An X-Ray diffraction analysis of low cycle bending fatigue was conducted on plain carbon steel (SAE 1008) in three conditions, as received (annealed) 20 percent prestrained, and strain aged. The decrease of the surface stresses at maximum and minimum strains, a reduction of the residual surface stress after unloading from the tension cycle, and a decrease of the half-value breadth during the first 20–30 cycles indicate rapid cyclic softening during the initial stages of strain cycling. After this initial period the microstructure of all specimens was found to be independent of the initial material condition. Similarly, the fatigue life as a function of strain range was also independent of the initial condition. X-Ray diffraction line analysis shows no changes for the major portion of the specimen life, from 0.1 Nf to 0.75 Nf. However, a linear relationship between half-value breadth and strain range is noted. It is suggested that this relationship can be utilized for monitoring and diagnostic purposes, particularly to determine the strain range to which a critical part has been subjected.

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