IEEE TRANSACTIONS ON COMPONENTS, PACKAGING, AND MANUFACTURING TECHNOLOGY-PART A, VOL. 19, NO. 3, SEPTEMBER 1996 295

Shape-Memory Alloy Mechanical Contact Devices Milenko BraunoviC, Senior Member, IEEE, and C . Labrecque

Abstract-The effect of current cycling on the performance of bolted aluminum-to-aluminum connections employing different mechanical contract devices has been studied. Current-cycling conditions and different initial contact loads (0.7 and 2 kN) have been used. The devices considered were lock-spring, conventional disc-spring (Belleville) and shape-memory alloy disc-spring wash- ers. The results show that the mechanical and electrical integrity of the connections is strongly affected by the joint configuration, which differs according to the mechanical device used. The CuAlMn shape-memory alloy disc-spring washer was found to significantly lower and stabilize the contact resistance in a bolted joint, even when incorrectly installed (low initial contact force). The observed effect is explained in terms of a strong temperature dependence of the shape-memory alloy mechanical properties (spring rate) over a narrow temperature range. The mechanism responsible for this dependence is reversible martensite-austenite phase transformation which enables the shape-memory alloy to act as a sensor and a force actuator. This is of significant practical importance since field experience has shown that very often joint failures are linked to the incorrect installation of the joints.

Index Terms- CuAlMn shape-memory alloys, contact resis- tance, disc-spring washers, current-cycling.

I. INTRODUCTION HE primary purpose of any contact device is to establish T and maintain the stable operation of a connector. A well-

designed contact device should have adequate mechanical strength to maintain the mechanical integrity of a connector under normal and overload conditions of operation. It should also establish and maintain a low contact resistance, thus preventing or minimizing the excessive heating of the joint during overlpads. In fact, the temperature rise of the joint should not exceed that of the conductor under normal or emergency conditions.

The conventional and most widely used method of joining aluminum conductors is bolting. This type of connection is easily made using a hand tool and steel hardware (bolts, washers). However, there are doubts concerning its reliability under operating conditions, primarily because the design of a connector for a given conductor is mainly empirical. Since no established criteria exist, the design is usually a result of trial and error and its effectiveness in maintaining the stable operation of an aluminum connection is assessed by its performance under current-cycling conditions. The situation becomes even more complex when dissimilar metals are used, since the difference in their physical, mechanical, and met-

Manuscript revised March 20, 1996. This paper was presented at the 41st IEEE Holm Conference on Electrical Contacts, Montreal, Canada, October 2 4 , 1995.

The authors are with IREQ, Institut de Recherche dHydro-Qu&bec, Varennes, QuCbec, J3X 1S1 Canada.

Publisher Item Identifier S 1070-9886(96)06792-3.

allurgical properties makes it difficult to make a satisfactory joint.

Previous work on aluminum connections has shown that a number of factors affect the stability of a joint, including oxidation [ 11, surface preparation of the contacting members [ll, [2], use of lubricants [2], [3], fretting [4], stress relaxation [ll, [51, [6], and use of different types of mechanical devices

One of the most serious drawbacks when using aluminum conductors is their tendency to creep and undergo stress relaxation. Creep or cold flow occurs when aluminum is subjected to a constant external force over a period of time. The rate of creep is stress- and temperature-dependent and is higher for aluminum than for copper or steel. Stress relaxation is also time-, temperature-, and stress-dependent but, unlike creep, is not accompanied by dimensional changes. It is evidenced by a reduction in the contact pressure in the joint, resulting in increased contact resistance possibly to the point of failure, especially in a bolted or screw type connection.

Thermal expansion is another important factor to be con- sidered when using aluminum, since it can produce a ther- moelastic ratcheting effect. During the heating cycle in a joint held by a steel bolt, excessive tightening of the bolt can plastically deform the aluminum conductor, which cannot regain its original dimension in subsequent cooling cycles. Repeated heating and cooling cycles can cause loss of the contact force in the joint, thus increasing its resistance and temperature. This, in turn, will enhance the oxide buildup at the contacting interface and cause even greater stresses and higher temperatures during the next heating cycle.

Deterioration of the electrical and mechanical integrity of a joint is usually accompanied by a temperature increase that can lead to stress relaxation, creep fretting, thermoelastic ratcheting, accelerated oxidatiodcorrosion, etc. Furthermore, since the contact resistance is inversely proportional to applied contact load, the loss of the latter during normal andor over- load conditions can further increase the contact temperature and accelerate the deterioration processes.

