THESIS
2002
xxi, 172 leaves : ill. ; 30 cm
Abstract
Fracture toughness is an important material parameter in engineering applications. Many analysis methods were developed, such as Linear Elastic Fracture Mechanics (LEFM) and Elastic-Plastic Fracture Mechanics (EPFM), however, both methods are not suitable for fracture evaluation of ductile fracture of polymers, particularly for thin film. Hence, an alternative analysis approach, namely Essential Work of Fracture (EWF) , was introduced and developed in recent years. This method is applicable to very ductile polymers, even for thin film. In this study, some controversial aspects on the EWF applications were investigated and discussed, including the basic prerequisites and ligament length criteria. Furthermore, the specimen thickness dependence of EWF parameters was also studied. Due to th...[
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Fracture toughness is an important material parameter in engineering applications. Many analysis methods were developed, such as Linear Elastic Fracture Mechanics (LEFM) and Elastic-Plastic Fracture Mechanics (EPFM), however, both methods are not suitable for fracture evaluation of ductile fracture of polymers, particularly for thin film. Hence, an alternative analysis approach, namely Essential Work of Fracture (EWF) , was introduced and developed in recent years. This method is applicable to very ductile polymers, even for thin film. In this study, some controversial aspects on the EWF applications were investigated and discussed, including the basic prerequisites and ligament length criteria. Furthermore, the specimen thickness dependence of EWF parameters was also studied. Due to the existence of buckling phenomenon when EWF approach were performed on very thin film, a new designed fixture was developed, which can successfully avoid the buckling problem. Based on the results with and without buckling, the influence of buckling on EWF parameters was examined.
The main purpose of this work aims to explore the relationships between the crystalline morphology and molecular structure and the fracture toughness of ductile polymers. Some commercial semi-crystalline polymers, such as Polypropylene, Polyethylene and Poly (ethylene terephthalate) (PET) with different rubber modifier, crystalline phase, lamellae orientation, crystallinity, lamellae thickness and perfection, etc. were applied to study the fracture behaviours, using analytical methods including EWF. The effects of those crystalline morphologies on the fracture properties were evaluated. Moreover, some amorphous copolymers with different molecular structures were also employed to investigate its influences on the fracture resistance under various strain rates. The various fracture responses for both semi-crystalline and amorphous polymers were mainly explained in terms of the energy consumption capability associated with deformation mechanisms and stress redistribution ability ahead the crack tip and around the ligament region. Based on the results of this study, some microstructure-toughness relationships were proposed, which are believed to be helpful to the development of new polymer materials with high fracture toughness.
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