Numerical investigation for erratic behavior of Kriging surrogate model

Kriging model is one of popular spatial/temporal interpolation models in engineering field since it could reduce the time resources for the expensive analysis. But generation of the Kriging model is hardly a sinecure because internal semi-variogram structure of the Kriging often reveals numerically unstable or erratic behaviors. In present study, the issues in the maximum likelihood estimation which are the vital-parts of the construction of the Kriging model, is investigated. These issues are divided into two aspects; Issue I is for the erratic response of likelihood function itself, and Issue II is for numerically unstable behaviors in the correlation matrix. For both issues, studies for specific circumstances which might raise the issue, and the reason of that are conducted. Some practical ways further are suggested to cope with them. Furthermore, the issue is studied for practical problem; aerodynamic performance coefficients of two-dimensional airfoil predicted by CFD analysis. Result shows that such erratic behavior of Kriging surrogate model can be effectively resolved by proposed solution. In conclusion, it is expected this paper could be helpful to prevent such an erratic and unstable behavior.     

Use of Au Nanoparticle-Filled PTFE Films to Produce Low-Friction and Low-Wear Surface Coatings

Polytetrafluoroethylene (PTFE) possesses exceptional lubricating properties; however, its uses are limited due to its high susceptibility to wear. In an effort to overcome this shortcoming, a great deal of focus is placed on creating PTFE composites that exploit the strengths of PTFE and also reduce or eliminate its weaknesses. This investigation explores the use of Au nanoparticle-filled PTFE films to produce low-friction and low-wear surface coatings. PTFE + Au nanoparticle composite films were produced by dip coating stainless steel substrates into a mixture of colloidal PTFE and Au nanoparticles. Tribological tests showed that the composite film has a wear life that is twice that of pure PTFE and possesses an average coefficient of friction that is up to 50 % lower. PTFE suffers delamination as a result of poor adhesion of the film to the substrate and tearing resulting from a dominant adhesive wear mode. PTFE + Au, on the other hand, shows no sign of delamination or adhesive wear. This change in wear mode caused by the addition of Au nanoparticles significantly increases the wear resistance and durability of PTFE.     

Unified synthesis of Watt-I six-link mechanisms using evolutionary optimization

A Watt-I mechanism can operate in eight different combinations of assembly modes and output link. In this paper, a novel approach is presented for carrying out unified optimum synthesis of various combination types of Watt-I mechanism, irrespective of whether identical or different ranges of variables are specified for different combination types. By carrying out unified synthesis the less suited combination types can be identified, leading to their elimination from the synthesis process. This results in a saving of the overall computational time. The presented approach can be implemented with most of the evolutionary optimization methods. In this paper, the Differential Evolution algorithm is chosen as the optimization method. Unified optimization results are presented for two problems. The proposed approach is general and can be used, with suitable modifications, to carry out unified optimum design of alternate mechanical systems which can perform a given task.     

Effects of moisture ingression on polymeric laminate composites and its prevention via highly robust barrier films

Three different fiber-reinforced composite test laminates were laid up using carbon-, glass-, and Kevlar-reinforced epoxy prepregs, and then four different hydrophobic barrier films were placed as the out-of-most ply (last ply) on top of the test laminates. The prepared samples were co-cured through an autoclave per recommended cure cycles. These hydrophobic barrier films included polyether ether ketone or PEEK (12.7- and 25.4-μm thicknesses), polytetrafluoroethylene or Teflon (25.4 μm), and polyvinyl fluoride (PVF) or Tedlar (25.4 μm). Tedlar films have been the only source used for moisture prevention in Aerospace composites, so the purpose of the present study was to determine other alternatives and their moisture ingression prevention characteristics. The tape adhesion tests conducted on the barrier films of the composite panels indicated that PEEK and Tedlar films were well bonded on the composite surfaces, while Teflon films failed the tape adhesion tests. The laminate composites that were co-bonded with barrier films were immersed in water up to 29 days, and then 3-point bend tests were conducted on each sample before and after immersion. Test results show that 25.4-μm thick PEEK and Tedlar films on the carbon, glass, and Kevlar laminate composites provided similar mechanical properties. Also, the laminates incorporated with barrier films exhibited significantly higher mechanical properties when compared to the same laminates without any barrier films. This study indicated that these barrier films considerably reduced moisture ingression into the laminate composite structures, which may be useful for applications in composite aircraft and wind turbines.     

