Mechanical Behavior of Shape Memory Alloy Composites
Seung Yong Yanga, Byeong Choon Goob and Hyung Jin Kimc
Korea railroad research institute, Uiwang, Kyunggi-do, South Korea [email protected],
Keywords: Shape memory alloy, CFRP, Hybrid composite
Abstract. Mechanical behavior of CFRP (Carbon Fiber Reinforced Polymer) containing Ni-Ti shape
memory alloy (SMA) is investigated by experimental methods. Tensile and bending fracture tests
were conducted to examine the strength of the composite for various volume fractions of the SMA.
Charpy impact test was used to study the toughness of the SMA/CFRP hybrid composite. Finite
element analysis was carried out for interface failure in bending test.
Introduction
SMA (Shape Memory Alloy) smart hybrid composites, in which SMA wires are embedded in
common composites such as CFRP or GFRP, are attracting much attention. SMA shows shape
memory effect and can undergo large elastic deformation depending on the operating temperature,
and by combining SMA with a conventional composite, one can obtain a hybrid composite with new
capacities such as damage detection and control in the composite [1].
Recently, to reduce the weight of railway vehicles, CFRP (Carbon Fiber Reinforced Polymer) is
studied and used [2]. In this paper, we conducted a preliminary research on mechanical behavior of
SMA/CFRP hybrid composites. First, material properties of NiTi shape memory alloy such as
transformation temperatures, residual strain, and recovery stress were obtained by differential
scanning calorimeter (DSC) and tensile tests [3]. To evaluate elastic modulus of the SMA/CFRP
composite, tensile tests were carried out for various volume fractions of SMA wire. Bending test was
used to examine the interface strength between SMA and CFRP of the hybrid composite. Charpy
impact test was also conducted to check the fracture behavior of the composite.
Characteristics of NiTi shape memory alloy
NiTi alloy was prepared by melting in a high-frequency vacuum induction furnace. The atomic
composition of the alloy was Ni50Ti50. The ingots were swaged and then drawn to wire specimens
with diameters of 0.6mm and 1.0mm, or pressed to plate specimens with thickness of 1.0mm at room
temperature. Finally they were heat-treated at 850oC for 1hour. The transformation temperatures were
measured by DSC. The heating and cooling rates were 10oC/min and the temperature range was from
-120oC to 150
oC. The shape memory effect and mechanical properties were measured using tensile
test. The wires were heated by immersing them in a silicon oil bath. The recovery stress was measured
during heating in a constrained state.
Fig. 1 shows DSC curve of the NiTi plate. The transformation temperatures were determined by a
line-intersection method. On heating and cooling two clear peaks were observed, and the
transformation temperatures were Ms = 35oC, Mf = 21
oC, As = 50
oC, and Af = 68
oC. The alloy is in the
martensitic phase at the room temperature.
Key Engineering Materials Vols. 297-300 (2005) pp 1551-1558Online available since 2005/Nov/15 at www.scientific.net© (2005) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/KEM.297-300.1551
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 130.15.241.167, Queen's University, Kingston, Canada-03/10/13,16:55:17)
Fig. 1 Measurement of transformation temperature by DSC
εxx
(%)
σxx
(MP
a)
0 1 2 30
100
200
300
400
500
600
T = 16oC
T = 29oC
T = 35oC
T = 50oC
T = 68oC
T = 77oC
ε
xx(%)
σxx
(MP
a)
0 20 40 60 80 1000
200
400
600
800
1000
1200
1400T = 3
oC
T = 24oC
T = 28oC
T = 75oC
Fig. 2 Stress-strain curves of NiTi shape
memory alloy at several temperatures
Fig. 3 Stress-strain curves of NiTi shape
memory alloy up to failure
εxx
(%)
σxx
(MP
a)
0 1 2 30
100
200
300
400
500
T = 16oC
T = 35oC
ε
xx(%)
σxx
(MP
a)
0 1 2 30
100
200
300
400
500
T = 68oC
T = 77oC
Fig. 4 (a) Tensile behavior of NiTi at T < Ms. After unloading, recovery stress builds up
due to heating, (b) Tensile behavior of NiTi at T > Af. After unloading,
recovery stress builds up due to heating
Fig. 2 shows the tensile behavior of the NiTi wire. The length of the specimen was 50mm and the
gauge length was 30mm. For temperatures 16oC, 29
oC and 35
oC, which are lower than Ms temperature,
about 2.5% residual strain was generated after unloading. This residual strain completely disappeared
after heating to 100oC which is higher than Af . As the test temperature became higher than the Ms
temperature, pseudoelastic behavior was observed and the residual strain decreased. The tensile
strength of the NiTi shape memory alloy can be measured from Fig. 3. For the test temperatures, the
tensile strength of the alloy was between 600MPa and 800MPa. Fig. 4 (a) and Fig. 4 (b) show the
1552 Advances in Fracture and Strength
recovery stresses produced by heating a constrained wire up to 100 oC. For the tests at temperature
below Ms temperature, around 150MPa recovery stress was generated (Fig. 4 [a]), and for the tests at
temperature above Af temperature (Fig. 4 [b]), smaller recovery stress was available since the residual
strain was smaller in these cases.
Mechanical behavior of NiTi/CFRP composite
NiTi wires and plate described in the previous section were used to fabricate SMA/CFRP hybrid
composites. The CFRP prepreg used in the present study is carbon epoxy fabric CF-327EPC (Hankuk
Fiber, Korea). NiTi/CFRP hybrid composites were manufactured by autoclave/vacuum bag degassing
process at 120oC for 2hours with a pressure of 1.8kgf/cm
2.
Tensile test was used to measure Young’s modulus of hybrid composite containing the SMA wires.
