The Interfacial Fracture Mechanisms And Toughening Study Of Foam Core Composite Sandwich Materials

Due to the combining mechanical features, e.g. high bending stiffness and low weight, and special functions, e.g. sound insulation, energy absorbability and heat control, foam core composite sandwich materials appear to be promising and have been widely used in aerospace, transportation and civil construction applications. However, the performance dismatch between face sheet and core always lead to such flaws as cracks and damages in the core or face/core interface during the processes of manufacturing or service. The loading capacity is subsequently reduced. Herein, this dissertation investigates the crack onset and growth process for the core and interface of composite sandwiches from macroscopic-mesoscopic views.In addition, a feasibility of face/core interfacial toughening with chopped fibers is studied by both experiment and mechanism analysis. This study is a sub-part of the project of National Basic Research Program of China (973 Project) "Advanced design optimization theory of ultralight porous materials and structures" (No.2006CB601205), National Science Foundation in China "Trans-scale study on dynamic initiation and propagation mechanism of interface crack for the composite sandwich materials" (No.10672027) and "Design optimization theory of multi-functional extra-weighted structures for the near space vehicle" (No.90816025). In addition, the financial support of the foundation (10702012) is greatly appreciated. The main research work can be summarized as follows:1. Study on the nonlinear fracture behavior based on the mesoscopic nonhomogeneity of the foam coreThe metallic and polymer foams are in common used cores for composite sandwich materials. Because of the nonhomogeneity of microscopic cells, the foams behave a class of materials with profound plastic compressibility and the yield strain or failure strength possess of a scattered characteristic. The yield strain and maximal cohesive traction are described by a Weibull type distribution function to study the influence of microscopic nonhomogeneity on the nonlinear fracture behavior. The hydrostatic pressure is also adopted in a continuum constitutive model to obtain the macroscopic constitutive relation. J-integral is calculated by finite element method to analysis the statistic features. Numerical examples are given to illustrate the effects of Weibull parameters on the constitutive relation and J-integral value. And the regularity of relative density on J-integral value is also obtained. 2. Study on the face/core interfacial crack initiation and growth for composite sandwich materials by considering the interfacial viscoelastic characteristic(1) For the sake of simplicity, most previous researchers assumed the composite sandwich material as a bi-material model consisting of the face sheet and the core. The face/core interface is simplified to ideal interface without thickness. However, for the interfacial fracture analysis of composite sandwich with a viscoelastic interface, that simplification is not adequately precise in some situations. Therefore, a novel three-phase medium model is established for a sandwich material with viscoelatic interfacial crack. A three-parameter standard solid material model is employed to describe the viscoelasticity of the interfacial layer. The geometric property is also taken into accout. A finite element procedure based on Rice J-integral and Kishimoto J-integral theories is used to analyze quasi-static and dynamic interfacial fracture behavior of the sandwich beam, respectively. The influence of viscoelastic, geometric characteristic of interfacial layer and the loading rates on the J-integral is discussed.(2) The cohesive zone model (CZM) not only combines the features of damage mechanics and fracture mechanics, but also presents many unique advantages, for example, the remesh is not necessary for the crack growth and the analysis precision is insensitivity to mesh size. So, cohesive zone model has been widely used for simulating interfacial crack growth. However, the commonly-used cohesive law is rate-independent, namely, the tractions within the cohesive zone depend only on the crack surface opening displacement, and are independent of the crack opening displacement rate. For sandwich materials, face/core interfacial crack within the adhesive layer is rate-dependent because of the interfacial viscoelastic characteristic. Based on the viscoelastic theory, the cohesive zone model in conjunction with different viscoelastic element is established to describe the characteristics of viscoelasticity for the adhesive layer. The parametric study shows that the influences of loading rate on the cohesive zone energy and strength are quite different for different models. Numerical examples are presented to demonstrate the efficacy of the rate-dependent cohesive model.3. The experimental and mechanism study of composite sandwich materials with chopped toughening interfaceThe study on face/core interfacial toughening of composite sandwiches is a worth project for theory and engineering. However, the published literatures on the interfacial toughening are rare for lack of effective experimental and trans-scale analysis mechanism. The sandwich beam specimens with and without chopped fiber toughening are manufactured by using the vacuum assisted resin injection process. And an experimental investigation is also performed to determine interfacial fracture toughness for the sandwich specimens. A meso-mechanical model considering energy dissipation is adopted to describe the process of single fiber peeling and pull-out as the crack growth. And a stochastic uniform distribution model is established to characterize the energy dissipation of overall chopped fibers per unit area. A finite element model is used to simulate the process of interfacial crack growth for a double cantilever beam. The nonlinear spring element is employed to account for bridging stress caused by the chopped fiber. And the energy release rate is calculated by the virtual crack closure technique. The typical numerical examples are performed to simulate the interfaceial crack of composite sandwich beams. The influence of meco-parameters on the interfacial toughness is also discussed. The experimental and numerical results indicate that the chopped fiber toughening is an effective technique to improve the interfacial toughness of the sandwich structures.The analytical model, theory, method and conclusions provided in this dissertation would contribute to better understanding of the interfacial failure and toughening mechanisms of composite sandwich materials.