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The fretting fatigue of titanium and some titanium alloys in a corrosive environment
Wear, 25 (1973) 171-175 0 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
171
THE FRETTING FATIGUE OF TITANIUM AND SOME TITANIUM ALLOYS IN A CORROSIVE ENVIRONMENP
R. B. WATERHOUSE and M. K. DUTTA department of ~etul~~~gy and ~uteriaZs Mence,
University of ~ottj~ghu~ (Gt. Britain]
(Received January 16, 1973)
SUMMARY
S-N curves have been determined in air and in 1% sodium chloride solution on specimens of commercial purity titanium (IMI 130) and the titanium alloys Ti-2.5Cu (IMI 230) and Ti-6Al-4V (IMI 318) with and without simultaneous fretting. IMI 130 and IMI 318 showed a reduction of about 16% in fatigue strength in corrosion fatigue whereas IMI 230 was immune. In the presence of fretting the two alloys showed reductions in fatigue strength of about 60:< in air and in sodium chloride, whereas IMI 130 only suffered a reduction of 39%. These results confirm previous results on aluminium alloys and stainless steel, that where a material relies on a protective oxide film to confer on it corrosion-resistant properties, fretting, by mechanically disrupting the protective film has a very deleterious effect on the fatigue strength of the material.
INTRODUCTION
Titanium and titanium alloys are attractive engineering materials from two points of view; firstly their excellent strength-weight ratios, and secondly their remarkable corrosion resistance. Both these attributes are of great importance in an application such as surgical implants where the strength-weight ratio must match that of bone, and the environment of the body fluids, which are saline, is particularly corrosive. The corrosion resistance of titanium and its alloys is due entirely to the protective nature of the oxide film on the surface. Any action which is likely to disrupt the film may have catastrophic efIects. Although normal fatiguing of the component may have some disruptive effect on the oxide film, fretting, which involves the rubbing together of two surfaces, is likely to have a much greater effect. In the case of implants the fretting arises most commonly‘between the underside of the heads of the screws which are used to secure the implant to sound bone and the countersunk holes in the implant through which the screws pass. As the patient uses the limb or. flexes the joint, relative oscillatory movement occurs at * Papei presented at the First World Conference on Industrial Tribology, New Delhi, December, 1972.
172
R. B. WATERHOUSE.
M. K. DUTTA
these points. There are reports that the tissue in the vicinity of titanium implants becomes discoioured with a dark particulate material, which suggests fretting action. Earlier work’ has shown that titanium alloys are very susceptible to fretting fatigue damage in air. The present investigation is concerned with the fretting fatigue of titanium materials in air and sodium chloride solution, and also with fatigue in air and corrosion fatigue. EXPERIMENTAL
The tests were carried out on four-point-loading rotating-bending fatigue machines. The specimens were machined from 9.5 mm diam. rod and were 356 mm in length. Two parallel flats were machined on the centre portion which lay between the two load-carrying bearings. In the fretting experiments two bridge pieces were clamped on to the lIats with a proving ring. The clamping pressure was kept constant throughout the tests and was 62.5 MN/m’. The corrosive environment was applied by dripping 1% sodium chloride solution on to the centre portion of the specimen. The experimental arrangement was similar to that used in previous investigations2 where it is described in more detail. The materials used and their mechanical properties are shown in Table I. TABLE I
C.p. Ti (IMI 130) Ti-2SCu (IMI 230) TidAl-QV (IMI 318)
U.T.S.
Elongation
Hardness
(MNIm’)
(%)
P’HN)
675 578 1050
14.3 26.5 13.7
265 228 335
In all the tests involving fretting the bridges were made of the same material as the specimen with which they were in contact. S-N curves were determined with a run-out value of 10’ cycles. RESULTS
The results are shown in Figs. 1, 2 and 3, and the fatigue strengths at 10’ cycles are recorded in Table II. The most resistant material in corrosion fatigue is Ti-2SCu (IMI 230) which has the same strength at 10’ cycles as in air. At shorter lives the corrosive atmosphere does have a small effect. C.p. Ti (IMI 130) shows a reduction in fatigue strength in corrosion fatigue of 150/, whereas Ti-6A1-4V (IMI 318) shows a slightly greater reduction of 17%. In the presencp of fretting large reductions in the fatigue strength are observed and these are largely independent of the nature of the environment, although at higher stresses fretting in. sodium chloride appears to be rather more severe in the cases of IMI 130 and 319, and very much more severe in the case of IMI 230. As has been observed in other work3, the reduction in fatigue strength by fretting is greater for the higher strength materials. The percentage reduction in the case of IMI 318 is 62.5x, whereas in IMI 130 it is 38.5%.
