| Pre-Tightened Teeth Connection(PTTC)is a new type of composite structure connection technology.The research shows that due to the single force transfer surface,the existing composite tooth profile is difficult to give full play to the bearing capacity of the connection.Therefore,based on the multi-stage tooth force transmission mechanism,this paper proposes an improved composite pre-tightened tooth connection structure,that is,improving the composite tooth configuration level to increase the force transmission surface of the composite tooth and embedding the appropriate thickness of the soft material so that the load of each force transmission surface is appropriately distributed to improve the bearing performance of the joint.Compared with the existing connection structure,the improved structure increases the load transfer surface of the joint,and the load of each load transfer surface is reasonably distributed by embedding soft material,so as to improve the bearing performance.In this paper,the variable parameters of the tooth level and the number of teeth are studied.The influence of different levels of teeth on the failure process,failure mode,load distribution ratio and ultimate bearing capacity of the joint is analyzed by experiments.Then,based on the geometric parameters of the experimental specimens,a progressive damage finite element model of the composite pre-tightened multi-stage tooth connection was established.The damage failure process of the joint was analyzed,and the mechanism of improving the bearing performance of the pre-tightened connection structure with multi-stage teeth and embedded soft layers was explored.Finally,based on the characteristic curve method,the theoretical calculation formula of the ultimate bearing capacity of the composite pre-tightened multi-stage tooth connection structure is derived,which provides a theoretical basis for subsequent research.The main research contents of this paper include :(1)The experimental study on the mechanical properties of composite pre-tightened tooth joints based on multi-stage teeth was carried out.The three-step configuration and the small sawtooth configuration were used respectively,and the soft layers with an appropriate thickness was embedded at the tooth end.Firstly,the load-displacement curve of the specimen is obtained by applying the displacement load to the multi-stage tooth connector by using the MTS test machine,and the crack propagation and failure process of the composite tooth under the tensile load are obtained by the high-speed camera.The crack propagation process and the final failure mode of the multi-stage tooth joint are analyzed.At the same time,the strain data between multi-tooth joints under tensile load were collected.Finally,the influence of level change on the ultimate bearing capacity of single-tooth and three-tooth connectors is analyzed.The results show that increasing the number of composite tooth layers can significantly improve the bearing capacity of the connector under the premise of embedding a suitable thickness of soft layers at the tooth end.Specifically,the ultimate bearing capacity of the three-step configuration connector is 28.7% higher than that of the traditional configuration,while the ultimate bearing capacity of the small sawtooth configuration joint is increased by 51.2%.For the threetooth joint,the increase of the level greatly improves the bearing performance of each tooth,and the load of each tooth is evenly distributed by embedding soft matter.Therefore,the ultimate bearing capacity of the multi-stage three-tooth joint is 89.2%higher than that of the traditional three-tooth joint.(2)The theoretical research on the force transmission mechanism of composite pretightened multi-stage tooth joint is carried out.Firstly,through the commercial finite element software ABAQUS and setting the Hashin composite failure criterion,the progressive damage analysis model of composite pre-tightened multi-stage tooth connection is established.The failure process,load displacement curve and load distribution ratio of three teeth of composite pre-tightened multi-stage tooth joints with different levels of teeth are calculated.Then,the correctness of the model is verified by comparing the calculation results of the prediction model with the experimental results.Finally,the force transmission mechanism of multi-stage single-tooth joint is analyzed by this model.The analysis points out that the multi-stage tooth structure set up by analogy to the biological suture line can effectively increase the bearing force transmission surface of the composite tooth.At the same time,embedding an appropriate amount of soft layers in the corresponding position can make each force transmission surface of the tooth distribute to an appropriate amount of load ratio under the external load,thus further improving the bearing capacity of the joint.This is also the mechanism of the improvement of the bearing performance of the joint by increasing the level of the composite tooth end under the action of the appropriate soft layers thickness.Compared with the three-step configuration,the small sawtooth configuration has more excellent bearing performance,mainly because the bearing area of the small sawtooth configuration is far away,which effectively avoids the adverse effects of stress coupling.(3)The calculation method of ultimate bearing capacity of composite pre-tightened multi-stage tooth joint is studied.The characteristic curve method is used to calculate the ultimate bearing capacity of composite multi-stage single-tooth connection structure,and the calculation model of ultimate bearing capacity of composite pre-tightened multi-stage tooth is established.Firstly,according to the geometric parameters of the multi-stage teeth,the corresponding finite element model of the pure teeth is established,and a fixed load is applied at one end of the pure teeth to find the corresponding feature points of each force transmission surface of the multi-stage teeth.Then,the corresponding composite material pre-tightened multi-stage tooth structure is established,and the external load is applied.By bringing the stress state of the feature point into Tsai-wu,it is determined whether the force transmission surface fails.If any force transmission surface fails,it represents the failure of the entire connection structure.For the prediction of the ultimate bearing capacity of the pre-tightened multi-stage multi-tooth joint,the load distribution ratio of the multi-tooth joint is adjusted by the soft layers to make the load of each tooth evenly distributed.Therefore,the predicted value of the ultimate bearing capacity of the preloaded multi-stage multi-tooth joint is obtained by multiplying the predicted value of the multi-stage single-tooth bearing capacity by the number of teeth,and the calculated theoretical value is compared with the finite element results.It is found that the difference of the ultimate bearing capacity of the single-tooth joint meets the engineering accuracy requirements,which proves the correctness of the theoretical model.Finally,the theoretical calculation method is applied to engineering examples to realize the rapid design of composite joints. |
