Wavelength Selective Multi-Material 3D Printing of Soft Active Devices Using Orthogonal Photoreactions.

Orthogonal photoreactions provide a unique way to locally and independently control (thermo)mechanical properties and functionality of polymer networks simply by choice of the wavelength. Herein, a library of acrylate functional coumarin monomers is synthesized, which are cured by sequence-dependent wavelength orthogonality. In the presence of a long wavelength absorbing photoinitiator, the monomers undergo rapid curing by visible light induced radical chain growth polymerization. Subsequent irradiation with light in the UV-A region selectively initiates the [2+2] photocycloaddition of the coumarin chromophores, which is confirmed by FTIR and UV-vis experiments. Through a well-targeted design, acrylate-based and thiol-acrylate resin formulations are prepared, whose fast curing rate, low viscosity, and prolonged storage stability enable the one-step fabrication of multi-material structures by digital light processing (DLP) 3D printing. By using a dual-wavelength printer, which operates at two different wavelengths (405 and 365 nm), objects comprising soft (ε = 22%, σ = 7.5 MPa) and stiff (ε = 2%, σ = 8.3 MPa) domains are printed with a single resin vat. Along with tensile properties, the wavelength selective change in the network structure features a local control of the glass transition temperature (ΔTg  = 17 °C) in the 3D-printed objects. Soft active devices are fabricated by dual-wavelength DLP 3D printing, with distinct domains having a higher Tg and the local programming of multi shapes is demonstrated.

Crack Propagation Analysis of Compression Loaded Rolling Elements.

The problem of crack propagation from internal defects in thermoplastic cylindrical bearing elements is addressed in this paper. The crack propagation in these elements takes place under mixed-mode conditions-i.e., all three possible loading modes (tensile opening mode I and shear opening modes II and III) of the crack are combined together. Moreover, their mutual relation changes during the rotation of the element. The dependency of the stress intensity factors on the crack length was described by general parametric equations. The model was then modified by adding a void to simulate the presence of a manufacturing defect. It was found that the influence of the void on the stress intensity factor values is quite high, but it fades with crack propagating further from the void. The effect of the friction between the crack faces was find negligible on stress intensity factor values. The results presented in this paper can be directly used for the calculation of bearing elements lifetime without complicated finite element simulations.

Size-Induced Constraint Effects on Crack Initiation and Propagation Parameters in Ductile Polymers.

Fracture mechanics are of high interest for the engineering design and structural integrity assessment of polymeric materials; however, regarding highly ductile polymers, many open questions still remain in terms of fully understanding deformation and fracture behaviors. For example, the influence of the constraint and specimen size on the fracture behavior of polymeric materials is still not clear. In this study, a polymeric material with an elastic plastic deformation behavior (ABS, acrylonitrile butadiene styrene) is investigated with regard to the influence of constraint and specimen size. Different single-edge notched bending (SENB) specimen sizes with constant geometrical ratios were tested. The material key curve was used to investigate differences in the constraint, where changes for small and large specimen sizes were found. Based on a size-independent crack resistance curve (J-R curve), two apparent initiation parameters (J0.2 and Jbl) were determined, namely, the initiation parameter Jini (based on the crack propagation kinetics curve) and the initiation parameter JI,lim (based on an ESIS TC 4 draft protocol). It was found that J0.2 and Jbl could be used as crack initiation parameters whereby Jini and JI,lim are indicative of the onset of stable crack growth.

Fracture of Thin-Walled Polyoxymethylene Bulk Specimens in Modes I and III.

Thin-walled polymeric components are used in many applications. Hence, knowledge about their fracture behavior in bulk is beneficial in practice. Within this study, the double cantilever beam (DCB) and out-of-plane double cantilever beam (ODCB) tests are enhanced to enable the testing of such bulk specimens in mode I and mode III on the basis of the J-integral. This paper then presents and discusses the experimental results following the investigation of a semicrystalline polymer (polyoxymethylen) under quasi-static load conditions. From the experiments, fracture energies of similar magnitude in both mode I and mode III were determined. In mode III, pop-in fracture was observed. Furthermore, the fracture surfaces were investigated regarding the mode I and mode III dominant crack growth mechanisms, based on the morphology of the tested material. For specimens tested in mode I, no signs of plastic deformation were observed, and the fracture surface appears flat. In mode III, some samples display a twisted fracture surface (twisting angle close to 45°), which indicates local mode I crack growth. A transfer of the presented methodology to other (more ductile) polymeric materials is deemed possible without further restrictions. In addition, the presented setup potentially enables an investigation of polymeric bulk specimens in mixed mode I+III.