291764 Crazing and Strain Localization of Polycarbonate Glass in Creep

Monday, October 29, 2012
Hall B (Convention Center )
Lilian Johnson, Department of Chemical, Biochemical, and Environmental Engineering, University of Maryland, Baltimore County, Baltimore, MD, Shiwang Cheng, University of Akron, Akron, OH and Shi-Qing Wang, Department of Polymer Science, University of Akron, Akron, OH

We apply constant tensile stress at different temperatures to examine the mechanical and structural responses of polycarbonate glass well below the glass transition temperature Tg. Unlike the more common displacement-controlled extension, the creep test allows the glass to develop structural failures on its time scales. In regime I of linear response, the applied stress is below a threshold that can cause failure of the primary structure made by intermolecular van der Waals bonds. Crazes form in regime II over time at higher applied tensile loads although PC is known to be ductile. At higher temperatures, crazes take a short time to form and can show up at a lower stress. Shear yielding and stable necking occur in regime III even when the applied stress is below the yield stress σy observed from a finite rate extension. The induction time for the strain localization decreases exponentially with increasing applied stress. The present work further elucidates the nature of both crazing and shear-yielding induced necking by examining the effect of melt-stretching. Recognizing that crazing arises as failure of the load transfer from the primary structure to the chain network, leading to localized structural rearrangements of the chain network through chain pullout, we show how melt-stretching completely suppresses crazing. Similarly, melt-stretched PC no long suffers shear yielding and necking and can extend uniformly during tensile stretching.

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