The results are summarized in Table 1. As Young’s modulus of the martensitic SMA alloy is
comparable with that of CFRP and the volume fraction (Vf) is small, the effect of the wires was not
significant. The hybrid composite shows linear stress-strain behavior until failure of CFRP at which
the stress level was about 700MPa. After breaking of the CFRP, fiber pull-out was observed.
Table 1 Young’s modulus of SMA/CFRP composites
Vf Young’s modulus
0 % 62.2GPa
1.21 % 61.7GPa
1.31 % 61.9GPa
Displacement (mm)
Fo
rce
(kg
f)
0 0.5 1 1.50
100
200
300
400
500
600
700
800
900
Displacement (mm)
Fo
rce
(kg
f)
0 0.5 1 1.50
100
200
300
400
500
600
700
800
900
Fig. 5 (a) Result of bending test of CFRP, (b) Result of bending test of SMA/CFRP hybrid composite
Hybrid composite was fabricated by laminating a SMA plate of 1 mm thickness in the CFRP for
bending test. The size of the bending specimens is width = 12mm, depth = 5.3mm, span = 21mm, and
the displacement loading rate was 2.12×10-5
m/s. Fig. 5 illustrates the force-deflection curves of the
three-point bending tests. For pure CFRP specimens (Fig. 5 [a]), the maximum force was about
600kgf, and for SMA/CFRP hybrid composites (Fig. 5 [b]), the maximum load was little bit lower
than the value of pure CFRP, and the load drop was not as large as pure CFRP. It apprears that the
SMA plate increases the ductility of the hybrid composite and after the initiation of fracture more
energy is required for the propagation [4].
Fig. 6 illustrates a numerical result for failure mode in bending test. A modified Gurson model [5,
6] was applied to the two interfaces between SMA and CFRP to evaluate the interface toughness. The
Key Engineering Materials Vols. 297-300 1553
numerical parameters for the interface was from material properties of epoxy, and the surrounding
bulk materials were assumed elastic. Plane strain finite element analysis was conducted. In the
Gurson model analysis, the damage evolution in the interface was accounted for in terms of void
volume fraction (vvf). Taking into accout brittle nature of epoxy, we assumed that the void volume
fraction of 0.00001 corresponds to the complete loss of stress carrying capacity of the material.
0 . 0 0 0 0 . 0 0 0 0 . 0 0 0 0 . 0 0 0 0 . 0 0 0 0 . 0 0 0 0 . 0 0 0 0 . 0 0 0 0 . 0 0 0 0 . 0 0 0 0 . 0 0 0 0 . 0 0 0 0 . 0 0 0
0 < v v f < 0 . 0 0 0 0 1
Fig. 6 Gurson model analysis of bending test
Impact test was conducted using a Charpy impact machine (Frank 565K) for various volume
fractions of the SMA wire. Free-standing SMA wires were embedded in host CFRP prepregs before
curing. The size of the specimen is 50mmx6mmx4mm without a notch. Fig. 7 illustrates the absorbed
energy in the tests. As the volume fraction of the SMA wire (Vf ) increases, the absorbed energy also
increases. Pure CFRP without SMA wire was broken completely, but the hybrid composite was only
bent as the embedded wires were not ruptured. Fig. 8 shows shape of the hybrid specimens before and
after the impact test. Considering that the hybrid composites did not break, the absorbed energy for
complete fracture should be higher than the measured values, and the hybrid composite is stronger
than pure CFRP against impact loading.
Test number
Ab
so
rbe
de
ne
rgy
(J)
0 1 2 3 4 5 60
1
2
3
4
5
6
7
Vf= 7.07 %
Vf= 6.54 %
Vf= 3.53 %
Vf= 3.27 %
Fig. 7 Result of Charpy impact test Fig. 8 SMA/CFRP composite specimens before
(above) and after (below) impact test. The size of
the specimen is 50mm×6mm×4mm
Summary
In this preliminary work for SMA/CFRP hybrid composite, material properties of NiTi shape memory
alloy were obtained. Elastic modulus of the hybrid composites was measured. Bending test and
Charpy impact test were conducted to examine fracture behavior of the hybrid composite. It was
shown that inserting SMA alloy in a conventional composite improves ductility of the composite, and
the hybrid composite becomes stronger against impact loading.
1554 Advances in Fracture and Strength
References
[1] R. Oishi, H. Yoshida, B. Jang, H. Nagai and Y. Xu: Proceedings of SPIE Vol. 4701 (2002)
[2] K.B. Shin, S.H. Cho and S.J. Lee: Korean Society for Composite Materials annual fall conference
(2004)
[3] Y. Xu, K. Otsuka, H. Nagai, H. Yoshida, M. Asai and T. Kishi: Scripta Mat. Vol. 49 (2003), p. 587
[4] G.E. Dieter: Mechanical Metallurgy (McGraw Hill, U.S.A. 1988), p. 476
[5] ABAQUS manual Version 6.3-1 (Hibbit, Karlsson & Sorensen, U.S.A. 2002)
[6] S.Y. Yang, B.C. Goo, and J.H. Kim: Korean Society for Railway annual spring conference (2004)
CD-ROM
Key Engineering Materials Vols. 297-300 1555
Advances in Fracture and Strength 10.4028/www.scientific.net/KEM.297-300 Mechanical Behavior of Shape Memory Alloy Composites 10.4028/www.scientific.net/KEM.297-300.1551
DOI References
[3] Y. Xu, K. Otsuka, H. Nagai, H. Yoshida, M. Asai and T. Kishi: Scripta Mat. Vol. 49 (2003), p. 587
doi:10.1051/jp4:2003993
