deformation behavior of niti/polymer shape memory alloy composites - experimental verifications

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DOI: 10.1177/0021998304040553

2004 38: 399Journal of Composite MaterialsGo Murasawa, Keiichiro Tohgo and Hitoshi Ishii

Experimental Verifications−Deformation Behavior of NiTi/Polymer Shape Memory Alloy Composites

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Deformation Behavior ofNiTi/Polymer Shape Memory Alloy

Composites – Experimental Verifications

GO MURASAWA*Department of Mechanical Engineering

Aoyama Gakuin University

Kanagawa 229-8558, Japan

KEIICHIRO TOHGO AND HITOSHI ISHII

Department of Mechanical Engineering

Shizuoka University

Hamamatsu 432-8561, Japan

(Received April 30, 2003)(Revised September 2, 2003)

ABSTRACT: Composites containing NiTi shape memory alloy (SMA) long-fiber,short-fibers or Ti long-fiber in a Polycarbonate (PC) matrix have been fabricated bythe injection molding technique. Also, prestrained SMA long-fiber/Epoxy matrixcomposites have been fabricated. The fracture behavior and thermo-mechanicaldeformation behavior are examined; (1) Fracture behavior – uniaxial tensile tests upto fracture for SMA long-fiber and short-fiber composite (SMAC). (2) Thermo-mechanical deformation behavior – tensile loading–unloading tests for Pseudoelastic(PE) long-fiber/PC matrix composites. Several thermo-mechanical loading tests forShape Memory Effect (SME) long-fiber/PC matrix and SME long-fiber/Epoxymatrix composites were used.The obtained results are as follows: (1) The stress–strain relation up to the final

fracture of the Shape Memory Alloy Composites (SMACs) showed the repeatedup-and-down of the stress which corresponds to the necking of the specimen, fiberfracture, and matrix fracture. The strain for the initiation of necking and the strainfor the fiber or matrix fracture in the SMACs were higher than those in the Ticomposite. This is attributed to the unique stress–strain relations accompanied bythe stress-induced martensitic transformation of the SMA fibers. (2) The SMACcontaining PE fiber and PC exhibited the pseudoelastic-like deformation undertensile loading–unloading. (3) The SMAC containing SME fiber and PC exhibitedthe large contraction by heating after tensile loading–unloading, but the compressiveresidual stress in the matrix expected in this process was not remarkable. However,

*Author to whom correspondence should be addressed.

Journal of COMPOSITE MATERIALS, Vol. 38, No. 5/2004 399

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compressive residual stress in the matrix may become greater by embeddingprestrained fiber in the matrix.

KEY WORDS: shape memory alloy composite, NiTi fiber, shape memory effect,pseudoelasticity, injection molding, polycarbonate, epoxy.

INTRODUCTION

CONSIDERABLE ATTENTION HAS been paid to shape memory alloys (SMA) with theirshape memory effect (SME) and pseudoelasticity (PE), and many investigations

are conducted on their basic performance and applications [1–7]. For instance, compositeswhich consist of SMA fibers and a metal or polymer matrix are promising as smartmaterials working under thermo-mechanical loading, and as structural materials withhigh mechanical performance [8–16]. Yamada et al. [8], Lagoudas et al. [9], and Kawaiet al. [10] proposed the constitutive equation of the shape memory alloy composite(SMAC) to predict its mechanical behavior. The present authors [11] proposed theconstitutive equation of SMAC based on Shear-Lag model, and analyzed the deformationof the material under thermo-mechanical loading. Furuya et al. [12], Shimamoto et al. [13]and Armstrong et al. [14] reported that the improvement of the yield strength andfracture toughness could be expected by compressive stress in the matrix created bythe contraction of SMA fibers due to transformation from martensite to austenite.Furthermore, to reveal the potentialities of the SMACs and to extend their application,the development of constitutive equations of the SMACs, the deformation analysisunder several thermo-mechanical loadings and the experimental verifications arenecessary.

