Research On The Damage Mechanisms And The Prediction Method Of Delamination In Fiber-reinforced Composite Laminates

Composite materials have many excellent properties such as higher specific strength and higher specific stiffness compared with metallic materials,thus having a broad application prospect in railway vehicle structures.But the current researches on the understanding of the damage mechanisms in composite structures and their modelling methods were not sufficient.The prediction methods for various damage modes,especially for the delamination,were also inadequate.The potential of the composite properties cannot be fully exploited in the design of complex composite load-bearing structures.Therefore,this thesis focuses on the delamination,which is the dominant damage mode in fiber-reinforced composite laminates.Based on the experimental analysis and finite element simulation methods,two main aspects of the research work are carried out,which are the analysis of damage mechanisms and the prediction method of delamination.The specific research contents and findings are as follows:(1)A finite element analysis method for analyzing matrix crack-induced delamination in composite laminates was developed to investigate the mechanisms of damage initiation and evolution in cross-ply laminates subjected to static longitudinal tensile loading.A macro-micro mechanical model was established in this method,and a special emphasis was put on the interlaminar resin-rich region,where the delamination was characterized by plastic deformation and cracking of the matrix and fiber/matrix interface debonding,which was consistent with the physical mechanism.The results of the finite element analysis showed that the model could completely describe the damage evolution process within the 90° plies of the orthotropic laminate,and the initiation and propagation of delamination induced by the transverse crack,as well as accurately reveal the interactions between different damage modes and their effects on the macroscopic axial stiffness and transverse Poisson’s ratio of the laminate.The analysis of the model influencing factors showed that the thickness of the interlaminar resin-rich region would affect the form of delamination.Transverse cracks in cross-ply laminates with thicker 90° ply are prone to be branched or oblique cracks,which did not affect the initiation of delamination from the tip of transverse cracks.And more significant degradations of axial stiffness and transverse Poisson’s ratio were caused by the damages in cross-ply laminates with thicker 90° ply.Introducing the thermal residual stress changed the location of damage initiation and damage evolution path,accelerated the speed of damage initiation and propagation,and more degradation of mechanical responses of the laminate was caused by delamination.(2)A qualitative study of the damage configurations of transverse cracks and the induced delamination in cross-ply laminates subjected to quasi-static axial tensile loading was carried out using a macro-micro finite element model.A representative volume element containing randomly distributed fibers was embedded in the 90° ply near the interlaminar region of the cross-ply laminate,to account for the heterogeneous interlaminar microstructure.Two damage modes,i.e.fiber/matrix interface debonding and matrix plastic deformation and cracking,were used to characterize the delamination based on the physical mechanism,which was also compared with the delamination simulation method based on the cohesive zone model.The effect of the microstructure and the constituent material properties on the damage configurations was investigated.Differences in the microstructure of the transverse plies were found to be the main cause of the propagation of transverse cracks to the interlaminar region in a straight or branched form.While the values of interlaminar interface strength and matrix fracture toughness would determine whether delamination was initiated before the transverse cracks reached the interlaminar region,i.e.whether the Cook-Gordon mechanism occurred.(3)The experimental study and finite element simulation analysis of the evolution of microscopic damage in cross-ply laminates under fatigue loading and its effect on the macroscopic response of the laminates were carried out.The fatigue tests on glass fiber/epoxy cross-ply laminates were carried out at two load levels.The test results showed that the damage evolution process of cross-ply laminates under fatigue loading and its effect on the macroscopic response of the laminates were similar to those under static loading.The random microstructures within the 90° plies caused the diversity of configuration of transverse cracks and delamination,which did not change the physical rule that the fatigue delamination initiated at the tip of the transverse crack.A method based on actual physical mechanisms was also proposed to simulate the damage mechanisms within the ply as well as the delamination damage in cross-ply laminates under fatigue loading An elastic-plastic fatigue damage model was used to describe the mechanical behavior of the matrix in the 90° plier and interlaminar resin-rich regions enrichment zones.And a fatigue cohesive zone model was applied to characterize the fatigue failure of the fiber/matrix interface.The key fatigue parameters in the two material models were determined and validated by rational methods.The method completely simulated the initiation and propagation process as well as the interactions of three damage modes,i.e.fiber/matrix interface debonding,matrix cracking and delamination,in cross-ply laminates under fatigue loading at the micro-scale.The effects of every damage mode on the macroscopic axial stiffness of the laminate were also accurately calculated.(4)A method based on a microscopic model with randomly distributed fibers for predicting the interlaminar cohesive strengths of unidirectional laminates was established.The method considered two different types of microstructures at the interlaminar interface and two yield criteria were adopted to determine the plastic deformation and damage initiation of the matrix,which were compared with the major principal stress criterion.The qualitative and quantitative analyses of the predicted cohesive strengths under three types of fracture loads showed that the three cohesive strengths,i.e.the normal cohesive strength N,the longitudinal shear cohesive strength S and the transverse shear cohesive strength T,were highly dependent on the type random distribution of the microstructure,the microscopic geometric characteristics described by the minimum fiber spacing and the fiber alignment angles,as well as the criteria for determining matrix damage initiation.(5)A method based on the mixed-mode three-linear cohesive zone model for predicting the delamination propagation considering the effect of large-scale fiber bridging was proposed.A three-linear cohesive zone model which was only applicable to mode I delamination was extended to the case of mixed-mode I/II delamination.A novel method for determining the bridging strengths in the cohesive zone model was developed.The so-called varied bridging strength method applied the maximum bridging stress of the bridging law(for the case of mode I delamination)or a moderate bridging strength(for the case of mixed-mode I/II delamination)to the first region of the pre-defined delamination path determined by the fracture process zone,and the smallest bridging strength calculated by the maximum crack opening displacement measured by the test to other regions.The load-displacement response curves for unidirectional laminates and multidirectional laminates of mode I delamination,and multidirectional laminates of mixed-mode I/II delamination were accurately simulated.Good agreement was achieved between the predicted results and the corresponding test results.95 figures,14 tables and 277 references.