This report outlines a qualitative concept of the nature of the behavior leading to fracture of metals under repeated applications of load, which has been formulated in terms of the phenomena proceeding on an atomic scale. The concepts involved are based on the results of recent investigations concerning the structure of polycrystalline metal aggregates, the formation of inherent stresses in them, and the nature of the processes of workhardening, thermal relaxation of bond energy, and of statistical disruption of atomic bonds.
Briefly, the theory indicates that fatigue damage is initiated within the unordered material at crystal boundaries, and progresses during repeated loading within the disordered atomic regions produced during slip and crystal fragmentation. The damage due to forced relaxation and to disruption of atomic bonds is counteracted to some extent by the spontaneous relaxations of some of the high energy bonds (energy dissipation) during unloading. The main conclusion arrived at is that the rate of relaxation of bond energy within the intercrystalline material is the principal single material factor determining the character and rate of progressive damage under repeated stress cycles.
Many of the phenomena observed in fatigue testing can be interpreted from this point of view, and the expected influence on fatigue strength, of variation of the principal parameters (time, temperature, previous strain-history, rest-periods, etc.) may be qualitatively predicted. Tests are proposed, and are being conducted to check predictions regarding the effects on fatigue life produced by thermal activation during rest periods at moderately elevated temperatures for several different materials and conditions of operation. An appendix is included to review certain concepts of the physics of matter as an aid (and as preliminary reading) for those who may not be fully familiar with some of the terminology used in the report.
Publisher
Department of Theoretical and Applied Mechanics. College of Engineering. University of Illinois at Urbana-Champaign
Series/Report Name or Number
TAM R 4
1967-0303
ISSN
0073-5264
Type of Resource
text
Language
eng
Permalink
http://hdl.handle.net/2142/112139
Sponsor(s)/Grant Number(s)
Office of Naval Research, U.S. Navy, Contract N6-ori-71, Task Order IV
Copyright and License Information
Copyright 1948 Board of Trustees of the University of Illinois
TAM technical reports include manuscripts intended for publication, theses judged to have general interest, notes prepared for short courses, symposia compiled from outstanding undergraduate projects, and reports prepared for research-sponsoring agencies.
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