Donadio, Federica (2023) Optimization of hybrid composite structures for applications under dynamic loading conditions. [Tesi di dottorato]

Anteprima

Testo DONADIO_FEDERICA_35.pdf Download (3MB) | Anteprima

Abstract

Fiber reinforced composites play a significant role in several structural and no-structural applications. This work wants to investigate on the hybrid composite structures. In particular it is focused on the possibility to optimize the composite structures under dynamic loading. Composites provide several advantages compared to traditional metallic counterparts for high-performance products that need to be lightweight but strong enough for tough loading conditions. These are often subjected to dynamic loading conditions under low and high velocity impacts. The impact energy involved in these dynamic loading events can cause damage modes such as matrix cracking, matrix splitting, delamination, fiber-matrix debonding, fiber micro-buckling and fiber pull-out. Interaction of these damages can severely reduce the load carrying capacity of such structures. In this scenario fibre hybridisation have been extensively acknowledged as a strategy that can lead to improved composite properties and performance, because it changes the material properties but also changes the damage propagation mechanisms leading to failure. Hybrid composites are laminates obtained by combining two or more different types of fibers within a matrix, so the behaviour of these materials results a balancing act between disadvantages and advantages of each single fiber. The growing awareness about environmental problems and the environmental legislations induce the industry and society to focus on environmentally friendly materials. Reinforcements and matrices obtained from renewable resources, such as natural fibres and biopolymers, are the main players of this green transition. Moreover, hybridization of natural and synthetic fibers represents an important way to reduce the carbon footprint of traditional composite materials, while maintaining good mechanical performance. The scientific community is really interested in all possible kinds of hybridization by varying matrix, thermoset and thermoplastic, and fibers, natural and synthetic. The materials considered in this project include the traditional synthetic fibers normally used for hybrid composites as carbon and glass fibers but also new organic materials that are getting through in many industries. We are interested in studying the impact behaviour at both low and high speed because laminated composite structures are more exposed to impact damage, for example during maintenance or manufacturing operations tools might be dropped on the structure but also they are susceptible to dynamic accidents during life service. Moreover driven by the necessity to assess damage tolerance and durability of composites, required above all by military and aerospace industry, ballistic impacts and an analysis of multi-hit impact are performed. On composite structures, common visual inspections cannot be useful to investigate the internal impact damages that could develop under load and could cause critical strength decreases. For this reason, different non-destructive and destructive testing methods were taken into account for the evaluation of composite damages. The confocal laser microscopy is used for the identification of damages like the indentation, the plastic deformation impressed on the surface of the material during the loading phase at the contact point. Ultrasonic spectroscopy, an analysis based on absorption phenomenon clearly related to the material structure and periodicity and advantage, is utilized to investigate the internal damage and to identify the presence of defects like delamination and porosities. Another very interesting non-destructive testing used within this research on the composite laminates is the Holographic Interferometry, that can accurately track delamination growth and can also provide information on the through-the-thickness distribution of delamination. Thesis objectives: Fibre reinforced composites play a fundamental role in structural applications, however the optimization of their use is still in progress. In particular although the hybridization of composite materials was a large field of study since the 80s, in the last years, hybridization has become a field of interest mainly due to the possibility of delaying and achieving a more gradual failure of composite materials by controlling the damage mechanisms. The final purpose of this work is to develop the knowledge on the mechanical behaviour of hybrid composites under dynamic loadings, especially the effects that including fibres of different types has in the impact behaviour. The main focus of hybridization in this work is the improvement of impact response in composite materials and understanding the fibre and matrix properties required to achieve this goal. The ultimate goal is to design a composite material able to adsorb the impact energy and a good damage tolerance without damaging the main composite properties such as resistance, attractive for several industrial applications. The first step of this research project has been to investigate the impact behaviour of conventional (carbon, glass, basalt) composite materials. Then, hybrid composite laminates were tested at low velocity impact at penetration and high-speed tests at different impact velocity were carried out at room and low temperatures. Another goal of this thesis is the development of totally bio hybrid composite material with good mechanical properties. The effect of repeated low velocity impacts at different energy levels on vinyl ester composite laminates was, then, investigated.

Downloads

Actions (login required)

Modifica documento