A complete real-time Ethernet solution for numerical control systems

This paper proposes a complete real-time Ethernet (RTE) solution including master and slave controllers for numerical control (NC) systems. To meet the development tendency of NC systems moving toward multi-axis coordination, high speed and high precision, one type of RTE architecture is built up with a dual ring topology. The function blocks of master/slave nodes are designed to realize the real-time communication capabilities using embedded CPU and field programmable gate array (FPGA) technologies. To reduce the minimum achievable cycle time, a cut-through transmission mechanism is employed to decrease the process latency at slave nodes, and the synchronization frame is optimized to shorten the frame duration. In order to synchronize the nodes of RTE-based NC systems accurately, a time synchronization strategy is scheduled and a proportion-integration (PI)-based phase-locked loop (PLL) is designed to keep the master clock and the slave clocks in step stably. The theoretical analysis and experimental results show that the proposed RTE solution can achieve good real-time performance with low process latency and high time-synchronization accuracy, which satisfies the real-time communication requirements of RTE-based NC systems in an economical way.     

Identifying Important Differences in Mass Spectra Generated by Secondary ion Mass Spectrometry (TOF–SIMS) in a Tribochemical Study

In order to better understand the lubricating properties of diesel fuel, species that were present on tribological surfaces were investigated using secondary ion time-of-flight mass spectrometry (TOF–SIMS). Traditionally, only certain species that are expected to be present at the interface are investigated and their presence or absence is used to make conclusions regarding the mechanism of lubrication. In this work, an alternative and complementary approach to data analysis and interpretation is proposed, previously demonstrated for TOF–SIMS and based on multivariate analysis methods, where the mass spectral data are investigated more comprehensively. The main objective was to interpret variation within and between different areas of a tested surface and ultimately to contribute to the understanding of the tribochemical reactions that occur at the interface. The validity of this approach was confirmed when the palmitate ion (which would normally be targeted) was shown to contribute significantly (together with other ions) to chemical differences between scratched and unscratched areas of the surface.     

Studies of the Kinetics of Lithium Grease Microstructure Regeneration by Means of Dynamic Oscillatory Rheological Tests and FTIR–ATR Spectroscopy

As part of this research, the kinetics of soap grease thickener microstructure regeneration during 24-h relaxation after 1-h shearing were studied. A lubricating grease, based on mineral oil ORLEN OIL SN-400, containing associated molecules of lithium 12-hydroxystearate was subjected to analysis. Rheological dynamic oscillatory and FTIR–ATR (Fourier Transform Infrared Attenuated Total Reflectance) spectroscopy were used to study the kinetics of thickener microstructure regeneration. Changes in the values of storage modulus G′ and loss modulus G″ of the investigated grease during its relaxation in the linear viscoelastic range were examined. In addition, the mechanical stability of the thickener microstructure at a variable oscillation frequency was tested, and changes in the cohesive energy of the grease versus the latter’s relaxation time were assessed. FTIR–ATR spectroscopy was used to study the physicochemical interactions between the associated molecules of lithium 12-hydroxystearate. Infrared bands were assigned to the vibrations of the particular functional groups of the grease components, and the spectral parameters in the absorption spectra in the range of 700–3,700 cm^?1 were examined. The studies carried out using the two investigative methods have revealed that the intensity of thickener microstructure regeneration was the highest in the first hour of grease relaxation after shearing. In this time interval, the largest changes in the vibrations of the main functional groups of the grease components were observed. In the next hours, the kinetics of thickener microstructure regeneration were substantially lower.     

Leonardo da Vinci’s Friction Experiments: An Old Story Acknowledged and Repeated

Leonardo da Vinci (1452–1519) is universally regarded as a brilliant polymath, designer, astronomer, artist, philosopher, and a visionary engineer of the Renaissance era. Interestingly, due to the delayed discovery of several caches of his notebook pages (as late as the 1960s), his immense contribution to the field of tribology has only recently surfaced. From these salvaged documents, da Vinci’s three notable observations that preceded the development of the laws of friction were uncovered: (1) Friction is independent of apparent contact area, (2) the resistance of friction is directly proportional to applied load, and (3) friction has a consistent value of µ = 0.25. In this work, we have attempted to construct a nearly faithful recreation of Leonardo da Vinci’s apparatus for measuring friction based on his notebook illustrations and investigate the conditions under which Leonardo da Vinci’s experiments produced his findings. Our experiments, performed roughly 500 years later, reproduced Leonardo da Vinci’s findings of friction coefficients with wood of µ = 0.25, but only under conditions of roughly cut and brusquely squared samples of dry wood that were handled and sullied by hand in a fashion typical of wood working but inconsistent with the modern laboratory practice. Thus, our interpretation of Leonardo da Vinci’s findings is that these first tribological studies were actually performed on roughly cut and unpolished samples that had been handled extensively prior to and during testing; Such a procedure of sample preparation is entirely reasonable for the time period and suggests an active, dusty, and dynamic laboratory environment.     