FRETTING
FATIGUE
173
OF TITANIUM
250 -
c.p.Ti(
-16
IMI 130)
-1L
"E \ g 200!! Li 0 z G g w 2
lso-
-12 u,
0
FATIGUE IN AIR
l
FATIGUE IN 1% NaCl
0
FRETTING-FATIGUE
0
FRETTING-FATIGUE
a
9 P N AIR
-Kl
IN 1% Ml
-a
100
I
I
105
106 CYCLES TO FAILURE
I 107
Fig. 1. S-N curves for c.p. Ti (IMI 130).
r
2.4
20
16 ‘ii
FATIGUE IN AIR FATIGUE IN 1 % NaCI FRETTING - FATIGUE IN AIR FRETTING - FATIGUE IN 1% NaCl
I
lo5
Fig. 2. S-N curves for Ti-2.SCu (IMI 230).
I
106 CYCLES TO FAUJJRE
I
107
174
R. B. WATERHOUSE, M. K. DUTTA
FATIGUE
IN AIR
FATIGUE IN 1%
NoCI
FRETTING - FATIGUE IN AIR FRETTING-FATIGUE
IN 1% NaCl
I
105
106 CYCLES
TO FAILURE
Fig. 3. S-N curves for Ti-6A1-4V (IMI 318).
TABLE II
(MN@)
IMI 230 (MN/m’)
iMI 318 (MN/m21
201 170 124 124
278 278 124 I24
371 309 139 155
IMI 130
Air fatigue Corrosion fatigue Fretting fatigue in air Fretting fatigue in NaCl
DISCUSSION
In general these results are very similar to results obtained on lSCr--SNi stainless stee14, another material which is corrosion resistant due to the presence of a protective oxide film on the surface. Where the surface damage is severe, as in the case of fretting, the nature of the environment appears to be of little consequence; air and sodium chloride are equally corrosive. This has also been found with aluminium and its alloys4. It is only when ah.nninium is fretted in vacua that a substantial increase in fatigue strength is observed5. Where the surface damaging process is less severe, as in the case of normal fatigue, the nature of the environment may or may not be significant. The properties of the oxide film on IMI 230 are such that it remains protective in both air and sodium chloride. The oxide films on IMI 130 and 318 are less protective in the presence of sodium chloride compared with air.
FRETTING FATIGUE OF TITANIUM
175
The assumption is that the sodium chloride must play some role in the breakdown of the oxide films on these two materials. Galvanostatic studies6 of the potential drop observed when the three materials are fretted in 1% sodium chloride show that there is some correlation between these figures and the reduction in fatigue strength: Potential drop on fretting, in volts for IMI 130 is 0.220; IMI 230, 0.255; IMI 318, 0.425. These figures give an indication of the chemical reactivity of the material when the oxide film is disrupted. CONCLUSION
Titanium and its alloys, like other materials which are corrosion resistant because of the presence of a protective oxide on the surface, suffer a severe decrease in fatigue strength under fretting conditions. Sodium chloride solution has a slightly greater detrimental effect than air. ACKNOWLEDGEMENTS
The authors wish to acknowledge financial support for this project from the Ministry of Defence, Procurement Executive. REFERENCES 1 D. W. Hoeppner and G. L. Goss, Research on the mechanisms of fretting fatigue, Proc. First Internat. Conf. on Corrosion Fatigue, Storrs, Conn. NACE, 1972. 2 R. B. Waterhouse and M. Allery, The effect of powders in petrolatum on the adhesion between fretted steel surfaces, ASLE Trans., 9 (1966) 179. 3 R. B. Waterhouse and D. E. Taylor, The effect of heat treatment on the fretting fatigue behaviour of a 0.7 per cent carbon steel, Proc. Instn. Mech. Engrs., 185 (1970-71) 691. 4 R. B. Waterhouse, M. K. Dutta and P. J. Swallow, Fretting fatigue in corrosive environments; Proc. Intern. Conf on Mechanical Behaviour of Materials, Kyoto, 3 (1972) 294. 5 A. J. Fenner and J. E. Field, A study of the onset of fatigue damage due to fretting, Trans. N.E.C. Instn. Engrs. Shipbfdrs., 76 (1960) 184. 6 R. B. Waterhouse and M. K. Dutta, to be published.
