Thesis title: 4D printed polymeric metamaterials: energy absorption performance and shape memory behavior.
Additive manufacturing technologies allow you to transform a virtual model into a finished product without the need to remove material (Ian Gibson, 2015). Additive manufacturing allows for pre-defined size products to be produced using a digital model or CAD (Ford, 2014). This technology, thanks to the use of bioplastic materials, allows to create objects with low environmental impact (Rishi Kumar et al., 2022). The environmental issue has been one of the most important in recent years. Therefore, the growing awareness of these issues is also influencing today’s industrial patterns. Another interesting aspect is the production of "reusable" products, which can be reused for the same application. This concept can be implemented by introducing a fourth variable: time, in 4D printing. In fact, a product which undergoes permanent deformation, for example through a collision, can recover its original form, totally or partially, by means of a thermal stimulus or other type (Dayyoub et al., 2022). These polymer materials are called Shape Memory Polymers (SMP).
Metamaterials, on the other hand, exploit the properties of SMPs and are engineered reticular structures, formed by repeated structural units, known as "meta-atoms" (Ding et al., 2022), arranged in a regular and orderly network. These units, which are considerably smaller than the wavelength of the radiation to be manipulated, such as visible light, microwave or sound waves, give the metamaterials their unique properties. They are also not designed to be isotropic, as they are mostly anisotropic and have properties that vary depending on the direction considered (Garcia et al., 2012).
Additive technology-based structures can be reused even after deformation, thus contributing to a more sustainable approach and reducing waste. This becomes even more relevant if reuse is combined with the use of bio-based materials such as polylactic acid (PLA). The use of PLA allows to reduce greenhouse gas emissions by 30% to 80% compared to traditional plastics, making the process more environmentally friendly (Rezvani Ghomi et al., 2021).
In the current scientific landscape there are many studies (Yousefi et al., 2023) (Zhang et al., 2020) (Hou et al., 2018), where different metamaterial structures are analyzed in terms of mechanical and absorption energy, considering a single deformation-shape deformation.
In addition, an analysis linking the geometry of the structure to the recovery properties of the original shape is missing at present, as well as an assessment of the energy absorption capacity as a function of progressive deterioration with increasing load cycles. These considerations could also be applied to structures with cellular configurations, subjected to dynamic as well as static tests (Li et al., 2020).
The present work aims to analyze the performance of cellular lattice structures, once subjected to various types of stress, such as compression, impact and bending, going to evaluate in addition to mechanical properties, also the energy absorption capacity. At the end of each study, the structure was chosen which, with the variation of cycles, retains most of its mechanical characteristics, showing limited damage and maintaining a high level of absorption energy. In addition, a mathematical model was added at the end of the experiment, with the aim of modelling the mechanical behaviour of the Lozenge Grid structure, which has proved to be a good absorber of energy both under compression and bending.
The thesis work was divided into 4 studies related to the analysis of lattice structures:
1. Analysis of compression stressed chiral structures with low and high displacement.
2. Impact stressed structures analysis, such as strut-based structures, Triply Periodic Minimal Surface (TPMS) and spinoidal structures.
3. Analysis of reticular structures, with negative stiffness and crossbow subjected to bending, with different mode of recovery of the form, both in water and in hot air.
4. Analysis of lattice structures stressed to bending, with recovery of shape at different time intervals.
Each activity has been developed for each academic year. The thesis was divided into nine chapters, the description of which is given here briefly.
Chapter 1: State of art
Chapter 2: Materials and methods
Chapter 3: Results and discussion: Analysis of the behaviour of chiral structures under compression stress
Chapter 4: Impact behaviour and energy absorption analysis of strut-based, TPMS and spinoidal structures
Chapter 5: Analysis of reticular structures, with negative stiffness and crossbow subjected to bending, with different mode of recovery of the form, both in water and in hot air.
Chapter 6: Analysis of lattice structures stressed to bending, with recovery of shape at different time intervals.
Chapter 7: Preliminary modelling of the mechanical behaviour of the Lozenge Grid structure.
Chapter 8: Conclusions
Chapter 9. Supplementary Information