  Theoretical analyses and experiments have been carried out to investigate
fracture behavior of glassy polymers. Our birefringence measurements quantify
the local stress buildup at cut tip during different stages of drawing. Based
on polymethyl methacrylate (PMMA), bisphenol A polycarbonate (PC) and
polyethylene terephthalate (PET), we find several key results beyond the
existing knowledge base. (1) The inherent fracture and yield strengths
sigma_F(inh) and sigma_Y(inh) differ little in magnitude from the breaking and
yield stress (sigma_b and sigma_y). (2) Stress intensification (SI) near a
pre-through-cut builds up deviates from the theoretical description of linear
elastic fracture mechanics (LEFM) upon approaching the cut tip. (3) SI meets a
natural cutoff below which stress ceases to increase. (4) The stress stip at
cut tip shows a trend of approximate linear increase with the far-field load s0
for all three polymers and different cut size a. (5) A characteristic length
scale P emerges from the linear relation between stip and KI. For these glassy
polymers, P is on the order of 0.1 mm. (6) Fracture toughness of brittle
polymers is characterized by critical stress intensity factor K_Ic =
sigma_F(inh)(2*pi*P)1/2, revealing relevance of the two crucial quantities. (7)
The critical energy release rate GIc for brittle glass polymers such as PMMA is
determined by the product of its work of fracture wF (of uncut specimen) and P.
(8) The elusive fractocohesive length Lfc defined in the literature as G_Ic/w_F
naturally arises from the new expression for G_Ic as stated in (7), i.e., it is
essentially P. These results suggest that a great deal of future work is
required to acquire additional understanding with regards to fracture and
failure behaviors of plastics.
