Focus on 4D Materials Design and Additive Manufacturing

The emergence of 4D printing has enabled the fabrication of various components that can change in response to external stimuli. Fused filament fabrication is one of the methods for creating shape-changing components using shape-memory polymer (SMP) filament. In order to exhibit the phenomenon of the shape memory effect, programming plays a crucial role. This article discusses two programming concepts, programming during printing (PDP) and programming after printing (PAP), for SMP processed by fused deposition modeling (FDM). We investigated the shape memory properties and other material behavior of PAP and PDP samples considering different thicknesses. We observe that PDP outperforms PAP in terms of shape memory properties based on various characterization tools like Fourier transform infrared spectroscopy (FTIR), x-ray diffraction (XRD), and field emission-scanning electron microscopy (FE-SEM), which are used for macro and microstructural features. Whereas, PAP shows better mechanical properties based on Nanoindentation analysis. The PDP samples achieved a maximum shape recovery of 99.25%, which is 44% higher than PAP for a 4 mm thick sample, and showed a 28% improvement in recovery compared to PAP for a 2 mm thick sample. Statistical analysis reveals significant differences in the means of recovery ratio and shape memory index between PAP and PDP, and no statistically significant difference is found for the fixation ratio. A shape recovery cycle life measurement has been carried out for a PDP bending actuator, which showed recovery until 140 cycles before complete failure. Finally, a working prototype demonstrating effectiveness of PDP and PAP for programming the same SMP in two different ways has been presented.

Controlling the printing parameters of four-dimensional (4D) printed actuators can be used to set the internal strain of the actuators. This approach can be utilised when using the fused deposition modelling method to develop 4D-printed actuators, allowing non-manual shape programming. However, there is a lack of comprehensive studies that investigate the effects of printing parameters on the actuation performance of 4D-printed actuators. In this study, the effects of four printing parameters on the bending angle of 4D-printed polylactic acid (PLA) actuators are reported. These printing parameters include the printing speed, printing temperature, ratio of passive-to-active layers, and layer height. In addition, these printing parameters are investigated while changing the height of the actuators. The results show that increasing the printing speed increases the internal strain while increasing the printing temperature, layer height, or actuator height has the opposite effect. Moreover, it is found that a ratio of passive-to-active layers of 50% maximises the strain while selecting a higher or lower ratio causes the opposite effect. Based on the results, four mathematical predictive models are developed to determine the bending angle induced in the actuators when printed based on each printing parameter. Then, a predictive model that relates all the printing parameters and actuator height to the bending angle is developed. The predictive model is based on the characterization results of 534 PLA actuators, providing an R-squared value of 0.98. Then, a finite element analysis model is developed to replicate the shape memory effect in actuators. To prove the accuracy of the proposed concept, two grippers with four and eight fingers are developed. The results show that the printing parameters can be used to control the bending angle of each finger based on the design specifications.

This study presents the design and development of a 2D auxetic filtering medium with programmable geometric features specifically designed to vary under in-plane tensile strain. This feature empowers the filtering medium to control the particles separation. A novel design and optimisation algorithm developed in Matlab^® determines the final optimized geometry of the filtering medium based on the desired particle size input. Upon thorough numerical investigation, an empirical relationship between the linear elastic in-plane tensile strain and aperture size of the proposed metamaterial is revealed. This empirical relation can be used in mechatronic and control systems to steer the proposed filtering medium. A prototype of such filtering medium capable of classification of particles of size 4 mm to 4.5 mm, when subjected to linear strain, is fabricated through fused deposition modelling process. The developed geometry configurations in this research are scalable, providing a potential cost-effective and efficient solution for industrial applications including reconfigurable filtration and segregation systems.

Shape-memory polymer (SMP)-based functional structures may now be produced more efficiently via four-dimensional (4D) printing, benefiting from the recent advances in multi-material three-dimensional printing technologies. Composite material design using 4D printing has opened new possibilities for customizing the shape memory property of smart polymers. This work studies a design strategy to harness desirable morphing by 4D printing multimaterial composites with a focus on the detailed finite element (FE) procedure, experimental results, and soft robotic application. Composites with bilayer laminates consisting of a SMP and a flexible elastomer are constructed with variable thickness ratios to control the self-bending of the composite. FE simulations are used to understand the underlying processes of composite materials and to generate accurate predictions for the experimental results, which reduces cost and development time. The application of 4D printing and multi-material composite programming is demonstrated with a soft robotic gripper for manipulating fragile objects.

The Cu-11Al-5Ni-4Fe wt% alloy was consolidated by additive manufacturing (AM) to determine the method applicability for producing shape memory alloy. The alloy was researched through compressive stress in three conditions: commercial (COM) (cast), as-built (AB), and AB heat treated (quenched). The results demonstrated that the AB sample acquired a reasonable superelasticity (SE) at room temperature (∼4%), which was improved to 6% after quenching. The COM sample damping capacity was better at high temperatures (350 °C) due to slip system activation at low stress (near 600 MPa), which resulted in a higher deformation energy dissipation. Due to the residual stress and null slip activation, the AB samples showed low damping capacity and low permanent strain at any temperature; however, they showed greater degree of SE. The AM technique of laser powder bed fabrication is concluded to be a viable option for producing printed parts with SE and damping properties.

