Universal testing machine for mechanical properties of thin materials

In this work, the design, construction, calibration and compliance measurement of a universal testing machine for tension tests of materials in film geometry are presented. A commercial load cell of 220 N and sensitivity of 1.2345 mV/V is used to measure the applied load. Material strain is measured by movement of the crosshead displacement of the machine with a digital indicator with 0.001 mm resolution and 25 mm maximum displacement, connected to a PC through an interface. Mechanical strain is achieved by an electric high precision stepper motor capable to obtain displacement velocities as low as 0.001 mm/s. The stress-strain data acquired with a GPIB interface are saved as a file with a home-made program developed in LabView 7.0. Measurements of the elastic modulus and yield point of a commercial polymer film (500 HN Kapton) were used to validate the performance of the testing machine. The obtained mechanical properties are in good agreement with the mean values reported by the supplier and with the values obtained from a commercial machine, taking into account the limitations of thin film testing and experimental conditions.