Modeling Techniques and Experimental Studies of Woven Materials

Woven materials are used in a wide variety of applications, including apparel, architectural structures, inflatable structures, parachutes, woven fabric composites, and body armors. In addition to these existing applications, exciting new micro- and nano-scale technologies such as nano-enabled fiber materials, mechanically active fibers, microfluidics, and microscopic electronic components are enabling advanced fabric technologies to be developed. Examples may include fabrics with microfluidic cooling systems embedded in the yarns, fabric armor panels that can become rigid at the touch of a button, and garments with embedded medical sensors or flexible computers interwoven among the yarns. In order to develop such new technologies, and to better understand and improve existing fabric applications, the mechanical behavior of woven structures must be understood and effective modeling techniques must be developed.

The mechanical behavior of woven fabrics is complex. Fabrics are anisotropic and highly nonlinear. The behaviors in different modes of deformation (e.g. extension along the yarns, in-plane shear, out-of-plane bending and twist) are coupled in complex manners. Both elastic and inelastic responses can occur, including the poisson-like crimp interchange effect, the yarn jamming effect which dramatically stiffens the mechanical response at large shear angles, and yarn slip, a complex and non-local phenomenon where yarns slide through the weave and redistribute stresses, while potentially causing weave unraveling or failure. The underlying mechanisms that control these behaviors exist at the length scale of the yarns and the weave (millimeters or less), or even at the scale of the individual fibers that make up the yarns (tens of microns); while the resulting macroscopic behaviors affect fabric systems that range in size from tens of centimeters in size (armor panels) to hundreds of meters (stadium roofs).

We are developing multi-scale continuum models to capture the macroscopic response of the fabric while simultaneously tracking the evolution of the underlying fabric mesostructure. The mesostructural configuration is described with a set of stored state variables that are related to the macroscopic deformation state using continuity and equilibrium arguments. For a given mesostructural configuration, the corresponding mesostructural-level and macroscopic forces can be determined. This continuum modeling approach is more computationally efficient for the analysis of large woven structures than trying to individually model every yarn or fiber, and is easier to interface with other material models for the design of multi-component systems. However, because it tracks the mesostructural evolution, it predicts the fabric response under general loading conditions far more faithfully than other homogenization techniques. It allows the effects of changes to the mesostructure (e.g. a different weave pattern, or the presence of mechanically active fibers or interwoven electronics) on the macroscopic response to be predicted, as well as the mesostructural level forces (yarn tensions, contact forces) that correspond to macroscopic loadings. These mesostructural forces are extremely important for predicting failure when designing advanced woven technologies (e.g. whether microfluidic tubes or embedded electronics will get crushed when a garment is folded, or whether the yarn tension will exceed the breaking strength of the yarn during a ballistic impact).

In support of our continuum modeling efforts, we are simultaneously developing detailed models of the yarns and weave in order to better understand the controlling phenomena. We are also conducting extensive experimental studies of all aspect of fabric behavior, including uniaxial tension tests on single yarns and on fabric strips in various orientations, compression tests on fabric coupons, shear frame tests on fabric panels, microscopy on the yarns and the fabric, bending and twist tests on the yarns and fabric, biaxial tension tests on fabric coupons, yarn pullout tests, yarn-to-yarn friction tests, indentation tests, ballistic impact tests, and various tests involving failure propagation within a fabric sample. These tests are used to develop accurate constitutive relations describing the various meso- and macroscopic deformation mechanisms of the fabric, measure the associated constitutive properties, and to validate the models that we develop by comparing simulation predictions to experimental observations in tests involving complex load conditions.