In order to make a satisfactory bolted joint, various palliative measures in the design have been considered. One measure is to use a suitable mechanical contact device combined with appropriate tightening of the joint, thus ensuring that all its members remain within their elastic limits under all expected operating conditions. The most widely used type of mechanical connector is a bolted joint consisting of a steel bolt with lock-spring and flat washers, but in recent years, a disc-spring (Belleville) and transition washers [8] have begun to replace it.

Previous studies on the effectiveness of different mechanical contact devices [7] have shown that the use of disc-spring

[71.

1070-9886/96$05,00 D 1996 IEEE

IEEE TRANSACTIONS ON COMPONENTS, PACKAGING, AND MANUFACTURING TECHNOLOGY-PART A, VOL. 19, NO 3, SEPTEMBER 1996

HEATING REVERTING TO INITIAL STATE

4

TWINNED MARTENSITE DEFORMED MARTENSITE

DEFORMATfQN

Fig. 1. Thermoelastic martensitic transformation and SME [lo].

(Belleville) washers combined with thick fat washers assures the most satisfactory mechanical stability of a bolted joint under stress-relaxation and current-cycling conditions. The same combination was also found to be the most effective in reducing the deleterious effects of thermoelastic ratcheting on the mechanical integrity of bolted aluminum-to-aluminum connections [9].

One of the most important characteristics of disc-spring washers is their ability to absorb elastic deformation caused by an outside load. Because of the space limitations imposed by the geometry of a bolted joint, disc-spring washers have to be as small as possible and made from materials having a high tensile strength and a high elastic limit. These materials should also have high dynamic-fatigue resistance and sufficient plastic deformation to allow fabrication of cold worked springs and to minimize spring failure undler sudden high load changes.

Disc-spring washers are generally made of spring steel such as high-carbon, chrome-vanadium, chrome-vanadium- molybdenum, tungsten-chrome-vanadium, and stainless steels. However, other resilient materials, such as silicon and phos- phor bronze, beryllium copper, inconel, and nimonic can also be employed. The force-djeflection characteristics of most disc-springs made of these materials obeys a linear rela- tionship between the force and the spring deflection. The spring rate is directly proportiional to the elastic modulus of the spring material which, in conventional materials, is not strongly temperature-dependent. Hence, the spring rate and the force-deflection characteristic of the disc-springs is, generally, independent of temperature.

More recently, it was shown that shape-memory alloys (SMA) can be used as a disc-spring material [lo]. The SMA’s have a high sensitivity to deformation over a narrow tem- perature range owing to the strong temperature dependence of their elastic modulus on temperature. This makes them ideal as disc-spring washers, since their spring rate shows a pronounced change with temperature, which means they can

typically 20°C

M, . Martensite start temperature

M, - Martensite finish temperature

4 -Austenite start temperature Start or reverse transformation of martensite

pV - Austenite finish temperature Finish of reverse transformation of martensite

TEMPERATURE

Hypothetical plot of property change as a function of temperature .. ~ - . for a martensitic transformation occurring in-a shape-memory alloy [I71

change the shape of the load-deflection curve, an advantage in many applications. This feature of SMA washers was used by Oberg and Nilsson [ 111 to demonstrate that, even in improperly installed bolted joints (low contact force), the heat developed by the joint looseness can be used to lower and stabilize the contact resistance in a bolted joint. At this point it is of interest to describe briefly some general features of the SMA’s.

The shape-memory effect (SME) refers to the ability of certain materials to “remember” a shape, even after severe deformation. When a material with shape-memory ability is cooled below its transformation temperature (martensite phase), it has a very low yield strength and can be deformed quite easily into a new shape. When it is heated above its transformation temperature, it undergoes a change in crystal structure which causes it to return spontaneously to its original shape (austenite phase). During this spontaneous, reversible and isotropic transformation process, as the atoms shift back to their original positions, a substantial amount of energy is released. A single 1 cm3 of SME alloy can exert enough force to move an object weighing 4650 kg!

This phase transformation is a first-order displacive trans- formation in which a body-center cubic phase (austenite), on cooling, transforms by a shear-like mechanism into marten- site, which is both ordered and twinned. The martensitic transformation is diffusionless, i.e., it involves a cooperative rearrangement of atoms over a short distance into a new stable crystal structure without changing the chemical nature of the matrix. This is schematically illustrated in Fig. 1.

The martensitic transformation starts at temperature M3 (martensite start) during cooling and is completed at tem- perature M f (martensite finish), when the whole austenitic phase has disappeared; the new phase formed is completely martensitic. Upon heating, the reverse transformation starts at A, (austenite start) when the martensite begins to disappear and i s completed at -4, (austenite finish). The different stages of this transformation may be followed by measuring such properties as electrical resistivity, length or volume changes as a function of temperature as illustrated in Fig. 2.