171
THE FRETTING FATIGUE OF TITANIUM AND SOME TITANIUM ALLOYS IN A CORROSIVE ENVIRONMENP
R. B. WATERHOUSE and M. K. DUTTA department of ~etul~~~gy and ~uteriaZs Mence,
University of ~ottj~ghu~ (Gt. Britain]
(Received January 16, 1973)
SUMMARY
S-N curves have been determined in air and in 1% sodium chloride solution on specimens of commercial purity titanium (IMI 130) and the titanium alloys Ti-2.5Cu (IMI 230) and Ti-6Al-4V (IMI 318) with and without simultaneous fretting. IMI 130 and IMI 318 showed a reduction of about 16% in fatigue strength in corrosion fatigue whereas IMI 230 was immune. In the presence of fretting the two alloys showed reductions in fatigue strength of about 60:< in air and in sodium chloride, whereas IMI 130 only suffered a reduction of 39%. These results confirm previous results on aluminium alloys and stainless steel, that where a material relies on a protective oxide film to confer on it corrosion-resistant properties, fretting, by mechanically disrupting the protective film has a very deleterious effect on the fatigue strength of the material.
INTRODUCTION
Titanium and titanium alloys are attractive engineering materials from two points of view; firstly their excellent strength-weight ratios, and secondly their remarkable corrosion resistance. Both these attributes are of great importance in an application such as surgical implants where the strength-weight ratio must match that of bone, and the environment of the body fluids, which are saline, is particularly corrosive. The corrosion resistance of titanium and its alloys is due entirely to the protective nature of the oxide film on the surface. Any action which is likely to disrupt the film may have catastrophic efIects. Although normal fatiguing of the component may have some disruptive effect on the oxide film, fretting, which involves the rubbing together of two surfaces, is likely to have a much greater effect. In the case of implants the fretting arises most commonly‘between the underside of the heads of the screws which are used to secure the implant to sound bone and the countersunk holes in the implant through which the screws pass. As the patient uses the limb or. flexes the joint, relative oscillatory movement occurs at * Papei presented at the First World Conference on Industrial Tribology, New Delhi, December, 1972.
172
R. B. WATERHOUSE.
M. K. DUTTA
these points. There are reports that the tissue in the vicinity of titanium implants becomes discoioured with a dark particulate material, which suggests fretting action. Earlier work’ has shown that titanium alloys are very susceptible to fretting fatigue damage in air. The present investigation is concerned with the fretting fatigue of titanium materials in air and sodium chloride solution, and also with fatigue in air and corrosion fatigue. EXPERIMENTAL
The tests were carried out on four-point-loading rotating-bending fatigue machines. The specimens were machined from 9.5 mm diam. rod and were 356 mm in length. Two parallel flats were machined on the centre portion which lay between the two load-carrying bearings. In the fretting experiments two bridge pieces were clamped on to the lIats with a proving ring. The clamping pressure was kept constant throughout the tests and was 62.5 MN/m’. The corrosive environment was applied by dripping 1% sodium chloride solution on to the centre portion of the specimen. The experimental arrangement was similar to that used in previous investigations2 where it is described in more detail. The materials used and their mechanical properties are shown in Table I. TABLE I
C.p. Ti (IMI 130) Ti-2SCu (IMI 230) TidAl-QV (IMI 318)
U.T.S.
Elongation
Hardness
(MNIm’)
(%)
P’HN)
675 578 1050
14.3 26.5 13.7
265 228 335
In all the tests involving fretting the bridges were made of the same material as the specimen with which they were in contact. S-N curves were determined with a run-out value of 10’ cycles. RESULTS
The results are shown in Figs. 1, 2 and 3, and the fatigue strengths at 10’ cycles are recorded in Table II. The most resistant material in corrosion fatigue is Ti-2SCu (IMI 230) which has the same strength at 10’ cycles as in air. At shorter lives the corrosive atmosphere does have a small effect. C.p. Ti (IMI 130) shows a reduction in fatigue strength in corrosion fatigue of 150/, whereas Ti-6A1-4V (IMI 318) shows a slightly greater reduction of 17%. In the presencp of fretting large reductions in the fatigue strength are observed and these are largely independent of the nature of the environment, although at higher stresses fretting in. sodium chloride appears to be rather more severe in the cases of IMI 130 and 319, and very much more severe in the case of IMI 230. As has been observed in other work3, the reduction in fatigue strength by fretting is greater for the higher strength materials. The percentage reduction in the case of IMI 318 is 62.5x, whereas in IMI 130 it is 38.5%.
