Paul V. Cavallaro

Mechanics of plain-woven fabrics for inflated structures

Pressurized fabric tubes, pressure-stabilized beams (known as air beams) and air-inflated structures are considered to be valuable technologies for lightweight, rapidly deployable structures. Design optimization of an inflated structure depends on a thorough understanding of woven fabric mechanics. In this paper the bending response of woven pressure-stabilized beams have been experimentally tested and analytically investigated. Additionally, the micromechanical effects of interacting tows have been studied through finite element models containing contact surfaces and nonlinear slip/stick conditions. Local unit cell models consisting of pairs of woven tows were created to characterize the effective constitutive relations. The material properties from the unit cell models were then used for the global continuum model subjected to 4-point flexure. An experimental set-up was designed and manufactured for testing of Vectran and PEN air beams. The air beam mid-span deflections were measured as functions of inflation pressure and bending load. Plots of the elastic and shear moduli with respect to the pressure and coefficient of friction have been generated. It was determined that the effective elastic and shear moduli were functions of inflation pressure, the material used and the geometry of the weave. It was shown that pneumatic or pressurized tube structures differ fundamentally from conventional metal structures.

Decrimping Behavior of Uncoated Plain-woven Fabrics Subjected to Combined Biaxial Tension and Shear Stresses

Tension structures continue to be of increasing importance to military applications requiring minimal weight, small packaging volumes and enhanced deployment operations. Presently, design methods for inflated fabric structures are not well established. Analysis tools for their efficient design lag behind those for conventional structures, partly because woven fabrics do not behave as a continuum. Changes in fabric architecture occur with loading and lead to several sources of nonlinear response. In particular, effective constitutive relationships must be developed that institute the combined effects of biaxial tensile stresses from inflation and shear stresses from bending for use in structural models. Through analysis and experiment, this study addressed these architectural changes, such as crimp interchange, and their effects on the mechanical properties of uncoated plain-woven fabrics. This was accomplished through meso-scale finite element analyses and material tests using a recently developed experimental fixture. The fixture facilitated testing of a wide variety of fabrics (woven, braided, knitted, etc.) subjected to combined biaxial tensile and shear stresses. The meso-scale models and swatch-level test results confirmed that: (1) crimp interchange profoundly influenced the fabric elastic and shear stiffnesses, as changes in crimp heights occurred with increasing biaxial tensions, (2) the shear modulus was highly dependent upon the biaxial tensions and compaction of the tows at the crossover points and (3) the shear modulus was highly nonlinear and was not monotonic with rotation and shear force. This study also presents analytical and experimental methods to ascertain the elastic and shear moduli of woven fabrics for use in evaluating the performance of air beams.

Advances in Fabric and Structural Analyses of Pressure Inflated Structures

Novel methods for analyzing the response of air inflated fabric structures are presented. The first method determines the global structural response of air inflated beam and arch structures. It employs a previously developed specialized finite element. The element was derived by minimizing the strain energy potential for a cylindrical membrane deforming about its pressurized state. Through the use of displacement approximations defining the motion of the beam’s cross section, analogous to classical beam theory, the energy principle is reduced to one dimension. However, the effect of the pressure is included in the formulation. Numerical results compare favorably to experimental data for air beams constructed from Vectran® . The second method is based on the micromechanics of plain-woven fabrics. It employs nonlinear kinematics to predict the load-displacement response of a biaxially loaded fabric. Based on the fabric strip model, this method includes the effects of crimp in nonlinear kinematic material behavior and estimates values of effective material properties in tension and shear.Copyright

Crimp-Imbalanced Protective (Crimp) Fabrics

This report documents research that was conducted to explore the unique concept of using crimp imbalance, which is a simple architectural modification achieved during the weaving process, as a potential mechanism to enhance fragmentation and ballistic protection levels of single-ply woven fabrics. It is shown in this report that crimp imbalance (1) can substantially influence the energy-absorption levels of single-ply fabrics for select fragment simulating projectile (FSP) velocities and friction coefficients; (2) can be tailored to controllably delay stress-wave propagations among yarn directions; and (3) can minimize reflections at the yarn crossover regions. This research, which used numerical models of single ply, plain-woven fabric, demonstrated that deviations in crimp contents can have significant effects on energy absorptions and projectile residual velocities; in short, optimal levels of crimp imbalance may exist for a specific ballistic threat type.Copyright

Effects of Crimped Fiber Paths on Mixed Mode Delamination Behaviors in Woven Fabric Composites

Abstract : This research investigated the fracture toughness and crack propagation behaviors of woven fabric polymer composite laminates subjected to single- and mixed-mode loadings using numerical models. The objectives were to characterize the fracture behaviors and toughness properties at the fiber/matrix interfaces and to identify mechanisms that can be exploited for increasing delamination resistance. The mode I and mode II strain energy release rates G(sub I) and G(sub II), the effective critical strain energy release rate Gc_eff, and crack growth stabilities were determined as functions of crimped fiber paths using mesoscale, two-dimensional multi-continuum finite-element models. Three variations of a plain-woven fabric architecture-each of which had different crimped fiber paths-were considered. The presence of mixed strain energy release rates at the mesoscale due to the curvilinear fiber paths was shown to influence the interlaminar fracture toughness and was explored for pure single-mode and mixed-mode global loadings. It was concluded that woven fabric composites provided a fracture toughness conversion mechanism (FTCM) and their toughness properties were dependent on and variedwith position along the crimped fiber paths. The FTCM was identified as an advanced tailoring mechanism that can be further utilized to improve toughness and damage-tolerance thresholds especially when the mode II fracture toughness G(sub IIc) is greater than the mode I fracture toughness G(sub Ic).