BRAUNOVIC AND LABRECQUE: SHAPE-MEMORY ALLOY MECHANICAL CONTACT DEVICES 291

TABLE I PHYSICAL DIMENSIONS AND SOME MECHANICAL

PROPERTIES OF CONTACT DEVICES USED IN THIS WORK

SMA DISC-SPRING BELLEVILLE GROVER

Di 12.8 10.5 10.2

De 23.7 24.0 16

t 2.2 3.0 2.5

ho 1.1 0.7

ho/ t 0.5 0.23

De/Di 1.85 2.28 1 57

Fm 7 kN** 19 kN

** Austenitic state (above Af)

Fm - Load at 100% deflection

Although the SME was observed as long ago as 1938, it was not widely recognized until 1962 when the nickel-titanium alloy was discovered and the first important applications were made. The SME is found in a number of alloys, but is characteristic of two particular groups: nickel-titanium (Ni- Ti) and copper-based alloys (Cu-Zn-A1 and Cu-Ni-Al) whose transition temperature is highly sensitive to the alloy composi- tion and thermo-mechanical treatment. In the case of Ni-Ti, the temperature can be varied from -200 "C-+lOO "C, whereas with copper-based alloys it can range from -105 "C to +200 "C.

Recently, considerable interest and R&D effort, particularly in Japan, have been devoted to ferrous-base alloys. The SME in these alloys is based on the stress-induced martensite. These alloys exhibit a very large transformation hysteresis and, in general, have less than 4% recoverable strain [12]-[16]. Although the commercial potential of these alloys has yet to be determined, this effort has opened up new classes of SMA for exploration.

SMA's are basically functional devices in that they are more important for what they can do (action) than for what they are (property). The practical applications of SMA's are numerous: they have been used successfully as thermal and electrical actuators, thermomechanical energy converters, electrical con- nectors, circuit breakers, and mechanical couplers, as well as in robotics applications, medicine, and other areas [17] and [18]. A comprehensive review on the use of SMA in electrical applications was given by Tautzenberger and Stockel [19], Braunovic [20], and an extensive bibliography of US patents on electrical applications of SMA's with exemplary claims can be found in NTIS special publications [21] and [22].

TABLE I1 TRANSFORMATION TEMPERATURES OF THE CuAlMn WASHER

Sample A, AI M,f'C) Mff'C)

1 23 37 13 -1 5 2 27 39 13 -7

In view of the SMA's unique ability to act as a force actuator, the present work was undertaken to evaluate the performance of a bolted joint with a shape-memory contact de- vice (washer) unda static (continuous) and dynamic (cycling) current conditions. This work is a part of a wider research program on the SMA electrical applications.

11. EXPERIMENTAL PROCEDURE

A. Aluminum Busbars

All the tests in this study were performed using busbars made of 1350 grade aluminum, a grade selected because of its widespread use in the electrical industry and, also, its inferior mechanical properties (creep and stress relaxation) compared with aluminum alloys. The samples, measuring 30 mm x 6 mm x 300 mm, were machined from 1350-H12 aluminum bars.

All contacting surfaces of the aluminum busbars were prepared following the same procedure. The overlapping joint surfaces were first machined and degreased by wiping them with cotton swabs soaked in a 50150 mixture of freon and methanol. The mating surfaces were then abraded with a fine stainless-steel brush. For each joint configuration tested, a new brush was used for each contact-device combination.

B. Joint Configurations and Contact Devices

The following joint combinations were used: 1) spring-lock washer (Grover) + thin flat washer; 2) disc-spring washer (Belleville) + thick flat washer; 3) SMA disc-spring (Belleville) washer + thick flat washer. The spring-lock (Grover) and disc-spring (Belleville) wash-

ers are commercially available and were obtained from a local hardware supplier and are made of C 1075 steel. The flat wash- ers were made of stainless steel. The physical dimensions and mechanical properties of the washers are given in Table I. The SMA disc-spring (Belleville) washer was made of CuAlMn alloys with the following chemical composition Cu-80%, A1-13%, Mn-3.7%, and Fe-1.8%. Its physical dimensions and mechanical properties are given in Table I.

The shape-memory properties, i.e., phase transformation temperatures of the CuAlMn alloy were determined using differential scanning calorimetry (DSC). This technique, based on the measurement of the latent heat of transformation, reveals discrete exothermic and endothermic reactions in the alloys occurring during heating and cooling and allows direct measurements of the transformation temperatures and energies as well as fractions of alloys transformed as a function of

298 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING, AND MANUFACTURING TECHNOLOGY-PART A, VOL 19, NO 3, SEPTEMBER 1996

HP 3497A Data acauision unit

TOP VIEW

Fig. 3. assembly.

