Low Normal Loads Research Articles

Microstructural and tribological properties of nanostructured Al6061-CNT produced by mechanical milling and extrusion

Wear properties of Al6061-CNT nanocomposites have been studied using dry sliding method. Nanostructured Al6061 powder has been prepared using 30 h mechanical milling of atomized powder, and mixed with different percent of CNT. The yielded mixture was subjected to mechanical milling for 0.5 and 4 h. Mixtures were cold compacted, homogenized and extruded to achieve 13 mm diameter round cross section bars. Relative density and hardness of specimens were determined for extruded samples. Maximum relative density was achieved for specimen without CNT which was about 99.848%, while maximum hardness of 236.1 HV was for specimen with 1.25 wt% CNT. Pin on disk method wear test results indicated that weight loss and wear mechanism strongly depends on normal load. At lower normal load, minimum weight loss achieved for Al6061-1.25 wt% CNT nanocomposite at abrasive and delamination mechanisms, while by increasing of normal load, minimum weight loss achieved for specimen without CNT that its wear mechanism was adhesive and delamination.

An Experimental Study on the Effect of Surface Texture on Contact Stiffness Using Soft Material

Contact interface is one of the most important factors in a mechanical contact because it is the place where friction, sound, and heat originate. It is therefore inevitable that modeling various phenomenon related to contact dynamics requires a proper representation of the contact interfaces. One of the methods in representing the behavior of two surfaces in contact is by using the parameter of contact stiffness. In this study, the effect of surface texture on contact stiffness is analyzed. the texture was used in order to reduce the randomness of surface roughness. the soft material was chosen to achieve a pure elastic contact thus preventing plastic deformation to the asperities. the analysis was conducted by using an indentation method employing a steel ball with a relatively small indentation force. the result show contact stiffness values of the textured surfaces were smaller than that of smooth surface. This is particularly observed at low normal load at which total deformation of the surface is relatively small compared to the asperities height. This decrease in the contact stiffness value of the textured surfaces can be related to the reduction in the real contact area.

Stuy on Fatigue Life of Aluminum Alloy Considering Fretting

To study the influence of fretting on Aluminum Alloy, a global finite element model considering fretting was performed using the commercial code ABAQUS. With which a new model for predicting fretting fatigue life has been presented based on friction work. The rationality and effectiveness of the model were validated according to the contrast of experiment life and predicting life. At last influence factor on fretting fatigue life of aerial aluminum alloy was investigated with the model. The results revealed that fretting fatigue life decreased monotonously with the increasing of normal load and then became constant at higher pressures. At low normal load, fretting fatigue life was found to increase with increase in the pad radius. At high normal load, however, the fretting fatigue life remained almost unchanged with changes in the fretting pad radius. The bulk stress amplitude had the dominant effect on fretting fatigue life. The fretting fatigue life diminished as the bulk stress amplitude increased.

Finite element simulation of low velocity impact loading on a sandwich composite

Sandwich structure offer more advantage in bringing flexural stiffness and energy absorption capabilities in the application of automobile and aerospace components. This paper presents comparison study and analysis of two types of composite sandwich structures, one having Jute Epoxy skins with rubber core and the other having Glass Epoxy skins with rubber core subjected to low velocity normal impact loading. The behaviour of sandwich structure with various parameters such as energy absorption, peak load developed, deformation and von Mises stress and strain, are analyzed using commercially available analysis software. The results confirm that sandwich composite with jute epoxy skin absorbs approximately 20% more energy than glass epoxy skin. The contact force developed in jute epoxy skin is approximately 2.3 times less when compared to glass epoxy skin. von Mises stress developed is less in case of jute epoxy. The sandwich with jute epoxy skin deforms approximately 1.6 times more than that of same geometry of sandwich with glass epoxy skin. Thus exhibiting its elastic nature and making it potential candidate for low velocity impact application.

A novel nonlinear nano-scale wear law for metallic brake pads.

