Strength Prediction Method Of Multi-bolt Composite Joint And Its Key Technologies

Mechanical joint is one of the most important load transfer elements in the field of aeronautics and astronautics.In bolted composite structures,stress concentrations developing around the holes everely reduce the strength of the structure,which directly affects the safety and life of the aircraft.To utilize the full potential of composite materials in structural elements,appropriate methods for predicting strength of multi-bolt composite joints must be developed.The current strength prediction method has the problems of complicated calculation process and low accuracy,and cannot meet the requirements of modern aircraft structural design.The research of strength prediction method with high efficiency and low cost for multi-bolt joint is of great significance to ensure the safety of the aircraft structure,to reduce the structural weight and life cycle cost,and to shorten the development cycle.The key techniques for strength prediction of multi-boltcomposite joint mainly lie in four aspects: 1)the selection of appropriate failure criterion for predicting the strength of notched laminate;2)the efficient and low-cost analysis method for bolt load distributions;3)the method for predicting strength of regular-arrangement multi-bolt composite joints;4)the method for predicting strength of irregular-arrangement multi-bolt composite joints.In this paper,the abovekey technologies are studied and the main contributions and innovations are listed as follows:1.The method of predicting the tensile strength of notched laminate under bypass loading or bearing loading is studied,respectively.Combined with analytical method and p-version finite element method,a new model based on Finite Fracture Mechanics is proposed to predict the tensile strength of notched laminate with arbitrary lay-ups.Failure is predicted when both stress-based and energy-based criteria are satisfied.The material properties required by the model are the laminate unnotched tensile strength and critical fracture toughness,which indicates that the model has the characteristics of less input parameters and more general application.The comparison with experimental data shows that the proposed model can most effectively predict the tensile strength of laminate with arbitrary dimensions and lay-ups.2.The method of predicting the bolt load distribution of multi-bolt composite laminates with high efficiency and low cost is studied.The analysis models are established by using both the spring-mass model and the p-version finite element method based on structural elements.In addition,an improved spring-mass model is proposed to calculate the load transferred in bolted repair by introducing the parameters,such as equivalent stiffness of the plate with damaged hole,the equivalent stiffness of the multi-column bolts and the stiffness of the joint.The predicted results of the two models are in good agreement with experimental data and the literature data.3.The method of predicting the failure strength of regularly arranged multi-bolt composite joint is studied.In this paper,a semi-analytical model based on Finite Fracture Mechanics is proposed to predict typical net-tension failure of multi-bolt composite joints.The stress distribution around the bolted hole under the coupled bearing load and bypass load is obtained by using bolt load ratio.The comparison between the prediction of the proposed model and experimental data shows that the prediction error is rather small.In addition,analysis results also show that the net-tension failure strength would increase with the decrease of the bolt diameter,the bolt load ratio of the first bolt and the joint width.4.The method of predicting the failure strength of irregularly arranged multi-bolt composite joint is studied.Aiming at predicting the typical net-tension failure of bolted composite repair,the Finite Fracture Mechanics model is employed combined with bolt load distribution and twostage analysis model.The analysis process is implemented in VB.NET together with the API of the p-version finite element method.Failure strength of the bolted repaired composite laminates with circular,elliptical and oblong cutouts are analyzed.Results show that the failure mode and failure location predicted by the proposed model are consistent with experimental results,and the failure strengths agree well with the experimental data.In addition,through the sensitivity analysis,it is found that the failure strength of the damaged hole mainly depends on the proportion of the load transferred by the patch and the greater the load ratio is,the greater the failure strength will be.However,the failure strength of bolt hole depends mainly on the bolt hole size.The larger the bolt diameter is,the less the failure strength is.Secondly,for repaired composite laminates with oblong cutouts,the closer the shape of the cutouts is to a circle,the higher the failure strength of the damaged hole is.Conversly,the closer the shape of the cutous is to a crack,the lower failure strength of the damaged hole is.In addition,in the symmetry plane perpendicular to the load direction,the bolt load ratio is rather small.The contribution of these bolts to the failure strength of the repaired structure is also small.In the bolted repair design,the bolts should be avoided being arranged near this symmetry plane.