Various types of instability in a neo-Hookean elastic solid are reviewed: expansion of small internal cavities when they are supersaturated with a dissolved gas or when the block is subjected to a dilatant stress; surface wrinkling caused by compression of a half-space parallel to the surface; and kinks that appear on twisting a stretched rod. Unrestrained growth of a cavity is predicted at a dilatant stress of 5E/6, where E is Young’s modulus [1]. Internal voids appear suddenly in rubber blocks when the dilatant stress approaches this value [2]. Failure often follows. The second instability is predicted to occur in the surface of a half-space at moderate compressions, between 33 and 55 % [3]. When a rubber block is bent, an instability would be expected at 44 % compression of the inner surface. Sharp creases appear suddenly, but at only 35 % compression, when bending is only about one-half of the predicted amount [4]. Such creases might pass undetected in service although they are potential failure sites. When a stretched rubber rod is twisted, kinks form suddenly at a critical amount of torsion, in good agreement with theory [5].
REFERENCES 1. A. E. Green and W. Zerna, “Theoretical Elasticity”, Oxford Univ. Press, 1954. 2. A. N. Gent, Rubb.Chem. Technol. 63, G49-G53 (1990). 3. M. A. Biot, “Mechanics of Incremental Deformations”, Wiley, New York, 1965. 4. A. N. Gent and I.-S. Cho, Rubb.Chem. Technol. 72, 253 (1999). 5. A. N. Gent and K.-C. Hua, J. Non-Linear Mech., 39, 483 (2004).
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