In the present paper, composites containing NiTi or Ti fibers in a polycarbonate matrixhave been fabricated by the injection molding technique. Furthermore, prestrainedNiTi(SME)/Epoxy composites have been fabricated. The fracture behavior and thermo-mechanical deformation behavior are examined; (1) Fracture behavior – uniaxial tensiletests up to fracture for SMA long-fiber and short-fiber composite (SMAC). (2) Thermo-mechanical deformation behavior – tensile loading–unloading tests for Pseudoelastic (PE)long-fiber/PC matrix composites. Several thermo-mechanical loading tests for ShapeMemory Effect (SME) long-fiber/PC matrix and SME/Epoxy matrix composites wereused. Based on these experimental results, the potentialities of the SMACs as structuraland functional materials are discussed from the point of view of the improvement ofstrength and ductility, the pseudoelastic behavior, the contraction behavior, andformation of compressive residual stress in the matrix by heating.

FABRICATION OF SHAPE MEMORY ALLOY COMPOSITE

The combination of fiber and matrix is an important factor to bring out variousproperties for SMAC, because there exists the correlation of mechanical property betweenSMA fiber and matrix in composite. In the present paper, the following conditions areconsidered for selection of materials:

1. Elastic strain of the matrix is smaller than transformation strain (6%) of SMA fiber.2. Strength of the matrix is lower than that of SMA fiber.

400 G. MURASAWA ET AL.



3. When composites are fabricated, fibers should not be heated to a high temperature fora long time.

4. It is possible to fabricate long-fiber-reinforced and short-fiber-reinforced composites.5. The matrix is transparent.6. It is possible to fabricate long-fiber-reinforced composites in which prestrained fiber is

embedded in the matrix.

As a result, NiTi alloy and polycarbonate (PC) were adopted as SMA fiber and matrixmaterials in the present study. Also, epoxy resin is adopted as a matrix material tofabricate composites including prestrained SMA fiber.

Shape memory alloy composites, which consist of SMA fibers and thermoplastic resin(PC), are fabricated by the injection molding technique (Figure 1(a)).

The SMA fibers are two kinds of NiTi fibers showing pseudoelasticity (PE) and shapememory effect (SME) at room temperature with diameter of 0.1, 0.3, 0.5, 0.7, and 1.0mm.

Ti fiber (annealed at 500�C, 30min) reinforced PC composite were also fabricated forthe comparison material. PE and SME wires underwent surface treatment using HNO3

solution (61%) during 90min and 60min. PC underwent preparatory drying at 120�Cduring 6 h. Figure 1(b) and (c) show the specimen configurations of SMA/PC composite.Long-fiber-reinforced composites as shown in Figure 1(b) were fabricated using the jigswhich made the wire fix into the mold. Short-fiber-reinforced composites as shown inFigure 1(c) were fabricated after mixing the PC pellet and SMA short fibers (diameter of0.1mm) which were cut at a length of about 1–2mm. The temperature to melt PC resin is380�C and the temperature of mold was 100�C during fabrication. During fabrication, thecondition of molding is good when foam in composites is not included. Also, short fibersmainly line the longitudinal direction in the composite, but some slanting fibers exist.

Shape Memory Effect fiber/Epoxy composites are fabricated by casting epoxy resin(Epoxy : Hardner¼ 80 : 20) into a mold fixed prestrained SMA fiber, then hardened at60�C, 12 h. Two types of composites are fabricated, one is the composite including nonprestrained fiber and the other is the composite including prestrained fiber. Prestrain of5% was given to the fiber in composites by using a tensile test machine before molding.The strain rate was 5%/min and the test temperature was 20�C. Specimen configuration ofthe composite is shown in Figure 1(d).

