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CHARACTERISTIC TEST OF SMA
Method for measuring the characteristics of the SMAs
We used the characteristics testing equipment shown in Figure 2 for material experiment of the SMA to be used in the joining jig. The measurement and experimental procedure are as follows:
Measurement conditions:
1) Length of sample material: 50 mm
2) Measured length: 80 mm
3) Elongation: 5% (1.5 mm)
4) Temperature measurement interval: 10℃
5) Temperature measurement range 20℃- 130℃
Experimental procedure:
(1) Tighten only the upper section of the chuck, mount a SMA sample to the chuck, and then heat the air from the fan to 130℃ using a heater, then apply the 130℃ air gas to the SMA sample to remove residual stress. (2) While maintaining the above state, tighten the lower section of the chuck to fasten the SMA sample to the chuck and set the elongation 5% of the sample length. Then lower and raise the temperature of the air gas between 130℃ and 20℃ repeatedly until the maximum and minimum values of the tensile force of the SMA sample become stable and the sample can trace the same hysteresis. (3) Measure the tensile force of the SMA sample using a force gauge while lowering and raising the temperature by 10℃ increments between 130℃ and 20℃elsius.
Figure 2. Characteristic testing equipment for shape memory alloy
Result of the characteristics test
The result of the characteristics test for the SMA is shown in Figure 3 in terms of the relationship of "temperature vs tensile force".
Figure 3. Result of the characteristics test
MODEL EXPERIMENT
Design of drawing jig
In the drawing jig of the SMA, the heat resistance of the SMA must be taken into consideration at two stages. At the first stage, the SMA needs to be able to endure heat-treatment at 500℃ in order to enable the SMA to memorize the SMA. At the second stage, the SMA needs to be able to endure a temperature of about 70℃ applied for eliminating the apparent strain of the alloy so as to restore the alloy to the original shape that the alloy memorized during the first stage. In this study, since two stages of heat-treatment are executed by electric heating, the insulation of the drawing jig must be taken into consideration at the second stage. We designed the drawing jig considering the above conditions.
MECHANICS OF THE DRAWING JIG
A drawing jig is an apparatus like a pair of scissors (Fig.4). Each edge of the apparatus is hinged to each floating body. Handgrip-side edges are combined using SMA. The perimeter of a sheet of a disk, whose center is on the axis of scissors, is involved with SMA. The disk is installed so that the length of the SMA will be longer and the jig stroke range wider. In this condition, two floating bodies are separate and distant, and as the stiffness of the SMA is low, the two floating bodies are softly restrained and comparatively free.
Electric conduction makes SMA reach a high temperature and shrink. As the blades of the scissors approaches each other gradually, so do the floating bodies. Here, since the stiffness of the SMA become stronger nearly in proportion to temperature rise, restraint force between floating bodies becomes gradually stronger and relative motion reduces. After SMA is heated long enough, the scissors are completely closed and the two floating bodies are so close to each other that they can be combined, behaving as if they were one body.
The entire configuration of the drawing jig is shown in Figure 4, of which dimensions are as follows: 1) Height: 175 mm; 2) Breadth: 116 mm; 3) Depth: 46 mm; 4) Drawing distance: 42 mm; 5) Radius of disc; 40 mm.
Figure 4. Drawing jig
Dimensions of the SMA
Dimensions of the SMA are as follows.
1) Type: Ni-Ti alloy
2) Diameter: 0.4 mm
3) Overall length (when loaded with SMA): 375.20 mm
4) Overall length (when drawing): 361.84 mm
5) Tensile strain (when SMA is linear): 8.9%
6) Electric resistance: 89x10-8Ω・m
Polycarbonate
In this study, since the SMA is heat-treated electrically, polycarbonate is used as an insulator. The polycarbonate used has a specific gravity ranging from 1.13 to 1.24, and a heat resistance of about 120℃elsius.
Water Tank Experiment
Floating Body Model
Dimensions of the floating body model, which were assumed to be on a scale of 1/80 of the real one, are as follows.
Scale: Length x width x depth = 1,100 x 600 x 100 (mm)
Material: Acryl Draft: 50 mm Weight: 28.91 (kgf)
Measuring Method for Floating Body Model Units
The motions of the floating body model units were measured by LED targets attached to the floating body model units at seven positions (points A through G) using sensor heads. Measured data were digitized and stored in a computer.
Layout of Experimental Equipment
The upper sections of floating body model units were equipped with at-sea joining jigs, LED targets, and sensor heads, In order to protect the floating body model units from being drifted by waves, springs and piano wires were laid out over the floating body model units as shown in Figure 5.
Figure 5. Layout of the experimental equipment (side view) in the water
tank (length: 17 m, breath: 9 m, depth: 0.7 m)
Experimental Conditions
Table 1 below shows the experimental conditions. WH in the Table means wave height.
Table 1. Experimental conditions
| λ/L |
Period (sec) |
Frequency(Hz) |
Nominal WH(mm) |
| Still water |
∞ |
0 |
0 |
| 0.80 |
0.55 |
1.818 |
25.0 |
| 1.25 |
0.69 |
1.440 |
| 2.00 |
0.88 |
1.140 |
| 2.50 |
0.98 |
1.015 |
| 6.00 |
1.52 |
0.658 |
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Procedures for Measuring the Oscillations of Floating Body Model Units
Carry out wave tests before measuring the motions of the floating body model units. Generate waves using the wave generating board, and switch on the constant current device after five seconds from the start of measurement. After 180 seconds from the start of measurement, switch off the constant current device. The measurement ends when 240 seconds have elapsed after the start of the measurement.
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