Low Shear Resistance Research Articles

Shear behaviour of continuous fibre-reinforced lightweight aggregate concrete beams

A series of shear tests on six fibre-reinforced lightweight aggregate concrete (FRLWAC) beams containing both steel and polypropylene fibres are presented in this paper. These beams were designed and cast with different boundary conditions, steel fibre contents, and shear-span-to-depth ratios (a/d). From the test programme, it was found that an addition of 0.5 % of steel fibre by volume significantly improved the shear behaviour of continuous FRLWAC beams and mitigated the concern of low shear resistance associated with lightweight concrete. The addition of steel fibre enhanced the direct strut strength of FRLWAC beams in shear and improved the efficacy of stirrups in transferring shear forces. The test results also suggested that continuity effect prevented sudden and complete loss of load-carrying capacity in FRLWAC beams failing in shear, although it did not significantly increase the shear-carrying capacity. A strut-and-tie model was proposed to predict the shear resistance of continuous FRLWAC beams. The contribution of steel fibre in enhancing the efficacy of compression struts and stirrups was considered in this model. This model was shown to be accurate in predicting the shear resistance of FRLWAC beams and was more accurate and reliable compared to available shear strength prediction provisions in different design codes. It also presents a viable alternative to a three-dimensional finite-element model for predicting the shear resistance of continuous FRLWAC beams.

Shear Behavior of High-Strength and Lightweight Cementitious Composites Containing Hollow Glass Microspheres and Carbon Nanotubes

In this study, an experimental program was conducted to investigate the shear behavior of beams made of high-strength and lightweight cementitious composites (HS-LWCCs) containing hollow glass microspheres and carbon nanotubes. The compressive strength and dry density of the HS-LWCCs were 87.8 MPa and1.52 t/m3, respectively. To investigate their shear behavior, HS-LWCC beams with longitudinal rebars were fabricated. In this test program, the longitudinal and shear reinforcement ratios were considered as the test variables. The HS-LWCC beams were compared with ordinary high-strength concrete (HSC) beams with a compressive strength of 89.3 MPa to determine their differences; the beams had the same reinforcement configuration. The test results indicated that the initial stiffness and shear capacity of the HS-LWCC beams were lower than those of the HSC beams. These results suggested that the low shear resistance of the HS-LWCC beams led to brittle failure. This was attributed to the beams’ low elastic modulus under compression and the absence of a coarse aggregate. Furthermore, the difference in the shear capacity of the HSC and HS-LWCC beams slightly decreased as the shear reinforcement ratio increased. The diagonal compression strut angle and diagonal crack angle of the HS-LWCC beams with shear reinforcement were more inclined than those of the HSC beams. This indicated that the lower shear resistance of the HS-LWCCs could be more effectively compensated for when shear reinforcement is provided and the diagonal crack angle is more inclined. The ultimate shear capacities measured in the tests were compared with various shear design provisions, including those of ACI-318, EC2, and CSA A23.3. This comparison showed that the current shear design provisions considerably overestimate the contribution of concrete to the shear capacity of HS-LWCC beams.

Tribological performance of ZrO2 nanoparticles as friction and wear reduction additives in aviation lubricant

Abstract High-performance aircraft engines require superior aviation oils to enhance their lubricating performance and prolong service life. Addition of nano-sized ceramic particles has been considered as a useful way to improve the tribological performance of base fluids. Up to now, few previous studies focused on the tribological properties of ZrO2 nanoparticles in aviation oil. The current study dispersed ZrO2 nanoparticles into PAO20 aviation base oil as lubricant additives. A dual-step method comprised of physical blending and ultrasonic dispersing was applied in the preparation of ZrO2 nanofluids. Oleic acid was utilized as surfactant to enhance the stability of ZrO2 nanofluids. Ball-on-plate reciprocating sliding wear tests were conducted to obtain the coefficient of friction and wear volumes. It was found that the PAO20 base oil produced the highest coefficient of friction of 0.278 and wear volumes of 2.305╳10-2 mm3. 5wt% ZrO2 nanofluids with 5wt% oleic acid showed the best lubricating performance. The coefficient of friction was reduced by 31.29%, and wear volume was reduced by 42.95%. In the examination of wear tracks, a physically embedded tribo-layer of ZrO2 nanoparticles and an oleic acid tribo-film with low shearing resistance were formed, which lowered the friction, and protected the mating surfaces against abrasive and adhesive wear. The results obtained in this study have applicable values in the development of high-performance aviation lubricants.

