Pre-Breakdown and Breakdown Behavior of Synthetic and Natural Ester Liquids under AC Stress

In the last decades, a large focus is being placed on the sustainability and safety of the power transformer spectrum. Ester liquids, which have interesting properties such as high fire point and biodegradability, are gaining needed attraction. Since in-service condition, thermal aging deteriorates the physicochemical and electrical properties of liquid dielectrics, it is important to study their long-term behavior. In this contribution, the pre-breakdown and breakdown behavior of ester fluids (synthetic and natural) under AC stress are investigated. Important characteristics, such as partial discharge pre-inception voltage, partial discharge inception voltage, breakdown voltage, average streamer velocity, and inception electric field, were assessed. The influence of the radius of curvature (of high voltage needle electrode) as well as the thermal degradation of typical ester liquids are also discussed. Mineral oil was also included in the tests loop as a benchmark for comparative purposes. It is found that the pre-inception voltage of ester liquids was, in most cases, higher than that of mineral oil. For a given radius of curvature, the streamer inception and breakdown voltages decreased with thermal aging. During the streamer initiation, the electric field at the electrode tip decreased with the increase in the radius of curvature. The velocity of the streamers seems to increase with the decrease in the radius of curvature. The period of vulnerability, the so-called “delay time”, seems to be independent of the aging or the radius of curvature for a given condition of the liquid.

Employees’ Post-service Plans and Options in Nigeria

One of the greatest challenges that faced typical employees throughout their working life is life after retirement. Despite government roles at providing social security, retirement epoch has been a tension provoking and overwhelming phenomenon in Nigeria and other emerging economies of Africa, Asia and Latin America. A close observation of many retirees in Nigeria society and the problems they are facing draw the attention of all and sundry. These problems seem to range from sudden death, dwindling status, lack of occupation, loss of the usual monthly salary, decreased strength and deteriorated health condition, physical disabilities, aging and anxiety about a residential home. In Nigeria, the regular non-payment of pensions and gratuities benefits has brought untold hardship and sudden death to many retirees, thereby making retirement something that is dreaded by workers. This problem is further compounded to lack of planning and management of post-service epoch and conditions. It is against this backdrop that this paper seeks to identify and discusses strategies available for managing post-service condition in Nigeria. The paper calls on Nigerian workers to embrace one of the options to remain moderately productive and healthy after retirement. The paper suggests, among others, that both employers and government should undertake regular pre-retirement training for their workers in order to save the country the embarrassment of seeing their retirees going to early graves or turning into destitute on the streets.

Investigation on the Micromechanisms of the Cracking of 316L Heat Exchanger Tubes

The cracking mechanisms of 316L heat exchanger tubes employed in power station were studied using optical microscope (OM) and scanning electron microscope (SEM). It is demonstrated that the hardness value, microstructure and tensile properties of selected #1 and #2 tube samples all meet the requirements of relevant standards, but the contents of Ni and Mo element of #1 tube are slightly lower than the standard requirements. The circumferential cracks on the two samples nucleate at the corrosion pits on the inner wall of the tubes, while Cl element was detected in the corrosion products of these pits. The cracks propagate from the inner wall to the outer wall along the circumferential direction of the tube, forming a dendritic crack morphology with both transgranular and intergranular propagation characteristics. Combined with the investigation of the service condition of the heat exchanger tubes and the analysis of the experimental results, it can be concluded that the main reason for cracking is the initiation of pin-corrosion when the content of chloride ion exceeds the standard during the service of the tubes, which will induce stress corrosion cracking, causing crack expansion through the wall thickness, and finally lead to leakage of the tube. In addition, from the point of view of materials, Mo is an important element to improve the pitting resistance of materials. The content of Mo element detected in the samples is lower than the standard requirement, which is also one of the reasons for the easy pitting corrosion of the inner wall of the pipe.

Construction types and typologies of existing masonry arch bridges in Croatia

Historical masonry arch bridges still form an important part of Croatian transportation network. There are approximately 680 masonry arch bridges and culverts currently being used for railways and roadways. Many of these bridges are relatively old (more than hundred years in most cases) but still in usage. Increasing vehicle load and speeds as well as deterioration due to anthropogenic and environmental influence have highlighted the need for reliable assessment of their service condition and regular maintenance. The aim of this study is to provide a review of existing masonry arch bridges in Croatia. Firstly, a historical review of bridges is given showing the time period in which they were built, indicating the materials and design principles used for their construction. Next, bridge typologies are presented as well as their detailed analysis of geometric characteristics for brickwork bridges. Finally, a short review of damages and their impact on serviceability of bridges is given. This review presents masonry arch bridges in Croatia and the need for reliable method of assessing their service condition in order to provide proper maintenance, repairing and retrofitting.

