Dental Research Management Centre (DRMC), Faculty of Dentistry with HIR/MoHE/Dent/11
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Everything You Always Wanted to Know About Shrinkage Stress But Were Afraid to Ask
Part 2 – A Simple Predictive Formula for Laboratory Shrinkage Stress Measurement

The shrinkage stress that a resin composite can generate is usually assessed using disc specimens attached to a uniaxial tensile-test or cantilever beam-bending equipment. It is generally recognized that shrinkage stress depends on the composite’s final elastic modulus, total shrinkage strain, dimensions of the specimen and stiffness of the stress-measuring equipment. However, the exact relationship is not known and there have been many contradictory results reported in the literature, notably, the shrinkage stress reducing with an increasing C-factor. Using a Maxwell-type viscoelastic model, a simple predictive formula for the laboratory-measured shrinkage stress has been derived. Comparisons of its predictions with results reported in the literature confirm the general applicability of this formula. These include the effect of machine stiffness on the relationships among shrinkage stress, material properties and specimen dimensions, the possible reduction in shrinkage stress with an increasing C-factor, and the effect of filler volume on shrinkage stress. The new shrinkage stress formula can therefore help resolve most, if not all, of the apparent contradictions in the reported results. It can also provide useful material design pointers for reducing shrinkage stress.

Bio-Sketch –

Professor Dr. Alex Fok
BEng (Manchester), BA (UK), MSc (Oxford), Ph.D (Manchester)

Dr Fok obtained his BEng and PhD, both in Mechanical Engineering, from the University of Manchester, UK. He has expertise in structural and stress analysis, having spent 4 years working as a structural analyst in the nuclear industry after gaining his PhD. Prior to moving to Minnesota, he was Senior Lecturer in Mechanical Engineering at the University of Manchester.

Dr Fok’s research activities cover a range of topics in solid mechanics, nuclear graphite technology and biomechanics, both at a fundamental level and related to practical applications. These include statistical analysis of brittle failure, fractal finite elements, micro-structural modeling of materials, material characterization using full-field strain measurement and inverse methods, multiple fracture from dynamic stresses, buckling of thin embedded shells and the biomechanics of dental restorations. The finite element method forms the main tool of his research.

In 2007, Dr Fok accepted the invitation of the University of Minnesota School of Dentistry to become Director of the Minnesota Dental Research Center for Biomaterials and Biomechanics (MDRCBB). Established as an industry-academic collaboration with funding from the 3M Foundation and 3M Dental Products Division, the MDRCBB works closely with the industry on the development of new dental products and biomaterials.

Dr Fok’s current research activities include shape optimization of dental restorations, shrinkage strain measurement using digital image correlation, nondestructive examination of interfacial debonding using acoustic emission and development of alternative bond tests for dental materials. Together with colleagues from the Dental School, he has recently been awarded an NIH grant looking at the possible effect of bacterial activities on the degradation of composite restorations. He has also been successful in securing two grants from the DOE, one with colleagues from the Department of Civil Engineering, to continue his work on nuclear.