Stiffness Parameters for Architectural Fabrics: An Analysis of Two Determination Procedures

Abstract

One important and challenging aspect of the design process for tensile membrane structures is the determination of biaxial material stiffness parameters. Coated textiles that are used as architectural fabrics display highly nonlinear and anisotropic stress-strain behaviour under biaxial tensile stresses. Nevertheless, in state-of-the-art structural analyses, the behaviour of these coated textiles is often simplified to a linear-elastic plane stress relationship, where the elastic constants are "tensile stiffness" and "Poisson's ratio." The elastic constants must be determined for each material using biaxial tensile tests. Several different biaxial test procedures to determine the elastic constants exist worldwide, and these procedures yield various sets of elastic constants with a wide spectrum of stiffness parameters. At the same time, design engineers have no guidelines to assess which set of parameters is appropriate for a specific design situation. This paper compares two different methods of determining stiffness parameters using theoretical and experimental analyses. The variation in structural analysis results due to stiffness parameters that were determined using different techniques is demonstrated using three types of PES/PVC materials from two material producers. Furthermore, this paper provides guidance regarding the manner of evaluation of sets of elastic constants and the modification of the evaluation of experimental biaxial tests, if required. Information is also provided concerning the applicability of the investigated procedures.Tensile or membrane structures are often incorporated into modern infrastructure applications as a means to achieve elegant structural forms with low self-weight. Examples can be found in bridges, long-span roofs and temporary or special structures. Due to their low self-weight and limited redundancy, these structural forms can present numerous unique design or construction challenges. Their structural response must be fully understood under the temporary configurations occurring during construction. The unique mechanical properties of non-traditional materials frequently used in tensile or membrane structures can also present specific challenges or constraints.This issue of Structural Engineering International contains a special series of six scientific papers to highlight some of the recent developments in the field of Tensile and Membrane Structures.The first two papers in this series examine different aspects of the design and performance of structures constructed from light, fabric membranes. Uhleman et al. present a study that contrasts different standardized protocols used to arrive at design parameters for the modeling of material properties of textile fabrics. A paper by Milosevic examines the influence of concentrated loading on the deformation characteristics of a membrane structure.Beam string structures, which have seen increased adoption for use as long-span roof systems, are notable because their active change in geometry during construction needs to be co...

The influence of test conditions and mathematical assumptions on biaxial material parameters of fabrics

“…The resulting material behaviour is thus not only highly nonlinear but its mechanical response also displays an important dependency on the ratio between the load applied in the warp direction and the load applied in the fill direction [1,3,10,11,12,13]. To incorporate this dependency in the design of tensile fabric structures multiaxial tensile tests are necessary to provide the designer with appropriate stress-strain data and/or material constants.…”

“…Do note that for both Poisson's ratios, results do exceed the typical upper limit of 0.5 regularly. Due to the strong 825 interaction between the two fibre directions, this is not uncommon for fabrics approximated through linear elastic orthotropic parameters [12,13,31,33] and has been shown to be possible for anisotropic elastic materials in general under the prerequisite of positive definiteness of the strain 830 energy density [34].…”

“…Perhaps the most significant one of the studied parameters is the applied load profile. Since fabrics experience a strong history-dependency in their structural response, the applied load is known to exercise a significant influence on the outcome of the biaxial tests [3,6,12,13,32].…”

Ram‐air kite airfoil and reinforcements optimization for airborne wind energy applications

“…Ram‐air kites are often made of ripstop material, which is a coated woven fabric with coarser warp and filling yarns at intervals so that tears will not spread. The modeling of woven fabric is difficult because of its nonlinear stress‐strain behavior and the load ratio dependence, which has a significant influence on the elastic constants . In this paper, the orthotropic Saint Venant‐Kirchhoff material model is used, which is valid for small strains.…”

Steel structures in interaction with non-metallic membranes

“…The supplied test data provides elastic modules for warp and fill directions and corresponding Poisson's ratio, valid for anticlastic type of structures. Recently a comparison of two approaches concerning simplified elastic constants for design of the membranes (Japanese MSAJ and the above European ones) was presented in Uhlemann et al (2015). The European approach was found to be more general and reasonable for PES/PVC material (but still far from satisfactory).…”