UNIVERSIDADE FEDERAL DE SANTA CATARINA

CURSO DE GRADUAÇÃO EM ENGENHARIA DE MATERIAIS

MATHEUS PAVARINI SCHIMIDT

STUDY OF STIFFNESS AND DAMPING IN UNIDIRECTIONAL AND TWILL

COMPOSITES INCORPORATING SHAPE MEMORY ALLOY WIRES

FLORIANÓPOLIS

2012

ii

UNIVERSIDADE FEDERAL DE SANTA CATARINA

CURSO DE GRADUAÇÃO EM ENGENHARIA DE MATERIAIS

MATHEUS PAVARINI SCHIMIDT

STUDY OF STIFFNESS AND DAMPING IN UNIDIRECTIONAL AND TWILL

COMPOSITES INCORPORATING SHAPE MEMORY ALLOY WIRES

Trabalho validado pelo Curso de Graduação em

Engenharia de Materiais da Universidade Federal de

Santa Catarina como ao equivalente ao trabalho de

conclusão de curso, requisito necessário para a

obtenção do título de Engenheiro de Materiais.

Avaliado por: Dylton do Vale Pereira Filho

FLORIANÓPOLIS

2012

iii

UNIVERSITY OF BIRMINGHAM

SCHOOL OF METALLURGY AND MATERIALS

MATERIALS ENGINEERING DEGREE

FINAL YEAR PROJECT

Study of Stiffness and Damping in

Unidirectional and Twill Composites

incorporating Shape Memory Alloy Wires

Student: Matheus Pavarini Schimidt

ID: 1210431

Supervisor: Dr. Martin Strangwood

Word count:7071

March 23th

, 2012

Table of Contents

1. Introduction ............................................................................................................................................ 1

2. Literature Review ................................................................................................................................... 3

2.1 Shape Memory alloy ......................................................................................................................... 3

2.1.1 Martensitic and Austenitic Transformation in NiTi alloys ............................................................ 3

2.1.2 One Way and Two Ways Shape Memory Effect ........................................................................... 4

2.1.3Memory effect ................................................................................................................................ 5

2.1.4 Superelasticity ............................................................................................................................... 6

2.2 Composites ....................................................................................................................................... 7

2.2.1 Fiber ............................................................................................................................................... 8

2.2.1.1 Carbon fiber ................................................................................................................................ 8

2.2.2 Matrix ............................................................................................................................................ 9

2.2.2.1 Thermoset ................................................................................................................................... 9

2.2.2.1.1 Epoxy ....................................................................................................................................... 9

2.2.3 Prepreg ..........................................................................................................................................10

2.2.3.1 Unidirectional Layers and Fabrics .............................................................................................10

2.3 Development of SMA Composites ..................................................................................................12

3. Methodology..........................................................................................................................................15

3.1 Beam Production .............................................................................................................................15

3.2 Three Point Bending ........................................................................................................................16

3.3 Cantilever Beam ..............................................................................................................................16

3.4 Polishing Test and Optical Microscopy ...........................................................................................17

4. Results ...................................................................................................................................................18

4.1 Mechanical Properties .....................................................................................................................18

4.2 Vibrational Caracteristic ..................................................................................................................21

4.3 Microstructure .................................................................................................................................23

5. Discussion .............................................................................................................................................24

5.1 Stiffness and considerations: ...........................................................................................................24

5.2 Damping and Considerations ...........................................................................................................27

6. Conclusions ...........................................................................................................................................30

7. References .............................................................................................................................................32

1

1. Introduction

Nowadays, sport science and aerospace industries are aiming to search for

materials that fit your needs, for example, on aerospace industries there are great efforts

to develop materials that present vibrational control for structured applications to reduce

the resonance effects in aircraft panel. In the most case, only one material not display the

characteristics required for the application.

It means the increase demands on the performances of materials with multiple

characteristic used in engineering applications as well as the necessity of lightweight

constructions in various applications can be fulfilled by development of so-called

adaptive, multifunctional and intelligent materials. In concert, researches into new

processes have been taken on purpose to improve these materials and their

characteristics.

Hybrid composite materials have proprieties of which, such as high damping

capacity, stiffness, superelastic behavior, may be varied in response to external or internal

stimuli. However, there are some limitations that difficult the application in industry:

strength and durability of the interface between SMA and matrix are of special

importance, since the functionality and durability of the SMA-composite may be

affected.

Shape memory alloys show a complex three-dimensional thermo mechanical

behavior with hysteresis, this behavior is influenced by a large number of parameters, the

failure of SMA-elements is completely different from the failure of conventional

materials. These open new perspectives to the development and improvement of this

material which are capable of reacting in a manner which will improve performance,

efficiency and reliability of the overall structure.

