Wujun Chen

Experimental study on uniaxial and biaxial tensile properties of coated fabric for airship envelopes

This article presents an experimental study to determine the tensile properties of the envelope fabric Uretek3216A under mono-uniaxial, uniaxial cyclic, and biaxial cyclic loading. First, the mono-uniaxial, uniaxial cyclic, and biaxial cyclic tests were carefully carried out on the envelope fabric, and the corresponding stress–strain behaviors were discussed. Then, the elastic constants were calculated from the experimental data of these tests and the influences of the uniaxial loading cycle and the determination options with different stress ratios were discussed. For the biaxial tests, the elastic constants were determined with and without the constraint of the reciprocal relationship to investigate its significance. Finally, a comparison of the elastic moduli between uniaxial and biaxial tensile tests was presented. Results show that the nonlinearity and orthotropy of the envelope fabric could be attributed to the mechanical properties and unbalanced crimp of their constitutive yarns, respectively. The elastic constants vary noticeably with the experimental protocols, as well as the determination options for biaxial tests, and then in the real design practice, elastic constants should be determined for specific loading conditions and stress distributions depending on the project’s needs.

An innovative methodology for measurement of stress distribution of inflatable membrane structures

The inflatable membrane structure has been widely used in the fields of civil building, industrial building, airship, super pressure balloon and spacecraft. It is important to measure the stress distribution of the inflatable membrane structure because it influences the safety of the structural design. This paper presents an innovative methodology for the measurement and determination of the stress distribution of the inflatable membrane structure under different internal pressures, combining photogrammetry and the force-finding method. The shape of the inflatable membrane structure is maintained by the use of pressurized air, and the internal pressure is controlled and measured by means of an automatic pressure control system. The 3D coordinates of the marking points pasted on the membrane surface are acquired by three photographs captured from three cameras based on photogrammetry. After digitizing the markings on the photographs, the 3D curved surfaces are rebuilt. The continuous membrane surfaces are discretized into quadrilateral mesh and simulated by membrane links to calculate the stress distributions using the force-finding method. The internal pressure is simplified to the external node forces in the normal direction according to the contributory area of the node. Once the geometry x, the external force r and the topology C are obtained, the unknown force densities q in each link can be determined. Therefore, the stress distributions of the inflatable membrane structure can be calculated, combining the linear adjustment theory and the force density method based on the force equilibrium of inflated internal pressure and membrane internal force without considering the mechanical properties of the constitutive material. As the use of the inflatable membrane structure is attractive in the field of civil building, an ethylene-tetrafluoroethylene (ETFE) cushion is used with the measurement model to validate the proposed methodology. The comparisons between the obtained results and numerical simulation for the inflation process of the ETFE cushion are performed, and the strong agreements demonstrate that the proposed methodology is feasible and accurate.

Mechanical Behaviors and Elastic Parameters of Laminated Fabric URETEK3216LV Subjected to Uniaxial and Biaxial Loading

A comprehensive experimental study of the laminated fabric URETEK3216LV subjected to mono-uniaxial, uniaxial cyclic and biaxial cyclic loading was performed to expose the detailed mechanical behaviors and determine proper elastic parameters for the laminated fabrics under specific stress states. The elastic modulus-strain curves and elastic parameter response surfaces were used to reveal the mechanical behaviors, and a weighted average method of integrals was proposed to calculate the elastic parameters for different stress states. Results show that typical stress-strain curves consist of three distinct regions during loading: crimp region, nonlinear transition region and yarn extension region, which is consistent with those of the constitutive yarns. The elastic parameters and mechanical behaviors of the laminated fabric are stress-state specific, and they vary noticeably with the experimental protocols, stress ratios and stress levels. The proposed method is feasible to evaluate the elastic parameters no matter what stress states the materials are subjected to, and thus it may offer potential access to obtain accurate design and analysis of the airship structures under different loading conditions.

Central tearing characteristics of laminated fabrics: Effect of slit parameter, off-axis angle, and loading speed:

Based on our prior study on uniaxial tearing tests, the influences of slit parameters, off-axis angles, and loading speeds on the tearing behaviors and strengths were discussed further. Results show that the tearing behaviors and strengths of the studied material are affected by above factors significantly, and typical tearing stress-displacement curves could be defined as four characteristic regions: a co-deformation region, a yarn extension region (or shear deformation region), a plateau region, and a post peak region. There is a W-shaped relationship between tearing strength and off-axis angle, with a small peak at 45°, and a new tearing strength criterion consisting of two parts, i.e., an inverted V-shaped part and a U-shaped part, was proposed for this laminated fabric. Comparisons between the experimental and the calculated results for the laminated fabric were performed, and the strong agreements demonstrate that the proposed criterion is feasible and accurate.