The details of all the composite specimens are shown in Table 1. Hereafter, two kinds ofSMACs are referred to as SME-composite and PE-composite, and furthermore the long-fiber-reinforced composites are represented by L-PE-C, L-SME-C, and L-Ti-C, and theshort-fiber-reinforced composites are represented by S-PE-C and S-SME-C, respectively.Also, for SME/Epoxy composite, composites including non prestrained and prestrainedSME fiber are represented by non prestrained composite and prestrained composite,respectively.

EXPERIMENTAL PROCEDURE

Tests About Deformation Behavior of Materials

Tensile loading tests are carried out at 17�C for fibers, PC, and Epoxy used in the presentstudy in order to investigate basic mechanical properties. Fibers were held with aluminumplates to make the effect of the grip section to reduce under tensile loading. Also, specimen

Deformation Behavior of NiTi/Polymer SMA Composites 401



configuration as shown in Figure 1 is used for PC and Epoxy testing. Based on theseexperimental results, the experiments of composite are conducted for deformation andfracture behavior, and thermo-mechanical deformation behavior.

Test about Deformation and Fracture Behavior of SMAC (SMA/PC Composite)

In order to investigate the deformation, damage, and fracture behavior of the composite,monotonic uniaxial tensile tests were carried out at 28�C, and the specimens were observedby CCD camera during the test. In the tensile test, cross-head speed was controlled with astrain rate of 0.5%/min, and three to five specimens were used for each test. Compositesused in this experiment were L-PE-C of three types (fiber diameter of 0.3, 0.5, and0.7mm), L-SME-C of four types (fiber diameter of 0.3, 0.5, 0.7, and 1.0mm), S-PE-C

Figure 1. Injection molding machine and specimen configurations: (a) injection molding machine; (b) long-fiber-reinforced composite (NiTi/PC); (c) short-fiber-reinforced composite (NiTi/PC); (d) long-fiber-reinforcedcomposite (NiTi/Epoxy) (prestrained fiber).




(fiber volume fraction of 2.8%) and S-SME-C (fiber volume fraction of 2.6%). Also, testswere conducted for L-Ti-C of two types (fiber diameter of 0.5 and 0.69mm).

Thermo-mechanical Loading Tests of SMAC (PE/PC Composite,

SME/PC Composite, and SME/Epoxy Composite)

For PE/PC composites, tensile loading and unloading tests are conducted at 28�C toinvestigate the effects of PE fibers on the deformation of the composites. Composites areL-PE-C of two types (fiber diameter of 0.5 and 0.7mm).

For SME/PC composites, the thermo-mechanical loading test was conducted toinvestigate the effects of SME fibers on the deformation of the composites as follows; (1)Tensile loading and unloading at 20�C, (2) Load-free during 10min at 20�C, (3) Heatingup to about 95�C and cooling down to 20�C under free-load condition, (4) Tensilereloading at 20�C. Heating and cooling rates in the thermal loadings were about 2.4 and0.3�C, respectively. These experiments are carried out for L-SME-C of four types (fiberdiameter of 0.3, 0.5, 0.7, and 1.0mm) and S-SME-C of two types (fiber volume fraction of3.0, 4.8%).

For SME/Epoxy composite, two kinds of thermo-mechanical loading tests wereconducted as follows; the first test was (1) Tensile loading and unloading at 20�C, (2)Load-free during 10min at 20�C, (3) Heating up to about 95�C and cooling down to 20�Cunder load-free conditions, (4) Tensile reloading at 20�C [for non prestrained composite].The second test was (1) Heating up to about 95�C and cooling down to 20�C under load-free conditions, (2) Tensile reloading at 20�C [for prestrained composite].

DEFORMATION BEHAVIOR OF MATERIALS

Figures 2 and 3 show the stress–strain relations of SMA wires (PE, SME), Ti wire, PCand Epoxy up to final fracture under uniaxial tension. As shown in these figures,

Table 1. Fibers used in the composites and fiber volume fraction of the composite.

