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Critical Entropy Threshold: An Irreversible Thermodynamic Theory of Fatigue

[+] Author Affiliations
P. W. Whaley

Oklahoma Christian University, Oklahoma City, OK

Paper No. IMECE2012-85435, pp. 1125-1135; 11 pages
doi:10.1115/IMECE2012-85435
From:
  • ASME 2012 International Mechanical Engineering Congress and Exposition
  • Volume 3: Design, Materials and Manufacturing, Parts A, B, and C
  • Houston, Texas, USA, November 9–15, 2012
  • Conference Sponsors: ASME
  • ISBN: 978-0-7918-4519-6
  • Copyright © 2012 by ASME

abstract

A theoretical model for material fatigue is described using irreversible thermodynamics to quantify fatigue damage by the generation of microplastic entropy. The microplastic entropy generated quantifies the microplastic deformation, commonly accepted as the mechanism of fatigue damage in polycrystalline materials. A stochastic model for microplastic deformation is utilized to calculate the expected values of tensile stress–strain, cyclic stress–strain, microplastic strain energy density and the microplastic entropy generated. When the cumulative microplastic entropy generated in cyclic loading exceeds the critical microplastic entropy threshold calculated from tensile tests, failure occurs. Calculated fatigue life with 99% tolerance limits (99% confidence) compares favorably to data for 6061-T6 aluminum rod and sheet specimens. Model parameters are determined from tensile tests and simple cyclic tests, decreasing the high cost of fatigue testing for parameter identification. This new theory has the potential to significantly decrease the cost of characterizing the fatigue properties of new materials.

Copyright © 2012 by ASME

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