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Anomalous Loss of Toughness of Work Toughened Polycarbonate University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Mechanical & Materials Engineering, Engineering Mechanics Dissertations & Theses Department of 12-2010 ANOMALOUS LOSS OF TOUGHNESS OF WORK TOUGHENED POLYCARBONATE Shawn E. Meagher University of Nebraska-Lincoln, [email protected] Follow this and additional works at: https://digitalcommons.unl.edu/engmechdiss Part of the Engineering Mechanics Commons, Mechanical Engineering Commons, and the Mechanics of Materials Commons Meagher, Shawn E., "ANOMALOUS LOSS OF TOUGHNESS OF WORK TOUGHENED POLYCARBONATE" (2010). Engineering Mechanics Dissertations & Theses. 14. https://digitalcommons.unl.edu/engmechdiss/14 This Article is brought to you for free and open access by the Mechanical & Materials Engineering, Department of at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Engineering Mechanics Dissertations & Theses by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. ANOMALOUS LOSS OF TOUGHNESS OF WORK TOUGHENED POLYCARBONATE by Shawn E. Meagher A THESIS Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Master of Science Major: Engineering Mechanics Under the Supervision of Professor Mehrdad Negahban and Professor Joseph Turner Lincoln, Nebraska December, 2010 ANOMALOUS LOSS OF TOUGHNESS OF WORK TOUGHENED POLYCARBONATE Shawn Edward Meagher, M.S. University of Nebraska, 2010 Advisers: Mehrdad Negahban and Joseph Turner Glassy polymers such as polycarbonate (PC) can be toughened through compressive plastic deformation. The increase in toughness is substantial, showing as much as a fifteen fold increase in the amount of dissipated energy during failure for samples compressed to 50% plastic strain. This toughness increase can be reversed through thermal aging at temperatures below the glass transition temperature (Tg = 147°C). The combined effect of plastic compression and thermal aging has been studied using Charpy, Single Edge Notch Bending (SENB), and Compact Tension (CT) tests. The tests mapped the response of samples cut along different orientations relative to the plastic compression as a function of plastic compression, thermal aging temperature and aging time. The Charpy and SENB tests were conducted on as received samples of PC that were 0%, 25% and 50% plastically compressed. The CT tests were done on annealed and quenched samples that were then compressed up to 35%. The Charpy samples were aged at isothermal temperatures between 105°C and 135°C. The SENB samples were aged at 105°C and 125°C. The CT samples were aged at 125°C. All samples were tested at room temperature. Three modes of failure were observed for the fracture surfaces, a low energy dissipation brittle fracture, high energy dissipation ductile fracture, and a mixed mode intermediate fracture. Toughening through plastic compression changed the failure mode of the PC from an initial low energy dissipation brittle fracture to an intermediate or high energy dissipation failure. Thermal aging reversed this process in some cases. This effect was mapped as a function of sample orientation, aging temperature and aging time. The dynamic Charpy test results were supported by the quasi-static SENB and CT results. The CT samples were small enough to provided samples in the third orientation, which had cracks propagating orthogonal to the direction of compression with crack surfaces that had a normal along the direction of compression. These samples failed with extremely low energy dissipation indicating that even though the compression improved the energy dissipation along the other two directions substantially (i.e., 5 to 15 times), it also resulted in a “weak” orientation. iv Acknowledgement I would first like to thank my advisers Mehrdad Negahban and Joseph Turner here at the UNL and Laurent Delbreilh and Jean Marc Saiter at the Université de Rouen. Their guidance and support has been instrumental to my success in this graduate program. I would also like to thank David Allen for introducing me to the international programs at UNL. Because of these men, I have been blessed with many opportunities that have helped me grow not only as an engineer, but also as a person. I also would like to thank the Army Research Lab for partially funding this project; as well as the Department of Education and FACE-PUF for their support. Finally, I would like to thank my parents, Michael and Judy, and my siblings. Without their loving support and encouragement, none of this would have been possible. v Table of Contents Acknowledgement ......................................................................................... iv Table of Figures ............................................................................................ vi Chapter 1. Introduction ................................................................................ 1 1.1 Polycarbonate ............................................................................................................ 2 1.2 Previous Work .......................................................................................................... 5 1.3 Tests Performed ........................................................................................................ 7 Chapter 2. Sample Preparation .................................................................... 9 2.1 Pretreatment .............................................................................................................. 9 2.2 Compression ........................................................................................................... 10 2.3 Thermal Aging ........................................................................................................ 11 2.4 Sample Machining .................................................................................................. 11 Chapter 3. Charpy Impact Tests ................................................................ 18 3.1 Introduction ............................................................................................................. 18 3.2 Procedure ................................................................................................................ 18 3.3 Results ..................................................................................................................... 21 3.4 Discussion ............................................................................................................... 30 Chapter 4. Single Edged Notched Bending ............................................... 35 4.1 Introduction ............................................................................................................. 35 4.2 Procedure ................................................................................................................ 35 4.3 Results ..................................................................................................................... 38 4.4 Discussion ............................................................................................................... 45 Chapter 5. Compact Tension Tests ............................................................ 48 5.1 Introduction ............................................................................................................. 48 5.2 Procedure ................................................................................................................ 49 5.3 Results ..................................................................................................................... 53 5.4 Discussion ............................................................................................................... 56 Chapter 6. Summary and Conclusion ....................................................... 58 Appendix A ................................................................................................... 61 SENB Load/Displacement Figures ............................................................................... 61 Appendix B ................................................................................................... 64 CT Load/Displacement Figures .................................................................................... 64 References ..................................................................................................... 70 vi Table of Figures Figure 1: PC monomer [2]. ................................................................................................ 2 Figure 2: Exaggerated change in orientation of PC polymer chains ................................. 3 Figure 3: Anisotropy of ultrasonic wave modulus measured in plastically compressed PC Samples [5]. ........................................................................................................................ 5 Figure 4: Uncompressed PC sheets & 50% (left) & 25% (right) compressed PC bars used to prepare charpy & SENB samples ................................................................................. 10 Figure 5: Compressed PC cylinders used to prepare CT samples, from left to right: 0%, 15%, 25%, 35% & 40% compression. .............................................................................. 11 Figure 6: Charpy sample geometry & notch diagram ...................................................... 12 Figure 7: SENB sample geometry & notch diagram ......................................................
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