Stress Distribution And Damage Initiation Analysis Of Composite Multi-pin Joints With Interference Fit

The mechanical joint is a common joining form of aeronautical constructions.However,the mechanical joint needs pre-drilling holes on structures,which destroys the continuity,leading to stress concentration,and does harmful to long life requirements.Interference fit technique is believed to be an effective method of life increasing for mechanical joints.It has been widely used in metal structures,and is starting to be employed in composite ones.The reliable application of joints depends on accurate stress prediction.The stress of composite interference fit joints is affected by many factors and is commonly uneven along the hole edge.The interference fit stress can affect pin-load stress by changing load path.For multi-pin joints,the stress analysis should also consider the effects of position and bypass load.The overlap of interference fit stress,pin-load stress and bypass stress makes the stress and damage prediction of composite interference fit multi-pin joints quite difficulty,which hinders the application.In order to support the application of the technique,the research on stress of composite interference fit multi-pin joints is needed.In this paper,stress distribution and damage initiation of composite multi-pin joints with interference fit is analyzed.Based to deformation relationships,an analytical stress analysis method is created for composite interference fit joints.Then,the stress calculation method for interference joints under tensile load is proposed.The action of pin load and bypass load in multi-pin joints is analyzed,and a model for stress analysis of these joints considering the effects of position is built.Generally considering interference stress,the overlap model of deformation and tensile load,and the effects of bypass load,an analytical model for stress distribution of composite multi-pin interference fit joins is proposed,and the damage around hole is analyzed.The main contents and results are as follows:(1)An analytical model for interference stress analysis of composite interference fit joints is proposed.Based on deformation relationships of the pin and hole,the displacement condition considering the pin's elastic deformation without slipper is built.Corresponding stress functions are determined on the basis of Lekhnitskii complex stress theory.The stress around holes is calculated and compared with finite element analysis results.The effects of material and interference value are discussed.Then the slipper deformation is considered and the effects are analyzed.Results show that the material property has apparent effects on stress distribution.Quasi isotropic plates obtain circle distribution,while for the 0 dominated plate the shape is like figure “8”.Interference value affects stress value,and the slipper deformation affects the stress distribution apparently.(2)The effects of interference deformation on loading path are analyzed,and a method for stress analysis of interference fit joints under tensile load is proposed.Based on motion relationships of the pin and hole,the hole region is divided into several parts and corresponding displacement conditions are created.Considering the effects of interference deformation of contact condition,a model based on spring equivalent method is built,and the load condition is given.According to the displacement and load condition,stress functions are determined and stress distribution around holes is calculated.Results of finite element method and experiments provided in published reference are used to verify the analytical method.The effects of material property are discussed.Combined with Hashin failure criterion,the damage initiation around hole is predicted.Results show that interference fit can reduce the stress increase brought by applied load.The effects of material property mainly reflect on hoop stress,and the main damage type is resin failure.(3)The effects of bypass load on stress distribution are analyzed,and an analytical model for stress calculation of neat fit multi-pin joints is proposed.Considering the load transfer characteristic,load condition of each pin is analyzed.For pins in different position,according to their deformation trait,the action model of pin load and bypass load is discussed.Then load condition is determined and load distribution can be known by classical spring-mass method.Stress functions are proposed satisfying the load condition,and stress around holes is calculated.Comparison with finite element results are experiments are conducted and the effects of pin number on load and stress are discussed.Combined with progressive damage analysis,damage initiation is predicted.Results showed that the first pin shares the highest stress lever,however the stress level of middle and last pins are the same.Damage model is mainly resin failure.Increasing pin number can decrease the pin load and stress level,but the stress decrease is relatively unapparent for the first pin.(4)An analytical model for stress calculation of composite multi-pin joints with interference fit is proposed.The stress distribution and damage initiation is analyzed.Based on the results of interference fit single pin joints and neat fit multi-pin joints,the load condition interference multi-pin joints is studied.Combining the effects of bypass load,interference stress and overlap relationship between interference deformation and tensile load,an analytical model for stress calculation is built.Stress distribution is calculated and compared with finite element results and experiments.The effects of material property and pin number are discussed.Both of progressive damage analysis and SEM method are employed in damage analysis.Then the combination of interference fit and neat fit of three pin joints is discussed.Results show that,material property has apparent effects of hoop stress.The first pin also gets the highest stress level,while for the pins in other position the stress is almost the same.The benefits of stress decrease brought by the increase of pin number are relatively small for the first pin position.Generally considering process and stress condition,interference fit applied on all pins is relatively effective.