The aim of this paper is to create novel 3D cubic negative stiffness (NS) structures (NSSs) with superior mechanical performances such as high energy absorption, shape recovery, super-elasticity, and reversibility. The conceptual design is based on an understanding of geometrical influences, non-linear buckling-type instability, snap-through mechanism, elasto-plastic deformation growth and plastic hinges. A finite element (FE) based computational model with an elasto-plastic material behavior is developed to design and analyze NSSs, saving time, material, and energy consumption. Material samples and meta-structures are 3D printed by selective laser sintering printing method. Material properties are determined via mechanical testing revealing that the printing process does not introduce much anisotropy into the fabricated parts. Experimental tests are then conducted to study the behavior of novel designs under loading–unloading cycles verifying the accuracy of the computational model. A good correlation is observed between experimental and numerical data revealing the high accuracy of the FE modeling. The structural model is then implemented to digitally design and test NSSs. Effects of the geometrical parameters of the negative stiffness members under three cyclic loading are investigated, and their implications on the non-linear mechanical behavior of NSSs under cyclic loading are put into evidence, and pertinent conclusions are outlined. In addition, the dissipated energy and loss factor values of the designed structures are studied and the proposed unit cell is presented for the energy absorbing systems. The results show that the structural and geometry of energy absorbers are key parameters to improve the energy absorption capability of the designed structures. This paper is likely to fill a gap in the state-of-the-art NS meta-structures and provide guidelines that would be instrumental in the design of NSS with superior energy absorption, super-elasticity and reversibility features.

Dielectric elastomer actuators (DEAs) usually suffer from rate-dependent viscoelastic nonlinearity, which manifests as hysteresis in their deformation cycles, leading to huge challenges in their modeling and control. In this work, we propose a model-free, proxy-based, sliding-mode tracking control approach to mitigate viscoelastic nonlinearity, achieving high-precision tracking control of DEAs. To this end, we first investigate the viscoelastic nonlinearity of DEAs, revealing its asymmetric and rate-dependent characteristics. Then, by combining the benefits of the PID control for small positioning errors and sliding-mode control for large errors, a proxy-based, sliding-mode tracking controller (PBSMC) is established. Finally, the stability of the controller is analyzed. To verify the effectiveness of the controller, several experiments are conducted to demonstrate the performance of DEAs in tracking sinusoidal trajectories under different frequencies. The experimental results demonstrate that with the PBSMC, the DEA can precisely track sinusoidal trajectories within a frequency range of 0.1 Hz–4.0 Hz by effectively minimizing the effect of inherent viscoelastic nonlinearity. Compared with open-loop tracking performance, the proxy-based, sliding-mode controlled DEA shows a significant reduction in maximum tracking errors from 45.87% to 8.72% and in root-mean-square errors from 24.46% to 3.88%. The main advantages of the proxy-based, sliding-mode control are: (a) it adopts a model-free approach, avoiding the need for complex dynamic modeling; (b) it can achieve high-precision tracking control of DEAs, thereby paving the way for the adoption of DEAs in several emerging applications.

The present work aimed to study the quasi-static compression behaviors of 3D printed continuous ramie fiber reinforced biocomposite corrugated structures (CFCSs) with excellent shape memory effects. The in-plane compression test was conducted to evaluate the effects of cell shapes, fiber volume fraction (f_v) and addition of fiber on the compression behaviors and energy absorption (EA) characteristics of the corrugated structures. The results showed that the compression property and EA capacity of the 3D printed CFCSs increased with decreasing f_v and the addition of continuous ramie yarn. The 3D printed continuous ramie fiber reinforced biocomposite with inverted trapezoid cell shape corrugated structures (CFITCSs) outperformed other cell shapes in the compression strength and specific EA. The analytical model for the in-plane compression strength of CFITCSs was derived, and predictions were in good agreement with measurements. In addition, continuous natural fiber reinforced composite structure for shape memory was proposed for the first time. The shape recovery testing results demonstrated that 3D printed CFCSs had the potential to be a key element of lightweight programmable smart systems.

Four-dimensional printing has set the stage for a new generation of soft robotics. The applications of rigid planar parallel robotic manipulators are also significant because of their various desirable characteristics, such as lower inertia, higher payload, and high accuracy. However, rigid planar parallel robots are heavy and require different actuators and components. This study introduces a novel technique to produce a light three degrees of freedom soft parallel manipulator at a low cost, which can be stimulated easily. This technique allows researchers to customize the actuator's design based on the requirement. The robot is made by 3D printing based on fused deposition modelling and a direct ink writing process. The design, development, and additive manufacturing of a soft parallel robot electrothermally driven by a linear silicon-based actuator and polylactic acid parts are presented. Silicon-based soft actuators replace the rigid conventional linear actuators in this study to drive the planar parallel manipulator. The actuation of actuators is conducted using simple heating compared to the conventional rigid actuator. Various heating approaches and configurations are compared and analysed to find the most suitable one for the effective linear stroke of the soft actuator. The finite element model is used to analyse the performance of the electrothermally silicon-ethanol soft actuators in ABAQUS. The kinematics of the planar parallel robotic manipulator are simulated in MATLAB to achieve its workspace. The final soft parallel robot mechanism and the active and passive links are fabricated and tested experimentally.