Fracture criterion for kinking cracks in a tri-material adhesively bonded joint under mixed mode loading

The fracture behavior of a composite/adhesive/steel bonded joint was investigated by using double cantilever beam specimens. A starter crack is embedded at the steel/adhesive interface by inserting Teflon tape. The composite adherend is a random carbon fiber reinforced vinyl ester resin composite while the other adherend is cold rolled steel. The adhesive is a one-part epoxy that is heat cured. The Fernlund-Spelt mixed mode loading fixture was employed to generate five different mode mixities. Due to the dissimilar adherends, crack turning into the adhesive (or crack kinking) associated with joint failure, was observed. The bulk fracture toughness of the adhesive was measured separately by using standard compact tension specimens. The strain energy release rates for kinking cracks at the critical loads were calculated by a commercial finite element analysis software ABAQUS in conjunction with the virtual crack closure technique. Two fracture criteria related to strain energy release rates were examined. These are (1) maximum energy release rate criterion (G_max) and, (2) mode I facture criterion (G_II = 0). They are shown to be equivalent in this study. That is, crack kinking takes place at the angle close to maximum G or G_I (also minimum G_II, with a value that is approximately zero). The average value of G_IC obtained from bulk adhesive tests using compact tension specimens is shown to be an accurate indicator of the mode I fracture toughness of the kinking cracks within the adhesive layer. It is concluded that the crack in tri-material adhesively bonded joint tends to initiate into the adhesive along a path that promotes failure in pure mode I, locally.     

Cover Picture: Bistable Nematic Liquid Crystals with Self‐Assembled Fibers (Adv. Mater. 6/2005)

The cover shows a recording process using nematic liquid crystal alignment. On p. 692, Kato and co‐workers report that reversibly bistable states have been achieved for nematic liquid crystals incorporating a small amount of self‐assembled fibers. Homeotropic monodomains and non‐aligned multidomains can be fixed and switched by thermal treatment in electric fields. The technique is applied to the formation of rewritable light scattering patterns, as shown on the cover.     

Inside Front Cover: Self‐Assembly of SiO2 Nanowires and Si Microwires into Hierarchical Heterostructures on a Large Scale (Adv. Mater. 8/2005)

The inside cover shows a scanning electron micrograph of a novel hierarchical heterostructures, as reported by Hu and co‐workers on p. 971. The heterostructures are formed from Si core microwires covered by dense, aligned SiO₂ nanowires, thus forming multiple junctions to the cores. The inset shows a schematic of the Sn‐catalyzed vapor–liquid–solid growth mechanism. These materials are envisaged to become important for optical fibers, low‐dimensional waveguides, scanning near‐field optical microscopes and high‐bandwidth optical signal processing devices.     

Cover Picture: Functionalized Gold Nanoparticles Mimic Catalytic Activity of a Polysiloxane‐Synthesizing Enzyme (Adv. Mater. 10/2005)

A system that acts as a biomimetic of the silica‐synthesizing enzyme found in a marine sponge is reported by Morse and co‐workers on p. 1234. Gold nanoparticles (GNPs) are functionalized with the same organic moieties that are found in the enzyme's catalytic site. Interaction between the nucleophilic (OH‐terminated) and hydrogen‐bonding (imidazole‐terminated) GNPs, as shown on the cover, is required for the hydrolysis of a silicon alkoxide precursor and subsequent polycondensation to form silica at a low temperature and near‐neutral pH. Replacement of either of the required functional groups by a non‐reactive methyl group abolishes catalysis in this synthetic system, as it does in the biological enzyme. Cover art provided by Peter Allen.     

Inside Front Cover: Self‐Assembled Highly Faceted Wurtzite‐Type ZnS Single‐Crystalline Nanotubes with Hexagonal Cross‐Sections (Adv. Mater. 16/2005)

Highly faceted wurtzite‐type ZnS nanotubes with hexagonal cross‐section morphologies have been self‐assembled via a controllable high‐temperature thermal‐chemical reaction route in work reported by Yin and co‐workers on p. 1972. The self‐assembly growth along c‐axis to highly hexagonal‐faceted ZnS single‐crystalline nanotubes is associated with the crystallographic characteristics, such as the non‐central and polar surfaces of the wurtzite‐type ZnS structure. The faceted ZnS nanotubes grow along the <0001> direction, and are closed by low‐index faces of non‐polar {1000} faces. The inside cover and insets show electron microscopy and scanning electron microscopy images of the faceted ZnS nanotubes.     

Cover Picture: Closing the Gap Between Self‐Assembly and Microsystems Using Self‐Assembly,Transfer, and Integration of Particles (Adv. Mater. 20/2005)

The cover shows 100 μm diameter glass spheres covered by a grid of hexagonally packed polystyrene beads. Wolf and co‐workers placed the 500 nm diameter polystyrene beads onto the larger spheres using the self‐assembly, transfer, and integration (SATI) process that they report on p. 2438. The cover illustrates the capability of SATI to process uneven surfaces in addition to the planar substrates discussed in the article. The carrier that holds the smaller beads deforms during their transfer onto the larger spheres, so that on the larger spheres patterned “caps” are formed. Using this process, which is compatible with standard microfabrication techniques, a variety of particle assemblies can be achieved.