On the Strain Measurement for Thermoplastics with Bi-Axial Extensometer in Thermo-Mechanical Testing: A Case of Characterizing Temperature and Physical Aging Effects on Polycarbonate

Background Characterizing the mechanical behavior of materials at elevated temperatures is critical for the design and development of polymer systems for use in complex operating conditions. The commonly used Dynamic Mechanical Analysis (DMA) does not apply to the investigation of the three-dimensional properties of materials. A combination of a mechanical testing system and an environmental chamber with extensometer measurements is more suitable for this purpose. However, the bi-axial extensometer suffers errors in measuring strains in thermoplastic specimens at elevated temperatures due to its characteristics. Objective This brief technical note analyzes the source of measurement errors with an extensometer and proposes a robust and straightforward experimental procedure for three-dimensional mechanical testing at high temperatures. Methods Two temperature and physical aging effects characterization experiments on polycarbonate at 120 degrees C were performed as a case study. Thermal and penetration drifts were analyzed and corrected in the axial and transverse measurements. Results The corrected experimental results for the two tests are nearly identical, attesting to the reproducibility of the proposed procedure. Furthermore, the bulk-related strain computed using the corrected strain increases monotonically with time, consistent with the thermodynamic principle, thus demonstrating the reliability of experimental results. Conclusions The methodology described in this work can serve as a protocol to guide the three-dimensional thermo-mechanical testing with a bi-axial extensometer.