Simplified schematic of the load cell and the current-cycling testing

temperature. Table I1 gives transformation temperatures for this alloy.

C. Dejection-Force Measurements The deflection-force characteristics of SMA washers were

determined at different temperature using an Instron 4204 universal tension-compression machine and a specially de- signed fixture made of Invar. The SMA washer was placed between the two Invar blocks, one block attached to the load cell, the other fixed to the base of the machine frame. The tests were conducted in an environmental chamber at 23, 60, 80, and 100 "C. Prior to compressing the washers, the temperature in the chamber was stabilized for 20-30 min. The applied force was measured with the machine load cell while the washer deflection was measured with the machine displacement transducer. The deflection-force characteristics were determined by compressing the washer until it reached its flattened position. The data from the machine were recorded using a HP3852 data logger and processed by a HP 745i computer.

D. Current-Cycling Test DC current-cycling tests were carried out on the joint

combinations described above. One current cycle consisted in rapidly heating the sample at 1000 A for 5 min (ON period) followed by cooling to room temperature during the next 15 min (OFF period). This cycling procedure was repeated three

DEFLECTION (%)

Fig. 4. different temperatures.

The force-deflection characteristics of a SMA disc-spring washer at

times followed by prolonged cooling to the room temperature during the next 45 min. Once a complete cycle (three heating cycles + prolonged cooling) at a selected current level was completed, the whole procedure was repeated.

Four pairs of bolted joints were tested simultaneously, two for each joint configuration used. The contact resistance, force and temperature were monitored continuously. The contact resistance was derived from the contact-voltage drop measured between the bolted busbars in a cross-rod configuration while changes in the contact force were monitored with a specially designed load cell comprising a capacitance gauge and a proving ring. The current-cycling tests were conducted at initial contact forces of 0.7 kN and 2 kN.

A simplified schematic of the load cell and the testing arrangements is shown in Fig. 3. The three parameters were recorded at regular time intervals using a HP3442 data logger whose output was transferred to an HP 7451 microprocessor for computer processing. The same microprocessor was used to control the ON and OFF switching of the power supply. When the joint temperature reaches 150 "C, the whole system shuts down.

111. RESULTS

A. Force-Dejection Characteristics The force-defleckion characteristics of a SMA disc-spring

washer at different temperatures are shown in Fig. 4. The most important feature of these results is that, as the tem- perature increases, the force generated by the disc-spring also increases. This temperature dependence is associated with the transformation of martensite into austenite, i.e., heating above the transformation temperatures A, and A f (Table I). The austenitic phase has a much higher elastic modulus than martensite and, hence, higher mechanical strength. Conse- quently, below the transformation temperature of the CuAlMn alloy, the disc-spring has a low spring rate while above this temperature it has a high spring rate. The importance and implications of this feature will be discussed later.

BRAUNOVIC AND LABRECQUE: SHAPE-MEMORY ALLOY MECHANICAL CONTACT DEVICES

1 0 -

0 5

299

-

LOCK - SPRING (GROVER) WASHER ", SMA DISC-SPRING (BELLEVILLE) WASHER 1201 . , I , , , I , I I . , DISC. SPRING ( BELLEVILLE) WASHER

1201 . , . , , , , , , , , ,

f ;;L/ 0 e

P O 0 200 400 6w BW 1WO 1200

3 O v , r , , , , , 24t 1 2 5 -

5 -

2 0 -

I O

0 5 -

I 8

1 2

0 8 L l 0 2w 4 w Mx) 8W lwo 1200

TlME (MINUTES)

(h) TIME (MINUTES )

(4 Fig. 5. Effect of current cvcline on the temuerature, contact resistance and force of bolted aluminum-to-aluminum joints with different mechanical contact

Initial contact foice 077 li~. devices.

I

4 c c E - 1 I -

B

SMA DISCSPRING (BELLEVILLE) WASHER

r-----7 ,20t , ,LOCK -SPRING ( G R 0 V E R ) y H E R , , , 90

Bo

30

60

30

Y 0 1 6

'30 ' " " " 1 "

30 , , , , , , I -

2 4

7 8

1 2 -

0 8

-

P -

0 200 4 w Mx) (Iw lwo 12w TlME (MINUTES )

00 " ' I " ' * ' ( '

(h)

0 0

30 I , I , , , , , , , ,

::- 0 6

0 2w 4w SM 8M 1wo 120.3 00

TlME (MINUTES )

(C)

contact resistance, and force of bolted aluminum-to-aluminum joints with different mechanical Fig. 6. contact devices. Initial contact force 2.0 Idv.