In the present work, molecular dynamics simulations were carried out to investigate the temperature distribution as well as the fundamental friction characteristics such as the coefficient of friction and wear in a disc-pad braking system. A wide range of constant velocity loadings was applied on metallic brake pads made of aluminium, copper and iron with different rotating speeds of a diamond-like carbon brake disc. The average temperature of Newtonian atoms and the coefficient of friction of the brake pad were investigated. The resulting relationship of the average temperature with the speed of the disc as well as the applied loading velocity can be described by power laws. The quantitative description of the volume lost from the brake pads was investigated, and it was found that the volume lost increases linearly with the sliding distance. Our results show that Archard's linear wear law is not applicable to a wide range of normal loads, e.g., in cases of low normal load where the wear rate was increased considerably and in cases of high load where there was a possibility of severe wear. In this work, a new formula for the brake pad wear in a disc brake assembly is proposed, which displays a power law relationship between the lost volume of the metallic brake pads per unit sliding distance and the applied normal load with an exponent of 0.62 ± 0.02. This work provides new insights into the fundamental understanding of the wear mechanism at the nano-scale leading to a new bottom-up wear law for metallic brake pads.

An Investigation into the Effect of Normal Load Frequency on Fretting Fatigue Behavior of Al7075-T6

ABSTRACTMost previous studies on fretting fatigue have been accomplished under constant normal loading and less attention has been paid to cyclic normal loading. An innovative test apparatus was specially designed and manufactured for fretting fatigue tests under cyclic loading in this work and the fretting fatigue behavior of Al7075-T6 was investigated at different normal load frequencies. A finite element model was developed to study the effect of normal load frequency on the contact stress distribution. It was found that the cyclic normal load has a more damaging effect on fretting fatigue life compared to constant normal load, particularly at lower frequencies. The results showed that at the normal load frequency of f = 1 Hz, fatigue life decreased by 52% in the high cycle fatigue regime and 28% in the low cycle fatigue regime. The experimental results also indicated that at the normal load frequency of 80 Hz, the fretting fatigue life converged to its corresponding life under constant normal load condition. The fracture surface and the fretting area of the specimens were examined using both optical and scanning electron microscopy (SEM). The experimental observations showed that the dominant partial slip condition with a wider slip region compared to constant normal loading, severe delamination, and higher oxidation rate due to the normal load release at each cycle, are the most important reasons for significant reductions in fretting fatigue life, under cyclic normal loading, especially for low normal load frequencies.

Experimental study on ignition mechanisms of wet granulation sulfur caused by friction

It is common to see fire accidents caused by friction during the storage and transportation of wet granulation sulfur. To study the sulfur ignition mechanism under friction conditions, a new rotating test apparatus is developed to reproduce friction scenes at lab scale. A series of experiments are performed under different normal loads. The SEM-EDS and the XRD were utilized to examine the morphologies and compositions of the tested specimens and the friction products. Experimental results show that these two methods are mostly in agreement with each other. The iron-sulfide compounds are produced and the proportion of iron-sulfide compounds is reduced with normal loads increasing, compared to the total number of the friction products. The facts implied by the integration analysis of friction products with the temperature changes of the near friction surface unveil an underlying mechanism that may explain sulfur ignition by friction in real scenarios. The sulfur ignition may be mainly caused by the spontaneous combustion of iron sulfide compounds produced by friction under low normal load with 200N. With the increase of normal loads, the resulting iron-sulfide compounds are decreasing and the high temperature from friction heat begins to play a major role in causing fire.

Optimal Allocation and Sizing of Capacitors for Distribution Systems Reinforcement Based on Minimum Life Cycle Cost and Considering Uncertainties

Background:The power and voltage losses have been greatly concerned in the distribution system planning and operation because low normal voltage, high impedance and load density of the system lead to increases in losses and decreases in bus voltages.Method:Therefore, an optimization model for the distribution system reinforcement improving efficiency of distribution systems by integrating capacitors is proposed. The optimal allocation and sizing of capacitors are determined concurrently with optimal upgrading sizing and timeframe of feeders and substations. The objective function of model is to minimize life cycle cost over reinforcement scheme including investment and operation costs of equipment in distribution systems (including feeders, substations and capacitors) and costs for purchasing energy from the electric market.Conclusion:The typical load curves for each day and season with uncertainties modeled by a Probability Distribution Function (PDF) and Time of Use (TOU) price applied to enhance accuracy of calculation results and suitability for real operation conditions. General Algebraic Modeling System (GAMS) is applied to undertake calculations in a test system.

