Key Engineering Materials Vol. 814

Abstract: In order to explore the mechanism of tribological fatigue fracture in PEEK (Poly-ether-ether-ketone) polymer mechanical element application, one-point contact type RCF (rolling contact fatigue) tests were carried out by using a PEEK shaft with an artificial defect. An alumina ball contacted a PEEK shaft specimen under maximum Hertzian stress 380 MPa. Flaking and internal fatigue crack propagation under the rolling track of the tested PEEK shaft were investigated through 2.5D layer observation method. The main fatigue crack occurred near the artificial defect on the rolling track of the PEEK shaft, and propagated into depth direction. In addition, the main fatigue crack branched due to internal shear stress. The branching crack as internal fatigue crack propagated into the ball’s rolling direction. After the linkage of the branching crack and another semicircular surface crack, the horseshoe-shaped flaking as tribological fatigue fracture occurred on the rolling track of the PEEK shaft.

Abstract: This paper presents a solution scheme for analysis of a multilayered elastic medium under axisymmetric loading and surface energy effects by adopting Gurtin-Murdoch surface elasticity theory. Love’s strain function and Hankel integral transform are employed to derive the general solutions, and the obtained solutions are employed in the determination of the stiffness matrix for each layer. The global stiffness equation of a multi-layered system is assembled by considering the continuity of traction and displacements at each layer interface. The numerical solutions to the global equation yield displacements and stresses at the interfaces of the layered medium under axisymmetric loading. The accuracy of the proposed solution scheme is verified by comparing with existing solutions. Selected numerical results are presented to demonstrate a significant influence of surface energy on elastic fields of a multilayered elastic medium under axisymmetric loading.

Abstract: This paper studies the aspect ratio (W/L), width (W) per length (L) of semiconductor resistor based on Hall effect current mode for horizontal magnetic field. At low concentration, 10¹⁴ cm^-3, W/L < 1, the length has direct effect to magnetoresistance. The W/L = 1, the large resistor provides magnetioresistance better than small device. The W/L ˃ 1, the width has inversely proportional to magnetoresistance. The %MR(B) is around 1 % at 0.5 T, 1 mA. The long resistor (W/L < 1) can create ΔR in the order of several kilo ohms and several hundred ohms for short resistor (W/L > 1). The contribution factors ρ (L/W) for high ΔR are low concentration and aspect ratio (W/L < 1). The high %MR(B) is contributed by high current density of short structure (W/L > 1). At high concentration 10¹⁷ cm^-3, aspect ratio and magnetoresistance are not sensitive to magnetic field because the Hall effect hardly occurs in high concentration material.

Abstract: In order to investigate the effect of pre-crack lengths on silicon nitride balls under cyclic pressure loads, the pre-crack lengths ranging from 400μm to 500μm were observed. Their growth behavior was compared to that of 200μm to 300μm pre-cracks. Furthermore, the initial threshold limits of their maximum stress intensity factors were measured.

Abstract: Addition of CeO₂ into ZTA and its effects on microstructure and mechanical properties were investigated. CeO₂ was detected with significant amounts only above 10 wt%. Viscosity was measured for slurry preparartions and characterization of mechanical properties of ZTA. Additions of CeO₂ of more than 10 wt% surpassed the solubility limit and formed Ce₂Zr₃O_10. Ce₂Zr₃O₁₀ increased the tetragonality factor, prevented excessive grain growth through a pinning effect, which is attributed to the segregation of Ce₂Zr₃O₁₀ to the grain boundaries and showed a peak in fracture toughness with a value of 9.3 MPam^1/2 with 10 wt% additions of CeO₂. Further additions of CeO₂ reduced ZTA’s mechanical strength. Maximum value of H_v was 17700 MPa with 10 wt% CeO₂. Porosities have been attributed as the underlying reason as to why theoretical density were always higher than measured densities.

Abstract: The objective of this study was to evaluate the antibacterial properties and pH changes of bioactive glasses and zinc oxide nanowire in different concentrations. Bioactive glasses (45S5 and 45S5F) were prepared in three concentrations of 10, 20, and 50 mg/ml and zinc oxide nanowire was prepared in 1 and 5 mg/ml concentrations. The materials were exposed to 500 ml brain heart infusion broth (BHI) with 1.5 x 10⁷ of S.mutans and S.sanguinis separately. Antibacterial properties were tested indirectly by collecting 100 ml of each sample and transferred into a 96 well-plate. The optical density (OD) was evaluated using spectrophotometry at 630 nm at 24h and 48h. The pH changes were recorded. The data were statistically analyzed by Kruskal-Wallis tests. The result showed that the pH changes were significantly different in the Bioactive glass samples, while zinc oxide nanowire showed stable pH. Antibacterial activity against S.mutans was significant lower for 45S5 at 50 mg/ml, 45S5F and zinc oxide nanowire in all concentrations at 24 h. While in 48 h, 45S5, 45S5F and zinc oxide nanowire showed significant antibacterial activity in all concentration except 45S5F at 10 mg/ml. Antibacterial activity against S.sanguinis was significant for 45S5 and 45S5F at 20 and 50 mg/ml and zinc oxide nanowire in all concentration at 48h. It can be concluded that Bioactive glasses (45S5 and 45S5F) exhibited antibacterial properties and pH changes depending on its concentration, while zinc oxide nanowire exhibited antibacterial properties at low concentrations with a constant pH value.

