John I. McCool

Silane treatment effects on glass/resin interfacial shear strengths

OBJECTIVE Methacrylic resin-based dental composites normally use a bifunctional silane coupling agent with an intermediary carbon connecting segment to provide the interfacial phase that holds together the organic polymer matrix with the reinforcing inorganic phase. In this study, fiber pull-out tests were used to measure the interfacial bond strength at the fiber-matrix interface. METHODS Glass fibers (approximately 30 microm diameter, 8 x 10 (-2)m length, MoSci) were silanated using various concentrations (1, 5 and 10%) of either 3-methacryloxypropyl-trimethoxysilane (MPS) or glycidoxypropyltrimethoxy-silane (GPS) in acetone (99.8%). Rubber (poly(butadiene/acrylonitrile), amine terminated, M(w) 5500) molecules were also attached to the fiber surface via GPS molecules. The resin was comprised of a 60/40 mixture of Bis-phenol-A bis-(2-hydroxypropyl)-methacrylate (BisGMA) and tri (ethylene glycol) dimethacrylate (TEGDMA). A bead of resin approximately 2-4 x 10(-3)m in embedded length was placed on the treated fibers and light cured. The load required to pull the fiber out of the resin was converted to shear bond strength. RESULTS Interfacial shear strengths were greater for silanated specimens compared with unsilanated, and for MPS compared with GPS. The same set of samples soaked in 50:50 (v/v) mixtures of ethanol and distilled water for a period of 1 month showed a decrease in properties. SIGNIFICANCE A positive correlation was found between the amount of silane on the filler surface and the property loss after soaking. Rubber treatment provided improvement in interfacial strength. 5% MPS samples had the highest strength both in soaked as well as unsoaked samples.

Fatigue of Restorative Materials

Failure due to fatigue manifests itself in dental prostheses and restorations as wear, fractured margins, delaminated coatings, and bulk fracture. Mechanisms responsible for fatigue-induced failure depend on material ductility: Brittle materials are susceptible to catastrophic failure, while ductile materials utilize their plasticity to reduce stress concentrations at the crack tip. Because of the expense associated with the replacement of failed restorations, there is a strong desire on the part of basic scientists and clinicians to evaluate the resistance of materials to fatigue in laboratory tests. Test variables include fatigue-loading mode and test environment, such as soaking in water. The outcome variable is typically fracture strength, and these data typically fit the Weibull distribution. Analysis of fatigue data permits predictive inferences to be made concerning the survival of structures fabricated from restorative materials under specified loading conditions. Although many dental-restorative materials are routinely evaluated, only limited use has been made of fatigue data collected in vitro: Wear of materials and the survival of porcelain restorations has been modeled by both fracture mechanics and probabilistic approaches. A need still exists for a clinical failure database and for the development of valid test methods for the evaluation of composite materials.

Inference on p{y<x} in the weibull case

The probability that a Weibull random variable Y is less than another independent Weibull random variable X is considered for the case where both X and Y have the same, but unknown, shape parameter. Tables,developed by Monte Carlo sampling, are presented whereby 90% confidence limits for this probability may be found in terms of its maximum likelihood estimate for 21 combinations of the size of the sample taken from each of the two populations and the ordered observation number at which the samples are type II censored. A normal approximation is also discussed and its accuracy vis-a-vis the exact values is examined as a function of sample size for a particular case

CONTROL CHARTS APPLICABLE WHEN THE FRACTION NONCONFORMING IS SMALL

It has been proposed that when the proportion of nonconforming product is extremely small that one base 3sigma control charts on the 0.2777th power of X, the number of items sampled until a nonconforming item is found. This choice of exponential renders..

Light curable dental composites designed with colloidal crystal reinforcement

OBJECTIVES Methods to prepare dental composites with a periodic filler arrangement were developed following a strategy of colloidal crystallization. The aims of this study were to determine the influence of suspension medium, silane treatment and amine additive on colloidal particle redispersion and subsequent ordering, and to evaluate the effect of filler ordering on mechanical properties of composites. METHODS Dry monodisperse silica particles (spherical, approximately 500-nm diameter) were redispersed in selected solvents and monomers (e.g. triethyleneglycol dimethacrylate, TEGDMA) to form sediments or dispersions with ordered particle arrangements. Ordering was evaluated by microscopy and mechanical properties of the composites were measured using compression tests (n=6). RESULTS A face-centered cubic packed structure could form in both the sediment from silica dispersions in polar solvents and stable dispersions in TEGDMA. Dimethylaminoethyl methacrylate (DMAEMA) was found to disrupt an ordered structure when non-silanized silica particles were used. Silanization with 3-methacryloxypropyl trimethoxysilane (MPS) promoted filler ordering. Standard compression tests on composites containing 60wt% silica in TEGDMA with or without DMAEMA indicated that DMAEMA had a clearly significant effect (p<0.05) on failure strain, compressive strength, and toughness, and a marginally significant effect on modulus (p=0.12). SIGNIFICANCE Significant increases in compressive strength (16%), failure strain (71%), and toughness (135%) were observed for composites with ordered filler compared to non-ordered composites.

