STRUCTURAL APPLICATION OF SMART MATERIALS

Dr. K. Muthumani, Assistant Director & Dr. R. Sreekala, Scientist
Structural Engineering Research Centre,
Chennai

Introduction

The development of durable and cost effective high performance construction materials and systems is important for the economic well being of a country mainly because the cost of civil infrastructure constitutes a major portion of the national wealth. To address the problems of deteriorating civil infrastructure, research is very essential on smart materials. This paper highlights the use of smart materials for the optimal performance and safe design of buildings and other infrastructures particularly those under the threat of earthquake and other natural hazards. The peculiar properties of the shape memory alloys for smart structures render a promising area of research in this field.

Materials and Application

Shape Memory Alloys(SMA)

The term shape memory refers to the ability of certain alloys (Ni – Ti, Cu – Al – Zn etc.) to undergo large strains, while recovering their initial configuration at the end of the deformation process spontaneously or by heating without any residual deformation .The particular properties of SMA’s are strictly associated to a solid-solid phase transformation which can be thermal or stress induced. Currently, SMAs are mainly applied in medical sciences, electrical, aerospace and mechanical engineering and also can open new applications in civil engineering specifically in seismic protection of buildings.

Its properties which enable them for civil engineering application are

Repeated absorption of large amounts of strain energy under loading without permanent deformation. Possibility to obtain a wide range of cyclic behaviour –from supplemental and fully recentering to highly dissipating-by simply varying the number and/or the characteristics of SMA components.
Usable strain range of 70%
Extraordinary fatigue resistance under large strain cycles
Their great durability and reliability in the long run.

Structural Uses

Active control of structures

The concept of adaptive behavior has been an underlying theme of active control of structures which are subjected to earthquake and other environmental type of loads. The structure adapts its dynamic characteristics to meet the performance objectives at any instant. A futuristic smart bridge system (An artist rendition) is shown below :Fig.1 (3)

(Courtesy: USA Today dt. 03.03.97). Sun and Sun (6) used a thermo mechanical approach to develop a constitutive relation for bending of a composite beam with continuous SMA fibers embedded eccentric to neutral axis. The authors concluded that SMA’s can be successfully used for the active structural vibration control. Thompson et al (3) also conducted an analytical investigation on the use of SMA wires to dampen the dynamic response of a cantilever beam constrained by SMA wires.

Fig.1

2) Passive control of structures

Two families of passive seismic control devices exploiting the peculiar properties of SMA kernel components have been implemented and tested within the MANSIDE project (Memory Alloys for New Seismic Isolation and Energy Dissipation Devices). They are

Special braces for framed structures and isolation devices for buildings and bridges. Fig.2.shows the arrangement of SMA brace in the scaled frame model and the reduced scale isolation system.

Fig-2

3) Smart Material Tag

These smart material tag can be used in composite structures. These tags can be monitored externally through out the life of the structure to relate the internal material condition . Such measurements as stress, moisture, voids, cracks and discontinuities may be interpreted via a remote sensor(6)

4) Retrofitting

SMAs can used as self-stressing fibres and thus they can be applied for retrofitting. Self-stressing fibres are the ones in which reinforcement is placed into the composite in a non-stressed state. A prestressing force is introduced into the system without the use of large mechanical actuators, by providing SMAs. These materials do not need specialized electric equipments nor do they create safety problems in the field. Treatment can be applied at any time after hardening of the matrix instead of during its curing and hardening. Long or short term prestressing is introduced by triggering the change in SMAs shape using temperature or electricity.

Introduction

Materials and Application

Shape Memory Alloys(SMA)

Its properties which enable them for civil engineering application are

Repeated absorption of large amounts of strain energy under loading without permanent deformation. Possibility to obtain a wide range of cyclic behaviour –from supplemental and fully recentering to highly dissipating-by simply varying the number and/or the characteristics of SMA components.
Usable strain range of 70%
Extraordinary fatigue resistance under large strain cycles
Their great durability and reliability in the long run.

Structural Uses

Active control of structures

Fig.1

2) Passive control of structures

Special braces for framed structures and isolation devices for buildings and bridges. Fig.2.shows the arrangement of SMA brace in the scaled frame model and the reduced scale isolation system.

