A Novel Trilinear-softening Cohesive Law For The Delamination Simulation In Composite DCB Laminates

Advanced carbon fiber reinforced composite materials are widely used in aircraft structure due to a series of excellent performance.Laminate composites are one of the commonly used configurations,but the fibers reinforced the laminates just in longitudinal and width directions,interlayer performance becomes a relatively weakness.During transverse loading,the layers are likely to degrade according to the applied force and crack occurs in interlayer regions.So,the delamination and propagation of laminates have become a key factor in the design of damage tolerance for composite structures.As an important reference indicator for damage tolerance design,numerical simulation is necessary to accurately predict the initiation and propagation of delamination.Previous studies have mostly focused on unidirectional laminates,corresponding experimental test standards and numerical simulation methods have been established.However,for engineering applications,multi-directional laminates with more complicated delamination and propagation mechanisms still need further research.The large-scale fiber bridging phenomenon that often occurs in multi-directional laminates delamination makes the numerical simulation process more difficult.The traditional bilinear cohesive zone model is no longer sufficient.In order to introduce the effect of fiber bridging in simulation,this thesis has carried out a series of work and achieved the following innovative results.Firstly,this thesis combines with the experimental phenomena and the realistic failure mechanism during the delamination process,and presents a novel trilinear-softening cohesive zone model.The novel cohesive model is superposing of a bilinear cohesive zone model for characterizing the damage response of matrix and a trilinear cohesive zone model for characterizing the damage response of fibers.Fracture toughness ratio m is defined to present the non-linear softening behavior of the bridging fibers in this model.The numerical results predicted by the proposed law have good agreements with the experimental results for four kinds of multidirectional laminates.Besides,relative to existing methods,this new cohesive law have less parameters required to be numerically determined and the simplicity for practical application.Secondly,four different stacking sequence T700/QY9511 laminate specimens are tested,and the initial crack tip opening displacement along with strain energy release rate during the crack propagation are obtained.And based on fracture toughness ratio m and the experimental data,this thesis establishes a parameter determination process.Finally,the new cohesive law is verified by numerical simulation.In summary,combining with experimental procedure,this thesis presents a new trilinear-softening cohesive zone model based on cohesive zone method,and establishes a reasonable parameter determination process.This model can effectively realize the numerical simulation of multidirectional composite laminates delamination under large scale fiber bridging,which can provide effective support for delamination numerical simulation work of multidirectional composite laminates.