Quasi-static Mechanical Analysis Research Articles

Preparation and Mechanical Properties of PTFE/SPT Composites Reinforced with Nanoparticles

Nanoparticles have been used with polymer to make composites having remarkable properties. An attempt was made in this direction, in order to enhance the mechanical and tribological properties of nanocomposites. In the paper, the polytetrafluroethylene (PTFE) polymer-based nanocomposites filled with serpentine (SPT) and reinforced with different nanoparticles such as nano-β-carborundum whisker (β-SiCw), nano-copper and nano-TiO2 were prepared by using cold briquetting and hot-press sintering technologies. Also, the mechanical performances of these nanocomposites were studied. The quasi-static tensile experiments and dynamic mechanical thermal analysis (DTMA) were carried out. The results obtained showed that the mechanical properties of these nanocomposites stronger depend on the variety of nanoparticle. Stress-displacement and stress relaxation curves indicate that theses composites are typical viscoelastic materials. These research findings are believed to be helpful for providing practical guide in applications.

A Quasi-Static Mechanics Analysis of Three-Dimensional Nanoscale Surface Polishing

A quasi-static mechanics analysis of nanoscale surface polishing that provides insight into the surface topography evolution and the removal of material at the asperity level is presented. The analysis is based on a three-dimensional stochastic model that accounts for multiscale (fractal) surface roughness and elastic, elastic-plastic, and fully plastic asperity deformation by hard abrasive nanoparticles embedded in the soft surface layer of a rigid polishing plate. Numerical results of the steady-state roughness of the polished surface, material removal rate, and wear coefficient are presented in terms of the apparent contact pressure, polishing speed, original topography and mechanical properties of the polished surface, average size and density of nanoparticles, and surface roughness of the polishing plate. Simulation trends are associated with elastic-plastic and fully plastic asperity contacts, responsible for irreversible topography changes (roughening effect) and material removal (smoothening effect), respectively. Analytical trends and predictions of the steady-state roughness of the polished surface and material removal rate are shown to be in good agreement with experimental results of nanoscale surface polishing (lapping) of magnetic recording ceramic heads.

Residual stresses in a stress lattice—Experiments and finite element simulations

In this work, residual stresses in a stress lattice are studied. The residual stresses are both measured and simulated. The stress lattice is casted of low alloyed grey cast iron. In fact, nine similar lattices are casted and measured. The geometry of the lattice consists of three sections in parallel. The diameter of the two outer sections are thinner than the section in the middle. When the stress lattice cools down, this difference in geometry yields that the outer sections start to solidify and contract before the section in the middle. Finally, an equilibrium state, with tensile stresses in the middle and compressive stresses in the outer sections, is reached. The thermo-mechanical simulation of the experiments is performed by using Abaqus. The thermo-mechanical solidification is assumed to be uncoupled. First a thermal analysis, where the lattice is cooled down to room temperature, is performed. Latent heat is included in the analysis by letting the fraction of solid be a linear function of the temperature in the mushy zone. After the thermal analysis a quasi-static mechanical analysis is performed where the temperature history is considered to be the external force. A rate-independent J 2 -plasticity model with isotropic hardening is considered, where the material data depend on the temperature. Tensile tests are performed at room temperature, 200 ° C, 400 ° C, 600 ° C and 800 ° C in order to evaluate the Young’s modulus, the yield strength and the hardening accurate. In addition, the thermal expansion coefficient is evaluated for temperatures between room temperature and 1000 ° C. The state of residual stresses is measured by cutting the midsection or the outer section. The corresponding elastic spring-back reveals the state of residual stresses. The measured stresses are compared to the numerical simulations. The simulations show good agreement with the results from the experiments.

Crosswind stability of a high-speed train on a high embankment

This work presents aerodynamic results of crosswind stability obtained numerically and experimentally for the leading control unit (class 808) of Deutsche Bahn AG's high-speed train Inter-CityExpress 2. The train model is on top of a 6m high embankment in accordance with the proposed European code for interoperable trains, the so-called technical specifications for interoperability. The purpose of the study is to convey the predictive accuracy that typical steady-state computational fluid dynamics-Reynolds average Navier-Stokes methods (industry standard) return and to contribute to the understanding of the aerodynamics for the current application. Attention is drawn to the aerodynamics around the train and embankment when subjected to a steady block profile crosswind of 30° yaw angle on the basis of the onset velocity far upstream the embankment. The Re (Reynolds number) of the embankment cases is 4.6 × 106. Calculated results are obtained with the commercial code STAR-CD, with exclusively hexahedral meshes with a total cell count of 13.5 × 106. Results are obtained when the train stands on the windward and leeward tracks on top of the embankment. These results are first compared with a flat ground case from a previous study. Then experimental data are obtained in a high-pressure wind tunnel with a model scale of 1:100. Re effects are compensated by raising the ambient pressure by a factor of 60, which increases the air density and thus the Re by a similar factor. Calculated results are in fair agreement with the experiments, where both the calculations and the experiments predict the leeward case to be the more critical one. In addition, the related consequences on the mechanical behaviour, i.e. the stability of the car, are briefly addressed by means of a quasi-static mechanical analysis. The results of the present study indicate that the 6m high embankment concerning the current train reduces the permissible crosswind speed with approximately 20 per cent.

