| High lock bolts are widely used in various aerospace equipment due to their selflocking,high strength,high fatigue resistance,vibration resistance,and ease of installation.However,due to the fact that bolts and mounting holes are often not coaxial during installation,resulting in an angle of the mounting surface(the angle between the lower surface of the bolt head and the surface of the assembly component),resulting in a decrease in the tensile strength of the bolt connection pair,which has become a major risk of premature failure of bolts in service.In this paper,TC4 high lock bolts are selected as the research object.Combined with the material structure characteristics,relevant mechanical properties,and finite element simulation analysis of the bolts,the influence of mounting surface angle on the tensile strength of assembly bolts during installation is studied,and the reasons for early fracture of bolt connection pairs are explored.The finite element model is used to successfully predict the tensile strength of connection pairs at different angles,and the fracture mechanism of bolts with mounting surface angle is obtained.The main research contents of this article are as follows:(1)Observation of bolt micromorphology and mechanical test: By observing the metallographic structure of the bolt section,it can be found that the TC4 titanium alloy studied in this article is typical α+β Type A alloy;The rolling process and solution aging heat treatment process make the hardness of different areas of the bolt different,with the hardness of the surface layer of the screw being 175 HV higher than that of the central area;The friction coefficients of TC4 titanium alloy,molybdenum disulfide,and threaded pairs were obtained through the reciprocating friction and SCHUTZ threaded fastener analysis system.The reason for the low friction coefficient of the coated bolt threaded pairs was explained.The stress-strain curves of TC4 titanium alloy were obtained using standard tensile tests,providing material constitutive parameters for finite element analysis.(2)Tensile test for installation process inspection: simulate the tension condition of bolts in service at different mounting surface angles by installing bolts on tensile plates at different angles(0°,3°and 5°)and conducting tensile tests.The results show that the maximum tensile load of the connecting pair decreases as the angle of the mounting surface increases,and when the angle is greater than 3°,the bolt fracture position changes from the thread to the bolt head.The reason for the decrease in tensile strength of bolts is that the stress on the lower surface of the bolt head has changed from a single axial load to a composite load of axial,tangential,and additional bending moments.The thread will bear the axial load and the load generated by the bending moment at the head simultaneously,exacerbating the stress concentration phenomenon at the bolt head and the thread,Bolt head fracture occurs because the increase in the angle of the mounting surface leads to excessive bending deformation caused by deflection and rotation,resulting in bolt head fracture.(3)Using ABAQUS modeling for simulation analysis,the fracture location and loaddisplacement curve of the bolted connection pair under three mounting surface angles of0°,3°and 5° were obtained.The experimental and simulated load-displacement curves and fracture location were consistent,verifying the accuracy of the finite element model of the connection pair;The stress distribution in each region of the bolt was obtained through simulation analysis,and the fracture mechanism of bolt connection pairs with different mounting surfaces was revealed through observation of experimental fracture morphology;In addition,the model was successfully used to predict the bolt fracture curve and location when the mounting surface angles were 1°,2°and 4 °,respectively.It can be seen that this model can effectively predict the tensile strength of bolt connections under different mounting surface angles,and guide the application of mounting bolts. |
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