Strength of whiskers

The enhanced strength of whiskers and natural fibers, by comparison to the strength of materials of the same composition in another morphology, could be a coincidental in these crystalline synthetic and mineral fibers, a particular crystal direction is parallel to the direction of the applied stress. However, the inverse diameter-strength relationship indicates that factors other than crystal structure contribute to the mechanical strength of fibrous materials. 

Brenner, S. (1956). Tensile strength of whiskers. J. Appl. Phys. 27 1481-1491. 

AK[c (VK R.) = increase in fracture toughness due to whisker reinforcement o/ = fracture strength of whiskers Vf = volume fraction of whiskers... 

Xiuyin Zhang, Wensheng Wu, Aifeng Tian. The effect of whisker fraction on flexural strength of whisker-PMMA composite reinforced with potassium titanate whiskers./owma/ of Clinical Stomatology, 25(3) 151-154, 2009. 

Reinforcements. The high modulus, high intrinsic strength, and temperature stabiHty make SiC, in the form of whiskers, platelets, and fibers, a promising candidate reinforcement material for metal, polymer, and ceramic matrix composites (qv). 

Fig. I.IB illustrates fibers typical of commercial asbestos, while Fig. l.ll shows Fiberglas and Fig. I.IJ silicon carbide whiskers. Some of the fibers in these examples are bent, occasionally through 180°, indicating considerable flexibility. Whiskers of other compounds can also bend but the tensile strength of these materials is their most remarkable feature. The measured values (Table 1.2) are at least ten times higher than those observed for the same compounds in bulk or in another morphology (Walker and Zoltai, 1979). The numerous investigations into the causes of this unique response have produced several hypotheses.

More than 250 years ago it was observed that the strength of small-diameter wires increased proportionally as the diameter decreased (Musschen-broek, 1727). The inverse relationship between elastic strain and diameter has also been noted for whiskers of brittle or ductile materials of less than 20 micrometers in diameter (Evans, 1972). 

Webb, W. W., H. D. Bartha, and T. B. Shaffer (1966). Strength eharacteristics of whisker crystals, macrocrystals and microcrystals, pp. 329-354. In Strengthening Mechanisms, Metals and Ceramics. Burke, J. J., N. L. Reed, and V. Weiss, Eds. Syracuse University Press, New York. 

The theoretical mechanical strength of perfect carbon fibers is between 14 and 20 X 10 psi, while the Young s modulus for graphite whiskers was calculated as 145 X 10 . Diamond (the most dense crystalline C modification at 3.5 g/cc, compared with graphite at 2 to 2.22 g/cc) has a Young s... 

Tensile Strength of Inorganic and Metallic Fibers and Whiskers 473... 

The tensile strength of materials is dependent on the treatment and form of the material. Thus, the tensile strength of isotropic bulk nylon-6,6 is less than that of anisotropic oriented nylon-6,6 fiber. Inorganics and metals also form fibers and whiskers with varying tensile strengths (Table 14.1). Fibers are generally less crystalline and larger than whiskers. 

Non-whisker SiC fibers are also of imporfance in MMCs, and they are currently available in two commercial forms Tyranno and Nicalon . As with the whisker form, the primary advantages of SiC fibers is their oxidation resistance and high-temperature mechanical property retention relative to other fibers. The high-temperature strength of three commercially available Nicalon SiC fibers is shown in Figure 5.111. 

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