The averaged value of the strain energy density over a well-defined volume is used to predict the static strength of U-notched specimens under mixedmode conditions due to combined bending and shear loads. The volume is centered in relation to the maximum principal stress present on the notch edge, by rigidly rotating the crescent-shaped volume already used in the literature to analyse U-and V-shaped notches subject to mode I loading. The volume size depends on the ultimate tensile strength σ u and the fracture toughness K IC of the material. In parallel, an experimental programme was performed. All specimens are made of polymethyl-metacrylate (PMMA), a material which exhibits quasi-brittle behaviour at −60 • C. Good agreement is found between experimental data for the critical loads to failure and theoretical predictions based on the constancy of the mean strain energy density over the control volume.

Major ampullate (MA) dragline silk supports spider orb webs, combining strength and extensibility in the toughest biomaterial. MA silk evolved ~376 MYA and identifying how evolutionary changes in proteins influenced silk mechanics is crucial for biomimetics, but is hindered by high spinning plasticity. We use supercontraction to remove that variation and characterize MA silk across the spider phylogeny. We show that mechanical performance is conserved within, but divergent among, major lineages, evolving in correlation with discrete changes in proteins. Early MA silk tensile strength improved rapidly with the origin of GGX amino acid motifs and increased repetitiveness. Tensile strength then maximized in basal entelegyne spiders, ~230 MYA. Toughness subsequently improved through increased extensibility within orb spiders, coupled with the origin of a novel protein (MaSp2). Key changes in MA silk proteins therefore correlate with the sequential evolution high performance orb spider silk and could aid design of biomimetic fibers.

A B S T R A C T A large bulk of static test results carried out on notched specimens are presented in a unified way by using the mean value of the strain energy density (SED) over a given finite-size volume surrounding the highly stressed regions. In plane problems, when cracks or pointed V-notches are considered, the volume becomes a circle or a circular sector, respectively, with R C being the radius. R C depends on the fracture toughness of the material, the ultimate tensile strength and the Poisson's ratio. When the notch is blunt, the control area assumes a crescent shape and R C is its width as measured along the notch bisector. About 900 experimental data, taken from recent literature, are involved in the local SED-based synthesis. They have been obtained from (a) U-and V-notched specimens made of different materials tested under mode I loading; (b) U-and V-notched specimens made of polymethyl-metacrylate (PMMA) and an acrylic resin, respectively, tested in mixed, I + II, mode; (c) U-notched specimens made of ceramics materials tested under mode I.The local SED values are normalized to the critical SED values (as determined from unnotched specimens) and plotted as a function of the R/R C ratio. A scatter band is obtained whose mean value does not depend on R/R C , whereas the ratio between the upper and the lower limits are found to be about equal to 1.6. The strong variability of the non-dimensional radius R/R C (ranging here from about zero to around 1000) makes stringent the check of the approach based on the mean value of the local SED on a material-dependent control volume.Keywords brittle fracture; critical radius; finite-size volume; notch; strain energy density. N O M E N C L A T U R Ea = notch depth E = Young's elastic modulus E 1 = strain energy on the control volume f ij , g ij = angular functions F(2α) = parameter to evaluate the strain energy density (blunt notches, R = 0) J = Rice's J integral l ch = characteristic length of the material K I , K U R,I , K V R,I = stress-intensity factor and notch stress-intensity factors for U-and V-shaped notches Correspondence: P. Lazzarin.

Spider silk is considered as the basis of a new family of high performance fibers that would reproduce the excellent mechanical properties of the silk, in particular its extreme toughness. However, it has been observed that the mechanical properties of spider silk are severely influenced by humid environments that give rise to significant decreases in its length and elastic modulus. The change from stiff to compliant tensile properties is associated with the glass transition from a glassy to a rubbery state; here, we have found that it depends on both temperature and relative humidity. The glass transition was identified at different temperatures and relative humidities by monitoring the variation of the elastic modulus and observing the emergence of supercontraction. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 994–999, 2006

Two fracture criteria are proposed and applied to blunt-notched components made of brittle materials loaded under mixed mode; the former is based on the averaged strain energy density over a given control volume, the latter on the cohesive crack zone model. In both instances use of the equivalent local mode I hypothesis is made. Only two material properties are needed: the ultimate tensile strength and the fracture toughness. Numerical predictions of rupture loads from the two criteria are compared with experimental measurements from more than 160 static tests with notched beams. The samples are made of PMMA and tested at −60 • C to assure a bulk behaviour almost linear elastic up to rupture. Notch root radii range from 0.2 to 4.0 mm and load mixicity varies from pure mode I to a prevailing mode II. The good agreement between theory and experimental results adds further confidence to the proposed fracture criteria.

The relationship between microstructure and mechanical properties has been investigated in Argiope trifasciata dragline silk fibers (major ampullate silk, MAS) by X-ray diffraction, Raman spectroscopy and tensile testing. We have analyzed three fractions of the material, i.e. amorphous, highly oriented nanocrystals and weakly oriented material, for different values of the macroscopic alignment parameter a, calculated as the relative difference between the length of the fiber and its length when supercontracted. Two distinct regimes have been identified: for low values of the alignment parameter a, microstructural changes are dominated by the reorientation of the nanocrystals; however, at high values (a > 0.5) of the alignment parameter, an increase in the fraction of the crystalline phase is revealed. The two regimes are also reflected in the mechanical behaviour, which can be explained by microstructural changes. This finding of the two distinct regimes in the microstructural evolution, which separates the reorientation and the increase in the crystalline phase, will be valuable to develop and validate molecular models of natural and artificial silk fibers, as well as to deepen our present knowledge of the origin of the outstanding properties of MAS fibers. In addition, we have analyzed the characteristics of the crystal lattice, and discussed the relationship between the percentage of short sidechain residues and the unit cell dimensions in different silks.

This paper provides a simple, albeit accurate, criterion for prediction of the rupture loads of brittle, or quasi-brittle, U-notched samples, where linear elastic fracture mechanics is not applicable because blunted notches do not exhibit stress singularities. Good agreement is found between numerical predictions and experimental results. The results of fracture tests from 18 different ceramic materials and a polymer (at −60 • C) are summarized and are used as a reference for checking the fracture criterion. Seven fracture criteria are reviewed and it is shown that all can be recast into the proposed criterion.

The mechanical behavior and microstructure of bioinspired fibers spun from solutions of recombinant spidroin-like proteins were extensively characterized, and compared with those of natural spider silk fibers. It is confirmed that high performance bioinspired fibers indistinguishable from natural spider silk up to large strains can be produced through genetic engineering and conventional spinning technologies. It is also found that fibers spun from spidroin-like proteins that contain different motifs of sequence exhibit variations in their microstructure in terms of crystallinity and chain alignment, but these differences are not reflected in distinct tensile properties. This similarity in terms of their mechanical behavior indicates that bioinspired fibers are largely independent of their exact sequence of recombinant proteins and, in particular, of their proline content. Finally, it is shown that the largest differences between natural and bioinspired fibers are found at very large deformations, marking the ultimate challenge in the synthesis of silk-like fibers.