Mechanical Behavior of Shape Memory Alloy Composites
Post on 18-Dec-2016
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
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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 .
Recently, to reduce the weight of railway vehicles, CFRP (Carbon Fiber Reinforced Polymer) is studied and used . 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 . 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 150oC. 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 = 21oC, As = 50oC, and Af = 68oC. 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
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Fig. 1 Measurement of transformation temperature by DSC
0 1 2 30
600T = 16 oCT = 29 oCT = 35 oCT = 50 oCT = 68 oCT = 77 oC
0 20 40 60 80 1000
1400T = 3 oCT = 24 oCT = 28 oCT = 75 oC
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
0 1 2 30
T = 16 oCT = 35 oC
0 1 2 30
T = 68 oCT = 77 oC
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, 29oC and 35oC, 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
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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 Youngs modulus of hybrid composite containing the SMA wires. The results are summarized in Table 1. As Youngs 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 Youngs modulus of SMA/CFRP composites Vf Youngs modulus
0 % 62.2GPa
1.21 % 61.7GPa
1.31 % 61.9GPa
0 0.5 1 1.50
0 0.5 1 1.50
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 2.1210-5m/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 .
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
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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.
0 1 2 3 4 5 60
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 50mm6mm4mm
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.
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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 Y. Xu, K. Otsuka, H. Nagai, H. Yoshida, M. Asai and T. Kishi: Scripta Mat. Vol. 49 (2003), p. 587doi:10.1051/jp4:2003993