Study Of Ultimate Strength Of Sandwich Composite Structures Based On Rate Dependence Adhesive Interface Model

As lightening and better acoustic and magnetic performance of new-type special vessels and the success of composite materials used in the fields of aircraft and aerospace,foam core sandwich composite materials which composed by laminated composite plates made of glass fiber cloth panels and rigid foam sandwich core appear to promising and have been widely used in ship design and construction due to high strength/stiffness-to-weight ratios,energy absorbability and designability.To further improve sandwich composite structures design performance,security and economical efficiency,the ultimate strength of sandwich composite structures need to be fully studied.Many fracture accidents occur in the case of lower than the strength of the material.The main reason is the discontinuity between face sheet and foam core always lead to deflects as cracks and damages in the bonding interface during the manufacture and service process.So,face/core interface failure is one of main failure foam of sandwich composite structures which will degenerate structural loading capacity.It is urgent and necessary to evaluation and analysis the mechanical properties of face/core interface.The cohesive zone model(CZM)which based on the elastic-plastic fracture mechanics has been extensively researched and used because the stress singularity at crack tip can be avoided and both the crack initiation and propagation are taken into account.Interface cracking modeled by CZM is the core of study on the ultimate strength of sandwich composite structuresHerein,the main research work of this paper has four following contents:Firstly,the principle of CZM and different cohesive zone laws was introduced.Among the widening class of cohesive force-separate laws this paper focus on the exponential cohesive zone law which has continuous relationship of stress and displacement.The exponential cohesive zone law of Xu and Needleman was investigated in mixed-mode loading and discussed the coupling between normal and tangential direction.It was concluded that only for q=1 a physically realistic coupling behavior can be obtained.Consequently,a cohesive zone law applicable to simulate the interface crack under 3D stress condition has been proposed based on the original exponential cohesive zone law of Xu and Needleman,and the corresponding failure criterion has also been presented.Numerous studies show that the bonding interface associated with the viscous properties of the loading rate-dependent.However,the commonly used cohesive laws are rate-independent,the stress within the cohesive zone only depend on the crack opening displacement,which are independent of the loading rate.On the basis of exponential cohesive zone law and generalized Maxwell model,a rate-dependent interface model was established.And the interface element was programmed by Fortran language in ABAQUS-UEL.According to a typical example,this interface model was proved to reflect the rate-dependent interface correctly.Considering the sandwich composite structures also have two or more failure modes,the ultimate strength analytical model was introduced by the combination of the rate-dependent adhesive interface model and progressive failure analytical method.A modified 3D Hashin interwoven fiber failure criteria were proposed to simulate the failure process of laminate face sheet while the mechanical behavior of adhesive interface is modeled by rate-dependent interface model proposed in previous chapter.An example was carried out to verify the validity of using such method to predict the ultimate strength of sandwich plate,and how face/core interface affect the ultimate strength of sandwich plate has been discussed.Finally,the ultimate strength test of sandwich composite L-joint under bending load was carried out,and experimental process and results were fully discussed.The finite element model of the L-joint under bending load were established by the proposed ultimate strength model of sandwich composite structures,and results were compared with the results of test and other interface models to prove the accuracy of proposed simulation method.Then,the influence of bracket size to the bearing capacity of the sandwich L-joint were analyzed by the strength analytical model to provide reference to the follow-up design.