Single wire fretting fatigue tests for electrical conductor bending fatigue evaluation

WEAR ELSEVIER

Wear X31-183 (1995) 537-543

Single wire fretting fatigue tests for electrical fatigue evaluation

conductor

bending

Z.R. Zhou a, S. Goudreau a, M. Fiset b, A. Cardou a ‘L%partement de Ghaie Mhnique, Universitt! Luval, Quibec, Que. GIK 7P4, Canada bL@atiement de Mines et M&a&&e, Universits? Luval, Qut%ec, Que. GIK 7P4, Canada

Received 3 May 1994; accepted 22 September 1994

Abstract

Overhead electrical conductors are often subjected to aeolian vibrations which may induce fretting fatigue damage of individual aluminium wires in suspension clamp regions. Many bending fatigue tests have been performed on electrical conductors. Depending on the test conditions, wire fracture may be found to occur in the external as well as internal layers. Individual wire fretting fatigue is very difficult to predict due to a conductor complex structure and dynamic mechanical

behaviour. The main objective of this work is to present experimental results obtained from tests on single wires under conditions simulating a typical conductor-clamp contact. A fretting fatigue test bench specifically designed for such simulation has been used on single H19 aluminium wires. They have been subjected to an initial minimal axial stress of 59 MPa. At the fretting point, a transverse compressive load of 130 N to 4000 N has been imposed, as well as an alternating displacement of 100 to 900 pm displacement amplitude. Cycling frequency has been kept at 10 Hz and test duration went up to 1.6~ 10’ cycles. All tests were performed in the stick-slip regime occurring in the plastically deformed contact zone. No global slip was allowed. Subsequent examination of the fretting scars at the contact surface and through the cross-section have been carried out by optical microscopy and scanning electron microscopy. The mechanical parameters influence is studied and comparison with results from complete conductor fatigue tests is discussed. Keywords:

Fretting fatigue; Electrical conductors; Aluminium

1. Introduction Under wind excitation, electrical transmission lines undergo so-called aeolian vibrations, which may lead to fretting fatigue problems in the suspension clamp regions. Aluminium wire fracture may imply a drastic reduction in the transmission line service life [l]. It has been found that three contact modes may induce fretting fatigue, depending on the loading conditions: (a) at inter-layer contact points; (b) at line contact between wires of the same layer; (c) or in narrow contact strips between outer layer wires and clamp. For the sake of clarity, some previous results on complete conductor fatigue testing are briefly recalled.

in Fig. 1. It is made of aluminium alloy. Two types of aluminium conductor steel-reinforced (ACSR) conductors have been tested namely, the DRAKE and the BERSIMIS [2]. Their cross-section is shown in Figs. 2(a) and 2(b). The DRAKE conductor is made of two aluminium layers and a steel core. The BERSIMIS conductor has three aluminium layers and a steel core. Aluminium wires are made of EC-H19 (Al, 99.5%; Si, 0.25%; Fe, 0.25%). The main experimental conditions during the fatigue tests were as follows: a constant Altematlng Amphtude

t

1.1. Conductor bending fatigue tests A large number of bending fatigue tests have been performed in this laboratory on overhead electrical conductors. The test bench has already been described [2,3]. A typical suspension clamp is shown schematically

Eisevier Science S.A.

SSDI 0043-1648(94)07045-S

Fig. 1. Suspension clamp with the conductor.

Z.R. Zhou et al. I Wear X81-183 (1995) 537-543

538

Alternating Amplitude

0 54

5x

Distance

Fig. 2. Cross-section at the suspension (b) BERSIMIS conductor.

centre:

(a) DRAKE

conductor;

t; P

e

E

0.78

0.58

Imposed

bending

0.26

amplitude

0.36

Imposed bending

Fig. 3. Percentage bending amplitude:

70

clamp center (mm)

(b)

(a)

0.10

66

62

from suspension

amplitude

(mm)

0.46

(mm)

of the wire breaks as a function (a) DRAKE; (b) BERSMIS.

of imposed

axial load on the specimens of 25% to 30% of their respective rated tensile strengths (RTS), an imposed displacement of 0.5-0.8 mm for the DRAKE and 0.2-0.5 mm for the BERSIMIS, amplitudes being measured at 89 mm (3.5 in) from the last point of contact between clamp and conductor. Tests were run up to 10’ cycles with a 10 Hz frequency. A test was stopped after at least four wire breaks had been recorded. 1.2. wire fracture behaviour Some results are shown in Fig. 3, which indicates the percentages of wire failure in each layer as a function of the imposed bending amplitude. For both conductors, a larger amplitude increases wire failures within the conductor (inner layers): inter-layer fretting dominates. On the contrary, for smaller amplitudes,