Effect of current cycling on the temperature,

B. Current-Cycling

The results of current-cycling tests in Figs. 5 and 6 show changes in the joint temperature, contact force and contact resistance as a function of time of current cycling at two initial contact forces, i.e., 0.7 N and 2 kN. The most characteristic feature of these results is that the performance of the joint under current-cycling conditions is significantly affected by the type of joint configuration. This is mainly reflected by changes in the contact force and contact resistance with time of current cycling.

The joint temperature of all configurations at 2 kN initial load showed a normal change with time of current cycling. The maximum temperature remained below 75 "C over the whole test period. However, for joints at the initial contact force of 0.7 kN, the maximum temperature attained in the combinations involving conventional and SMA disc-spring washers was approximately 90 "C and remained more or less at that level over the whole test period. In the bolted joint involving a

lock-spring washer, the contact temperature showed no such tendency but continued to increase with current-cycling time to exceed 90 "C at the later stages of cycling.

'The changes in the contact force and contact resistance with current-cycling time were more indicative of the joint configuration than the corresponding changes in the contact temperature. The contact resistance of a configuration involv- ing a lock-spring washer at 0.7 kN initial force not only increases with cycling time but also shows no tendency of ever reaching a steady state. The situation is improved somewhat at higher initial contact force (2 kN), since the tendency of the contact resistance to increase with current cycling-was slightly reduced. The contact force in both cases, i.e., initial contact loads of 0.7 and 2 kN, showed the same type of behavior-it initially decreases and then reaches a stable value.

In the case of a joint with a conventional disc-spring, the contact resistance at initial contact loads of 0.7 and 2 kN showed the same behavior-it remained practically unaffected

300 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING, AND MANUFACTURING TECHNOLOGY-PART A, VOL. 19, NO. 3, SEPTEMBER 1996

by the current-cycling and stable over the whole test period. This was also observed for the contact force variations with the cycling time. However, force variations during the ON periods of cycling were slightly higher in a joint bolted with an initial force of 2 kN than with 0.7 kN.

The contact resistance of a bolted joint involving a SMA disc-spring washer showed the most interesting behavior. Within the first few cycles of current-cycling, the contact resistance decreases rapidly and then stabilizes with further cycling at a value approximated one-half its initial value. Although the contact resistance of the joints with different initial contact forces showed practically the same behavior with the current cycling, the drop in the contact resistance was more pronounced in the joints bolted with 0.7 kN than with 2 kN.

Another interesting feature of the joints with SMA wash- ers is the contact force behavior, which takes the form of pronounced but stable changes in the contact force with current-cycling. Again, these changes in the contact force are more pronounced at 0.7 kN initial contact force than at 2 kN. Plausible explanations for the observed effect are discussed below.

IV. DISCUSSION From the results, it is obvious that some mechanical-contact

devices have a considerable influence on the performance of bolted aluminum-to-aluminum connections, whereas others have very little or no effect at all. The characteristic feature of the results is that the contact resistance and contact force are the parameters most sensitive to the type of device, the joint configuration and current cycling, whereas the contact temperature is affected to a lesser degree.

The results of current-cycling tests clearly show that con- siderable improvement in the mechanical integrity and, hence, the performance of bolted joints can be achieved using con- ventional disc-spring instead of spring-lock washers. However, the most dramatic improvement was observed when a SMA washer was used. The beneficial effect of SMA washers is emphasized by the fact that, even with a low initial contact force (improper installation), the heating caused by reduced contact force generates an additional stress in the joint that appreciably increases its mechanical integrity and lowers and stabilizes the contact resistance, thus, preventing the joint from failing. This is of significant practical importance, since field experience has shown that very often joint failures are linked to poor or inadequate installation.

Although a detailed mechanism responsible for the observed effect is beyond the scope of this work, it is obvious that the significant improvement in the mechanical and electrical integrity of bolted joints with SMA washers, even under the least favorable conditions (loose joint), is due to the strong temperature dependence of their mechanical properties (stress- strain) over a narrow temperature range.

The mechanical properties of an SMA in the martensitic state are very different from those of conventional materials. This difference is best illustrated in Fig. 7 showing the stress- strain curves for the martensite and the austenite. The curve

HIGH TEMPERATURE

U) ‘A 0 W E + cn

old TEMPERATURE

STRAIN &M Fig. 7. Stress-strain curves for martensite and austenite

for austenite resembles that of a conventional material but that for martensitic SMA can be arbitrarily divided into three well defined stages.