The present report details the research project of investigating the stiffness and

damping in composite materials consisting of epoxy resin reinforced by unidirectional

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and twill carbon fibers embedded Nitinol wires as actuator. First of all, the literature

surrounding the SMA composites research and its components is revised, then, the

experimental procedure is laid out. Finally, the results and discussions on the findings

will be presented, followed by conclusions and future work.

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2. Literature Review

2.1 Shape Memory alloy

Shape memory alloys are metallic alloys that display two unique qualities, shape memory

effect (SME) and superelasticity, both of which are related to the material phase of the

SMA (Lagoudas 2008).

The material phase is dependent on temperature, consequently, the temperature of the

SMA has a great effect on its mechanical properties. At relatively high temperatures, the

alloy is in an austenite phase while at relatively low temperatures, it is in a martensite

phase (Brinson 1993). Thus, SMA can be used as an actuator because of the large

recovery stress and strain caused by the phase transfer from the martensite phase to the

austenite, which lead to their wide use in the medical and sport sciences.

The six characteristic transformation temperatures that define SMA’s transformation are:

Ms ,Mf, Rs , Rf , As, Af which are martensite start temperature, martensite finish

temperature, R -phase start temperature, R- phase finish temperature , Austenite start

temperature and Austenite finish temperature (Machado 2002).

2.1.1 Martensitic and Austenitic Transformation in NiTi alloys

According to (Zhang and Sehitoglu 2004), at high temperatures, the lattice shows the

austenite matrix formed by body-centered cubic, B2, and phase product, consisting of

martensite monoclinic B19 '. The martensite B19 'can be obtained by a transformation

step B2→ B19'. However in other conditions such as cold work, thermal cycling or a

third element in the alloy provides the appearance of the rhombohedric R-phase, which

appears between martensite and austenite phases, resulting in a second stage of

processing stage B2→R → B19. The crystal structure of the R-phase is rhombohedric.

The main features of martensitic transformations are:

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• The martensitic phase can be either a substitutional and interstitial solid solution;

• The chemical composition of the martensitic phase is the same phase austenitic matrix;

• Transformation is accompanied by a dimensional variation and on surfaces is polished

possible to observe the relief;

2.1.2 One Way and Two Ways Shape Memory Effect

Generally, SMA materials can be plastically deformed at some relatively low

temperature, and upon exposure to some higher temperature will return to their shape

prior to the deformation. Materials that exhibit shape memory only upon heating are

referred to as having a one-way shape memory. Some materials also undergo a change in

shape upon re cooling. These materials have a two-way shape memory (ASM

International Handbook Committee 1990).

Figure 1: Demonstration of shape memory effect (a-c) and two-way shape memory effect

(d-g) in Ti-50 at% Ni alloy (Otsuka and Wayman 1998).

The figure 1 explains this phenomenon .When the applied stress is small, the specimen

reverts to the original shape completely by SME (a-c). However, when the applied stress

is too large (d), irreversible slip occurs, and the shape does not revert to the original one

even after heating above Af [(c) and (e )]. However, in the next cooling cycle, the

specimen elongates automatically as shown in (f). Then, if heating and cooling is

repeated, the specimen changes its shape between (g) and (f), respectively. The specimen

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now remembers the shape of (f) in the martensitic state. This is called two-way shape

memory effect (TWME). In contrast to this, the previous SME is sometimes called the

one-way shape memory effect (K. Otsuka and Wayman 1998).

2.1.3Memory effect

The shape memory effect is a unique property of certain alloys exhibiting martensitic

transformation. Even though the alloy is deformed in the low temperature phase, it

recovers its original shape by the reverse transformation upon heating to a critical

temperature called the reverse transformation (Otsuka and Wayman 1998). Thus, this

property depends basically of two factors: temperature and stress.

Processed NiTi alloys, commonly, present this transformation at temperatures close to

room temperature.

.

(Shaw and Kyriakides 1995) described the memory effect occurs as follows: at low

temperature the alloy has a martensitic structure (B19'-monoclinic) and can apparently be

plastically deformed, about as much as 3% to 8% than in other alloys. But this

deformation "permanent" can be recovered due to the increased temperature. This

recovery mechanism occurs by the transformation of the structure martensitic in an

austenitic structure (B2 - cubic body-centered).

Figure 2: Transformation of austenite to martensite in function of temperature.

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2.1.4 Superelasticity

The other unique property of SMAs is called superelastity. This property, differently of

the shape memory effect, occurs at higher temperatures when the alloy has austenitic

structure.

The phenomenon of superelasticity is closely linked to shape memory effect and is a

manifestation of the transformation of austenite to martensite induced for stress. That is

when the alloy is at a temperature where the austenite is stable, applying a stress within

certain limits causes a instability at this stage, gradually passes into the martensitic

structure self-accommodated induced for stress. However, when the unloading occurs,

this martensite induced for stress is reversed again to austenite, returning the sample to its

original size. This is usually involved a lot of deformation beyond that expected for the

elastic regime of the alloy. Thus, the phenomenon receives the designation of

superelasticity. The phenomenon can be seen in Figure 3.

