Viscoelastic behavior of HDPE polymer using tensile and compressive loading

Large containers for liquids, exposed to different static loadings, are mainly made of high-density polyethylene (HDPE). The viscoelastic response of HDPE under long-term tensile and compressive creep is investigated. Monotonic experiments under tension are performed over a wide range of strain rates. In these experiments, the transition in the damage mechanisms (development and propagation of contraction in the HDPE specimen) is analyzed. The monotonic tensile behavior of the HDPE is found to be nonlinear and depends on the strain rates. It is observed that both elastic modulus and plastic flow stress present an increase with displacement speed due to the viscoelastic behavior of HDPE. A similar observation can be made for monotonic compressive tests by developing a new experimental device that ensures accurate measurement of the strain. Such a device makes use of an extensometer of compressive displacement of the specimen. In addition, the long-term behavior of HDPE is evidenced through creep and relaxation tests at an imposed range respectively of lower stresses and strains. It is shown that the normalized curves, associated with these tests, can be represented by a single curve characterizing the compressive creep compliance or relaxation stresses versus time. The linearity of the viscoelastic behavior is confirmed within the linear domain of the monotonic compressive and tensile tests.     

Finite element modelling of an energy-storing prosthetic foot during the stance phase of transtibial amputee gait

Energy-storing prosthetic feet are designed to store energy during mid-stance motion and to recover it during late-stance motion. Gait analysis is the most commonly used method to characterize prosthetic foot behaviour during walking. In using this method, however, the foot is generally modelled as a rigid body. Therefore, it does not take into account the ability of the foot to deform. However, the way this deformation occurs is a key parameter of various foot properties under gait conditions. The purpose of this study is to combine finite element modelling and gait analysis in order to calculate the strain, stress and energy stored in the foot along the stance phase for self-selected and fast walking speeds. A finite element model, validated using mechanical testing, is used with boundary conditions collected experimentally from the gait analysis of a single transtibial amputee. The stress, strain and energy stored in the foot are assessed throughout the stance phase for two walking speed conditions: a self-selected walking speed (SSWS), and a fast walking speed (FWS). The first maximum in the strain energy occurs during heel loading and reaches 3J for SSWS and 7J for FWS at the end of the first double support phase. The second maximum appears at the end of the single support phase, reaching 15J for SSWS and 18J for FWS. Finite element modelling combined with gait analysis allows the calculation of parameters that are not obtainable using gait analysis alone. This modelling can be used in the process of prosthetic feet design to assess the behaviour of a prosthetic foot under specific gait conditions.     

A fuzzy-based negotiation approach for collaborative planning in manufacturing supply chains

The coordination of the planning operations across the manufacturing supply chains (MSC) is considered as a major component of supply chain management. As centralized coordination requires relevant information sharing, alternative approaches are needed to synchronize production plans between partners of MSC characterized by decentralized decision making systems with limited information sharing. In this paper, a bi-level fuzzy-based negotiation approach is proposed in order to model collaborative planning between MSC partners. During negotiation, each manufacturer is optimizing a bi-objective planning model. In order to generate optimal production plans, a genetic algorithm is used. To evaluate the exchanged proposals and the satisfaction degree of each partner, the fuzzy logic approach is adopted in the both negotiation levels. The main result of the developed approach consists in a collaborative decision making mechanism allowing the MSC partners to define their optimal production plans while considering the whole negotiating process with the pre-negotiation and post-negotiation stages. Computational tests done for different MSC structures show the effectiveness of the proposed mechanism, which ensures the satisfaction of the manufacturers and the optimality of the final solution. By comparing the results with the ones obtained considering a centralized planning approach, it is shown that the developed negotiation mechanism yields to near optimal solutions with insignificant gaps from the global optimal solutions.     

Resilient model approximation for Markov jump time-delay systems via reduced model with hierarchical Markov chains

In this paper, the resilient model approximation problem for a class of discrete-time Markov jump time-delay systems with input sector-bounded nonlinearities is investigated. A linearised reduced-order model is determined with mode changes subject to domination by a hierarchical Markov chain containing two different nonhomogeneous Markov chains. Hence, the reduced-order model obtained not only reflects the dependence of the original systems but also model external influence that is related to the mode changes of the original system. Sufficient conditions formulated in terms of bilinear matrix inequalities for the existence of such models are established, such that the resulting error system is stochastically stable and has a guaranteed l₂-l_∞ error performance. A linear matrix inequalities optimisation coupled with line search is exploited to solve for the corresponding reduced-order systems. The potential and effectiveness of the developed theoretical results are demonstrated via a numerical example.     

A New Preventive Strategy for Better Remediation of Marine Biofouling by an Eco-friendly Physical and Morphological Modification Process

Silicon - In this study, a new preventive technique based on the morphological modification of nets is tested in order to enhance their fouling resistance when immerged in seawater. For this...     

Effect of Sprayed Solution Flow Rate on the Physical Properties of Anatase TiO2 Thin Films

Titanium dioxide (TiO₂) thin films were synthesized on glass substrates by spray pyrolysis. The effect of solution flow rate on the physical properties of the films was investigated by use of x-ray diffraction (XRD), scanning electron microscopy, atomic force microscopy (AFM), and spectrophotometry techniques. XRD analysis revealed the tetragonal anatase phase of TiO₂ with highly preferred (101) orientation. AFM images showed that grain size on top of TiO₂ thin films depended on solution flow rate. An indirect band gap energy of 3.46 eV was determined by means of transmission and reflection measurements. The envelope method, based on the optical transmission spectrum, was used to determine film thickness and optical constants, for example real and imaginary parts of the dielectric constant, refractive index, and extinction coefficient. Ultraviolet and visible photoluminescence emission peaks were observed at room temperature. These peaks were attributed to the intrinsic emission and to the surface defect states, respectively.     

Templated Solid‐State Dewetting of Thin Silicon Films

Thin film dewetting can be efficiently exploited for the implementation of functionalized surfaces over very large scales. Although the formation of sub‐micrometer sized crystals via solid‐state dewetting represents a viable method for the fabrication of quantum dots and optical meta‐surfaces, there are several limitations related to the intrinsic features of dewetting in a crystalline medium. Disordered spatial organization, size, and shape fluctuations are relevant issues not properly addressed so far. This study reports on the deterministic nucleation and precise positioning of Si‐ and SiGe‐based nanocrystals by templated solid‐state dewetting of thin silicon films. The dewetting dynamics is guided by pattern size and shape taking full control over number, size, shape, and relative position of the particles (islands dimensions and relative distances are in the hundreds nm range and fluctuate ≈11% for the volumes and ≈5% for the positioning).