Research On Fatigue Strength Of Aerospace Composite Bolt Structures

Carbon Fiber Reinforced Plastic(CFRP)is widely used in aircraft structures due to a series of advantages,such as high specific strength,high specific stiffness,good fatigue resistance,designability of material layup and so on.With a large number of aircraft structural parts and connecting components,composite structural connection technology has inevitably become a key problem in aviation applications.The composite structure of countersunk bolt connection is of large bearing capacity and detachable,and can guarantee good aerodynamic shape,which is employed in bearing components of important aircraft composite skin structures.Countersunk bolt connections require holes drilling on composite materials,which destroys fiber continuity,and the installation angle error of countersunk bolt leads to the uneven stress distribution of countersunk bevel.As a result,the area around the hole of composite materials becomes the “weak point” of structural fatigue design.Therefore,investigating the fatigue strength and damage of composite components of countersunk bolt connection are urgent to be solved.Based on the engineering problems in aviation applications,this dissertation conducts research on the fatigue damage characteristics of countersunk inclined planes and the fatigue wear behavior around holes in low temperature environments focusing on three point bending fatigue strength of composite components connected with countersunk bolts.(1)Stress distribution of countersunk hole inclined surface of countersunk bolt connection composite componentsBased on the numerical simulation analysis of finite element method,the influence of countersunk bolt installation angle error and composite fiber laying angle on the contact stress distribution and first principal stress distribution of counterbore inclined surface is studied.The results indicate that under the same preload,regardless of the installation angle error and fiber laying angle,the contact stress and first principal stress of the counterbore inclined plane increase along the direction of hole depth,and the maximum contact stress and first principal stress are located at the bottom edge of the counterbore inclined plane.The contact stress and first principal stress increase gradually,when the installation angle error changes from positive to negative.The contact stress is distributed uniformly along the circumferential direction of the hole,while the first principal stress is distributed in a "W" shape along the circumferential direction of the hole.The position of the first principal stress around the hole is related to the fiber layer angle.The maximum first principal stress locates at perpendicular direction to the fiber,and the minimum first principal stress locates at the 45 ° angle direction with the fiber.This achievement supplies theoretical guidance on the drilling process of aviation composite materials.(2)The effect of preload on the distribution of cracks around the hole under three-point bending loadBased on finite element numerical simulation,the influence of preload force on the strength and stiffness of bolted composite members under the action of three-point bending static load is analyzed,and the cause,crack form and crack distribution of material cracks under different preloads are studied when the three-point bending static load is damaged.Through comparison with the test,it is found that the maximum damage position on the surface of the specimen coincides with the test fracture position.Too large or too small preload torque will reduce the strength and stiffness of the bolted composite components.Through finite element numerical simulation,it is found that with the increase of preload,the relationship of fiber cracks and matrix cracks is not obvious.However,the number of interlaminar cracks is significantly reduced,and the form,distribution and number of cracks are basically consistent with the test results.Meanwhile,under static load of three-point bending,the changes in the strength and stiffness of the specimen have little effect on the crack formation.(3)The three-point bending fatigue characteristics of composite components connected by countersunk bolts were studied under normal temperature environmentThrough fatigue test and Ncode software fatigue simulation,the three-point bending fatigue damage around countersunk hole of composite material is studied.Both tests and simulations show that the fatigue danger zone of composite components connected by countersunk bolts is located at the lower part of the perimeter of the countersunk bevel,which is consistent with the maximum contact stress and the first principal stress position of the countersunk slope.The bending fatigue life is in the order of 106 cycles,and the main manifestation of damage is fiber delamination caused by matrix fracture,followed by fiber delamination around the countersunk hole and fibrous burr-like protrusions appearing along the boundary of the slope,which is consistent with the research results of peripore crack distribution results.This achievement is of great significance to the design optimization and process improvement of aviation countersunk head bolts connecting composite components.(4)Experimental research on the three-point bending fatigue wear behavior at room temperature and low temperature environmentsThrough the comparison of the three-point bending test process and results of different preload torque specimens at room temperature and low temperature environment,the fatigue wear behavior of bolted composite components in low temperature environment was analyzed.It is found that the wear phenomenon of the contact position between the nut and the composite material is obvious,and the wear phenomenon is mainly reflected as annular white wear marks,local fracture and delamination of fibers,matrix defects etc.The greater the preload torque,the greater the friction in the wear area,and the more serious the damage is to the specimen.In the low temperature environment,the preload torque decreases,and the wear condition is more serious,which is mainly because the composite material should be distributed between layers in the low temperature environment and the low temperature embrittlement has greater impact on fatigue wear.This finding has certain guiding value for the reliability design of aircraft composite connections.