Hideyasu Andoh

Failure mechanisms and recent improvements in ZnO arrester elements

ZnO arrester elements can fail by a number of mechanisms, including electrothermal instability and thermal stress-induced mechanical cracking in this brittle ceramic. Breakdown channels at the edge of the electrode are among the most common failure mechanisms at high currents. Such failures take the form of a melt puncture of the element from the electrode edge to the opposite electrode or to the edge of the disk. This failure mechanism limits the energy absorption capability of ZnO arrester elements.

A Statistical Approach to Prediction of ZnO Arrester Element Characteristics

ZnO arrester elements consist of ZnO grains with dimensions in the range of 10 to 100 ¿m, the boundaries between which form double Shottky junctions with conduction voltages in the range of 3.5 V. A fraction of the grains contain no conducting boundaries with other grains, which results in the percolation path for current across the ZnO element being a statistical parameter that is a function of the fraction of nonconducting grains, which also affects the nonlinear properties of the element. In this paper, we use a simple statistical approach to predict the effect of the fraction of nonconducting grains on the nonlinear properties of the element. This computationally simple approach gives results that are comparable to far more complex approaches, which require solving a network of nonlinear resistive elements.

Electro-thermal-mechanical computations in ZnO arrester elements

ZnO nonlinear dielectric is a complex composite of power dissipating grain boundaries, conducting ZnO grains, cavities, and nonconducting grain boundaries. Optimization of the material requires sophisticated approaches to modeling the various electro-thermal-mechanical phenomena in the macroscopic approximation of a homogeneous nonlinear material, the microscopic approximation of grains and grain boundaries, and in statistical approximations to determine the effects of statistically distributed phenomena such as nonconducting grains. In this contribution, we have provided examples of each of these approaches. Through application of studies such as these, substantial improvements have been achieved in ZnO element characteristics.

Stress Singularity for Free-Edge of Interface of Copper/Aluminium Friction Welded Joint with Intermetallic Compounds.

The friction welding for copper and aluminium is useful to minimize the diffusion thickness at the interface between copper and aluminium. The minimization of diffusion thickness is required for maintaining the joint strength. However, it is well known the growth of brittle intermetallic compounds, such as CuAl2, CuAl and Cu9Al4, during the use at high-temperature. The effect of intermetallic compound layers on joint strength characteristics has not fully been clarified. In this study, elastic properties of intermetallic compound layers were investigated using depth-sensing micro-indentation technique. And stress singularities at the intersection of free surfaces and the interface between copper and aluminium were investigated using singular analysis. It was informed that the no free-edge stress singularities existed at the nelghborhood of apex angles of 55°and 125°. It was also confirmed using finite element analysis that the reduction of stress concentration could be observed by paying attention to the condition of no free-edge stress singularities in case of the copper and aluminium friction welded joint with brittle intermetallic compound layers.

Effect of Interface Shape of the Strength for Carbon Steel/PMMA Adhered Joint

The adhered joint composed of polymer and metal is a useful and practical method to take advantage of the both material properties. However, there are some basic problems for the material mismatch due to the different Youngs modulus. Inthis study, the stress singularity at the intersection between the free surface and the interface betweenpoly-methyl-methacrylate (PMMA) and structural carbon steel (JIS SS400) was analyzed to clarify the joint strength using a finite element method. As a result, the stress singularity was found above 130 degree of PMMA apex angle and between 35 and 41 degree of PMMA apex angle. In this experiment, it was confirmed that the no free-edge stress singularity was useful conditions to insure the reliability of bending strength for the adhered joint composed of SS400 and PMMA.