Friction force models play a fundamental role for simulation of mechanical systems. Their choice affects the matching of numerical results with physically observed behavior. Friction is a complex phenomenon depending on many physical parameters and working conditions, and none of the available models can claim general validity. This paper focuses the attention on well-known friction models and offers a review and comparison based on numerical efficiency. However, it should be acknowledged that each model has its own distinctive pros and cons. Suitability of the model depends on physical and operating conditions. Features such as the capability to replicate stiction, Stribeck effect, and pre-sliding displacement are taken into account when selecting a friction formulation. For mechanical systems, the computational efficiency of the algorithm is a critical issue when a fast and responsive dynamic computation is required. This paper reports and compares eight widespread engineering friction force models. These are divided into two main categories: those based on the Coulomb approach and those established on the bristle analogy. The numerical performances and differences of each model have been monitored and compared. Three test cases are discussed: the Rabinowicz test and other two test problems casted for this occurrence.

Review and comparison of dry friction force models

Superoxide dismutase 1 (Sod1) has been known for nearly half a century for catalysis of superoxide to hydrogen peroxide. Here we report a new Sod1 function in oxidative signalling: in response to elevated endogenous and exogenous reactive oxygen species (ROS), Sod1 rapidly relocates into the nucleus, which is important for maintaining genomic stability. Interestingly, H2O2 is sufficient to promote Sod1 nuclear localization, indicating that it is responding to general ROS rather than Sod1 substrate superoxide. ROS signalling is mediated by Mec1/ATM and its effector Dun1/Cds1 kinase, through Dun1 interaction with Sod1 and regulation of Sod1 by phosphorylation at S60, 99. In the nucleus, Sod1 binds to promoters and regulates the expression of oxidative resistance and repair genes. Altogether, our study unravels an unorthodox function of Sod1 as a transcription factor and elucidates the regulatory mechanism for its localization.

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Superoxide dismutase 1 acts as a nuclear transcription factor to regulate oxidative stress resistance.

The antioxidant, anti-inflammatory, and cytotoxic activities of water and ethanol extracts of 14 Chinese medicinal plants were investigated and also their total phenolics and flavonoid contents measured. The antioxidant activity was evaluated in a biological assay using Saccharomyces cerevisiae, whereas the radical scavenging activity was measured using the 2,2-diphenyl-1-picrylhydrazyl (DPPH) method. Total phenolics and flavonoid contents were estimated by Folin-Ciocalteu and aluminum chloride methods, respectively. The anti-inflammatory activities of the plant extracts were determined by measuring the inhibition of production of nitric oxide (NO) and TNF-α in LPS and IFN-γ activated RAW 264.7 macrophages. Their cytotoxic activities against macrophages were determined by Alamar Blue assay. Four plants, namely, Scutellaria baicalensis, Taxillus chinensis, Rheum officinale, and Sophora japonica, showed significant antioxidant activity in both yeast model and also free radical scavenging methods. The ethanol extract of S. japonica showed highest levels of phenolics and flavonoids (91.33 GAE mg/g and 151.86 QE mg/g, respectively). A positive linear correlation between antioxidant activity and the total phenolics and flavonoid contents indicates that these compounds are likely to be the main antioxidants contributing to the observed activities. Five plant extracts (S. baicalensis, T. chinensis, S. japonica, Mahonia fortunei, and Sophora flavescens) exhibited significant anti-inflammatory activity by in vitro inhibition of the production of NO and TNF-α with low IC 50 values. These findings suggest that some of the medicinal herbs studied in this paper are good sources of antioxidants.

Antioxidant and anti-inflammatory activities of selected medicinal plants containing phenolic and flavonoid compounds.

This note presents a modification of the integrated friction model structure proposed by Swevers et al. (2000), called the Leuven model. The Leuven model structure allows accurate modeling both in the presliding and the sliding regimes without the use of a switching function. The model incorporates a hysteresis function with nonlocal memory and arbitrary transition curves. This note presents two modifications of the Leuven model. A first modification overcomes a recently detected shortcoming of the original Leuven model: a discontinuity in the friction force which occurs during certain transitions in presliding. A second modification, using the general Maxwell slip model to implement the hysteresis force, eliminates the problem of stack overflow, which can occur with the implementation of the hysteresis force.

Technical Note: modification of the Leuven integrated friction model structure

This study is aimed at examining and comparing several friction force models dealing with different friction phenomena in the context of multibody system dynamics. For this purpose, a comprehensive review of present literature in this field of investigation is first presented. In this process, the main aspects related to friction are discussed, with particular emphasis on the pure dry sliding friction, stick–slip effect, viscous friction and Stribeck effect. In a simple and general way, the friction force models can be classified into two main groups, namely the static friction approaches and the dynamic friction models. The former group mainly describes the steady-state behavior of friction force, while the latter allows capturing more properties by using extra state variables. In the present study, a total of 21 different friction force models are described and their fundamental physical and computational characteristics are discussed and compared in details. The application of those friction models in multibody system dynamic modeling and simulation is then investigated. Two multibody mechanical systems are utilized as demonstrative application examples with the purpose of illustrating the influence of the various frictional approaches on the dynamic response of the systems. From the results obtained, it can be stated that both the choice of the friction force model and friction parameters involved can significantly affect the simulated/modeled dynamic response of mechanical systems with friction.

