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In this work, we investigate toughening mechanisms in soft polymers reinforced with stiff fibers, particularly focusing on the effect of the strength of the fiber-matrix bond on the toughness. We print polydimethylsiloxane with short milled glass fibers using direct ink writing, an extrusion-based 3D printing method. This process produces composites with aligned fibers. Fibers can be treated with acid prior to printing, which improves the strength of the fiber-matrix bond. This results in higher yield stress and toughness. The higher toughness of the composites can be attributed to intrinsic mechanisms such as matrix deformation and fiber pullout, as well as to extrinsic mechanisms like mechanical dissipation. The intrinsic toughness of the composites is analytically estimated using a micro-mechanical model and experimentally measured by stretching the composites in the direction of fiber alignment. Finally, we demonstrate partial healing of the fiber-matrix bond after initial pre-stretch. Thermal treatment of the damaged composites results in partial recovery of stiffness and toughness.

In this work, we report 3D printable soft composites that are simultaneously stretchable and tough. The matrix of the composite consists of polydimethylsiloxane containing octuple hydrogen bonding sites, resulting in a material significantly tougher than conventional polydimethylsiloxane. Short glass fibers are also added to the material. Prior to solvent evaporation, the material possesses a viscoelastic yield stress making it suitable for printing

Spatial variations in fiber alignment (and, therefore, in mechanical anisotropy) play a central role in the excellent toughness and fatigue characteristics of many biological materials. In this work, we examine the effect of fiber alignment in soft composites, including both “in-plane” and “out-of-plane” fiber arrangements. We take inspiration from the spatial variations of fiber alignment found in the aorta to three-dimensionally (3D) print soft, tough silicone composites with an excellent combination of stiffness, toughness, and fatigue threshold, regardless of the direction of loading. These aorta-inspired composites exhibit mechanical properties comparable to skin, with excellent combinations of stiffness and toughness not previously observed in synthetic soft materials.

The effect that twin activity in Magnesium alloys has in both local and global strain is investigated in this article. The need to investigate this effect is related to difficulties in associating the spatially varying twinning with both plasticity and failure of this particular class of alloys. To address this need, mechanical testing both inside and outside the Scanning Electron Microscope was conducted and it was coupled with Digital Image Correlation (DIC) deformation and Electron Backscattered Diffraction (EBSD) texture measurements. In addition volume recordings of Acoustic Emission (AE) and surface measurements of microstructural changes were used to further complement the in situ tracking of twinning activity throughout monotonic and cyclic mechanical loading experiments. The definition of appropriate monitoring regions allowed the direct correlation of evolving twin activity with associated deformation in Magnesium alloys, for the first time to the authors’ best knowledge, from which twinning contributions to strain localizations were computed.

A crack terminating at an arbitrary angle to the interface between two neo-Hookean sheets is investigated under plane stress conditions using finite deformation theory. The asymptotic crack-tip deformation and stress fields are analyzed as a function of the ratio of the moduli and the angle of the crack relative to the interface. Full-field numerical calculations and experimental studies validate the analytical results. A stretch-based crack growth criterion is developed using crack-tip field solutions. Such criterion can predict the delay of crack growth through the bi-material interface observed in experiments and can be extended to any heterogeneity and material.

Strain localization bands (SLBs) that appear at early stages of deformation of magnesium alloys have been recently associated with heterogeneous activation of deformation twinning. Experimental evidence has demonstrated that such “Luders-type” band formations dominate the overall mechanical behavior of these alloys resulting in sigmoidal type stress–strain curves with a distinct plateau followed by pronounced anisotropic hardening. To evaluate the role of SLB formation on the local and global mechanical behavior of magnesium alloys, an integrated experimental/computational approach is presented. The computational part is developed based on custom subroutines implemented in a finite element method that combine a plasticity model with a stiffness degradation approach. Specific inputs from the characterization and testing measurements to the computational approach are discussed while the numerical results are validated against such available experimental information, confirming the existence of load drops and the intensification of strain accumulation at the time of SLB initiation.

Advances in fabrication techniques have led to a proliferation of studies on new mechanical metamaterials, particularly on elastic and linear phenomena (for example, their phonon spectrum and acoustic band gaps). More recently, there has been a growing interest in nonlinear wave phenomena in these systems, and particularly how geometric parameters affect the propagation of high-amplitude nonlinear waves. In this paper, we analytically, numerically, and experimentally demonstrate the propagation of cnoidal waves in an elastic architected material. This class of traveling waves constitutes a general family of nonlinear waves, which reduce to phonons and solitons under suitable limits. Although cnoidal waves were first discovered as solutions to the conservation laws for shallow water, they have subsequently appeared in contexts as diverse as ion plasmas and nonlinear optics, but have rarely been explored in elastic solids. We show that geometrically nonlinear deformations in architected soft elastic solids can result in cnoidal waves. Insights from our analysis will be critical to controlling the propagation of stress waves in advanced materials.