Particle aerosol generation and potential altering in airflow used for acute/repeated inhalation toxicity testing

Reproducible aerosol generation in combination with stable aerosol properties are essential prerequisites for compliant performance of acute or repeated inhalation toxicity tests of particulate materials according to OECD TG 403, 436, 412, or 413. A frequent problem of powder aerosol generation is the formation of coarse agglomerates with low shear resistance, which are beyond the tolerable size range but not detected by the prescribed aerodynamic measurement techniques by cascade impactor as the measurement conditions cause a disintegration into smaller fragments. But such agglomerates are observed during the transport to the inhalation chambers. These effects particularly apply to high mass concentrations and low-density powders, i.e., pyrogenic oxides. This study describes the transport influence in the airflow on the change of powder aerosols and on their respirability.A simplified short tube set-up was developed for the aerosol transport pre-tests, which allows the determination of the optimal aerosol formation conditions for the inhalation tests. The particles were measured with low shear using laser diffraction measurement or optical particle counters. The calculation of the aerodynamic particle sizes prescribed in the guidelines requires knowledge of the effective particle density of the porous aerosol particles. A practicable method for determining these is presented and described. In the outlook, first low concentration measurements show that clear agglomeration effects can also occur at particle concentrations around 20 mg/m³.

Structural superlubricity at the interface of penta-BN2.

Two-dimensional (2D) materials have been widely used as lubricants due to their weak interlayer interaction and low shear resistance for interlayer sliding. Composed entirely of five-membered rings, penta-BN2 monolayer has excellent thermal and mechanical stability, higher hardness and a negative Poisson's ratio. In this work, we investigate the frictional properties at both the commensurate and incommensurate contacting interfaces of penta-BN2 by adopting the molecular dynamics (MD) simulation method. Our calculations demonstrate robust superlubricity at the incommensurate contacting interface of penta-BN2. The ultra-low friction is explained by the potential energy surface (PES) fluctuations, interlayer binding energy and out-of-plane motion amplitude of the sliding layer. In addition, our calculations show that the anisotropy of friction at the commensurate contacting interface is more obvious compared with that at the incommensurate contacting interface. Finally, the influences of the size of the Moiré pattern, normal force, temperature and sliding velocity on the friction are examined. Our results show that 2D penta-BN2 is a promising solid lubricant, enriching the family of 2D lubrication materials.

Preparation and Antifiltration Performance of Organic Zirconium Crosslinker for Guar Gum‐Fracturing Fluid

Aiming at the characteristics of low shear resistance and high filtration loss of guar gum‐fracturing fluid, a high‐efficiency organic zirconium crosslinker was prepared. In addition, the effects of different factors on the filtration coefficient and apparent viscosity of fracturing fluid were studied, and this study explores the micro mechanisms of various factors affecting the filtration coefficient from a molecular dynamics perspective. The results show that the antifiltration ability of guar gum‐fracturing fluid is inversely proportional to the fluid viscosity. The increase of crosslinker content and reservoir pressure is beneficial to improve the viscosity and antishear ability of guar gum‐fracturing fluid. When the crosslinker content is 0.25% and pressure is 30 MPa, the fracturing fluid parameters are 145 mPa·s and 2.1 × 10−2 m3 · min1/2, respectively, while the increase of reservoir temperature and the shear rate reduces the rheology and shear resistance of guar gum‐fracturing fluid, so that the viscosity and filtration coefficient of fracturing fluid at 180°C and 200 s-1 are 123 mPa·s and 2.3 × 10-2 m3·min1/2, respectively. This work not only paves a new avenue for synthesizing organic zirconium as a crosslinker but also provides an efficient measure to reduce the filtration of guar gum‐fracturing fluid on the oil and gas reservoirs.

