High Normal Force Research Articles

Enhanced adhesion and thermal stability of thick (Al,Cr)2O3 coatings on hot work steel

Al2O3 coatings deposited by chemical vapor deposition (CVD) at temperatures T ˃ 1000 °C are commonly used for wear and oxidation protection in manufacturing processes. Physical vapor deposition (PVD) gained also great interest to synthesize crystalline Al2O3 at lower temperatures by modification using Cr. High Speed PVD has the potential to synthesize thick crystalline (Al,Cr)2O3 coatings. However, previous studies showed that increasing adhesion strength is necessary for corresponding applications. This study evaluated various interlayer systems to enhance the adhesion strength between (Al,Cr)2O3 functional layer and steel X40CrMoV5-1. AlCrN interlayers of varying thicknesses were deposited, as well as two graded AlCrNg interlayers at different bias voltages. Afterwards, AlCrN/(Al,Cr)2O3 coatings were produced. Hereby, the functional layer thickness is varied. The AlCrNg/(Al,Cr)2O3 compounds show no spallation at the edge of Rockwell imprints and scratches at higher normal forces. The thermal stability of the coatings was investigated by annealing in vacuum at T = 1000 °C and T = 1200 °C. Moreover, no diffusion processes leading to chemical and structural changes was obtained after annealing. Therefore, thick (Al,Cr)2O3 coatings, s ≈ 30 μm, with a graded AlCrNg interlayer provide high potential for protecting hot-work steel against thermal loads up to T = 1200 °C.

Solid-State Additive Manufacturing of AA6060 Employing Friction Screw Extrusion Additive Manufacturing

AbstractSolid-state additive manufacturing constitutes a worthwhile alternative for many precipitation-based aluminium alloys that cannot be processed through fusion-based approaches due to metallurgical problems. In this work, the newly developed solid-state friction screw extrusion additive manufacturing (FSEAM) approach has been employed to study the processability of AA6060 T6 as a function of the printing velocity. Various wall-like builds were fabricated at printing velocities of 100 mm/min to 250 mm/min by deposition of 50 layers of 1 mm thickness, 14 mm width, and 150 mm length. No major defects were observed in cross section, and the microstructure showed equiaxed grains with an average size of 2–4 μm for all builds. Promising mechanical properties were obtained for all tensile test specimens extracted in the deposition direction of the layers. For specimens extracted in the build direction, only the builds fabricated with sufficiently high normal force to ensure proper bonding between successively deposited layers showed similar results to the deposition direction. The relatively high temperatures obtained during fabrication led to the dissolution of strengthening precipitates for the 150–250 mm/min samples as indicated by serrated yielding effects. Post-manufacturing heat treatment of these samples was successful, partially restoring the feedstock hardness.

Experimental and numerical simulation study on flow behavior of asphalt mixture using smart aggregate

A meso-scale mechanical analysis approach, which is essentially based on the use of smart aggregate sensor, was employed to inquire the dynamic responses of smart aggregate in cold recycled asphalt mixtures during gyratory compaction, including as well an investigation over the critical influence factors. The discrete element method (DEM) was further applied to study the interaction behaviors of smart aggregate with the surrounding particles. The laboratory test results showed that smart aggregate is an effective method to monitor the contact force and rotation variation between the aggregates, and the changes in the mechanical and physical properties of the mixtures can be characterized by the monitoring results of smart aggregate. In addition, the results of DEM analyses show that the mixtures with 55% coarse aggregates have higher normal force and lower kinetic energy during gyratory compaction, revealing the influence mechanism of the effect of coarse aggregate content on smart aggregate monitoring results at the meso-scale.

Experimental study of the direct shear characteristics of cement grout under constant normal loading and stiffness boundary conditions

Grouting cable is one of the most critical technologies in mine support. However, there are few studies on the effect of post-peak residual strength of cement grout under direct shear testing, which is of great significance for evaluating the bearing capacity of grouting cables in the whole life cycle. Hence, in this study, we carried out a systematic direct shear test to reveal the shear characteristics of cement grout under both constant normal loading (CNL) and constant normal stiffness (CNS) boundary conditions. The test results show that, under the same initial normal stress, the peak shear strength of the cement grout obtained under the CNS boundary conditions is greater than that under the CNL boundary conditions. The high initial normal force reduces the effect of boundary conditions on the post-peak mechanical shear properties of the cement specimens. Moreover, under CNS boundary conditions, the acoustic emission activity of the sample has an obvious signal before the peak shear strength, accompanied by multiple large ringing counts, indicating that the internal damage of the cement grout starts before the peak intensity. This work enhances the understanding of cement grouting fracture characteristics in mining support engineering.

