Tensile strengths,sigma_b,of gum and N115-filled natural rubber testpieces,with and without edge pre-cuts,have been determined.At low crosslink density,the regular (uncut) sigma_b of filled and gum vulcanizates is similar.However,at high crosslink density,the gum NR becomes brittle,while the corresponding filled rubber remains strong and resistant to cut growth.It is proposed that the tightly linked gum does not strain-crystallize appreciably during stretching,but that its filled ounterpart does.Carbon black appears capable of inducing crystallization in a newtork that alone remains amorphous during extension.Filled vulcanizates of various crosslink densities have similar normal tensile strengths (approx = 30 MPa),but strengths differ,sometimes more than two-fold,if a pre-cut is present.Lightly crosslinked specimens containing a small cut have strengths that depend vey weakly on cut size,c.Furthermore,these develop long longitudinal cracks from which catastrophic rupture initiates.With larger cuts,strength decreases more rapidly with increasing c,there is lesslongitudinal crack growth,and rupture initiates near the original cut tip.In contrast,the strength of a highly crosslinked vulcanizate is sensitive to small cuts and testpieces exhibit minimal longitudinal cracking before failure.
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