Strength Of Composite Sandwich Plate Fastened With Countersunk Bolts

Due to its excellent mechanical properties and weight saving potential,composite sandwich plate has been used in marine structure design and manufacture.The bearing capacity of composite structures,our main concern,is mainly determined by the strength and reliability of their bolt joints,which is the main connection form in such structures.While most of the current researches focus on protruding bolt joint,only a few researchers pay attention to the bearing problem of countersunk bolt joint,which is widely adopted on underwater structures,because a smooth external surface is usually required.Thus analyzing the strength of countersunk bolt joint is significant for predicting the performance of underwater composite structure as well as expanding its application range.Firstly,a general analysis method of composite joint using ABAQUS is introduced and validated by experimental data in the previous literatures.And a series of finite element models are developed based on typical joint forms in a real structure,which is a composite sandwich plate with mechanical fastened joints.Analysis of their stress distribution in the joint area shows that the maximum shear stress of countersunk bolt joint is larger than that of protruding bolt joint under the same condition,while other stress components in both joints are in similar range.And certain stress components multiply when the bolt pretension increases or the h/a values of countersunk bolt and hole are different.Secondly,a progressive failure analysis method is implemented to investigate the ultimate strength of composite joint,and USDFLD and VUSDFLD user subroutines are written for static and dynamic analysis respectively using FORTRAN programming language.The predictions of initial failure and ultimate failure by different failure criterion show less than 4% difference,however,the critical load is significantly affected by degradation coefficient,hole-edge distance and ply stacking sequence.The critical load of countersunk bolt joint is about 14% larger than that of protruding bolt joint with the same dimension,and delamination problem of composite plate is more severe in countersunk bolt joint.Lastly,a few simulation methods of composite plate with multiple fasteners are explored,and quasi-static method turns out to be much more efficient in solving such kind of problem,nevertheless,the accuracy needs to be improved.During the experiment of composite sandwich plate under static load,large amount of local damage is found on the contact area between loading ends and composite plate.The FEM result indicates that such damage is initially caused by Z direction matrix failure,which induces Y direction fiber failure,and eventually leading to cracks on the whole plate as well as a 23% decrease of critical load when compared with the uniform load case.