2D FEM simulation and experimental verification of Al 7075‐T6 during turning process

AbstractIn this work, temperature distribution, effective stress and strain distribution in Al 7075‐T6 material and plastic flow of materials during turning process have been studied using Deform 3D v11.0 software. The four different points (P1, P2, P3, and P4) were chosen at the different locations in the cylindrical workpiece and the graph has been plotted with different parameters namely, temperature distribution, effective stress and strain. For temperature distribution, a good agreement has been found between simulation and experimental results at P4. The following results obtained from the simulation: In tertiary zone, the maximum temperature of specimen reaches upto 142 °C at points P1 due to friction between the tool rack face and the lower surface of chip. Maximum effective stress value reaches upto 650 MPa and 704 MPa at P1 and P4 respectively in the first sticking zone due to the combined effect of high normal force by tool edge and ploughing force. The effective strain value at P1 and P4 obtained, strain at P4 reaches maximum compare to P1 due to friction and unven temperature distribution. Additionally, chip analysis has been performed with microscopic image.

Fracture energy for orthogonal cutting in unidirectional CFRP at different cutting directions

A unidirectional carbon fibre reinforced polymer (CFRP) laminate is a composite material made up of strong parallel carbon fibres incorporated in a polymer matrix such as epoxy to provide high stiffness and strength in the fibre direction of the laminate. Unfortunately, the interlaminar or intralaminar plane of this material has a low resistance to damages as the fracture toughness of a unidirectional CFRP laminate is related to the energy dissipation during the orthogonal cutting. The aim of this study is on cutting a unidirectional CFRP along the longitudinal or transverse directions, characterizing orthogonal cutting forces and the related fracture energy. Orthogonal cutting is performed using braised carbide tools for a range of cutting depth of 10-100 ³m with a rake angle of 30° to quantify the cutting forces and to observe the fracture mechanisms. The fibre orientations have a significant impact on surface bouncing-back. For some fibre orientations, the energy balance model is applicable, deducting the reasonable value of fracture toughness due to high normal force (F t). Fibre subsurface damage and cutting forces during cutting are found to be strongly influenced by the cutting depth. The input energy of cutting is released in form of new surface energy, fibre breakage, high bending energy, and chip fracture energy.

Lateral friction surfacing: experimental and metallurgical analysis of different aluminum alloy depositions

Lateral friction surfacing, a solid-state deposition process, is a novel friction surfacing technique. In this approach, frictional heat and plastic deformation result in deposition of consumable material from the radial surface of a tool onto a substrate. This paper presents a comprehensive assessment of lateral friction surfacing of AA2011, AA6061, and AA7075 aluminum alloys, with particular focus on the impacts of process parameters on the coating properties. The influence of process variables such as tool rotational speeds, normal applied forces, and type of consumable materials was investigated on the process temperature, physical, and metallurgical characteristics of the deposits using optical microscopy, infrared thermography, scanning electron microscopy, and EDS. This study exhibits that the lateral friction surfacing approach enables the deposition of ultra-thin and smooth layers of different aluminum alloys. Furthermore, the temperature generated in this technique was low enough to avoid plasticizing the substrate and intermixing between the consumable material and substrate, which mitigates the thermal impacts on the grain structures and metallurgical characteristics. The lateral friction surfacing performance of the different alloys can be partially explained by their material properties. High input energy provided by high normal forces and tool rotational speeds may result in failure in the deposition process of materials with lower thermal conductivity and melting point, which emphasizes on limitations for the process parameters during the process.

Friction maps and wear maps of Ag/MoS2/WS2 nanocomposite with different sliding speed and normal force

The friction maps and wear maps for Ag/MoS2/WS2 nanocomposite under different sliding speeds and normal forces were constructed using the Response Surface Classification Study Method. The investigation found that the lower normal force and the lower sliding speed led to the ploughing wear, and the higher normal force and the higher sliding speed resulted in the oxidation delamination. The lower normal force and the higher sliding speed facilitated the formation of the transfer film. However, the sliding speed of 1.0 m/s and the normal force of 0.25 N showed a stable low friction coefficient and wear rate. The wear mechanism, friction coefficient and wear rate of Ag/MoS2/WS2 nanocomposite were predicted by the wear maps with different sliding speeds and normal forces.

