The Vickers indentation fracture toughness test, or VIF, is addressed by considering its origins and the numerous equations that have been applied along with the technique to estimate the fracture resistance, or the K Ic of ceramics. Initiation and propagation of cracks during the VIF test are described and contrasted with the pre-cracking and crack growth for internationally standardized fracture toughness tests. It is concluded that the VIF test technique is fundamentally different than standard fracture toughness tests. The VIF test has a complex three-dimensional crack system with substantial deformation residual stresses and damage around the cracks. The VIF test relates to an illdefined crack arrest condition as opposed to the rapid crack propagation of the standardized fracture toughness tests.Previously published fracture toughness results employing the VIF technique are reviewed. These reveal serious discrepancies in reported VIF fracture toughness values. Finally, recent fracture resistance measurements by the VIF technique for the Standard Reference Material SRM 2100 are presented. These are compared with standardized test results for the same material. It is concluded that the VIF technique is not reliable as a fracture toughness test for ceramics or for other brittle materials. What the VIF actually measures in terms of fracture resistance cannot be readily defined. It is recommended that the VIF technique no longer be acceptable for the fracture toughness testing of ceramic materials.
Objectives To review the history, theory and current applications of Weibull analyses sufficient to make informed decisions regarding practical use of the analysis in dental material strength testing. Data References are made to examples in the engineering and dental literature, but this paper also includes illustrative analyses of Weibull plots, fractographic interpretations, and Weibull distribution parameters obtained for a dense alumina, two feldspathic porcelains, and a zirconia. Sources Informational sources include Weibull's original articles, later articles specific to applications and theoretical foundations of Weibull analysis, texts on statistics and fracture mechanics and the international standards literature. Study Selection The chosen Weibull analyses are used to illustrate technique, the importance of flaw size distributions, physical meaning of Weibull parameters and concepts of “equivalent volumes” to compare measured strengths obtained from different test configurations. Conclusions Weibull analysis has a strong theoretical basis and can be of particular value in dental applications, primarily because of test specimen size limitations and the use of different test configurations. Also endemic to dental materials, however, is increased difficulty in satisfying application requirements, such as confirming fracture origin type and diligence in obtaining quality strength data.
Objectives-To demonstrate the effectiveness of in vivo replicas of fractured ceramic surfaces for descriptive fractography as applied to the analysis of clinical failures.Methods-The fracture surface topography of partially failed veneering ceramic of a Procera Alumina molar and an In Ceram Zirconia premolar were examined utilizing gold-coated epoxy poured replicas viewed using scanning electron microscopy. The replicas were inspected for fractographic features such as hackle, wake hackle, twist hackle, compression curl and arrest lines for determination of the direction of crack propagation and location of the origin.Results-For both veneering ceramics, replicas provided an excellent reproduction of the fractured surfaces. Fine details including all characteristic fracture features produced by the interaction of the advancing crack with the material's microstructure could be recognized. The observed features are indicators of the local direction of crack propagation and were used to trace the crack's progression back to its initial starting zone (the origin). Drawbacks of replicas such as artifacts (air bubbles) or imperfections resulting from inadequate epoxy pouring were noted but not critical for the overall analysis of the fractured surfaces.Significance-The replica technique proved to be easy to use and allowed an excellent reproduction of failed ceramic surfaces. It should be applied before attempting to remove any failed part remaining in situ as the fracture surface may be damaged during this procedure. These two case studies are intended as an introduction for the clinical researcher in using qualitative (descriptive) fractography as a tool for understanding fracture processes in brittle restorative materials and, secondarily, to draw conclusions as to possible design inadequacies in failed restorations.
IntroductionFailure analysis includes examination of a fractured component in order to investigate the circumstances surrounding a failure event with the expectation of eventually elucidating the cause of failure, whether it was a result of design deficiency, material deficiency (fabrication process) or in situ stress-induced conditions.Fractography includes the examination of fracture surfaces that contain features resulting from the interaction of the advancing crack with the microstructure of the material and the stress fields. The description and interpretation of fracture markings used to understand failure events of brittle materials are summarized in a comprehensive, new book currently available, a NIST recommended practice guide for Fractography of Ceramics and Glasses 1 (free download at http://www.ceramics.nist.gov/pubs/practice.htm) as well as by the ASTM C1322-05a, Standard practice for fractography and characterization of fracture origins in advanced ceramics 2 .The key tools for performing fractography on failed parts are the binocular stereomicroscope and the scanning electron microscope (SEM). The stereomicroscope is often used to carry out the preliminary inspection of the fractured ceramic piece. Its separate optical paths with two eyepieces provide slightly different viewing angles to the left and right eyes. In this way the stereomicroscope produces a 3-D visualization of the sample being examined allowing better understanding of the spatial relationships of the observed fracture features which are best viewed by varying the angle of the illumination. In addition, it retains all color, surface roughness and reflectivity information of the fractured compound. The SEM, on the other hand, is an excellent complementary tool to the stereomicroscope analysis allowing for highresolution close-ups of predetermined regions of interest. In many instances, an optical stereomicroscope examination is enough for an interpretation.Very few papers in the dental literature have been published using a standardized approach of crack feature recognition to failure analysis of fractured ceramic restorations 3-10 . Hence, the *Corresponding author: Susanne S. Scherrer,
The uniaxial strength of engineering ceramics is often measured by the well‐known flexure strength test method there is a risk that flexure data are not representative of the properties of fabricated components. Reliability estimates for components based upon statistical extrapolation techniques from flexure data may not be valid. This paper reviews the problem and judges the usefulness of flexure data for design purposes. It is shown that some of the limitations of flexure data apply; to other modes of testing, including direct tension testing
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