Thermal mechanical performance test on suspended insulators for transmission line
This study described some basic performance tests and accelerated ageing test by cool-heat cycling methods and thermal mechanical performance test methods on alumina porcelain insulators (new and aged) used for transmission lines in Korea. There were no failures in electrical and mechanical performance tests such as high voltage strength, flashover voltage, impact strength, etc. in any of the samples. However in the case of accelerated ageing tests above 90 /spl deg/C temperature gradient, fracture phenomena occurred by a thermal shock in the aged samples with low aluminous porcelain body. It was found that sample A was more severely aged than other samples. According to results of the HRB test and microstructural analysis, it was reasoned that insulator bodies with the matrix reinforced with alumina crystalline phase have advantages in the suppression of crack propagation. Cool-heat aging and mechanical thermal aging tests show that a temperature gradient is more effective for acceleration of the aging than variation in the number of cycles.
High voltage insulator mechanical load limitations to dielectric material damage
The paper discusses the mechanical load limitations to dielectric material damage of high voltage suspension type insulators. For ceramic insulators, discussed are the design aspects of cap-and-pin insulators that play a role in the limits of time-load and thermal mechanical loads to prevent damage to the dielectric material. Brittle fracture, weathershed seals, interfaces between dielectric materials, and temperature, are discussed in terms of limiting the mechanical loads of non-ceramic insulators in order to prevent dielectric material damage.
Numerical analysis of surface crack propagation in flexible pavements using XFEM and cohesive zone model
Surface initiated (top-down) cracking in the longitudinal direction of pavements has become a predominant mode of failure in thick asphalt pavements. Most previous researchers studied crack propagation using the traditional Linear Elastic Fracture Mechanics (LEFM) method by calculating the stress intensity factor around the crack tip. In this paper, Extended Finite Element Method (XFEM) was used to study the key factors causing surface initiated crack propagation under surface tension and/or shear. A two-dimensional (2-D) finite element (FE) model for a two-lane flexible pavement was built to predict pavement responses and crack propagation potential under various scenarios. It was found that surface tension is the major driving force for crack propagation when the crack is at some distance away from tire loading. The results show that the distance from the location of crack to tire loading, the depth and direction of the crack, and the modulus ratio between the hot-mix asphalt (HMA) layer and the base layer have significant influences on crack propagation under surface tension. On the other hand, this study shows that shear stress can induce surface initiated crack propagation near the loading area as the shear resistance of material becomes low at the high temperature.