Influence of δ ferrite on impact toughness of M50NiL bearing steel

M50NiL bearing steel is widely used in the aerospace field because of its excellent properties. δ ferrite would affect the impact toughness of steel, but there is little research on it in M50NiL bearing steel. In this study, the effect of the δ-ferrite content on the impact toughness of M50NiL bearing steel and the corresponding mechanism were investigated through microstructure characterization and mechanical property testing. The results indicated that the impact toughness of the steel at room temperature, as well as the energy needed for crack initiation and crack propagation, improved with δ-ferrite content reduction. This suggests that the presence of δ ferrite in M50NiL steel is detrimental to its impact toughness and that reducing the δ-ferrite content can suppress crack initiation and crack propagation, significantly improving the impact toughness. The adverse effect of δ ferrite on impact toughness is mainly attributed to the fact that δ ferrite is prone to crack initiation and cannot effectively prevent crack propagation due to its low strength and shear resistance. Additionally, the difference in microstructure and strength between the δ ferrite and martensite matrix may weaken the interface, which is conducive to crack initiation and rapid propagation.

High-temperature oxidation-wear properties of Ru-doped cermet

The oxidation-wear properties of WC-Co cermet at high temperatures were remarkably enhanced by Ru addition for increasing life expectancy of cermet tools in high-speed cutting processes. The oxidation-resistant mechanisms of Co with substitutional Ru were disclosed by calculating the energy barrier for oxygen adsorption and diffusion in Co with and without Ru doping. Particularly, the oxidation of WC was effectively inhibited due to the suppression of reactions between cobalt oxides and WO3. Thus less cracking occurred in the Ru-containing cermet as its tribolayer contained fewer oxides with low shear resistance, which in turn protected the subsurface materials from further oxidation.

Review of Graphene-Based Materials for Tribological Engineering Applications

Graphene-based materials have great potential for tribological applications. Graphene’s unique properties such as low shear resistance, high stiffness, and thermal conductivity make it an attractive material for improving the properties of lubricants in a wide range of industrial applications, from vehicles to house refrigerators and industrial machinery such as gearboxes, large compressors, etc. The current review aims to give an engineering perspective, attributing more importance to commercially available graphene and fully formulated lubricants instead of laboratory-scaled produced graphene and base oils without additives. The use of lubricants with graphene-based additives has produced e.g., an increase in mechanical efficiency, consequently reducing energy consumption and CO2 emissions by up to 20% for domestic refrigerators and up to 6% for ICE vehicles. Potential effects, other than purely friction reduction, contributing to such benefits are also briefly covered and discussed.

Macroscopic ultra-low friction and wear enabled by carboxylated graphene with glycerol

Reducing wear and even achieving superlubrication in industrial applications using simple yet effective methodologies has been a longstanding research objective. In this study, we propose a convenient approach by employing a combination of carboxylated graphene (Gr-COOH) coating with glycerol as lubricants. Compared to the non-lubricated state, this strategy enabled the achievement of superlubrication with an exceptionally low friction coefficient of 0.002, while simultaneously reducing wear volume of substrate by 4 orders of magnitude, and the wear of the counterpart balls are nearly zero. The transfer of Gr-COOH to the ball surface led to the formation of a low shear resistance interface with the Gr-COOH present on the substrate. Nanoscopic experiments with atomic force microscopy revealed that the substrate surface lubricated by this method exhibited more hydrophilic properties. The formation of a featured hydrophilic interface enables the persistence of glycerol molecules in the contact region for an extended duration, thereby facilitating continuous elastohydrodynamic lubrication. This proposed lubrication methodology holds broad utilization potential as a wear-reduction strategy across various industrial fields.

