Scripta Materialia 50 (2004) 249–253

www.actamat-journals.com

Can shape memory alloy composites be smart?

V. Michaud *

Laboratoire de Technologie des Composites et des Polym�eeres (LTC), Ecole Polytechnique F�eed�eerale de Lausanne (EPFL),

CH-1015 Lausanne, Switzerland

Accepted 4 September 2003

Abstract

Shape memory alloys and their composites are often associated with the term smart material. In probing whether this is justified,

we first attempt to define the term, then provide an opinion on the potential and remaining hurdles for SMAs and their composites

to constitute smart materials, based on recent research.

� 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Adaptive composites; Smart materials; Shape memory alloys; Smart structures

1. Introduction

Scanning through the body of literature concerning

the applications of shape memory alloys within the past

10 years, two main domains clearly emerge: bio-medical

applications, and ‘‘smart’’ structures or devices. In terms

of actual market shares, the bio-medical field is largelydominant, and mostly relies on the superelastic effect at

human body temperature. Among the other tradition-

ally cited smart material candidates, including elec-

trostrictive and piezoelectric ceramics or polymers,

magnetostrictive materials, and electro-rheological flu-

ids, the so-called smart materials based on SMAs are

often still at the stage of prototypes or small niche ap-

plications. One may therefore legitimately ask whetherSMAs and their composites do indeed have potential as

‘‘smart’’ materials and, if so, what are the main hurdles

that prevent them from massive integration in engi-

neering usage.

What is a ‘‘smart material’’? The adjective ‘‘smart’’ in

its first sense meant ‘‘causing a sharp sting’’––initially,

thus, this word’s meaning was restricted to the descrip-

tion of objects or materials [1]. Somewhat similarly tothe word ‘‘sharp’’, it then evolved to encompass human

attributes, such as ‘‘forceful’’, ‘‘brisk’’, ‘‘spirited’’ or

‘‘fashionable’’. The more recent acceptation, of present

interest, is mostly found in American English to describe

* Tel.: +41-21-693-4923; fax: +41-21-693-5880.

E-mail address: veronique.michaud@epfl.ch (V. Michaud).

1359-6462/$ - see front matter � 2003 Acta Materialia Inc. Published by E

doi:10.1016/j.scriptamat.2003.09.016

objects or materials by analogy with these human at-

tributes, taking the meanings ‘‘operating by automa-

tion’’ and ‘‘imitation in a device of human intelligence’’,

thus returning to its original inanimate objects of de-

scription. In the scientific literature, smart materials

are often interchangeably called intelligent materials or

adaptive materials, and all authors seem to agree thatthere is no clear definition of these terms [2–7].

If we consider the evolution of structural materials

for the past 40 years, in particular for composite mate-

rials, the trend is moving from the search for high spe-

cific properties, to the need to maintain high properties

while reducing manufacturing and production costs, fi-

nally towards a greater flexibility and functionality of

the part. In a broad sense, smart materials and struc-tures hence represent a subset of the field of functional

materials, mostly from the angle of structural materials

with added functions, which can sense and respond to

environmental stimulus in a predetermined fashion, and

go back to their original state when the stimulus is re-

moved. One could argue that this is the definition of any

elastic tensile coupon loaded in tension and subse-

quently released; hence the concept needs to also specifythat the response of the material is not according to

usual laws of physics or mechanics, but rather optimized

towards a given goal dictated by the application, for

example optimization of the drag of an aeroelastic

structure. Also, the definition should include the feature

that the response is not attained using mechanical force

alone, but rather through functions directly built into

the structure, which respond to external forces resulting

lsevier Ltd. All rights reserved.

250 V. Michaud / Scripta Materialia 50 (2004) 249–253

from electrical, thermal or magnetic fields. A relatively

consensual definition of a smart material can thus be

proposed as a structural material that inherently con-

tains actuating, sensing and controlling capabilities builtinto its microstructure. A smart structure or system is

thus an assembly, which presents the previously men-

tioned characteristics through the combination of vari-

ous materials. In this article, we will mostly restrict the

discussion to smart materials.

