Monitoring the calibration status of a universal testing machine through the implementation of acoustic methods : development of equipment and a suitable interface

Abstract

The calibration of a universal testing machine (UTM) verifies the accuracy of the system instruments responsible for obtaining force and displacement measurements. This process involves comparing the instrument to equipment that has already been calibrated to a known traceable standard. The limit of accuracy is then certified and the traceability of the measurements is determined. There are several internationally recognized standards that are used to calibrate the cross-head speed and displacement (ASTM E2658 and E2309, respectively), strain and load rate (ASTM E2309), measurement of tension, compression (ASTM E4) and dynamic force (ASTM E467). The current study aims to monitor the calibration status of UTMs through the implementation of acoustic methods. A methodology is developed whereby a reference sample is initially identified with suitable material properties, enabling it to be used continuously. The sample is used simultaneously with acoustic instruments to check its natural frequencies, which enables the monitoring of the UTM calibration status. An algorithm is developed that enables the user to interact with the system, thus forming an interface and helping the user to check the calibration status of the equipment. The entire system is validated to check if the equipment and the inbuilt algorithm can predict the calibration status of the machine. It was found that the geometric constraints imposed on the sample influence the output from the algorithm, and hence correct values should be fed to the system. Our sample never lost its elastic characteristics through continuous use, demonstrating that it can be used to continuously monitor the machine’s status.