Figure 3: Stress-strain curve of the nickel-titanium alloy to 70 ° C associated with the

scheme of microstructural changes (Shaun and Kyriakides 1995).

In figure 3 there is the graph of stress by strain for the NiTi alloy in the field of stability

of austenite The passage (o-a) shows an elastic deformation of austenite. At the point (a)

an austenite begins to suffer macroscopic elongation becoming unstable. Thus beginning

the process of transformation to martensite induced by stress, getting the two coexisting

phases to the point (b) where the alloy begins to deform permanently. Thus, there is a

decrease in tension in section (b) the stress induced martensite becomes unstable and

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begins to revert back to austenite.

Figure 4: Stress-strain hystersis for superelastic SMA.

Figure 4 shows the transformation stresses in corresponding to the phase transformation.

Additionally, one can see that when loading, the stiffness of the composite is dramatically

decreased during phase transformation. In this figure 4, σMAf ,σMAs , σAMs , and σAMf

are the martensite to austenite finish stress, martensite to austenite start stress, austenite to

martensite start stress, and the austenite to martensite finish, respectively.

2.2 Composites

Composites are considered to be combinations of materials differing in composition or

form on a macro scale (Composite Materials Handbook et al 2002). Normally, they are

made of reinforcing fibers and matrix materials.

A composite is designed to display a combination of the best characteristics of each of

the constituent materials (William 2003). Consequently, the composite’s performance is

superior to those of its constituents acting independently. Thus, the properties of a

composite are different from those of its constituents

They have excellent engineering properties e.g. light weight, corrosion resistance, high

strength, stiffness, and control characteristics (Tsai and Suong 2003).

Because of this, their uses are increasing broadly with application in relevant fields such

as the space and aviation industries, sporting goods and mechanical parts.

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In this Thesis will be address the constituents and characteristics (orientation, properties,

etc.) of the composite used in the experiments.

2.2.1 Fiber

Reinforcements are important constituents of a composite material and give

all the necessary stiffness and strength to the composite.

Fibre reinforced polymers have already gained large interest and increased application in

diverse fields, mainly due to their inherent high stiffness-to-weight ratio, corrosion

resistance and controlled anisotropic properties. Therefore, the integration of these extra

properties in SMA-composite might offer unique opportunities in many fields of

industrial activities (Rudy Stalmans 2002). There are multiple different reinforcing

materials that have been studied to be used as reinforcement with SMAs in composites,

most of which are conventional fibers, such as carbon, aramid, and glass fibers.

Some of the common types of reinforcements include:

• Continuous unidirectional carbon.

• Woven fabric, the most used: Twill, Plain, Satin.

• Multidirectional fabric (stitch bonded for three-dimensional properties).

Figure 5: Woven Fabrics-Twill Fiber

2.2.1.1 Carbon fiber

Carbon fibers are usually obtained from the pyrolysis of PAN (Polyacrylonitrile) which

show high strength and stiffness, and are widely used in the production of parts with great

9

structural responsibility. Moreover, exhibit a diversity of physical and mechanical

characteristics, thus allowing carbon based composites to have specific engineered

properties and retain their high strength-to-weight and stiffness-to-weight at elevated

temperatures.

2.2.2 Matrix

Matrix surrounds the fibers and thus protects those fibers against chemical and

environmental attack and gives rigidity to the structure. The matrix transfers the load to

Fibers, thus, for the fibers to carry maximum load, the matrix must have a lower modulus

and greater elongation than the reinforcement (Sanjay K. Mazumdar 2001).

The Matrix selection is performed based on chemical, thermal, electrical, flammability,

environmental, cost, performance, and manufacturing requirements, can be classified into

thermosetting and thermoplastic.

2.2.2.1 Thermoset

Thermoset are brittle in nature and offer greater thermal and dimensional stability, better

rigidity, higher electrical, chemical and solvent resistance. They are generally used with

some form of filler and reinforcement. Materials once cured with thermoset cannot be

remelted or reformed (Sanjay K. Mazumdar 2001). Thermoset resins provide easy

processability and better fiber impregnation because the liquid resin is used at room

temperature for various processes such as filament winding, pultrusion, etc.

The most common resin materials used in thermoset composites is epoxy.

2.2.2.1.1 Epoxy

The remarkable mechanical and chemical characteristics of epoxy resins recommend

application when it is necessary to achieve high performance laminations (AL-Qureshi

1990), allowing for a broad range of properties and processing capabilities. It exhibits

low shrinkage as well as excellent adhesion to a variety of substrate materials.

Epoxies are generally brittle and the most widely used resin materials and are used in

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many applications, from aerospace to sporting goods, based composites; provide good

performance at room and elevated temperatures.

Table 1: Resin Properties

2.2.3 Prepreg

Prepreg is the composite industry’s term for continuous fiber reinforcement pre

impregnated with a polymer resin that is only partially cured (Callister 1985).

