Interfacial failure of a dental cement composite bonded to glass substrates.

OBJECTIVES: To measure the interfacial fracture toughness and investigate failure mechanisms of dental cements bound to soda-lime glasses elastically equivalent to dental ceramics, as loading angle changes from 0 to 20 degrees . METHODS: Two half-circle glass discs received surface treatment were bound using dental cement (3M RelyXTM ARC BLBL) to make Brazil-nut sandwich specimens for interfacial toughness testing. Before bonding the two half-circle glass disks, 8% hydrofluoric acid (HF) was applied on the surfaces to bond for 2 min, washed thoroughly for 1 min under tap water and air dried. The surfaces were further treated using silane primer Monobond-s (Vivadent, Liechtenstein) for 60s and air dried. Interfacial toughness as a function of mode mixity was measured using an Instron testing machine by changing loading angels from 0 to 20 degrees . The interfacial fracture surfaces were examined using SEM and EDX to determine the failure modes when loading angles change. RESULTS: Interfacial toughness increases from approximately 1 to 8 J/m/m when loading angle increases from 0 to approximately 20 degrees . Increasing deformation and fracture in dental cement occur when loading angle increases. SIGNIFICANCES: Increasing interfacial toughness can be attributed to more deformation and fracture of dental cement when loading angle increases. Brazil-nut sandwich samples are shown to provide a promising alternative method to evaluate bond strength and interfacial failure for dental restoration. Research was supported by NIH (NYU/PHS No. F5262-07).     

Modeling of water absorption induced cracks in resin-based composite supported ceramic layer structures

Cracking patterns in the top ceramic layers of the modeled dental multilayers with polymer foundation are observed when they are immersed in water. This article developed a model to understand this cracking mechanism. When water diffuses into the polymer foundation of dental restorations, the foundation will expand; as a result, the stress will build up in the top ceramic layer because of the bending and stretching. A finite element model based on this mechanism is built to predict the stress build-up and the slow crack growth in the top ceramic layers during the water absorption. Our simulations show that the stress build-up by this mechanism is high enough to cause the cracking in the top ceramic layers and the cracking patterns predicted by our model are well consistent with those observed in experiments on glass/epoxy/polymer multilayers. The model is then used to discuss the life prediction of different dental ceramics.     

Substrate creep on the fatigue life of a model dental multilayer structure

In this article, we investigated the effects of substrate creep on the fatigue life of a model dental multilayer structure, in which a top glass layer was bonded to a polycarbonate substrate through a dental adhesive. The top glass layers were ground using 120 or 600 grit sand papers before bonding to create different subsurface crack sizes and morphologies. The multilayer structures were tested under cyclic Hertzian contact loading to study crack growth and obtain fatigue life curves. The experiment results showed that the fatigue lives of the multilayer structures were impaired by increasing crack sizes in the subsurfaces. They were also significantly reduced by the substrate creep when tested at relatively low load levels, i.e. P(m) < 60 N (P(m) is the maximum magnitude of cyclic load). But at relatively high load levels, i.e. P(m) > 65 N, slow crack growth was the major failure mechanism. A modeling study was then carried out to explore the possible failure mechanisms over a range of load levels. It is found that fatigue life at relatively low load levels can be better estimated by considering the substrate creep effect.     

Mechanical properties of functionally graded hierarchical bamboo structures

This paper presents the results of a series of multi-scale experiments and numerical models concerning the mechanical properties of moso culm functionally graded bamboo structures. On the nano- and microscales, nanoindentation techniques are used to study the local variations in the Young’s moduli of moso culm bamboo cross-sections. These are then incorporated into finite element models in which the actual variations in Young’s moduli are used to model the deformation and fracture of bamboo during fracture toughness experiments. Similarly, the measured gradations in moduli are incorporated into crack bridging models that predict the toughening observed during resistance curve tests. The implications of the results are discussed for the bio-inspired design of structures that mimic the layered, functionally graded structure of bamboo.     

Adhesion and cohesion in structures containing suspended microscopic polymeric films

This paper presents a novel technique for the characterization of adhesion and cohesion in suspended micro-scale polymeric films. The technique involves push-out testing with probes that are fabricated using focused ion beam techniques. The underlying stresses associated with different probe tip sizes were computed using a finite element model. The critical force for failure of the film substrate interface is used to evaluate adhesion, while the critical force for penetration of the film determines cohesion. When testing a standard material, polycarbonate, a shear strength of approximately 70 MPa was calculated using the Mohr–Coulomb theory. This value was shown to be in agreement with the results in the literature. The technique was also applied to the measurement of adhesion and cohesion in a model drug-eluting stent (the Nevo? Sirolimus Eluting Coronary Stent) containing suspended microscopic polymeric films in metallic Co–Cr alloy reservoirs. The cohesive strength of the formulation was found to be comparable with that of plastics such as those produced by reaction injection molding and high-density polyethylene.     

Juvenile delinquency in Bendel state, Nigeria

A study of the juvenile delinquency in Bendel State of Nigeria, over a period of 12 years,has revealed an upsurge in its prevalence. More males were convicted than females, and more females under the age of 14 years were convicted than those in the 14- to 18-year-old age group. Most of the convictions resulted from crimes against property. Many juveniles in remand homes came from broken homes. Some ways of solving the problem are discussed.     

Interactions between MC3T3-E1 cells and textured Ti6Al4V surfaces

This paper presents the results of an experimental study of the interactions between MC3T3-E1 (mouse calvarian) cells and textured Ti6Al4V surfaces, including surfaces produced by laser microgrooving; blasting with alumina particles; and polishing. The multiscale interactions between MC3T3-E1 cells and these textured surfaces are studied using a combination of optical scanning transmission electron microscopy and atomic force microscopy. The potential cytotoxic effects of microchemistry on cell-surface interactions also are considered in studies of cell spreading and orientation over 9-day periods. These studies show that cells on microgrooved Ti6Al4V geometries that are 8 or 12 microm deep undergo contact guidance and limited cell spreading. Similar contact guidance is observed on the surfaces of diamond-polished surfaces on which nanoscale grooves are formed due to the scratching that occurs during polishing. In contrast, random cell orientations are observed on alumina-blasted Ti6Al4V surfaces. The possible effects of surface topography are discussed for scar-tissue formation and improved cell-surface integration.