The impact of coulombic interactions among polar molecules and metal substrates on flow and lubrication properties

In the current work we present an extensive study on the impact of short- and long-range interactions between solids and liquids on the hydrodynamic and lubrication behaviour of a tribological system. We have implemented a coarse grain molecular dynamics description of two ionic liquids (ILs) as lubricants which are confined by two infinitely long flat iron solids and which are subjected to a shearing flow. The impact of surface polarizability and molecule geometry on the ion arrangement under shearing has been studied in detail. The results have revealed two regimes of lubrication, with a liquid phase being present under low normal loads, while solidification of the ILs, accompanied by a steep rise of normal forces and significant wall slip is observed at small plate-to-plate distances.

Experimental Study of the Shear Strength of Bonded Concrete–Rock Interfaces: Surface Morphology and Scale Effect

The shear strength of the concrete–rock interface is a key factor to justify the stability of a hydraulic structure foundation. The Mohr–Coulomb failure criterion is usually used as shear strength and evaluated by extrapolating shear tests results carried out in a laboratory on small-sized samples. This paper presents an experimental study on the concrete–rock interface shear behavior. The effect of rock surface morphology on shear behavior was studied by performing laboratory direct shear tests on prepared square samples with a previously characterized rock surface. The scale effect and the test conditions were also studied by comparing the results to those obtained by performing usual laboratory shear tests on cored samples at lower scale. The tested interfaces were composed of the same concrete and granite and have a natural rock surface. The results displayed that the peak shear strength is strongly dependent on the concrete–rock bonding, the rock surface morphology and the applied normal load. A new surface morphology description tool was developed in order to characterize the main waviness. Moreover, the concrete–rock shear behavior at medium scale was reproduced by a 2D finite elements model to study the stress distribution along the sheared interface. Under low normal load, the concrete–rock adhesion is thus progressively mobilized according to the waviness on the rock surface and the local shear failure mechanisms depend on the type of this main waviness. Consequently the shear strength of a concrete–rock interface must be analyzed with respect to the various morphology aspects on its rock surface.

Effect of hardness, microstructure, normal load and abrasive size on friction and on wear behaviour of 35NCD16 steel

Wear and friction properties depend on many parameters such as normal load and material hardness. However, coupled contributions of microstructure and abrasive particle size are less investigated. A contribution in this field is proposed with wear tests performed between a 35NCD16 steel and abrasive silicon carbide grains ranging from 35µm to 200µm under different normal loads (50–110N). To vary hardness and microstructure, samples are subjected to water quenching and tempering at various temperatures (200–600°C). These heat treatments result in a martensitic microstructure with various levels of carbon contents and carbide precipitations. For a given microstructure and hardness, the friction coefficient decreases with increasing the normal load and/or decreasing the abrasive particle size. However, the wear rate increases with increasing the normal load and/or the abrasive particle size. For high normal loads, the results also reveal that the friction coefficient decreases when the hardness and the martensite volume fraction increase. Nonetheless, for low normal loads, a transitional microstructure from martensite laths to carbides and equiaxed grains, for a tempering around 400°C, impacts the friction coefficient. Whatever the normal load, this transitional microstructure also influences the wear rate.

The effect of in-service work hardening and crystallographic orientation on the micro-scratch wear of Hadfield steel

This work presents the micro-scratch wear analysis of a sample of Hadfield steel previously used in a jaw crusher machine. The in-service work hardening material derived from dynamic high loads during the abrasive contact on the steel surface. The scratch resistance of hardened samples was evaluated by micro-scratch tests on a cross-section surface. Scratches were made on a deformed layer and on an undeformed material. In the undeformed region, the two crystallographic planes 001 and 111 were analyzed. The scratch tests were performed under various normal loads (from 20 mN to 250 mN) with a diamond cone stylus of 60° and a 5 μm radius. The electron microscopy techniques (FEG, FIB, and EBSD) were applied to characterize the microstructure, the wear micromechanisms, and the change of the submicrostructure due to scratches and crystallography. Additionally, an optical interferometry analysis was done for the topography characterization of the scratches. The microhardness profile analysis showed a variation of the hardness values from 300 to 700 HV0.3, which resulted from the twinned grains found near the worn surface. Low loads (20 mN and 50 mN) corresponded to mild wear regime, with a wear regime transition at 100 mN. A severe wear regime corresponded to higher loads (150 mN, 200 mN, and 250 mN). A significant variation of the friction coefficient along with the severely hardened layer for low normal loads was not observed. On the other hand, high loads resulted in significant variations of the friction coefficient, without correlation with microhardness profile. Additionally a strong relationship was observed between the scratch wear and the individual grain crystallographic orientation. For scratches in the (001) plane microploughing was established as the predominant wear micromechanism, whereas for (111) plane microcutting was identified as the micromechanism responsible for wear, presenting a higher friction coefficient than for the (001) plane.

