Investigation and enhancement of the mechnanical properties of the fabric reinforced hybrid composites

Since the discovery of steel, metal materials have been the most preferred types of materials for engineering applications. However in the last century, there has been an ever-inreasing demand for novel materials with high specific mechanical properties, high stiffness-to-weight ratio and high strength-to-weight ratio which especially led the researchers to discover new materials by combining existing materials, namely known as composites. Most of the composite structures used in advanced applications are textile reinforced composites. The primary aim of this work is to create a novel generation hybrid composite structure with improved mechanical properties. When a hybrid structure comprising two or more different types of fibers is used, desirable properties of a fiber eliminate the undesirable properties of the other. As a result, a balance of performance and cost can be achieved with appropriate material design. It is also known that the fiber-matrix interface has a prominent role in the mechanical performance of textile reinforced composites. This contribution mainly relates to the ability of the interface to transfer mechanical load to the matrix reinforcement material during loading. It has been shown in the literature that the long-term performance and fatigue life of composites are effectively controlled by micromechanical damage processes occurring at the fiber-matrix interface. In this thesis, hybrid composite structures were manufactured by vacuum assisted resin transfer molding method using three different woven fabrics (jute, E-glass and carbon) as the reinforcing material and polyester resin as the matrix material. It was aimed to reduce the cost by adding jute fabric while by using E-glass and carbon fabrics to improve the mechanical properties. In addition, it was observed that previous studies on textile reinforced composites have generally used fiber or unidirectional structures as the reinforcement material. In this study, woven fabric structures were preferred because they show better impact strength and more stable structure. By using these materials firstly one type of fabric reinforced composite materials and nine hybrid composite materials in the form of binary combinations of these three different fabrics were produced. All composite structures comprise four layers of fabric. The void ratios, mechanical properties (tensile and impact strength) and thermal properties (dynamic mechanical analysis, thermogravimetric analysis and differential scanning calorimetry analysis) of these twelve different composite structures were investigated. When previous studies were examined, it was appeared that there were many studies about investigation of the mechanical properties of textile reinforced composites. In those studies, the effects of changing the reinforcement materials (different fiber types) and the alignment of the reinforcement material (fabric) on the mechanical properties of composite materials were investigated. Within the scope of this thesis, tests were performed with samples that taken from both directions of composite material, taking into consideration the weft and warp direction of the fabric, which is an important factor for the woven fabrics. The obtained results showed that the analysis results of the samples taken from these directions had a big difference and the fact that there was not any study considering the weft and warp directions of reinforcing material displays the deficiency in the field of textile reinforced composites. Since produced composite specimens were thought to be suitable for use in the automotive industry, it has been concluded that thermal analysis other than mechanical tests were also appropriate for these materials. These tests had been carried out in order to understand how the increase in the temperature of the internal parts during the operation of the automobiles influences the structure of the material and its thermo-mechanical properties. The results of the mechanical tests showed that the highest tensile strength result was obtained with the carbon fabric reinforced composite structure while the E-glass reinforced composite material was attained for the highest impact strength. The jute fabric reinforced sample had the lowest values in both test results. When the hybrid specimens were examined, it was understood that the best results in tensile strength tests were obtained with constructions in which the high tensile strength fabrics placed to the outer layers of the composite structure, whereas the higher impact strengths were achieved by placing fabrics which have high impact strength to the inner layers. At the second part of the thesis study, the effects of addition of nano filler materials on the mechanical properties of composite structures were examined. In this study, carbon nanotubes with a combination of excellent mechanical, electronic and thermal properties had been preferred as nanofiller materials. It was seen that in previous studies in order to improve the mechanical properties of composite materials, the most widely used type of multi-walled carbon nanotube (MWCNT) was pristine MWCNT and that there was not any study about MWCNTs with hydroxyl and carboxyl functional groups. In this thesis, pristine MWCNT, MWCNT-OH and MWCNT-COOH were used as the nano-filler material in order to determine how the MWCNTs with different functional groups affect the mechanical properties compared to pristine MWCNT. Thirty-six composite specimens were produced by adding these three nano-filler materials to twelve different constructions. Mechanical and thermal analysis of those thirty-six samples were performed and the results were compared with those of not nano-filler added composite materials. When examining the changes in mechanical properties, it was found that all three types of MWCNTs positively affected the tensile and impact strengths, and it was understood that the best results were obtained with pristine MWCNT added samples. In addition, void ratios, which have major effects on the mechanical properties of composite materials, were also calculated and it was found that void ratios were decreased by the addition of different types of MWCNTs. Dynamic mechanical analysis (DMA), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) analysis were performed as thermal analysis. All three tests were applied to composite samples as well as polyester resin and fabric samples. When the literature was examined, no study has been found on the application of dynamic mechanical analysis to fabric structures. It was seen from the DMA results that, the carbon fabric structure has higher moduli (storage and loss moduli) than the others and that with increasing temperature there is a decrease in the moduli of carbon and E-glass fabrics, while an increase in the modulus of jute fabric structure. Dynamic mechanical analysis results of composite specimens showed that the moduli were higher in the samples taken from the warp direction of the material and the MWCNT addition affected the modulus values positively. Thermogravimetric analysis of the fabrics revealed that the jute fabric structure was passed through two degradation stages at about 255°C and 345°C, and that the carbon fabric started to degrade at around 520°C, while E-glass fabric did not show any degradation till to 600°C. When the thermogravimetric analysis results of composite structures were considered, different types of MWCNTs had slightly increased the degradation temperature of composite materials. DSC analysis of both fabric and composite materials showed that the jute-containing samples showed an endothermic reaction at around 55°C due to the impurities and humidity in the jute fabric. Apart from the TGA results, it has been determined that the addition of MWCNTs has nearly no effect on the degradation temperature of the samples. The significance of the mechanical test results (tensile and impact strength) were examined by full factorial analysis method with the help of MINITAB software program and it was observed that the four selected factors (yarn material type, fabric direction, fabric stacking sequences and MWCNT type) had statistically significant effects on mechanical properties. It was seen that the most effective factor on the mechanical properties of the composite structure was the yarn (material) type, while the least effective one was the type of MWCNT. In addition, the bonding strength of reinforcement and matrix materials were investigated via scanning electron microscope images. The hybrid composite structures that produced as a result of this work is expected to be a suitable material for the automotive industry due to its mechanical properties and lightweight. As it provides less energy and fuel consumption in vehicles, lightness has become a necessity for automobiles. By taking into account the decline in fossil fuel supplies and global warming, it is clear that how lightness of a vehicle is an important factor for the environment. Within the scope of this doctoral dissertation, the below subjects that have not been carried out in previous works were studied on. These are;  Dynamic mechanical analysis of fabrics was carried out,  It has been proved that the mechanical and thermo-mechanical properties of the woven fabric reinforced composites vary considerably accordingly to the warp/weft directions of the fabric reinforcement,  Instead of epoxy resin that was used in almost all studies about E-glass and carbon reinforced composites, polyester resin which is lighter and cheaper than epoxy was utilized in this study to demonstrate that polyester based composites can also achieve sufficient mechanical properties for automotive industry.  It has been examined how the hydroxyl and carboxyl functionalized MWCNTs contribute to the mechanical properties of materials compared to pristine MWCNT and it has been proved that pristine MWCNT had higher effect on mechanical properties of textile reinforced hybrid composites. In the light of these studies, it is thought that the results obtained from this doctoral dissertation will guide the following studies about fabric reinforced polymer composite materials.