Study On Impact Damage Prediction In Fiber Reinforced Composite Laminated Structures

It has been widely accepted that fiber reinforced polymer composites offer a number of important technical advantages over metals, such as high specific strength, high specific stiffness and specific energy absorption. Now more and more fiber reinforced polymer composites are included in lightweight design of crashworthy structure of vehicles (such as bumpers, frontal side rails). Till now, a great deal of research on the impact response and damage mechanisms of fiber reinforced polymer composites have been conducted, which makes an important contribution to the application of fiber reinforced polymer composites in the automotive industry. The energy absorption and damage mechanisms of fiber reinforced polymer composites are very complex, which are influenced by many factors, and are distinctly different from those of metals.There are many numerical models for predicting the impact response and damage in composite structures, however, there are still some shortages in the existing numerical models that need to be further studied: there are various damage modes such as matrix cracking, delamination and fiber breaking composite structures during impact load, and the occurrence and growth of damages are coupled. The numerical models based on traditional strength theory cannot correctly predict the whole damage process of composite structures during impact. It is necessary to establish a constitutive model to accurately describe the damage mechanisms, and to successfully design damage tolerant composite structures, it is also necessary to have a reliable analysis tool that can capture the intricate details of impact events.Based on the experimental and theoretical results of the low-velocity impact response and damage mechanisms of laminated structures, the damage in composite laminate is classified into two categories: the intra-lamina damage and the inter-lamina damage. The laminated structure is schematized by an assemblage of sub-laminates bonded by thin interface layers in the transverse direction. Based on these features, an efficient mechanical model is established for analyzing the response of composite laminates. The sub-laminates are modeled by first order shear deformable Mindlin shell elements, and the intra-lamina damage is confined to take place in sub-laminates. The delamination damage is confined to take place in interface layers. A new reliable analysis tool has been established to capture the intricate details of impact events based on the explicit finite element program ABAQUS/Explicit. The following three aspects of efforts are performed:(1) Study on the mechanical model for analyzing delamination process of composite laminated structuresA three dimensional interface element is used to model the interface layers between composite laminas. Based on the relative displacement between the two neighboring laminas, the mathematical formulation of a interface element is derived. Based on the cohesive zone model and traction-separation law of the interface, a general mathematical model describing the delamination process is put forward: a scalar damage variable is introduced and the degradation of the interface stiffness is established; a damage surface which combines stress-based and fracture-mechanics-based failure criteria is set up to derive the damage evolution law. The delamination model is implemented into a commercial finite element package, ABAQUS/Explicit, via its user subroutine VUMAT.(2) Study on the constitutive model of the composite lamina based on continuum damage mechanicsA constitutive model accounting intra-lamina damage behavior is derived for a thin composite lamina. The lamina is assumed to be in a state of plane stress. The model is derived within a thermodynamic framework and the failure behavior is modeled using continuum damage mechanics. Three damage variables associated respectively with the stress in the fiber, transverse and shear directions, are used. The total strain is decoupled into elastic and inelastic parts based on the strain additive decomposition assumption. The inelastic behavior of the matrix material and in-plane shear is modeled. The difference in the material response in tension and compression is modeled. The numerical method is derived using the iterative predictor-corrector algorithm and implemented in a commercial finite element code, ABAQUS/Explicit, via its user subroutine VUMAT. Based on the proposed damage model for composite lamina, the parameters of the model are derived from a series of standard stress-strain curves from composites specimen tests for a certain kind of glass-fabric-reinforced epoxy composite.(3) Study on low-velocity impact damage prediction of composite laminated structuresA finite element model is established for predicting the intra-laminar and inter-laminar damage of laminated structures based on the proposed composite laminates mechanical model. The parameters for intra-laminar model and inter-laminar model are derived from a series of standard mechanical tests. The numerical model is applied to predict the response and damage mechanisms of glass-fabric-reinforced epoxy strips impacted at different velocities by a steel impactor. The numerical predictions of structural response and failure modes agree well with those observed in tests. The numerical model is then used to predict the impact damage of a car bumper at different impact velocities.In the dissertation, an innovative numerical tool for analyzing the impact response and damage mechanisms of fiber reinforced polymer composite structures is brought forward. This numerical tool is applied to predict the complex response of composite laminated structures. It is proved that the research fruits in this dissertation have theory and practice significance.