Slow speed friction behaviour of a-C:H with different fs-laser micro-patterns against diamond tip in hyaluronic acid

Slow speed and different normal forces were chosen for investigation of the friction under short impact sliding. The base material is a hydrogenated diamond-like carbon coating (a-C:H). The coating was modified by femtosecond laser micro-patterning (µ-patterning). Three different µ-patterns were generated in the hydrogenated diamond-like carbon (DLC) coating and compared with a-C:H in the “as-deposited” condition. The sliding behaviour was demonstrated without an intermediate medium and with fluid addition. In the latter case, hyaluronic acid gel (HYA) was added. Gentle or deep dimpling by local femtosecond laser ablation resulted in a significantly more variable coefficient of friction (COF) curvature. During dry sliding of the diamond tip against the laser µ-patterned a-C:H, a zig-zag shape of the COF curve was observed. For deep laser µ-patterned DLC under high normal force of 10 N, sudden switches to very low COF down to 0.02 occurred when HYA was added. For hydrogenated DLC with gentle laser µ-patterning, the lowest COF were observed in this study under hyaluronic gel lubrication and low normal forces of 1 N. A volume expansion of the DLC and negative coefficients of friction were observed after loading with 1 N for some coating variants. For laser-induced periodic surface structured (LIPSS) DLC, at low loads of 1 N the COF became higher while at high normal forces like 10 N, a positive trend of LIPSS on the friction behaviour were seen under hyaluronic gel addition. Laser µ-patterning effected graphitization of small or large regions of the a-C:H coating. The defect-rich graphite sections on laser µ-patterned DLCs influenced the crystallisation behaviour of HYA and its distribution.

Tribology of hard particles lubricating soft surfaces

Soft materials often have interesting and unexpected frictional behavior owing to their deformable nature. We use soft polydimethylsiloxane (PDMS) surfaces lubricated by hard glass spheres to study how this deformability influences particle-based lubrication. For particles between 100 and 2000 $\ensuremath{\mu}\mathrm{m}$ in size, we observe a nontrivial rate dependence and three frictional regimes: (I) a rolling friction regime where the rolling particles keep the surfaces apart sufficiently to give low friction coefficients---this is mainly found for large particles and smooth surfaces; (II) a sliding friction regime with high friction coefficients where the surfaces are partially in contact, which is found for small particles, rough surfaces, and high normal forces; (III) a PDMS-PDMS contact regime where the particles are fully inserted into surfaces and the surfaces are in contact. We interpret the friction dynamics in terms of the Hertzian contact deformation effects in the indentation of the PDMS surfaces.

Longitudinal joints of tubbings with newly designed high‐strength reinforcement

AbstractContinuous tunneling using a tunnel‐boring machine is a very popular method when it comes to the construction of large tunnel projects. The use of precast reinforced concrete elements, called tubbings, is a highly economical way to secure the tunnel post excavation. Unfortunately, a loss of the load‐bearing capacity has been observed in the past in the joints between the tubbings. Reasons for that are among others the required reduction of the cross section in the longitudinal joint area and, above all, the high normal compressive forces that arise due to soil pressure. This paper describes experimental testing of longitudinal joints of tubbings conducted at TU Wien. The increase of the load‐bearing capacity of a newly designed and optimized tubbing with high‐strength joint‐compressive reinforcement, which was developed and patented by the Institute of Structural Engineering at TU Wien, is presented. The high‐strength steel is characterized by its high yield strength of 670 MPa, as well as the very large available bar diameters of up to 75 mm. This new optimized tubbing enables the production of thinner tubbings compared to conventional designs, and can thus achieve a reduction in material, transport and excavation costs.

Fretting test rig with variable normal force

Fretting is small amplitude reciprocating sliding between surfaces, and it may quickly causes surface cracks, which can continue growing under cyclic loads, until the structure breaks entirely as a result of the fretting fatigue. Fretting can also produce hardened wear particles as a result of adhesive wear, which then accelerates abrasive wear. In this case, the community uses the term fretting wear. The design of heavily loaded contacts, susceptible to fretting, is a difficult task because there is no generally accepted design guide. More extensive fretting research is needed to create them. This paper introduces detailed design phases for a equipment (rig) for a variable normal force fretting test. Supporting high radial and normal forces such that there is minimal run-out between the specimens was the most significant design challenge. The combination of a hydrostatic radial bearing and elastic torque shaft was selected for the detail design phase based on FE-analyses, calculations, and overall evaluation. The frame of the test rig consists of the main frame, which supports mainly the normal force and two torque frames, which support torque cylinders. Many solutions, which were found to be working in the current "ring-ring" apparatus of Tampere University, could be utilized in the new test rig like the tapered connections of the specimens, the elastic rod of the torque lever, axial displacement plate, and contact pressure adjustment system. The designed test rig enables fretting tests with 0 Hz to 20 Hz cycle frequency so that normal and tangential force or displacement can be controlled independently of each other. The normal force cannot change from compression to tension dynamically, but the adhesive force of the contact can be measured by slowly increasing the tension force. The designed fretting test rig fulfills all essential requirements, which were set.