Long Fiber Composite Short Fiber Composite

Diameter ofFiber / (mm)

Fiber VolumeFraction Vf (%)

Diameter ofFiber / (mm)

Fiber VolumeFraction Vf (%)

PE 0.3 0.71 0.1 2.80.5 2.00.7 3.85

SME 0.3 0.71 0.1 2.60.5 2.0 3.00.7 3.85 4.81.0 7.85

Ti 0.5 2.00.69 3.68

*SME/Epoxy composite (prestrained long-fiber-reinforced composite), Diameter of fiber: 1.0mm, Fiber volume fraction:3.27%.




SMA wires show the stress–strain relation with large deformation at constant stress due tostress-induced martensite transformation, then, stress increase followed by plasticdeformation, finally, wires are fractured. As observed, deformation behavior due totransformation details, PE wires showed the highest transformation stress and the mostdistinct start and finish stress of transformation with knee point. On the other hand, SMEwires showed the lowest transformation stress and the most smooth stress–strain relationfrom start to finish of stress transformation. Ti wires showed that the fracture strain wasas large as that of SMA wires by heat treatment. PC exhibited the stress decrease due tonecking at one side of specimen after maximum stress, then, the deformation underconstant stress with necking propagating, and the stress increase after finish of neckingpropagation followed by the fracture. Experimental results obtained from uniaxial tensileloading test at 17�C, that is, Young’s modulus, transformation stress (0.2% proof stressfor Ti), tensile strength, ductility, are shown in Table 2. Figure 4 shows the stress–strainrelation of SMA wires under tensile loading and unloading at 17�C. PE wires exhibited

Figure 2. Stress–strain relations of shape memory alloys and Ti at 17�C.




strain recovery during unloading (Pseudoelasticity), but that was not complete afterunloading. Also, large strain remains after unloading for SME fiber, but transformationstrain in this residual strain recovers by heating over austenite finish temperature (Shapememory effect).

DEFORMATION AND FRACTURE BEHAVIOR OF SHAPE MEMORY

ALLOY COMPOSITES (SMA/PC COMPOSITE)

Stress–strain Relations and Fracture Process

Figures 5 and 6 show the stress–strain relations of PE-, SME-composites and PC underuniaxial tension. In these figures, Figure (b) is an enlargement of Figure (a) at the part ofthe early stress–strain curve.

Figure 3. Stress–strain relation of polycarbonate at 17�C.

Table 2. Mechanical properties of the constituent materials at 17�C.

Diameterof Fiber/ (mm)

Young’sModulus E

(GPa)

TransformationStress

(*:0.2% Proof Stress)(MPa)

TensileStrength(MPa) Ductility (%)

PE 0.3 56.6 543 1330 12.80.5 40.0 432 1140 13.90.7 34.1 415 1400 19.3

SME 0.3 18.9 134 1220 12.30.5 9.9 99.3 1140 15.10.7 12.5 88.3 1230 17.8

Ti 0.5 48.9 214* 399 22.40.69 56.4 171* 385 20.3

PC — 2.4 — 60 79.1

*Data obtained from experimental results (Figures 2, 3).




Figure 5. Stress–strain relations of PE-composites at 28�C.

Figure 4. Stress–strain relations of SMA at 17�C.




The long-fiber-reinforced composites show the stress–strain relations with repeatedincrease and decrease of the stress up to the final fracture at the relatively large strain,while the short-fiber-reinforced composites show the low fracture strain. These stress–strain relations of long-fiber-reinforced SMAC (L-SMAC) are related to the damage andfracture process. For example, stress decreases correspond to necking of the matrix, fiberfracture or matrix fracture. The effect of fiber on deformation behavior of Short-fiberreinforced SMAC (S-SMAC) is not remarkable, because short fibers have low loadcarrying capacity. Therefore, the L-SMACs are discussed in detail.