Air-inflated fabric structures

Air-inflated fabric structures are categorized as pretensioned structures. They are capable of many …

Structural Analyses & Experimental Activities Supporting the Design of a Lightweight Rigid-Wall Mobile Shelter

Lightweight rigid-wall shelters used in mobile military operations are often constructed of sandwich panels comprised of thin face sheets and thick, yet ultra light core materials to minimize weight while maximizing structural integrity. The key structural advantage of sandwich panel construction (SPC) versus homogeneous panel construction (HPC) is the potential for up to an order of magnitude weight reduction while matching equivalent bending stiffnesses. Additional advantages include increases in damping, acoustic and thermal insulation, and possibly ballistic protection performance for a given areal weight density. However, these advantages come at a cost, which often impact the design and manufacturing complexities of critical joints used to connect the sandwich panels in a box-like assembly. Furthermore, stiffnesses of these joints are often difficult to characterize and their finite values significantly influence panel deflections and rotations. While mobile rigid wall shelters must be certified for several transport loading environments including rail impact (vehicle mounted and dismounted), drop shock, mobility and external air transport (EAT), the present effort, addresses survivability against conventional air blast effects. This study employed combined experimental and analytical approaches at the material and sub-structural levels to (1) generate accurate shelter models, (2) validate the material- and sub-structural models and (3) maximize the shelter’s global performance against a conventional air blast event early in the design stage to avoid costly physical tests. The material level tests focused on the mechanics of the assembled constituents that formed the sandwich panel and the benchmarking of an appropriate finite element to predict the displacement, stress and strain responses. The sub-structural level tests focused on loading a structurally representative shelter section to determine the joint behaviors and stiffnesses for model benchmarking purposes. Finally, a complete rigid-wall mobile military shelter model was constructed and its modal behavior was characterized followed by its complete dynamic response to an air blast event.

Bending Behavior of Plain-Woven Fabric Air Beams: Fluid-Structure Interaction Approach

A swatch of plain-woven fabric was subjected to biaxial tests and its material characterization was performed. The stress-strain relations of the fabric were determined and directly used in finite element models of an air beam, assumed constructed with the same fabric, subjected to inflation and bending events. The structural responses to these events were obtained using the ABAQUS-Explicit[1] finite element solver for a range of pressures including those considered typical in safe operations of air inflated structures. The models accounted for the fluid-structure interactions between the air and the fabric. The air was treated as a compressible fluid in accordance with the Ideal Gas Law and was subjected to adiabatic constraints during bending. The fabric was represented with membrane elements and several constitutive cases including linear elasticity and hyperelasticity were studied. The bending behavior for each constitutive case is presented and discussions for their use and limitations follow.Copyright

ENERGY ABSORPTION MECHANISMS IN HARD EPOXY RESIN MATRIX COMPOSITE LAMINATES SUBJECTED TO DYNAMIC LOADING EVENTS

Three energy absorption mechanisms of severe dynamic loading events are numerically investigated using a nite element model of a cross-ply unidirectional (UD) composite laminate. In this study, the inelastic energy absorption mechanisms associated with damage at the interfacial and constituent levels were numerically characterized through three admissible failure modes: ber breakage, matrix shearing, and ber/matrix debonding(delamination) (i.e., cohesive failure). The UD composite was constructed of ultrahigh molecular weight polyethylene (UHMWPE) bers separately reinforced with a rigid (epoxy) matrix material. The energy absorption capacities of these damage mechanisms were contrasted for three dierent dynamic loading cases including blast, shock, and ballistic impact at three dierent energy levels. Energy loss due to cohesive failure was observed in all three loading cases and energy levels. Furthermore, energy loss due to matrix failure was observed at all energy levels for the blast case, but only for the highest energy level in the shock and ballistics. There was energy loss due to ber failure in the blast and in the highest energy ballistics impact case. However, there wasn’t any ber damage in the shock case.

Mechanics of Air-Inflated Drop-Stitch Fabric Panels Subject to Bending Loads

Abstract : Rapid deployability and mobility of lightweight structures, namely inflatable structures, are of growing significance to the military and space communities. When deployment and rigidity are driven by pressure (for example, air or fluid) and materials such as textiles, elastomers, and flexible composites are used for the structure, significant load-carrying capacity per unit weight (or per-unit stowed volume) can be achieved. Specifically, the pressurized air directly provides the stiffness to support structural loads, thus eliminating the requirement for heavy metal stiffeners that are used in conventional rigid structures. The technologies, materials, and system behaviors for these inflatable structures, however, are not sufficiently understood. Furthermore, predictive performance and analysis methods and test standards have not been adequately established because the structural behaviors of inflatable fabric structures often involve coupled effects from inflation pressure such as fluid-structure interactions, thermo-mechanical coupling, and nonlinear constitutive responses of the fabrics all of which can restrict the use of conventional design, analysis, and test methods. The research documented in this report focuses on the mechanics of air-inflated drop-stitch fabric panels that are subject to bending loads. Both analytical and experimental methods are used: the results of experimental four-point bend tests conducted at various inflation pressures were used to validate the analytical method, and predicted and experimentally obtained data such as deflections, wrinkling onset moments, ultimate loads, and pressure changes were compared.