In Stage I, on exceeding a first yield point, a plateau is formed whereby a strain of several percent can be accumulated with only little stress increase. The deformation in the plateau region is due to the stress-induced growth of one martensite orientation at the expense of an adjacent, unfavorably oriented. This process, called “detwinning” is fundamentally different from the conventional deformation by gliding and can be recovered by heating above the transformation temperature. Stage I1 is usually linear, although not purely elastic. It is believed that the deformation mechanism in this stage is a combination of elastic deformation of the detwinned martensite and the formation of new orientations of martensite intersecting those already present, which, in turn, provides additional heat-recoverable strains. Stage 111 corresponds to the onset of irreversible plastic deformation and the material is plastically deformed in the conventional way.

The results presented show that current cycling considerably reduces contact force. The consequence of this force reduction will be contact deterioration. Hence, maintaining the contact load and keeping all the members of the joint well within their elastic limits are of prime importance for the mechanical and, also, electrical integrity of a joint.

As mentioned above, the use of SMA disc-spring washers produced the most dramatic improvement in contact resistance behavior during current cycling. Hence, it is of interest to invoke some basic features of electrical contacts in order to provide some plausible explanations for the observed effect of SMA washers.

It has been established [23], that real surfaces are not flat but comprise many asperities. Hence, when contact is made be- tween two metals, surface asperities of the contacting members will penetrate the natural oxide and other surface contaminant films, thereby establishing localized metallic contacts and, thus, conducting paths. As the force increases, the number and the area of these small metal-metal contact spots will increase as a result of the rupturing of the oxide film and extrusion of metal through the ruptures. These spots, termed a- spots, are small cold welds which provide the only conducting paths for transfer of electrical current. Current passing across a contact interface is therefore constricted to flow through

301

INlTlAL CONTACT FORCE 0.7 kN 4 4 , I 1 . I I 1 3 \ . I . ,

-i 2e-4 i DISCSPRING WASHER

LOCKSPRING WASHER E

1 e 4 x

le-3 -

5 . 4 z SMA DISCSPRING WASHER 8

&+or ' I ' I ' I ' 1 ' I ' I 0 200 400 600 800 1000 1200

BRAUNOVIC AND LABRECQUE: SHAPE-MEMORY ALLOY MECHANICAL CONTACT DEVICES

Fig. 8. Calculated contact area (A,) of bolted aluminum-to-aluminum joints as a function of current-cycling time for two initial contact loads 0.7 kN and 2 kN.

TIME (MINUTES )

these a-spots. Hence, the electrical resistance of the contact due to this constricted flow of current is called "constriction resistance" and is related to the basic properties of metals such as hardness and electrical resistivity. Holm [23] has shown that the constriction resistance for a single a-spot can be expressed as

(1)

where p 1 and pz are resistivities of the contacting metals, n is the number of a-spots and a is the radius of the metal-to-metal contact area. If the two contacting metals are the same, then the constriction resistance becomes

(2) P 2na

P1+ P2 R, = ___ 4na

R c - --.

Since the metals are not clean, the passage of electric current may be affected by the presence of thin oxide, sulphide and other inorganic films which are usually present on metal surfaces. Hence, the total contact resistance of a joint is the sum of the constriction resistance (R,) and the resistance of the film ( R f )

R = R, f Rf

( 3 )

where cr is the resistance per area of the film. Both tunneling and fritting are considered as operative mechanisms for the current transfer across the film. In most practical applications, the contribution of these films to the total contact resistance is of minor importance, since the contact spots are usually created by the mechanical rupture of surface films.

The actual contact area where conduction takes place can be calculated by considering the contact as one large circular

INITIAL CONTACT FORCE 2.0 kN

2e-3 - DISCSPRING WASHER

oe+o

4 e 4 Lt 1 LOCKSPRING WASHER

rl

SMA DISCSPRING WASHER

i o e o l ' I ' I ' I ' I ' ' J

0 200 400 600 800 lo00 1200

TIME (MINUTES )

composite area comprising several discrete small areas of radius a (a-spots) with an electrical resistance R,

where p is the bulk resistivity of a conductor busbar) and a, is the composite radius and A,

(4)

(aluminum the contact

area attained in a bolted joint with different contact devices. Using these expressions and the data for the contact resis-

tance values from the current cycling tests (Figs. 5 and 6), the calculated contact area A, for two initial contact loads is illustrated in Fig. 8. It is evident that in the case of a bolted joint with a SMA washer the contact area is dramatically increased and maintained throughout the current cycling tests by the action of a SMA washer. On the other hand, the bolted joint with a lock-spring washer showed just the opposite-it decreases as the current cycling progresses. This is a clear indication of the inability of a lock-spring washer to suppress the deleterious effect of stress relaxation and, hence, overheat- ing of a joint. The joint with a conventional disc-spring had a somewhat similar behavior to that with a SMA washer.