First of all, as examined stress–strain relation (Figures 5, 6(b)) at the range of smallstrain up to initial necking, the stress–strain relations of PE-composite exhibit the higherstress with increasing fiber volume fraction, while those of SME-composite exhibit thesame behavior as PC.

Those are attributed to the difference of Young’s modulus between PE and SME fiber incomposite.

Also, PE-composites show the only stress decrease corresponding to the finish of thephase transformation. On the other hand, SME-composites show the monotonic increaseof stress from start to finish of the phase transformation.

Furthermore, it is noticed that L-SMACs have the larger stress and strain at initialnecking than those of PC, though there exists some differences of stress–strain relationbetween PE-composites and SME-composites before initial necking.

Figure 6. Stress–strain relations of SME-composites at 28�C.




(a)

(A)(A) (B)(B) (C)(C) (D)D) (E)(E)

0.7 1.0

0.30.5

(b)

(c)

(F)(F) (G)G) (H)(H) (I)(I)

(J)(J) (K)K) (L)(L)

φφ

φφ

Figure 8. Schematic illustrations of fracture process of PE-, SME-, and Ti-composites: (a) PE-composite;(b) SME-composite; (c) Ti-composite.

Figure 7. Comparison of the stress–strain relations of PE-, SME-, Ti-composites, and PC at 28�C.




Figure 7 shows the comparison of the stress–strain relations of the long-fiber-reinforced

PE-, SME-, Ti-composites with fiber diameter of 0.7mm and PC. Furthermore, Figure 8

shows the schematic illustrations of the damage and fracture process of PE-, SME-, and

Ti-composites based on the observation by CCD camera. Each illustration corresponds to

each stage denoted by A–L on the stress–strain relations in Figure 7. The arrows (N) and

(J) in Figure 7 indicate the fracture of fiber and the fracture of the matrix, respectively. As

shown in Figure 7, Ti-composites show fiber fracture after initial stress decrease due to

necking, and its fracture strain is lower than that of SMACs. It is interesting to note in

Figure 7 that the strain at fiber- or matrix-fracture of two SMACs is considerably higher

than that of Ti-composite, in spite of almost the same in the ductility of SMA and Ti fibers.

This results from the difference in the damage and fracture process of these composites as

shown in Figure 8. The damage and fracture process of each material are as follows:

(a) PE-composites Initiation of the debonding of the fiber–matrix interface occurs at the

middle part of specimen, the strain corresponding to the start of phase transformation

of PE-fiber (stage A). The debonding evolves through the specimen with the same space.

Then, the necking of the specimen develops from the debonding at one end of specimen

(stage B) and at another end of the specimen (stage C), the fiber fracture occurs at the

necking (stage D). During the damage and fracture process, the stress yields this repeated

increase and decrease. Finally, the matrix is fractured at the site of fiber fracture (stage E).

(b) SME-composites The debonding of the fiber–matrix interface and necking occur at

one end of the specimen as the strain corresponds to the completion of the phase

transformation of SME-fiber (stage F). Furthermore, the debonding and necking occur at

the other end of the specimen (stage G). In the case of thin fiber, the fiber is broken (stage

H) and then the matrix is fractured (stage I) at the initial debonding and necking site. In the

case of thick fiber, the matrix is fractured at the initial debonding and necking site (stage H).

(c) Ti-composites The debonding and necking occur at one end of the specimen, the

deformation corresponds to the start of necking of PC (stage J). Then, the stress decreases,

and the fiber fracture at the debonding and necking site (stage K) is followed by the matrix

fracture (stage L).

It can be considered that the above-mentioned differences of the damage and fracture

process among two SMACs and Ti-composite are due to the differences of stress–strain

relation among two SMA wires and Ti wire. It is not easy for SMACs to undergo

concentrated deformation at the debonding site due to high hardening rate of SMA fiber

at the range of large deformation, while it is easy for Ti composite to undergo

concentrated deformation at the debonding site due to low hardening rate of Ti fiber. It is

suggested that large deformation ability and high hardening rate at large strain of SMA

fiber have potentialities to be a strengthened and toughened composite. Also, multiple

debondings are observed during phase transformation of fiber before initial necking for

PE-composites, while debonding is not observed up to initial necking for SME-

composites. This is attributed to different stress level of phase transformations for PE

and SME wire.




