# A Tunable Metamaterial Joint for Mechanical Shock Applications Inspired by Carbon Nanotubes
## 3 Experimental Characterization of the UV Resin After-Curing Mechanical Behavior  
For the purpose of the current research, a photosensitive resin was utilized as a raw material. The final parts were printed by using a 3D printer with Liquid Crystal Display (LCD) irradiation [32]. The LCD-based printing is a method of creating 3D objects using a light-emitting device and, thus, is very similar to the well-known stereolithography (SLA) method [33]. SLA aims at the curing of a specific target point of a layer of the component to be printed. The curing is achieved through the utilization of a focused laser beam, the size of which affects the resolution of the printed product [33]. Similarly, the LCD-based additive manufacturing process uses UV light to cure the resin raw material. However, according to the working principle of the photosensitive 3D printer with LCD light irradiation, specific image signals are provided by a computer program and displayed on a screen driving circuit. During the process, transparent zones appear on the screen before the initiation of the irradiation. During irradiation, the UV lights transmit through the LCD screen to form the part cross-section. A transparent film is set at the bottom of a liquid tank, which is placed on the surface of the liquid crystal screen. The UV light is radiated by the LCD screen to the photosensitive liquid resin via the transparent film. Thus, the resin is polymerized by the curing reaction and then solidified. The UV resin used in this study was provided by \"Anycubic\" and is a mixture of monomers, acrylates, and polymerization initiators. The limited information provided by the supplier refers to a resin that consists of 30-60% polyurethane acrylate, 10-40% acrylate monomer, and 5-10% photo-initiator. The specific resin selection was made only for testing and validation reasons. However, it was preferred over other similar materials due to its good specific strength and damping properties. Thus, in the future, according to the contribution and findings of the present study, a more appropriate polymeric medium could be adopted, which could be characterized by even more effectual mechanical properties, providing an improved version of the joint. Various specimens were prepared and printed using the \"Creality LD-002H UV Resin 3D Printer\", according to the ASTM standards for the testing of the tensile properties of plastics. Figure 3a-c presents the configuration of the used 3D printer, five additively manufactured specimens, and the utilized tensile testing equipment, respectively. In addition, Table 1 summarizes some basic specifications regarding the adopted 3D-printing machine and process parameters for manufacturing the standardized specimens. Three different groups of five specimens were prepared, corresponding to an after-print curing time of 1.5, 3.0, and 4.5 h. The measurements proved that the extended curing process leads to a higher mechanical performance regarding both the stiffness and tensile strength of the 3D-printed specimens. Consequently, the higher fracture toughness was observed for 4.5 h, and the obtained stress-strain curves for this case, using a small strain rate of about 1 mm/min at room temperature, are given in Figure 4. For this case, an average stress-strain behavior was extracted, denoted in Figure 4 with a red dotted line, to be inserted in the FEM-based models for the numerical simulation of the proposed joint. The specific average stress-strain behavior was expressed by an elastic modulus and a tensile strength of about \(E=842\) MPa and \(\sigma_{\text{UTS}}=76\) MPa, respectively. The fracture occurred at a strain of about \(\varepsilon_{\text{fr}}=15\)% after a distinct strain-hardening region. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{1}{c}{**Creality LD-002H UV Resin 3D Printer**} & \multicolumn{2}{c}{ \begin{tabular}{c} **ASTM Standardized 3D Printed** \\ **Specimens** \\ \end{tabular}