Simple shear plastic deformation behavior of polycarbonate plate II. Mechanical property characterization C.K.-Y. Li, Z.-Y. Xia, H.-J. Sue * Polymer Technology Center, Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843-3123, USA Received 19 August 1999; received in revised form 1 November 1999; accepted 4 November 1999 Abstract It has been demonstrated in Part I of this research that the high degree of plastic deformation caused by the equal channel angular extrusion (ECAE) process leads to a nearly uniform molecular orientation across the extruded polycarbonate (PC) plate. In this paper, the effectiveness of the ECAE process in altering the mechanical property of PC is investigated. Significant improvements in flexural modulus and fracture toughness are observed, and are found to be closely related to the ECAE-induced molecular orientation. Microscopy observations of the fracture process show that the crack propagation direction is controlled both by the ECAE-induced molecular orientation direction and by the maximum principal stress direction, which results in the formation of hackles. Potential advantages of ECAE for fabricating polymer parts are addressed.  2000 Elsevier Science Ltd. All rights reserved. Keywords: Equal channel angular extrusion; Molecular orientation; Polymer strengthening 1. Introduction It has been shown in Part I of this series that the solid- state, equal channel angular extrusion (ECAE) process is effective in shear-orienting macromolecular chains in poly- carbonate (PC) plates [1]. It has also been demonstrated that ECAE can be used to tailor micrometer and/or nanometer scale anisotropy via different extrusion routes [1–5]. This paper focuses on studying how the mechanical property of PC is affected by ECAE under two different routes: route A, and route C (Fig. 1). The uniqueness of ECAE is that it imposes a uniform, through-thickness simple shear deformation on the extru- date without altering the dimensions of extrudates. Depend- ing on whether the polymer is a semi-crystalline polymer or an amorphous polymer, extrusion is usually performed at temperatures below the melting temperature (T m ) or the glass-transition temperature (T g ) of the polymer to slow down molecular relaxation. This leads to a significant level of molecular and/or micro-domain orientation in the polymer extrudate. The uniformity and degree of molecular orientation observed in ECAE cannot be easily achieved by any existing conventional processing means, especially when the polymer extrudate is thick (2 mm). The FEM modeling results obtained previously indicate that both simple shear and the high strain rate gradient across the channel intersection plane of the ECAE die (Fig. 1) are responsible for the macroscopic orientation during the ECAE process [1]. However, a thorough under- standing of both mechanics and molecular orientation behavior is still needed so as to predict the molecular orien- tation during the ECAE process. According to Heymans [6], all reasonably successful descriptions of the deformation behavior in polymers require the knowledge of at least two components of strain. He maintained that the Brown–Windel model [7], which divides polymer deformation into a segmental orientation part and a chain extension part, is currently the most convin- cing model to address molecular orientation. The necessary deformation conditions for obtaining high segmental orien- tation are opposite to what lead to high chain extension [8], for example, high levels of segmental orientation are achieved by rapid stretching at temperatures close to, but below T g , which will result in low molecular chain extension. While the largest chain extension, on the other hand, is achieved at high temperatures (above T g ) by slow stretching. The discrepancy in macroscopic shear plane angle and in microscopic molecular orien- tation is discussed in this paper. The main objective of this study is to investigate the effectiveness of ECAE in improving mechanical properties of PC. The relationship between the fracture process and the ECAE-induced molecular orientation is established. Polymer 41 (2000) 6285–6293 0032-3861/00/$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S0032-3861(99)00837-X * Corresponding author. Tel.: +1-409-845-5024; fax: +1-409-862-3989. E-mail address: hjsu@acs.tamu.edu (H.-J. Sue).