PSI - Issue 42

F. Cesarano et al. / Procedia Structural Integrity 42 (2022) 1282–1290 F. Cesarano, M. Maurizi, C. Gao, F. Berto, F. Penta, C. Bertolin / Structural Integrity Procedia 00 (2019) 000 – 000

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models, prototypes, or mock-ups of a proposed device. However, due to the ability of 3D printing technology to quickly build intricate or complex shapes, it soon became a primary fabrication method. 3D printing has evolved to bring out a novel concept called 4D printing, which is 3D printing with a time dependency [1]. This time dependency is achieved by printing with intelligent material [1]. The smart materials are called shape-memory materials, consisting of hydrogels, ceramics, metals, alloys, and polymers. They change shape due to exposure to specific stimuli, such as temperature, light, humidity, and electromagnetic radiation [1]. This phenomenon has been studied in the scientific literature concerning different fields of application, e.g., in van Manen et al. 2017 [2], the strains induced by thermal stress are analyzed for polylactic acid (PLA) components obtained from 3D printers. Based on this analysis, the authors propose self-assembling origami for various applications. In Lantada and Rebollo, 2013 [3], the reversibility of transformations for continuous applications in the industrial field is analyzed; in Lee et al., 2017 [4], various types of activation methods for shape-memory materials are compared to exploit better types of stimuli response (e.g., fabrics and paints) that may be implemented easily, quickly, and economically. Finally, in Yu et al., 2020 [5], similar tests are carried out as in van Manen et al. 2017 [2], both for PLA and Carbon Fibre composite PLA components, to evaluate the change in shape memory effect. Another fascinating aspect of this technology to consider is the reversibility of the transformation (transition from temporary to permanent shape) and the two-way characteristic (i.e. in the transition from temporary to permanent shape, different shapes can be obtained) of the generic shape memory material [6]. In Ke et al., 2020, [7], the difference between one-way and two-way SME mechanisms is explained in detail, and it is emphasized that one-way SMPs require, in some way, an external stimulus to reprogram the SME again after the SMPs have restored their original form after the temporary one. In contrast, bidirectional SMPs can transform from their original form to their temporary shape so that bidirectional components can be reused after each recycling [7]. In general, there are several strategies for reversible 4D printing. The most common is a two-layer or gradient structure (sandwich components); this additive composite printing technology utilizes different properties between layers (such as additional materials that exploit physical shape changes and water absorption) to achieve reversible shape changes. This strategy consists of a complex programming step to generate specific internal stresses that provide a driving force for reversible shape deformation. In practice, a support material, such as hydrogels, is used to store the stresses released after the activation of the SME in the transition from temporary to original shape [7]. Hence, 4D printing is a combination of four primary variables: the type of process used to print (e.g. fused deposition modelling (FDM) or stereolithography (SLA)), the material chosen, the stimulus applied, and the programming parameters (such as one- and two-way shape memory effect) [6]. Through the right combination of these four parameters, it is, therefore, possible to obtain 4D components with totally different properties, which can be used in various sectors and for different types of applications [6]. Consequently, it is easy to see that 4D printing is an accessible technology with many possibilities for further development and a high potential for application in many different fields, from aerospace to the design and development of everyday objects. However, this application is still in its infancy, as most of the works proposed in the literature represent ideas and design concepts which demonstrate this technology's potential and still need to be further developed through careful experimentation and analysis [6]. This status of knowledge is due to the wide combination of printing methods and innovative materials that can be used in 4D printing, which, combined with the different temporal variations and activation methods (stimuli), give rise to an infinite number of possible applications and types of 4D printing to be investigated. In the literature, therefore, there are qualitative rather than quantitative relationships between the various parameters that can be modified during the programming phase of the print and the activation phase of SME. In this context, the present work aims to find a suitable experimental method to study the SME associated with viscoelastic behaviour by analyzing the response of the smart material to a homogeneous thermal increase stimulus through various time temperature combinations and different programming parameters; this will be done, using PLA (one-way SMP) associated with the FDM technology (which is the most widely used combination in 3D and 4D printing [6]). After this brief introduction, the paper follows with the explanation of materials and methods (section 2) and the results and validation paragraph (section 3), where two subsections are presented: validation and development of the experimental methodology (section 3.1) and preliminary experimental results and their analysis through FEA (section 3.2). Finally, the conclusions and outlooks (section 4) briefly discuss the outcome and the further research steps.