The aim of this work is to investigate the mechanical performance of hardened cement paste (HCP) under compression at the micrometre length scale. In order to achieve this, both experimental and ...numerical approaches were applied. In the experimental part, micrometre sized HCP specimens were fabricated and subjected to uniaxial compression by a flat end tip using nanoindenter. During the test, the load-displacement curves can be obtained. In the modelling part, virtual micrometre sized specimens were created from digital material structures obtained by X-ray computed tomography. A computational compression test was then performed on these virtual specimens by a discrete lattice fracture model using the local mechanical properties calibrated in the authors' previous work. A good agreement is found between the experimental and numerical results. The approach proposed in this work forms a general framework for testing and modelling the compression behaviour of cementitious material at the micrometre length scale.
This paper presents an experimental investigation on the short-term creep recovery of cement paste at micrometre length scale. Micro-cantilever beams were fabricated and tested with 8 different ...loading series using the nanoindenter. It is found that cement pastes show high recovery ratios (>80%) even subjected to very high stress levels. Relatively lower recovery ratios and non-linear creep were also observed for w/c 0.4 samples under high stress levels. A good agreement is found between the results predicted using the linear superposition principle and the experimental results except for the measured non-linear creep in w/c 0.4 samples. It is suggested that the short-term creep recovery may be associated with the microscale stress redistribution or the reversible internal water movement. The observed non-linear creep under the highest stress level may be due to the higher density of microcracks generated during the loading stage, which may further promote the water transfer.
This study proposes an experimental method for studying the short-term creep behaviour of cement paste at micro-scale. The micro-bending tests on miniaturized cantilever beams were used to ...characterize the viscoelastic properties of cement paste. The effects of w/b ratio, the type of binder and the stress level on the microscopic creep behaviour were investigated. It is found that the short-term creep of cement paste at microscale can be satisfactorily described by a power-law function. A linear viscoelastic behaviour has been observed in different cementitious systems at the microscale with the stress level up to 67.9%. When compared with the creep results in microindentation tests and conventional macroscopic tests, the obtained creep compliance function in this study is found to be both qualitatively and quantitatively representative of the macroscopic results. This experimental study underlines the importance of microstructural effect on the creep behaviours of cementitious materials at microscale.
Researchers have widely investigated Oriental lacquers to identify the chemical composition and have elucidated corresponding polymerization mechanisms using rigorous analytical techniques. However, ...researchers generally test the physical properties of Oriental lacquers by conventional methods that are perhaps overly simplistic. Here, we propose accurate and quantitative methods for evaluating the physical properties of Korean, Vietnamese, and Myanmarese lacquer films using atomic force microscopy (AFM), a nanoindenter, and a 90° peel tester. We obtained surface images of the lacquers in accordance with drying time using scanning electron microscopy and AFM. The Korean lacquer film exhibited fast hardening speed, enhanced hardness, and strong adhesion strength compared with the other lacquers, although the Myanmarese lacquer film had a smoother surface than the Korean lacquer film. We used our characterization approach for evaluating a mixed Korean/Myanmarese (50/50
/
) lacquer. Our proposed measurement techniques for Oriental lacquer films provided results that agreed with qualitative results from conventional tests. Force-distance curves in AFM and force-displacement with nanoindenter for Oriental lacquer films showed more accurate and quantitative data on the mechanical properties. Thus, researchers will find our approach useful when they optimize the chemical compositions and improve the physical properties of Oriental lacquer films for industrial applications.
•Drawing the material flow lines in ploughing scratch by using FEM results.•The focal point method is proposed to model the material flow lines.•Describing the evolution of material flow lines in ...terms of material properties and local friction coefficient.•Calculating the friction coefficients based on the material flow lines.
In nanoscratch experiments, the frictional shear force vector applying on each element of the contact surface between the nanoindenter and material is in the direction of material flow lines or relative displacement vectors. In this paper, the material flow lines in the nanoscratch test around the Berkovich nanoindenter in the face-forward orientation are investigated. The finite element simulation of nanoscratch test is conducted to calculate and track the trajectory of the material points located on the material surface. The material flow lines around the nanoindenter are qualitatively and quantitatively explained and a new method entitled focal point method is proposed to describe the material flow lines in the contact surface. The evolution of the material flow lines in terms of the material properties and local friction coefficient is studied. The adhesive friction coefficient is calculated using the pattern of flow lines obtained by the focal point method. There is a close agreement between the predicted friction coefficients and those obtained from the finite element simulation and experiment, so that the error percent is about 5.7%.
