Micromechanics of fresh and 30-year-old Nephila inaurata madagascariensis dragline silk

We report tensile tests for the dragline silk of Nephila madagascariensis golden silk spider strained at 7% min(-1). The average Young's modulus (8.2 GPa), work of fracture (250 MJ m(-3)) and ultimate strain (28%) fall in the same range as for most previously tested spider silks. The average strength is rather low yet (645 MPa in the engineering convention), even in comparison with other Nephila spiders having similar secondary structures (as controlled by polarized Raman microscopy). The scattering of stress/strain failure values is often seen as an obstacle for the unequivocal characterization of silks, but our study evidences a great shape similarity between normalized tensile curves. The so-called strain-hardening region may be divided into a nonlinear regime (nLHR) and a linear one (LHR). The nLHR tends to disappear on averaged spectra, suggesting that it does not correspond to any specific mechanism. Instead, we propose that the plastic deformation to linear hardening transition threshold changes from one point of each fiber to the other, as a result of local heterogeneities. No major aging effect was found comparing the average properties of fresh and 30-year-old fibers, but the lower mechanical variability of the latter reveals a structural consolidation over time, to the detriment of the plastic deformation plateau. The relative extension of this plateau and both hardening regimes, the deviation to linearity in the nLHR and a newly defined epsilon(T) strain may be used to compare the mechanical properties of silk fibers as a function of age, origin or testing conditions.