A chamber composed of precision Mylar drift tubes

The design of a drift chamber composed of three layers of thin-walled (0.125 mm) precision Mylar tubes is described. Twenty six chambers of this type with areas from 1 × 1 to 2.5 × 2 m² comprising 4392 drift tubes have been produced for experiments at the 70-GeV accelerator at the Institute for High Energy Physics. The chamber design, the assembling procedure, and test results are presented.     

Scientific basis of creation of elastic structures with special vibroacoustic properties

A new scientific approach for the work is characterized, the results of theoretical and experimental research obtained recently by the department of Theoretical and Applied Acoustics are revealed in the article. Prospects for their development are outlined.     

Mechanical and Tribological Properties of Ekonol Blends as Frictional Materials of Ultrasonic Motors

While high friction coefficients and good wear resistance are antagonistic properties of most materials, these properties are expected to promote excellent torque-speed characteristics and extend the life span of ultrasonic motors. Blending is an accepted technique for modifying tribological applications. p-Hydroxybenzoic acid polymer (Ekonol) blends with different compositions, and proportions were prepared through mechanical blending. Poly(tetrafluoroethylene) (PTFE), poly(etheretherketone), and poly(phylenesulfide) (PPS) were selected as dispersed phases. The mechanical properties of the blends were investigated, and their tribological performance was tested using a block-on-ring wear meter. The worn surfaces of Ekonol blends were observed using a scanning electron microscope to elucidate the relevant wear mechanisms. Results showed that the dispersed phases have distinct effects on the impact strength and hardness, as well as friction coefficient and wear rate, of the blends. Curves of hardness and friction coefficient versus the dispersed phase content showed apparent similarities, which indicates that hardness influences the friction of polymer blends in contact with carbon steel. Worn tracks on the surfaces of different polymer materials showed that the dominant wear mechanism transforms from fatigue and abrasion into adhesion with the addition of a dispersed phase; delamination was observed in the transfer films, especially those formed by the Ekonol/PTFE and Ekonol/PPS blends.     

The effect of high-dispersion fillers on adhesive and frictional properties of ethylene-vinyl acetate copolymer

The adhesive strength of adhesive joints between metal surfaces has been improved considerably by modifying an adhesive based on a copolymer of ethylene with montmorillonite vinyl acetate. It as been proved that the static (adhesive joint) and dynamic (frictional interaction) adhesion modes are correlated. It has been shown that the surface potential found by a modified Kelvin-Zisman method increases with increasing adhesion of the metal-polymer joint.     

Variance of Particle Size: Another Monitor to Evaluate Abrasive Wear

The effect of variance of particle size plays a dominant role in the wear rate of materials. In the present research, such effect is investigated using computer simulation, in which Monte Carlo method has been combined to calculate the wear rate of the materials. Abrasive wear rates of materials with the same particle average size exhibited a rapid increase with increasing variance of particle sizes distribution from small variance. Then wear rates maximize and decrease slightly with further increase of variance. The relationship between particle variances and hardness is also investigated. The particle size effect is explained via comparison of wear pattern of different particle sizes.     

The Role of Functional End Groups of Perfluoropolyether (Z-dol and Z-03) Lubricants in Augmenting the Tribology of SU-8 Composites

In an earlier work, we demonstrated the development of SU-8 composites using perfluoropolyether (PFPE) as lubricant filler which reduced friction coefficient by ~7 times and enhanced wear life of SU-8 by more than four orders of magnitude. In this work, we have investigated the role of chemical bonding between SU-8 and PFPE molecules using two types of PFPE lubricants (i.e., Fomblin^® Z-dol and Z-03) in improving the tribological properties of the composite. Z-dol has polar (–OH) end groups whereas Z-03 has non-polar (CF₃) end groups. SU-8 with Z-dol (SU-8 + Z-dol) films yielded ~8 times greater wear life than SU-8 with Z-03 (SU-8 + Z-03) films and more by four orders of magnitude than pure SU-8. The nature of the films was analyzed in detail by chemical and physical characterization techniques like X-ray photoelectron spectroscopy, water contact angle and thermo-gravimetric analysis. The results validated the role of polar end functional group of Z-dol in covalent binding with SU-8 upon UV plasma treatment that resulted in improved tribological properties.     