In this work, we systematically study the mechanics of short glass fiber composites with a soft, hyperelastic matrix (polydimethylsiloxane). The fiber orientation can be controlled by fabricating the materials with direct ink writing, an extrusion-based 3D printing method. We specifically characterize the damage evolution in these materials when subject to tensile and cyclic loading by developing a micromechanical model, a continuum model, and by performing experiments. The stress-stretch behavior and damage evolution are highly-dependent on both volume fraction and fiber orientation. We focus primarily on loading that is parallel with the fiber alignment, which shows rich complexities as a function of volume fraction, including variable softening and non-monotonic increase of the yield strength. Unlike most prior work, we use in situ optical measurements and digital image correlation during mechanical loading to quantify the highly-nonhomogeneous stretch field at macroscopic and microscopic length scales, and to simultaneously visualize the mechanisms underlying the damage evolution and complexities in the stress-stretch response, namely fiber-matrix debonding. The micromechanical model is based on modifications to the classic shear lag model, which include incorporation of fiber length distribution as a function of volume fraction and use of Weibull distributions to account for the probability of fiber-matrix debonding. The continuum model is based on hyperelastic strain energy with damage parameters for both matrix and fibers. The combination of the two models is able to capture the stress-stretch behavior and damage evolution well, and also the Mullins-like, history-dependent cyclic loading behavior we observe in experiments.

International audience; Soft mechanical metamaterials can support a rich set of dynamic responses, which, to date, have received relatively little attention. Here, we report experimental, numerical, and analytical results describing the behavior of an anisotropic two-dimensional flexible mechanical metamaterial when subjected to impact loading. We not only observe the propagation of elastic vector solitons with three components-two translational and one rotational-that are coupled together, but also very rich direction-dependent behaviors such as the formation of sound bullets and the separation of pulses into different solitary modes. Ongoing advances in digital manufacturing technologies are enabling fabrication of systems with an unprecedented level of compositional and structural complexity [1-3]. This remarkable control of geometry has stimulated major advances in the design of mechanical metamaterials-designer matter with unique mechanical properties that are dictated by their engineered structure [4,5]. While initial efforts in the field have focused on systems with unusual linear properties, such as negative Poisson's ratio [6-8], negative stiffness [9,10], and negative thermal expansion [11,12], large deformation and nonlinearities have been recently embraced as a means toward new functionalities, including programmability [13], energy absorption [14], and shape transformation [15]. Moreover, it has been shown that highly deformable mechanical metamaterials can be designed to support the propagation of a variety of nonlinear waves with large displacement amplitudes [16-19], providing a convenient platform to study non-linear wave physics. However, to date the investigation of the nonlinear dynamic response of flexible metamaterials has been limited to one-dimensional (1D) systems. Here, we investigate the nonlinear dynamic response of a 2D flexible mechanical metamaterial comprising a periodic arrangement of squares connected at their vertices by thin ligaments [18,20,21]. Remarkably, our experiments and analyses reveal that several new physical phenomena emerge when subjecting the structure to low-energy impacts. First, our system supports the propagation of elastic vector solitons with three polarization components-two transla-tional and one rotational. Second, we investigate the effect of the anisotropy of the medium on the 2D nature of the soliton and find that such anisotropy plays a crucial role, leading to rich new nonlinear effects. For example, for propagation at 45°from the symmetry axis, a distinct focusing effect is observed. The pulse does not spread along either direction, suggesting that sound bullets may exist in our system. Moreover, we find that for most other propagation angles the wave separates into two distinct solitary modes, each following a principal direction of symmetry. While 2D nonlinear elastic waves have been previously studied in granular media [22-25], the monolithicity and printability of our system allow facile control of the architecture, and hence control of the system's nonlinear dynamic response, providing a powerful platform to explore, visualize, and engineer new wave phenomena. We start by studying experimentally the response of a 2D circular sample with 30 squares along its diameter when excited with an impactor [see Fig. 1(a)]. Our sample is fabricated out of polydimethylsiloxane using direct ink writing, an extrusion-based 3D printing approach [26].

Defects play a major role in determining the mechanical properties of materials. Examples of this span from dislocations, grain boundaries, and precipitates in metallurgy to voids and imperfect interfaces in natural composites. These can enable complex failure modes and balance competing mechanical properties. With the increasing adoption of additive manufacturing in both research and industry, there are unprecedented opportunities for controlling the internal composition and structure of a material system. However, the very nature of forming a material in increments produces a set of characteristic defects (e.g., void formation due to incomplete merger of new material), which results in local heterogeneity and anisotropy. Rather than seeking to prevent these characteristic defects, we utilize them to improve the damage resistance of printed structures by systematically controlling their distribution within a single material. Inspired by the well-known Bouligand structure found in tough natural materials, we use a helical build sequence in which each layer is added at a defined pitch angle relative to the previous layer. The resulting helical defect distribution can guide the crack tip during fracture and enhance damage resistance. Microstructural evidence from crack surface images and X-ray microtomography reveals intricate mixing of twisting and branching cracks during failure, and is explained via an analytical model. Since these improvements are obtained solely from the defect distribution within a single material, the findings of this work could improve the failure characteristics of a broad range of printed materials (metals, ceramics, polymers, and composites).

Materials 9(8), 662 (2016). doi:10.3390/ma9080662 special issue: "Acoustic Waves in Advanced Materials", Published by MDPI, Basel