MXenes in tribology: Current status and perspectives

MXenes are an emerging class of new two-dimensional materials, which have been widely used in energy storage, catalysis, sensing, biology, and other fields due to their unique structure and properties. The distinct structure, low shear resistance, and easy-to-modify ability endow MXenes with particularly superior lubrication potentials. This review highlights the research status and applications of MXenes lubrication which are categorized into solid lubricants, lubricant additives, and reinforcement phase parts, summaries their influencing factors and lubrication mechanisms of MXenes lubrication, points out some unexplored research fields and unsettled questions, and then puts forwards possible solutions and prospects for future research. The lubrication advances and potentials of MXenes are fully verified. Predictably, the emerging MXenes lubricants will exhibit remarkable application prospects in advanced manufacturing such as machining industries, automotive industries, micro/nanoelectromechanical systems, and spacecraft components. This paper offers a comprehensive review about research status and applications of MXenes in tribology, highlights influencing factors and lubrication mechanisms of MXenes lubrication, points out some unexplored research fields and unsettled questions, and puts forward possible solutions and prospects for future research. • MXene is an emerging lubricating material for reducing friction and wear. • MXene used in solid lubrication, liquid lubrication and as a reinforcing agent in polymer/metal matrix are comprehensively reviewed. • Tribological properties and lubrication mechanism of MXene-based materials are demonstrated. • The challenges and prospects of MXene in tribology are discussed and proposed.

Macroscale superlubricity by a sacrificial carbon nanotube coating

Superlubricity, i.e., coefficient of friction (COF) below 0.01, was earlier limited to microscale in controlled environments in the earlier literature and more recently realized for macroscale sliding of ceramic or carbon surfaces lubricated by water or other polar fluids. However, there is lack of report of superlubricity for the most common bearing system, i.e., steel-steel contact in non-polar oil lubrication. Here we present ultra-low COF of 0.001–0.007 by using a sacrificial coating composed of vertically-aligned carbon nanotubes (CNTs) for macroscale steel-steel sliding under minimum quantity lubrication (MQL) of a polyalphaolefin (PAO) oil in the ambient environment. Raman spectroscopy and electron microscopy analyses detected graphene-containing tribofilms on both the steel surfaces, which was produced by the fractured CNT flakes and metallic wear debris during running-in. The in situ formed graphene-graphene contact interface presumably possesses a low shear resistance leading to superlubricity. The presence of oil, despite as little as one droplet, has proven to be crucial. Such a superlubricity performance has shown good sustainability in extended testing of more than 500,000 cycles and strong ability of accommodating changes in sliding conditions. Results here demonstrate feasibility with fundamental insights for achieving ambient environment macroscale superlubricity.

Shear strength of bentonite–polymer composite geosynthetic clay liners and geomembranes

The interface shear strength between bentonite–polymer composite (BPC) geosynthetic clay liners (GCLs) and textured geomembrane (GM) was compared with the interface shear strength between conventional sodium (Na) bentonite GCLs (NaB GCLs) and the same GM. Four BPC GCLs were used that had polymer loading ranging from 0.12 to 1.09 g/kg. The peak shear strength of the BPC GCL/GM interface ranged from nearly identical to the interface strength of the NaB GCL/GM to as much as 14.4 times lower. The lower strengths of the BPC GCL/GM interface are attributed to the hydrated polymer gel that migrated out of the GCL and into the interface. The hydrated gel has very low shear resistance, reducing both peak and large-displacement interface shear strengths. The reduction in shear strength increased with increasing polymer loading of the BPC GCL and increasing normal stress. Inclusion of a woven slit-film geotextile in the interior of one of the BPC GCLs reduced migration of the polymer into the interface, leading to higher interface shear strength than obtained with the same BPC GCL without a slit-film geotextile.

A sulfonated modification of PEEK for ultralow friction

Polyether ether ketone (PEEK) is a widely used material for friction pairs due to its excellent mechanical strength, good wear resistance, and chemical inertness. However, some modifications are necessary when PEEK is used as a water-lubricated friction pair. In this study, a novel sulfonation method was developed to design a water-lubricated friction pair with ultralow friction, good wear resistance, and high loading capacity. PEEK powders were sulfonated using ClSO3H and sintered to form bulk plastic. The sulfonated PEEK (SPEEK) plastic exhibited good tribological properties. At a low sliding speed, the friction coefficient was smaller than 0.02 when a 3 wt% NaCl solution was used as the lubricant. The order of magnitude of the wear rate was as low as 10−8 mm3/(N·m). The mechanism of friction reduction was mainly hydration lubrication. The negatively charged −SO3− groups on the friction pair can adsorb hydrated Na+ cations by electrostatic interactions. These hydrated Na+ cations have a high load capacity and low shearing resistance. The ultralow wear mechanism observed in this study is possibly due to ultralow friction properties of the friction pairs prepared through the proposed sulfonation and thermoforming procedures.

