Graphene has attracted tremendous attention as a promising additive in lubricants due to its unique lamellar structure and excellent mechanical strength. Yet, unlike its use in oil and water lubricants, the amount of graphene additive should be considered when it is introduced into the grease which is a two-phase colloid. In this work, graphene was added into the lithium grease in different concentrations, and lubrication behaviors were investigated using a four-ball testing method under various operating conditions. Prior to the four-ball friction tests, graphene and grease materials were characterized by scanning (SEM) and transmission electron microscopy (TEM), X-ray diffraction (XRD), atomic force microscopy (AFM), and Raman spectroscopy. Friction test results demonstrate that the graphene concentration in grease varies at different tribological contact conditions to reach the optimum lubrication behavior. On the basis of the results from friction tests and worn scar morphology analysis, a lubrication mechanism was proposed to better understand the interactions among grease elements, e.g. graphene, thickener, and base oil, during the shearing process. It is believed that thickener soap actively participates in the lubrication process at low speeds by releasing enough oil into the friction contacts under high load. Meanwhile, less graphene concentration is required to strengthen the base grease by inhibiting and avoiding severe wear. High rotational speeds negatively affect the “oil-bleed” capability of thickener under lower contact loads due to the churning loss at high centrifugal force. Thus, extra graphene additives are required to retain more oil and separate the contact surfaces. This, in turn, promotes the formation of protective tribofilm on the interface which is the key to the enhancement of antifriction and antiwear performance.

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This paper presents structural studies of a medium scale composite wind turbine blade construction made of epoxy glass fiber for a 750kW rated power stall regulated horizontal axis wind turbine system. The complex geometry of the blade with a skin-spar foam sandwich structure was generated by utilizing commercial code ANSYS finite element package. Dimensions of twist, chord and thickness were developed by computer program. NREL S-series airfoils with different chord thickness are used along current blade cross-sections. The current design method uses blade element momentum (BEM) theory to complete satisfactory blade design and can be carried out using a spreadsheet, lift and drag curves for the chosen aerofoil. According to composite laminate theory and finite element method, optimal blade design was obtained. The focus is on the structural static strength of wind turbine blades loaded in flap-wise direction and methods for optimizing the blade cross-section to improve structural reliability. Moreover, the natural frequencies and modal shapes of the rotor blade were calculated for defining dynamic characteristics. Structural analysis was performed by using the finite element method in order to evaluate and confirm the blade to be sound and stable under various load conditions.

Frictional and fretting wear behaviors of Inconel X-750 alloy against GCr15 steel ball were investigated in dry contact condition with ∼60% air humidity. Fretting tests were run at the high frequency tribosystem SRV 4 in room temperature and ball-on-flat contact configuration were adopted with the relative oscillatory motion of small displacement amplitude (40 μm). Sliding regimes, wear volumes, frictional properties, and material damage mechanisms were studied with regard to different normal loading and test durations. After the tests, the worn surface morphologies were analyzed by three-dimensional (3D) optical surface profiler, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) to distinguish fretting running conditions and material responses for different test cases. It was found that the material removals by abrasive and adhesive wear, debris formation and oxidization, and wear delamination were the main damage mechanisms under the lower normal load where the full slide or gross slip regime (GSR) was dominant between the contact surfaces. On the other hand, fretting regime was found to be a stick-slip or a partial slip at greater loads where damage mechanisms were correlated with deformed asperities, fatigue cracks, and thick layer removal due to highly concentrated cyclic stresses. Time dependence was crucial during GSR where the wear volume increased substantially; however, the wear volumes and scars sizes were consistent over time because of stick-slip effects under the higher normal load.

Tribological behavior of the Inconel X-750 alloy disk subjected to fretting against the GCr15 steel ball was investigated in an ambient laboratory air with relative humidity of 55–65%. A high-frequency oscillating Optimol SRV 4 tribometer was employed to execute dry fretting tests in the partial and gross slip regimes under constant 100 N normal load. Tests were carried out for 10, 30, and 90 minutes, and the friction forces vs. displacement amplitudes were monitored during the test duration. Posttest examinations were conducted utilizing advanced tools such as 3D optical surface profiler, scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX). The main objective was to obtain wear scar evolutions, frictional properties, and degradation mechanisms under the different running conditions over time. It was found that fretting wear behaviors of friction pairs were strongly influenced by fretting regimes. Degradation evolutions were greatly influenced by fretting time during partial slip regimes, i.e., evolving from asperity deformation and slight damage to the fatigue crack and material transfer. However, the combination of adhesive, abrasive, delamination, and wear oxidation mechanisms was repeated during the entire gross slip fretting process.