Evaluating Weibull Endurance Data by the Method of Maximum Likelihood

The method of maximum likelihood has been found to offer many advantages over existing methods for analyzing Weibull distributed rolling bearing fatigue data. The purpose of this paper is to provide the rolling bearing engineer with what he needs to use the method of maximum likelihood for, 1) Unbiased estimation of Weibull L10 life and shape parameter. 2) Setting confidence limits and testing hypotheses about Weibull L10 life and shape parameter. 3) Unbiased estimation of the ratio of two population L10s and shape parameters. 4) Testing the significance of differences between two estimates of L10 life or shape parameter. Presented as an American Society of Lubrication Engineers paper at the ASLE/ASME Lubrication Conference held in Houston, Texas, October 14–16, 1969

Lifetime predictions for resin-based composites using cyclic and dynamic fatigue

Because dental restorative materials undergo fatigue in use, testing is often performed in the laboratory to evaluate material responses to cyclic loading. The purpose of this study was to compare the lifetime predictions resulting from two methods of fatigue testing: dynamic and cyclic fatigue. Model composites were made in which one variable was the presence of a silanizing agent, and specimens tested in 4-point flexure. Cyclic fatigue was carried out at a frequency of 5 Hz, while dynamic fatigue testing spanned seven decades of stress rate application. Data were reduced and the crack propagation parameters for each material were calculated from both sets of fatigue data. These parameters were then used to calculate an equivalent static tensile stress for a 5-year survival time. The 5-year survival stresses predicted by dynamic fatigue data were approximately twice those predicted by cyclic fatigue data. In the absence of filler particle silanization, the survival stress was reduced by half. Aging in a water-ethanol solution reduced the survival stresses by a factor of four to five. Cyclic fatigue is a more conservative means of predicting lifetimes of resin-based composites.

Testing for Dependency of Failure Times in Life Testing

A method is proposed for testing whether the failure of the first specimen in a test machine that contains multiple specimens shortens the time to failure of the other specimens in that unit. The time to failure is modeled as a two-parameter Weibull distribution. Two estimates of the shape parameter are constructed: an estimate based on the first failures in each of k identical test units, and an estimate based on the ratios of the second to the first failure times within each unit. The supposition is that these ratios will be shortened if the first failure has influenced the second, causing the estimate of the shape parameter constructed from these ratios to tend to be larger than the estimate obtained from the set of first failures. The null distribution of the ratio of the two shape parameter estimates was determined by Monte Carlo sampling for the case of k units containing exactly two specimens each with k = 5(1)20. The power is determined under the assumption that the first failure causes the remaining failures to occur according to a Weibull distribution with the same shape parameter as the original failure distribution but with a scale parameter reduced by the factor c < 1. Numerical results are given for the case where k = 10 and c = .1(.1).9. The article contains a numerical example based on simulated data and a second example based on actual data obtained from fatigue testing of ball bearings. A comparable test is developed for the case where the Weibull shape parameter is assumed to be known.

Weibull models of fracture strengths and fatigue behavior of dental resins in flexure and shear

In estimating lifetimes of dental restorative materials, it is useful to have available data on the fatigue behavior of these materials. Current efforts at estimation include several untested assumptions related to the equivalence of flaw distributions sampled by shear, tensile, and compressive stresses. Environmental influences on material properties are not accounted for, and it is unclear if fatigue limits exist. In this study, the shear and flexural strengths of three resins used as matrices in dental restorative composite materials were characterized by Weibull parameters. It was found that shear strengths were lower than flexural strengths, liquid sorption had a profound effect on characteristic strengths, and the Weibull shape parameter obtained from shear data differed for some materials from that obtained in flexure. In shear and flexural fatigue, a power law relationship applied for up to 250,000 cycles; no fatigue limits were found, and the data thus imply only one flaw population is responsible for failure. Again, liquid sorption adversely affected strength levels in most materials (decreasing shear strengths and flexural strengths by factors of 2-3) and to a greater extent than did the degree of cure or material chemistry.

Inference on the Weibull Location Parameter

A simple method is proposed for testing the 2-parameter Weibull distribution against the 3-parameter alternative. It is based on the ratio of the maximum likelihood estimate of the shape parameter calculated with the observations censored at the r1th or..