Equilibrium Configuration Analysis of Non-rigid Airship Subjected to Weight and Buoyancy

The general forces applied on the airship hull and their effects on the overall structural behavior are evaluated according to the classical engineering method. The static and differential pressure moment are observed to constitute great parts of the whole bending moment. A numerical procedure is proposed on the basis of pre-stressed pneumatic membrane structure, which assumes a basic pressure to be the pre-stress, then applies buoyancy and constraints, and finally calculates the equilibrium configuration. The airship hull is assumed to be either close or open, i.e. constant mass or constant pressure, respectively. Finally, a 25-m demonstration airship is investigated comprehensively. Seven models are created to investigate the structural behavior of the airship subjected to various loading conditions. Interest structural behavior and numerical characteristics are demonstrated.

Comparison of Constant-Force Increment and Constant-Rate Loading Protocols in Biaxial Tests of Coated Fabrics

AbstractBiaxial tension testing is an increasingly accepted tool to understand the mechanical behavior of architectural coated fabric membrane materials. However, different loading protocols in bia...

Full-strain range characteristics of the Poisson’s ratio for coated biaxial warp-knitted fabrics under bias tensile loading

It is undeniable that full-strain range characteristics of Poisson’s ratios on the behaviors of a fabric composite are crucial. However, there have been relatively few papers devoted to this subject. In this study, tensile tests of seven bias angles with a 15° increment are conducted on a typical coated biaxial warp-knitted fabric (BWKF) to estimate the Poisson’s ratios in full-strain range. By utilizing the digital image correlation technique, experimental results are processed, and detailed responses of strain contours and Poisson’s ratios are determined for specific strain states. Then, two typical types, that is recession and peak types, of the Poisson’s ratio strain curves are proposed and their characteristics evolving with the strain, bias angle and stress state are discussed. Results show that characteristics of Poisson’s ratios and strain contours vary noticeably with the yarn orientations, strain and stress levels. As the strain increases, the Poisson’s ratio of peak type first ascends markedly, and then descends moderately after arriving at a peak, generating three distinct stages: the ascending, peak and post-peak stages; in contrast, the recession type only experiences a downward trend, resulting in two characteristic stages: the recession and stable stages. In addition, there is an M-shaped relationship between the Poisson’s ratio and bias angle, with a local valley at 45°, which is not consistent with results in previous works. This investigation could provide new insights into the orientation dependence, Poisson effect and warp–weft interaction mechanism of BWKFs.

Response surface analysis of biaxial mechanical properties and elastic parameters for woven fabric composites

This paper describes a self-developed MATLAB program for achieving complex deformation properties and accurate elastic parameters of woven fabrics. Using this program, the response surface analyses of deformation and elastic parameters were conducted to reveal the variation and evolution in stiffness, Poisson’s ratio, and orthotropy of the laminated woven fabric composite under different loading/unloading stages, cycles, stress levels and ratios, which is a further development of our prior work. Results show that elastic parameters and orthotropic characteristics of the material vary noticeably with the loading/unloading stages, cycles, stress levels, and ratios. As higher values are required to model the remarkable warp–weft interaction and large negative strains, Poisson’s ratios in boundary regions with a maximum of 1.0 (loading) or 1.5 (unloading) are outside the bounds normally expected for these parameters. In addition, the proposed method is feasible to evaluate elastic parameters for any stress state, and thereby provides new insights into underlying deformation mechanisms of woven fabric composites.

Fracture failure analysis and bias tearing strength criterion for a laminated fabric

This paper deals with the fracture failure analysis on plain woven laminated fabrics used in stratospheric airship structures. A series of uniaxial tensile and central slit tearing tests were carefully conducted on bias specimens, and the corresponding tensile and tearing properties, including failure mechanisms and material strengths, of a laminated fabric were discussed. Results show that laminated fabrics are typical direction-depended materials, and their failure characteristics vary greatly with the bias angles. Typical tearing stress–displacement curves of the laminated fabric could be defined as four characteristic regions: a co-deformation region, a yarn extension region (or shear deformation region), a plateau region, and a post peak region. Among bias specimens, there are many obvious differences in tearing behaviors in terms of maximum displacement, damage mode, curve slope, and number of stress peaks, which could be attributed to the material orthotropy and different failure mechanism of constituent materials. Unlike results of tensile strength, there is a W-shaped relationship between tearing strength and off-axis angle, with a local strength peak at 45° angle. Based on the Tsai-hill criterion, a new tearing strength criterion consisting of two parts, including a U-shaped and an inverted V-shaped part, is proposed for this laminated fabric. Comparisons between the experimental and calculated results for the laminated fabric are performed, and the strong agreements demonstrate that the proposed criterion is feasible and accurate.