Daxu Zhang

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.

Prediction of failure envelopes and stress–strain curves of fiber composite laminates under triaxial loads

This article presents a methodology for predicting the stress–strain response and failure behavior of fiber composite laminates. First, a stress transfer method is proposed by using the concept of the state space equation. The generalized plane strain deformation is assumed in the model. The state space equation was derived by using Fourier series expansions in the width direction and the layer refinement technique through the thickness. Secondly, Christensen’s stress-based failure criteria are adopted to predict failure of individual plies. Finally, the applications of the methodology are shown by predicting the failure envelopes and stress–strain curves of the test cases provided by the organizers of the Second World-Wide Failure Exercise (WWFE-II).

Prediction of failure envelopes and stress strain curves of fiber composite laminates under triaxial loads: comparison with experimental results

This paper represents the authors’ contribution to Part B of the second world wide failure exercise, in which the failure predictions and stress–strain curves of 12 test cases are compared with experimental data. The paper briefly reviews the methodology presented in second world wide failure exercise Part A, which includes a semi-analytical stress analysis model and Christensen’s failure criteria. A further development in this paper is to introduce a new failure criterion developed also by Christensen and the application of the criterion in conjunction with the stress model. These failure criteria have no adjustable parameters and only depend upon a minimal number of measurable failure properties. Comparisons between the theoretical predictions using different criteria and the experimental results provided by the second World-Wide Failure Exercise organizers are shown in diagrams. It appears that Christensen’s failure criteria for anisotropic fiber composite materials predict the failure strength satisfactorily in most of the test cases. Christensen’s Polynomial invariants theory compares also well with the experimental results. However, the layup-dependent strength properties required in the criterion are sometimes very difficult to evaluate without performing experimental tests.

Form-finding model shows how cytoskeleton network stiffness is realized.

In eukaryotic cells the actin-cytoskeletal network provides stiffness and the driving force that contributes to changes in cell shape and cell motility, but the elastic behavior of this network is not well understood. In this paper a two dimensional form-finding model is proposed to investigate the elasticity of the actin filament network. Utilizing an initially random array of actin filaments and actin-cross-linking proteins the form-finding model iterates until the random array is brought into a stable equilibrium configuration. With some care given to actin filament density and length, distance between host sites for cross-linkers, and overall domain size the resulting configurations from the form-finding model are found to be topologically similar to cytoskeletal networks in real cells. The resulting network may then be mechanically exercised to explore how the actin filaments deform and align under load and the sensitivity of the network’s stiffness to actin filament density, length, etc. Results of the model are consistent with the experimental literature, e.g. actin filaments tend to re-orient in the direction of stretching; and the filament relative density, filament length, and actin-cross-linking protein’s relative density, control the actin-network stiffness. The model provides a ready means of extension to more complicated domains and a three-dimensional form-finding model is under development as well as models studying the formation of actin bundles.

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.

Effects of in-plane extension on transverse thermal conductivity of a carbon–carbon 8-harness satin weave composite

A finite element-based technique for coupled thermo-mechanical analysis of woven Ceramic Matrix Composites sheets is presented for the prediction of the degradation of transverse thermal transport behaviour with in-plane extension. The thermal conductivity–strain characteristics have been determined, at the tow level, from the properties of the constituent elements, and then extended to tows and composite. The non-linear thermal conductivity-extension behaviour of the tow has been discretised by multi-linear curves, and implemented in a user-defined subroutine in ABAQUS to model the behaviour of the homogenised orthotropic unidirectional tow and its matrix. By using this approach, an 8-Harness Satin weave HITCO C/C composite unit cell has then been analysed. The variation of through-thickness thermal conductivity degradation with in-plane extension has been predicted and compared with the results of experiments. Very good agreement has been achieved. Two classes of behaviour have been experimentally observed: one that exhibits a brittle response, and another that shows a quasi-ductile behaviour. Both classes of behaviour have been predicted and shown to relate, respectively, to strain localisation and instantaneous pull-out deactivation, without localisation being invoked. These responses are reflected directly in the predicted and experimental rates of decay of transverse thermal conductivity with axial extension. It is advocated that the reduction in transverse thermal conductivity with extension and damage can be used as a Structural Integrity Monitor for CMC operational components.