Fracture mechanical behavior of polymers: 1. Amorphous glassy state

Abstract

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.