DAVID M. STEPP
Adjunct Assistant Professor
Department of Mechanical Engineering and Materials Science

Contact Information
189 Hudson Hall Annex
919-549-4329, 919-660-5325 phone
919-660-8963 fax
david.stepp@duke.edu

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Education

	PhD	A Positron Annihilation Lifetime Study of Shock Loading in Tantalum, Mechanical Engineering and Materials Science, Duke University, 1998
	MS	A Chi-Square Goodness of Fit Analysis of Positron Annihilation Lifetime Spectra, Mechanical Engineering and Materials Science, Duke University, 1995
	BS	Engineering, Harvey Mudd College, 1993

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Experience

	1999-present	Chief, Mechanical Behavior of Materials Branch, U.S. Army Research Office
	1999-present	Adjunct Assistant Professor, Mechanical Engineering and Materials Science, Duke University
	1999-present	Assistant Research Professor, Mechanical Engineering and Materials Science, Duke University

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Selected Publications

1. J.A. King, M. Grundmeyer, D.M. Stepp, and D. Hugo, 1993. “Water Tree Morphology,” IEEE Transactions on Electrical Insulation, 28(3), pp. 415-419.
2. D.M. Stepp, J.A. King, J. Worrall, A. Thompson, and D.E. Cooper, 1996. “High-resolution Study of Water Trees Grown in Silver Nitrate Solution,” IEEE Transactions on Dielectrics and Electrical Insulation, 3(3), pp. 392-398.
3. D.P. Garg, M.A. Zikry, G.L. Anderson, and D. Stepp, 2002. “Health Monitoring and Reliability of Adaptive Heterogeneous Structures,” Structural Health Monitoring, 1(1), pp. 23-39.
4. D.M. Stepp, 2002. “Damage Mitigation in Ceramics: Historical Developments and Future Directions in Army Research,” Ceramic Transactions, 134, pp. 421-428.
5. D.M. Stepp, P.L. Jones, and G.W. Pearsall. “Positron Iterative Fit: A Statistical Approach to PALS,” under revision for publication, Nuclear Instruments and Methods in Physics Research – Section B: Beam Interactions with Materials and Atoms.
6. B.J. Ward, G.W. Pearsall, and D.M. Stepp. “Extending the Limits of Small Volume Fracture Toughness Testing in Polycarbonate,” currently under preparation for submission to Journal of Materials Science.
   

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Short Research Interest Descriptor

My research interests include the design and characterization of soft materials, particularly elucidating structure-property relationships through the use of experimental analysis and computational simulations and the rapid mechanical characterization and of very low volumes of materials.

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Research Interest

My primary research efforts are currently focused on characterizing both the microstructure and mechanical behavior of BPA-polycarbonate in order to elucidate the intrinsic relationships governing deformation and fracture behavior; the ultimate goal of the effort is to establish a fundamental basis for designing transparent polymeric materials with optimal toughness. Various structural and optical characterization tools, such as polarized light microscopy, DSC, AFM, SEM, sputter coating, and positron annihilation lifetime spectroscopy are being utilized to establish a robust understanding of the amorphous chain structure in polycarbonate, including the effects of free volume upon this structure. The effects of these microstructural changes are being correlated with measurements of fracture toughness, tensile strength, glass-transition temperature, and a topographic analysis and interpretation of the fracture surfaces that result from tensile and fracture-toughness testing. Future directions for this effort include the use of atomistic and molecular modeling approaches to elucidate the chain conformations and motions associated with chain folding and spherulite formation and single-molecule mechanical characterization.

My research is also developing new methodologies for improving the accuracy of positron annihilation spectroscopy, which includes a family of non-destructive testing techniques derived from the observation and analysis of gamma rays associated with positron-electron annihilations. This effort is currently focused on the continued development of a new fitting algorithm that implements the chi-square goodness of fit test, both to correctly determine the best fit for a given positron annihilation lifetime spectrum and to measure the goodness of this best fit statistically. By measuring the statistical goodness of the results in this way, this unique algorithm has been demonstrated to provide substantially improved discernment of global solutions from local ones. Further refinement of this approach is expected to make possible a dramatic increase in the precision of positron annihilation results and thereby enable a more detailed analysis of the free volume and microstructure of polycarbonate than has previously been possible.

 

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