Date(s) - 03/16/2021
3:00 pm - 4:00 pm
John Mecholsky, Ph.D.
Professor, Associate Chair
Department of Materials Science & Engineering
University of Florida
Dr. John (Jack) Mecholsky is a Professor at the University of Florida in the Department of Materials Science & Engineering. He served as the Associate Chair from 2005-2010 and 2017-Present, the Chair of the Faculty Senate in the 2009-2010 academic year and served on the Board of Trustees for the University of Florida (2009-2010). He is a Fellow of the American Ceramic Society (ACerS). He served on the Board of Directors of the American Ceramic Society from 2006 to 2009. He is known as an international expert in the fractographic analysis of brittle materials. While on sabbatical leave (1995-1996) he served as the Associate Director for Materials at the Office of Naval Research in London (UK) and (in 2006) as a Guest Researcher at the National Institute for Standards and Technology ) and at the Cavendish Laboratory in Cambridge University (2013). As a recipient of the UF’s Faculty Enhancement Opportunity (FEO) award he spent two months at Imperial College in London (2010) as a visiting researcher. He won the Teacher of the Year Award in 2006 and the Graduate Advisor of the Year Award in 2009. Prior to 1990 he held a joint appointment at the Penn State University in the Materials Science Department as an Associate Professor and as a Research Associate in the (U. S. Navy’s) Advanced Research Laboratory. From 1979-1984 he was a member of the technical staff at Sandia National Laboratories in Albuquerque NM. He worked at the Naval Research Laboratory in Washington, D.C. from 1972 – 1979 as a Ceramic Research Engineer. While finishing his graduate degrees he was a structural research engineer at the Naval Ship Research & Development Center (formerly the David Taylor Model Basin) from 1967-1972. He helped design the pressure hull for the Deep Submergence Search Vehicle (DSSV) and the escape hatch for the Deep Submergence Rescue Vehicle (DSRV) [shown in the movie “Hunt for Red October”]. He developed new fractographic techniques used in the failure analysis of dental FPDs, optical fibers, infrared transmitting radome materials, and of ferroelectrics. He also developed equations for the analysis of failure by laser irradiation of ceramic materials. He holds patents for the development of a laser hardened composite material and a bioactive tapecast multi-layer ceramic/metal composite. He has published over 200 technical papers and is the co-author of “Fracture of Brittle Materials: Testing and Analysis” (Wiley Pub. 2019).
Failure Analysis of dental crowns and bridges can be extremely useful for preventing future failures and guiding the design of reliable prosthetics. The combination of fracture mechanics, quantitative fractography and fractal geometric analysis offers a powerful tool for failure analyses.
Fracture mechanics is based on the relationship between the fracture toughness, KIC, strength, s, and failure initiating flaw size, c: KIC = Y s (c) ½ where Y is a load and crack geometric parameter. There are standard tests for establishing fracture toughness and the values are known or can be established for dental ceramics. Thus, the strength can be determined for an unexpected failure by measuring the fracture initiating flaw size.
Fractography provides the methodology for finding the fracture origin and determining or estimating the size of the fracture initiating flaw.
Fractal geometry is a non-Euclidean geometry that provides a means for determining a numerical value for the tortuosity of a fracture surface at all length scales. This value is called the fractal dimension, D. The fractal dimension is related to the toughness of the material:
KIC µ D*1/2 where D* is the fractal dimensional increment. Thus, the fracture toughness can be estimated from a measurement on the fracture surface. This is important in forensic analyses because, for example, it can determine if a dental crown was fabricated properly.
This presentation will focus on describing the elements for each of the approaches and will show how these all blend into a complementary procedure for understanding failures in components. Several specific examples will show how veneers govern the behavior of crowns and how a combination of stress and failure analyses determine the location of failure and the magnitude of strength in three and four-unit bridges.