An experimental study on crack healing of various glassy polymers

The validity of a simple tube model proposed by the senior author for crack healing recovery of various polymeric materials such as PMMA(Polymethyl-methacrylate) and Homalite-100(A Polyester) has been tested. The experimental results of recovery in terms of healing fracture toughness were compared with the theoretically predicted trends of recovered fracture toughness. The agreements between these results were found to be reasonable within the limitation of experimental method. After healing the PMMA CT (compact tension) specimens with various solvents having different solubility parameters(δ), the healing recovery (R) was measured by both the method of maximum load and the method of caustics. The relationship between R and solubility differences (Δδ) between a solven and a glass polymer was found to be R∝(Δδ)?1/4.     

Influence of viscosity modifiers on the rheological properties of synthetic oils

The study of the effect that the molecular weight and concentration of the polymer gelling additive PMMA (polymethylmethacrylate) introduced into the synthetic base oil PAO6 has on viscosity. The empirical dependence of the viscosity of the thickened oil on the content of additives has been established and the coefficients of the thickening efficiency of additive have been determined. An experimental study of the viscosity of synthetic oils on the shear rate has been carried out at different contents of viscosity modifier PMMA with different molecular weights in a base of PAO6. The ranges of velocities have been determined at which the oil exhibits properties of non-Newtonian fluid. It has been shown that, with an increase in the molecular weight of PMMA of additive at the same concentration in its base, the non-Newtonian behavior of oil begins to emerge at lower shear rates. It has been found that the use of an acoustic arsenal of the operational control of the viscosity of lubricating oils based both on the piezoelectric and the magnetoelastic interaction is promising for the operational control of the viscosity of lubricating oils.     

Starvation and Reflow of Point Contact Lubricated with Greases of Different Chemical Formulation

Starved film thickness measurements are presented in grease-lubricated point contact under pure rolling condition. The starvation process of different types of greases is recorded and analysed in detail. Results show that the film decay rates for lithium greases and SiO₂ greases are quite similar regardless of the rolling speed. Urea grease shows strong adsorbability to the metal surface, which can be clearly observed during the test. Oil separation tests and scanning electron microscopy observations proved the differences in structure and stability of the tested grease. A close relationship between starved film thickness and oil bleeding rate is found in the present research.     

Development of self-lubricating antifriction materials based on polytetrafluoroethylene and modified zeolites

The influence of zeolite treated by engine oil on the thermophysical and operational characteristics, as well as on the structure of polytetrafluoroethylene-based composites, is studied. It is established that filling PTFE by zeolite preliminarily soaked with M-8V oil leads to a decrease in the mass wear rate by to 600 times and a decrease in the coefficient of friction by twofold. New materials with self-lubricating effects for wear-resistant multipurpose friction units with high load capacity used in mechanisms that operate in the Russian Far North are developed.     

Optimization of cryogenic cooled EDM process parameters using grey relational analysis

This paper presents an experimental investigation on cryogenic cooling of liquid nitrogen (LN₂) copper electrode in the electrical discharge machining (EDM) process. The optimization of the EDM process parameters, such as the electrode environment (conventional electrode and cryogenically cooled electrode in EDM), discharge current, pulse on time, gap voltage on material removal rate, electrode wear, and surface roughness on machining of AlSiCp metal matrix composite using multiple performance characteristics on grey relational analysis was investigated. The L₁₈ orthogonal array was utilized to examine the process parameters, and the optimal levels of the process parameters were identified through grey relational analysis. Experimental data were analyzed through analysis of variance. Scanning electron microscopy analysis was conducted to study the characteristics of the machined surface.     

Identification of suitable process parameters for friction surfacing of mild steel with AA6063 aluminium alloy

The present study is aimed at identifying suitable process parameters for friction surfacing of IS2062 mild steel with AA6063 aluminium alloy and obtain relationships between the input parameters and the process response that can be used for sound coating with adequate strength and ductility. Factorial experimental design technique was used to investigate and select the parameter combination to achieve adequate strength and ductility. Mechatrode rotational speed, axial load and substrate traverse speed were observed to be the most significant factors on the process response. Based on the results, it was observed that (i) lower axial load and traverse speed with high rotational speed and (ii) higher axial load and traverse speed with lower rotational speed produced sound coatings. It has also been observed that sound coatings could be obtained in a narrow set of parameter range as the substrate-coating materials are metallurgically incompatible and have propensity to form brittle intermetallics.