Recent Developments in Starch Modification by Organic Acids: A Review

AbstractTreatment or processing of starch with organic acids (citric, stearic, succinic, and malic acids) can be used to obtain starch properties like low retrogradation, desirable viscosity, shear resistance, and high resistant starch. Esterification, cross linking, and hydrolysis of starch may occur after modification with organic acids (citric, succinic acid, malic acid) whereas a complex between starch and stearic acid may be formed on use of stearic acid. The conditions of reactions namely duration, starch acid concentration ratio, and temperature may influence physicochemical and structural properties of starch. Citric acid, succinic acid, stearic acid, and malic acid are regarded as "GRAS" and thus the starch citrate, starch succinate, starch stearate, and starch maleate are widely used in manufacturing of starch films, blends, nanoparticles, fat replacer, viscosity enhancer, and many other products. This review paper discusses the reaction conditions of organic acid modification of starches along with their influence on physicochemical and structural characteristics.

Investigation of butadiene-elastomer-based high modulus materials reinforced by basalt, glass, and carbon fabrics

A relevant task in improving the properties of elastomers is to increase their strength and stiffness, which affect the reliability and durability of rubber products. The paper presents a technology for manufacturing high-modulus materials based on SKD-V butadiene rubber and reinforcing layers of fabrics from basalt, glass, and carbon fibers. The results of studying elastic strength properties reveal a significant increase in the ultimate strength of reinforced samples in comparison with an unmodified elastomer. The increase in tensile strength varies from 1.7 to 2.8 times. The addition of reinforcing layers reduced the elongation value by 25 to 47 times compared to rubber without reinforcement. High tensile strength and low elongation increase shear resistance. The wear resistance testing of elastomers coated with reinforcing fabrics shows a decrease in abrasion resistance reduced by a factor of 5.8. Abrasion wear and interaction between the reinforcing filler and the polymer are studied by electron microscopy. The study of the microstructure shows a weak contact between the fiber and the elastomeric matrix. Lack of contact during the abrasion process causes destruction of the fibers on the abrasive surface and their further separation. Due to the combination of high tensile strength and low elongation, the reinforced materials obtain high modulus properties combined with lateral mobility.

Effect of Partial Saturation on Ultimate Bearing Capacity of Skirted Foundations

Skirted foundations are one of the solutions proposed to increase the bearing capacity of the soil. They assist in increasing the load and depth of failure in weak ground or soils with low shear resistance and reducing the foundation settlement if a soil improvement method cannot be applied or the cost of implementing deep foundations increases. This study examined and investigated the extent of soil bearing of skirted foundations on sandy soils and studied the effect of soil saturation cases and three cases of water content reduction to measure the matric suction value of unsaturated soil. A physical model was created to simulate the strip foundation and compare these cases (dry-fully saturated-partially saturated). It was found that the soil load carrying capacity in the case of unsaturated soil is the highest, where matric suction is at a depth of 450 mm, followed by the dry case and then the saturated case as it represents the weakest state of the soil.

Experimental Study on Seismic Performance of Steel Frame Sheathing with Concrete and Plasterboard Composite Wall

The current cold-formed steel (CFS) wall is limited to the low-rise cold-formed thin-walled steel structure, which is inappropriate for multi-story constructions due to its low shear resistance and poor corrosion resistance of outer sheathing board. Therefore, a new composite wall of CFS frame sheathing with concrete and plasterboard was proposed, the cast-in situ concrete layer was used for the outer surface in response to the needs of corrosion resistance, and the plasterboard was utilized for the inner surface to reduce weight. This paper presented a quasi-static test on the seismic behavior of one CFS wall and three composite-walls’ sheathings with concrete and plasterboard to obtain different failure modes and investigate the influence of different sheathing and opening sizes. Combined with the finite element model, a large parametric study of composite walls was conducted to analyze the impact of load ratio, reinforcement ratio, sheathing board, and concrete and steel strength. Test and finite element analysis (FEA) results demonstrate that the seismic performance of composite walls with concrete and plasterboard is satisfactory considering their shear strength, ductility, and energy dissipation. Load ratio, opening size, and reinforcement ratio significantly affect the seismic performance of the composite wall, while the concrete and steel strength and sheathing board have minimal effects. The seismic design suggestions of composite walls are given based on this study.

