4,345 research outputs found
The Formulation of the Quadratic Failure Criterion for Transversely Isotropic Materials: Mathematical and Logical Considerations
No full textThe quadratic function of the original Tsai–Wu failure criterion for transversely isotropic materials is re-examined in this paper. According to analytic geometry, two of the troublesome coefficients associated with the interactive terms—one between in-plane direct stresses and one between transverse direct stresses—can be determined based on mathematical and logical considerations. The analysis of the nature of the quadratic failure function in the context of analytic geometry enhances the consistency of the failure criterion based on it. It also reveals useful physical relationships as intrinsic properties of the quadratic failure function. Two clear statements can be drawn as the outcomes of the present investigation. Firstly, to maintain its basic consistency, a failure criterion based on a single quadratic failure function can only accommodate five independent strength properties, viz. the tensile and compressive strengths in the directions along fibres and transverse to fibres, and the in-plane shear strength. Secondly, amongst the three transverse strengths—tensile, compressive and shear—only two are independent
Relating quantitative soil structure metrics to saturated hydraulic conductivity
No full textSoil structure affects saturated hydraulic conductivity (Ks) by creating highly conductive macropores that preferentially transmit soil water. In this study, we explore the relationship between Ks and macropores in an Oxyaquic Vertic Argiudoll in northeastern Kansas. Macropores were quantified from an excavation wall using multistripe laser triangulation (MLT) scanning. Soil water contents were measured at four depths within a soil lysimeter installed within 2 meters of the MLT-scanned soil profile and adjacent to an Ameriflux tower monitoring precipitation, air temperature, and solar radiation. Selected hydraulic properties of soil horizons within the lysimeter were optimized to water content data using a Markov chain Monte Carlo technique in combination with the mobile-immobile water (MIM) model in HYDRUS-1D. Estimates of Ks varied between 4198 cm d-1 in the A horizon and 0.6 cm d-1 in a 2Btss2 horizon with strongly expressed wedge structure. Approximately 87% of the variation in Ks was explained by the geometric mean of the widths of pores quantified with the MLT technique and modified by the coefficient of extensibility (COLE). The use of COLE allows the widths of the macropores obtained at dry conditions to be approximated at saturation. Two models that predict Ks from either texture or water retention data resulted in Ks estimates that were similar to each other, but significantly lower than Ks values predicted with MIM in horizons where structural pores dominate water flow. This technique shows a great deal of promise in better understanding and predicting the relationship of soil structure to water flow.Peer reviewe
Unification and parameterisation of 2D and 3D weaves and the formulation of a unit cell for composites made of such preforms
No full textIn this paper, the method to unify the definition of woven architectures in textile composites over a wide range of variations has been established. This was achieved via parametrisation of the weaves, which was conducted employing a limited set of clearly defined parameters. Detailed description of these parameters has been given, based of which a wide range of 2D and 3D weaves can be reproduced. The intended use of the parametrisation developed is to facilitate virtual testing of composites through numerical material characterisation based on use of unit cells (UC). The process as presented in this paper delivers a fully parametrised UC in a straightforward way. After implementing the parametrisation as a computer code and combining it with the piece of software UnitCells©, it has been fully developed as a methodology. Its capability as a design tool for 3D woven composites is demonstrated via a range of case studies
A failure criterion for genuinely orthotropic materials and integration of a series of criteria for materials of different degrees of anisotropy
No full textExisting failure criteria for orthotropic materials are subject to an underlying assumption which cause contradictions when applied to genuinely orthotropic materials that are significantly anisotropic in elasticity as well as in strengths. For such materials, there is lack of consistent failure criteria to support their applications in engineering structures. A general quadratic failure criterion tends to leave undetermined coefficients for interactive terms. A rational approach is adopted in this paper based on mathematical and logical considerations to determine these coefficients as the objective of this paper. Considerations are based on the intrinsic characteristics of the quadric surfaces introduced by the quadratic failure criterion. These coefficients must take the values as obtained, leaving no alternatives if logic prevails. The obtained criterion integrates for the first time a range of criteria separately formulated for materials of different degrees of anisotropy, from genuinely orthotropic, through transversely isotropic, cubically symmetric, to completely isotropic ones with different or identical tensile and compression strengths
Visualization studies on evidence-based medicine domain knowledge (series 2): structural diagrams of author networks.
