Omid Eslami

Grade:  Master

Thesis Title:

Study of mixed-mode fracture toughness of thermosetting based polymeric hybrid micro/nanocomposites reinforced with thermoplastic phase exhibiting self-healing performance under thermal actuation

Year: Sept. 2019 - Jun. 2022.

Abstract:

With the development of nano science and considering that nano materials have characteristics such as small dimensions and as a result less defects, unique physical and mechanical properties, they were used as reinforcements of polymer composites. These materials are susceptible to microcracks for a long time and with high reliability by being exposed to environmental stresses or using polymer materials in structural applications. This defect causes a decrease in the mechanical properties, loss of durability and useful life of these materials and ultimately leads to failure and failure of the part. The heal of these defects is the most important issue for polymer matrix composites in advanced structures. For this reason, engineers have been inspired by natural self-healing to heal various materials, including composites. However, the use of self-healing mechanisms requires expensive basic materials and numerous and time-consuming construction steps, which regardless of the fact that these researches are still in the initial stage of their development and commercial application, as well as regarding the design and construction of mechanical structures in accordance with the mentioned applications, this methods are not cost-effective in terms of time and money. On the other hand, introducing a new formulation in the construction of self-healing structures and parts that can introduce self-healing parts with new technology with the approach of high-scale production capability with regard to economic efficiency is one of the necessities of research development and application of this class of materials. Also, most of the studies have investigated the biological self-healing properties or the appearance and visual properties of the samples. While in the design and manufacture of mechanical components by composites, the topic of mechanical properties and mechanical response of samples and parts, especially their impact resistance and fracture toughness properties, plays a fundamental role in the development of practical products in this field. Another issue in the field of self-healing composites is the lack of sufficient reference and extensive research on the properties of fracture toughness in mixed modes. While in many applications of self-healing composites, such as those seen in bone or structural implants, samples or parts are subjected to tensile or shear loads at the same time. For this research, it was tried to use common and economical materials and heal mechanism in such a way that it does not require high production costs of other methods. In this research, unsaturated polyester resin (UPR) is the reinforcing matrix and self-healing phase, and it is also responsible for power transmission. The secondary and restorative phase is polyvinyl chloride (PVC) microchips, which is the restorative component of the main phase with thermal stimulation. Carbon black microparticles (CB) and graphite nanoplate (GNP) were used as micro and nano reinforcements to change mechanical and thermal properties and create heat transfer. The experimental results on mechanical properties showed that PVC microchips had better results than pure UPR matrix polymer on properties such as impact resistance, tensile modulus, fracture toughness in 1, 2 and mix mode at low loads. The loading of CB microparticles had a positive effect on properties such as impact resistance, fracture toughness in 1, 2 and mix mode and improved these mechanical properties in some cases. Also, the loading of GNP nanoplate had an increasing effect on properties such as tensile modulus, strength and flexural modulus in some cases. The experimental results on the healing efficiency showed that the loading of PVC microchips increases the healing efficiency in the properties of impact resistance and mix mode fracture toughness in the first phase composite. Arcan disc and butterfly sample were used to extract the stress intensity factor in the mixed mode, so that the required geometric factor were extracted with the help of numerical modeling and applied to the stress intensity factor relationships. According to the numerical results, it was determined that until the loading angle of 70 degrees, the fracture toughness is dominant in mode 1, and after this loading angle, the fracture toughness of mode 2 becomes dominant. It was also found that the geometric stress intensity factor in mode 1 increases with the increase in the ratio of crack length to sample width, but the ratio of crack length to sample width does not have a great effect on the geometric stress intensity factor in mode 2 and includes minor changes.

Keywords: Self healing, Thermal healing mechanism, Mixed mode fracture toughness (I/II), Hybrid Micro/nanocomposites, Mechanical property, Impact resistance.