Fig. 4. Variation from suspension

of plastified mark width clamp centre.

as a function

of distance

wire breaks are mostly found in the outer layer. They also take much longer periods to occur. In the outer layer, most of the wire breaks have been found to nucleate at points of contact with the keeper of the suspension clamp, that is, at fretted zones on the conductor upper external surface. This contact zone is apparently the critical one in terms of conductor fretting fatigue since it is the one which is activated for amplitudes normally found in the field. It is also a difficult one to study since lower amplitudes mean much longer tests. Further investigation [3,4] has shown that wire-keeper contact zones are highly plastified strips due to the clamping pressure. Contact strip width vs. distance from suspension clamp centre is shown in Fig. 4 for four aluminium wires. It is of the order of 1 mm, and is slightly smaller near the edge, indicating a lower pressure. After cycling, metallographical examination has shown some slight fretting in this zone (near the keeper’s edge). Apart from this very localized zone, no evidence of relative slip could be observed on the contact strips. In order to better characterize this fretting fatigue mode, a new test bench has been designed and used to simulate the wire-keeper system, allowing independent control of the main parameters and also, an easier monitoring of fretting crack nucleation and propagation.

2. Experimental procedure 2.1. Fretting fatigue test set-up

The fretting fatigue rig (Fig. 5) is actuated by a motor-eccentric system (1). A slider (2) is connected with the eccentric and can move horizontally. A strain gaged clamp (3) is fixed to the slider and holds one end of the aluminium wire specimen. The other end of the specimen, about 1 m long, is held with gaged clamp (4), fused to framework (5). Displacement amplitude is controlled by eccentric (1) and can be varied by switching shafts. Here, eccentricities ranging from 100 to 900 pm have been used.

Z.R. Zhou et al. / Wear 181-183

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539

u Fig. 5. Fretting

fatigue

,a,

test bench.

A static tensile load is imposed on the aluminium wire. It is adjusted via the screw-spring system (6), with the eccentric in the zero position (the initial displacement is equal to zero). After that, the aluminium wire is fixed by a frame (5). The static tensile stress could be any value compatible with the material strength. However, its initial minimal level has been kept constant in all tests, at 59 MPa, which corresponds to the theoretical stress in aluminium wires when the DRAKE conductor is subjected to a 25% RTS axial load. It should be noted that the material softens during the cyclic testing, and, consequently the minimal load decreases. This is particularly true for large imposed displacement amplitude. A transverse load is imposed through a loading screw (7). Its value is given by load cell (8), located on plate (9). High precision rods (10) are used to insure proper, frictionless, vertical motion of the loading system. The load is applied to the wire with two cylindrical blocks (11). Their radius of curvature has the same value (0.35 m) as the one measured on the keeper edge. They are made of the aluminium 6061T6. Its yield strength is about 1.7 times that of aluminium wire specimen. The surface roughness is comparable with the clamp’s. With the selected tensile stress and transverse load, the cyclic amplitude is applied to the end of the specimen. The cycling frequency is 10 Hz, the same as in the bending tests. Tests have been conducted up to 1.6 x 10’ cycles. 2.2. Transverse loading calibration In the actual conductor-clamp system, it is of course very difficult to know the pressure level exerted by the keeper on a given wire. It is not a constant and may vary from one test to the other, depending on the bolt tightening conditions. What is known, though, is the contact strip pattern. It has been estimated that the radial load per unit length of contact strip was about 170 N mm-‘. Static tests on single wires have been performed with the same 59 MPa static tensile stress. Resulting plastic marks are elongated ellipses. Their major axis length a and minor axis width w were measured by optical microscopy. Their value as a func-

2,x,

Load per unu length (N!mm)

(bl 2

I

II

J

59

MPa

/

Fig. 6. Static mark as a function of the load elliptic mark length; (b) elliptic mark width.

per unit

length:

(a)

tion of applied load per unit length (load/a) is summarized in Figs. 6(a) and 6(b). From this figure, one can see that a and w are roughly proportional to the load per unit length. In order to show the influence of the static tensile stress on the size of the plastic marks, another series of static tests has been performed with a 12 MPa stress. Corresponding curves are shown in Fig. 6. They tend to show the low sensitivity of the mark geometry with respect to that parameter. Comparing the mark width w in the single wire tests and the contact strips mean width in the conductor-clamp tests, one finds that a contact load of 140-220 N mm-l should be imposed in the single wire tests. The 170 N mm-’ value previously found falls in that range. The corresponding transverse load should then range from 1000 to 3500 N. Finally, one should insure that no gross slip takes place at the contact point. Thus, for a given cyclic amplitude, several tests were made with decreasing values of the transverse load. The upper value was high enough so that gross slip was completely prevented.

