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The purpose of this note is to report some preliminary results from a study of near-threshold fatigue crack growth in a rail steel under mode III loading and, on the basis of these results, to make some general observations concerning the nature of mode III fatigue threshold. Table 1 contains the chemical composition and mechanical properties of the standard carbon rail steel used in this investigation.Edge-notched bar specimens having square cross-section were machined from the head of a heavy haul rail.Mode III fatigue tests were then conducted on these using a novel test rig developed at the University of New South Wales [i], which allows any desired combination of mode II and mode III loading conditions to be applied to the test specimen.The mode III fatigue threshold value was determined by an increasing load technique.Each specimen was tested for 4 million cycles at a given load range, with load ratio R_>0. If no evidence of fatigue cracking was found, either by the use of a specimen protection device sensitive to changes in the compliance of the specimen or by visual inspection of the notch with a monocular eyepiece, the specimen was re-tested at a higher load range.The results obtained by following this procedure are shown in Fig. I, from which a mode III fatigue threshold value for standard carbon rail steel of AKIIIt h = 18.8 ± 2.0 MPa/m is indicated.It is well established from torsion tests on cylindrical, circumferentially-notched specimens [2] that fatigue crack propagation under mode III loading generally leads to a macroscopically flat (mode III shear) fracture surface for high stress intensity ranges and short crack lengths, while a "factory roof" morphology (resulting from the growth of mode I branch cracks) develops at low stress intensity ranges.For a given applied loading, the growth rates of both types of crack diminish rather than increase with increasing crack length due to interference between the crack faces, and a transition commonly occurs from a macroscopically flat to a factory roof type of fracture surface. This transition has been taken to define a pseudo mode III threshold for macroscopically flat fatigue crack growth [3], the value of which is in general considerably higher than the fatigue threshold corresponding to factory roof cracking. Pook and Sharples [4] have suggested from consideration of a mode Ibranch at a mode III crack that the ratio of this latter mode III threshold to the mode I threshold for a given material should theoretically be 1.35. The value of AK . reported in the present note is larger by a factor of about 1.4 than ~I t~ " Table I, in e mean value of AK h for rail steel glven in t fairly good agreement with this pre~iction. 29 (1985) R45 R46 Examination of the fatigue fracture surfaces by scanning electron microscopy revealed the presence of a factory roof morphology in all cases, as expected from the considerations outlined above. However, the fracture surfaces of some specimens also exhibited a number of small, macroscopically flat regions where fatigu...

Concrete reinforced by galvanized reinforcing bars requires the addition of inhibitor anions to passivate the zinc; otherwise evolution of hydrogen reduces the bond strength. In the present study, zinc in saturated calcium hydroxide solution, which is the principal electrolyte formed on initial hydration of cement, was found to passivate in the presence of either sodium chromate or chromic oxide. In both cases chromate ions were reduced instead of water so that there was no evolution of hydrogen. The passive film consisted of zinc chromate and chromic oxide, but calcium hydroxo-zincate was also produced, which assisted in passivating the zinc surface. In the case of chromic oxide a smaller concentration of chromate ions was present and passivated the zinc to a lesser extent. While 70 ppm of sodium chromate was sufficient to passivate the zinc, at least 300 ppm of chromic oxide was necessary to achieve the same degree of passivation. Chloride ions in the presence of chromate ions compete for the zinc surface and a critical concentration of chromate is necessary. This observation has been correlated with the passivation of galvanized reinforcing bars in concrete and it is considered that passivation is best achieved by the addition of 70-ppm sodium chromate to the concrete mix.