The last decade has been characterised by the progressive development of several biaxial testing equipment and procedures for the mechanical characterisation of coated fabrics. The few standards available in this field led to the development of several testing machines based on different loading principles to reproduce a biaxial stress state in a fabric. The most common approach is based on the symmetrical loading of a cruciform sample by means of four actuators, one on each side. However, other concepts have been successfully applied such as the “floating frame”, where the upper reaction frame is mounted on spherical bearings and is free to move in the plane of the fabric, and the use of rigid square frame with batteries of independent servomotors on each side . Two recent interlaboratory studies confirmed a fairly good correspondence in the values of stiffness obtained by the different laboratories.
The financial cost of each type of biaxial testing machine is directly related to the size, the accuracy and the level of flexibility of the testing equipment. However, the level of complexity and the absence of off-the-shelf products lead to costs which are not affordable by small research institutions, manufacturing companies and engineering offices. This research aims to unveil the potential of the new generation of single-board microcontrollers (e.g. Arduino and Raspberry-Pi) for a reliable and affordable biaxial testing machine which can drastically reduce the current costs associated to the biaxial characterisation of coated fabrics and foils.
The paper presents the development of a data logger based on Raspberry-Pi and able to record the data measured by four load cells a and three extensometers applied to a biaxial cruciform sample obtained from ETFE foils commonly used in Architecture. The results obtained from the experimental tests have been compared in order to estimate the level of reproducibility and accuracy which can be achieved through the cost-effective and affordable equipment.