Properties of Deformation and Strength

Figure 9 shows the properties of deformation and strength for long-fiber-reinforced

composite as a function of fiber volume fraction. The prediction based on mixture law and

the experimental results of S-SMAC also draw in Young’s modulus–fiber volume fraction

relation Figure 9(a). Young’s modulus of composite show good agreement with that based

on law of mixture. Figure 9(b) shows stresses for initial debonding and necking, and

maximum stress of composite as a function of fiber volume fraction. Necking immediately

occurs after the start of debonding for SME-composite and Ti-composite. As shown in

Figure 9(b), stresses of initial debonding and necking, and maximum stress increase with

increasing fiber volume fraction for all of the composites, especially it is remarkable for

PE-composite. Figure 9(c) shows strains for initial debonding and necking, and fiber or

matrix fracture strain of composite as a function of fiber volume fraction. As shown in

Figure 9(c), strains for initial necking of SMAC are larger than that of Ti composite. It can

Figure 9. Young’s modulus (a), stresses for initial debonding and necking, and maximum stress (b), andstrains for initial debonding and necking, and fiber or matrix fracture strain (c) as a function of fiber volumefraction on PE-, SME- and Ti-composites.




be understood that there exists the potentiality that made the necking of composite delay

by using SMA fiber as a reinforcement. Also, fracture strain for SMAC is larger than that

for Ti composite.

THERMO-MECHANICAL DEFORMATION BEHAVIOR

OF SHAPE MEMORY ALLOY COMPOSITES

Pseudoelastic Fiber-reinforced Composite (PE/PC Composite)

Figure 10 shows the stress–strain relations of PC, PE-fiber, and PE-composites under

tensile loading and unloading. PC exhibits the strain recovery due to the viscosity during

unloading. On the other hand, the composites show the strain recovery due to the

martensite to austenite transformation of the fiber in addition to the viscosity of the matrix

under unloading. The pseudoelastic-like behavior of the composites is remarkable with

increasing fiber volume fraction.

Shape Memory Effect Fiber-reinforced Composite

(SME/PC Composite and SME/Epoxy Composite)

Figure 11 shows the stress–strain relations of SME fiber, PC, L-SME-C, and S-SME-C

under thermo-mechanical loading. In those figures, oab, bc and cd correspond to the

Figure 10. Stress–strain relations of PC, SMA(PE), and PE-composites under tensile loading–unloading at28�C.




tensile-loading and unloading, load-free followed by heating and cooling under free-loadcondition, and tensile-reloading, respectively. As shown in Figure 11(a) for SME fiber, thestress-induced martensitic transformation occurs during tensile-loading. Then, the strain of4.2% remains after unloading, and this residual strain recovers up to 2.2% due to shapememory effect by heating. It is imagined that the austenitic transformation of the SMEfiber does not complete by heating up to 95�C, as a result, the strain of 2.0% still remainsafter heating. As shown in Figure 11(b), (c), and (d), the strain recovery is observed in PCand SME composites during load-free, heating and cooling. Figure 12 shows details of thedeformation of the composites between b and c as a function of fiber volume fraction.As shown in Figure 12, the strain recovery in SME fiber (Vf¼ 1.0) is attributed to theaustenitic transformation by heating, and that in PC (Vf¼ 0.0) is due to the viscosity duringload-free condition. In the composites, the matrix expands, while the fiber contracts byheating. As a result, an amount of contraction in the composites increases with an increasein the fiber volume fraction. Figure 13 shows the comparison of the stress–strain relationsof the composites between tensile loading (oa) and tensile reloading (cd). The stress–strainrelation under tensile-reloading exhibits higher stress than those under tensile-loading, butit is not remarkable. It is presumed that the compressive residual stress in the matrix bycontraction of SME fiber is relatively low in the present composites, because PC used as thematrix material exhibits strain recovery due to viscosity during unloading.