2. Are shape memory alloys smart?

The purpose here is not to review the properties of

SMAs, as these are well known [8]. Rather, the goal is to

briefly review their sensing and actuating capabilities

and limitations. Most SMAs inherently exhibit a ther-mally or stress-driven thermoelastic martensitic trans-

formation, which confers them three main interesting

properties: (i) the potential to switch from high to low

damping characteristics through temperature or stress

change, (ii) a superelastic behavior in the austenitic

state, and (iii) the shape memory effect upon heating

from a deformed martensitic state, with a possible two-

way shape memory effect upon training. Hence, SMAscan be used as temperature sensors, since they can

change shape, electrical conductivity, stiffness, and

damping characteristics when passing the transforma-

tion temperature, which is dictated by the alloy com-

position. Importantly, most of the alloys present a

rather large transformation range, which makes the

change in properties gradual as well. This could be

beneficial for sensing temperature over a given range;however, the hysteretic behavior implies that the paths

for increasing and for decreasing temperature are not

identical. Ni–Ti alloys exhibiting the R-phase transfor-

mation, which presents a lower hysteresis and a steeper

dr=dT slope, may allow an on–off response over a

temperature range [9,10]. Another interesting sensing

capability of the SMA, not often mentioned, is the al-

most linear evolution of electrical resistivity with strainat a constant temperature, which could make these

materials strain sensors [11].

Shape memory alloys are however more often cited

for their actuation capabilities, which are most striking

and stem from their shape memory behavior. Indeed,

SMAs deformed in the martensitic state exhibit large

recovery strains (up to 8%), or large recovery stresses if

constrained (up to 800 MPa), when heated above theirtransformation temperature. This effect has led to the

commercial application of fastening or clamping de-

vices, and to the development of self-deploying struc-

tures [8]. The shape memory effect is however restricted

to one-shot applications, unless the SMA is coupled to

an elastic device which forces the recovery to the initial

position. SMA springs have also demonstrated their

potential for thermally activated valves, in particular

when acting against a steel spring to force return to the

initial position [12]. The two-way shape memory effect

could be a solution to make the actuation reversible;however, in both cases, if high precision is needed in

terms of activation magnitude versus the number of

cycles, the issues of thermal fatigue and drift in the

response are still not completely solved, even though

R-phase and Ni–Ti–Cu alloys seem to provide better

stability [13]. In all cases, accurate prediction of the

thermomechanical behavior of the SMA is needed to

design the actuator, taking into account the non-linearand hysteretic behavior of the alloy. Another limitation

arises from the kinetics of the actuation, which are

governed by heat transfer to or from the surroundings.

These materials are thus only restricted to very low

frequency activation, by opposition to piezo-electric

ceramics. A promising competition in terms of fre-

quency is the recent development of ferromagnetic

SMAs. As an example, NiMnGa alloys can achievemagnetically controlled strains up to 10%, with fre-

quencies in the range of 200 Hz or more [14,15]. Ap-

plications as linear motors, valves and pumps are now

proposed.

In summary, shape memory alloys can be considered

as smart materials in a restricted sense, since they un-

dergo a phase transformation, with marked variations in

physical or mechanical properties, induced by a thermalor possibly for ferromagnetic alloys by a magnetic field.

Their range of applications is thus very limited, com-

prising mainly niche markets where very elegant solu-

tions can be found to specific problems, such as

fastening devices, temperature-sensitive valves, or low-

temperature damping plates [16]; however, in such ap-

plications competition with small motorized devices

could be fierce. Another drawback of SMAs for struc-tural applications is their high density (6–8 g/cm3 for

most common alloys) coupled with their cost. To be

used in wider application ranges, SMAs need to be

coupled to a structure or integrated into another mate-

rial, possibly with added sensing capabilities. A first

simple approach, as already mentioned in this section, is

to attach SMA wires or plates to an already existing

structure acting as an elastic spring [8,17]. This is a firststep towards the coupling of SMA, but does not really

provide a smart material in the strict sense.

