| Advanced composite materials are widely used in aerospace,automotive,shipbuilding and pressure vessel manufacturing due to their high strength,light weight,strong corrosion resistance and easy tailoring design.Although the integral forming technology of composite material has been developed more mature,it can effectively reduce the strength reduction of the integral structure caused by the connection part.However,for some large and complex structures,the bolt connection between components still needs to be considered.For composite bolt joints,the characteristic curve method can predict the strength and failure mode quickly and accurately.In addition,composite materials are subject to the effects of temperature and humidity in the external environment.These environmental factors can lead to changes in the mechanical properties of the material,which can affect the safety and stability of the structure,so it is important to consider the effects of temperature and humidity in the strength analysis of composite bolt joints in the design and manufacture of composite structures.In this paper,based on the classical characteristic curve method,the influence of hygrothermal condition on the failure of composite bolted joints was studied.The specific research work is as follows:(1)According to the working condition of the main bearing structure in large aircraft,seven different hygrothermal conditions were identified as influencing factors,and the rule of the influence of hygrothermal condition on the characteristic length of composite bolted joints was revealed by using the progressive damage analysis method,and the characteristic length equation and characteristic curve equation considering the hygrothermal effect were established,and then the characteristic curve method considering the influence of hygrothermal condition was developed.By comparing the characteristic length and characteristic curves under seven working conditions,the influence of the hygrothermal condition on the characteristic curves is analyzed.The results show that the cold temperature and cold condition increases the characteristic length and characteristic curves,which will enhance the strength of composite bolt joints;the elevated temperature and wet condition decreases the characteristic length and characteristic curves,which will reduce the strength of composite bolted joints.(2)Numerical simulations based on progressive damage analysis were used to reveal the effects of width-to-diameter ratio and end-to-diameter ratio on the failure loads as well as the characteristic length and characteristic curves of composite bolted joints under cold temperature and dry,room temperature and dry,elevated temperature and wet condition.The results show that the effect of the width-diameter ratio on tensile failure and tensile characteristic length is obvious,where the tensile damage load increases linearly with the increase of the width-diameter ratio and the tensile characteristic length increases non-linearly with the increase of the width-diameter ratio.The effect of enddiameter ratio on bearing failure and compressive characteristic length is more obvious,both of them show a non-linear increase with the increase of end-diameter ratio.(3)The characteristic curve method,which takes into account the hygrothermal effect,was used to predict the failure of the typical two-bolt joint,three-bolt joint and four-bolt joint in cold temperature and dry,room temperature and dry,elevated temperature and dry condition.Firstly,the effect of the hygrothermal condition on the bolt load distribution of the multi-bolt joints is investigated using the 3D finite element method.The results show that the cold temperature and dry condition increases the load ratio of the critical hole in the multi-bolt joints,while the elevated temperature and dry condition decreases the load ratio.Based on the bolt load distribution,the multi-bolt joint is simplified to the single-bolt joint and the characteristic curves obtained in the previous study for the cold temperature and dry,room temperature and dry,elevated temperature and wet condition are used to predict the failure load and failure modes of the multi-bolt joints under these three typical working conditions.In summary,this paper presents a series of research works on the characteristic curve method for failure prediction of composite bolted joints based on the classical characteristic curve method,taking into account the influence of hygrothermal condition,which can provide reference for strength analysis and structural design of composite bolted joints under hygrothermal condition. |