3. Fretting

fatigue

results

3.1. Surface degradation examination

Recordings of the axial load on both sides of the clamped specimen show that it takes a few cycles for

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the alternating stress Aa to stabilize. Evolution of Aq (eccentric side) and A+ (fixed end) in the first 300 cycles is shown in Fig. 7. Aa, tends towards the value imposed by the slider motion, while ha, tends towards a small constant value. The residual alternating value AU, is very small and this side of the aluminium wire can be considered as static. There is no gross slip at the contact point which is thus in the stick-slip regime. In this regime, cyclic stress amplitude is constant after the first few cycles. As shown in Fig. 8, ha,, and AuSz are proportional to the displacement amplitude. Thus, the displacement amplitude can also be expressed in terms of stress amplitude Aa,. After a test is completed, the fretting mark is examined. As shown in Fig. 9, it can usually be divided into three zones: a no-slip zone (l), on the fixed-end side; micro-slip fretted zone (2), on the cycling side;

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zone 3 /

Fig. 9. Typical elliptic fretting mark after the test.

(a)

-

P=ZOOO

N

P=lOOO

N

1

7” ,,

10” Number

200

3””

20”

3””

of cycles

Cb)

I

0

10”

0

Number ofcycles

(c) Fig. 10. Surface degradation: (a) 38 MPa; (b) 60 MPa; (c) 100 MPa.

Fig. 7. Cyclic stress amplitude variation in the first cycles: (a) fixed end side; (b) the eccentric side.

0 0

200

400

Imposed

displacemenr

*cm

6Oll

amplaude

(pm)

Fig. 8. Variation of cyclic stress amplitude in steady state as a function of imposed displacement amplitude.

zone (3), at the edge of the fretted zone, where aluminium lamellae are observed to be extruded towards the eccentric. Such zone is not observed for lower number of cycles. Zones (2) and (3) sizes depend strongly on stress amplitude. This is particularly true for fretted zone (2). Part of the contact edge on the eccentric side is shown in Fig. 10 for various stress amplitudes. It can be seen that fretted zone size increases with Au,,. For AU,, of the order of 10 MPa, zones (2) and (3) are almost absent, even at high number of cycles (10’). Conversely, at high ho,,, fretted zone (2) covers almost completely the contact mark, indicating a near gross-slip regime.

Z.R. Zhou et al. I Wear 181-183

A transverse load decrease has a similar effect as amplitude increase, and leads to an increase in the fretted zone. This is shown in Fig. 11: with a 500 pm amplitude and 3500 N force, particle detachment is limited to the extreme contact edge, while a 1000 N force is hardly sufficient to prevent gross-slip. The combined influence of normal load P and tensile stress amplitude Au, is presented in Fig. 12. Under

541

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line AB, gross-slip takes place. Above line CD, fretting is negligible. Between lines AI3 and CD, and for a given normal load, the fretting zone increases with Au,,. For a given cyclic amplitude, it decreases when normal load P increases. 3.2. Fretting crack behaviour After a test, the contact region is cut out from the aluminium wire. Its cross-section is polished for optical microscopy examination. A residual “bathtub” profile is always observed due to the local plastic indentation. No detached particle can be observed from the central part of the contact to the fixed end edge; in the fretting zone, some particle detachment and wear can be noted towards the eccentric side edge. However, two degradation mechanisms are also apparent in Figs. 13(a) and 13(b): (a) delamination; (b) fretting cracking. The effect of the three control parameters (transverse load, cyclic amplitude, number of cycles) on crack nucleation and propagation is presented below. For a given amplitude, the fretting zone depends upon the normal force P. However, little influence on fretting cracking has been noted for 1000 N
(a)

12.5pll1 2 mm

(a)

-(cl Fig. 11. Surface

degradation:

(a) 3500 N; (b) 2000 N;

(c)

1000 N.

(b) Fig. 12. Fretting

map in the stick-slip

regime.