Supercritical carbon dioxide foaming of elastomer/heterocyclic methacrylate blends as scaffolds for tissue engineering

This study reports on the supercritical carbon dioxide (scCO2) foaming of rubber toughened heterocyclic methacrylates for potential applications as non-degradable scaffolds in tissue repair and engineering. Porous blends of styrene–isoprene–styrene copolymer elastomer (SIS) and tetrahydrofurfuryl methacrylate (THFMA) at three SIS/THFMA compositions that ranged from methacrylate to elastomer rich were foamed and characterised in terms of their morphological, mechanical and biological properties. The results showed that the foaming factor (FF) was dependent on blend composition and the foaming conditions demonstrating that the process was tuneable. A greater FF, resulting in higher open and total porosities, was obtained for THFMA rich formulations, which were demonstrated by a predominantly open pore structure. Quasi-static and dynamic mechanical analysis (DMA) showed that the foamed SIS/THFMA blends gave distinct behaviours according to their compositions which were in the range of mechanical properties of soft tissues. The loss modulus and mechanical loss tangent through DMA gave two transition regions associated with the glass transition temperatures of poly(THFMA) and polystyrene components in the blends, along with a reduction in storage modulus. Cell adhesion and spreading in terms of neuroblastoma (human neuron-like SH-SY5Y) cells and ovine meniscal chondrocytes were demonstrated for scaffolds with THFMA rich formulations confirming their suitability for tissue engineering.

Sensitivity analysis of the thermomechanical response of welded joints

A computational procedure is presented for evaluating the sensitivity coefficients of the thermomechanical response of welded structures. Uncoupled thermomechanical analysis, with transient thermal analysis and quasi-static mechanical analysis, is performed. A rate independent, small deformation thermo-elasto-plastic material model with temperature-dependent material properties is adopted in the study. The temperature field is assumed to be independent of the stresses and strains. The heat transfer equations emanating from a finite element semi-discretization are integrated using an implicit backward difference scheme to generate the time history of the temperatures. The mechanical response during welding is then calculated by solving a generalized plane strain problem. First- and second-order sensitivity coefficients of the thermal and mechanical response quantities (derivatives with respect to various thermomechanical parameters) are evaluated using a direct differentiation approach in conjunction with an automatic differentiation software facility. Numerical results are presented for a double fillet conventional welding of a stiffener and a base plate made of stainless steel AL-6XN material. Time histories of the response and sensitivity coefficients, and their spatial distributions at selected times are presented.

Computational mechanical analysis of composite bondlines under environmental moisture loads

A combined moisture diffusion and mechanical computational analysis methodology was developed as part of an independent research program at Lockheed to enhance our understanding of nonlinear and coupled physical processes in materials. The physical behavior of interest is the mechanical response of composite material systems to environmental conditions which can change the materials' moisture content. In particular, it is desirable to know what set of environmental conditions will affect the integrity of a partial adhesive bondline when a bondline gap (no adhesive) is present. The Lockheed proprietary finite element analysis code DIAL was modified to analyze this phenomena in two stages. First, the temporal moisture partial pressures in the materials are calculated from initial and environmental boundary conditions and second, these partial pressures are then passed to a quasi-static mechanical analysis where the moisture data at each time step is treated as a dilatation (swelling) load. The resulting stress and strain state is then calculated, as well as gap closure, if any. Demonstration of this methodology was on a relatively simple 2-dimensional axisymmetric example problem. The results show primarily compressive stresses caused by the positive dilatation (swelling) induced by moisture intrusion of the structure. The stresses concentrate at adhesive bondlines, and bondline gap behavior (i.e., the absence of a contiguous adhesive bond at material interfaces) is correctly simulated. It should be noted that the behavior of the gap and any resultant stress/strains due to a change in moisture is problem dependent, and this example problem is meant as a demonstration of methodology.