INVESTIGATIONS ON THE FRACTURE BEHAVIOR AND NOTCH SENSITIVITY OF INJECTION MOLDINGS

BERNARD CHUKWUEMEKA OGAZI-ONYEMAECHI

2010-10-06T01:08:14Z

2010-10-06T01:08:14Z

2010-09-24

The prevention of catastrophic failures in engineering components depends largely on the accurate prediction of their fracture mechanism. Much research efforts have been made to understand the fracture toughness of polymeric materials and composites but these are limited to investigations conducted on samples introduced with single-depth standard V-notches. However, there is inadequate understanding on the notch sensitivity of injection moldings especially when a hierarchical skin-core structure is present. Therefore, this thesis focused on characterizing the fracture toughness of injection molded polyethylene terephthalate (PET) and glass fiber reinforced polypropylene (GFPP) composites, both of which are known for being notch sensitive. Chapters 3 and 4 of this thesis were dedicated to understanding the effects of standard V-notch and sharper hairline cracks, respectively, on the fracture mechanism of neat- (V-PET) and recycled-PET (R-PET) injection moldings. The PET specimens exhibited very distinct skin-core structures and the depths of these regions were well defined. Upon tensile test, the specimens would remain semi-ductile as long as the V-notch depth was within the skin region. The skin region was tougher due to its lower crystallinity and would therefore, yield extensively during tensile loading. However, brittle fracture was imminent when the notch penetrated the skin into the core, which had higher crystallinity than the skin. Nevertheless, except upon annealing, the toughness of the skin region was compromised when sharper hairline cracks were introduced. The dependence of crack-tip temperature on stress intensity and notch sensitivity was investigated in Chapter 5. High crack-tip temperatures would indicate full ligament yielding, which provided the necessary requirements for the application of essential work of fracture principles in characterizing the fracture toughness of the specimens. Chapters 6 and 7 of the thesis were dedicated to investigating the effect of polypropylene (PP) molecular weight on the fracture toughness of glass fiber reinforced PP (GFPP) composites. Elastic plastic fracture mechanics have revealed that GFPP containing the lowest molecular weight PP (J-3000GP) exhibited the highest fracture toughness due to its high interfacial shear strength that resulted in more fiber breakage and better resistance to crack propagation.

所属：大学院工芸科学研究科博士後期課程先端ファイブロ科学専攻；取得大学：京都工芸繊維大学 ; 取得学位：博士（学術）; 学位授与年月日：2010-09-24 ; 証書番号：博甲第５８３号

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