No full textOBJECTIVE: To investigate the output of evidence-based medicine (EBM) researchers in China and elsewhere by examining the EBM domains they work within and the networks that exist among them; using visualization methods to analyze these relationships. This maps the current situation and helps with the identification of areas for future growth. METHODS: We used co-citation matrixes with Pathfinder networks and hierarchical clustering algorithms, and constructed a co-author matrix which were analyzed with a whole network approach. The analyzed matrixes were visualized with the UCINET program. RESULTS: Much of the development of EBM has been centered around three authors, David Sackett, Gordon Guyatt and L Manchikanti, within three different clusters. The main authors of EBM articles in China were divided into nine academic domains. The relations among core authors of articles indexed by the Science Citation Index (SCI) was loose. There was a stronger co-authorship network among core authors in the Chinese literature, with three groups and 21 cliques. Nine distinct academic communities appeared to have formed around Li Youping, Liu Ming and Zhang Mingming. CONCLUSION: The EBM literature contains several key clusters, with universities in high-income countries being the source of the majority of articles. Outside China, McMaster University in Canada, the original home of EBM, is the dominant producer of EBM publications. In China, Sichuan University is the main source of EBM publications. The EBM cooperation network in China is comprised of three major groups, the largest and most productive in this sample is led by Li Youping with Liu Ming, Zhang Mingming, Li Jing, Wang Li, Wu Taixiang, and Liu Guanjian as central members
On the objectivity of the nonlinear along-fibre shear stress–strain relationship for unidirectionally fibre-reinforced composites
No full textThe nonlinear along-fibre shear stress–strain relationship for unidirectionally fibre-reinforced composites has been investigated in this paper aiming at its applications in general 3D stress conditions in a consistent manner. So far, such relationship has only been addressed in plane stress conditions. In this paper, it has been shown that its straightforward generalisation to 3D stress states lacks objectivity, which is a basic requirement for all theoretical studies of physical problems. A new formulation is proposed based on the stress invariants and the complementary strain energy guided by the rational theoretical framework of nonlinear elasticity. A consistent and objective stress–strain relationship has been obtained and verified through an example of application to a torsion problem.</p
Efficient characterization of mechanical nonlinearity in triaxially braided composites by a novel unit cell
No full textA range of nonlinear factors were investigated based on a novel unit cell model with a reduced domain to characterize the mechanical nonlinearity exhibited by braided composites in this study. These factors include geometric nonlinearity, nonlinear shear behaviour, plasticity within the matrix and transverse fibre direction, as well as failure and damage evolution. This novel size-reduced unit cell for braided composites, based on non-orthogonal translational symmetry principles, has been developed to streamline the analysis and tackle convergence challenges. This novel size-reduced unit cell is formulated by leveraging geometric features, as well as the symmetry and anti-symmetry of relative displacement boundary conditions. In order to evaluate the efficacy of these unit cells in characterizing elastic and strength properties, a benchmark study was conducted. The results demonstrate that the novel size-reduced unit cell has the ability to reliably predict the stiffness and strength responses of the 2D3A braided composite at the mesoscale. The unit cell's predictions, which include nonlinear stress–strain curves, modulus, and strength, exhibit a robust analytical performance and a satisfactory agreement with experimental results.</p
Design strategy for 3D layer-to-layer angle interlock woven composites
No full textA design strategy for 3D layer-to-layer angle interlock woven composites has been established by employing a set of three key properties of the weave (KPoWs): the global fibre volume fraction, the interlocking angle and the ratio of the weft tow volume to the warp tow volume. Using analytically derived expressions of the KPoWs, their variation trends relative to the manufacturing parameters have been revealed. At the same time, via a range of systematic computational material characterisation exercises, the KPoWs were shown to be sufficient for representing the woven reinforcement as far as the elastic behaviour predictions are concerned, because the effective elastic properties were found to follow consistent variation trends with the KPoWs. As a result, through use of KPoWs, manufacturing parameters have been associated with the effective elastic properties in a systematic manner. This offer means for obtaining a desirable elastic behaviour of 3D woven composites via variation of their internal architecture. The design method developed is the alternative to trial-and-error-based selection method conventionally adopted for this type of materials. As an example of application of the proposed method, a woven composite with balanced weft and warp properties has been designed
Controllable parameters as the essential components in the analysis, manufacturing and design of 3D woven composites
No full textThe comprehensive design feasibility for woven composites has been established by identifying the missing component in the design tool for these materials, the so-called controllable parameters. These are the parameters that are involved, directly or indirectly, in manufacturing of woven preforms, and they include the tow densities, the number and the size of filaments in a tow and intra-tow fibre volume fractions. Controllable parameters have been related through a simple procedure to the conventional geometric parameters of woven composites, which eliminates the need for their costly, inefficient, and often unreliable measurements. The controllable parameters provide sufficient representation of woven geometry in terms of both modelling and manufacture, thus offering common terminology to the woven composite designers and manufacturers. This also allows to naturally involve the practical considerations and manufacturing restrictions in material design exercises. With controllable parameters being incorporated in woven composite design framework, a direct analogy can be drawn to classical lamination theory-based design for conventional laminates. The wide applicability of the design tool has been demonstrated via a series of systematic material characterisation exercises carried out with woven composites of sufficiently different internal architectures and constituents, which also showed good predictive capability of the models involved
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