 1

t

I

I

1

t

‘

I

20

30

40

so

60

70

80

So

Fig. 7. (a) Variation of the alternating stress at constant fatigue life with the testing temperature. (b) Dependence of the fatigue endurance limit on the sea water temperature.

and C, can be considered as the ratio of the endurance limit (crJ of the material in the working medium at any temperature (T) to the endurance limit (aJ of the same material in air at a temperature of 20°C (room temperature). Accordingly, this factor can be obtained from Fig. 8 if the working medium is sea water. Further work is needed to establish a design nomogram similar to that in Fig. 8 to cover most of the practical working media at different tem~~tures. On the micromechanism frontier, it is worth noting that the onset of damage in a cyclically stressed ductile metal is generally associated with a free surface. A general overview of the corrosion fatigue micromechanisms is illustrated in Fig. 9. Fatigue cracks will be initiated at the surface in rotating bending as the stress is maximum at the surface, while corrosion will tend to occur at scratch lines on specimen surfaces, which are exposed to the aggressive environments; therefore, corrosion fatigue crack initiation is a relatively difficult process, which is insidious in nature and leads to unexpected failures with little prior warning. In most in-service corrosion fatigue failures, the number of cycles required to initiate corrosion cracks is much greater than the number of cycles required to propagate the crack to ultimate final fracture. Corrosion fatigue crack initiation is to some extent a random, poorly characterized process which tends to occur at

Fatigue behaviour of a commercial aluminium alloy

305

I& 0

s!

B TEMPERATURE (1) ,

IN %

Fig. 8. The relationship between the ratio of [the fatigue endurance limit (u,) in sea water at temperature (T)/the fatigue endurance limit in air at temperature of 2O”Cj and the sea water testing temperature.

discontinuities and corrosion pits. During cyclic loading, a dislocation substructure is produced, the nature of which is affected by the amplitude of the strain cycle, the stacking fault energy and the temperature of testing; all of these influence the ease of cross slip. Upon the progress of deformation, dislocations multiply and gradually produce, from interactions, a debris of jogged and tangled dislocations, prismatic loops, dipoles and multipoles. The agglomeration of debris is influenced by cross slip; a higher incidence of cross slip caused by increase in strain range, temperature or stacking fault energy, produces a cellular arrangement of dislocation debris, while a lowering of cross slip incidence favours the formation of bands of debris. Also, it is argued that a dislocation escaping at the surface creates a step which may, especially in corrosive environments, oxidize rapidly and prevent annihilation of the step during a subsequent half cycle. Typical scanning electron micrographs of the fracture surfaces at different temperatures and stress levels are presented in Fig. 10. It is clear from this figure that in the aluminium alloy under consideration, under exposure to sea water, corrosion fatigue cracks frequently originate at the site of pitting or intergranular corrosion, where increasing the temperature and decreasing the applied stress creates an increase in the density of pitting and/or the intergranular corrosion fatigue cracks. 4. CONCLUSIONS A specially designed experimental set-up has been used to conduct fatigue tests of a commercial aluminium alloy in sea water at different temperatures. Comparison of the fatigue endurance limits obtained in air with those obtained in sea water, for the aluminium alloy, indicated a significant decrease in the fatigue endurance limit upon changing the testing medium from air to sea water. Increasing the temperature of the sea water from 20 to 80°C resulted in a change in fatigue endurance limit from 0.58 ou to 0.3 ou. Thus, incorporation of an environmental factor (C,) in a generalized empirical estimation formula for the endurance limit is clearly presented. Fracture surface observation using scanning electron microscopy revealed a great dependence of the pitting and/or intergranular corrosion fatigue cracks on the testing temperature and the applied cyclic stress levels.

M. R. BAYOUMi

306

I

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Fatigue behaviour of a commercial aluminium alloy

307

REFERENCES [I] M. G. Fontana, Corrosion Engineering, Third Edition. McGraw-Hill, New York (1987). [2] P. A. Schweitzer (Ed.), Corrosion and Corrosion Protection Handbook. Marcel Dekker, New York (1989). [3] H. H. Uhlig and R. R. Mears, in Corrosion Handbook (Edited by H. H. Uhlig), pp. 21-35. John Wiley, New York (1948). (41 B. F. Brown, Corrosion fatigue in naval structure. International Meeting on Corrosion Fatigue, Storm, CT (14-18 June 1971). [5] B. P. Haigh, J. Inst. Metals 18, 55-61 (1917). [6] D. J. McAdam, Jr., Proc. ASTM 26, 224-230 (1962). [7] T. Magnin, D. Desijardins and M. Puiggali, Corrosion Sci. 29, 567-576 (1989). [8] M. P. Mueller, Corrosion 38, 431437 (1982). [9] G. Butler and H. C. K. Isom, Corrosion and its Prevention tit Water. Robert E. Krieger, Huntington, New York (1978). [IO] I. S. McCollough and A. A. Van Haute, Corrosion 30, 47-52 (1971). [11] J. L. Gray, Proc. Inst. Mech. Engrs 186, 739-745 (1972). [l2] V. Rollins, B. Arnold and E. Lardner, J. Corrosion 5, 3345 (1970). [l3] M. Kowaka and T. Tudo, The Japan-U.S.A. Seminar on Passsivity (1975). [14] C. P. Dervenis, E. I. Meletis and R. F. Hochman, Mater. Sci. Engng 102, 151-160 (1988). [15] M. Rebiere and T. Magnin, Mater. Sci. Engng 128, 99-196 (1990). [l6] D. J. Duquette and H. H. Uhlig, Trans. Am. Sot. Metals 61, 449456 (1969). [l7] D. J. Duquette and H. H. Uhlig, Trans. Am. Sot. Metals 62, 839-845 (1969). [l8] S. C. Srivastaya and M. B. Ives, Corrosion 45, 488493 (1989). [19] Z. Szlarska, S. Smialowska and J. Gust, Corrosion Sci. 19, 753-761 (1979). [20] L. Felloni, G. P. Cammarota and G. Palombraini, Br. Corrosion JI 13, 167-175 (1978). [21] A. Moccari, Corrosion fatigue of type 304 stainless steel in H,SO, + NaCl at 25°C. Ph.D. thesis, The Ohio State University, OH (1974). [22] M. R. Bayoumi, On the mechanics and mechanisms of fracture in stress corrosion cracking of aluminium alloys. Submitted to J. Mater. Sci. [23] M. R. Bayoumi, On the development of plastic zone and crack growth of short cracks at notches in biaxial fatigue. Bull. Faculty Engng, Assiut Univ., Egypt 16, 115-128 (1988). [24] A. U. Malik, S. Basu, I. Anijani and S. Ahmed, Corrosion behaviour of N&resistant cast irons in seawater. Proceedings of Corrosion and Its Control Symposium (pp. 163-179), Department for Chemical Engineering, King Saud University, Riyadh, Saudi Arabia (1618 May 1992). [25] K. W. J. Treadaway, C. L. Page and N. R. Short, The prediction of reinforcement corrosion: from laboratory studies to exposure trials, in Durability of Construction Materials (Edited by J. C. Maso), Volume 3, pp. 1323-1329. Chapman & Hall, London (1987). [26] Standard methods of tension testing of metallic materials. E8-82, ASTM Annual Book of Standards, American Society for Testing and Materials, Philadelphia, PA (1981). [27] R. C. Juvinal, Fundamentals of Machine Component Design, Second Edition. John Wiley, New York (1991). [28] J. E. Shigley and C. R. Mischke, Mechanical Engineering Design, Fifth Edition. McGraw-Hill, New York (1989). (Received 23 July 1992)