Formation mechanism of the lubrication film on the plasma sprayed NiCoCrAlY-Cr2O3-AgMo coating at high temperatures

A self-lubricating NiCoCrAlY-Cr2O3-AgMo composite coating was deposited by atmospheric plasma spray technology. Phase composition, microstructure and thermal behavior of this coating were characterized by X-ray diffraction (XRD), Scanning electron microscopy (SEM) and Thermogravimetric Analysis (TGA) respectively. Tribological tests of this coating were conducted at 500°C with different normal loads and sliding velocities. The results showed that the AgMo-containing composite coating exhibited lower friction coefficients around 0.3 at 10N normal loads due to the formation of Ag2MoO4. However, Ag2MoO4 could not be formed under lower normal load at the critical temperature of 500°C. Further investigation on the long-term wear behaviors of the coating revealed that the excellent friction reduction and wear resistance performance of the AgMo-containing composite coating at elevated temperatures could be ascribed to a liquid Ag2MoO4 film spreading on the upper surface and a dense oxide protective layer underneath.

Effect of normal load on the crack propagation from pre-existing joints using Particle Flow Code (PFC)

In this paper, the effect of normal load on the failure mechanism of echelon joint has been studied using PFC2D. In the first step, calibration of PFC was undertaken with respect to the data obtained from experimental laboratory tests. Then, six different models consisting various echelon joint were prepared and tested under two low and high normal loads. Furthermore, validation of the simulated models were cross checked with the results of direct shear tests performed on non-persistent jointed physical models. The simulations demonstrated that failure patterns were mostly influenced by normal loading, while the shear strength was linked to failure mechanism. When ligament angle is less than 90o, the stable crack growth length is increased by increasing the normal loading. In this condition, fish eyes failure pattern occur in rock bridge. With higher ligament angles, the rock bridge was broken under high normal loading. Applying higher normal loading increases the number of fracture sets while dilation angle and mean orientations of fracture sets with respect to ligament direction will be decreased.

Wear Resistance Limited by Step Edge Failure: The Rise and Fall of Graphene as an Atomically Thin Lubricating Material.

Owing to its intrinsically lubricious property, graphene has a high potential to be an atomically thin solid lubricant for sliding interfaces. Despite its ultrahigh breaking strength at the nanoscale, graphene often fails to maintain its integrity when subjected to macroscale tribological tests. To reveal the true wear characteristics of graphene, a nanoscale diamond tip was used to scratch monolayer graphene mechanically exfoliated to SiO2 substrates. Our experimental results show that while graphene exhibited extraordinary wear resistance in the interior region, it could be easily damaged at the step edge under a much lower normal load (∼2 orders of magnitude smaller). Similar behavior with substantially reduced wear resistance at the edge was also observed for monatomic graphene layer on graphite surface. Using molecular dynamics simulations, we attributed this markedly weak wear resistance at the step edge to two primary mechanisms, i.e., atom-by-atom adhesive wear and peel induced rupture. Our findings shed light on the paradox that graphene is nanoscopically strong yet macroscopically weak. As step edge is ubiquitous for two-dimensional materials at the macroscale, our study also provides a guiding direction for maximizing the mechanical and tribological performance of these atomically thin materials.

Effect of martensite volume fraction and abrasive particles size on friction and wear behaviour of a low alloy steel

Friction tests between a ferrite-martensite dual phase steel, with different martensite volume fractions (48–98%), and an abrasive paper with different sizes (15–200µm) under various normal loads (50–110N) are performed.It is highlighted that low normal loads minimize the wear rate whereas high normal loads minimize the friction coefficient. The lowest abrasive particle size minimizes the wear rate whereas an intermediate one (35µm) minimizes the friction coefficient. The wear rate and the friction coefficient decrease with increasing the martensite volume fraction. Conversely to the wear rate, for high normal loads, the friction coefficient becomes insensitive to the martensite volume fraction. It is also noticed that there are combined effects of martensite volume fraction and abrasive particle size on wear mechanisms.