A novel high-shear and low-pressure grinding method using specially developed abrasive tools

In this work, a novel grinding method using a special abrasive tool was first proposed to achieve high tangential grinding force and low normal grinding force. The abrasive tool was developed with flexible composites to remove workpiece materials under “high-shear and low-pressure” grinding mode. The concept of the high-shear and low-pressure grinding method was introduced in detail. Grinding tests were carried out on a precision grinder with the developed abrasive tool for single crystal silicon specimens. The results showed that the grinding force ratio between tangential force and normal force for the proposed grinding method was ranged between 0.9 and 1.3, which was three times larger than that of the conventional grinding method. It was verified that the developed abrasive tool possessed the grinding characteristics of high tangential grinding force and low normal grinding force. After grinding of silicon specimens for 120 min, the value of surface roughness decreased from 429.20 to 32.91 nm under the selected grinding conditions. The surface quality of the silicon specimens was greatly improved after grinding.

Relationships among rheological, sensory, and wear behaviors of cheeses.

Studying rheological and sensory behaviors of cheese provides structural and texture-related information that could be useful for a better understanding of the complex wear behaviors of cheese and their relationships with cheese industrial and oral processing behaviors. Thus, the objective of this study was to determine the relationships of rheological and sensory properties with cheese wear. Rheological tests including large amplitude oscillatory shear, strain sweeps at different temperatures (5, 15, and 25°C), and large-strain compression at room temperature (22 ± 2°C) were conducted for cheeses with varying fat contents (40, 50, 52, and 54% fat-in-dry-matter aged for different periods (15, 30, 45, and 60 d). Descriptive sensory analysis was used to evaluate cheese sensory texture attributes. Overall, fat content, testing temperature, and aging time had significant impact on cheese viscoelastic parameters. Higher temperature, aging time, and fat content led to lower rigidity and greater extent of nonlinear viscoelastic behaviors in the cheeses. Mass loss showed negative correlations with critical strain (γc ), critical stress (σc ), complex modulus (G* ), and fracture stress, but had positive correlations with phase angle (δ) and fracture strain. Sensory data showed that texture attributes were affected by cheese fat content and aging time and had significant correlations with mass loss at high normal force and sliding speed. This study showed that rheology and sensory data can be used to provide fundamental information on the wear behaviors of cheese and other soft materials.

Experimental investigation on high-shear and low-pressure grinding process for Inconel718 superalloy

In this work, we reported a novel grinding method with high tangential grinding force and low normal grinding force using specially developed grinding tools. The tools were made of flexible composites based on the principle of liquid body armor and the shear thickening mechanism of non-Newtonian fluid. During grinding, abrasive particles are capable of generating a “hydro-cluster effects” under reverse tangential load, which lead to the decreased normal grinding force and the increased tangential grinding force. Hence, workpiece materials are removed under “high-shear and low-pressure” grinding mode. A serial of grinding experiments were carried out on Inconel718. The results showed that the novel grinding tool had an excellent grinding performance on Inconel718 workpieces. The value of surface roughness decreased from Ra 473.7 nm to Ra 153.0 nm under the optimal grinding parameters, i.e., wheel speed of 1 m/s, workpiece speed of 2000 mm/min, and grinding depth of cut of 180 μm. The surface defects of the Inconel718 workpiece were gradually removed. Meanwhile, the uniformed grinding textures were generated. The surface of the grinding tool had residual wear debris, and there was a little loss of grains after 240 grinding cycles.

Effect of laser shock peening on tribological properties of magnesium alloy ZK60

The influence of laser shock peening (LSP) on tribological properties of magnesium alloy ZK60 was investigated adopting different laser power density. Surface wear morphology, elemental composition and content in wear debris were observed and analyzed using SEM and EDS. Meanwhile, surface roughness, profiles, microhardness, and microstructure were also characterized. It was proven that the roughness increased from 0.32 μm to 9.3 μm, microhardness improved by 39%, the number of grains augmented from 1045 to 1461, and wear rate decreased by 17.6% in maximum. Adhesive wear and oxidation wear were the main mechanisms of the original sample, and the wear mechanism of modified specimens was dominated by abrasive wear. In conclusion, LSP can improve wear resistance under the higher normal force.

