Irregularities are inherent to virtually all fibers, including the conventional textile fibers, the high-performance brittle fibers and newly developed nano-fibers. These irregularities can fall into two main categories: dimensional or geometrical irregularity (external) and structural irregularity (internal). For natural fibers such as wool, diameter variation along fiber length is a typical example of fiber dimensional or geometrical irregularity, while the presence of flaws and defects within a fiber signifies structural irregularity. The irregularities of fibers are bound to have major impact on the MECHANICAL behavior of fibers. In recent years, there has been a growing awareness of the importance of this particular fiber attribute – fiber irregularity, particularly the within-fiber diameter variation. Instruments have been developed to accurately measure fiber diameter variations. In addition, with the development of computing technology, numerical modeling technique has been applied to examine the impact of fiber irregularity on fiber MECHANICAL PROPERTIES. This paper reviews research progress in the area of fiber geometrical irregularity and its effect on important fiber MECHANICAL PROPERTIES.

Structural fire safety capacity of concrete is very complicated because concrete materials have considerable variations. In this paper, constitutive models and relationships for concrete subjected to fire are developed, which are intended to provide efficient modeling and to specific fire-performance criteria of the behavior of concrete structures exposed to fire. They are developed for unconfined concrete specimens that include residual compressive and tensile strengths, compressive elastic modulus, compressive and tensile stress-strain relationships at elevated temperatures. In this paper, the proposed relationships at elevated temperatures are compared with experimental result tests and pervious existing models. It affords to find several advantages and drawbacks of present stress-strain relationships and using these results to establish more accurate and general compressive and tensile stress-strain relationships. Additional experimental test results are needed in tension and the other main parameters at elevated temperatures to establish well-founded models and to improve the proposed relationships. The developed models and relationships are general, rational, and have good agreement with experimental data.