3. Are shape memory composites smart?

The next step to produce a smart material is to pro-

duce a composite material containing a shape memory

alloy in a form that is compatible with the composite

processing route. To get the best response out of the

shape memory effect, thin wires or ribbons span along

the entire composite part, whereas for damping or

0

10

20

30

40

50

0 5 10 15

1/1/12/1/23/1/34/1/42/1/2/1/2no wire

Stre

ss a

t 90˚

C (

MPa

)

Wire volume fraction (%)

Fig. 1. Stress exerted on the extremities of a rigid clamping frame by

Kevlar–epoxy composites with embedded NiTiCu wires, 150 lm in

diameter, initially pre-strained to 3%. Each composite, with a different

NiTiCu wire volume fraction is heated to 90 �C, i.e. above As. n=m=nrepresents the number of layers of prepreg/number of layers of wires/

number of layers of prepregs. 2/1/2/1/2 is a composite with 2 layers of

wires, separated by 2 layers of prepreg in the middle. Geometrical

arrangement is not seen to exert an effect on the activation stress.

0.0

1.0

2.0

3.0

4.0

5.0

6.0

20 40 60 80 100 120

Stre

ss (

MPa

)

Temperature [˚C]

Fig. 2. Recovery stress versus temperature for a neat epoxy sample

containing 8 NiTiCu wires 150 lm in diameter, initially pre-strained to

0.3%, clamped in a rigid frame. The squares represent the third heating

of the sample, the circles the fifth. The starting point of both curves is

at a pre-stress of 3 MPa.

V. Michaud / Scripta Materialia 50 (2004) 249–253 251

electrical properties, particles or foams could be suit-

able. Most examples of composites containing SMAs

are designed for shape change, and hence incorporate

wires or ribbons, as reviewed in Refs. [3,5]. Recentprogress in the manufacturing of SMA wires has made

available high quality wires, with thin oxide layers and

diameters below 0.2 mm. This allows the direct inte-

gration of SMA wires into fiber-reinforced polymer

composites without losing the structural integrity of the

matrix material. The resulting composite materials ex-

hibit, albeit to a lower extent, the advantages resulting

from the incorporation of pre-strained SMAs: high re-versible strains, high damping capacity, large reversible

changes in mechanical and physical characteristics, and

most importantly, the ability to generate extremely high

recovery stresses. Internal coupling to the host material

ensures the reversibility. Moreover, their specific prop-

erties and compositions are compatible with existing

structures, e.g., composite wing structures, rotor blades,

composite automotive body parts, and composite sportequipment. Their main drawback is again the need to

heat up the structure to activate it, and the relatively

slow response, since heat transfer controls the kinetics.

As a result, composite materials with embedded SMA

wires should demonstrate effects such as a shape change,

a controlled overall thermal expansion or a shift in the

natural vibration frequency upon thermal activation,

making them potentially ‘‘smart’’ materials. Until re-cently, however, the reliable production of composites

with SMAs was still a challenge, as pointed out by

Boller [12]. Some of the main critical issues have been,

besides the availability of thin reliable wires, the control

of manufacturing processes, the control of interfacial

adhesion in a highly stressed couple, and the prediction

of the resulting properties. These have evolved in the

past years to reach a stage where composites can bereliably produced at the laboratory scale, and their

properties predicted for very simple configurations [18–

22]. As an example, Kevlar–epoxy composites contain-

ing embedded pre-strained NiTiCu wires in their neutral

axis were shown to exhibit negative strain upon heating,

or if clamped at their extremities, recovery stresses. Fig.