Characterization of air plasma-sprayed yttria-stabilized zirconia coatings deposited with nitrogen

Air plasma-sprayed (APS) yttria-stabilized zirconia (YSZ) conventional and nanostructured coatings were deposited with nitrogen as the primary plasma gas. Nanostructured coatings were deposited as a function of critical plasma spray parameter (CPSP). Microstructural characteristics, e.g., phase distribution, porosity, microhardness, percentage of partially melted (PM) regions, surface roughness, and wear performance, were evaluated for deposited coatings. Effect of CPSP on wear performance of coatings was most prominent at highest normal load considered. Conventional and nanostructured coatings deposited with nitrogen showed different trends for specific wear rate as a function of normal load. While specific wear rate for conventional coatings was nearly constant, it increased with increasing load for nanostructured coatings. Thus, even though nanostructured coatings provided better wear resistance at lower normal loads, conventional YSZ coatings become more wear resistant at higher loads. For example, at highest normal load considered, specific wear rate of conventional coatings was around 12 % smaller than that for corresponding nanostructured coatings.

Numerical Simulation of Shear Behaviour of Non- Persistent Joints under Low and High Normal Loads

In this paper, the effect of rock bridge surface on the shear behavior of planar non-persistent joints under low and high normal loads has been investigated using particle flow code in 2 Dimensions. PFC2d was calibrated with respect to the data obtained from experimental laboratory tests to ensure the conformity of the simulated numerical models response. The models consisting non-persistent joint were simulated and tested by shear loading under low and high normal loads. The discrete element simulations demonstrated that the failure pattern was mostly influenced by normal load, while the shear strength was linked to the failure pattern and failure mechanism. The failure patterns were found reasonably similar to the experimentally observed trends.

Evolution Process of Natural Rock Joint Roughness during Direct Shear Tests

AbstractSurface roughness is one of the most important parameters that have significant influence on the mechanical and hydraulic characteristics of rock joints. To investigate the evolution of surface roughness under normal and shear loads, direct shear tests and numerical modeling based on the discrete-element method (DEM) were performed; bright area percentage (BAP), which places emphasis on the contact areas, was used as an index to describe the roughness. The results demonstrate that the surface roughness of rock joints decreased gradually as the shear displacement increased, and then roughness experienced a significant reduction in the residual stage and remained constant. Furthermore, the evolution of surface roughness was found to depend on the normal stress and shearing direction. Reduction in the degree of joint roughness with high normal loads was lighter than that with lower normal loads, and roughness varied with shear directions. The numerical simulation method proposed in this study has the...

Wear and corrosion properties of Mo surface-modified layer in TiNi alloy prepared by plasma surface alloying

In order to improve the wear resistance and restrain nickel release of TiNi alloys, the Mo modified layers on TiNi substrates were obtained using the double glow plasma surface alloying technique. Scanning electron microscopy (SEM), glow discharge optical emission spectroscopy (GDOES) and X-ray diffraction (XRD) were employed to investigate the morphology, composition and structure. Microhardness test and scratch test were performed to analyze the microhardness and coating/substrate adhesion. Tribological and electrochemical behaviors of the Mo modified layers on TiNi were tested by the reciprocating wear instrument and electrochemical measurement system. The Ni concentrations in Hanks’ solution where surface electrochemical tests took place were measured by mass spectrometry. The surface-modified layer contained a Mo deposition layer and a Mo diffusion layer. The X-ray diffraction analysis revealed that the modified layers were composed of Mo, MoTi, MoNi, and Ti2Ni. The microhardnesses of the Mo modified layers treated at 900 °C and 950 °C were 832.8 HV and 762.4 HV, respectively, which was about 3 times the microhardness of the TiNi substrate. Scratch tests indicated that the modified layers possessed good adhesion with the substrate. Compared with as-received TiNi alloy, the modified alloys exhibited significant improvement of wear resistance against Si3N4 with low normal loads during the sliding tests. Mass spectrometry displayed that the Mo alloy layers had successfully inhibited the Ni release into the body.