V. A. Stepanenko

The effect of low temperatures on the rate and microfractographic features of the development of a fatigue crack in low-carbon steel

Several investigations of the development of a fatigue crack at low temperatures have indicated that with a decrease in temperature the rate of the growth of the crack decreases [1-3]. However, as a result of tests of mediumand low-carbon steels at temperatures of +20 to --160~ it has been established that tnheeffect of reduced temperature on the growth rate of a crack depends substantially upon the value of the amplitude of stress [1-4] and that the growth rate of a crack decreases at low amplitudes and increases at higher ones. In addition, in [5] it was shown that in 15G2AFDps steel at reduced temperature the growth rate of a crack does not always change steadily. Until a certain temperature (--95~ it decreases at all amplitudes and then depending upon the level of stresses either continues to decrease or increases and then again decreases. Regrettably the complex configuration of a crack in the cylindrical samples used in these investigations very much complicates a quantitative analysis of the stresses in the vicinity of its contour. A similar inconsistent temperature relationship of the propagation rate of a crack was observed in testing high-temperature tempered 65G steel [6]. Until now the reasons for this behavior of materials have not been studied.

Use of linear and nonlinear fracture mechanics for assessing resistance to crack propagation in 15Kh2NMFA structural steel

This note considers a possibility of application of linear and nonlinear fracture mechanics for the assessment of resistance to crack propagation in the 15Kh2NMFA steel of a special melt using the results obtained in testing specimens of different sizes.

Mechanism of propagation of a fatigue crack in nickel by the method of quantitative stereoscopic fractography

Conclusions1.Using the method of stereoscopic fractography, continuous relief profiles were studied on matching fatigue fracture surfaces.Direct evidences of projection-depression and depression-projection correspondence were obtained. The nonidentity of the profiles on opposite sides of a fracture was noted.2.The features of fatigue groove structure were studied. The narrow sides of a fatigue groove on opposite fracture surfaces were faced to different sides and not to the same, as was earlier established by other authors for aluminum alloys. It was shown that these narrow sides are points of contact of opposite matching fatigue grooves.3.On the basis of matching the opposite profiles, conclusions were drawn on the character of propagation of a fatigue crack. A new mechanism of the crack propagation is proposed according to which the advance of a crack during a load cycle consists of two stages, the stage of development of the transverse shear crack and the stage of plastic blunting of the crack tip. In accordance with this mechanism, all of the advance of the crack occurs in the loading stage but the unloading stage also plays an important role in preparation of the material for subsequent shear failure.4.Within the limits of the proposed model, an explanation is given for various experimental facts regarding the propagation of a fatigue crack. In particular, the fact of a decrease in the ratio of the height of a groove to its width with an increase in the load amplitude and also effects observed in variations of unloading of a sample are explained.

Use of stereoscopic fractography for analyzing resistance to the development of cracks

Conclusions1.The relationship between the height and width of the stretch zone [Eq. (11)] may be considered acceptable, at least, in comparatively ductile failure.2.With a 25 times increase in test sample thickness there is practically no change in the height of the stretch zone.3.At low temperatures a correlation has been established between the height of the stretch zone and fracture roughness which is predicted by mathematical models of the opening at the crack tip. A linear corrlation coupling factor between 2h and KIc2/Eσy equal to 0.9 corresponds to the results of this work.4.At quite high temperatures the height of the stretch zone is less than 0.9(KIc2/Eσy. This is explained, frist, by the increase in deformation strengthening with an increase in temperature and, second, by the fact that in this case the values of KIc are determined in the presence of a macroscopic increase in crack length.5.The subcritical crack growth in the investigated steel has the character of jumps and stops. The presence of secondary stretch zones is apparently caused by delay and blunting of the crack in front of a macroscopic jump and their area is quantitatively related to the R-curve at a given temperature.

The failure of polycrystalline iron under plane stress conditions at low temperatures

Conclusions1.The mechanism of failure of commercial iron is determined by the temperature-force conditions used in testing. At normal temperatures the metal fails by slip without any crack formation, irrespective of the stress conditions: at low temperatures failure is preceded by the development of trans-and intercrystalline microcracks, the space orientation of which depends on the form of the stress state.2.The statistical distributions of the number and total length of isolated microcracks with respect to their orientation have been constructed from data obtained by the metallographic examination of ruptured specimens. The results show that the orientation of the majority of the microcracks formed in the initial stages of plastic flow is similar to that of the planes on which the maximum tensile stresses act, although microcracks having a substantially different orientation have also been observed.3.Microcracks do not grow in a straight line within a single grain, but appear as a zigzag line. The distribution of the number and total length of the straight portions of the microcracks with respect to orientation indicates that the mean length of these portions is commensurate with the mean distance between the nonmetallic inclusions. The reason for the change in direction of the microcracks evidently lies in the interaction of the elastic-stress field at the tip of the crack with the elastic-stress field surrounding a nonmetallic inclusion.4.At low temperatures the appearance of a second tensile stress has a significant effect, leading to a reduction in the ductility of the iron. The effect was most clearly observed at −100°C, at which temperature the maximum slip before failure in uniaxial tension considerably exceeded the maximum slip in biaxial tension. The suggestion is put forward that this effect is due to the smaller degree of opening of the nucleated microcracks in the presence of a second tensile stress.

The mechanical properties and failure of mild steel in tension

Conclusions1.The temperature range of blue brittleness is 100° wider in fine-grained mild steel than in coarsegrained, and extends approximately from +100 to +300°C.2.With all three grain sizes investigated maximum ductility occurs in the temperature range from −100 to +20°C. Maximum permanent elongation occurs at −50°C, and maximum relative transverse contraction after failure occurs in the range from −100 to +50°C. In this range transverse contraction is slightly sensitive to temperature.3.The methods of optical and electron fractography have revealed, even in tests at very low temperatures, traces of plastic deformation in the fracture, with patterns typical of slip and twinning. The appearance of cleavage tongues is evidently due to the intersection of a cleavage crack with deformation twins generated by a high-stress zone at the tip of a moving crack.