FRETTING
FATIGUE
173
OF TITANIUM
250 -
c.p.Ti(
-16
IMI 130)
-1L
"E \ g 200!! Li 0 z G g w 2
lso-
-12 u,
0
FATIGUE IN AIR
l
FATIGUE IN 1% NaCl
0
FRETTING-FATIGUE
0
FRETTING-FATIGUE
a
9 P N AIR
-Kl
IN 1% Ml
-a
100
I
I
105
106 CYCLES TO FAILURE
I 107
Fig. 1. S-N curves for c.p. Ti (IMI 130).
r
2.4
20
16 ‘ii
FATIGUE IN AIR FATIGUE IN 1 % NaCI FRETTING - FATIGUE IN AIR FRETTING - FATIGUE IN 1% NaCl
I
lo5
Fig. 2. S-N curves for Ti-2.SCu (IMI 230).
I
106 CYCLES TO FAUJJRE
I
107
174
R. B. WATERHOUSE, M. K. DUTTA
FATIGUE
IN AIR
FATIGUE IN 1%
NoCI
FRETTING - FATIGUE IN AIR FRETTING-FATIGUE
IN 1% NaCl
I
105
106 CYCLES
TO FAILURE
Fig. 3. S-N curves for Ti-6A1-4V (IMI 318).
TABLE II
(MN@)
IMI 230 (MN/m’)
iMI 318 (MN/m21
201 170 124 124
278 278 124 I24
371 309 139 155
IMI 130
Air fatigue Corrosion fatigue Fretting fatigue in air Fretting fatigue in NaCl
DISCUSSION
In general these results are very similar to results obtained on lSCr--SNi stainless stee14, another material which is corrosion resistant due to the presence of a protective oxide film on the surface. Where the surface damage is severe, as in the case of fretting, the nature of the environment appears to be of little consequence; air and sodium chloride are equally corrosive. This has also been found with aluminium and its alloys4. It is only when ah.nninium is fretted in vacua that a substantial increase in fatigue strength is observed5. Where the surface damaging process is less severe, as in the case of normal fatigue, the nature of the environment may or may not be significant. The properties of the oxide film on IMI 230 are such that it remains protective in both air and sodium chloride. The oxide films on IMI 130 and 318 are less protective in the presence of sodium chloride compared with air.
FRETTING FATIGUE OF TITANIUM
175
The assumption is that the sodium chloride must play some role in the breakdown of the oxide films on these two materials. Galvanostatic studies6 of the potential drop observed when the three materials are fretted in 1% sodium chloride show that there is some correlation between these figures and the reduction in fatigue strength: Potential drop on fretting, in volts for IMI 130 is 0.220; IMI 230, 0.255; IMI 318, 0.425. These figures give an indication of the chemical reactivity of the material when the oxide film is disrupted. CONCLUSION
Titanium and its alloys, like other materials which are corrosion resistant because of the presence of a protective oxide on the surface, suffer a severe decrease in fatigue strength under fretting conditions. Sodium chloride solution has a slightly greater detrimental effect than air. ACKNOWLEDGEMENTS
The authors wish to acknowledge financial support for this project from the Ministry of Defence, Procurement Executive. REFERENCES 1 D. W. Hoeppner and G. L. Goss, Research on the mechanisms of fretting fatigue, Proc. First Internat. Conf. on Corrosion Fatigue, Storrs, Conn. NACE, 1972. 2 R. B. Waterhouse and M. Allery, The effect of powders in petrolatum on the adhesion between fretted steel surfaces, ASLE Trans., 9 (1966) 179. 3 R. B. Waterhouse and D. E. Taylor, The effect of heat treatment on the fretting fatigue behaviour of a 0.7 per cent carbon steel, Proc. Instn. Mech. Engrs., 185 (1970-71) 691. 4 R. B. Waterhouse, M. K. Dutta and P. J. Swallow, Fretting fatigue in corrosive environments; Proc. Intern. Conf on Mechanical Behaviour of Materials, Kyoto, 3 (1972) 294. 5 A. J. Fenner and J. E. Field, A study of the onset of fatigue damage due to fretting, Trans. N.E.C. Instn. Engrs. Shipbfdrs., 76 (1960) 184. 6 R. B. Waterhouse and M. K. Dutta, to be published.