1 illustrates this effect, by representing the recovery

stress exerted by such a composite when heated to 90 �Con the extremities of a rigid clamping frame, as a

function of wire volume fraction [13,23,24]. The straight

line represents a very simple model taken from a force

balance:

rtotal ¼ �EcompacompDT þ VSMArSMA; ð1Þ

where rtotal is the recovery stress exerted by the clamped

composite on the rigid frame, Ecomp, acomp are the

modulus and coefficient of thermal expansion, respec-

tively, of the composite material and VSMA, rSMA are the

volume fraction of wires, and the recovery stress in one

wire, respectively. The fairly good agreement of this very

simple model with experimental results shows that, in a

first approach, the recovery stress behavior is simply

linked to that of the SMA wire through its volume

fraction. This indicates in turn that models describingthe behavior of the wire can be incorporated quite

simply into a composite model to predict the composite

response, as now demonstrated in various cases [19,25].

It is worth mentioning that this simple approach is also

demonstrated for the strain response of a free beam and

the resonance vibration response of a clamped beam

[24,25]. It is only valid, however, if the host composite

material remains fully elastic, with a constant modulus.An example of the detrimental effect of matrix visco-

elasticity is provided in Fig. 2, plotting the stress re-

covery behavior as a function of temperature of a

-0.16

-0.12

-0.08

-0.04

0

0.04

0.08

0 4 8 12 16 20 24

Time (hours)

10

30

50

70

90

110

130

150Strain

Temperature

Specimen removed from oven

mouldreleased from

Specimen

Tem

pera

ture

(˚C

)

Fig. 3. Evolution of the strain, measured by an optical fiber sensor in a

Kevlar–epoxy host composite containing 4% NiTiCu wires, 150 lm in

diameter, initially prestrained to 3%, during post-cure at 140 �C of the

composite in the manufacturing frame.

-0.2

-0.18

-0.16

-0.14

-0.12

-0.1

-0.08

20 40 60 80 100 120

Com

posi

te s

trai

n (%

)

Temperature (˚C)

Fig. 4. Evolution of composite strain versus temperature measured by

an embedded fiber Bragg grating sensor during heating and cooling of

a free-standing Kevlar–epoxy composite with 5% embedded NiTiCu

wires initially prestrained to 3%.

252 V. Michaud / Scripta Materialia 50 (2004) 249–253

clamped epoxy–NiTiCu composite, that was insuffi-

ciently post-cured [24]. The sample was put in tension in

the testing frame to avoid buckling effects, since the

thermal expansion of the host material is positive in thiscase. It was found that when the composite was heated

to 115 �C (squares in Fig. 2) the stress exerted by the

composite on the rigid frame initially decreases, due to

the thermal expansion of the host resin. When passing

the transformation temperature (50 �C), the stress in-

creases again because the wires start to contract. When

cooling down the sample, however, the behavior is not

reversible, and the stress continues to increase. Thisbehavior is yet to be fully understood, but may arise

from the fact that the stress in the matrix relaxes by

viscoelasticity close to the glass transition temperature

of the resin, and there is no elastic stress then opposing

the matrix to follow a cool-down behavior dictated

mostly by thermal contraction only. When the com-

posite is heated to 80 �C, a reversible behavior is ob-

served, with the same starting behavior up to 50 �C,followed by a reduced effect of the wires, probably re-

sulting from the internal stress redistribution at each

cycle. Such a behavior is yet to be modeled accurately,

but should be avoided at all cost to maintain the re-

versibility generally desired for a smart material.

A few laboratory scale structures incorporating ‘‘re-

liable and predictable’’ SMA composites are hence now

available to demonstrate the feasibility of the concept.Once again, however, these materials are smart only in

the same, restricted, sense defined for neat SMAs, as

their sensing capability is limited to temperature, which

is not the required physical quantity to sense when one

aims to achieve a controlled shape or vibration fre-

quency change. These composites are also quite difficult

to manufacture, because pre-strained wires have to be

individually maintained during processing of the com-posite to prevent them from recovering their shape; still,

ideas are proposed to alleviate this need [26].

