Experimental And Numerical Analysis On Damage Evolution Of Composite Laminates

On account of the complexity of damage mechanism of carbon fiber-reinforced polymer (CFRP) composite laminate, the progressive damage process of composite laminate with a hole subjected to tensile loading was investigated by using an analysis method which combined both test method and numerical simulation Both acoustic emission (AE) and digital image correlation (DIC) method were used to monitor damage progress in real-time synchronously, in order to obtain AE signals generated in internal damaged composites and strain fields evolution on specimen surfaces. Peak frequency of AE was extracted to distinguish damage modes of composites. According to the local feature of strain fields generated by damage, The statistical standard deviation value of local strain field was selected for a good description of damage degree. A constitutive model for progressive damage analysis of composite laminates was established based on continuum damage mechanics (CDM), which was employed to simulate the progressive damage of composite laminates. The main contents of the dissertation are as follows:(1) In order to do deep analysis on damage mechanism of CFRP composite laminates,the parameter values of AE signals obtained from seven kinds of specimens with different layers have been analyzed statistically in the process of tensile test, so as to study the corresponding relation between composites damage modes including fibre/matrix debonding, delamination, matrix cracking, fibre pull-out, fibre breakage and five kinds of distribution ranges of AE peak frequency values.(2) With respect to the question about other damage models implying in single AE signal except for damage mode characterized by peak frequency, the decorrelation ensemble empirical mode decomposition (DEEMD) method was put forward, which could eliminate the mode mixing problem that appear in the sifting process of empirical mode decomposition (EMD), and which detach the peak frequency feature of several damage models from AE signal effectively,and on this basis the assessment method about damage modes of composite laminate was set up using AE peak frequency based on DEEMD. This study has directive significance to damage monitoring of composite laminates in AE practical application.(3) A new experimental characterization method of damae evolution of composite laminates with an open hole was set up. Based on the full-field measurement of digital image correlation and experimental data analysis, the strain value that is used to describe local damage evolution was chosen as a key parameter,and on this basis the damage variable was proposed to characterize the damage evolution of composites based on the statistical standard deviation value of the strain band. There could be qualitative and quantitative characterization in composite damage by using AE accumulative energy events and accumulative events of all damage modes, the damage evolution model was built based on AE accumulative effect, and the damage evolution equation was also derived.(4) A three-dimensional (3D) finite element model of composite laminates progressive damage analysis has been established based on CDM. The model contain four kinds of damage modes of composite laminates under three-dimensional stress states, four kinds of damage modes include matrix damage, fibre breakage, delamination and fibre pull-out. Damage initiation is controlled by failure criterion, which adopts the mixed failure criterion composed by Linde failure criterion and Hashin failure criterion, failure criterion chooses strain as parametric description. The damage evolution is controlled by damage variable, which adopts the nolinear evolution model with modified exponential form. It has been considered that the strain dissipated energy has an effect on damage variable evolution. The computational formula of the strain dissipated energy was given,and irreversible growth pattern was introduceed into damage variable model. The proposed composite laminates constitutive equation, damage initiation criterion and damage evolution criterion have been implemented in the finite element analysis (FEA) process,through coding the ansys parameter design language (APDL) of the finite element analysis software ANSYS. The progressive damage numerical simulations have been performed to investigate damage evolution of unidirectional laminate and bidirectional laminate with a hole, the evolutionary process of some kinds of damage modes in laminates has been obtained. The results of FEA have a good agreement with experiments. It validates the proposed 3D FEM.