The laminations of the prepreg result in the laminate. Thus, the laminate is defined how

the superposition of layers, playes or sheets, made of unidirectional layers, fabrics or

mats, with proper orientations in each ply (Suong and Tsai 2003).

Figure 6: Composite formed by three pre-impregnated layers, fiber orientation with

0°/90°/0°.

2.2.3.1 Unidirectional Layers and Fabrics

The advantages of unidirectional (one direction of reinforcement) layers are: they have

high strength and stiffness (provides the ability to tailor the composite properties in the

desired direction), the load transmission of the fibers is continuous over large distance,

low fibre weights ≈ 100 g/m.

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Fabric (several directions of reinforcement) layers can be used to make highly contoured

parts in which material flexibility is key. The advantages are: possibility to shape

complex form using the deformation of the fabric; strength and stiffness in two

directions; very good handling characteristics.

Tabel 2: Properties of carbon fiber Fabric (Twill) X Unidirectional. (Performance

Composites 2009).

Fibres 0° (UD), 0/90° (fabric) to loading axis,

Dry, Room Temperature, Vf = 60% (UD), 50%

(fabric)

Units

Standard High Modulus Carbon Fiber (HMCF)

Standard High Modulus Carbon Fiber (HMCF)

Carbon

Fiber

Carbon

Fiber

Fabric Fabric UD UD

Young’s Modulus 0° GPa 70 85 135 175

Young’s Modulus 90° GPa 70 85 10 8

In-plane Shear Modulus

GPa 5 5 5 5

Major Poisson’s Ratio 0.10 0.10 0.30 0.30

Density g/cc 1.60 1.60 1.60 1.60

Most fabric constructions offer more flexibility for the lay-up of complex shapes than

straight unidirectional layers offer. For aerospace structures, tightly woven fabrics are

usually the choice for aril weight considerations, minimizing resin void size, and

maintaining fiber orientation during the fabrication process.

Example of fabrics is twill as seen previously, a type of textile weave with a pattern of

diagonal parallel ribs (in contrast with a satin and plain weave).

Figure 7: Fabric X Unidirectional prepreg.

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2.3 Development of SMA Composites

Composite materials containing SMA wires has attracted great attention as materials able

to adapt their shape, thermal behavior or vibrational properties during service (Jan

Schrooten et al 2002).In particular, the inherent properties of the high damping capacity

and the superelastic behavior (high reversible strains, up to 8%) of SMA’s (i.e. Nitinol)

are unique properties not offered by any other material. Moreover, the high cost of

Nitinol is one of the motivating factors for research in the field of SMA composites.

Figure 8: Theoretical model of the SMA composite beam.

This alloy was discovered in 1962 by Willian J Buehler and coworkers .They found that

an alloy consisting of equal numbers of nickel and titanium atoms showed the

transformation that leads to shape memory (Srinivasan 2001). Although there are

several SMAs (copper-, iron-,silver-, and gold-based alloys), Nickel-Titanium (NiTi) is

considered the SMA with the most engineering significance owing to its ductility at low

temperature, high degree of shape recovery capability, large pseudoelastic hysteresis,

corrosion, fatigue resistance, biomedical compatibility, and relatively high electrical

resistance (Turner 2001).

A new application of SMA was developed when (Rogers et al. 1989) introduced the

concept of embedding SMA actuators in composite laminates for structural control. This

type of material was named shape memory alloy hybrid composite material (SMAHC).

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The increase of the resistance against impact damage by embedding SMA-wires in

polymer composites has been the subject of preliminary studies, in which composite-ply

delamination can be effectively reduced by embedding SMA-wires (Rudy Stalmans

2002).

Many research efforts have been dedicated to the development of stiffness and vibration

control concepts (J.-E. BIDAUX et al 1997) .The high recovery stresses generated by

prestrained SMA-wires during heating can induce significant stiffness and vibration

modifications of clamped SMA-composite beams. It has been also studied to what extent

the high damping capacity of SMA-wires influences the overall damping response of the

SMA-composite (Rudy Stalmans 2002).

Dong-Ying Ju and Akira Shimamoto (1999) observed a large change of damping in

epoxy matrix beam embedded SMA fiber of Nitinol due to the effect of the reverse phase

transformation from martensite to austenite with an application of current. Thus,

conclude that the jump variation of stress/strain due to the reverse phase transformation

can also cause frequency variation of the composite beam.

Studies of Mooi (1992) have shown bending stiffness increases of up to 100% using

embedded SMA wires activated by electrical current heating.

On the other hand (BIDAUX et al 1993) observed that the bending stiffness of the SMA-

composite decreases when the material undergoes its martensitic transformation. Its show

that. The behavior of embedded thin SMA wires has been found to be different from the

one of plain SMA wires. It might be an effect of the interaction between the SMA

material and the matrix in the composite. A good deal of attention can be held at this

detail, the behavior of the SMA-composite is not merely the superposition of thematrix

and the SMA behavior respectively but results in new properties.