The beneficial action of the SMA washer is a direct con- sequence of the martensite-austenite phase transformations occurring as the joints heat, which changes their mechani- cal properties. Furthermore, this transformation imparts the SMA's with the unique ability to perform a dual function-that of a sensor and also of an actuator. This clearly opens up a wide range of applications since the SMA actuators combine large motions, high forces and small size, thus providing a very efficient energy output within a confined space.

302 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING, AND MANUFACTURING TECHNOLOGY-PART A, VOL. 19, NO. 3, SEPTEMBER 1996

It should be pointed out, however, that an important problem that plagues the application of SMA’s is the alteration of their characteristics induced either by cyclic deformation (fatigue) and phase changes or by aging at application temperatures. The fatigue life of SMA’s depends strongly on the mode of cyclic deformation, since the interfaces between different phases or between martensitic variants, grain boundaries, precipitates, grain size, and other lattice defects participate in the deformation process. Because of the large number of parameters and their strong influence on the SME, improving the fatigue properties of SMA is a very complex task.

The aging of SMA, often called amnesia, is manifested by the changes in the transformation temperatures, increased hysteresis and stabilization of the martensitic phases which entails the loss of SME. The rate at which aging progresses is a function of the SMA application temperature and force and the effects differ in cause according to the aging temperature.

Two types of aging are distinguished: initial aging, in the martensitic phase, and long term aging, in austenitic phase. Initial aging corresponds to a diffusion-induced rearrangement of atoms that find their equilibrium states, and thus, stabilize the martensite and raise the transformation temperatures M3 and A,. The atom rearrangement is accelerated by the presence of a high concentration of quench-in vacancies. This can be eliminated or suppressed by step-quench which, in turn, will slow down the kinetics of this type of aging.

In conclusion, it can be said that the main objective of this work was to demonstrate the great flexibility and adaptability of the SMA technology in a particular electrical application. The constantly increasing applications requiring such adapt- ability will eventually make the SMA technology an essential part of the electrical and other industries.

One of the basic needs in the electrical industry is for long-term unattended devices and systems that can be used in different environments, i.e., generation, transmission, and dis- tribution of energy. SMA’s can provide applications with self- inspecting and self-identifying capabilities that can determine the adaptive response based on the application (environment) and/or damage to the particular device or system.

The ability of SMA’s to alter the task, scope, objective, and design of a specific device or system will undoubtedly have a strong impact on the design philosophy in the future. For example, a device or system comprising an SMA can detect temperature changes or short-circuit conditions on the network and react autonomously, by changing the load pattern or interrupting the circuit before failure of the system or unacceptable degradation of performance occws.

On the other hand, it should be pointed out that the metallurgy and production technology of SMA materials are very complex and still not fully understood within the bounds of the present knowledge. There are still haunting questions regarding their long-term thermal and mechanical stability, fatigue resistance, higher recoverable strains, high-temperature alloys, reduction in hysteresis, effect of dynamic stresses, and electrical fatigue, and also the development of cost-effective, inexpensive processing, and fabrication techniques.

These problems compounded by the poor information and lack of engineering data for SMA’s are probably the main

cause for the slow penetration of SMA technology in the industry. It is indeed surprising that, despite some 10000 patents issued world-wide on SMA covering the variety of devices and applications as well as the processing methods, so few large-scale applications exist.

Understanding the complex behavior of SMA materials calls for an open, intensive discussion between metallurgists, physicists, and mechanical engineers and scientists, to improve our knowledge of the factors influencing the properties and behavior of SMA materials. Fundamental and applied know- how combined will provide the necessary conditions for re- producible, cost-effective industrial processing and production of SMA materials.

In view of the unique properties of SMA’s (superelasticity, damping, SME), and also the vast area of applications based on the SMA ability to perform such functions as detection- activation, fail-safe control, damping, etc., it is not difficult to find areas of R&D of interest to the electrical industry and utilities. The main aim of such an R&D effort would be directed toward improved service reliability and reduced frequency of maintenance interventions.

V. CONCLUSION

The results show that the mechanical and electrical integrity of the connections is strongly affected by the joint configuration, which differs according to the mechanical device used. The CuAlMn shape-memory alloy disc-spring washer was found to significantly lower and stabilize the con- tact resistance in a bolted joint even when incorrectly installed (low initial contact force). The observed effect is explained in terms of a strong temperature dependence of the shape-memory alloy me- chanical properties (spring rate) over a narrow temper- ature range. The mechanism responsible for this depen- dence is reversible martensite-austenite phase transfor- mation which enables the shape-memory alloy to act as a sensor and a force actuator.

