Three-dimensional tomography of composite fracture surfaces.

The goal of this project was to image the three-dimensional fracture interface of a dental composite with the use of X-ray tomography. With the use of the Advanced Photon Source (APS) at Argonne National Laboratory, three-dimensional images were obtained of the crack interface of a dental composite material that had been subjected to three different treatments: a control, cycled in air, and cycled in a 50/50 mixture by volume of ethanol and distilled water. The cycle-loaded treatments were for 100,000 cycles at a load between 80 and 100 N at 5 Hz. The crack interface extended over 28 slices for the control, 96 for the air-cycled specimen, and 83 slices for the 50/50 solution cycle specimen. It would appear that the fatiguing of the specimens allowed for an increase in the crack interface as demonstrated by the 3D tomographical analysis. This volume increase in the crack interface is attributed to a separation of the filler fiber from the resin matrix. Three-dimensional tomography provides an excellent method to observe crack interfaces of dental composites subjected to different types of mechanical and environmental conditions.

Fracture surface examination of dental ceramics using fractal analysis.

OBJECTIVE: The purpose of this study was to measure the fractal dimensional increment of the fracture surface of six dental ceramics, and to measure the correlation of the fractal dimensional increment with fracture toughness. The six dental ceramics investigated were: a traditional porcelain (Finesse[F], Ceramco, Burlington, NJ, USA), four leucite reinforced pressable ceramics (Finesse Pressable [FP], Ceramco; Empress [E], Ivoclar; New Albany, NY, USA and two shades of OPC [OA2 and OI40], Jeneric-Pentron, MA, USA), and a lithium disilicate containing pressable ceramic ([LD] Ceramco). METHODS: The fracture surfaces examined and the measured fracture toughness (K(IC)) values are from a previous study . An impression was taken of the fracture surface, poured up in epoxy, coated with gold, a second layer of epoxy added, and then the surface polished at an angle to the fracture surface. This fracture interface (slit island) was observed in an optical microscope with an imaging system from which an image of the fracture line was digitized. The slit island perimeter was then measured using five different length rulers. The fractal dimensional increment (D*) was determined using a Richardson technique. A second experiment on three ceramics investigated whether there was a correlation between fracture toughness and the values of D* measured by an alternative technique using a confocal microscope. RESULTS AND SIGNIFICANCE: Fracture toughness was well correlated with D* using either technique.

Characteristics of period doubling in the human cone flicker electroretinogram.

Electroretinogram (ERG) responses of the cone system to a flickering stimulus can exhibit a cyclic variation in amplitude. This phenomenon of synchronous period doubling has been attributed to a nonlinear feedback mechanism within the retina that alters response gain. The aim of the present study was to investigate intersubject variability in period doubling in the ERG of the human cone system, and to assess the implications of this variability for signal processing within the retina. Period doubling was examined in a group of 12 visually normal subjects, using sinusoidal full-field flicker and harmonic analysis of the ERG waveforms. For all subjects, the ERG responses to 32-Hz flicker (a frequency commonly used clinically) were characterized by a harmonic component at the stimulus frequency and at higher harmonics that were integral multiples of the stimulus frequency, as expected. In addition, six of the subjects showed period doubling at 32 Hz, characterized by harmonic components at integer multiples of a frequency that was half the stimulus frequency (the subharmonic). However, the subharmonic itself did not exceed the noise level. These findings suggest that the subharmonic is generated prior to or at the site that produces the nonlinear higher harmonics of the ERG response, and that a subsequent band-pass filter attenuates this subharmonic. Examination of harmonic components of the subjects' ERG waveforms at other stimulus frequencies, as well as a cycle-by-cycle analysis of the ERG waveforms, suggested that individual differences in period doubling may be due to intersubject variation in the strength of the hypothesized feedback signal and/or the time constant of its decay.

Perceptual organization of two-dimensional patterns.

The perceptual organization of image patterns is considered from 2 standpoints. First, a theoretical framework is presented from which computational models of perceptual organization can be constructed and tested. Second, a specific computational model for perceptual organization of line images is described. In this model, input images are first processed by a dense array of neurons that have properties consistent with recent analyses of single-neuron responses in primary visual cortex. Then, complex image structure is discovered by interleaved pattern-matching and grouping processes constrained by a generalized uniqueness principle. A series of 3-pattern grouping experiments was performed to test a restricted version of the model and to estimate critical parameters. Using the estimated parameters, an extended version of the model was tested by generating predictions for a series of "textbook" perceptual organization demonstrations.