Cement paste is the main binding component in concrete and thus its fundamental properties are of great significance for understanding the fracture behaviour as well as the ageing process of ...concrete. One major aim of this paper is to characterize the micromechanical properties of cement paste with the aid of a nanoindenter. Besides, this paper also presents a preliminary study on the fatigue behaviour of cement paste at the micrometer level. Miniaturized cantilever beams made of cement paste with different water/cement ratios were statically and cyclically loaded. The micromechanical properties of cement paste were determined based on the measured load-displacement curves. The evolution of fatigue damage was evaluated in terms of the residual displacement, strength, and elastic modulus. The results show that the developed test procedure in this work is able to produce reliable micromechanical properties of cement paste. In addition, little damage was observed in the cantilever beams under the applied stress level of 50% to 70% for 1000 loading cycles. This work may shed some light on studying the fatigue behaviour of concrete in a multiscale manner.
This study investigated the compressive strength and drying shrinkage of mortar made with recycled fine aggregate carbonated in air or water. Recycled fine aggregate carbonated in air showed a ...decrease in water absorption and an increase in Oven - dry density. Mortar using recycled fine aggregate carbonated in air showed an increase in compressive strength and a decrease in drying shrinkage. On the other hand, the recycled fine aggregate carbonated in water had the same water absorption and decreased in Oven - dry density. The compressive strength of mortar using recycled fine aggregate carbonated in water decreased and drying shrinkage increased. No clear relationship was found between the water absorption of the recycled fine aggregate and the compressive strength of the mortar, and between the water absorption of the recycled fine aggregate and the drying shrinkage of the mortar.To elucidate the cause, cement paste was carbonated under the same conditions as the recycled fine aggregate and analyzed.The results showed that carbonation in air increased the modulus of elasticity of the cement paste compared to that before carbonation. On the other hand, when carbonated in water, the modulus of elasticity decreased due to leaching of calcium ions. It was estimated that the recycled fine aggregate carbonated in water decreased the modulus of elasticity of the cement paste on the aggregate surface, resulting in a decrease in compressive strength and an increase in drying shrinkage.
Ni-B alloy coatings were electrodeposited using a Ni plating bath containing dimethylamine borane (DMAB) as boron sources. Hard chromium coatings were electrodeposited from conventional Cr (VI) bath ...for comparison. Ni-B alloy coatings were heat treated at different temperature from 200°C to 400°C using vacuum annealing furnace. Crystal structures and surface morphologies of coatings were evaluated by XRD, SEM and TEM, respectively. Their mechanical and tribological properties were tested by microhardness tester, scratch tester and friction wear testing machine, respectively. Wear tracks morphologies were observed through SEM to analyze the wear mechanism of coatings. Nano-hardness and elastic modulus were tested by Nanoindenter. Results show that the hardness of as-plated Ni-B alloy coatings is 700–800Hv, and it increases to 980±28Hv after heat treatment at 300°C for 20min. The scratch tests show that the Ni-B coatings have a good adhesion to the substrate in both as-plated and heat-treated conditions. Tribological tests show that the 300°C-heat-treated Ni-B coatings exhibit a lowest wear rate under different oil-lubricated sliding conditions, the wear resistance of which is higher than that of hard chromium coatings. The excellent wear resistance is attributed to its high ratio of hardness to elastic modulus (H/E).
•High hardness Ni-B coatings were prepared as alternative for Cr coatings.•The wear resistance of 300°C-heat-treated Ni-B coatings is obviously higher than that of hard chromium coatings.•Coatings with high H/E exhibit good resistance to both abrasion and fatigue.•Ni-B coatings heat-treated at 300°C and having the highest H/E exhibit best wear resistance.
MEMS biochemical sensors based on nanomechanical cantilevers (NMC) translate bio/chemical interactions into nanomechanical motion that can be detected by various transduction techniques. Polymer ...Nanomechanical Sensors have proven to be better candidates than the conventional silicon MEMS sensors owing to better sensor performance, ease of fabrication and cost effectiveness. In this paper, the development of novel polymer nanomechanical cantilever sensor platform with a high gauge factor Indium tin oxide (ITO) as piezoresistor for electrical transduction has been discussed. With simple and cost-effective polymer MEMS process integration, low temperature deposited ITO has been incorporated for efficient electromechanical transduction of SU-8 microcantilever devices. The fabricated SU-8/ITO microcantilevers were experimentally characterized for extracting the mechanical, electrical, and electromechanical properties. These SU-8/ITO devices exhibited strong negative piezoresistive behavior with the experimentally extracted gauge factor of ITO as −779. These devices show displacement sensitivity of 37 ppm/nm, which is nearly 30 times that of reported values of SU-8 cantilevers with nanocomposites as strain sensitive layer. The surface stress sensitivity is found to be 49.8 ppm mN/m −1 . One of the potential applications of this polymer MEMS cantilever for room temperature detection of hydrogen gas has also been demonstrated. 2019-0266
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