ACKNOWLEDGMENT

The successful execution and completion of this work would not have been possible without the technical assistance of D. Gagnon and L. Rodrigue of IREQ. L. Kelley read the manuscript and made valuable comments.

REFERENCES

R. D. Naybour and T. Farrell, “Degradation mechanisms of connectors on aluminum conductors,” Proc. ZEE, vol. 120, p. 273, 1973. R. L. Jackson, “Significance of surface preparation for bolted aluminum joints,” Proc. IEE, vol. 128, p. 45, 1981. M. Braunovic, “Evaluation of different types of contact-aid compounds for aluminum-to-aluminum connectors and conductors,” in Electrical Con<ucts--1984, IIT, Chicago, IL, 1984, p. 97. __, “Fretting corrosion between aluminum and different plating materials,” in Electrical Contacts--1977, IIT, Chicago, IL, 1977, p. 223. -, “The effect of electrical current on the stress relaxation of aluminum wire conductors,” in Proc. ICSMA-7, London, U.K.: 1985, p. 619. __, “Effect of current cycling on contact resistance, force and tem- perature of bolted aluminum-to-aluminum connectors of high ampacity,” IEEE Trans. Comp., Hybrids, Munufact. Technol., vol. CHMT-4, p. 57, 1981.

BRAUNOVIC AND LABRECQUE: SHAPE-MEMORY ALLOY MECHANICAL CONTACT DEVICES 303

-, “Effect of different types of mechanical contact devices on the performance of bolted aluminum-to-aluminum joints under current cycling and stress relaxation conditions,” in Electrical Contacts-1 986, Boston, 1986, p. 133. R. L. Jackson, “Electrical performance of aluminudcopper bolted joints,” Proc. IEE, vol. 129, p. 177, 1982. M. Braunovic and M. Marjanov, “Thermoelastic ratcheting effect in bolted aluminum-to-aluminum connections,” IEEE Trans. Comp., Hy- brids, Manufact. Technol., vol. 11, p. 54, 1988. T. Waram, “Design considerations for shape-memory Belleville wash- ers,” Ruychem Tech., Documentation. A. Oberg and S. Nilsson, “Shape-memory alloys for power connector applications,” in Electrical Contacts-I 993, 1993, p. 225. K. Tsuzaki, M. Ikegami, Y. Tomota, Y. Kurokawa, W. Nakagawara, and T. Maki, “Effect of thermal cycling on the Martensitic transformation in an Fe-24Mn-6Si shape-memory alloy,” Mater. Trans. JIM, vol. 33, p. 263, 1992. T. Maki and K. Tsuzaki, “Transformation behavior of €-Martensite in Fe-Mn-Si shape memory alloys,” in Proc. ICOMAT-92, Monterey, 1992, p. 1151. S. Miyazaki and K. Otsuka “Development of shape memory alloys,” ISlJ Intl., vol. 29, p. 353, 1989. T. Maki, S. Furutani, and I. Tamura, “Shape-memory effect related to thin-plate Martensite with large thermal hysteresis in ausaged Fe-Ni-Co- Ti alloys,” ISIJ Int., vol. 29, p. 438, 1989. K. Tsuzaki, Y. Antsume, Y. Kurokawa, and T. Maki, “Improvement of the shape-memory effect in Fe-Mn-Si alloys by the addition of carbon,” Scripta Met. Mat., vol. 27, p. 471, 1992.

T. W. Duerig, K. N. Melton, D. Stockel, and C. M. Wayman, Eds., Engi- neering Aspects of Shape-Memory Alloys. London, U.K.: Butterworth, 1990. L. MacDonald Schetky, “Shape-memory alloy applications,” in fnter- metallic Compounds, J. H. Westnbrook and R. L. Fleischer, Eds. New York Wiley, 1995, vol. 2, p. 527. P. Tautzenberger and D. Stockel, “The use of shape-memory alloys in switchgears,” in Electrical Contacts-1984, IIT, Chicago, IL, 1994, p. 449. M. Braunovic, “Use of shape-memory materials in distribution power systems,” CEA Rpt. SD-294A, 1994. “Shape-memory alloys: Electrical applications,” NTIS PB94-884434, 1984. “Zero and low insertion force connectors,” NTIS PB94-851391, 1994. R. Holm, Electrical Contacts. New York: Springer-Verlag, 1961.

Milenko Braunovic (M’73-SM’92), photograph and biography not available at the time of publication.

C. Labrecque, photograph and biography not available at the time of publication.