Also (G. Faiellaa et al 2009) addressed an effect related to the loss of pre-strain in the

SMA elements during the actuation due to a partial debonding between wires and matrix

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in pre-strained Nitinol wires as actuating elements embedded in an thermoset resin pre-

preg carbon fibres composite.

(Farhan Gandhi and Wolons 1998) observed when as excitation frequency increases, the

loss modulus (a measure of the damping) undergo a rapid initial decrease. As the cycling

strain amplitude increases, the storage modulus (a measure of the stiffness) initially

undergoes a rapid decrease, implying that the material softens with increasing motion

amplitude.

Due to the complexity arising from the non- linear and hysteretic thermo mechanical

responde of SMA elements, (Petr Sittner et al 2002) demonstrated the design of SMA

polymer composite using SMA composite model. This design helped to find optimal

material parameter of the SMA wires and of the polymer matrix as well as optimal

composite fabrication parameter.

(Stalmans et al 2006) related that The expected performances of engineering materials

which are manifested through formidable characteristics and better properties could be

achieved by the combined excellent features of shape memory materials and host

matrices because they showed behavior that reported interfacial quality, internal stress

distribution and reproducibility of the behavior . Nevertheless, one-way shape memory

effect cannot be properly applied to repeated actuation because structures cannot recover

their original shape after the SMA wire is cooled to room temperature. Besides,

commonly the martensite finish temperature is too low to utilize the two-way SMA effect

without the residual stresses in the SMA actuator (Maenghyo and Kim 2004).

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3. Methodology

3.1 Beam Production

Initially, three samples, two unidirectional (0°/90 °) and one twill carbon fibre

reinforced epoxy composite panels of 150mmx20mm dimensions and three samples,

two unidirectional (0°/90 °) and one twill carbon fibre pre-preg panels containing

embedded Nitinol (SMA) wires as actuators of dimensions 150mmx10mm ,were

produced (Table 3 ). The fibre volume fraction of all the samples was set to 55%.

Table 3: Samples orientation and geometry

Samples Orientation Width Lenght

UD 0°/0°/0°/0° 20mm 150 mm

UD 0°/90°/90°/0° 20mm 150 mm

Twill 0°/90° 20mm 150 mm

UD ( with Nitinol wires) 0°/0°/0°/0° 10mm 150 mm

UD ( with Nitinol wires) 0°/90°/90°/0° 10mm 150 mm Twill ( with Nitinol wires) 0°/90° 10mm 150 mm

Then, the panels were constructed by hand-lay-up and cured at 125 °C in vacuum-

bagging with a heating rate of 0.5 °C/min from room temperature and a constant

pressure of 4 bar has been applied during all the curing cycle.

The SMA wires were present in the form of 0.224 mm diameter aligned along the

beam’s longitudinal axis introduced between the first and second ply ,with a

transformation temperature (Af- austenite finish) of 25°C , fabricated by Memory-

Metalle ( the nitinol was annealed to be possible the transformation of austenite to

martensite at room temperature). Each Nitinol sample contained 3 actuators

proportionally spaced. The beams were produced using four plies in thickness.

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3.2 Three Point Bending

The material was tested on an Instron 5566 tensile test machine at the room temperature

and the support span length was set to 90 mm. The beam was supported towards either

end and loaded in the middle. Finally, the load F started with 5 kN and the speed of head

was 100 mm/min. An AD converter and computer were used to obtain the load-deflection

curves.

3.3 Cantilever Beam

The damping property at super-elastic status was evaluated by measuring the vibration

amplitude during the experiment. Each panel of fiber carbon composite with or without

Nitinol wires was fitted with a Kyowa uniaxial strain gauge (type KFG) to monitor the

oscillation and clamped for a distance of 20 mm from one end. The strain gauge was

connected to a signal converter (supplied by Kyowa) and this linked to a computer. The

beams were slightly perturbed at the free end and the signal from the strain gauge was,

then analysed by Fast Fourier Transform. Panels were oscillated and the amplitude was

altered to change strain. Thus was possible to measure the natural frequency and the

damping ratio of the beams.

Figure 9: Scheme cantilever beam system.

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3.4 Polishing Test and Optical Microscopy

In order to measure the real stress on the wires of Nitinol, beams containing the SMA

were cut off, prepared and subjected to polishing test. And then, were examined in optical

microscopy.

The samples were prepared using acrylic resin and cured for 30 minutes at room

temperature. Then were submitted for stage of rough grinding, used water as a lubricant,

with silicon carbide (SiC) paper in the following sequence of grit size: 240- followed by

400- and 800-grit papers. Then, the samples were cleaned with soap, water, ethanol,

respectively, and finally, with ultra-sonic cleaning bath containing industrial methelated

spirits (IMS).