Fig-2

3) Smart Material Tag

4) Retrofitting

SMAs can used as self-stressing fibres and thus they can be applied for retrofitting. Self-stressing fibres are the ones in which reinforcement is placed into the composite in a

They make active lateral confinement of beams and columns a more practical solution. Self stressing jackets can be manufactured for rehabilitation of existing infrastructure or for new construction

5) Self-healing

Experimentally proved self-healing behavior (5) which can be applied at a material micro level widens their spectrum of use. Here significant deformation beyond the first crack can be fully recovered and cracks can be fully closed.

6) Self-stressing for Active Control

Can be used with cementitious fibercomposites with some prestess, which impart self-stressing thus avoiding difficulties due to the provision of large actuators in active control which require continuous maintenance of mechanical parts and rapid movement which in turn created additional inertia forces.

In addition to SMA’s some other materials such as polymers can also be temporarily frozen in a prestrained state that have a potential to be used for manufacturing of self-stressing cementitious composites (4).

7.Structural Health Monitoring

Use of piezo transducers, surface bonded to the structure or embedded in the walls of the structure can be used for structural health monitoring and local damage detection. Problems of vibration and UPV testing can be avoided here. Jones et. al., (7) applied neural networks to find the magnitude and location of an impact on isotropic plates and experimented using an array of piezo-transuders surface bonded to the plate.

Substitute for steel?

It is reported that (4) the fatigue behaviour of CuZnAl-SMA’s is comparable with steel.If larger diameter rods can be manufactured. It has a potential for use in civil engineering applications. Use of fibre reinforced plastics with SMA reinforcements require future experimental investigations.

CARBON FIBRE REINFORCED CONCRETE(CFRC)

Its ability to conduct electricity and most importantly, capacity to change its conductivity with mechanical stress makes a promising material for smart structures .It is evolved as a part of DRC technology(Densified Reinforced Composites).The high density coupled with a choice of fibres ranging from stainless steel to chopped carbon and kelvar, applied under high pressure gives the product with outstanding qualities as per DRC technology. This technology makes it possible to produce surfaces with strength and durability superior to metals and plastics.

SMART CONCRETE

A mere addition of 0.5%specially treated carbon fibres enables the increase of electrical conductivity of concrete. Putting a load on this concrete reduces the effectiveness of the contact between each fibre and the surrounding matrix and thus slightly reduces its conductivity. On removing the load the concrete regains its original conductivity. Because of this peculiar property the product is called “Smart Concrete”. The concrete could serve both as a structural material as well as a sensor.

The smart concrete could function as a traffic-sensing recorder when used as road pavements. It has got higher potential and could be exploited to make concrete reflective to radio waves and thus suitable for use in electromagnetic shielding. The smart concrete can be used to lay smart highways to guide self steering cars which at present follow tracks of buried magnets. The strain sensitive concrete might even be used to detect earthquakes.

Active railway track support

Active control system for sleepers is adopted (3)

to achieve speed improvements on existing bridges and to maintain the track in a straight and non-deformed configuration as the train passes With the help of optimal control methodology the train will pass the bridge with reduced track deflections and vibrations and thus velocity could be safely increased. Fig(3) shows various positions of the train with and without active railway track support. Fig-3

Active structural control against wind

Aerodynamic control devices to mitigate the bi-directional wind induced vibrations in tall buildings are energy efficient, since the energy in the flow is used to produce the desired control forces. Aerodynamic flap system(AFS) is an active system driven by a feedback control algorithm based on information obtained from the vibration sensors(3).The area of flaps and angular amplitude of rotation are the principal design parameters. fig.(4) shows an active aerodynamic control device.

CONCLUSION

The technologies using smart materials are useful for both new and existing constructions. Of the many emerging technologies available the few described here need further research to evolve the design guidelines of systems. Codes, standards and practices are of crucial importance for the further development.

ACKNOWLEDGEMENT

The author thanks the Director, SERC for the constant encouragement and support rendered in preparing this paper and also for giving permission to publish the paper. The kind support and guidance of all the team members of Structural Dynamics Laboratory is gratefully acknowledged.

REFERENCES

DuerigT.W, Melton K.N, Stoeckel D., Wayman C.M., Engineering aspects of shape memory alloys, Butterwort heinemann Ltd:London,1990.
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Jones,R.T.,Sirkis.J.S.,andFriebele,E.J.(1997)Detection of impact location and Magnitude for Isotropic plates Using Neural Networks,Journal of Intelligent material systems and Structures,7,pp90-99.