Abstract: Orthodontic elastic bands are commonly made from natural rubber because they provide high resiliency at a reasonable cost. However, hypersensitivity related to protein present in latex have been reported in some patients which has led to increased usage of non-latex elastic alternatives. Therefore, the assessment of their mechanical properties is of importance. The objective of this study was to compare the physical and mechanical properties of three commercial latex and non-latex type orthodontic elastic bands. Samples of latex and non-latex type orthodontic elastics from manufacturers – AO (6.5oz), MASEL (6.0oz), GAC (6.0oz), with 3/16-inch diameter were selected. Firstly, the physical characteristics (width, cross-sectional thickness, and inner diameter) of the elastic bands were determined, following which their mechanical properties [initial extension force (F0), 24 h-residual force (F24), percentage of force decay, force exerted at 3 times the inner diameter (F3xID) and breaking force] were tested. The data were analyzed with Mann-Whitney U test and multiple comparisons among the groups were done with Kruskal-Wallis Test (p< 0.05). Significant differences were found in the physical characteristics and mechanical properties among each brand and type of elastics. AO elastic bands had significantly low F0 and F24 compared with the others. While the percentage of force decay at 24 h was greatest in AO followed by MASEL and GAC. Non-latex type elastics showed greater force decay than latex type ones, approximately 30-40% and 20-30% of the initial force in non-latex and latex type elastic, respectively. AO elastics showed the highest F3xID and also the lowest breaking force. Overall, non-latex type elastics exhibited lower breaking force compared to latex type ones. Wide variations were observed in the physical and mechanical characteristics among same manufacturer and same elastic type. All commercial brands presented higher F3xID than that stated by the manufacturers. Non-latex type elastics showed greater force decay over 24 h than latex type ones. The differences in the properties between the 2 types of the elastics could be due to the differences in their structure and polymers composition.

Abstract: In Vietnam, the medicinal plant Scoparia dulcis is used for the treatment of bronchitis, gastric disorders, antidiabetes, hypertension, hepatitis etc.1 In this paper, experiments was designed to evaluate in vitro anticancer and anti-inflammatory activities and isolate phytochemicals from methanol extracts of Scoparia dulcis whole plants. The chemical investigation of methanol fraction of Scoparia dulcis led to the isolation of benzoxazinone (1), phenylethanoid (2), flavone (3), and lignan (4) glycosides. The bioactivity results indicated that crude ethanol and ethyl acetate extracts had potent cytotoxic activity toward the HepG2 cancer cell with IC50 of 47.03 μg/mL and 36.04 μg/mL, respectively. Interestingly, crude ethanol and ethyl acetate extracts had the NO inhibitory activity, with inhibition of 56.9 % or 74.7 % at 30 μg/mL and 97.3% or 82.9 % at 100 μg/mL, respectively. In addition, the hexane extract at concentrations of 100 μg/ml demonstrated anti-inflammatory activity through the inhibition of nitric oxide production of 76.7%.

Abstract: In this paper, chitosan and caffeic acid were used as starting materials to prepare chitosan caffeates by reflux-heating and freeze-drying. The structures of chitosan caffeates were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and potentiometric titration. At the same time, the physical properties of chitosan caffeates were tested and the hemostatic properties were evaluated. The results showed that four chitosan caffeates with different mass ratios of chitosan and caffeic acid (1:1, 1:2, 1:4, 1:6) had been successfully prepared, which enhanced the water solubility. FTIR analysis demonstrated that caffeic acid had been successfully grafted onto chitosan chains. XRD showed that the crystal form of chitosan changed to some extent and the chain had some regularity in some directions, but its crystallinity reduced. Chitosan caffeates, particularly mass ratio of 1:1, showed excellent hemostatic properties and even better than chitosan and the positive control (Yunnan Baiyao), which were expected to be developed as an effective biomaterial for hemostasis.

Abstract: In orthodontics, nickel-titanium wires are used for teeth alignment and leveling. For leveling the curve of Spee, reversed curve archwires are often used to increase the vertical force needed to correct a deep bite.Objectives: The aims of this study were to investigate and compare the mechanical properties (unloading force, stiffness, springback, and surface hardness) of the pre-formed plain and reversed curved NiTi archwires.Materials and Methods: NiTi wires of dimensions 0.016x0.022 inch were divided into two groups, Group 1 - plain and Group 2- reversed curve NiTi archwires. For each type of the archwire, load-deflection curve obtained from a three-point bending test, performed by a Texture Analyser (TA.XT.plus, Stable Micro System, United Kingdom) with 5 kg load cell at room temperature, was used to analyze unloading force, springback, and stiffness. Surface hardness was measured by Vickers micro-hardness tester. Kruskal-Wallis test was used to analyze the variables of this study.Results: The results showed that the unloading force of each deflection point of the reversed curve NiTi archwire was more than the plain archwire. The means of unloading force, stiffness, and springback were 2.42 N, 2.76 N; 0.28 N/mm, 0.49 N/mm; and 2.94 mm, 2.98 mm for the plain archwire and reversed curve NiTi archwire, respectively. The properties of reversed curve NiTi archwire were significantly higher (p<0.05) than the plain NiTi archwire, except the springback. The surface hardness of the plain archwire was significantly higher (p<0.05) than reversed curve NiTi archwire in each segment.Conclusion: The reversed curve NiTi archwire had more unloading force and stiffness than plain NiTi archwire. For the correction of deep curve of Spee in orthodontic treatment, clinicians must be aware of the vertical force needed during intrusion of lower incisors or the wires should be used in the later leveling and aligning stage.