Determination of multiaxial stress rupture criteria for creeping materials:A critical analysis of different approaches

Materials in engineering applications are rarely uniaxially-loaded.In reality,failures under multiaxial loading has been widely observed in engineering structures.The life prediction of a component under multiaxial stresses has long been a challenging issue,particularly for high temperature applications.To distinguish the mode of failure ranging from a maximum principal stress intergranular damage to von Mises effective stress rupture mode a multiaxial stress rupture criterion(MSRC)was originally proposed by Sdobyrev and then Hayhurst and Leckie(SHL MSRC).A multiaxial-factor,α,was developed as a result which was intended to be a material constant and differentiates the bias of the MSRC between maxi-mum principal stress and effective stress.The success of the SHL MSRC relies on accurately calibrating the value ofαto quantify the multiaxial response of the material/geometry combination.To find a more suitable approach for determining MSRC,the applicability of different methods are evaluated.Given that the resulting analysis of the various approaches can be affected by the creep failure mechanism,princi-ples in the determination of MSRC with and without using continuum damage mechanics approaches are recommended.The viability of uniaxial material parameters in correlating withαthrough the analysis of available data in literature is also presented.It is found that the increase of the uniaxial creep dam-age tolerance parameterλis accompanied bythe decreaseof theα-value,whichimplies thatthe creep ductility plays an important role in affecting the multiaxial rupture behavior of materials.

Highly stereoselective construction of polycyclic benzofused tropane scaffolds and their latent bioactivities: bifunctional phosphonium salt-enabled cyclodearomatization process

Structurally fused heterocycles encompassing a centerpiece of benzotropane are significant scaffolds with a plethora of promising biological activities,but such molecular architectures pose a long-standing daunting synthetic challenge.Herein,we reported a highly efficient asymmetric cyclodearomatization of 2-nitrobenzofurans with cyclic azomethine ylides by employing bifunctional phosphonium salts as phase-transfer catalysts.Under optimized reaction conditions,a diverse array of polycyclic benzofused tropane derivatives with four contiguous 4°/3°stereocenters were readily synthesized in both high yields and diastereoselectivities with up to>99%ee.The practicality and utility of this protocol were further demonstrated by the scaled-up reaction and facile elaborations.Moreover,preliminary investigations into their antitumor activities were also presented.

In situ phosphonium-containing Lewis basecatalyzed 1,6-cyanation reaction:a facile way to obtainα-diaryl andα-triaryl acetonitriles

We present a phosphonium-containing catalyst generated in situ from phosphine and tert-butyl acrylate that serves as an unusual Lewis base catalyst.It was applied for the promotion of a remote 1,6-cyanation reaction of p-quinone methides and fuchsones employing trimethylsilyl cyanide as the cyanide source.A diverse range ofα-diaryl andα-triaryl acetonitriles was obtained in high yields under mild reaction conditions with low catalyst loading(5 mol%).The practicality and utility of this protocol were demonstrated via the gram-scale preparation and facile elaboration of products.Mechanistic investigations(in situ NMR and ESI-MS analysis)were employed to characterize the active zwitterionic phosphonium intermediate,which was the“true”active catalyst.