Αριθμητική και πειραματική διερεύνηση της αστοχίας συνδέσεων με κόλλα σύνθετων υλικών με μηχανισμούς διακοπής της αποκόλλησης με έμφαση στην προσομοίωση της εξέλιξης της αποκόλλησης σε κόπωση

In the present thesis, a numerical and experimental investigation of composite bonded joints failure with crack stopping mechanisms was performed based on Cohesive Zone Modeling method for the simulation of debonding in composite bonded joints under fatigue loads. The crack stopping mechanisms that were studied was the corrugation, which belongs to the category of geometry modifications and bolts which belong to the category of through-the-thickness reinforcements. Part of the thesis was implemented in the framework of the European research project BOPACS. Initially, an extensive experimental campaign was performed for the characterization of the adhesive joints and the export of input parameters and validation data for the numerical models. The campaign contains Mode-I fracture toughness tests under pseudo-static and dynamic loads using Double Cantilever Beam specimens, Mode-II fracture toughness tests using under pseudo-static and dynamic loads using End Notched flexure tests, Mixed-mode I+II fracture toughness tests under pseudo-static loads and tensile axial loading of Cracked Lap Shear specimen under pseudo-static and dynamic loads. Supplementary to the aforementioned mechanical tests, in the present thesis, experimental results of tests performed by partners in BOPACS were used. The adhesive joints that were studied in the present thesis consist of CFRP plates and epoxy adhesive either film or paste adhesive. For the debonding simulation, Cohesive Zone Model method and Virtual Crack Closure Technique which have been successfully used in simulation of delamination of composite joints, were chosen. For the development of the Finite Elements models and the analyses, the commercial codes ΑNSYS and LS-DYNA were utilized. Where necessary, apart from the debonding simulation, the damage of the composite plates was simulated using progressive damage modeling method. This method implements an enriched group of Hashin-type criteria and a gradual degradation method which is based on strain softening. In the present thesis, for the simulation of the dobonding growth under fatigue loading, was developed a modified Cohesive Zone Model method which is based on the gradual degradation of the strength of the cohesive elements according to the elapsed fatigue cycles. Two degradation methodologies were developed; the first one is based on the fully developed cohesive zone and the second one on the effective element length. The second one has the advantage that is less mesh dependent. For the implementation of the modified Cohesive Zone Model method for fatigue loading, a parametric user-defined subroutine was created in LS-DYNA (MAT_USER_DEFINED_MATERIAL_MODEL) using Fortran programming language. The subroutine was created parametrically in order to be easily applied in bonded joints of different geometry and materials. All numerical models concerning specimens and structural elements were validated by comparing the numerical results from pseudostatic and dynamic analyses (fatigue and impact) to experimental results. The comparison was performed by means of load-displacement curves, debonding growth, strains and debonded area from impact loading. In all cases, good agreement was observed. Using the numerical models, the effectiveness of two debonding stopping features was studied and parametric analyses were performed concerning their geometric feutures. Concerning corrugation feature, numerical results revealed that in case of double cantilever beam (Mode-I loading) under pseudo-static loading, debonding growth is stopped in the area of corrugation. Furthermore, the results of the parametric study showed that there is not important influence of the diameter and the height of the corrugation in debonding growth stopping capability. However, corrugation is considered inefficient due to the fact that pseudo-static analyses and fatigue experiments in cracked-lap shear specimen showed debonding initiation and catastrophic failure on the area of corrugation in the early stages of loading. The bolt mechanism was at first studied in cracked-lap shear specimen with one bolt under pseudo-static and dynamic loads. In both cases the analyses showed important debonding retardation in debonding growth in the area of bolt. Based on these positive results the study extended to the structural element of the wide single lap shear. Initially, in the structural element considered artificial initial debonding defect. Two rows of bolts were modeled, each one at a distance from the edge of the defect.. For all studied cases, numerical results showed that the presence of bolts has an important influence in debonding retardation which leads to increased maximum load and maximum applied displacement of the joint. The influence of bolts is greater as its position is closer to the edge of the artificial initial defect. Afterwards, in order to examine a more realistic scenario of initial defect, a low velocity impact simulation was performed. In this case, since bolts were quite away from initial defect and the debonding growth was very fast the presence of the bolt had not important influence in debonding growth that was created under impact loading. From the results of the study concerning hybrid joints, arises that bolts could be a promising debonding stopping feature in adhesive joints. The main innovation of the present thesis is located on the development of a numerical methodology for the simulation of debonding growth and estimation of the residual strength of composite bonded joints of structural elements under pseudo-static and dynamic loads (fatigue and impact). The methodology was successfully applied in complex geometry joints with debonding stopping features and can be applied in different joint geometry and adhesive/adherent materials.