Comparing double-step and penalty-based semirecursive formulations for hydraulically actuated multibody systems in a monolithic approach

The simulation of mechanical systems often requires modeling of systems of other physical nature, such as hydraulics. In such systems, the numerical stiffness introduced by the hydraulics can become a significant aspect to consider in the modeling, as it can negatively effect to the computational efficiency. The hydraulic system can be described by using the lumped fluid theory. In this approach, a pressure can be integrated from a differential equation in which effective bulk modulus is divided by a volume size. This representation can lead to numerical stiffness as a consequence of which time integration of a hydraulically driven system becomes cumbersome. In this regard, the used multibody formulation plays an important role, as there are many different procedures for the constraint enforcement and different sets of coordinates to choose from. This paper introduces the double-step semirecursive approach and compares it with a penalty-based semirecursive approach in case of coupled multibody and hydraulic dynamics within the monolithic framework. To this end, hydraulically actuated four-bar and quick-return mechanisms are analyzed as case studies. The two approaches are compared in terms of the work cycle, energy balance, constraint violation, and numerical efficiency of the mechanisms. It is concluded that the penalty-based semirecursive approach has a number of advantages compared with the double-step semirecursive approach, which is in accordance with the literature.

     

The resistance of nickel and iron aluminides to cavitation erosion and abrasive wear

The abrasive wear and cavitation erosion resistance of several alloys based on the Intermetallic compounds Ni₃Al and Fe₃Al have been investigated. The erosion resistance of the nickel aluminides is relatively insensitive to alloying with iron or chromium and is comparable with or superior to that of many commercial erosion-resistant alloys; the abrasive wear resistance is found to be decreased by alloying, despite increased room temperature strength and refined grain size. Preliminary results for the iron aluminides indicate increased resistance to abrasive wear with increasing alloy content. It is suggested that the abrasive wear process causes temperature increases in the damage zone that are sufficient to cause the elevated temperature properties of the alloys to become dominant. Under these conditions, the wear resistance can be related to the tendency to disorder, either thermally or through plastic deformation.     

A Non-Incremental Finite Element Procedure for the Analysis of Large Deformation of Plates and Shells in Mechanical System Applications

In this investigation, a non-incremental solution procedure for the finite rotationand large deformation analysis of plates is presented. The method, whichis based on the absolute nodal coordinate formulation, leads to plateelements capable of representing exact rigid body motion. In thismethod, continuity conditions on all the displacement gradients areimposed. Therefore, non-smoothness of the plate mid-surface at the nodalpoints is avoided. Unlike other existing finite element formulationsthat lead to a highly nonlinear inertial forces for three-dimensionalelements, the proposed formulation leads to a constant mass matrix, andas a result, the centrifugal and Coriolis inertia forces are identicallyequal to zero. Furthermore, the method relaxes some of the assumptionsused in the classical and Mindlin plate models and automatically satisfiesthe objectivity requirements. By using a generalcontinuum mechanics approach, a relatively simple expression for theelastic forces is obtained. Generalization of the formulation to thecase of shell elements is discussed. As examples of the implementationof the proposed method, two different plate elements are presented; oneplate element does not guarantee the continuity of the displacementgradients between the nodal points, while the other plate elementguarantees this continuity. Numerical results are presented in order todemonstrate the use of the proposed method in the large rotation anddeformation analysis of plates and shells. The numerical results, whichare compared with the results obtained using existing incrementalprocedures, show that the solution obtained using the proposed methodsatisfies the principle of work and energy. These results are obtainedusing explicit numerical integration method. Potential applications ofthe proposed method include high-speed metal forming, vehiclecrashworthiness, rotor blades, and tires.     