Nanotribology of hydrogels with similar stiffness but different polymer and crosslinker concentrations

HypothesisThe stiffness has been found to regulate hydrogel performances and applications. However, the key interfacial properties of hydrogels, like friction and adhesion are not controlled by the stiffness, but are altered by the structure and composition of hydrogels, like polymer volume fraction and crosslinking degree. ExperimentsColloidal probe atomic force microscopy has been use to investigate the relationship between tribological properties (friction and adhesion) and composition of hydrogels with similar stiffness, but different polymer volume fractions and crosslinking degrees. FindingsThe interfacial normal and lateral (friction) forces of hydrogels are not directly correlated to the stiffness, but altered by the hydrogel structure and composition. For normal force measurements, the adhesion increases with polymer volume fraction but decreases with crosslinking degree. For lateral force measurements, friction increases with polymer volume fraction, but decreases with crosslinking degree. In the low normal force regime, friction is mainly adhesion-controlled and increases significantly with the adhesion and polymer volume fraction. In the high normal force regime, friction is predominantly load-controlled and shows slow increase with normal force.

Characterizing wear behaviors of edible hydrogels by kernel-based statistical modeling

Hydrogels are used in multiple industries; although hydrogel deformation and wear behaviors can be a good indicator for their functionality, there are no available models for characterizing hydrogel deformation–wear behaviors. This study characterized wear behaviors of casein (3–5% w/w, 200 mM NaCl), κ-carrageenan (0.8–1.2% w/w, 100 mM NaCl), and whey protein gels (13% w/w, 50–150 mM NaCl) at room temperature (20 °C) under several normal forces (0.1–1.5 N). A kernel-based model was used to characterize hydrogel wear behavior by separating the deformation–wear process deformation-dominant, constant wear rate, and failure regions. Higher normal force and lower polymer concentration showed higher constant wear rates and lower durability. The kernel model was able to characterize hydrogel wear-deformation behaviors and is useful for industrial applications.

Probing of Nanoscale Friction and Mechanical Characteristics of Cotton Fiber’s Surface

The surface topography and nanomechanical attributes of two samples of cotton fibers, namely, A and B, were characterized with various operation modes of an Atomic Force Microscope (AFM). The surface topography and friction images of the fibers were obtained in contact mode. The nanomechanical properties images—i.e., adhesion and deformation—were obtained in force tapping mode. The results indicate that the surface nanomechanical and nanoscale frictional properties of the fibers vary significantly between two samples. The plots of friction versus normal force of the fibers’ surface from both samples are fitted to the equation of single-asperity, adhesion-controlled friction. Nevertheless, within the range of the applied normal force, the friction curves of sample A surfaces show a characteristic transition phase. That is, under low normal forces, the friction curves closely conform with the Hertzian component of friction; after the transition takes place at higher normal forces, the friction curves follow Amontons’ law of friction. We demonstrated that the transition phase corresponds to a state at which the cuticle layer molecules are displaced from the fibers’ surface. The average adhesion force of the samples is consistent with the average friction signal strength collected under low normal forces.

Experimental Study on Shear Behavior of Bolted Cement Mortar Blocks under Constant Normal Stiffness

In this paper, the influence of bolting on shear mechanical behavior of cement mortar specimens were studied. First, a shear test apparatus which could provide CNS (constant normal stiffness) condition was designed. Then, more than 140 bolted and non-bolted cement mortar blocks (100 × 100 × 100 mm) were prepared. The strengthening effect of bolts with diameters of 0 – 10 mm on cement mortar specimens was studied under different initial normal loads ranging from 20 kN to 60 kN and constant normal stiffness of 420 kN/mm. Two simplified models of shear behavior of reinforced cement mortar blocks were proposed. The most important influence of bolt on the cement mortar blocks was to strengthen the post-peak shear strength rather than the peak shear strength, and the strengthening effect was reduced under high initial normal force. The cohesion of cement mortar blocks was reduced while internal friction angle was increased in residual sliding stage by bolts. The dilation of reinforced cement mortar specimens in post-peak shear stage was significantly reduced due to bolting, and the larger the bolt diameter, the greater the reduction in the block dilation.