Strength Model And Optimal Design Of The NiTiNb Shape Memory Alloy Connector System

Major: Material Science Postgraduate: Zhou Xuchang Advisor: Zeng Guangting Huo YongzhongAs a new type of shape memory alloy pipe coupling, NiTiNb pipe coupling has a good prospects on the connecting pipe .The study on the strength of NiTiNb connnetor system and design to optimize its structure have great meaning to its application on the connecting pipe of the steam maker in nuclear reactor on naval vessels. In this paper, the relaxation resistance at high temperature of the NiTiNb alloy and its recovery characteristics with various constrains were studied, subsequently the mathematic model for the strength of the NiTiNb pipe connector was proposed to investigate the temperature dependence of the strength of the connector, and optimal design for the pipe coupling was brought forward.The relaxation resistance of the NiTiNb alloy affects the strength of the NiTiNb pipe coupling at high temperature, so that the relaxation experiment at various temperatures and various initial stresses was preformed to investigate the relaxation resistance of the NiTiNb alloy. The Results show that NiTiNb alloy has good relaxation resistance in the temperature range 300~400C and the initial stress range 260~360 Mpa ,therefore the NiTiNb pipe coupling can work safely under the condition of temperature above 300C and initial stress no more than 360Mpa.The connecting force between pipe coupling and connected pipe is resultedfrom the shape memory alloy's recovery stresses during its transformation to parent phase when the alloy is heated with constrains. The recovery stress and recovery temperature of NiTiNb shape memory alloy with diverse constrains were studied .In the experiment, the specimen with the same pre-deformation was heated to recover freely to some extent, then was holded to maintain its length and heated on .The results show that the temperature at which recovery stresses begin increase, and maximal stress and the temperature at which the maximal stress arrive decrease as free recovery strain rise. And to the specimens with various pre-deformation, the larger strain is, the higher temperature at which recovery stress start and the maximal stress arrive is, and the lower recovery stress is. When the recovery stress is low, the stress increase sharply along with heating, and dependence of the stress on the temperature is approximately linear. A simplified recovery stress model was obtained by summarizing the rules of the results.The stress state in the stainless steel pipe - titanium alloy pipe - NiTiNb shape memory alloy connector system is analyzed by using the theory of solid mechanics and the recovery rules of NiTiNb shape memory alloy with constrains on the basis of analysis of the structure of the connector system and the all steps in the preparation and the service. A mathematical model for the strength was proposed .The temperature dependence of connecting force and the maximal stress intensity in the pipe connector in the process of preparation and service was studied by programming and computing .The computing results show that the difference of heat expansile coefficient between the coupling material and the pipe material leads to the strength of connector system at room temperature less than the strength at high temperature. And due to the heat expansile coefficient of the stainless steel rather high, the connecting force between the stainless steel pipe and the pipe coupling in the room temperature is much less than that between the titanium alloy pipe and the pipe coupling .The results accord with the tensile experiments well.Finally, the dependence of dimensions of the coupling on the strength of the connector system is analyzed. Base on the analysis, the design of the pipe coupling is optimized by bringing forward a dimension scheme of the coupling whichincrease the connecting force and keep the maximal stress intensity within the safe scope.