Development of efficient numerical models for the simulation of low velocity impact and compression after impact on composite structures
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Fiber reinforced composite materials are nowadays used in several industrial applications that pursue structural weight reduction to reduce fuel consumption. The stiffness-to-weight and strength-to-weight ratios of composite materials made them an excellent choice for aerospace applications. However, impact loads is one of the major design concerns of aeronautical structures made by laminate composite materials. It is especially the case of Low Velocity Impact (LVI) events that despite leading to barely visible impact damage can significantly reduce the mechanical performance of composite structures. Reliable numerical models can help in reducing the actual amount of physical tests that are time-consuming and costly. Nevertheless, impact simulations are computationally intensive and their application in large composite structures is limited. Furthermore, the numerical models require many parameters that affect their efficiency, accuracy, objectivity and robustness. The present thesis aims to define a clear and efficient methodology to build reliable numerical models for the LVI and Compression After Impact (CAI) simulation of composite structures that can be applied in challenging applications of scientific and industrial interest. Firstly, the present work describes a methodology to simulate LVI and CAI on composite laminates that is validated experimentally at the coupon level. The key definitions are discussed and especial attention is devoted to the definitions that affect the computational efficiency. Novel formulas, which are useful for optimum mesh discretization, are proposed to predict the cohesive zone length of composites undergoing delamination under pure fracture modes. A numerical benchmark of different finite element types and interaction technologies commonly used in the literature is performed to compare their computational performance and accuracy. Furthermore, criteria to efficiently define cohesive numerical parameters are proposed. Numerical simulations can help in the understanding of the damage sequence of polymer based composite laminates during an impact event, which can be a difficult task to perform experimentally when dealing with a large number of plies. The proposed methodology is applied to predict the LVI and CAI of thin ply fabric laminates, which is a computationally challenging case due the large number of plies and interfaces involved. The numerical results indicate that matrix cracking effects can be assumed negligible for the studied thin ply laminate while delamination and especially the fiber traction separation law shape are important for accurate predictions. Finally, the methodology is applied for the prediction of relatively large composite sub-components with the aim to show that the proposed methodology enables analyses at larger scales. It is demonstrated the potential of the methodology and employed techniques to address problems of industrial interest such as the strength prediction of both undamaged and damaged stiffened panels
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