John Jy-An Wang

The Lifetime Estimate for ACSR Single-Stage Splice Connector Operating at Higher Temperatures

The power transmission conductor system consists of: the aluminum conductor, the steel-core supporting material, and the splice connector. The splice connector connects the aluminum conductor to form a continuing current transmission line. The splice connector region of a conductor system is more sensitive to material aging during service. This is due to the material discontinuity and the crimped connectors forming mechanism. The objective of this project is to develop a protocol to evaluate the integrity of a full tension single-stage splice connector (SSC) assembly operated at high temperature. The project focuses on thermal mechanical testing, thermal cycling simulation and the effective lifetime of the SSC system. The investigation indicates that thermal cycling temperature and frequency, conductor cable tension loading, and the compressive residual stress field within a SSC system have significant impact on SSC integrity and its associated effective lifetime. The developed governing equation and its application to assure the adequate service life of transmission lines are also discussed in the paper.

An Innovative Technique for Bi-Material Interface Toughness Research

A material configuration of central importance in microelectronics, optoelectronics, and thermal barrier coating technology is a thin film of one material deposited onto a substrate of a different material. Fabrication of such a structure inevitably gives rise to stress in the film due to lattice mismatch, differing coefficient of thermal expansion, chemical reactions, or other physical effects. Therefore, in general, the weakest link in this composite system often resides at the interface between the thin film and substrate. In order to make multi-layered electronic devices and structural composites with long-term reliability, the fracture behavior of the material interfaces must be known. Unfortunately, none of the state-of-theart testing methods for evaluating interface fracture toughness is fully conformed to fracture mechanics theory, as is evident from the severe scatter in the existing data, and the procedure dependence in thin film/coating evaluation methods. This project is intended to address the problems associated with this deficiency and offers an innovative testing procedure for the determination of interface fracture toughness applicable to thin coating materials in general. Phase I of this new approach and the associated bi-material fracture mechanics development proposed for evaluating interface fracture toughness are described herein. The effort includes development of specimen configuration and related instrumentation setup, testing procedures, and postmortem examination. A spiral notch torsion fracture toughness test (SNTT) system was utilized. The objectives of the testing procedure described are to enable the development of new coating materials by providing a reliable method for use in assessing their performance.

Thermal Expansion Study and Microstructural Characterization of High-Performance Concretes

Ultra-high performance concrete (UHPC) is a family of emerging materials for building and construction applications. Behavior of UHPCs at high temperature is very important to their reliability and safety. In the current study, two UHPC materials were studied using the thermomechanical analysis (TMA) technique between room temperature and 800°C. Both reversible and irreversible phase transformations were observed from the TMA results, which were likely attributable to the α-β quartz transformation and the dehydroxylation transitions, respectively. Thermal expansion coefficients exhibited significant variations in different temperature regimes. Postmortem scanning electron microscopy (SEM) examinations revealed extensive cracking in the heated samples. In addition, microporosities were observed in the calcium-silicate-hydrate (C-S-H) phase as a result of phase changes during heating.

CAVITATION DAMAGE STUDY VIA A NOVEL REPETITIVE PRESSURE PULSE APPROACH

Cavitation damage can significantly affect system performance. Thus, there is great interest in characterizing cavitation damage and improving materials’ resistance to cavitation damage. In this paper, we present a novel methodology to simulate cavitation environment. A pulsed laser is utilized to induce optical breakdown in the cavitation media, with the emission of shock wave and the generation of bubbles. The pressure waves induced by the optical breakdown fluctuate/propagate within the media, which enables the cavitation to occur and to further develop cavitation damage at the solid boundary. Using the repetitive pulsed-pressure apparatus developed in the current study, cavitation damage in water media was verified on stainless steel and aluminum samples. Characteristic cavitation damages such as pitting and indentation are observed on sample surfaces using scanning electron microscopy. The synergistic effect of combining cavitation and the laser heating/water cooling induced thermal cycling fatigue to the target surface damage was also demonstrated in the report.Copyright

A New Test Method for Determining the Strength and Fracture Toughness of Cement Mortar and Concrete

The Spiral Notch Torsion Fracture Toughness Test (SNTT) was developed recently to determine the intrinsic fracture toughness (KIC ) of structural materials. The SNTT system operates by applying pure torsion to uniform cylindrical specimens with a notch line that spirals around the specimen at a 45° pitch. KIC values are obtained with the aid of an in-house developed three-dimensional finite-element computer code, TOR3D-KIC. The SNTT method is uniquely suitable for testing a wide variety of materials used extensively in pressure vessel and piping structural components and weldments. Application of the method to metallic, ceramic, and graphite materials has been demonstrated. One important characteristic of SNTT is that neither a fatigue precrack nor a deep notch are required for the evaluation of brittle materials, which significantly reduces the sample size requirement. In this paper we report results for a Portland cement-based mortar to demonstrate applicability of the SNTT method to cementitious materials. The estimated KIC of the tested mortar samples with compressive strength of 34.45 MPa was found to be 0.19 MPa √m.Copyright

A New Approach for Bimaterial Interface Fracture Toughness Evaluation

A material configuration of central importance in composite materials or in protective coating technology is a thin film of one material deposited onto a substrate of a different material. Fabrication of such a structure inevitably gives rise to stress in the film due to lattice mismatch, differing coefficient of thermal expansion, chemical reactions, or other physical effects. Therefore, in general, the weakest link in this composite system often resides at the interface between the thin film and the substrate. In order to make multilayered electronic devices and structural composites with long-term reliability, the fracture behavior of the material interfaces must be known. This project offers an innovative testing procedure of using a spiral notch torsion bar method for the determination of interface fracture toughness that is applicable to thin coating materials in general. The feasibility study indicated that this approach for studying thin film interface fracture is repeatable and reliable, and the demonstrated test method closely adheres to and is consistent with classical fracture mechanics theory.

The Integrity of ACSR Full Tension Splice Connector at Higher Operation Temperature

Due to the increase in power demand and limited investment in new infrastructure, existing overhead power transmission lines often need to operate at temperatures higher than those used for the original design criteria. This has led to the accelerated aging and degradation of splice connectors, which have been manifested by the formation of hot-spots that have been revealed by infrared imaging during inspection. The implications of connector aging is two-fold: (1) significant increase in resistivity of the splice connector (i.e., less efficient transmission of electricity) and (2) significant reduction in the connector clamping strength, which could ultimately result in separation of the power transmission line at the joint. Therefore, the splice connector appears to be the weakest link in the electric power transmission lines. This paper presents a protocol for integrating analytical and experimental approaches to evaluate the integrity of a full tension single-stage splice connector assembly.

Determination of Interfacial Fracture Toughness for Thin Film Coating Materials

A material configuration of central importance in composite materials or in protective coating technology is a thin film of one material deposited onto a substrate of a different material. Fabrication of such a structure inevitably gives rise to stress in the film due to lattice mismatch, differing coefficient of thermal expansion, chemical reactions, or other physical effects. Therefore, in general, the weakest link in this composite system often resides at the interface between the thin film and substrate. In order to make multi-layered electronic devices and structural composites with long-term reliability, the fracture behavior of the material interfaces must be known. This project is intended to address the problems associated with interface fracture toughness evaluation and offers an innovative testing procedure for the determination of interface fracture toughness applicable to thin coating materials in general.Copyright