Physicochemical Characterization of Thirteen Quinoa (Chenopodium quinoa Willd.) Varieties Grown in North-West Europe-Part II.

Quinoa cultivation has gained increasing interest in Europe but more research on the characteristics of European varieties is required to help determine their end use applications. A comparative study was performed on 13 quinoa varieties cultivated under North-West European field conditions during three consecutive growing seasons (2017–2019). The seeds were milled to wholemeal flour (WMF) to evaluate the physicochemical properties. The WMFs of 2019 were characterized by the highest water absorption capacity (1.46–2.06 g/g), while the water absorption index (WAI) between 55 °C (2.04–3.80 g/g) and 85 °C (4.04–7.82 g/g) increased over the years. The WMFs of 2018 had the highest WAI at 95 °C (6.48–9.48 g/g). The pasting profiles were characterized by a high viscosity peak (1696–2560 mPa.s) and strong breakdown (−78–643 mPa.s) in 2017. The peak viscosity decreased in 2018 and 2019 (823–2492 mPa.s), while breakdown (−364–555 mPa.s) and setback (19–1037 mPa.s) increased. Jessie, Summer Red, Rouge Marie, Vikinga, and Zwarte WMFs were characterized by low WAIs and high shear resistance. Bastille WMF developed high viscosities and, along with Faro WMF, showed a high breakdown. The wide variation in physicochemical properties suggests that the potential food applications of WMFs depend on the variety and growing conditions.

Atomistic modeling of interface strengthening in Al-Si eutectic alloys

Al-Si cast alloys are usually composed of α-Al and Al-Si eutectic. Si flakes and Al matrix generally hold cube-on-cube orientation relationship with the primary interface (111)Al∥(111)Si. Extensive experimental studies demonstrated that Si flakes cannot significantly improve mechanical properties of Al-Si cast alloys. We hypothesize that the weak strengthening effect associated with Si flakes might be attributed to thermomechanical properties of Al-Si interfaces besides their morphologies. To characterize Al-Si interfaces with a large lattice mismatch (> 30%), we proposed the quasi-coincident site lattice (Q-CSL) as reference lattice, and demonstrated that the Q-CSL Al-Si coherent interface has three characteristic coherent structures, one stable and low energy structure and two metastable and high energy structures. The translation vectors for the same type of coherent Q-CSL structures are consistent with three displacement shift complete (DSC) vectors. The two metastable structures can be obtained by shifting the low energy structure with three partial DSC vectors. Semi-coherent interface is composed of the low energy Q-CSL patches and three sets of interface misfit dislocations with Burgers vectors same as the DSC vectors. Atomistic simulations revealed that Al-Si interface exhibits low shear resistance. Ideal shear strength of the Q-CSL coherent interface is 110 MPa and semi-coherent interface is 20 MPa. The low shear resistance is attributed to the glide of interface misfit dislocations. Al-Si interface also exhibits low formation and migration energies of point defects. Owing to low shear strength and low formation and migration energies of point defects, interface sliding or shear readily happen under mechanical loading or during dislocation-interface interactions. Lattice dislocations can cross slip onto or climb along Al-Si interfaces. These reactions decrease the number of accumulated dislocation loops around Si flakes and promote nucleation and emission of lattice dislocations from Al-Si interfaces to matrix, consequently reduce the repulsive force on approaching dislocations and weaken Si flakes strengthening effect. In situ tension and compression tests in a scanning electron microscope reveal relatively weak strengthening effect due to Si flakes, consistent with the computed dislocation interaction with interfaces and shear behavior of interfaces.