After that, the SMA composite is applied automated polishing (the speed grinder polisher

was supplied by Buehler). In the rough polishing there are two steps: the 6 µm polish is

recommended for the first and ¼ µm with using mechanically alumina (aluminum

oxide Al2O3) abrasive particles in suspension for the final step and platen speeds less

than 120 rpm for the both process . Between the two steps the samples were cleaned and

submitted the ultra-sonic cleaning bath containing IMS (industrial methelated spirits).

Then, the sample were analysed using MI7100 MEIJI optical microscope.

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4. Results

In this section, the mechanical properties of the materials will firstly be presented in

order to discuss the effects that have on the properties and behaviour of NiTi SMAs ,

followed by the vibrational caracteristic wich revealed the damping behaviour. Finally,

the micrographs will illustrate the position of Nitinol on composite.

4.1 Mechanical Properties

Figure 10: Graphic load(N) x deflection(mm) to 0°/0° /0° /0° samples unidirectional (a )

without Nitinol (b) with Nitinol.

Figure 10 shows the results of the load in 2 mm of the deflection on sample the

0 °/0 °/0 °/0 ° unidirectional carbon fiber in resin epoxy (a) without and (b) with

embedded Nitinol wires as actuator. On this graphic it can be observed that the

relationship between the load and deflection are linear for both cases and the sample with

SMA wires has lower load than the sample without Nitinol wires. From the graphics (b) it

can be seen as well that the characteristic associated with NiTi SMAs was pronunced.

19

Figure 11: Graphic load(N) x deflection(mm) to 0°/90°/90°/0° samples unidirectional (a )

without Nitinol (b) with Nitinol.

Figure 11 represents that the characteristic associated with NiTi SMAs was expressed on

composite with layers 0 °/90 °/90 °/0 ° unidirectional as well and reduced the load in the

same 2 mm of extension. As previously seen, this phenomenon was observed on samples

with 0 °/0 °/0 °/0 ° unidirectional plies, however, the Nitinol effect on 0 °/90 °/90 °/0 °

samples was 25% smaller than beam with plies 0 °/0 °/0 °/0 ° unidirectional.

Figure 12: Graphic load(N) x deflection(mm) to twill samples unidirectional (a ) without

Nitinol (b) with Nitinol.

Figure 12 shows the load behaviour obtained from twill composites with and without

Nitinol wires. Thus, can be seen that samples twill allow smaller load that samples

unidirectional.

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Table 4 : Mechanical Properties

Samples F (N) E (Gpa) Strain

(%) ơ (Mpa)

Frequency (s-1)

Q K

(N/mm)

0°/0°/0°/0° (ud) 43 158,4663 0,16% 199,5535 98,58307 0,064539 21500

0°/0°/0°/0° (Nitinol) 18 91,52134 0,17% 158,637 102,7861 0,16508 9000

0°/90°/90°/0° (ud) 28 109,0203 0,15% 168,941 80,39311 0,097638 14000

0°/90°/90°/0°(Nitinol) 15,9 88,86298 0,17% 148,7632 83,06485 0,108214 7950

0°/90° (twill) 8,5 57,42555 0,13% 74,01515 52,46574 0,098735 4250

0°/90° (Nitinol- twill) 4,9 45,39825 0,15% 65,91153 56,76031 0,158368 2450

This table shows the values of Force, Young Modulus, strain, stress, damping factor and

stiffness on samples of the resin epoxy reinforced by unidirectional or twill carbon fiber

without and with embedded Nitinol wires.

An important relationship is an increase of strain associated to a decrease of Young

modulus is observed in the samples with the same orientation.

Figure 13: Stiffness behaviour on samples with and without Nitinol wires.

The results presented in graphic confirm the significant reduction in the stiffness of the

SMA based composite beams. The Young Modulus dropped on samples with NiTi

wires(see table 4), consequently, is observed an decrease of the stiffness of the 58.14%,

43.12% and 42.35% on Nitinol samples with 0 °/0 °/0 °/0 °, 0 °/90 °/90 °/0 °, twill

respectively.

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Table 5: Real Stress on Nitinol wires

Nitinol Samples ơ (Mpa) ơ0

real (Mpa)

0°/0°/0°/0° (ud) 158,637 301,4225076

0°/90°/90°/0° (ud) 168,941 290,3894963

0°/90° (twill) 74,01515 145,8544089

Table 5 show real stress on Nitinol fiber. Considering that the host matrices showed

behavior that reported internal stress distribution, the stress is 50wt% higher on Nitinol

wires than superficies composites.

4.2 Vibrational Caracteristic

Figure 14: Damping behaviour on 0°/0°/0°/0° unidirectional samples (a ) without Nitinol

(b) with Nitinol.