A survey and comparison of several friction force models for dynamic analysis of multibody mechanical systems

An enhanced formulation to model spatial revolute joints with radial and axial clearances

Friction exists in most mechanical systems, and it can have a major influence on their dynamic performance and operating conditions. As a consequence of frictional contact phenomena, energy is dissipated and the state of a system can change slowly and rapidly, depending on the nature of the contact, continuous or impact condition. Other effects associated with friction in mechanical systems are the vibration and noise propagation of the system components, nonlinear systems’ behavior and wear. Overall, the knowledge of the friction regimen, as well as the frictional forces developed at the interface of mechanical parts in contact with relative motion, is crucial for the dynamic analysis of mechanical systems, and has consequences in the design process. Thus, this work is a review of the modeling and analysis of frictional effects in multibody systems with the purpose of better understanding and obtaining accurate responses. In this process, pure dry sliding friction, stick–slip effect, viscous friction, Stribeck effect, and frictional lag are some of the main phenomena associated with friction, which are addressed in depth. Overall, the friction models can be divided into two main groups, namely the “static friction models” and the “dynamic friction models”. The static models describe the steady-state behavior of the relation friction-force/relative-velocity, while the dynamic models allow for the capturing of more physical responses and properties by using extra state variables. In a simpler manner, the static and dynamic friction models differ mostly in the modeled frictional effects, implementation complexity, and computational efficiency. Hence, this research is aimed at analyzing in detail the role of friction modeling in the dynamic response of multibody system, as well as addressing the importance of friction models selection for accurately describing the friction related phenomena. Demonstrative application examples, which include friction in ideal mechanical joints and systems involving contact–impact events, including an example of rolling contact will be considered and investigated to illustrate the main assumptions and procedures adopted in this work. The results from this study indicate that in most cases, a static friction model, which accounts for static friction and avoids the discontinuity at zero velocity, is a suitable choice. A more advanced dynamic friction model has to be developed to be utilized for systems containing high variations of normal load, namely with impact conditions.

Modeling and analysis of friction including rolling effects in multibody dynamics: a review

It is known that the dynamic equations of motion for constrained mechanical multibody systems are frequently formulated using the Newton–Euler’s approach, which is augmented with the acceleration constraint equations. This formulation results in the establishment of a mixed set of partial differential and algebraic equations, which are solved in order to predict the dynamic behavior of general multibody systems. The classical solution of the equations of motion is highly prone to constraints violation because the position and velocity constraint equations are not fulfilled. In this work, a general and comprehensive methodology to eliminate the constraints violation at the position and velocity levels is offered. The basic idea of the described approach is to add corrective terms to the position and velocity vectors with the intent to satisfy the corresponding kinematic constraint equations. These corrective terms are evaluated as a function of the Moore–Penrose generalized inverse of the Jacobian matrix and of the kinematic constraint equations. The described methodology is embedded in the standard method to solve the equations of motion based on the technique of Lagrange multipliers. Finally, the effectiveness of the described methodology is demonstrated through the dynamic modeling and simulation of different planar and spatial multibody systems. The outcomes in terms of constraints violation at the position and velocity levels, conservation of the total energy and computational efficiency are analyzed and compared with those obtained with the standard Lagrange multipliers method, the Baumgarte stabilization method, the augmented Lagrangian formulation, the index-1 augmented Lagrangian, and the coordinate partitioning method.

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On the constraints violation in forward dynamics of multibody systems

Chromosomal DNA damage can be a result of several processes and agents of endogenous or exogenous origin. These cause strand breaks or oxidized bases that lead to strand breaks, which relax the normally supercoiled genomic DNA and increase its electrophoretic mobility. The extent of DNA damage can be assessed by single cell gel electrophoresis, where the chromosomal DNA migration distance correlates with the extent of DNA damage. This technique has been used for a variety of applications with several organisms, but only a few studies have been reported for Saccharomyces cerevisiae. A possible reason for this absence is that low cellular DNA content could hamper visualization. Here we report an optimization of the comet assay protocol for yeast cells that is robust and sensitive enough to reproducibly detect background DNA damage and oxidative damage caused by hydrogen peroxide. DNA repair was observed and quantified as diminishing comet tail length with time after oxidative stress removal in a process well described by first-order kinetics with a tail length half-life of 11 min at 37 °C. This is, to our knowledge, the first quantitative measurement of DNA repair kinetics in S. cerevisiae by this method. We also show that diet antioxidants protect from DNA damage, as shown by a three-fold decrease in comet tail length. The possibility of assessment of DNA damage and repair in individual cells applied to the model organism S. cerevisiae creates new perspectives for studying genotoxicity and DNA repair.

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Filipe Marques

Papers

A survey and comparison of several friction force models for dynamic analysis of multibody mechanical systems

An enhanced formulation to model spatial revolute joints with radial and axial clearances

Modeling and analysis of friction including rolling effects in multibody dynamics: a review

On the constraints violation in forward dynamics of multibody systems

Measuring oxidative DNA damage and DNA repair using the yeast comet assay.