Potential further developments require conferring a

sensing potential to the composite material by incor-

porating sensors, additionally to the SMA. A promising

path is to further incorporate thin fibrous sensors, which

do not alter the structural integrity of the whole struc-

ture, and have a diameter in the range of that of theSMA wires. Preliminary experiments have been per-

formed to incorporate a fiber optic sensor with Bragg

gratings in a hybrid Kevlar–epoxy–SMA composite

[24,27]. These measure in-situ the strain in the composite

during processing as well as during activation. A first

outcome is a measurement of the internal stress state in

the composite after processing, since this will potentially

influence the initial stress state of the wires prior to ac-tivation, and hence their activation potential. Fig. 3

presents the evolution of the strain during post-cure of a

Kevlar–epoxy composite. The residual strain is signifi-

cant, corresponding to about 30 MPa compressive stress

in the host composite, translating into 250 MPa tensile

stress in the SMA wires. This agrees with results ob-

tained on similar materials using Raman spectroscopy

[28,29]. No current predictive model for SMA composite

takes into account the residual stresses from processing,

and this may be a need for further design with these

materials, as the activation potential and interfacialstress are affected by residual stresses. Finally, Fig. 4

displays the evolution of in-situ measured internal strain

as a function of temperature for the same material, left

free-standing. A stable behavior is obtained up to 100

�C, displaying a monotonically increasing contraction

and a low hysteresis. A laboratory scale strain-con-

trolled system could thus be manufactured with this

beam, by adding a controlled loop to adjust strain to agiven target, and maintain the value to compensate for

an applied stress [24], bringing the potential of such

composites closer to that of a more broadly defined

smart material.

V. Michaud / Scripta Materialia 50 (2004) 249–253 253

4. Concluding remarks

Bulk shape memory alloys present temperature and

strain sensing properties, as well as striking actuationproperties in terms of strain or recovery stress, which

make them smart materials, albeit only in a restricted

sense with limited applications. To increase their appli-

cation range as smart structural materials, one needs to

combine these alloys produced in the form of wires or

ribbons with a host structural material. Composite

materials containing embedded SMA wires are now

reaching the point where they can be manufactured at alaboratory or prototype scale with good reproducibility

and the desired properties for simple configurations.

Progress is still needed to design complex structures and

ensure long term reliability. The resulting materials are

however intrinsically not smart as they often lack an

adequate sensing potential apart from temperature. The

next level of refinement to produce a true smart com-

posite is thus to add sensing materials, in a minimallyintruding form, together with the actuating SMAs. This

is technically possible with fiber optic sensors, piezo-

electric fibers or any strain sensitive sensors; however,

manufacturing is still far from reaching an industrial

stage, and additional modeling work is required to take

into account manufacturing induced stresses, as well as

complex load cases. Finally, these materials will always

be restricted to application where frequency of activa-tion is not high, and where heating of a whole structure,

or at least part of it, is allowed during service. A

promising exploration path to supersede these limita-

tions may well be the further development of ferro-

magnetic SMAs, and more importantly, of composite

materials including these as fine fibers.

Acknowledgements

Recent research on adaptive composite at LTC was

funded by the Swiss Office F�eed�eeral de l’Education et de

la Science, in the frame of the ADAPT Brite/EuRamProject, supported by the European Commission. Part-

ners of the project are acknowledged for their active

collaboration. Mr. J.A. Balta of LTC, Dr. F. Bosia and

Mr. G. Dunkel of the Applied Mechanics and Reli-

ability Analysis Laboratory are acknowledged for their

contribution to the presented research results. In addi-

tion, I would like to express my gratitude to Dr. P.

Kronenberg of the Applied Computing and MechanicsLaboratory, and to Dr. M. Facchini and Mr. D. Alasia

of the Laboratory of Metrology at EPFL for lending

equipment and providing scientific advice regarding

fiber optic sensor integration.

References

[1] Webster’s new collegiate dictionary, Merriam-Webster Inc., 2003.

[2] Wei ZG, Sandstr€oom R, Miyazaki S. J Mater Sci 1998;33:3743–62.

[3] Wei ZG, Sandstr€oom R, Miyazaki S. J Mater Sci 1998;33:3763–83.

[4] http://www.smartmaterials.info/materials.