In the figure 14, it is found the damping behavior of the Nitinol wires on samples

0 °/0 °/0 °/0 °. The graphic amplitude (voltage) against time (seconds) shows an increase

of damping on samples with SMA actuators. Also, is possible observed a decrease in

amplitude on samples with Nitinol

22

Figure 15: Damping behaviour on 0°/90°/90°/0°unidirectional beam (a ) without Nitinol

(b) with Nitinol.

Figure 15 can see a reduced of amplitude on samples 0 °/90 °/90 °/0 °. Consequently, an

increase in damping was relates on samples with SMA wires. Comparing samples,

observed a bigger damping on samples 0 °/0 °/0 °/0 °than 0 °/90 °/90 °/0 °.

Figure 16: Damping behaviour on twill panels (a ) without Nitinol (b) with Nitinol.

In twill samples, the graphic result not showed a damping behavior deep comparing with

unidirectional samples. However, the results related an increase of damping in samples

twill as well.

23

4.3 Microstructure

(a) (b)

Figure 17: (a) Nitinol wires between the composite plies, (b) distance between the

Nitinol wires edge and the superficies of the composite.

In order the identify the distance between the Nitinol wires and the edge of the sample,

was carried out the microscopy measure. How the picture show the distance between the

edge of Nitinol wires and the sample superficies is 321,61 µm, and 522.36 µm between

the center of the sample and the superficies of the first ply of carbon fiber how can see in

figure 17 (b).

Figure 18: Position of the Nitinol into the composite and illustration of the stress on

composite superficies and the real stress on Nitinol wires.

24

5. Discussion

5.1 Stiffness and considerations:

Stiffness is an important property in SMA composite, it expresses the capacity of a

mechanical system to sustain loads without excessive changes of its geometry

(deformations), and can be modified (enhanced) only by proper selection of the

component geometry (shape and size) and its interaction with other components. The

principal effects of Stiffness on performance of mechanical systems are due to influence

of deformations on static and fatigue strength, wear resistance, efficiency (friction

losses), dynamic/vibration stability, and manufacturability (Eugene 1999). SMA’s, i.e.

Nitinol, exhibit unique property of high stiffness.

The Stiffness is defined by:

K = P/ δ

Where, the (P) is load in Newton, and (δ) is deflection in mm.

Stiffness effect can be seen in the results section, it is because the austenite (stronger

phase - the two phases have different elastic modulus) transform in martensite upon

stress. For the the pseudoelastic effect to occur, the stress need to be between 200Mpa

and 350 Mpa (according to the Nitinol wires supplier). Thus, as showed in table 5, only

Nitinol wires with orientation 0 °/0 °/0 °/0 ° and 0 °/90 °/90 °/0 ° have the stress in this

interval. Therefore, a reduction of stiffness (of 58% in 0 °/0 °/0 °/0 ° and 43% in

0 °/90 °/90 °/0 °) was observed in unidirectional samples with Nitinol wires due the

pseudoelasticity.

However, on twill carbon fiber embedded Nitinol samples the stress is not enough to

stimulate the transformation of the austenite in martensite which characterize the

pseudoelasticity effect. As discussed on literature review, strength and stiffness are in two

25

directions on twill fibres. Thus, the Young modulus is smaller than unidirectional

samples. Consequently, the load transmission is on several directions, and therefore, the

stress is lower compared to unidirectional beam as shown in the results section on table

3 and 5. Nevertheless, an increase of the 42% on stiffness was observed, probably,

because the inclusion of the wires on the composite or not complete transformation of

austenite to martensite. Thus, from now all the effects associated with twill carbon

fiber/epoxy embedded Nitinol wires will be considered the two effects mentioned.

The three point bending test is a very useful test for determining the stiffness of

engineering materials. Moreover with this test, we can obtain the stress.

One of the properties of SMAs resulting from reversible phase transformations is the

stress induced pseudoelasticity.

Its definition is Flexural Strength Test (ASTM D 790 and ISO 178) which shows the

maximum stress developed when a bar-shaped test piece, acting as a simple beam, is

subjected to a bending force. On three-point bending the applied bending moment varies

along the length of the specimen, thus, the strains and stresses become dependent on axial

(ASM Handbook Committee 2000).

Figure 19: set-up of three point bending.

The flexural stress (ơ) at the outer fibers at mid-span in three-point bending was

calculated from the following expression:

26

The mechanical property of Young modulus is related to the stiffness and can be

formulated. The flexural modulus (Et) for three-point bending (Fig. 19) is calculated as

(ASM Handbook Committee 2000):

Where, st is the deflection in mm, E is the bending modulus of elasticity in MPa. L is the

support span; b, the width of the test beam; d, the thickness of the test beam. All these

three are in mm. P is the Force in Newtons.

As shown in the results section a decrease the Young modulus between samples with and

without Nitinol , it is because the pseudoelasticity effect is related to the transition of

the austenite to martensitie on unidirectional panels, as previously seen. Opposite effect is

observed by (Rustighi et al 2005) which related an increase in the elastic modulus in the

transition from the low temperature martensitic state to the high temperature austenitic

phase.

