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This paper presents the design and fabrication of a magnetic double network (DN) composite, which consists of permanent magnet chains embedded in an elastomer matrix, and was capable of large yet fully recoverable deformation. The initially connected magnets served as reusable sacrificial components in the composite. The strong magnetic attraction between neighboring magnetics endowed the composite with the high strength while the compliance of the elastomer matrix provided the high extensibility. Having a similar mechanism as DN gels, the composite was found to be significantly tougher than either of the constituents. The nonlinear behavior in the composite separated it into two coexisting phases - a softer phase with separated magnet links and a stiffer phase with connected magnet links - which led to the stress plateau on the tensile curve. Further stretching was manifested by the growth of the disconnected softer phase at the expense of the linked stiffer phase, until all magnets were separated. The unloading curves appeared drastically different from the loading curves, as the force needed to separate two magnets was much higher than the force at which two separated magnets snapped back. Such asymmetry between loading and unloading was the main cause of the hysteresis in the stress-strain curve and the energy dissipation. To further understand the physical mechanism and the damage process of the magnetic DN composite, a simple model was developed to examine the deformation and damage dissipation process of composite. With very few parameters, the model predictions agree qualitatively with the measured properties of the material, and the difference can be further reduced by accounting for the interfacial friction/adhesion, a second means of energy dissipation. With a combination of desired properties including high stretchability, self-healing, and high toughness, the magnetic DN composite is a viable candidate for various applications.

Printed active composites (PACs) are capable of deforming from an initial shape to a target shape via spatial arrangements of active materials within a passive matrix. Multi-material polymer printers allow precise placement of multiple materials in the design space. However, Single active material is difficult to satisfy the demand in the high-precision matching of more complex target shape. Hence, a multi-material topology optimization approach for the design of PACs is proposed to achieve a target shape under a given stimulus. A multi-material interpolation function for active materials is established, and continuum mechanics modeling is used for simulating active material behaviors on a voxel basis to compute deformations for a given material distribution. The adjoint method is used for sensitivity analysis. Numerical simulations, of different target shapes with the same initial shape, verify the effectiveness of the method. The proposed method is feasible in the conceptual designs of PAC material distributions benefit with stable convergence, ease of implementation, and low computational costs.

Inspired by the toughening mechanism of double-network (DN) hydrogels, a soft composite consisting of a fabric mesh and VHB tape layers was fabricated. The composite was as stiff as the fabric mesh, and as stretchable as the VHB tape. At certain compositions, the composite was significantly stronger and tougher than the base materials. The extensibility and toughness of the composite can be attributed to a damage delocalization mechanism similar to that of the DN gels. In the partially damaged regions, the fabric mesh fragmented into small islands, surrounded by the highly stretched VHB tapes. Accommodated by the finite sliding at the interface, the large deformation of the composite is highly non-affine. Just as the DN gels, the coexistence of the partially damaged and intact regions resulted in a stable necking in the composite when subjected to uniaxial tension. The propagation of the necking zone corresponded to a plateau on the stress-stretch curve. During cyclic loading, the composite also exhibited stress hysteresis with almost recoverable strain, similar to that in a DN gel. To rationalize these observations and to better understand the underlying physical mechanism, a simple 1D model has been developed for the damage evolution process in the composite. The predictions of the model have achieved good agreement with the measured properties of the composite of various compositions. Furthermore, the composite itself may also be regarded as a macroscopic model when studying the properties and toughening mechanism of the DN gels.

Consisting of stretchable and flexible cell walls or ligaments, soft elastic foams exhibit extremely high fracture toughness. Using the analogy between the cellular structure and the network structure of rubbery polymers, this paper proposes a scaling law for the fracture energy of soft elastic foam. To verify the scaling law, a phase-field model for the fracture processes in soft elastic structures is developed. The numerical simulations in two-dimensional foam structures of various unit-cell geometries have all achieved good agreement with the scaling law. In addition, the dependences of the macroscopic fracture energy on geometric parameters such as the network connectivity and spatial orientation have also been revealed by the numerical results. To further enhance the fracture toughness, a type of soft foam structures with nonstraight ligaments or folded cell walls has been proposed and its performance studied numerically. Simulations have shown that an effective fracture energy one order of magnitude higher than the base material can be reached by using the soft foam structure.