Structured fabrics with tunable mechanical properties

Structured fabrics with tunable mechanical properties

Structured fabrics, such as woven sheets or chain mail armors, derive their properties both from the constitutive materials and their geometry. Their design can target desirable characteristics, such as high impact resistance, thermal regulation, or electrical conductivity. Once realized, however, the fabrics' properties are usually fixed. Here we demonstrate structured fabrics with tunable bending modulus, consisting of three-dimensional particles arranged into layered chain mails. The chain mails conform to complex shapes, but when pressure is exerted at their boundaries, the particles interlock and the chain mails jam. We show that, with small external pressure (about ninety-three kilopascals), the sheets become more than twenty-five times stiffer than in their relaxed configuration. This dramatic increase in bending resistance arises because the interlocking particles have high tensile resistance, unlike what is found for loose granular media. We use discrete-element simulations to relate the chain mail's micro-structure to macroscale properties and to interpret experimental measurements. We find that chain mails, consisting of different non-convex granular particles, undergo a jamming phase transition that is described by a characteristic power-law function akin to the behavior of conventional convex media. Our work provides routes towards lightweight, tunable and adaptive fabrics, with potential applications in wearable exoskeletons, haptic architectures and reconfigurable medical supports.

Smart fabrics are wearable materials that sense and respond to environmental stimuli, varying their properties, and/or measuring and communicating data to external recording devices. Their applications include medical monitoring, wearable computing and energy harvesting. They are usually fabricated by integrating'smart' components, such as flexible electronic circuits for sensing and computing, phase-changing materials for thermal regulation or photovoltaic materials for solar energy harvesting into conventional fabrics. However, most of these solutions focus on sensing and data communication. Fabrics with adaptable or tunable mechanical properties could provide mechanical feedback to human bodies and perform functions such as joint assistance, support and haptic perception. Here, we introduce a type of architected fabric, consisting of chain mail layers with designed particles, that can reversibly and gradually switch between soft and rigid states. Such a fabric, with its tunable mechanical properties, is a promising candidate for smart wearable applications.

Unlike other fabrics that are woven or knitted with continuous material (such as fibers and wires), the basic building blocks of our designed fabrics are discrete, granular particles. Assemblies of granular particles or laminar structures are known to undergo changes in their mechanical properties during jamming. Jamming is a phase transition that does not rely on temperature changes, as in ordinary materials, but is instead controlled by local geometric constraints in granular matter. The jamming transition enables disordered granular systems to switch reversibly between deformation with fluid-like plasticity or with solid-like rigidity, accompanied by a change of packing fraction. Jamming has been used to create smart materials with adaptive mechanical properties, for example, in soft robotic grippers, impact absorption materials and reconfigurable architectures. However, conventional granular materials are dense and large volumes are needed to provide enough mechanical stiffness when jammed. In addition, assemblies of convex particles do not support tensile forces. This limits their bending stiffness and tensile strength, making them unsuitable for wearable fabrics.

Fabrication and characterization

Inspired by ancient chain mail armor and topologically interlocked elements, we have designed a structured fabric consisting of two layers of interlocked granular particles. In our material, each particle is a hollow, three-dimensional structure, constructed from connecting trusses, designed to reduce the overall density and enhance contacts between elements. We chose octahedral particles because their ninety-degree rotational symmetry enables the formation of a square two-dimensional lattice in the interlocked configuration, and their sharp corners increase contacts between layers. The chain mail is created by rotating neighboring particles ninety degrees with respect to each other and by topologically interlocking all particles without forming solid connections. The interlocked lattice is manufactured using a selective laser sintering method, which prints the lattice in one piece without extra supports. We stack two chain mail layers to increase the number of contacts. Like chain mail armor, the resulting structured sheets can freely bend, fold and drape over curved objects. We avoid three-dimensional-printing chain mail with particles interlocked through thickness (that is, the two layers are stacked but are not physically linked) to allow sliding between layers, which results in higher flexibility.

To tune the effective mechanical properties of our fabric, we seek to trigger jamming between the interlocked particles, by applying variable compression at the boundary. We enclose the two layers in an airtight, flexible envelope, where the layers are weakly coupled and can still bend easily. To induce jamming, we apply a confining gauge pressure (pumping air out of the envelope), which causes confinement stress at the fabrics' boundaries. This increases the particles' overall packing fraction, triggering a jamming transition that increases the bending modulus and turns the fabrics into load-bearing structures.

To quantify the change in mechanical properties as a function of increasing confining pressure, we perform three-point bending tests and calculate the apparent elastic bending modulus of the fabrics. In these experiments, the samples are supported at two edges, and a line-shaped indenter is applied to the center of the top layer. The measured force-displacement curves ('displacement controlled' in Figure two a) show an initially linear regime, at small indentation depths, governed by the elastic behavior of the jammed granular structure. As indentation increases, a nonlinear response is observed, probably due to frictional sliding and local rearrangement of the particles. Although our fabrics are discrete and strongly anisotropic, we use an apparent elastic bending modulus E star as a parameter to compare the fabrics' mechanical properties under different conditions. E star can be calculated as:

E star equals K L cubed over four H cubed

Here, K is the stiffness of the initial elastic regime from the three-point bending test, and L, B and H are the length, width and height of the fabrics before the three-point loading tests. As the internal confining pressure increases from zero kilopascals to about ninety-three kilopascals, the apparent bending modulus increases monotonically, from about one point four megapascals to about thirty-six point three megapascals, by over twenty-five times. We note that the plateau reached by the apparent bending modulus at high confining pressures depends on our experimental setup (that is, the choice of the envelope). Different confinement solutions may allow higher bending moduli to be reached. It is important to emphasize that the jamming effect is controlled largely by contact topology. Compared to other variable-modulus materials, such as shape memory alloys, magnetorheological materials or electroactive polymers, our smart fabrics have the advantage of high modulus tunability without the need for applying large temperature changes or high electrical or magnetic fields, which are undesirable in wearable applications.