[5] Roytburd AL, Slutsker J, Wuttig M. In: Comprehensive compo-

site materials, vol. 5. Elsevier; 2000. p. 507–22 [chapter 5.23].

[6] Gandhi MV, Thomson BS. Smart materials and structures.

Chapman and Hall; 1992.

[7] Boller C. Encyclopedia of materials, Science and technology.

Elsevier Science Ltd; 2001. p. 1376–82.

[8] Otsuka K, Wayman CM. Shape memory materials. Cambridge,

UK: Cambridge University Press; 1998.

[9] Bidaux J-E, M�aanson J-AE, Gotthardt R. In: Mater Res Soc

Symp, vol. 459. 1997. p. 107–17.

[10] Leclercq S, Lexellent C, Tobushi H, Lin PH. Mater Trans JIM

1994;35(5):325–31.

[11] Airoldi G, Pozzi M, Riva G. In: Mater Res Symp Proc, vol. 459.

1997. p. 459–65.

[12] Boller C. In: Materials for smart systems III. Mater Res Soc Symp

Proc, vol. 604. Materials Research Society; 2000. p. 87–98.

[13] Schrooten J, Michaud V, Parthenios J, Psarras G, Galiotis C,

Gotthardt R, M�aanson J-AE, Van Humbeeck J. In: Mater Trans

JIM. Special Issue on Smart Materials––Fundamentals and

Applications � 2002 The Japan Institute of Metals 2002;

43(5):961–73.

[14] Tellinen J, Suorsa I, J€aa€aaskel€aainen A, Aaltio I, Ullako K.

Proceedings of 8th International Conference ACTUATOR 2002,

Bremen, Germany, June 2002.

[15] Kakeshita T, Ullako K. MRS Bull 2002;27(2):105–9.

[16] Scherrer P, Bidaux JE, Kim A, M�aanson J-AE, Gotthardt R. J

Phys IV France 1999;9, pr. 9-393-400.

[17] Brinson LC, Huang MS, Boller C, Brand W. J Intell Mater Syst

Struc 1997;8:49–56.

[18] Turner T. In: Rao VS, editor. SPIE 8th Annual Symposium on

Smart Structures and Materials, SPIE vol. 4326, Paper no. 4326-

24, Newport Beach, March 2001.

[19] Turner T. In: Jacobs JH, editor. SPIE 7th Annual Symposium on

Smart Structures and Materials, SPIE vol. 3991, Paper no. 3991-

47, Newport Beach, March 2000.

[20] Schrooten J, Michaud V, Zheng Y, Balta J-A, M�aanson J-AE.

Proceedings of SPIE Smart Structures and Materials, 17–21

March 2002, San Diego, USA, paper 4698-59, 2002.

[21] Michaud V, Schrooten J, Parlinska M, Gotthardt R, Bidaux JE.

SPIE Smart Structures and Materials, 17–21 March 2002, San

Diego, USA, paper 4698-60, 2002.

[22] Simpson J, Boller C. SPIE Smart Structures and Materials, 17–21

March 2002, San Diego, USA, paper 4698-45.

[23] http://www.mtm.kuleuven.ac.be/Research/ADAPT/index.html.

[24] Balta J. Adaptive composites with embedded shape memory alloy

wires. EPFL. PhD thesis 2003, no. 2781.

[25] Sittner P, Michaud V, Schrooten J. In: Mater Trans JIM. Special

Issue on Smart Materials––Fundamentals and Applications �2002 The Japan Institute of Metals 2002;43(5):984–93.

[26] Xu Y, Otsuka K, Yoshida H, Nagai H, Oishi R, Horikawa H,

Kishi T. Adv Eng Mater 2002;4(N9):683–6.

[27] Michaud V, Balta JA, Bosia F, Botsis J, M�aanson J-AE.

Proceedings of ICCM 14, July 2003, San Diego, USA, paper

no. 1621.

[28] Parthenios J, Psarras GC, Galiotis C. Composites Part A

2001;32:1735–47.

[29] Psarras GC, Parthenios J, Galiotis C. J Mater Sci 2001;36:

535–46.