Fatigue of iron and of Fe-Si 5% down to liquid helium temperature and the relationship between fatigue and brittleness

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Abstract
Fatigue of Armco iron and of Fe-Si 5% is studied at liquid nitrogen and liquid helium temperatures in symmetric torsion, and by estimating the plastic strain amplitudes from internal friction measurements and from comparison with the static stress-strain curves. It is shown that, contrary to previous experiments of MacCammon and Rosenberg, iron in pure shear still shows a definite fatigue curve also at 4°K. The corresponding plastic strain amplitude at the fatigue limit is of the order of 10−4. For Fe-Si 5%, on the other hand, fatigue at 4°K almost disappears, and this disappearance seems associated with a critical value of the strain to fracture of the order of 10−5. In the representation of Manson and Coffin of the plastic strain amplitude as a function of the number of cycles it is shown that the slope of the curves decreases with increasing brittleness and for the silicon alloy becomes almost zero at liquid helium temperature. The results obtained on the relationship between fatigue and brittleness can be interpreted by considering fatigue as being necessarily associated with dislocation multiplication as already proposed by Mason.

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