To obtain the strain, r, of the specimen under three-point test, the following expression

applies:

Where, D is deflection in mm, and r is the strain in %.

As already noted, there was an increase of strain on the composite when added Nitinol.

This suggests recovery of strain proporcionate by pseudoelasticity effect on

27

unidirectional panels with SMA wires.

5.2 Damping and Considerations

Damping is the capacity of a mechanical system to reduce intensity of a vibratory

process. The damping effect for a vibratory process is achieved by transforming

(dissipating) mechanical energy of the vibratory motion into other types of energy, most

frequently heat, which can be evacuated from the system (Eugene 1999).

SMA wires exhibit good damping properties at pseudo-elastic and super-elastic statuses.

The Damping to shock in SMA is more efficient in the transition zone (martensite-

austenite), whereas damping of continuos vibration is readily solved in the martensitic

domain. According to literature, optimum damping properties would be obtained by

SMA volume reiforced of about 10% (François Boussu et al 2002).

Figure 20: damping capabilities (I) martensite (III) austenite + martensite (II) austenite

(François Boussu et al 2002).

The resonance amplitude is inversely proportional to the damping parameter and the

energy dissipation is directly proportional to the square of the vibration amplitude.

For the present composites with SMA fibers, logarithmic decrement method is used to

measure damping in time domain, which can be evaluated by measuring the vibration

amplitude during the experiment. A classical damping equation for vibration beams is

given by:

28

Where, xn, xn+1 are the amplitude of sine wave with logarithm damping in different

intervals.

According to the results and theory, an increase of damping was observed comparing

unidirectional and twill samples with and without Nitinol. Nevertheless, analyzing

Nitinol unidirectional samples, an increase of damping is due to transformation of

austenite in martensite. This effect can be seen on figure 20.

For determine the amplitude- and therefore – was carried out the cantilever beam testing,

explained above.

Figure 21: Set out cantilever beam test.

For a simple elastic beam problem with uniform cross-sectional area, a well-known

to determine the natural frequencies of the bending modes of the composite beam,

the following theory is used (ASHBY et al 1992).

Where f is a frequency in (s-1) , 3.5156 is a constant relative to vibration bound

29

condition, E is young modulus in Pascal, I is the moment in m, and is the relation

mass by length in Kg/m.

As already seen, SMA wires offered an increase of frequency in all samples.

Chatter resistance (stability in relation to self-excited vibrations) of machine tools and

other processing machines is determined by the criterion KQ (K - effective stiffness and

Q- damping, e.g., logarithmic decrement) in many cases dynamic stiffness and damping

are interrelated, such as in mechanical joints and materials , the stiffness increase can be

counterproductive if it is accompanied by reduction of damping. In some cases, stiffness

reduction can be beneficial if it is accompanied by a greater increase in damping (Eugene

et al 1999).

30

6. Conclusions

In this project, a study for evaluating the stiffness and damping on SMA composites is

introduced. The present project considers mechanical properties and vibrational

characteristics to describe the results, thus, as conclusions of the present project, the

following can be draft:

Comparing samples with and without Nitinol wires, the results of the three point bending

presented a slightly decrease in properties as Young Modulus and stress, on the other

hand, an increase in strain was observed.

For unidirectional composites embedded Nitinol wires, the change in stiffness is

associated with the transformation of the austenite in martensite. Thus, the

pseudoelasticity effect is more significant in 0°/0°/0°/0° samples than 0°/90°/90°/0°, what

showed a decrease in stiffness of the 58.14% and 43.12% was observed on 0°/0°/0°/0°

and 0°/90°/90°/0°, respectively.

Damping properties increase substantially and the amplitude decrease on Nitinol samples.

It is because occur the transformation of austenite in martensite, characterizing the

pseudoelastic effect on unidirectional samples.

Twill carbon fiber/epoxy with Nitinol embedded showed a decrease in stiffness and an

increase in damping but it’s not associated with pseudoelastic effect, because the stress

on twill sample was not enough to stimulate the transformation of austenite in martensite.

Thus, this means these effects can be due the inclusion of the wires on the composite or

not complete transformation of austenite to martensite.

The real stress on Nitinol wires is practically 50% bigger than the stress on the

superficies of the carbon fiber. This real stress characterizing the stress necessary to occur

the transformation of the austenite in martensite- and therefore- the pseudoelastic effect.

31

Further work can be done on the influence of quantity of Nitinol wires and the

temperature in unidirectional carbon/epoxy with Nitinnol wire. Also, the interaction

between the Nitinol wires and the matrix and their effects on unidirectional and twill

carbon/epoxy could be investigated. Moreover, the influence of another reinforcing fibre

(glass fibre/epoxy) will be more suitable than carbon fiber for some applications.

32

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