Influence of hygrothermal environment on thermal and mechanical properties of carbon fiber/fiberglass reinforced matrix composite materials
The absorption and diffusion of water in a carbon fiber/glass fiber hybrid composite was investigated. Water-sorption experiments, mechanical property tests and dynamic mechanical analysis (DMA) were performed after immersion in water at different temperatures for up to 32 weeks. The moisture uptake mechanism exhibited by the hybrid fiber system was determined to be more complex than the single fiber type. Weight-change profiles for the composites fitted the theoretical Fickian diffusion curve during the initial immersion time, but diverged substantially as time progressed. The shear properties and the glass transition temperature (Tg) were sensitive to the effects of hygrothermal environment, and values for both decreased with increasing water uptake. Microscopic inspection of water-soaked samples showed no cracking when the absorption was less than saturation. The thermal and mechanical properties were mostly retained (after drying), provided
ACCELERATED AGING AND DURABILITY OF COMPOSITE RODS FOR POWER TRANSMISSION LINES
Transmission lines typically consist of electrically conductive Al wires that are supported by a stranded steel cable core, and the lines are hung from lattice towers. Figure 1 shows a section of conventional ACSR overhead conductor (aluminum conductor/steel reinforced). Despite widespread use, ACSR lines have inherent limitations, one of which is the high CTE (coefficient of thermal expansion) values of the aluminum and steel wires (23 and 11.5x10/°C). When amperages increase and the line temperature rises, linear expansion causes substantial sag between towers. The extent of sag limits the operating temperature (because safety dictates that minimum clearances be maintained), and ultimately the distribution capacity of the electrical grid. Although the design of transmission lines has not changed substantially in the past 100 years, new designs have appeared in recent years. In one such design, the stranded steel core is replaced with a solid composite rod of unidirectional glass and carbon in epoxy, as shown in Figure 1. The advantages of the composite conductor (ACCC, aluminum conductor/composite core) include a low CTE value (nearly an order of magnitude less than steel), strength that is nearly 1.5 times the steel wire used in ACSR conductor (2158 MPa vs. 1240 MPa), and a density roughly 1.3 that of steel [1]. The lower CTE value results in greatly reduced sag at high service temperatures [1]. In terms of electrical performance, the ACCC cable has a higher electricity transmission capacity (ampacity), because (1) the round Al wires are replaced with trapezoidal wires, filling interstitial spaces, and (2) the operating temperatures can be well over 100°C because the sag is nearly eliminated by the low-expansion composite core. Figure 1. The Composite core conductor (ACCC, left) and the conventional steelreinforced conductor (ACSR) commonly used today. The ACCC conductor is intended for sustained operation up to 180°C. Thus, the longterm durability of the composite core is a key performance issue, and the effects of thermal exposure (25°C-200°C), moisture, and cyclic loads on tensile strength have been investigated [2]. Retention of tensile strength is critical, since the core supports nearly all of the mechanical loads. The effects of accelerated aging on strength, glass transition temperature, and microstructure has been investigated. Water uptake and thermal oxidation experiments have shown that while both processes can lead to strength degradation, the service conditions anticipated for the composite are more moderate, and lifetimes are expected to be more than adequate [3]. This report will focus on accelerated aging experiments and degradation mechanisms, as well as the implications for long-term durability.
CRIMINAL PROSECUTION FOR PRENATAL INJURY
A case of maternal assault resulted in fetomaternal hemorrhage and subsequent neonatal death, leading to a verdict of criminally negligent homicide against the assailant.