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Effect of microstructure on mixed‐mode (mode I + II), high‐cycle fatigue thresholds in a Ti‐6Al‐4V alloy is reported over a range of crack sizes from tens of micrometers to in excess of several millimeters. Specifically, two microstructural conditions were examined—a fine‐grained equiaxed bimodal structure (grain size ∼20 µm) and a coarser lamellar structure (colony size ∼500 µm). Studies were conducted over a range of mode‐mixities, from pure mode I (Δ K II /Δ K I = 0) to nearly pure mode II (Δ K II /Δ K I ∼ 7.1), at load ratios (minimum load/maximum load) between 0.1 and 0.8, with thresholds characterized in terms of the strain‐energy release rate (Δ G ) incorporating both tensile and shear‐loading components. In the presence of through‐thickness cracks—large (> 4 mm) compared to microstructural dimensions—significant effects of mode‐mixity and load ratio were observed for both microstructures, with the lamellar alloy generally displaying the better resistance. However, these effects were substantially reduced if allowance was made for crack‐tip shielding. Additionally, when thresholds were measured in the presence of cracks comparable to microstructural dimensions, specifically short (∼200 µm) through‐thickness cracks and microstructurally small (< 50 µm) surface cracks, where the influence of crack‐tip shielding would be minimal, such effects were similarly markedly reduced. Moreover, small‐crack Δ G TH thresholds were some 50–90 times smaller than corresponding large crack values. Such effects are discussed in terms of the dominant role of mode I behaviour and the effects of microstructure (in relation to crack size) in promoting crack‐tip shielding that arises from significant changes in the crack path in the two structures.
The epitaxial liftoff approach has been attracting increasing interest as an alternative to lattice-mismatched heteroepitaxy. A thin-film GaAs double heterostructure injection diode laser fabricated on a glass substrate by the epitaxial liftoff technique is reported. This presages the integration of the two major optical communication materials, III-V semiconductor crystals with SiO/sub 2/ glass.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">></ETX>
The accuracy of the nonlinear static procedure (NSP) using the lateral force distributions specified in the FEMA-356 document (ASCE 2000), now standard in engineering practice, and the recently developed dynamics-based modal pushover analysis (MPA) procedure (Chopra and Goel 2002) was evaluated in the companion paper by Goel and Chopra (2003). The median seismic demands were computed by these procedures for six SAC buildings, each analyzed for 20 ground motions, and compared with ‘‘exact’’ results obtained from nonlinear response history analysis (RHA).
The scandium alkyl Cp*(2)ScCH(2)CMe(3) (2) was synthesized by the addition of a pentane solution of LiCH(2)CMe(3) to Cp*(2)ScCl at low temperature. Compound 2 reacts with the C-H bonds of hydrocarbons including methane, benzene, and cyclopropane to yield the corresponding hydrocarbyl complex and CMe(4). Kinetic studies revealed that the metalation of methane proceeds exclusively via a second-order pathway described by the rate law: rate = k[2][CH(4)] (k = 4.1(3) x 10(-4) M(-1)s(-1) at 26 degrees C). The primary inter- and intramolecular kinetic isotope effects (k(H)/k(D) = 10.2 (CH(4) vs CD(4)) and k(H)/k(D) = 5.2(1) (CH(2)D(2)), respectively) are consistent with a linear transfer of hydrogen from methane to the neopentyl ligand in the transition state. Activation parameters indicate that the transformation involves a highly ordered transition state (DeltaS++ = -36(1) eu) and a modest enthalpic barrier (DeltaH++ = 11.4(1) kcal/mol). High selectivity toward methane activation suggested the participation of this chemistry in a catalytic hydromethylation, which was observed in the slow, Cp*(2)ScMe-catalyzed addition of methane across the double bond of propene to form isobutane.
Achiral polysiloxanes with para-alkoxybenzoic acid side chains can bind many molecules of an optically active trans-stilbazole with hydrogen bonds. The resulting, self-assembling system 1 has ferroelectric properties which can be regulated by the number of stilbazole units per polymer molecule and the length of the (CH2)n spacer.
ADVERTISEMENT RETURN TO ISSUEPREVCommunication to the...Communication to the EditorNEXTThe Largest Discrete Oligo(m-aniline). An Exponential Growth Strategy Using Palladium-Catalyzed Amination of Aryl SulfonatesJanis Louie and John F. HartwigView Author Information Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut 06520-8107 Cite this: Macromolecules 1998, 31, 19, 6737–6739Publication Date (Web):September 3, 1998Publication History Received8 July 1998Published online3 September 1998Published inissue 1 September 1998https://pubs.acs.org/doi/10.1021/ma981073uhttps://doi.org/10.1021/ma981073urapid-communicationACS PublicationsCopyright © 1998 American Chemical SocietyRequest reuse permissionsArticle Views608Altmetric-Citations52LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-AlertscloseSupporting Info (2)»Supporting Information Supporting Information SUBJECTS:Aromatic compounds,Hydrocarbons,Monomers,Oligomers,Oxidation Get e-Alerts
