Mechanics of Dynamic Fracture in Polycarbonate

Abstract

Fracture toughness of brittle amorphous polymers (e.g. PMMA) has been reported to decrease with loading rate at low rates and increase abruptly to close to 5 times its static value at very high loading rates. Dynamic fracture toughness that is much higher than the static values has attractive technological possibilities. However, the reasons for the sharp increase remain unclear. Motivated by these observations, the present work focuses on the dynamic fracture behavior of Polycarbonate (PC), which is also an amorphous polymer but unlike PMMA, is ductile at room temperature. The objective of this paper is to investigate if PC also shows a behavior similar to PMMA, with a view to understanding the mechanics of the increase. Towards this end, a combined experimental and numerical technique is adopted. Dynamic fracture experiments at varying loading rates are conducted on single edge notched (SEN) specimens using Hopkinson bar with ultra high speed imaging (> 105 fps) to observe dynamic processes during fracture. Concurrently, 3D dynamic finite element simulations are performed using a well calibrated material model for amorphous polymers. Based on the experimental observation and numerical studies, mechanics behind dynamic fracture in PC is explained in detail. It has been concluded that the final fracture toughness remain invariant with loading rate. However, the void initiation toughness is higher in dynamic loading compared to that in static, due to rapid expansion of void leading to radial crazes emanating from it. With further studies on void dynamics, a mean stress based criterion for void initiation and then plastic strain based criterion for final fracture of PC is also established.