Evaluation of Prestressing with a Shape Memory Alloy
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287 Mat. Res. Soc. Symp. Proc. Vol. 503 0 1998 Materials Research Society
oppose its strain recovery. The consequent behavior (Duerig et. al., 1989) is as depicted in Figure 2. When heated from room temperature Td, past As, the phase transformation to Austenite begins. Due to the constrain effect, the strain recovery from Cp to Cr is gradual (depending on the amount of constraint). The actuation stress also increases gradually from zero at As to a maximum value of Gr at Md. Md is the maximum temperature at which some transformation still occurs, or the maximum temperature at which martensite can still be stress induced. It can be seen that as the temperature rises, larger actuation stresses are generated up to a maximum of Yrat Md. This is the situation that occurs when the Nitinol strands are gradually heated to activate prestressing in our model beams. TEST
SPECIMENS
NiTi alloys for our research (binary, straight annealed, oxide finish) were obtained from Shape Memory Applications Inc., (Santa Clara, CA, SMA inc., 1997). Some of their properties are listed in Table 1, others are available on their web-page. The properties of our material were in between those at the cold-worked and the fully annealed stage. Strands were made from 4 single 0.64 mnm wires in a controlled manner. These 4-wire strands were stretched past their yield point to a strain of 5% and embedded in mortar beams. An optical microscope (200x) was used to observe the wires after their pullout from the mortar. It was observed that the surface of the individual wires that were was clean, implying relatively (compared to steel fibers) weak bond. Such clean surface is also observed in Nylon or Carbon fibers embedded in concrete; which also have poor bond with hydration products (Lange et al., 1996). However, the 4 wire strands show cement paste adhering to the crevices in between the wires (Figure 3). This mechanical interlock is responsible for the good bond observed in the strands tested. A mold for making the model prestressing beams (30.5 cm x 2.5 cm x 1.3 cm) was made out of wood specimens measuring could be cast. Holes for inserting the nitinol prestressing strands were left 1.9 mm from the bottom of each mold. Mortar beams were cast with a single 4-wire strand, and with 2 4-wire strands. They were moist cured for 24 hours, removed from the molds, and moist cured up to the time of testing. A three-point bending test rig was used along with a dial gage (0.001) to measure deflections at the center point. The shape memory strands were activated by heating them a 2 channel, 25 Amp., 25 volt variable capacity voltage source. RESULTS The results of the tests are shown in Figures 6a and 6b, corresponding to the 2strand and 1-strand specimens respectively. When the voltage (4.0 volts) is first applied, the thermal expansion of the strands precede their phase transformation. Hence, in the first minute the deflection is downwards. Once phase transformation starts, the beams starts to camber, eventually reaching a stable deflection after 8 minutes, wh
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