Fibre mortar composites in fire conditions

A small-scale test series was carried out using the heating system (radiant exposure) of a cone calorimeter to detect any differences in the way different fibres affect the thermal properties of a standard mortar. The fibres were different polypropylene, polyacrylonitrile, aramide, carbon or steel. Fibres affect the release of moisture from the fibre mortar material. Local pressures caused by water vaporization due to rapid heating can be decreased by incorporating fibres. Fibres have a weak insulating effect. However, use of polyacrylonitrile fibres in mortar may increase the risk to spalling under rapid thermal exposure such as fire. The moisture level in specimens is highly significant for their thermal properties and hence their fire behavior.     

High strain rate deformation of Mo and Mo-33re by shock loading: Part II. Rates of defect generation and accumulation of plastic strain

The kinetics of plastic flow in Mo and Mo-33Re under shock loading conditions has been examined by characterizing the substructure quantitatively and relating these observations to defect generation rates and the dynamic strength of the materials. The finite dislocation generation rates, apparently limited to the order of 10 21 m⁻² s⁻¹ result in a maximum plastic strain rate on the order of 10 6 s⁻¹ and a maximum dislocation velocity of ∼3 x 10 2 m s⁻¹. The maximum shear stress imposed on the materials is estimated to be ∼μ/50, at short pulse durations. The estimated shear stresses, dislocation velocities, and production of dislocations as a function of strain are found to be consistent with conventional deformation mechanisms, and require no novel material behavior such as homogeneous nucleation of dislocations or supersonic dislocation velocities.     

Effect of composition and deformation temperature on the annealing behavior of stacking faults in α-Cu-Ge alloys

The disappearance of stacking faults in filings of α-Cu-Ge alloys (5.6, 6.7, 7.7, and 8.5 at. pct Ge) prepared at various deformation temperatures has been studied with X-ray diffraction. The faults anneal out in two stages, the second of which is diffusion controlled. The time necessary to anneal out all of the faults depends on both alloy composition and deformation temperature. The dependence of the number of faults on the deformation temperature has also been determined.     

Mass transport in the cathode of a free-breathing polymer electrolyte membrane fuel cell

In small fuel cell applications, it is desirable to take care of the management of reactants, water and heat by passive means in order to minimize parasitic losses. A polymer electrolyte membrane fuel cell, in which air flow on the cathode was driven by free convection, was studied by experimental and modelling methods. The cathode side of the cell had straight vertical channels with their ends open to the ambient air. A two-dimensional, isothermal and steady state model was developed for the cathode side to identify the limiting processes of mass transport. The modelled domain consists of the cathode gas channel and the gas diffusion layer. Experimental data from current distribution measurements were used to provide boundary conditions for oxygen consumption and water production. The model results indicate that at the cell temperature of 40 °C the performance of the cell was limited by water removal. At the cell temperature of 60 °C, the current distribution was determined by the partial pressure of oxygen.     

Wood stove use and other determinants of personal and indoor exposures to particulate air pollution and ozone among elderly persons in a Northern Suburb

A six‐month winter‐spring study was conducted in a suburb of the northern European city of Kuopio, Finland, to identify and quantify factors determining daily personal exposure and home indoor levels of fine particulate matter (PM_2.5, diameter <2.5 µm) and its light absorption coefficient (PM_2.5abs), a proxy for combustion‐derived black carbon. Moreover, determinants of home indoor ozone (O₃) concentration were examined. Local central site outdoor, home indoor, and personal daily levels of pollutants were monitored in this suburb among 37 elderly residents. Outdoor concentrations of the pollutants were significant determinants of their levels in home indoor air and personal exposures. Natural ventilation in the detached and row houses increased personal exposure to PM_2.5, but not to PM_2.5abs, when compared with mechanical ventilation. Only cooking out of the recorded household activities increased indoor PM_2.5. The use of a wood stove room heater or wood‐fired sauna stove was associated with elevated concentrations of personal PM_2.5 and PM_2.5abs, and indoor PM_2.5abs. Candle burning increased daily indoor and personal PM_2.5abs, and it was also a determinant of indoor ozone level. In conclusion, relatively short‐lasting wood and candle burning of a few hours increased residents’ daily exposure to potentially hazardous, combustion‐derived carbonaceous particulate matter.