Identification Of Inhibiting Factors for Road Construction Using Sem Application In Kaway XVI Sub-District

Road construction is one thing that is always in line with technological advances and human thinking. Because the road is an important facility for humans, in order to reach an area. Continuous use of the road will reduce the service life of the road to the economic life of the road. To restore the service condition of the road, it is necessary to carry out road maintenance. Kaway XVI Sub-district is one of the connecting lines for Pidie Regency - West Aceh Regency, the road will continue to be built and maintained to support the agricultural and fishery sectors. The purpose of this study was to identify the inhibiting factors for road construction and maintenance as well as the relationship between these factors and indicators in Kaway XVI District using Structural Equation Modeling (SEM). The results of the study indicate that the quality factor has a positive effect on Spatial Planning and the Environment (TRL), and financing with a correlation number of 0.25 and 0.17. Regulations and Policies (PK) have a positive effect on financing of 0.60. Quantity has a positive effect on politics with a correlation number of 0.07. Politics has a positive effect on Regulations and Policies (PK) with a correlation number of 0.73, but negatively on TRL with a correlation number of -0.09. TRL has a positive effect on Human Resources (HR) with a correlation number of 0.81. HR has a significant positive effect on politics with a correlation of 0.67. Based on the research path diagram, the most decisive indicators both in quality and quantity for the construction and maintenance of roads in support of the Agriculture and Fisheries Sector are TRL, Financing, Politics and PK.

Influences of Brass Surface Morphology on Leidenfrost Effect during Liquid Nitrogen Cooling

Cooling in liquid nitrogen is a typical service condition of high-temperature superconducting wire, and the variation of boiling stages on the wire protective layers such as the brass layers could be crucial for the quench behavior of superconducting devices. In this study, the influence of brass surface morphology (parameters of surface roughness and fractal dimension) on the Leidenfrost effect (including the wall superheat at critical heat flux and the wall superheat at Leidenfrost point, which are respectively characterized by the temperatures of ΔTCHF and ΔTLP) was studied. The surfaces of brass samples were polished by sandpaper to obtain different morphologies, which were characterized by using white light interferometer images, and the boiling curves were recorded and analyzed by Matlab with lumped parameter method. The experimental results demonstrated that the surface morphology of brass samples could influence the ΔTLP significantly, but had no clear relationship with the ΔTCHF. Moreover, the multi-scaled analysis was carried out to explore the influencing mechanism of surface microstructure, the relationship between ΔTLP and scale was more clear when the scale was small, and the fractal dimension was calculated and discussed together with surface roughness. The findings of this study could be instructive for surface treatment of superconducting wires to suppress quench propagation.

Review of Existing Methods for Evaluating Adhesive Bonds in Timber Products

Gluing is an integral part of the majority of production processes in the timber industry. The effectiveness of adhesive application, glue bond development and glue penetration into the wood structure is becoming more and more important as more structural glued timber products are used in construction and other applications. The continued increase in utilisation of mass timber products (MTPs) such as CLT, glulam and LVL in tall timber buildings requires an accurate and in-depth understanding of adhesive roles and their performance effectiveness during the life span of any of those products in relation to the type of loading applied, environmental effects (e.g. RH and temperature) and in-service condition of elements (e.g. exposure to major wet events and degradation from decay). This review aims to provide a comprehensive summary of existing imaging and other visualisation methods used to assess the glue line properties and examine the performance of glue lines in relation to factors such as species, product type and environmental conditions during manufacture and in-service life.

Residual Mode Vector-Based Structural Damage Identification with First-Order Modal Information

Damage identification based on the change of dynamic properties is an issue worthy of attention in structure safety assessment, nevertheless, only a small number of discontinuous members in existing structure are damaged under service condition, and the most remaining members are in good condition. According to this feather, we developed an effective damage location and situation assessment algorithm based on residual mode vector with the first mode information of targeted structure, which utilized the quantitative relationship between first natural modes of global structure with the change of the element stiffness. Firstly, the element damage location is determined with exploitation of the sparseness of element stiffness matrices based on the discontinuity of damaged members. Then, according to the distribution characteristics of the corresponding residual mode vector, the nodal equilibrium equation about the damage parameter is established based on the residual mode vector, and the damage coefficients of structural elements are evaluated with the proposed equations. Two numerical examples are given to verify the proposed algorithm. The results showed that the proposed damage identification method is consistent with the preset damage. It can even accurately identify large-degree damages. The proposed algorithm only required the first-order modal information of the target structures and held few requirements of analysis resource; hence when compared with existing methods, it has obvious advantages for structural damage identification.