Experimental Study On Recovery Stress Of Nitinb Shape Memory Alloy

Different levels of damages have come into being on most of the concrete components with the elapse of time,which means that the resistance of structure could not meet the requirement of service.So,the retrofits of them need to be taken seriously.Till now,the retrofit methods of these diseases could be divided in to two categories,passive and active constraint measures.But,the first one could not show an active restraint to the structure while the large tension equipment has to be used in the second method.Faced with these problems,a method that could show an active restraint to the structure and easy to achieve is really necessary.There is a large recovery stress of prestrained SMA wires after heated.Even cooled to room temperature,the recovery stress is still larger than 400 MPa.Based on these above,a new reinforcement method which is easy to come true was put forward in this paper.The thermally triggered recovery stress of prestrained SMA wires is utilized to apply large active confinement pressure at the concrete components in this method.However,how to control the recovery stress of this material is not clear till now.In order to get a better way to use the recovery characters about this material,the main works in this paper are as follows.1)The results of the mechanical properties of Ni TiNb shape memory alloy are as follows:(1)As the improving of the ambient temperature while pre-stressing,the ultimate stress and elongation of this material were grown continuously.(2)Restrained the two sides of SMA wires that have been pre strained before under a low temperature,the recovery stress would improve by the rising of the ambient temperature.In consideration of the mechanical properties of steel bar in components,250? has been seemed as the best thermal excitation temperature in this paper.(3)The recovery ratio of NiTiNb shape memory alloy will decline as the rising of the pre strain temperature when the pre-strain is 16%.However,even the environment temperature reached to 50?,the recovery ratio of this material is still higher than 50%.Meanwhile,the temperature has less influence on the recovery stress of this material.(4)The recovery stress of the material pre-strained under room temperature is higher than those pre-strained in a low temperature,but the elongation will dropped a lot.And under a tensile stress of 600 MPa with amplitude,the material was still in an elastic state,while those pre strained under a low temperature(Ms+30?)was in superelastic state.2)The research results of the bonding properties between this material and polymer mortar are as follows:(1)The bonding strength of smooth SMA wires were very low,only 1 to 1.4MPa.And it would be smaller as the improving of the bonding length,but the displacement of the free side and pullout side would be improved at the same time.(2)The SMA wire could be anchored in the polymer mortar with less relative slide until an ensile failure occurred on the wire under the constraint of self-tapping screw anchorage which has been redesigned.In a word,using the anchorage designed in this paper to constraint the NiTiNb SMA which have a pre-strain about 16% under room temperature and heated the SMA to 250? is a good way to enhance the concrete components.