Understanding of the time-dependent fracture phenomena of polycarbonate

摘要

It is generally recognized that the excellent toughness of poly (bisphenol A carbonate), (PC), arises from the yielding process which allows large scale deformation. However, there is a change in the failure mechanism of PC from ductile to brittle depending on temperature, rate of strain, thickness, ... At present, the general practice is to design for strength and toughness without considering the aspects of time. The time dependene of the micromechanisms of deformation and fracture has been observed in many engineering polymers ranging from amorphous and semi-crystalline polyethylene terephthalate, poly-styrene, and polyvinyl chloride. The same problem is also recognized in non-polymeric materials, for example, a change in the mechanism from ductile failure of ligaments between voids to transgranular and intergranular microcracking in chromium steel under creep. In this paper, a new approach for modeling of the time dependent ductile-birttle response in PC is proposed. This approach is an extension of our recent work, where in the new model of the necking process in PC as a double glass transition was proposed. The draw stress in this model is a material parameter that defines a boundary between yielding and microcracking / crazing under uniaxial tension. Experimental evidence from delayed yielding under creep are provided to support this concept. A simple stress-temperature-time map of various failure mechanisms is proposed.