Nondestructive Analysis On 4D-Printed Hygroscopic Actuators Through Optical Flow-Based Displacement Measurements

The many advantages of additive manufacturing are particularly noticeable in the fabrication of 4D-printed actuators. Through the selection of specific printing properties and materials, hygroscopic wood polymer composites (WPCs) can be produced and their reaction to humidity can be preprogrammed to achieve the greatest deformations in the shortest time. This responsive behavior makes 4D-printed WPCs suitable for architectural applications, where they can act as passive airflow controllers to improve hygrometric conditions in indoor environments. Image analysis methods have been proven to be reliable to select the best combinations of materials and properties for hygroscopic WPCs but, in some cases, they provide only information on the curvature angles and the instrumentation and software can be expensive. This paper presents an optical flow method for tracking the displacements through a free and open-source software. Starting from a time-lapse video of the sample immersed in water, the analysis returns a matrix composed of the 3D displacement values for each pixel in consecutive frames and the velocity of the displacement, with their visual representation. Such image analysis techniques proved to be suitable to assess the different hygro-responsive behavior under water of 4D-printed WPCs through low-cost equipment without altering their configuration.