Do Natural Silks Make Good Engineering Materials?
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Do Natural Silks Make Good Engineering Materials? Natalie A. Morrison2, Fraser I. Bell2, Alexandre Beautrait2, Joanne Ritchie2, Christopher Smith2, Iain J. McEwen2, and Christopher Viney1, 2 1
School of Engineering, University of California at Merced, P.O. Box 2039, Merced, CA 95344, USA 2 Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, Midlothian, Scotland ABSTRACT Fast relaxation of stresses lower than the yield stress is demonstrated in Bombyx mori (silkworm) cocoon silk and Nephila clavipes (spider) major ampullate silk (MAS; dragline). Stress relaxation and creep make natural silk unsuitable as a long-term load-bearing material. Instead, silk-like materials are better suited to applications in which energy dissipation is important, and in which high loads need to be withstood on a once-off basis for only very short periods of time. Examples might include use as a ballistic material that arrests the penetration of fragments from the explosion of a pressure vessel, an aircraft luggage container, or a tyre. Treatment in a domestic microwave oven is shown to significantly reduce the rate of stress relaxation in both silkworm cocoon and spider MAS. Except for ductility, the tensile properties of cocoon silk measured in constant strain rate experiments are enhanced by this treatment. Initial experiments on MAS suggest that the tensile properties of this material also are enhanced by exposure to microwaves, in this case with the exception of initial modulus. INTRODUCTION Much of the widespread interest in natural silk as a blueprint for new engineering materials has its origin in the high strength, stiffness and toughness exhibited by silk fibres in constant strain rate tests [1-3]. These tests typically are completed within minutes; they do not duplicate realistic in-service load histories, and they do not adequately probe the long-term behaviour of the sampled material. Mechanical testing regimes that are specifically designed to explore creep of, or stress relaxation in, silk reveal a rather less promising outlook: both effects are significant, and must restrict the possible applications of silk [4-6]. A limiting creep stress (approximately one fifth of the conventionally measured fracture stress) can be identified: if samples are loaded smoothly and quickly to a constant stress lying above the limiting creep stress, they break within a few seconds of the stress being applied [5]. Silk also suffers from significant moisture sensitivity. The load-bearing properties of silk are decreased [7], and stress relaxation and creep are greatly increased [4, 6], by exposure to a moist environment. Creep of moist MAS is easily measured at stresses as low as 5% of the conventional yield strength [6]. In the present study, we demonstrate directly the occurrence of significant and rapid stress relaxation in samples of B. mori cocoon silk and N. clavipes MAS. We also demonstrate that the rate of stress relaxation is reduced markedly by exposure to kitchen microwave radi
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