The general nature of fracture in polymers, when subject to alternating loads as distinct from static or steadily increasing loads, is reviewed; and the molecular mechanisms and micromechanics aspects of the fatigue fracture process are discussed. Some attention is given to thermal fatigue, where fracture results primarily from a large specimen temperature rise due to hysteresis heating. However, primary emphasis is devoted to mechanical fatigue, in which fracture is a result of initiation and propagation of a crack, as a result of the periodic nature of the applied load.Attention is given to the important internal, or material, variables such as polymer structure, molecular weight, crosslinking, and filler or diluent type and content; and to significant external variables such as stress or stress intensity factor amplitude, mean stress, temperature, frequency and environment. Various methods that can be utilized to provide significant degrees of enhancement in the fatigue resistance of polymers are outlined and discussed.
Polypropylene aggregates, consisting of interwoven arrays of single-crystal lamellae, have been obtained by crystallization of samples of isotactic polypropylene from dilute solutions held at constant temperatures between 85° and 115°C. The solvent used was α-chloronaphthalene with polymer concentrations ranging from 0.4% to 0.01% by weight. The resulting crystalline aggregates were examined by means of phase contrast and electron microscopy. Similar crystal aggregates have been studied by Khoury and found to be incipient spherulites. These aggregates tend to occur in the form of open weave structures, with characteristic cusps. Lath-like lamellar crystals, with well-defined faces, are found at the edges of these structures. These edge lamellae produce a single-crystal electron-diffraction pattern corresponding to the monoclinic structural form of isotactic polypropylene. On the basis of the experimental findings, it is suggested that folding occurs in monoclinic polypropylene only along a given set of parallel fold planes, viz. {010}. Preliminary results of a study of the fracture characteristics of the crystal lamellae tend to support the suggested hypothesis.
The possible reinforcing effects of six different types of filler particles on composites based on the thermoplastic polypropylene have been examined. It is found that significant increases in elastic modulus and tensile strength can he obtained by addition of 210 percent by volume of glass fibers. Ceramic whiskers, based on alumina and silicon carbide, also lead to increases in modulus but to decreases in strength and ductility. Additional measurements were made with composites prepared from two sizes of spherical glass heads and from carbon spheres. For the glass beads, an increase in modulus was obtained but strength and elongation to fracture decreased. Carbon spheres were ineffective as a reinforcing agent. The possible effects of nonuniform mixing, of size and shape of filler particles, and of surface coatings are discussed.
Odors had been a chronic operational problem at the City and County of Honolulu's Sand Island WWTP since it began service in 1979. A field survey identified the release of hydrogen sulfide as the major odor source and concluded that off-gas collection and treatment afforded the best methods for abatement. A pilot program was initiated that tested caustic scrubbing and a chelated iron catalytic absorbent. The latter was selected based on performance, economics, and chemical-handling requirements.The full-scale system was designed to treat 10.9 m 3 /s (23 000 scfm) of air and achieve a 95% reduction in hydrogen sulfide levels, which were expected to range from an average of 75 ppm (V/V) to a peak of 150 ppm. The design used two fiberglass packed towers operating in series followed by activated carbon adsorbers. The system has been in service since June 1986 and has shown itself to be effective in controlling odors.zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Water Environ. Res., 64, 120 (1992).
The effects of thermal treatment at temperatures in the 177–250°C. range for 2 hr. on solution‐grown single crystals of poly‐4‐methylpentene‐1 have been studied by using electron microscopy. Crystals were grown both from 0.02% pentyl acetate solution at 110°C. and from 0.02% toluene solutions at temperatures less than 80°C. A number of distinct types of crystals have been obtained in the same or in separate solutions depending on the crystallization conditions. These crystals have some morphological characteristics in common such as a square outline with well‐defined faces. The contrasting features include such things as a marked difference in relative size, with the smaller crystals showing extensive overgrowth and collapse markings. Observations by means of differential scanning calorimetry (DSC) on the melting behavior of these various crystals show the presence of distinct and characteristic melting points. The effects of a given thermal treatment depend on the type of crystals used, with the larger crystals showing greater stability. The first effects are the appearance of lines, notches at the edges, and holes. An increase in temperature results in an increase in these effects with the formation of fibrillar structures. Consideration is given to the influence of molecular conformation and molecular chain folding on the observed results.
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