Christos G. Papakonstantinou

Comparative study of high temperature composites

Abstract Two classes of composite made using either ceramic matrix with high temperature fibers or carbon/carbon have been used for various applications that require high temperature resistance, over three decades. However, their use has been limited to special applications because of the high costs associated with fabrication. Typically the composites are cured at more than 1000°C, and in most instances the heating has also to be carried out in controlled environments. In addition, because of the high processing temperature, only certain type of expensive fibers can be used with the ceramic matrices. A recently developed inorganic matrix, called polysialate can be cured at temperatures less than 150°C, making it possible to use carbon and glass fibers. Composites made using carbon, glass and combinations of carbon and glass fibers have been tested in bending and tension. This paper presents the comparison of processing requirements and mechanical properties of carbon/carbon composites, ceramic matrix composites made with silicon carbide, silicon nitride and alumina fibers and carbon/polysialate composites. The results indicate that carbon/polysialate composite has mechanical properties comparable to both carbon/carbon and ceramic matrix composites at room and high temperatures. Since the polysialate composites are much less expensive, the authors believe that it has excellent potential for more applications in aerospace, automobile and naval structures.

Wireless sensor network based model for secure railway operations

The current state of the art in detecting immediate and long-term railway track problems involves both inspectors walking the track lines and train cars instrumented with accelerometers and ultrasonic sensors that are capable of detecting wear of the rail and breakages. Additionally, a widespread practice of sensing rail continuity by using the tracks to complete simple circuits is in place. In this paper, we propose a fundamentally different approach to improve the current practices in railway operations using wireless sensor network (WSN). The primary technical and scientific objectives of the system introduced in this paper are to generate innovative solutions for a number of the issues facing the railroad community through the development of a system based on WSN. The objectives from a railroad perspective include finding new approaches to reduce the occurrence rate of accidents and improving the efficiency of railroad maintenance activities

Flexural response of inorganic hybrid composites with E-glass and carbon fibers

By far, carbon and glass fibers are the most popular fiber reinforcements for composites. Traditional carbon composites are relatively expensive since the manufacturing process requires significant heat and pressure, while the carbon fibers themselves are inherently expensive to produce. In addition, they are often flammable and their use is restricted when fire is a critical design parameter. Glass fabrics are approximately one order of magnitude less expensive than similar carbon fabrics. However, they lack the stiffness and the durability needed for many high performance applications. By combining these two types of fibers, hybrid composites can be fabricated that are strong, yet relatively inexpensive to produce. The primary objective of this study was to experimentally investigate the effects of bonding high strength carbon fibers to E-glass composite cores using a high temperature, inorganic matrix known as geopolymer. Carbon fibers were bonded to E-glass cores (i) on only the tension face, (ii) on both the tension and compression faces, or (iii) dispersed throughout the core in alternating layers to obtain a strong, yet economical, hybrid composite laminate. For each response measured (flexural capacity, stiffness, and ductility), at least one hybrid configuration displayed mechanical properties comparable to all carbon composite laminates. The results indicate that hybrid composite plates manufactured using 3k unidirectional carbon tape exhibit increases in flexural capacity of approximately 700% over those manufactured using E-glass fibers alone. In general, as the relative amount of carbon fibers increased, the likelihood of precipitating a compression failure also increased. For 92% of the specimens tested, the threshold for obtaining a compression failure was utilizing 30% carbon fibers. The results presented herein can dictate future studies to optimize hybrid performance and to achieve economical configurations for a given set of design requirements.

Mechanical Behavior of High Temperature Hybrid Carbon Fiber/Titanium Laminates

The aim of this paper was to investigate the tensile and flexural properties of hybrid laminates made with titanium sheets and high modulus carbon fiber composites. Grade II titanium was used, which exhibits great high-temperature performance and creep resistance, low weight, and high strength. An inorganic fireproof matrix, known as geopolymer, was used to fabricate the high modulus carbon fiber composites. Previous studies have shown that these composites are strong, durable, lightweight, and can exhibit excellent performance up to 400°C. In the present study, a number of specimens were tested in uniaxial tension and four-point bending after exposure at elevated temperatures. The results indicate that the addition of carbon fibers can reduce the weight and increase the stiffness of the pure titanium. Moreover, the hybrid laminates are stronger and stiffer than the sum of the individual strengths and stiffnesses of the parent materials. An important finding is that the interlaminar bond is strong, and as a result no delamination failures were observed.

Analysis and Design Recommendations of FRP-Strengthened Prestressed Concrete Beams

This paper describes how there are a limited number of experimental results available on the retrofit of prestressed concrete structural elements. In addition, there is a lack of analytical models that deal with the flexural performance of such elements. This paper addresses the latter problem by presenting a methodology for analysis and design of prestressed concrete flexural elements strengthened with externally bonded, fiber reinforced composites. The method provides systematic suggestions on the analysis and design of strengthened prestressed concrete beams with both bonded and unbonded tendons. The method can be used to determine the flexural capacity and to compute stresses and strains in concrete, tendons, and externally bonded fiber reinforcement. Compared to existing experimental data on carbon strengthened beams, the model provides very good prediction of the flexural performance of strengthened prestressed beams. The equations are also applicable to other fiber types including glass, steel, and aramid.