Therefore, thermo-mechanical loading tests are conducted for SME/Epoxy compositesto investigate the effect of strain recovery during unloading and load-free process on

Figure 11. Stress–strain relations for SMA(SME), PC, SME-composites under thermo-mechanical loading.




compressive residual stress in matrix after heating. Thermo-mechanical loading tests areconducted using two kinds of composites (non prestrained composite and prestrainedcomposite) as follows; for non prestrained composites, (1) Tensile loading and unloadingat 20�C, (2) load-free during 10 min at 20�C, (3) heating up to about 95�C and cooling

Figure 12. Deformation of SME-composites during load-free, heating, and cooling after tensile loading–unloading.

Figure 13. Comparison of stress–strain relations of SME-composites under tensile loading and tensilereloading: (a) L-SME-C; (b) S-SME-C.




down to 20�C under load-free condition, (4) tensile reloading at 20�C. For prestrainedcomposites, (1) heating up to about 95�C and cooling down to 20�C under load-freecondition, (2) tensile reloading at 20�C. Figure 14 shows strain–temperature curves ofcomposites under thermo-loading.

Strain–temperature behavior is much different between non prestrained composite andprestrained composite. Non prestrained composite shows thermal expansion, whileprestrained composite shows slight thermal expansion under beginning of heating. Also,prestrained composite shows greater shrinkage than that of non prestrained compositeunder heating. It seems that residual stress in the matrix existed before thermo loadingcaused these differences. Also, Figure 15 shows the stress–strain relations as follows; (1)tensile-loading of non prestrained composite, (2) tensile-reloading of non prestrainedcomposite (after tensile-loading, unloading, load-free, heating, and cooling), (3) tensile-

Figure 14. Comparison of strain–temperature relations of non prestrained composite and prestrainedcomposite under thermo-loading: (a) non prestrained composite; (b) prestrained composite.




reloading of prestrained composite (after heating and cooling). Three composites show thematrix fracture form and the different deformation behavior as shown in Figure 15.

Three differences may be attributed to internal stress in matrix before tensile-reloading.Tensile-reloading (3) exhibits higher stress than those of the other process. From theresults of the matrix fracture form, it is found that prestrained composite possess thelargest compressive stress in the matrix. Therefore, compressive residual stress in matrixmay become greater by embedding prestrained fiber in the matrix.

CONCLUSIONS

Composites containing NiTi and Ti fibers in a PC matrix have been fabricated bythe injection molding technique. Also, prestrained NiTi/Epoxy composites have beenfabricated. Damage and fracture process under uniaxial tension and deformation behaviorunder thermo-mechanical loading were examined on these composites. The resultsobtained in the present investigation are summarized as follows.

1. The stress–strain relation up to the final fracture of the SMACs showed the repeatedincrease and decrease of the stress which corresponds to the necking of the specimen,fiber fracture, and matrix fracture. The strain for the initiation of necking and the strainfor the fiber or matrix fracture in the SMACs were higher than those in the Ticomposite. This is attributed to the unique stress–strain relations accompanied by thestress-induced martensitic transformation of the SMA fibers.

2. The SMAC containing PE fiber exhibited the pseudoelastic-like deformation undertensile loading–unloading.

Figure 15. Comparison of stress–strain relations of non prestrained composite and prestrained compositeafter thermo-loadings.




3. The SMAC containing SME fiber exhibited the large contraction by heating aftertensile loading–unloading, but the compressive residual stress in the matrix expected inthis process was not remarkable. However, compressive residual stress in matrix maybecome greater by embedding prestrained fiber in matrix.

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deformation behavior of niti/polymer shape memory alloy composites - experimental verifications

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