Fig. 13. Cross-section cracking.

degradation:

(a) delamination;

(b) fretting

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Z.R. Zhou et al. / Wear 181-183

is an elongated ellipse, almost a strip. Increasing the load increases its length. Load per unit lengthp varies only from 140 to 220 N mm-l, a much smaller relative variation. Besides, the fretting conditions are always limited in the sticking regime. For a given cyclic amplitude and normal load, a crack can nucleate and propagate from the fretted zone of the contact edge as the number of fretting cycles increases. For example, at 900 pm amplitude, no crack is observed after 5 x lo4 cycles; after 2 x lo5 cycles, the same fretting conditions yield a 250 pm deep crack (Fig. 14(a)); after 1.5 X lo6 cycles, a 600 pm crack is obtained (Fig. 14(b)). However, the most critical parameter for fretting cracking appears to be cyclic stress amplitude ha,,. In fact it plays a double role. Firstly, as in conventional fatigue, it is the controlling parameter for crack nucleation and propagation. Secondly, micro-slip increases rapidly with Au,, and fretting damage increases accordingly.

(1995) 537-543 Ae,i

(MPa)

Fig. 1.5. Cyclic stress amplitude number of cycles.

to crack nucleation

as a function

of

The combined effect of stress amplitude and number of cycles can be summarized in a diagram similar to Wiihler’s fatigue diagram (Fig. 15). Here, crack nucleation is considered to be over for a 50 pm crack length (longer than grain size). Thus, Fig. 15 gives Aa,, to crack nucleation as a function of fretting cycles N. On the left of line AB, a crack cannot nucleate because of the low number of fretting cycles. On the right of AB, such a crack will nucleate and propagate up to final fracture. However, under a 20 MPa cyclic amplitude, fretting is too slight and its influence on fatigue is negligible. This compares with a previous study [5] on single aluminium wire fatigue behaviour, where it was noted that a low cyclic amplitude (<300 pm) in our test did not induce fretting fatigue but, rather, classical fatigue behaviour.

4. Comparisons and conclusions

2oopl (b) Fig. 14. Fretting crack nucleation cycles; (b) at 1.5 X 10’ cycles.

and

propagation:

(a) at 5X 104

A new fretting fatigue test bench has been designed specifically for single aluminium wire testing. Contact conditions similar to those encountered in a conductor-clamp system have been imposed. Compared with full-size test results on conductor-clamp systems, it has been found that the fretting pattern obtained in single wire testing such as, fretting zone size, delamination phenomena, crack nucleation, propagation modes, as well as the order of magnitude of number of cycles to crack nucleation, are analogous. It thus seems possible to use such simple test in order to understand and, possibly, to predict, complete conductor-clamp fretting fatigue behaviour. Needless to say, such prediction capability would be of great practical value as full-size conductor testing is very costly. The work presented here has concentrated on wirekeeper contact in the stick-slip regime. Further work will deal with wire-wire contact, occurring between layers. Fatigue in the gross slip and mixed regimes could also be studied, as well as multiple contact points,

Z.R. Zhou et al. I Wear 181-183

influence of cycling frequency, lation for variable amplitude.-

and damage accumu-

543

References VI [21

Acknowledgements [31 The authors wish to thank Y. Jean, C. DalpC, M. Larouche and J.P. Tremblay for their technical help. Financial support from the National Sciences and Engineering Research Council of Canada as well as collaboration from the Hydro-Quebec Power Utility, are gratefully acknowledged.

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[41 [51

C.B. Rawlins, Transmission Line Reference Book, Electric Power Research Institute, Palo Alto, CA, 1979, pp. 51-80. A. Cardou, L. Cloutier, J. Lanteigne and P. M’Boup, Fatigue strength characterization of ACSR electrical conductors at suspension clamp, Electric Power Syst. Res., 19 (1990) 61-71. Z.R. Zhou, A. Cardou, M. Fiset and S. Goudreau, Fretting fatigue in electrical transmission lines, Wear, I73 (1994) 179-188. Z.R. Zhou, A. Cardou, M. Fiset and S. Goudreau, Fretting pattern in clamp-conductor contact, Fatigue Fract. Eng. Mater. Strut., 17 (6) (1994) 661-669. J. Lanteigne, L. Cloutier and A. Cardou, Fatigue life of aluminium wires in all aluminium and ACSR conductor, Report Electrical Association). to the CEA, 1986 (Canadian