| Περίληψη: | The constant requirement of aerospace and space industries to enhance the structural efficiency as well as the increasing need to protect people and structures from various threats, including explosions and collisions, have driven to the usage of high-performance materials. Composite materials belong to that category due to their high specific stiffness and strength. However, they present some disadvantages such as their high raw material cost, the complexity of manufacturing, the difficulty in repairing as well as the susceptibility to impact damage and ply separation. This vulnerability of composites can result significant damage or even perforation which will lead to the degradation of their post-impact residual strength. In some specific applications including the defence protection systems and space shields, para-aramid fabric materials are often used either as reinforcement in composites for delamination and impact resistance or as pure dry fabric layers for protection from fragments.
The current thesis focuses on the numerical prediction of mechanical behavior of composites and dry fabric materials to impact loading. The objectives and the innovative elements of present investigation are summarized as follows:
1. Development of an experimentally validated methodology for establishment of a robust and accurate numerical model appropriate for prediction of fabrics behavior to ballistic impact.
2. Development of road map, starting from quasi-static material characterization tests, for the numerical prediction of residual strength of a CFRP laminate subjected to low and high-velocity impact. An innovative algorithm is proposed for calibration of the used material models.
3. Numerical simulation of hypervelocity impact response of a CFPR laminate and the produced secondary debris cloud using a FE-SPH hybrid model.
The detail content of the dissertation, that is composed of seven chapters, is presented below:
In Chapter 1, an overview of applications of composite and textile materials is provided. Specific emphasis was also given to impact threats in defence, aerospace and space sectors. Finally, the objectives and the innovative elements of present PhD thesis are shortly given.
Chapter 2 focuses on the fundamentals of composite and textile materials giving some useful introductory elements. The mechanical behavior of these materials to impact loading is marked and discussed.
Chapter 3 provides elementary data about the numerical methods in the field of computational solid mechanics, and it explains the difference between Implicit and Explicit time integration scheme. Finally, an overview of contact algorithms used in case of impact phenomenon is given.
Chapter 4, initially, investigates the simulation of quasi-static response of fabric materials subjected to uniaxial tensile loading. The purpose is to simulate the fabrics mechanical behavior accurately determining the quasi-static elastic and failure properties of fabric yarn and to rank, from the analysis point of view, the parameters which influence the fabric behavior. Afterwards, the current work studies the dynamic response of dry fabric materials subjected to ballistic impact loading. The aim is to simulate the dynamic behavior of fabric in order to capture the maximum fabric deformation, the perforation limit and the absorbed energy in case of fabric perforation considering the previously determined elastic and failure properties of yarns.
In Chapter 5, an innovative material model calibration procedure is proposed. The main target of this procedure is to approximate the best combination of material model parameters that will represent the experimental response of CFRP material to both quasi-static and dynamic loading. For the validation of used modeling technique and the proposed calibration algorithm, low and high-velocity impact tests at the impact energy level of 30 J were carried-out and simulated. Afterwards, the damaged specimens were tested to compression after impact loading according to AITM standard.
In Chapter 6, the hypervelocity impact response (HVI) of a carbon fiber reinforced polymer composite (CFRP) and the produced secondary debris are investigated using a hybrid FE-SPH model in LS-DYNA. The aim is to reproduce numerically the CFRP material response to hypervelocity impact and fragments cloud, to investigate the applicability of SPH modeling technique on composite materials and to determine the suitable numerical solution parameters. A verification procedure for modeling of composite laminate using SPH methodology is proposed. The investigation starts with some typical quasi-static tests in order to ensure the stability and efficiency of the SPH kernel function under quasi-static loading. Afterwards, the developed methodology is applied to hypervelocity impact response of CFRP laminate. The perforation limit, the crater diameter as well as the secondary debris cloud are numerically calculated; then they are correlated with the published HVI experimental results on CFRP plates.
Finally, in Chapter 7, the main conclusions of current work are summarized, and some proposals for future work are drew.
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